The role of rice rhizobacteria in plant growth promotion and defense against Magnaporthe oryzae infection

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University of Delaware
The natural interactions between plant roots and their rhizospheric microbiome are vital to plant fitness, modulating both growth promotion and disease suppression. Naturally occurring root-associated rhizospheric bacteria were isolated from California field grown rice plants (M-104), eleven of which were taxonomically identified by16S rRNA gene sequencing and fatty acid methyl ester (FAME) analysis. Using an experimental hydroponic set-up, the isolates were inoculated into the rice growth media to test for growth promotion properties. A cocktail of all eleven isolates increased rice biomass by 91%. Treatment with a single isolate identified as an Arthrobacter oxydans species (EA201) increased biomass, as well as shoot phosphorus (P) and iron (Fe) concentration. Bacterial isolates were tested for biological control activity against the devastating foliar rice fungal pathogen, Magnaporthe oryzae pathovar 70-15. In vitro, a Pseudomonas isolate, EA105, displayed antibiosis through impairing 70-15 appressorium formation, the specialized structure needed for infection. EA105 also displayed inhibition of fungal growth through diffusible compounds (65% inhibited) and through volatile compounds (51% inhibited). Although hydrogen cyanide (HCN) is a volatile commonly produced by biocontrol Pseudomonads, the activity of EA105 seems to be independent of its HCN production. Characterization of volatiles produced by EA105 was done using SPME-GC-MS and included alcohols, hydrocarbons, ketones, and sulfur-containing compounds. During in planta experiments, an induced systemic resistance (ISR)-like response was seen in soilgrown rice by priming roots with EA105 and a Pantoea agglomerans isolate (EA106). EA105 reduced the number of blast lesions formed by 33% and EA106 by 46%. The application of natural bacteria as biocontrol agents in combination with current disease protection strategies could create a synergistic and long-lasting approach to sustainable agriculture.