Environmental Resources Management

#### **Chapter 2**

## Environmental Education and Community-Based Natural Resource Management in Zambia

*Delphine Inonge Milupi, Liberty Mweemba and Kaiko Mubita*

#### **Abstract**

This chapter analyses the relationship between Environmental Education (EE) and Community-Based Natural Resources Management (CBNRM) in Zambia. It examines the roles that EE could play in the management of natural resources. The chapter begins with the general introduction on CBNRM. It further analyses the concept of Environmental Education by explaining its aims and objectives. The chapter further deliberates on the history of CBNRM as an approach to natural resource management in Africa and Zambia in particular. The chapter concludes by giving an analysis of the roles played by EE in CBNRM in Zambia and recommends the incorporation of EE in the sustainable management of natural resources in the country. Doing so would provide every person with opportunities to acquire the knowledge, values, attitudes, commitment and skills needed to protect and improve the environment.

**Keywords:** community participation, community resource boards and game management areas, environmental education, rural community development, community-based natural resource management (CBNRM)

#### **1. Introduction**

There is a growing interest in rural community development and conservation of natural resources in Africa in recent years that has led to the development of the community-based natural resources management [1]. This is as a consequence of recognised general failures of centralised approaches to natural resources management to arrest lasting losses of biodiversity all over the world during colonial and post-independence periods led to a search for an alternative 'Community-Based Natural Resources Management' (CBNRM). The concept of CBNRM rose specifically to address the objectives of environmental, social, economic as well as social justice. CBNRM has been adopted as a positive approach to the management of natural resources management in many countries. This is because the approach integrates wildlife conservation and rural development goals [2, 3].

The concept of CBNRM rose specifically to address the objectives of environmental, social, economic as well as social justice. CBNRM has been adopted as a positive approach to the management of natural resources management in many countries.

This is because the approach integrates wildlife conservation and rural development goals [2, 4, 5] noted that benefits to natural resource reliant on communities that are closely dependent on wildlife management. The approach according to [5] identifies that local communities could be motivated to embrace sustainable practices to natural resource management. According to the assumption of the foundation of CBNRM, the local communities are interested and willing to adopt and implement conservation programmes so as long as they are legally entitled to ownership of resources and its related benefits [6]. In view of these benefits, CBNRM stresses social fencing as a tool for conserving the natural resources in question and perpetuating the flow of benefits associated with it. In this context, the natural environment is treated as part of the community and perceives the community as part of the landscape. There is gratitude of the interdependence of physical environments with the community. Identifying the interdependence of community well-being and ecosystem health, there is need to strengthen the capacity of communities to have the ability to speak in decisions about planning and design of conservation initiatives affecting them at local level.

The natural environment plays a huge role in supporting peoples' livelihoods, the health and welfare at large and people in turn have faith in their source of income, food and other resources. This is the case for many across the world who live in inaccessible villages with little access to outside markets. They depend on the land to offer them with enough food to feed their families through the year and enough money so that they can meet the expense of medical care, clothing and shelter.

In Zambia, the rural livelihoods depend significantly on the use of wildlife and other natural resources, harvesting forest community and on small-scale agriculture [7]. Further observation by [7] indicates that Zambia is one of Africa's most resourcerich countries with its two-thirds of land area being forested and nearly 40% of the land area being restricted within a network of national parks and forest reserves, as well as co-managed areas that overlap with customary community lands. These resources have to be sustainably managed. The local people who are in direct use of these resources have to be involved in planning and sustained yield practices. One of the most influential strategies to use to engage the local communities in sustainable use of natural resources is through Environmental Education (EE). It is against this background that this chapter emphases on the role of Environmental Education in Community-Based Natural Resource Management in Zambia. This is because the local communities have to be educated about the importance of sustainable management of natural resources in the country.

#### **2. What is environmental education (EE)?**

In order to understand how environmental education can be used as a strategy to natural resources management amongst the local communities in Zambia, there is need to clarify the concept. Environmental Education (EE) is a process that allows individuals to discover environmental issues, engage in problem solving and take action to improve the environment. EE refers to organised efforts to teach about how natural environments function and, particularly, how human beings can manage their behaviour and ecosystems in order to live sustainably. EE is sometimes used more broadly to include all efforts to educate the public and other audiences, including print materials, websites, media campaigns, etc. Related disciplines include outdoor education and experiential education [8]. One of the most important definition of EE given by UNESCO states that EE is a process that increases people's knowledge and

#### *Environmental Education and Community-Based Natural Resource Management in Zambia DOI: http://dx.doi.org/10.5772/intechopen.108383*

consciousness about the environment and related challenges, advances the necessary skills and expertise to address the challenges, advances the essential skills and expertise to address the challenges and fosters attitudes, motivations and obligations to make informed decisions and take responsible action [8].

The visibility of environmental problems and amplified awareness of their consequences have made environmental issues prominent in Zambia. At the beginning of the twenty-first century, EE, conservation and management emerged on the global policy stage [9, 10]. Most international declarations and conventions to combat environmental problems call for amplified environmental awareness among the population through EE [11]. Global environmental politics will only be successful if decision-makers are backed by an environmentally aware population. The concept of sustainable development needs to be decisively anchored in people's consciousness in order to effect behavioural change. This calls for EE that is an important tool to eliminate environmentally harmful forms of behaviour and learn how to safeguard the earth [11, 12]. Sound EE consists of learning from personal and conveyed experience in everyday situations (situational orientation), learning in connection with one's own direct action (action orientation) and incorporation of the subject matter into the socio-political context (problem orientation) [12]. Many ecosystems are controlled by human activity, and none is free from human influence [13, 14].

For individuals to worry themselves with environmental issues, they must first be aware that environmental problems occur. Without such awareness, society will not understand the need for change; will tend not to support it and maybe reluctant to participate in the process. The result of the insensible activity of economic systems according to [15, 16] often leads to environmental degradation. Awareness of environmental risks and the importance of responding to reduce or eliminate such risks is crucial to society. Awareness helps in achieving environmental literacy across economic sectors in all regions. The magnitude of environmental degradation or the sense of how environmental problems were becoming worse was not known for many years by most people. This let to continued contribution to the problem by society inadvertently [17]. As a result, society inadvertently continued to contribute to the problem [17]. Environmental literacy achieved through EE which is part of an effective strategy to protect the earth's resources therefore helps us learn from our mistakes [13]. This chapter therefore analyses the links between EE and CBNRM in Zambia. It examines the roles that EE could play in natural resources management.

The concept of Environmental Education can also be made clear through its aims, objectives and principles.

#### **3. Environmental education aims, objectives and principles**

Environmental education is a concept incorporating a vision of education that seeks to empower people of all ages to undertake responsibility for creating a sustainable future, an understanding of and concern for stewardship of the environment in its broadest sense and the knowledge to contribute to ecologically sustainable development. The overall goals of EE are to increase people's knowledge about the environment and environmental issues and to influence attitudes and behaviours [18]. The term is often used to indicate education within the school system; however, it is used more broadly to include all effort to educate the public and other audiences, including the print materials, websites, media campaigns and many other efforts used in education.

