Natural rice rhizospheric microbes suppress rice blast infections.

BACKGROUND: The natural interactions between plant roots and their rhizospheric microbiome are vital to plant fitness, modulating both growth promotion and disease suppression. In rice (Oryza sativa), a globally important food crop, as much as 30% of yields are lost due to blast disease caused by fungal pathogen Magnaporthe oryzae. Capitalizing on the abilities of naturally occurring rice soil bacteria to reduce M. oryzae infections could provide a sustainable solution to reduce the amount of crops lost to blast disease. RESULTS: Naturally occurring root-associated rhizospheric bacteria were isolated from California field grown rice plants (M-104), eleven of which were taxonomically identified by 16S rRNA gene sequencing and fatty acid methyl ester (FAME) analysis. Bacterial isolates were tested for biocontrol activity against the devastating foliar rice fungal pathogen, M. oryzae pathovar 70-15. In vitro, a Pseudomonas isolate, EA105, displayed antibiosis through reducing appressoria formation by nearly 90% as well as directly inhibiting fungal growth by 76%. Although hydrogen cyanide (HCN) is a volatile commonly produced by biocontrol pseudomonads, the activity of EA105 seems to be independent of its HCN production. During in planta experiments, EA105 reduced the number of blast lesions formed by 33% and Pantoea agglomerans isolate, EA106 by 46%. Our data also show both EA105 and EA106 trigger jasmonic acid (JA) and ethylene (ET) dependent induced systemic resistance (ISR) response in rice. CONCLUSIONS: Out of 11 bacteria isolated from rice soil, pseudomonad EA105 most effectively inhibited the growth and appressoria formation of M. oryzae through a mechanism that is independent of cyanide production. In addition to direct antagonism, EA105 also appears to trigger ISR in rice plants through a mechanism that is dependent on JA and ET signaling, ultimately resulting in fewer blast lesions. The application of native bacteria as biocontrol agents in combination with current disease protection strategies could aid in global food security.

Native plant and microbial contributions to a negative plant-plant interaction.

A number of hypotheses have been suggested to explain why invasive exotic plants dramatically increase their abundance upon transport to a new range. The novel weapons hypothesis argues that phytotoxins secreted by roots of an exotic plant are more effective against naïve resident competitors in the range being invaded. The common reed Phragmites australis has a diverse population structure including invasive populations that are noxious weeds in North America. P. australis exudes the common phenolic gallic acid, which restricts the growth of native plants. However, the pathway for free gallic acid production in soils colonized by P. australis requires further elucidation. Here, we show that exotic, invasive P. australis contain elevated levels of polymeric gallotannin relative to native, noninvasive P. australis. We hypothesized that polymeric gallotannin can be attacked by tannase, an enzymatic activity produced by native plant and microbial community members, to release gallic acid in the rhizosphere and exacerbate the noxiousness of P. australis. Native plants and microbes were found to produce high levels of tannase while invasive P. australis produced very little tannase. These results suggest that both invasive and native species participate in signaling events that initiate the execution of allelopathy potentially linking native plant and microbial biochemistry to the invasive traits of an exotic species.