**5.2 Phosphorus uptake**

 Phosphorus is the second most important nutrient for plants after nitrogen. It is a crucial mineral for the growth and development of plants. It is one of the essential components of a live cell since it serves as the primary structural support for DNA,

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

RNA, and ATP [68]. Phosphate is frequently supplied to the soil in the form of phosphatic fertilizers. However, plants only use a small portion of this nutrient since a large portion of it is quickly converted to insoluble complexes in the soil that plants cannot utilize. With the help of phosphatase enzymes, cyanobacteria can solubilize and mobilize the insoluble organic phosphates present in the soil, for example, ferric phosphate, aluminum phosphate, tricalcium diphosphate, and hydroxyapatite into soluble forms and improve the bioavailability of phosphorus to the plants [69]. The use of cyanobacteria in crop fields plays a significant role in the mobilization of inorganic phosphates by extracellular phosphates and the excretion of organic acids. Cyanobacteria enhanced the decomposition and mineralization of phosphate and transformed it into readily available soluble organic phosphates.

#### **5.3 Degradation of agrochemicals**

Control of agricultural pests and weeds depends on the use of agrochemicals, for example, pesticides, fungicides, bactericides, insecticides, and herbicides. This leads to maintaining global food production by killing agricultural pests, but at the same time, these pesticides pollute the environment. Biological intervention for many beneficial microorganisms, including cyanobacteria, is involved in removing the chemical residues [70]. Cyanobacteria can be used to get rid of various pollutants, such as heavy metals, pesticides, chemical fertilizers, and crude oil [71]. Cyanobacteria are also able to remove heavy metals from water bodies and can reduce the increase in nitrates and phosphates from agricultural fields [72]. Intensive use of pesticides leads to an imbalance in the environmental system, especially in soil, water, and air. Currently, the use of beneficial microorganisms, especially cyanobacteria, is considered the best way to eliminate pesticides and chemicals that pollute agricultural soil. Cyanobacteria have the ability to break down pesticides at a faster rate. This requires some processes, such as adding the necessary nutrients or organic materials, to accelerate the rate of decreasing pollutants by the cyanobacteria, which have growth activities that exceed the chemical roads in addition to being environmentally friendly [73].

Among the different compounds used for agricultural applications the phosphorous-organic pesticide category. The random use of such chemicals causes many environmental problems. It also poses a great danger to other organisms, such as birds, fish, animals, and humans. As a result, it is highly recommended that these hazardous chemicals be removed from the environment in an appropriate manner. Cyanobacteria are one of the best applications of beneficial microorganisms because it breaks down toxic chemicals into nontoxic compounds. The widespread appearance of cyanobacteria in the polluted area is a contributing factor, making them a better candidate for biological decomposition [8].

### **6. The role of cyanobacteria in controlling phytopathogens**

Plants can be attacked by bacteria, fungi, viruses, and nematodes at different stages of growth, causing severe harm to the root system, stem, leaves, and fruits. Chemical pesticides were the best approach to decrease the damage of these diseases, but pesticides have many negative effects over time. The use of biological alternatives, for example, cyanobacteria, has become a necessity to preserve the safety of the environment and the quality of crops. The major strategies used by cyanobacteria to

attack plant pathogens are antibiosis, the release of chemical compounds that may have the potential to inhibit a variety of phytopathogens, competition for space, and activation of plant defense responses. Cyanobacteria are distinguished by producing a huge number of bioactive substances ( **Figure 5** ). Thus, cyanobacteria provide a significant, safe alternative to avoid the harmful effects resulting from chemical control. It is a critical tool in sustainable agriculture [ 7 ].

 Several plant fungi can be effectively controlled by cyanobacterial extracts, for example, *Fuarium* spp., *Verticillium* spp., *Alternaria* spp., *Penicillium* spp., *Botrytis cinerea* , *Rhizoctonia solani* , and *Sclerotinia sclerotiorum* [ 6 ] *.* Two orders of cyanobacteria, the Nostocales and Oscillatoriales, are very effective against fungal pathogens. Among Nostocales, two species, *Anabaena minutissima* and *Anabaena variabilis* are active in preventing the spread of airborne diseases [ 74 , 75 ]. Airborne fungal pathogens produce a significant number of spores, which are considered the main source of spread. Therefore, inhibiting spore germination could play an effective role in controlling the disease and preventing secondary infection. Spraying of *A. minutissima* on cucurbit plants can reduce the symptoms of powdery mildew caused by *Podosphera xanthii* ; also, infected areas of cucumber leaves and spore production decreased by 31% and 47%, respectively [ 75 ], while the disease was inhibited by 25% on zucchini [ 74 ]. *A. variabilis* has effective antibiosis against *R. solani* and *F. moniliforme* pathogens that infect tomato seedlings [ 76 ]. Also, *A. variabili* s, *N. linckia* , and *N. commune* have the same antibiosis effect on tomato wilt disease caused by *F. oxysporum* f. sp. *lycopersici* [ 76 – 78 ]. *Anabaena* sp. has an antibiosis against *P. xanthii* , which causes powdery mildew in zucchini plants [ 79 ]. *N. entophytum* and *N. muscorum* considerably decreased the activity of *R. solani* in soybean by an antibiosis mechanism [ 24 ]. Additionally, *Oscillatoria agardhii* has an antibiosis against *F. solani* , *Macrophomina phaseolina* , and *R. solani* , which cause the damping off disease in lupine seedlings [ 80 ].

