Elucidation of molecular and biochemical determinants in a natural rice rhizospheric isolate to attenuate rice blast pathogen Magnaporthe oryzae.

摘要

Rice, a staple food crop world-wide, suffers devastating yield losses as a result of blast disease caused by fungal pathogen Magnaporthe oryzae. Currently there are no methods for eradicating blast, and strategies for controlling the disease are flawed; their efficacy is limited, resistance develops rapidly, and many pose environmental and health hazards. A relatively new concept for increasing crop yields is the use of beneficial microbes to enhance growth or reduce disease. Microbial crop solutions are often safer and longer-lasting alternatives to chemical treatments, with considerable results. Microbes were isolated from rhizospheric soil of California field-grown rice plants. One isolate in particular, Pseudomonas chlororaphis EA105, was striking in its antifungal activity, drastically reducing vegetative growth of M. oryzae and almost completely halting the formation of M. oryzae's appressoria, a structure which is required for penetration into the host. When rice plants were root-treated with EA105 prior to M. oryzae infection, there were fewer lesions and the size of lesions was reduced. Plant defense mechanisms are typically mediated through salicylic acid (SA), jasmonic acid (JA), and/or ethylene (ETH). In plants which were treated with EA105, there was induction of genes involved in JA and ETH signaling while the expression of genes involved in salicylic acid (SA) signaling were largely unaffected. In addition SA, JA, and ETH, another critical plant hormone, abscisic acid (ABA) was also investigated. Some phytopathogens, including M. oryzae, have evolved mechanisms to trigger increased ABA biosynthesis in plants as part of the virulence process. EA105 prevented M. oryzae from up-regulating NCED3, the key enzyme involved in ABA biosynthesis. Similarly, EA105 prevented M. oryzae from up-regulating a putative rice beta glucosidase that is likely involved in activating conjugated inactive forms of the hormone. ABA appears to function not only as a suppressor of plant defense, but also a promoter of pathogenesis in M. oryzae through the acceleration of spore germination and appressoria formation. Spores and mycelia of M. oryzae produced ABA, though at levels lower than in plants, further indicating that ABA also plays an important role in fungi. However, even with the addition of exogenous ABA, EA105 was able to counter the virulence-promoting effects of this compound. In summary, EA105 can directly antagonize fungal growth and pathogenesis as well as increase host resistance to blast, mediated through JA and ETH signaling, and through the suppression of ABA-related susceptibility. Therefore, EA105 shows promise as a biocontrol solution which may reduce the severity of blast, a disease which threatens global food security.