**4.1 Promoting plant growth**

 Cyanobacteria will actively promote seed germination, plant growth, and development due to their ability to produce some of the plant hormones, such as auxins, cytokinins, and gibberellins, by the genera *Anabaena, Anabaenopsis* , and *Calothrix* [ 20 , 21 ]. Cyanobacteria have the ability to increase root and stem growth, dry weight, and yield in wheat [ 8 , 20 ]. The cyanobacteria used in wheat cultivation showed effective results on the appearance of plants in terms of increasing plant height, dry weight, and a number of grains of the wheat crop, in addition to some important positive changes in increasing the bio-carbon content of the beneficial microbial mass [ 22 ]. The effects of cyanobacteria on rice crop growth have demonstrated that cyanobacterial inoculation can improve rice seed germination and growth parameters [ 23 ]. According to Osman et al. [ 24 ], the amount of growth-promoting secondary

 *The role of cyanobacteria in improving plant growth and stimulating the response of defense systems.* 

*Perspective Chapter: Cyanobacteria – A Futuristic Effective Tool in Sustainable Agriculture DOI: http://dx.doi.org/10.5772/intechopen.109829*

metabolites varies depending on the cyanobacterial strain. While *Oscillatoria angustissima* had higher quantities of gibberellic acid, and *Nostoc entophytum* had higher levels of auxin and cytokinin. Cyanobacterial extracts improved nutrient uptake, and plant development in lettuce, red beet [25], tomato [26], and cucumber [27]. In a broader sense, cyanobacteria are used as commercial bioinoculants to promote plant development because of their greater biodiversity, ability to survive in a variety of conditions, faster growth rate, and simpler nutritional requirements [28].

#### **4.2 Nitrogen fixation**

Nitrogen is the most important element needed for plant growth and is a key ingredient for successful cultivation on reclaimed land. Biological atmospheric nitrogen fixation by microorganisms is the main source of soil nitrogen [29]. Cyanobacteria have the ability to fix atmospheric nitrogen through specific cells called heterocysts that possess the nitrogenase enzyme. *Nostoc Linkia*, *Anabaena variabilis*, *Aulosira Fertilissima*, and *Calothrix* SP are the most efficient cyanobacteria in the soil's air nitrogen fixation [30]. Cyanobacteria get established permanently in the field after being applied for three to four subsequent crop seasons [31]. The growth characteristics of *Oryza sativa* were enhanced by the addition of *Nostoc commune* and *Nostoc carneum* as sources of cyanobacteria with chemical fertilizer [32]. Spraying the foliar of *Salix viminalis* L. three times with *Anabaena* sp. and *Microcystis aeruginosa* improved photosynthesis, stomatal conduction, and intracellular CO2 concentration [33]. The application of *Nostoc entophytum* and *Oscillatoria angustissima* on *Pisum sativum* L. decreased the chemical fertilization to 50% [34]. The addition of cyanobacterium *Phormidium ambiguum* to sandy soil increased nitrate content by 15% more than the untreated soil after 90 days. Furthermore, the use of *Scytonema javanicum* improved the N content in slit loam, sandy loam, loamy sand, and sandy soils by 11, 10, 14, and 55%, respectively, the effect of cyanobacteria in biological crust formation and N supplementation for any sort of soil [35].

#### **4.3 Bio fertility**

In modern agriculture, microbes play a vital role in determining fertility and soil structure [36]. Cyanobacteria have potential use in agriculture as biofertilizers. Maintaining soil fertility using renewable bioresources is the main requirement of sustainable agriculture to reduce the need for synthetic fertilizers.

Among such resources, cyanobacteria are the most promising candidates. In the rhizosphere, cyanobacteria can be directly inoculated in the soil or can be used as a coating on seeds, but in both cases, their survival should be guaranteed. Although the use of agricultural chemical nitrogen fertilizers was a solution to all agricultural problems related to food production and increasing agricultural crop production, many environmental problems have arisen as a result of the excessive use of these chemical fertilizers in intensive farming systems. The high prices of chemical fertilizers have led to a decrease in the profit of agricultural crops, and the shortage of chemical fertilizers is a major problem facing farmers in developing countries, which makes researchers try to search for bio-alternatives to expensive chemical fertilizers [37]. Recently, there has been much interest in linking primary field crops in agriculture, especially cereal crops, such as wheat and rice, and organisms as a source of biofertilizers, such as cyanobacteria.