#### **Figure 5.**

 *Mechanisms of antimicrobial activity of cyanobacteria against phytopathogens.* 

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

The presence of diisooctyl adipate, extracted from *N. piscinale* and *A. variabilis*, is one strong indication that cyanobacteria contain chemical compounds active against *R. solani*, the causative agent of rice sheath blight, which causes severe damage in rice fields in China [81]. *Anabaena* spp., *Scytonema* spp., and *Nostoc* spp. have antifungal and toxic activity against soil-borne fungi [82]. *Rhizopus stolonifer*, *Phytophthora capsici*, *Pythium ultimum*, *Botrytis cinerea*, *Colletotrichum gloeosporoides*, *Fusarium oxysporum*, and *Alternaria solani* are all considerably inhibited by *Nostoc commune* methanolic extracts [83]. Additionally, methanolic extracts of *Spirulina platensis* effectively prevent the growth of *Helminthosporium* spp., *Alternaria brassicae*, *Aspergillus flavus*, and *Fusarium moniliforme* [84, 85].

Cyanobacteria can produce enzymes that directly act against the pathogen's cell wall. *Anabaena* sp. and *Calothrix elenkinii* can produce chitinases and chitisonases against pathogens, *F. moniliforme*, *F. solani*, *F. oxysporum*, *A. solani*, *M. phaseolina*, and *R. solani* and significantly reduce disease [86, 87]. Endoglucanases and glucosidases are two other enzymes that *Anabaena* sp. and *C. elenkinii* release. These enzymes can disrupt the cell walls of different plant pathogens by degradation of chitin and glucan, respectively [88]. In addition, Gupta et al. [89] reported that the antifungal properties of cyanobacteria are attributed to the production of endoglucanase, chitosanase homologs, and benzoic acid. Benzoic acid has the ability to interfere with fungal cell functioning, alter many parts of the cell, and has an effect on the respiration of the fungal cell [90]. Cyanobacteria can compete for space in the rhizosphere by forming biofilms at the roots and blocking sites of infection for soil pathogens, such as *Anabaena* sp., against *R. solani* in cotton roots [91].

On the other hand, they activate the defensive responses of the plant directly against fungal pathogens, such as *A. variabilis* or *A. laxa*, which enhance the activity of defense and pathogenesis-related enzymes in tomato roots against *F. oxysporum* f. sp. *lycopersici* [92], or by the activation of systemic resistance, such as *N. muscorum* and *A. oryzae,* that increase total phenol content and the activities of peroxidase, superoxide dismutase, and polyphenol oxidase enzymes in tomato leaves against *A. solani* [93].

The ability of cyanobacteria to combat various plant pathogenic bacteria and their ability to release compounds into the environment has been extensively studied [94]. The mechanism underlying the bactericidal action of cyanobacteria is attributed to the presence of tannins, amino acids, phenolics, alkaloids, carbohydrates, and fatty acids, which may cause bacterial membrane deterioration that eventually allows cells to leak, lowers nutrition intake, and prevents cellular respiration [95]. *Pseudomonas aeruginosa* is capable of infecting the roots of *A. thaliana* and *Ocimum basilicum*, causing plant death [96]. Nostoc sp. was effective in controlling *P. aeruginosa* due to the presence of long-chain fatty acids [97]. Additionally, *Anabaena flos-aquae* can completely suppress *Ralstonia solanacearum,* which causes brown rot disease in potatoes due to the production of antibiosis that is released into the environment [98]. Yanti et al. [99] found that cyanobacteria were able to stop *Ralstonia syzygii* subsp. *indonesiensis*, which is the cause of many vascular diseases in different crops.

Cyanobacteria possess antibiosis mechanisms against plant pathogenic nematodes that include paralysis, death, accelerating egg hatching, and inhibiting gall formation against plant harmful nematodes. *Heterodera cajani*, *Heterodera avenae*, *Meloidogyne graminicola*, *Meloidogyne incognita*, and *Rotylenchulus reniformis* can all be immobilized and killed by aqueous extracts of *Synechococcus nidulans* [100]. *Nostoc calcicola*, *Spirulina* sp., and *Anabaena oryzae* can lessen the quantity of nematode galls and egg masses in the cowpea rhizosphere [101]. *M. incognita* and *M. triticoryzae* are

nematostatically inhibited by *Aulosira fertilissima* [102]. Additionally, *M. incognita* eggs are inhibited from hatching by the cyanobacteria species *Anacystis nidulans*, *Oscillatoria fremyii*, and *Lyngbya* sp. [103]. Furthermore, *M. incognita* in the tomato rhizosphere can be eliminated by an aqueous extract of *Calothrix parietina* [104]. Additionally, *Microcoleus vaginatus* has the capacity to lower *M. incognita* populations in the tomato rhizosphere and reduce root galling [105]. By coming into touch with plant roots, cyanobacteria can trigger several nematode defense mechanisms in plants. In order to combat *M. incognita*, *S. platensis* increases the catalase activity in the roots of banana plants [106] and stimulates the production of the plant defense compound jasmonic acid in tomato plants [107].
