A rice rhizosphere plant growth-promoting Streptomyces corchorusii isolate antagonizes Magnaporthe oryzae and elicits defense responses in rice.

Aims: Rice blast, caused by Magnaporthe oryzae, is one of the most devastating diseases of rice (Oryza sativa L.). The aim of this study was to investigate the biocontrol potential of rice rhizosphere actinomycetes against M. oryzae Guy 11, and elucidate the antagonistic mechanisms.

Methods and results: An isolate characterized as a Streptomyces corchorusii strain (Sc75) using the 16S rRNA gene exhibited superior antifungal activity. Sc75 had an inhibitory effect of 69.25% ± 0.15% against M. oryzae and broad antifungal activity on other fungal plant pathogens in the dual culture assay. Its cell-free culture filtrate inhibited fungal growth and reduced mycelial mass. Also, the ethyl acetate crude extract completely inhibited conidia germination and appressoria formation on the hydrophobic coverslips and detached leaf at a concentration of 20 mg/ml. Its volatile organic compounds (VOCs) suppressed fungal growth by 98.42%. GC-MS analysis of the VOCs identified butanoic acid, 2-methyl-, methyl ester; di-tert-butyl peroxide; furan, 2-pentyl-; and undecanoic acid, 10-methyl-, methyl ester as the main components. In the greenhouse experiment, the disease severity was reduced and growth promotion was evident. Molecular investigation revealed that Sc75 upregulated defense-related genes involved in the synthesis of jasmonic acid, salicylic acid signaling pathway, and led to callose deposition and ROS production in the leaves. Finally, Sc75 produced hydrolytic enzymes, siderophore, indole acetic acid, gibberellic acid, phosphate solubilization, and 1-aminocyclopropane-1-carboxylate deaminase.

Conclusions: The rice rhizosphere soil harbors actinomycetes that can be explored as biocontrol agents against fungal pathogens such as M. oryzae. The isolate Sc75 had superior antifungal activity against M. oryzae and other selected plant pathogenic fungi. It showed remarkable antagonistic activity through direct antibiosis, production of VOCs, antifungal metabolites in the culture filtrates and crude extracts, and produced enzymes. In addition, the isolate promoted plant growth, reduced rice blast disease index in the greenhouse experiment, and elicited defense-related responses. Sc75 is a promising candidate for future exploration as a biofungicide and a biofertilizer.