The level of endogenous JA is critical for activation of SA- and JA-defensive signaling pathway in japonica rice cultivar Ziyu44 upon Magnaporthe oryzae infection

The molecular mechanism of durable and broad-spectrum resistance to rice blast disease in japonica rice remained unknown. The Oryza sativa subs. japonica cultivar, Ziyu44, a local japonica rice in Yunnan Province of China, has shown durable broad-spectrum blast resistance for more than 30 years, which provides an opportunity for us to explore the molecular basis of broad-spectrum resistance to rice blast in japonica rice. In this study, we conducted a comparative study of mycelium growth, appressorium formation, the accumulation of endogenous salicylic acid (SA), endogenous jasmonic acid (JA) and hydrogen peroxide (H 2 O 2 ), the expression of SA- and JA-associated genes between resistant rice variety Ziyu44 and susceptible rice variety Jiangnanxiangnuo (JNXN) in response to Magnaporthe oryzae ( M. oryzae ) infection. We found that appressorium formation and invasive hyphae extention were greatly inhibited in Ziyu44 leaves compared with that in JNXN leaves. Both Ziyu44 and JNXN plants maintained high baseline levels of endogenous SA that showed a little change following infection by M. oryzae and low baseline levels of endogenous JA that showed a marked increase in Ziyu44 plants but not in JNXN upon M. oryzae infection. The expression levels of key genes involving JA- and SA-signaling pathway OsCOI1b, OsNPR1 , OsMPK6 as well as pathogenesis-related ( PR ) genes OsPR1a , OsPR1b and OsPBZ1 (also known as OsPR10a ) were markedly up-regulated in resistant rice Ziyu44 compared to susceptible rice JNXN. Taken together, our results indicate that the sharply increased accumulation of endogenous JA in rice Ziyu44 plants in response to M. oryzae infection plays crucial roles in enhancing accumulation of H 2 O 2, the expression of SA- and JA-associated defense genes, activating the SA-JA defense signaling, and mediating broad-spectrum rice blast resistance in rice Ziyu44.