A Study on the Role of Rhizospheric Bacteria in Promoting Plant Growth and Alleviating Biotic Stress in its Host

Abstract

Cabbage is susceptible to various fungal pathogens such as Xanthomonas campestris, Alternaria brassica, Rhizoctonia solani, etc. Various fungicides have been recommended such as chlorothalonil and tebuconazole, which are highly toxic to both soil and aquatic ecosystem and may induce tumors in mammalian cells. Bio-inputs can be the most sustainable and eco-friendly approach to mitigate plant diseases. In the present study, two rhizospheric bacterial isolates Bacillus amyloliquefaciens AG1B and B. subtilis AG2B showed antagonistic activity against Alternaria brassicicola AG1F. Previously reported endophytic bacteria, B. subtilis Scb-1 also showed similar antagonistic activity against the same pathogenic fungi. The cell-free supernatant of the bacterial isolates showed a significant reduction in fungal growth. Microscopic studies revealed deformed hyphae and conidia with bubble formation when co-cultured with the antagonist bacterial isolates. Lipopeptides are bioactive compounds that pose substantial challenges to the structural integrity of fungal cell walls and are regarded as antifungal in nature. LC-MS analysis showed that co-cultivation of B.amyloliquefaciens AG1B with A. brassicicola AG1F results in the secretion of several lipopeptide and polyketide compounds such as surfactin, iturin, etc. Similar results were also obtained when B. subtilisAG2B and B. subtilis Scb-1 were co-cultured with A. brassicicola AG1F, respectively.The complete genome of the potential bacterial isolate; B. amyloliquefaciens AG1B with a percentage inhibition of 76.47 % against A. brassicicola AG1F was reconstructed through de novo assembly, revealing a genome size of 3.89 Mbp. This genome encompasses genes associated with plant growth promotion, lipopeptides and polyketides synthesis, and genes that confirm its affiliation with rhizobacterial interactions. Differential expression analysis of key genes such srfAB, ItuA, fenF etc., involved in the synthesis of lipopeptides and polyketides during the dual culture condition showed significant transcriptional up-regulation, validating the results of metabolite profiling. Moreover, the bio-primed seeds of cabbage with the potential bacterial isolates, B. amyloliquefaciens AG1B, B. subtilis AG2B and B. subtilis Scb-1 respectively, showed better germination percentage, increase in root, and shoot length. Foliar application of the potential bacterial isolates B. amyloliquefaciensAG1B, B. subtilis AG2B and B. subtilis Scb-1 efficiently resulted in a reduction of disease severity up to 80%. Thus, multifaceted bio-inputs like B. amyloliquefaciens AG1B, B. subtilis AG2B and B. subtilis Scb-1 can be used for making bio-formulation for sustainable management of fungal diseases. newline