Antifungal and defense elicitor activities of pyrazines identified in endophyticPseudomonas putidaBP25 against fungal blast incited byMagnaporthe oryzaein rice

Black pepper-associated endophyticPseudomonas putidaBP25 displayed volatile-mediated antagonism against rice blast fungusMagnaporthe oryzae1637. The major chemical fraction identified in the volatiles ofPseudomonas putidaBP25 was 2-methylpyrazine and 2-ethyl-3,6-dimethylpyrazine; both of them inhibited all developmental stages such as conidial germination, mycelial growth, and sporulation ofM. oryzae. To validate the antifungal activity of 2-methylpyrazine and 2-ethyl-3,6-dimethylpyrazine on blast disease,in plantaexperiments were conducted on rice seeds and seedlings. Blast disease incidence and disease severity on volatile-treated seedlings were significantly reduced as compared to mock. Seedlings that emerged from volatile-exposed seeds were analyzed for expression of candidate defense gene,OsPAD4,OsEDS1,OsPDF2.2,OsPR3, andOsPR1.1 by quantitative real-time PCR (qPCR). Seedlings exposed to volatiles showed significant induction of theOsPAD4 gene with both 2-ethyl-3,6-dimethylpyrazine and 2-methylpyrazine. Direct antifungal activity of pyrazine coupled with its defense elicitation capability can be harnessed for rice seed disinfection to ensure healthy, vigorous, and disease-free transplants.

Automated summary

The endophytic Pseudomonas putida BP25 associated with black pepper exhibited volatile-mediated antagonism against the rice blast fungus Magnaporthe oryzae 1637. The major chemical components identified in the volatiles were 2-methylpyrazine and 2-ethyl-3,6-dimethylpyrazine, both of which inhibited the developmental stages of the fungus. In planta experiments on rice seeds and seedlings demonstrated that these compounds reduced blast disease incidence and induced the expression of defense genes in the plants.