#### **4. Environmental education aims**

The aim of EE according to UNESCO [19] clearly shows the economic, social, political as well as ecological interdependence of the modern world, in which decisions and actions by different countries can have international repercussions. EE therefore helps to spread a sense of responsibility and unity among countries and regions as a base for a new international order which will warranty the conservation and improvement of the environment.

At the grass root level, the main aim of EE is to engage individuals and communities in appreciating the complex nature of the natural and the built environments. In addition, EE aims to accelerate citizens to acquire knowledge, values, attitudes and practical skills to contribute in a responsible and effective way in solving social problems and in the management of the environment.

Therefore, the necessary steps for Environmental Education include:


#### **4.1 Important outcomes of environmental education**

An Effective Environmental Education programme requires the regular use of learner-centred, interactive teaching and learning strategies in various educational systems that may include the informal, formal and non-formal. The outcome of this type of education is the:


#### **4.2 Main goals of environmental education**

UNESCO [19] outlines the goals of EE as follows:

a.Nurture clear awareness of and concern about, economic, social, political and ecological interdependence in urban and rural areas;


These goals can lead to successful educational process that promotes enhanced awareness, expression of interest and behavioural changes. Positive attitudinal change is one of the most important objectives of education. Next we examine one approach where EE may find relevance.

#### **4.3 Origin of community-based natural resource management (CBNRM)**

Global biodiversity is endangered by several factors such as extensive biodiversity loss, over exploitation of species, pollution, invasive species and climate change. Biodiversity loss appears to be severe in the equatorial region of the world where the world's greatest biodiversity and species endemism are intense [20]. In order to prevent biodiversity loss, conservationists have tried finding ways of inhibiting biodiversity loss including the fences-and-fines approach which failed because of excluding the human dimension aspects of wildlife management [21]. This steered the search for a viable and sustainable alternative approach to wildlife management by conservationists [22]. The approach whereby rural communities are given ownership rights, custodianship and management responsibilities for the resource became popular in the 1960s, and it was named BNRM, also called community-based conservation (CBC) [21].

Community-based natural resource management (CBNRM) is an approach to environmental protection in rural areas that attempts to integrate the goals of conservation, sustainable development and community participation [21, 23], further define CBNRM as the management of resources that include forest, land, water as well as wildlife by resource users in order for them to benefit. Projects of CBNRM, particularly in eastern and southern Africa, frequently focus on conservation of wildlife, but in principle the method may be used for management of a range of natural resources [24]. The concept of CBNRM arose and gained popularity in the early 1980s as an alternative to resource regimes that were generally perceived to be failing [25]. The approach has been applied widely in the developing world, including Zambia. As an approach that seeks to achieve both biodiversity conservation and socioeconomic objectives, CBNRM has received much support in the past years because of its attempt to integrate the goals of conservation, sustainable development and community participation [21]. Since then CBNRM has been extensively promoted in recent years as an approach for pursuing biological conservation and socioeconomic objectives.

Nabane and Matzke [22] noted that CBNRM approach gives communities full or partial control over resolutions concerning natural resources, such as water, forests, pastures, communal lands, protected areas and fisheries. The degree of CBNRM control ranges from community consultations to joint management or to full obligation for decision making and benefit collection, using tools such as joint management plans, community management plans, stakeholder consultations and workshops and communal land tenure rights. Community-based institutions are key to any CBNRM project, and selecting and building the capacity of local institutions are critical [22]. The selection process in these local institutions must always try to ensure transparency and accountability and minimise conflict at all cost. Together with decentralisation reforms, CBNRM approach guarantees stakeholder participation, increases

sustainability and provides a forum for conflict resolution and as such the approach often leads to more equitable and more sustainable natural resource management than one that does not have stakeholder participation [21].

#### **4.4 Understanding CBNRM**

CBNRM approach is based on the idea that if conservation and development can be concurrently achieved, the benefits of both are served [26]. The main objective for CBNRM approach was to create, through the bottom-up participatory approach, conditions whereby a maximum number of community members stand to benefit from a sustainable management and utilisation of wildlife resources [25]. According to a typology by [27], community participation in natural resource management has a variety of approaches that range from passive to active participation (**Table 1**). Community and participation are therefore fundamental concepts underlying the theory of CBNRM. The participation also includes education which promotes active participation of the local communities in managing their natural resource [28]. Participation in CBNRM can take the form of direct democracy, in which all individuals belonging to a community participate themselves, or in the form of representative democracy, in which elected leaders speak for their constituents [29, 30].

In wildlife management, a combination of active form of participation which includes functional participation, interactive participation as well as self-mobilisation


#### **Table 1.** *Showing typology of participation.*

*Environmental Education and Community-Based Natural Resource Management in Zambia DOI: http://dx.doi.org/10.5772/intechopen.108383*

and passive participation (participation for material incentives, participation by consultation and participation in information giving and passive participation) is encouraged. Active participation involves all stakeholders in decision making process at all the stages of the project, whereas passive participation does not [29]. In passive participation people participate by being told what is going to happen or has already happened [31]. The active participation of stakeholders in natural resource decisionmaking and use increases economic and environmental benefits and, therefore, leads to a sustainable management of natural resources.

#### **4.5 Community-based natural resources management in Zambia**

In Zambia, CBNRM programme was introduced by the Zambian Government in 1987, and it comprised wildlife in game management areas and National Parks [32]. The idea was to improve the livelihood of local people in rural communities living in national parks and game management areas and also to create awareness in the local communities regarding the importance of conserving wildlife resources [33]. The CBNRM initiative in Zambia was initiated in Lupande Game Management Area (LGMA) under the project called Lupande Integrated Resource Development Project (LIRDP). There are 35 Game Management Areas (GMAs) and 20 national parks in Zambia (**Figure 1**), representing 30% of the total territory of protected areas. National parks are intended for the protection and enhancement of wildlife, ecosystems and biodiversity [34]. No human settlements are allowed in the national park, only photographic safaris, also known as non-consumptive wildlife use, are allowed.

GMAs act as buffer zones between the national parks and farming areas [30]). GMAs are intended to promote sustainable harvest of wildlife through hunting as an alternative to other economic activities not compatible with wildlife protection. GMAs also offer wildlife viewing but allow human settlements and licensed hunting (consumptive wildlife use). The LIRDP project promoted the sustainable use of fisheries, water, wildlife, forestry and agriculture. Later on, the initiative evolved into Administration Management Design (ADMADE) that was replicated in other GMAs [35], and today a total of 63 Community Resource Boards (CRBs) have been formed country-wide [36]. The CRBs are a form of co-management model currently active in the wildlife sector in Zambia where local communities have been given opportunity to actively participate in and benefit from natural resource management [37].

