Isolation, characterisation and identification of plant growth promoting bacteria exhibiting activity against Fusarium pseudograminearum in chickpea and wheat

The main constraints to Australian chickpea and wheat production include several factors such as drought, biotic and abiotic stresses such as crown rot, salinity and cold; which totally contribute to losses of 10-70%. It has been found that there are practices that are helpful in controlling these stresses, such as the tolerant varieties, pesticides and crop rotation, transgenic crops and conventional breeding techniques, but these methods are not completely successful. It can be thus said that new methods need to be developed in order to minimise the biotic as well as abiotic stresses in chickpea. Plant growth promoting bacteria (PGPB) potentially represent one such novel approach and are the focus of this research. The generally perceived mechanisms of PGPB which result in reduced plant stress include competition (with a plant pathogen) for an ecological niche, secretion of inhibitory bioactive compounds, and secondary metabolic induction of systemic resistance in the plant host to a range of soil born-pathogens and abiotic stresses. The current study focuses on the potential use of PGPB to enhance the tolerance of chickpea and wheat to crown rot caused by Fusarium pseudograminearum. Two strains, NM-12 and NM-33, identified as Bacillus subtilis and Stenotrophomonas rhizophila were isolated from the rhizosphere soils in Victoria, Australia (Perry Bridge). The beneficial bacterium, Bacillus subtilis and Stenotrophomonas rhizophila were analysed for their direct plant growth promoting effects. Direct antagonistic effect on Fusarium pseudograminearum was demonstrated by a dual culture assay and culture filtrate assays together with estimation of spores and fungal biomass dry weight in vitro. To identify the mechanisms underlying the inhibition of the fungus by the two isolates, the bacterial exudates were assessed for the presence of a range of potential antifungal products, including lytic enzymes, hormones, antibiotics and other secondary metabolites. Strain NM-12 was shown to produce indole acetic acid (IAA) at different concentrations even at 6% salt concentration. In comparison, no IAA production was observed by strain NM-33. Further, siderophore production was moderate under control conditions but significantly increased at high salt concerntration (6%). In contrast, β- glucanase production was observed under normal as well as high salt concentrations. Interestingly, NM-12 which exhibited enhanced ability to suppress the fungal pathogen was found to possess genes encoding cyanide production and 1-Aminocyclopropane-1-carboxylate (ACC) deaminase both of which are indirectly responsible for plant growth promotion. In conclusion, the two bacterial isolates, Bacillus and Stenotrophomonas were found to be capable of promoting growth and improving the survivability of chickpea and wheat plants exposed to crown rot. These findings could be potentially extended to other crops to improve crop productivity under biotic stress