3.2 Pi starvation activates JA signaling
It has been reported that Pi starvation elevates the JA pathway inArabidopsis , and furthermore that the JA pathway plays a positive role in rice resistance to Xoo (Yamada et al. , 2012; Khanet al. , 2016); consequently, we hypothesized that the JA pathway may be involved in Pi starvation-induced rice resistance to Xoo . To test our hypothesis, we firstly examined the expression of JA responsive genes using RT-qPCR. As shown in Figure 2a and b, transcript levels of genes involved in JA biosynthesis (OsLOX1 ,OsLOX2 and OsAOS2 ) (He et al. , 2017), JA signaling (OsMYC2 , OsJAmybOsJAZ2 , OsJAZ5 ,OsJAZ10 and OsJAZ12 ), and JA-responsive pathogen-related (PR) genes (OsPR1a , OsPR1b , OsPR5 and OsPR9 ) (Agrawal et al. , 2000a; Agrawal et al. , 2000b; Rakwal & Komatsu, 2000; Deng et al. , 2012) were all significantly increased in the leaves of Pi-starved plants relative to Pi-sufficient plants. To confirm whether activation of JA-related gene expression led to elevated endogenous hormone contents, we measured MeJA and JA-IIe concentrations in Pi-starved plants over three days. The results showed higher MeJA content in inoculated leaves of Pi-starved plants than in those of the mock control, whereas no significant change of JA-Ile content, the bioactive form in JA signaling, was detected between Pi-starved plants and to Pi-sufficient plants (Figure 2c and S1a). Interestingly, the expression of OsMYC2, the key regulator in JA signaling, was also altered upon Pi starvation (Figure 2a and b), gradually increasing in a time-course manner during Pi starvation (Figure 2d). Together, these results suggested that the activation ofOsMYC2 expression by Pi starvation, not the increase in JA-Ile content, was due to the interaction of Pi starvation and JA signaling.