Due to the adaptation of cyanobacteria to most different environmental conditions, it is widely used in increasing soil fertility and improving soil quality and structure, so it has become one of the most important biofertilizers [38]. The effect of cyanobacteria supplementation on growth, productivity, and physical properties of sandy soil under greenhouse conditions was tested. Sood et al. [39] found that there was a lot of ecological and metabolic diversity in cyanobacteria and that their structural-functional flexibility led to even more diversity. The use of cyanobacteria is one of the inexpensive applications in agriculture, which legalizes the use of chemical fertilizers. Cyanobacteria are one of the most important improvers that increase organic matter, amino acids, vitamins, and auxins in the soil, reduce soil salinity and phosphate deposits, and increase productivity in rice crops [40].

Cyanobacteria are emerging microorganisms for sustainable agricultural development. It can contribute about 20–30 kg of N per hectare, as well as soil organic matter, which is quite important for economically weak farmers who cannot invest in expensive chemical nitrogen fertilizers. The diazotroph group is the cyanobacteria most widely used for the development of biofertilizers and is capable of controlling the nitrogen deficiency in plants and improving the aeration of the soil and the water holding capacity. The most efficient nitrogen-fixing cyanobacteria are *Nostoc linkia*, *Anabaena variabilis*, *Aulosira fertilissima*, *Calothrix* sp., *Tolypothrix* sp., and *Scytonema* sp., which are normally present in the rice crop cultivation area.

#### **4.4 Protection against abiotic stress**

Abiotic stress on plants can be caused by a variety of factors, such as temperature, droughts, light, and soil-related factors, including salinity, presence of heavy metals, and soil acidity [41, 42]. Cyanobacteria induce diverse changes in response to elevated soil salinity by synthesis and accumulation of protective substances, maintaining low intracellular ion concentrations, and expression of so-called salt stress proteins [43]. *Anabaena torulosa* and *Anabaena* sp., exhibited anti-saline action by suppression of some expressed proteins, enhancement of other proteins, and expression of specialized salt stress proteins [44]. The effect of the extracellular products of *Scytonema hofmanni* on the growth of rice plants under salt stress was clearly demonstrated. These extracellular products made rice plants able to cope with stress caused by high salt concentrations. Comparison with the effects of plant gibberellic acid indicates that *S. hofmanni* produces gibberellin-like plant growth stimuli [45]. Another way to increase the sensitivity of plants to salinity stress is through the expression of cyanobacterial flavodoxin within them. This can induce multiple resistances in plants; it has been shown that it can reduce salt stress in *Medicago truncatula*. Adding cyanobacterium *Aphanothece* sp. and *Arthrospira maxima* led to improve tomato plant growth and increase the content of chlorophyll and nutrients essential content, such as nitrogen, phosphorous, and potassium, under saline stress [46]. Reduce the effects of salt stress on sweet pepper plants increase in growth, as well as in the water content of the plants by using a liquid extract of *Roholtiella* sp. [47].

The reduction of the harmful effect of abiotic stresses on plants was observed by cyanobacteria, which has a direct effect on the soil or an indirect effect through the activation of specific responses in plants [48]. Concerning salinity stress, the mechanisms of cyanobacteria depend on increasing the plant's ability to tolerate salinity through nitrogen fixation; the production of extracellular polysaccharides, compatible solutes, hormones, and antioxidative enzymes; the active export of ions; and the effects on the microbial community [49]. Rice plants showed an effective response to abiotic stress after treatment of rice roots with *Oscillatoria acuta* and *Plectonema boryanum*. That results in regular increases in the activity of peroxidase, phenylalanine ammonia-lyase, and phenylpropanoid [50]. Furthermore, rice plants

*Perspective Chapter: Cyanobacteria – A Futuristic Effective Tool in Sustainable Agriculture DOI: http://dx.doi.org/10.5772/intechopen.109829*

showed an increase in tolerance to salinity after inoculating roots with strains isolated from saline soils, such as *Nostoc calcicola*, *Nostoc linkia*, and *Anabaena variabili*s [51]. In salt-affected soils, *N. punctiforme* enhanced the physical composition, nutritional status, and microbial activity, leading to noticeably higher growth and yield [52].