#### **4.6 The role of environmental education in CBNRM**

Community participation is one of the key principles of CBNRM. This, therefore, confirms that participation of local resource users in the management of their resources could lead to sustainable management of natural resources [30]. In Zambia however, a large proportion of community participation in wildlife resource management lacks necessary knowledge and understanding of the world [38]. Some scholars such as [38] suggested the need to purposefully put in place a new, open, transparent and robust participatory approach called 'EE in CBNRM' in the GMAs so as to improve capacity building available in rural areas. The introduction of EE in natural resource management according to [38] would improve the nature of participation currently prevailing in the natural resource programme which lacks the necessary tools of understanding the world.

The introduction of EE would further enhance positive community participation in wildlife resource management in the GMAs. This is because EE is a vision of knowledge that allows community participants in natural resource management to have sufficient knowledge that will allow them to contribute to the sustainable management of natural resources. Furthermore, EE promotes behavioural change in learners by motivating people to act in a responsible way that does not exploit the resource base. In other words, EE helps to create a sense of empathy for the environment [38].

The introduction of EE in wildlife resource management in the GMA would also enable the local community to have a broader and a more complete understanding of reality. EE would also help decrease uncertainty and unlikelihood of unpremeditated consequences [38]. Finally, the incorporation of EE in Zambia would further strengthen the weak CBNRM of wildlife resources being experienced in the country and reduce loss of biodiversity experienced in most areas of the country [39]. This is because the main objective of EE is to help individuals acquire the knowledge, values, attitudes and practical skills that will assist them to participate in a responsible and effective way in solving social problems, as well as in the management of the environment. Hence, individuals and communities would appreciate complex nature and the built environment.

#### **5. Conclusion**

The above account demonstrates a link between EE and CBNRM in that while EE on the one hand aims to raise environmental awareness among human populations and to provide opportunities to acquire the knowledge, values, attitudes and skills needed to protect the environment, CBNRM on the other hand is expected to provide

#### *Environmental Education and Community-Based Natural Resource Management in Zambia DOI: http://dx.doi.org/10.5772/intechopen.108383*

a scientific basis about environmental management decisions. This is because EE is a disseminator of ecological concepts [40]. EE plays an important role in natural resource management as it is a facilitator of the use of ecological knowledge that could promote sustainable utilisation of natural resources. People's preferences for action and their social and cognitive features must be considered for the successful environmental policies that activate action on environmental degradation. Worldwide environmental politics will therefore only fulfil its tasks if policy-makers in different nations are sustained by a population whose environmental awareness and willingness to behave in an environmentally appropriate manner permits them to demand solutions to global environmental problems.

### **Author details**

Delphine Inonge Milupi\*, Liberty Mweemba and Kaiko Mubita The University of Zambia, School of Education, Lusaka, Zambia

\*Address all correspondence to: inongemilupi69@gmail.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 3**

## Phosphate Solubilizing Rhizobacteria as Sustainable Management Strategy in Agrobiology

*Aqsa Tariq and Ambreen Ahmed*

#### **Abstract**

Phosphorous limits agricultural productivity due to its limited plant availability. Use of synthetic phosphate fertilizers disturbs soil fertility and ecosystem ecology as it contaminates environment. Plants have developed certain mechanisms to respond to P-scarcity, which involve release of specific chemical messengers through root exudates that attract rhizospheric phosphorbacteria to colonize plant root vicinity. Thus, use of phosphate-solubilizing bacteria/rhizobacteria (PSB/PSR) as biofertilizers is a safer approach toward sustainable agrobiology. These PSR are capable of solubilizing soil phosphate from insoluble to plant available form. Due to instability and slow movement of available phosphates in soils, they readily get incorporated with soil particles or chelates as metal complexes. In this scenario, PSR provide continuous chain of soluble phosphate to plants. PSR direct plant root system architecture toward available phosphate zones in soils. Moreover, there is an increased number of roots, root hair and lateral root, increase root absorbing surface area by increasing contact to soil particles. Hence, PSR-based root system morphology is a significant trait in measuring their agronomic efficiency. Moreover, PSB also possess phytostimulatory properties that significantly contribute to agricultural efficiency. Hence, the use of phosphate-solubilizing bacteria can improve crop productivity by increasing soil P-mobility and soil fertility.

**Keywords:** biofertilizers, phosphate solubilizing rhizobacteria, inorganic phosphorus, plant growth promotion, agrobiology

#### **1. Introduction**

Global food security greatly depends on soil fertility and agricultural sustainability. Most of the soils with high sorption capacity have finite phosphorous (P) resources which is far away from meeting the agricultural P demand, thereby, limiting agricultural fertility and productivity [1]. On the average, 0.05% (w/w) phosphorous is present in agricultural soils out of which only 0.1% is available to plants. Mostly inorganic phosphates (Pi) are present in higher concentrations but due to highly reactive nature of P-anions, it readily gets immobilized via complex formation with various mineral cations (Fe3 + , Mg2 + , Ca2 + , and Al3 + ) [2]. Hence, application of animal

manure in traditional farming technique solves P-deficiency problems to some extend but this leads towards an unbalance ratio between various nutrients especially nitrogen and phosphorous in relation to relative crop demand and results in overfertilization [1]. Lower plant accessible P-concentrations and higher immobility in soils make it an essential mineral needed to be applied exogenously in the form of fertilizers. Therefore, conventional agricultural practices rely on high input of chemical fertilizers to boost crop productivity. Among various fertilizers, phosphate fertilizers are the major contributor to environmental contamination. Concentration of various metals in potassium and nitrogen fertilizers is significantly low as compared to phosphate fertilizers therefore, these are not regarded as serious threat to soil and environmental [3]. On the other hand, phosphate fertilizers contain traces of various metals including heavy metals i.e., cadmium (Cd), lead (Pb), arsenic (As), strontium (Sr), chromium (Cr), zinc (Zn) and radioactive metals such as thorium (Th), uranium (U), radium (Ra) etc. [4]. Consumption of such crops deteriorate our ecosystem by accumulating in agricultural soils and becoming part of food chain. Moreover, soil erosion facilitates the entry of P in waterbodies where it causes uncontrolled growth of algal blooms, deplete oxygen and cause risk to aquatic life. Even very low P- concentrations (10–20 μgPL−1) can support luxurious growth of algal bloom. In addition, drinking highly eutrophicated water adversely affects human health [4].

