Impact of the tripartite interaction between rice, sheath blight and diverse crop-associated endophytes on phenotypic and biochemical responses in rice

Endophytes stimulate plant growth and inhibit phytopathogens. Most of the known endophytes are host-specific and only a few strains are effective for practical field use. Thus, this study focuses on the evaluation of endophytes viz., Bacillus pseudomycoides strain HP3d, Paenibacillus polymyxa strain PGSS1, B. velezensis strain A6 and P42 isolated from diverse crop ecosystems for their potential to promote plant growth and induce systemic resistance against sheath blight disease in rice. The endophytes were studied for plant growth promoting traits in vivo conditions and were found to exhibit ammonia (light to strong), siderophore (yellow zone on the CAS agar plate), indole-3-acetic acid (15.20-22.19 μg mL^-1) production and phosphorus solubilization (1.2-1.5 cm). In the glasshouse, when applied individually and in combinations through various methods like seed treatment, seedling dip, and foliar spray these endophytes significantly reduced lesion size (2.06-2.37 fold) and ShB severity (2.60-2.58 fold), enhancing growth parameters viz., shoot (1.09-1.11 fold), root (1.02-1.20 fold), number of tillers (1.2-1.6 fold), shoot (80.58-82.64 %) and root (62.01-66.66 %) dry matter over untreated control. Consequently, enzyme activity viz., polyphenol oxidase (2.20-3.00 U^-1min^-1g^-1), peroxidase (0.31-0.35 min^-1g^-1), superoxide dismutase (118.50-123.00 Ug^-1 FW), and phenylalanine ammonia lyase (0.84-0.90 min⁻¹g⁻¹FW) was found to increase up to the fourth day after the pathogen challenge and subsequently decrease thereafter. Chlorophyll content post inoculation of ShB declined over time but endophyte treated plants exhibited lesser reductions over uninoculated control. Field trials corroborated the in vitro findings, demonstrating reduced ShB (1.71-1.88 fold decrease in PDI) and enhanced growth (1.1-1.2 fold increase in shoot length) over untreated controls. The combined application of seedling dip, seed treatment, and foliar spray proved to be the most optimum treatment. The findings highlight the potential of diverse crop-derived endophytes, emphasizing their non-host specificity and effectiveness as broad-spectrum bioagents in actual field conditions.