Comparative studies of cyanobacterial associations with crop plants

Abstract

Cyanobacteria are very sociable organisms having ability to form symbiotic relationships with a variety of organisms from all other domains of life. Their association with plants is of prime importance. Very less work is done on development of new artificial symbiotic associations between cyanobacteria and crops of agricultural importance. Present study deals with formation of association between cyanobacteria, from different taxonomic classes and different economically important crops. Cyanobacteria, mainly isolated from rice fields were used. These cyanobacteria were co-inoculated with ten days old seedlings for one week. Roots were observed by light microscopy, fluorescent microscopy and confocal laser scanning microscope (CLSM) to locate the presence of cyanobacteria in roots. Chlorophyll-a content from the roots was estimated to quantify the extent of cyanobacterial association with roots. Extracellular polysaccharides (EPS) from cyanobacteria was extracted and characterized for comparison, to find component actively involved in the formation of association. Indole-3-acetic acid was determined in media to view its role in colonization. Cyanobacterial strains were identified by ribotyping. Isolated cyanobacteria were both unicellular and filamentous, Identified as Synechocystis sp., Chroococcidiopsis sp., Oscillatoria sp., Phormidium sp. and Arthrospira sp. on the basis of 16S rRNA. Filamentous cyanobacteria showed significantly more attachment with roots and prefer the entire root. Root tips were not invaded in beginning but in later stages cyanobacterial growth was also present on tips. Unicellular strains were entangled in root hairs in beginning and in later stages colonize the rhizoplane also. After ten days cyanobacteria were also present inside the roots. Cyano-plant association is initiated by EPS, produced by cyanobacteria and strength of attachment depends on extent of cyanobacterial EPS released. It was observed that attachment of cyanobacteria to roots depends more on quality of EPS not on quantity of cyanobacteria. Arthrospira sp. producing maximum EPS showed very less attachment while Oscillatoria sp. and Phormidium sp. were strongly attached. Comparison of EPS components will also show some interesting results (in progress). It was also observed that strains producing indole-3-acetic acid showed more colonization with the roots in comparison with the strains that lack this ability. IAA was also measured in the medium after co-inoculation. These strains have tryptophan dependent pathway for IAA synthesis, so presence of IAA without tryptophan showed IAA synthesis activity in presence of roots. Cyanobacteria can be used in agricultural fields that not only make associations with plants, can also improve the plants growth and the rhizospheric conditions.