Rock phosphate is the naturally occurring source of phosphate used for the manufacturing of various phosphate (P) fertilizers such as triple superphosphate (TSP), monoammonium phosphate (MAP), diammonium phosphate (DAP), and NPK mixtures [5]. Apatite, basic constituent of phosphate rock, is incorporated with various metals and radionuclides which later become distributed in environment by the application and formation of these fertilizers. Sometimes, besides commercially available P- fertilizers, its by-product phosphogypsum (PG) is also used to fertilize agricultural lands having potential environmental risk [6]. This uneven distribution of various metals in soil adversely affects its physiological properties which, in turn, affect nutrient availability to plants. This, in the longer run, reduces soil biodiversity and fertility by disrupting soil microbiota as these are very sensitive to environmental variations. The soil microorganisms play crucial role in regulating soil fertility as they are involved in nutrient cycling (particularly P- cycle) hence, maintaining plant health and crop productivity. Hence, keeping in view all the agrobiologically and environmental sustainability concerns, a greener and cleaner approach should be needed to compete this challenge. In this regards, utilization of phosphate solubilizing microbes (PSM) is the best possible solution. Phosphorous solubilization capacity of soil microbes have been extensively studied from the perspective of their utilization in agro-ecosystems and development of biological fertilizers. For this purpose, molecular prospects of bacterial transformation of organic phosphates through various mechanisms have received a great deal of attention. First ever report on plant growth improvement via. Inoculation using phosphate solubilizing microorganisms was published in 1948. Since then, after so many decades, there is no general agreement among the scientific communities on the benefits of these microbes in crop production, hence, their use is still limited. The current chapter summarizes the agricultural accountability and significance of phosphate solubilizing rhizobacteria (PSR) and the strategies acquired by these microscopic creatures to solubilize phosphate and the genetic aspects for better understanding of phosphate mineralizing mechanisms. This would lead scientific community to understand their nature that would be beneficial for the development of commercially available formulations used in agriculture.

*Phosphate Solubilizing Rhizobacteria as Sustainable Management Strategy in Agrobiology DOI: http://dx.doi.org/10.5772/intechopen.108657*

#### **2. Soil phosphorous dynamics and accessibility**

Phosphorous is an important macronutrient constituting about 0.2–0.8% of plant dry weight [7]. Phosphorus is crucial in various plant metabolic processes including energy generation and transformation during developmental processes such as germination, flowering, root expansion, photosynthetic activities, nitrogen fixation, carbohydrate metabolism, enzymatic activities etc. In addition, it is integral part of various structural and functional macromolecules such as adenosine triphosphate, proteins, nucleic acids, lipoproteins etc. [8]. In soil, phosphorous is present in two basic chemical forms i.e., organic (Po) and inorganic forms (Pi). Primary sources of inorganic phosphates include stable P minerals such as apatite (Ca5[PO4]3(OH,F,Cl)), variscite (AlPO4 2H2O) and strengite (FePO4 2H2O). These minerals have P structural element and are very stable and considered as huge P- reservoirs existing naturally in soils. However, the phosphate liberation from these minerals is a gradual process, regulated particularly by soil pH [9]. Optimum pH for P availability to plants is 5.5–7. At high or low pHs', it forms chelates and become unavailable for plants [10, 11]. Under acidic conditions, adsorption of P on Fe and Al oxides and hydroxides (gibbsite and goethite) is increased. On the other hand, in alkaline conditions, Ca serves as primary P precipitated site. P can also readily bound with soil particles or adsorbed with cations to form complexes such as aluminum phosphate (AlPO4), iron phosphate (FePO4), and calcium phosphate (Ca3(PO4)2) etc. These secondary sources of Pi are the major phosphorus sources for young plants [12, 13].

In addition, compounds originated mainly from soil organic matter (plant and animal residues and manure) are the source of organic phosphates. They include wide range of compounds varying in terms of their bioavailability and solubility. These compounds are categorized as various phosphate esters such as phospholipids, sugar phosphates, inositol phosphates, nucleic acids; phosphonatases such as C–P bonded compounds; and phosphoric acid anhydrides (adenosine di- and tri- phosphates) [14, 15]. Important organic source of P is soil microorganisms. Soil microbes have potential to inlock soil phosphorous by absorbing and incorporating in their cellular structures such as nucleic acids, coenzymes) or stored as polyphosphates which temporarily act as immobilized P-pool. This temporarily locked P can later be released into soil solutions through mineralization process [16]. Rhizobacteria accumulate polyphosphates or polymers of phosphoric acids under unfavorable conditions which serves as P-reserves within bacterial cells. These P-reserves are considered as high energy reserves providing anhydrides and can easily be used as energy source by releasing Pi [17]. Various enzymes are involved in consumption and degradation of accumulated polyphosphates. Poly P kinase catalyzes the synthesis of polyphosphates within microbes. Similarly, polypases (exopolypase (PPX)) and polyphosphatespecific kinases (polyP-fructokinase and polyP-glucokinase) are involved in phosphate utilization and degradation [9]. Bacterially mediated P-cycling process releases accumulated phosphorous back to the soil. However, the P- liberation from biomass is highly dependent on available soil carbon and phosphorous and composition of microbial communities [18, 19]. Po constitute almost 30–65% of the soil out of which 3–14% become immobilized into soil microbial biomass [20]. Plant roots can efficiently uptake Orthophosphates (H2PO4 − /HPO4 2–) but due to its weak stability and highly reactive nature, it loses its efficiency and becomes yield limiting factor in most of the agricultural soils [21, 22]. In addition to the microbial, plant and animal residues, a large quantity of xenobiotics (detergents, pesticides, antibiotics etc.) released in the environment also serves as source of organic P. These high molecular

weight organic compounds are resistant to chemical hydrolysis or biological degradation, thereby, the locked P within them is useless for plants unless converted to Pi or orthophosphates. However, some PSR studied have ability to degrade such complex compounds and release P from these sources [23].

Strong P- fixing capacity of soils and immobilization of soil P pool via precipitation, chelation or complex formation causes P scarcity in soils. Despite all these factors, P availability is generally a balanced process including desorption and adsorption mechanisms. Various rhizospheric phenomena particularly biological processes play critical role in soil P dynamics and its availability to plants. Both plants and rhizospheric biota contribute to bioavailability of P at root-soil interference by regulating specific signaling molecules such as release of H<sup>+</sup> , chelation and ligand exchange etc. All these rhizospheric activities contribute to P-cycling process to improve P availability in agricultural soils [24].

