Cyanobacteria and Their Beneficial Interactions in the Rhizosphere: Agricultural Significance and Insights

Cyanobacteria are a fascinating microbial group, whose members co-exist in symbiotic, parasitic, or mutualistic associations with various other prokaryotic and eukaryotic organisms. Their photosynthetic and diazotrophic traits among other structural-functional adaptability make them successful in colonizing diverse and extreme environments. These organisms are a rich source of novel metabolites - hydrolytic enzymes, phytohormones such as indole acetic acid and several bioactive molecules with biocidal activity. Hence these organisms are of immense agricultural, industrial, biotechnological, and pharmacological significance.

The cyanobacterial colonization of the rhizosphere is a less investigated niche. Our studies showed that the rhizosphere of different rice and wheat varieties supported both non-heterocystous and heterocystous cyanobacteria with the colony forming units ranging from 10²-10⁵ per g soil. Further systematic evaluation was undertaken under controlled, greenhouse conditions, and field trials with promising rhizospheric isolates and their consortia/ combinations with bacteria, and novel Anabaena based biofilm inoculants in diverse crops - rice, wheat, maize, cotton, chickpea and selected vegetables. Using light-, scanning electron (SEM) and confocal microscopic analyses, and DNA based fingerprinting profiles, first reports were generated on the inter- and intracellular colonization of cyanobacteria in roots and stem tissues, both under natural- and artificially inoculated conditions. Their interactions with phytopathogenic fungi-Fusarium, Rhizoctonia, and Macrophomina spp. resulted in reduced disease severity, and improved growth and crop yields.

The cyanobacterial inoculants characteristically modulated the eubacterial, archaeal and cyanobacterial communities of the host plants, both in the phyllosphere and rhizosphere, which was evident from the phospholipid fatty acids (PLFA) analyses, PCR-denaturing gradient gel electrophoresis (DGGE) profiling of metagenomes and qPCR analyses of microbial abundances and expression patterns of functional genes. The modulation of both the host plant physiologies and their microbiomes due to their application favourably influences the macro-, micronutrient mobilization and uptake, growth promotion, and crop yields.