Plant germination under drought stress can be enhanced by the use of cyanobacteria [53], moreover, it enhances the growth and development of plants in arid lands [54]. *Microcoleus* sp. and *Nostoc* sp. are capable of increasing germination and seedling growth of *Senna notabilis* and *Acacia hilliana* under drought stress [55]. Similar results were achieved in lettuce plants cultivated in barren soils after the addition of *Spirulina meneghiniana* and *Anabaena oryzae* [56].

Heavy metals can be effectively removed from agricultural soil and water by cyanobacteria [57]. Many cyanobacterial species, including *Anabaena variabilis*, *Nostoc muscorum*, *Aulosira fertilissimia*, and *Tolypothrix tenuis*, may absorb and remove Cr, Cu, Pb, and Zn [58], whereas *Oscillatoria* sp. and *Synechocystis* sp. can remove Cr, this was linked to increasing wheat growth [59]. Applying *Spirulina platensis* can hasten seed germination and boost plant growth by preventing Cd from moving from roots to shoots [60]. *Synechocystis* sp. and *Phormidium* sp. are capable of absorbing and removing systemic insecticide from the soil [61]. The addition of *S. platensis* in the soil can induce the biosynthesis of some amino acids, which can protect plants from the negative effects of the herbicide [62]. Cyanobacteria contribute to stimulating the release of plant hormones, such as salicylic acid or jasmonic acid, which have an effective role in protecting plants from biotic and abiotic stresses by stimulating gene expression of specific proteins [63]. Cyanobacteria lead to increased nitrogen and carbon content, state of soil aggregation, water retention, decrease in pH, exchangeable sodium, and decrease in heavy metals, as well as microbial flora reconstitution which in turn all have an effective role in reducing salt stress [49, 53].

### **5. The role of cyanobacteria in soil resilience**

Soil health is seriously threatened in many parts of the world due to salinization, groundwater pollution from acidification, and excessive use of chemical fertilizers and pesticides. Cyanobacteria are essential for maintaining the health of the soil by enhancing soil physicochemical properties, including aggregation, aeration, and nutrient release patterns [20]. Additionally, cyanobacteria contribute to the fixation of nitrogen, excretion of biologically active compounds, increase soil biomass and organic matter, improve water-holding soil capacity, and improve soil phosphate bioavailability, moreover, cyanobacteria are alternative low-cost and eco-friendly that ensure soil sustainability (**Figure 4**).

#### **5.1 Cyanobacteria improve physical properties of soil**

In the upper crust of soil, the growth of cyanobacteria produces exopolysaccharides and extracellular polymers that alter the chemical composition and improve the physical properties of soil, which in turn promote beneficial microbial growth and strengthen soil structure [56]. Some cyanobacteria secrete mucilage or slime, which increases the availability of nutrients, and enhances soil structure that creates an ideal environment for the growth of advantageous microorganisms and plays a part in enhancing soil characteristics. The cyanobacteria *Nostoc muscorum* excrete exopolysaccharides and enhance saline soil stability [64]. Cyanobacteria can contribute to

#### **Figure 4.**

 *An overview of the cyanobacterial role in sustainable agriculture and environmental safety.* 

the improvement and recovery of infertile soils by releasing holding and aggregation of soil particles together, the accumulation of organic content, and an increase in the water-holding capacity of the upper soil layer [ 65 ]. Rossi et al. [ 66 ] reported that the addition of cyanobacteria to the soil will improve soil properties and texture by adjusting soil stabilization, nutrients, moisture-holding capacity, and crust formation. The micromorphological characteristics of soil were improved after 6 weeks of the application of cyanobacteria combined with polysaccharides [ 67 ]. Chamizo et al. [ 35 ] demonstrated that the application of cyanobacteria can improve dry land functions through restoration and reestablishment. Cyanobacteria contribute to improving the properties of hard-to-cultivable lands, such as calcareous and saline soils, and making them suitable for cultivation.