#### **3. Plant starvation responses**

Plants uptake P- through roots by simple diffusion. The absorbed P- ions actively move across the plasmalemma against concentration gradient developed by existence of low orthophosphates. Plant response vary greatly from species to species in P-deficiency response. Generally, plants cannot respond and absorb soil P efficiently (plant P uptake rate: 10–12 to 10–15 m2 s −1) due to its low mobility. This causes the formation of phosphate depleted areas adjacent to plant roots. Therefore, plants need subsidiary system that can help plants to receive optimum P requirement by developing nutrient pool around the plant roots [25]. Plants have developed generally various physiological, biochemical and morphological adaptations to respond P- scarcity and to endeavor P acquisition efficiency. These genetic modifications acquire by plants can be categorized as plant P- acquisition efficiency: capacity to absorb soluble P and Putilization efficiency: capacity to utilize and assimilate the absorbed P. These include high expression of P transporters, carbon metabolism, secretion of various organic acids such as oxalate, citrate and malate), modification in root architecture, enhanced production of acid phosphatases and phytases. Modifications in root architecture is foremost response substantially studied in plants [26, 27]. A preferential allocations metabolic budget towards roots undoubtedly results in greater root hair formation and clustering of roots, providing greater surface area for P absorption but, on the other hand, it decreases root to shoot ratio resulting in reduced plant growth. However, greater root system allows plants for greater and easier nutrient acquisition [28]. Besides modifications in root architecture, root signaling is also significantly important parameter affecting P- acquisition efficiency. Under P-scarcity, release of organic acids by plant roots help to solubilize the nearby immobilized P-pool. Moreover, plants also release P- scavenging enzymes that also help in soil P- cycling mechanism. For instance, release of acid phosphatase catalyzes Pi hydrolysis process to release Pi from Po residues [29]. In addition, plants enhance cellular P utilization efficiency by increasing activity of high affinity Pi/H+ symporters (PHT1 gene family) associated with plasma membranes [30, 31]. Plants also regulate alternate metabolic pathways e.g., glycolysis pathways, tonoplast pyrophosphatase, and various respiratory electron transport pathways [32]. Despite of all these modifications in plants for improved P acquisition efficiency under P stress conditions, plants still are unable to full fil their P- demand, therefore, plants tend to establish symbiotic interactions with soil microbiota especially rhizobacteria to cope up with P- scarcity.

*Phosphate Solubilizing Rhizobacteria as Sustainable Management Strategy in Agrobiology DOI: http://dx.doi.org/10.5772/intechopen.108657*

#### **4. Phosphate solubilizing rhizobacteria (PSR): biological revolution**

Rhizosphere is hotspot for various plant beneficial bacteria with potential to solubilize immobilized P sources (di- and tricalcium phosphates, hydroxyapatite, and rock phosphate). These rhizobacteria are known as phosphate solubilizing rhizobacteria (PSR). PSR are copious in nature. Various rhizobacteria belonging to genera *Paraburkholderia*, *Ralstonia*, *Burkholderia*, *Curtobacterium*, *Arthrobacter*, *Cronobacter*, *Massilia*, *Pseudomonas*, *Enterobacter*, *Bacillus*, *Serratia*, *Pantoea*, *Rhizobium*, *Klebsiella*, *Ochrobactrum*, *Staphylococcus*, *Arthrobacter*, *Acinetobacter* have phosphate solubilizing potential [33–40]. Visualizing the formation of clear halo zones around bacterial colonies on various phosphate media indicates their phosphate solubilizing ability. Quantitative analysis of P-solubilizing potential of PSR using rock phosphates (RP) and various Al-, Ca- and Fe- complexes revealed their efficiency to mobilize soil Phosphate for plant use. However, the extent to solubilize phosphorous is highly dependent on bacterial species. Agronomic efficiency of RP significantly increased using suitable PSR. This improvement is attributed to the positive effects of PSR on soil P-availability [20]. These microbes play significant role in P acquisition and nutrient management in soils and hence, serve as potential biofertilizers [41]. In addition, PSR exhibit diverse abilities and exert synergistic effect on plant growth and development besides solubilizing soil phosphate. They enhance plant growth by various plant growth promoting mechanisms including production of plant growth stimulating phytohormones such as auxins, gibberellins, cytokinins and various compounds such as siderophores, 1-aminocyclopropane-1-carboxylate (ACC) lytic enzymes, hydrogen cyanide (HCN), exopolysaccharides that lock up soil nutrients for plant availability and protect them from various unfavorable conditions [42]. Moreover, PSR also act as biocontrol agents protecting plants from pathogenic attacks by producing wide variety of antifungal compounds including certain phenolics and flavonoids [43]. The phosphate solubilization mechanisms are summarized in **Figure 1**.

#### **4.1 Unearthing the mechanisms of P-solubilization: molecular insight**

#### a.Inorganic phosphate solubilization

Principle mechanism of inorganic phosphate solubilization acquired by PSR is release of mineral dissolving compounds such as protons (H+ ), siderophores, organic acids (OAs), carbon dioxide (CO2) and hydroxyl ions (OH− ). Production of low molecular weight organic acids is common mechanism shared by PSR. Rhizobacteria produce these organic acids either during carbon metabolism through intercellular phosphorylation or through direct oxidation of glucose to gluconic acid and sometimes to 2-ketogluconic acid via quinoprotein glucose dehydrogenase (GDH), an enzyme involved in direct oxidation pathway in periplasmic space [44]. Pyrrolo quinoline quinine (PQQ ) (product of pqq) acts as a cofactor which is essential for the activity of GDH. These organic acids lower down soil pH. Under alkaline conditions, soil P precipitates as Ca2+ phosphates and its solubility increase with decreasing soil pH. Increase in soil pH causes the formation of di- and tri- Pi (PO4 3− and HPO4 2−) [45]. The production of organic acids acidifies the surroundings and cellular environment by liberating H+ in the vicinity of plants which regulates the accumulation of other cations that directs to P solubilization by substitution of H+ for Ca2+. For instance, assimilation of NH4 + along with H+ causes P solubilization [46]. Moreover, there is no evidence of a correlation between pH and solubilized P [47]. The P- solubilizing efficiency of

#### **Figure 1.**

*Rhizospheric interactions between PSR and plants and their impact on plant growth. 1- Plant releases certain chemical messengers ( ) that attract beneficial PSR which in turn colonize plant roots and fight off pathogenic bacteria (PB). 2- Bacteria receive sugar chemical messengers ( -glucose, -fructose) by sugar transporters (ST) which activate synthesis of phosphatases (Pase). 3- Synthesis of ALPs solubilize organic phosphate to inorganic phosphate that can be taken up by plants. Moreover, accumulation of H+ on bacterial surface also facilitates P-solubilization process by releasing Pi from various soil minerals. 4- Chemical messengers also activate pqq involved in sugar metabolism as a result of which it forms various organic acids (OAs). 5- Synthesized OAs lower soil pH which favors P-solubilization by PSR. 6- PSR also utilizes tryptophan released in root exudates to synthesize bacterial Indole acetic acid (IAA) that is released in the vicinity of plant roots and taken up by plants. 7- Pi present in phosphate fertilizers interacts with soil particles and form chelates before it is taken up by plants thus minimizing the advantage taken by the plants from the application of phosphate fertilizers.*

PSR greatly depends on the strength and type of acid production. Variable nature of OAs leads them to respond differently. For example, di- and tri forms of carboxylic acids are more efficient as compared to monobasic or aromatic acids. In the same way. Aliphatic acids are more efficient as compared to fumaric, phenolic, or citric acids [48]. Moreover, the quantity of OAs produced is correlated to the concentration of soluble P. Hence, OAs production in P-deficit soils is greater as compared to the P- sufficient soils [49]. OAs produced by majority of PSR are glutaric, citric, propionic, lactic, glyoxalic, malonic, glicolic, 2-ketogluconic, oxalic acid, glyconic acid, acetic acid, malic acid, fumaric acid, succinic acid, tartaric acid, butyric acid, and adipic acids [50]. Among these OAs, gluconic and 2-ketogluconic acid are most commonly produced OAs. Gram-negative bacteria oxidize glucose to gluconic acids for mineral P- solubilization. Gluconic acids chelate the cations bounded with phosphate via OH− or carboxyl (–COOH) groups making phosphate accessible to plants [51]. Pyrroloquinoline quinone-dependent periplasmic glucose dehydrogenase (PQQ-GDH), is responsible for the production of gluconic acid from glucose. PQQ-GDH is also responsible to produce gluconic acid. In most of the Gram-negative bacterial species, biosynthesis of PQQ is regulated by five genes comprising pqq operon (pqqA-BCDE) [52]. Until now, 11 pqq genes have discovered so far in various bacterial genera, however, pqqF and pqqG existing at proximal or distil end of operon are commonly found [53]. Various PSR genera exhibit this mechanism including *Pseudomonas*, *Enterobacter*, *Acinetobacter*, *Pantoea*, *Klebsiella*, *Rahnella*, *Serratia*, *Erwinia*, *Citrobacter*, *Burkholderia* and *Gluconobacter* [54, 55]. Another P-solubilizing mechanism acquired

#### *Phosphate Solubilizing Rhizobacteria as Sustainable Management Strategy in Agrobiology DOI: http://dx.doi.org/10.5772/intechopen.108657*

by some PSR is release of H+ microbes which release H+ at their surfaces helping cation exchange via H+ translocation or ATPase leading to the release of P from inorganic minerals (Ca-P) [9]. Production of chelating compounds and inorganic acid by some bacteria is also source of mineral P solubilization, however, effectiveness of these compounds is very less compared to other mechanisms of P-solubilization [43].

#### b.Organic phosphate mineralization

Mineralization of organophosphates highly depends on the environmental conditions. Alkaline conditions favor this process. Phosphate decomposition by PSR from organic substances is correlated with P- content of their biomass. This biological event plays an important role in solubilization of organic P and regulating P-cycling events in nature. These phosphate solubilizing bacteria secrete various enzymes responsible for organic P mineralization. Among these enzymes, phosphatases and phytases are important. Phosphatases (phosphohydrolases) belonging to the class phosphomonoesterases, dephosphorylate phosphoester or phosphoanhydride bonds present in organic compounds [56]. They can either be alkaline or acidic phosphomonoesterases (ALPs), however, acidic phosphatases are important and play significant role in decomposition having optimum catalytic activity. ALPs can mineralize up to 90% of organophosphatases, however, phytate is resistant to them [57]. The key ALPs encoding gene found in phosphorbacteria is *pho* (phoX, phoA, and phoD). Among these phoD is widely distributed among various PSR. However, phoD abundance has shown no correlation with the P-availability. phoD can mineralize phosphate even under low concentrations but causes immobilization of P in bacterial biomass under application of P fertilizers [58]. ALPs are categorized as specific acid phosphatases (SAP) and non-specific acid phosphatases (NSAP). Examples of SAP with different activites are: nucleotidases, hexose phosphatases, and phytases [59]. Several bacterial species have been known for their potential to produce phosphatases such as Pseudomonas sp., *Klebsiella aerogenes*, *Burkholderia cepacia*, *Enterobacter cloacae*, *Pseudomonas fluorescens*, *Enterobacter aerogenes*, *Proteus mirabalis*, *Citrobacter freundi*, and *Serratia marcenscens* [9].

The enzyme phytase is responsible for releasing phosphorous locked in soil organic compounds such as seeds or pollens that were stored as phytate (inositol polyphosphate). Phytates are great source of phosphorous containing 60–80% of soil P. Phytates contain strong and stable ester bonds that can easily be hydrolyzed by PSR. They completely hydrolyzed phytates to lower molecular weight isomers of inositol polyphosphate and inorganic phosphates [60]. Several phosphorbacteria have been known for having their potential to produce phytases such as *Bacillus*, *Pseudomonas*, *Enterobacter*, *Pantoea*, and *Escherichia coli* [61, 62]. Four types of phytases identified so far from PSR are: β-propeller phytase (BPP; alkaline phytases), histidine acid phosphatase (HAP; acid phytases), protein tyrosine phytase (PTP; cysteine phytase) and purple acid phosphates (PAP; metalloenzyme) [63]. Acidic nature of these enzymes enhances their efficiency under various pH conditions. Some rhizospheric P- solubilizing *Bacillus* and *Streptomyces* also tend to produce phosphoesterases, phosphodiesterases and phospholipases to mineralize organophosphates [64].

#### **4.2 Impact of exogenous P on phosphobacterial activities.**

Soil phosphorous status directly influences plant metabolic activities, root exudates and carbon availability for rhizospheric microbes. Low soil P levels causes plants to activate P- stress responsive mechanisms involving various transcriptional and physiological changes that indirectly affect its associated rhizobacterial communities [65, 66]. P- fertilizers are the yield limiting determinants of soil fertility which influence by disturbing soil nutrient equilibrium. The aggressive use of these fertilizers affects nutrient availability for biological processes and plant uptake [67]. Application of P- fertilizer significantly changes phosphorous turnover efficiency by recruiting rhizobacterial families and regulating bacterial genes involved in P cycling [68, 69]. P-fertilizers can shift soil microbial communities affecting soil biodiversity [70]. Environmental phosphate affects all the phenomena of inorganic P- solubilization, organic P mineralization, P-uptake and transport and plant responses. Phosphorbacteria respond differently to available phosphate conditions. Shifting of various phosphorbacteria in response to P fertilizer indicates their P- availability based selection criteria. Some bacteria such as Actinobacteria prefer high P areas whereas *Moraxellaceae* and *Pseudomonadaceae* prefer low phosphate soils. Similarly, bacterial genera *Bacillus*, *Clostrodium* and *Alicyclobacillus* have shown negative correlation with soil P-content [71]. Moreover, besides affecting rhizospheric bacterial taxonomy, soil nutrient also affects bacterial potential to solubilize immobilized phosphate. *Burkholderia* and *Collimonas* exhibit nutrient poor soils having efficiency for mineral decomposition to fulfill their nutritional demand [72]. *Burkholderia* is described as low phosphate responsive taxon. It is abundantly present in P deficient soils where it switches its interactions with plants i.e., commensal to opportunistic and utilize the stored inorganic shoot phosphate [73]. Nutrient acquisition ability of phosphate solubilizing bacteria makes them more competitive in nutrient poor soils [73].

Soil P-status leads to the upregulation of various P-solubilizing enzymes. Expression of gene (gcd) responsible for glucose dehydrogenase synthesis is suppressed under greater soil P levels through feedback mechanism. Moreover, plants growing under P-deficit conditions release certain signals through root exudates that influence P-solubilizing activity of PSR. The expression of pqq genes is increased by detecting root signals of plant growing under P-deficient conditions [74]. Moreover, the production of phosphatases is regulated by the availability of nitrogen and phosphorous. In the presence of sufficient nitrogen, their production is enhanced. On the other hand, phosphorous supply decreases their production [75]. This negative feedback creates strong correlation between exogenous P and phosphatases to increase P mineralization. Similarly, inorganic phosphate supply reduces the activity of phoD [76]. Under acidic conditions activity of acidic phosphatases and abundance of phoC are negatively correlated with P availability, whereas exogenous P- supply exceeds no significant effect on abundance and activity of alkaline phosphatases [77]. In some cases, long term P- fertilization causes bacterial dormancy leading to inactivation of bacterial P-solubilizing potential [74]. However, there are some controversies in bacterial response to available phosphorous. Sometimes PSR show no response to P-fertilization and the composition of soil bacterial communities remain uninterrupted [78]. Moreover, shift in bacterial communities in response to exogenous P supply is controlled by various biotic and abiotic factors such as nutrient level, drought, pH etc. hence, it is considered that rhizospheric microbial communities are initially determined by soil conditions, then scrutinized by root exudates and finally shaped by alterations in soil physiology [79].

#### **5. Agronomic efficiency of phosphobacteria**

Various plant traits have been extensively studied to develop an agronomic framework for the evaluation of PSR effects on crop yield parameters. These traits serve as

*Phosphate Solubilizing Rhizobacteria as Sustainable Management Strategy in Agrobiology DOI: http://dx.doi.org/10.5772/intechopen.108657*

applicable indicators for evaluating the efficiency and potential of phosphorbacterial biofertilizers in agricultural fields (**Table 1**).

#### **5.1 Plant-phosphobacterial interactions**

Various plant physiological activities are involved in efficient use of soil phosphorous. Release of ions, organic acids and enzymes through root exudates favors plant, to recruit microbial communities especially PSR beneficial for their growth [88]. Soil microbes have affiliation with C-containing compounds, target plant root exudates and response chemotactically to plant chemical messengers. Several rhizobacteria especially phosphate solubilizing bacteria prefer to occupy the plant root zones. For instance, *Oceanobacillus*, *Massilia*, *Arthrobacter*, *Lactococcus* and *Bacillus* are recruited in the vicinity of wheat root zone to get benefit from organic acids released in the form of root exudated [89]. Similarly, some phosphorbacteria such as *Bacillus* sp. enhance root colonization in response to plant secreted organic acids. Plant root exudates activate root colonizing genes present in phophobacteria. This, in turn, significant in establishing plant-PSR interactions which is crucial in P-acquisition by plants [90, 91].



#### **Table 1.**

*Phosphate solubilizing rhizobacterial efficiency in agriculture.*

#### **5.2 PSR mediated regulation of phosphate related genes in plants**

Phosphate solubilizing bacteria can either directly or indirectly trigger the expression genes responsible for Pi movement. These PSR regulate the expression of P transporters either by modulating the expression of plant metabolic genes (pheromone producing genes) or sometimes by increased P-supply in the vicinity of plant roots. Plants have two types of phosphate transporters (PHT) for the regulation of intracellular optimum phosphate concentrations. The high affinity transporter (PHT1) activates in roots whereas, low affinity transporter (PHT2) is responsible for Pi transfer in shoots, flowers, leaves etc. [92]. Phosphorbacteria regulate various phosphate related genes within plants in response to environmental conditions especially during low P supply. Plants growing in P deficit soils have shown upregulation of PHT1. The PSR *Pseudomonas putida* increased the expression of AT5G43350 gene responsible of the production of PHT1 in *Arabidopsis thaliana* [93]. Under combination of P and salt deficiency, PSR upregulated the expression of AT1G80050 gene responsible for the production of PHT2 in A.

*Phosphate Solubilizing Rhizobacteria as Sustainable Management Strategy in Agrobiology DOI: http://dx.doi.org/10.5772/intechopen.108657*

thaliana. Contrary to this, the expression of gene (PHO2) responsible for Pi accumulation in shoots was down regulated. This phenomenon is referred to the fact that PHO2 is responsible for Pi signaling under low P supply [93]. Similarly, phosphate solubilizing *Bacillus* sp. enhanced P-acquisition in wheat plant by upregulating PHT1 transporter [94]. On the other hand, P solubilizing *Pseudomonas* sp., *Klebsiella* sp., *Stenotrophomonas* sp., *Serratia* sp. and *Enterobacter* sp. have been shown to down regulate the expression of Pi transporter in inoculated plants, however, plant growth is enhanced with improved P acquisition and biomass [95]. These changes in molecular patterns positively influence plant P acquisition that ultimately improved crop yield and productivity.

#### **5.3 Effect of PSR on root system architecture**

Root system is a paramount, fitness determining component of a plant. Phosphobacteria can modulate root system architecture through various mechanisms in favor of P acquisition. Modified root system stimulate enhanced root absorptive capacity for nutrients uptake [96]. Generally, under P scarcity, plants have adopted root modifications such as increased root biomass, greater number of roots, enhanced root length and surface area. This extensive, denser root system with larger surface area help plants in detecting localized higher phosphate content [97]. Moreover, spatial parameters in root architecture are important under P- stress. Sometimes for the P- acquisition, PSR affect plant roots to develop shallow root system by decreasing primary root growth and inducing laterals root formation. Thus, development of shallow and more proximate roots favor plants to acquire P from topsoil [98]. This phenomenon of detecting local phosphate concentration by modifying root system is termed as 'P-mining'. Under low P supply, inoculation of phosphate solubilizing *Bacillus megaterium* and *P. fluorescens* inhibited primary root formation and initiated lateral root and root hair formation in *A. thaliana* [99]. PSR also have positive influence on the development of root hair of inoculated plants. Plants with longer root hair are found to be more efficient in P- acquisition under P deficiency. Plants treated with phosphate solubilizing *Pseudomonas* sp. strongly influence root hair formation by increasing number of root hair and length of root hair [100]. Root functions related to phosphate foraging such as number of roots, root hair, lateral roots, frequency of root tips, branching intensity etc. have shown to be increased under the influence of PSR [101].

#### **5.4 Mechanisms adopted by PSR for plant growth promotion**

Phosphate solubilizing bacteria follow several other mechanisms influencing plant growth directly or indirectly such as production of phytohormones, quorum sensing signals, production of various enzymes etc. These mechanisms act synergistically, helping plant to better adopt the environmental conditions with improved growth yield. Some of these mechanisms affecting directly are described below.

#### *5.4.1 Nitrogen fixation*

Some phosphate solubilizing *Rhizobia* spp. with nitrogenase (nif) gene have potential to fix nitrogen. N is important macromolecule so PSR with potential N-fixing ability can significantly help plants to cope with its nitrogen demand having improved nitrogen acquisition [82]. Leguminous plants have developed symbiotic relation with nitrogen fixing rhizobacteria and modify plant roots by developing root nodules where these bacteria convert environmental nitrogen into ammonia (plant available form of N) [102]. However, some non-nodule forming N-fixing phosphate solubilizing bacterial species such as Pseudomonas sp. also regulate legume-rhizobia symbiosis for improving the plant nitrogen levels. Increased ACC activity in Pseudomonas sp. trigger nodulation process in rhizobia [103, 104].

#### *5.4.2 Siderophore production*

These are iron chelating compounds secreted by some PSR bacteria to reduce inter- or intra-cellular iron that can be utilized by the associated plants. Due to iron scarcity, Phosphate solubilizing e.g., *Pseudomonas fluorescence,* can produce different kinds of siderophores i.e., pyoverdine pyochelin, and pseudobactin [105]. This phenomenon positively influences plant growth. For instance, Pseudomonas fluorescence produce pyoverdine which form complex with iron (pyoverdine-Fe) that can be easily taken up by plants. Iron acquisition is more important under stress conditions. Siderophores also help to alleviate the stress imposed on plants [106].

#### *5.4.3 Phytohormone production*

Phosphate solubilizing bacteria have potential of producing various plant hormones such as auxins, cytokinins, gibberellins, and ethylene. PSR release these hormones via interconnected series of signaling network and affect plant physiological activities [107]. Tryptophan present in plant root exudates acts as principle signaling molecule to produce bacterial Indole Acetic Acid (IAA). PSR generally detoxify tryptophan, or its analogs present in root exudates that cause IAA production [108]. Bacterial phytohormones can alter plant hormonal balance which is positively correlated with plant health. Many species of phosphate solubilizing *Bacillus* and *Pseudomonas* exhibit potential to produce auxin that triggers formation of lateral root and root hair in inoculated plants [109]. Moreover, auxin stimulates seed germination, enhance photosynthetic rate and produce other plant growth related metabolites [110, 111]. Similarly, gibberellins and cytokinins stimulate wide variety of plant processes such as seed germination, cell elongation etc. which play important role in plant growth and development. Various genera of phosphate solubilizing bacteria can produce phytohormones such as *Rhizobium*, *Pantoea*, *Azotobacter*, *Paenibacillus*, *Rhodospirillum*, *Bacillus*, *Pseudomonas*, *Microbacterium*, *Plantibacter*, *Sanguibacter*, *Buttiauxella*, *Microbacterium*, *Erwinia* [96, 112–114].

#### *5.4.4 ACC-deaminase production*

Sometimes bacterial IAA stimulate ACC synthase enabling the of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase using S-adenosyl methionine precursor which is also the intermediate of ethylene production in higher plats. ACC deaminases have potential to cleave ACC to ammonia and -ketobutyrate that act as nutrient for plants. This enzyme is also responsible for the reduction of plant stress ethylene, thus alleviating stress effects imposed on plants. Plants inoculated with ACC producing PSR have shown increased shoot system [115]. Moreover, ACC deaminase producing PSR can also stimulate nodulation process. Various PSR have potential to produce ACC such as *Achromobacter*, *Azospirillum*, *Enterobacter*, *Acinetobacter*, *Serratia*, *Bacillus*, *Burkholderia*, *Pseudomonas* etc. [105].

*Phosphate Solubilizing Rhizobacteria as Sustainable Management Strategy in Agrobiology DOI: http://dx.doi.org/10.5772/intechopen.108657*

#### *5.4.5 Bacterial cyanide biosynthesis*

Some phosphate solubilizing bacterial have hydrogen cyanide (HCN) production potential which is a volatile compound and protect plants from various biotic stresses including allelopathic effects. Moreover, they also protect other harmful rhizobcateria by colonizing plant roots. Most of the phosphate solubilizing *Pseudomonas*, *Bacillus*, *Serratia*, *Enterobacter*, *Pantoea* can produce HCN [116, 117].

#### *5.4.6 Indirect methods of plant growth promotion*

Various indirect mechanisms are also adopted by phosphate solubilizing rhizobacteria such as production of various antifungal compounds, antibiotics and lytic enzymes. Different antifungal compounds such as proteases, lipases, cellulases and chitinases degrade cell wall of pathogens. Different P solubilizing *Pseudomonas* and *Bacillus* species can produce antifungal compounds. These compounds can protect plant from various plant pathogens [118]. Hence, these phosphorbacteria can also act as biocontrol agents in agricultural fields. Moreover, some PSR can also release enzymes that act as antibiotics, protecting plants from other pathogenic bacteria. Thus, inducing plant systemic responses (ISR). *Bacillus* sp. can produce various compounds such as difficidin, bacillaene, rhizocticinsn chlorotetain, bacilysin, and mycobacillin. ISR positive plants can response stronger and faster to pathogenic attack due to their induced defense system [119, 120].

#### **6. Future perspectives**

Efficiency of phosphate solubilizing rhizobacteria as biofertilizer, biopesticides, phytostimulaors and bioremidiators have now become research priority owing to their importances as environmentally safe plant growth promoting agents. Various genera of rhizospheric bacteria are capable of solubilizing soil phosphate by either releasing organic acids or enzymes. But there is a need to investigate further indepth mechanisms for bacterial phosphate solubilization and their interactions with root exudates for the development of suitable biofertilizer. Also, study about the knowledge of impact of these biofertilizers on soil microbiota is necessary as the rhizobacteria are important candidate of P-cycling mechanism. Moreover, plant growth promotion by rhizobacteria is a complex network of mechanisms functioning synergistically, thereby particular interaction between phosphate solubilization and its influence on root morphology needs to be investigated. In addition, the interactions and coordination between various rhizobacterial traits and their impact on agronomic parameters should be considered as top priority research for sustainable agriculture economically. Hence, commercializing these biofertilizers can be a promising tool for agricultural sustainability.

#### **Conflict of interest**

Authors declare no conflict of interest.

*Sustainable Management of Natural Resources*

#### **Author details**

Aqsa Tariq and Ambreen Ahmed\* Institute of Botany, University of the Punjab, Lahore, Pakistan

\*Address all correspondence to: ambreenahmed1@hotmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Phosphate Solubilizing Rhizobacteria as Sustainable Management Strategy in Agrobiology DOI: http://dx.doi.org/10.5772/intechopen.108657*

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Section 3
