Genome-wide identification of WRKY45-regulated genes that mediate benzothiadiazole-induced defense responses in rice

Abstract

BackgroundThe rice transcription factor WRKY45 plays a crucial role in salicylic acid (SA)/benzothiadiazole (BTH)-induced disease resistance. Its knockdown severely reduces BTH-induced resistance to the fungal pathogen Magnaporthe oryzae and the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo). Conversely, overexpression of WRKY45 induces extremely strong resistance to both of these pathogens. To elucidate the molecular basis of WRKY45-dependent disease resistance, we analyzed WRKY45-regulated gene expression using rice transformants and a transient gene expression system.ResultsWe conducted a microarray analysis using WRKY45-knockdown (WRKY45-kd) rice plants, and identified WRKY45-dependent genes among the BTH-responsive genes. The BTH-responsiveness of 260 genes was dependent on WRKY45. Among these, 220 genes (85%), many of which encoded PR proteins and proteins associated with secondary metabolism, were upregulated by BTH. Only a small portion of these genes overlapped with those regulated by OsNPR1/NH1, supporting the idea that the rice SA pathway branches into WRKY45- regulated and OsNPR1/NH1-regulated subpathways. Dexamethazone-induced expression of myc-tagged WRKY45 in rice immediately upregulated transcription of endogenous WRKY45 and genes encoding the transcription factors WRKY62, OsNAC4, and HSF1, all of which have been reported to have defense-related functions. This was followed by upregulation of defense genes encoding PR proteins and secondary metabolic enzymes. Many of these genes were also induced after M. oryzae infection. Their temporal transcription patterns were consistent with those after dexamethazone-induced WRKY45 expression. In a transient expression system consisting of particle bombardment of rice coleoptiles, WRKY45 acted as an effector to trans-activate reporter genes in which the luciferase coding sequence was fused to upstream and intragenic sequences of WRKY62 and OsNAC4. Trans-activation of transcription occurred through a W-box-containing sequence upstream of OsNAC4 and mutations in the W-boxes abolished the trans-activation.ConclusionsThese data suggest a role of WRKY45 in BTH-induced disease resistance as a master regulator of the transcriptional cascade regulating defense responses in one of two branches in the rice SA pathway.

The WRKY45-Dependent Signaling Pathway Is Required For Resistance against Striga hermonthica Parasitism

“…5C; Supplemental Fig. S6B), which is consistent with a previous report on rice infected with Magnaporthe oryzae (Nakayama et al, 2013). Remarkably, WRKY45-kd plants exhibited disease susceptibility, characterized by a large number of S. hermonthica seedlings that attached and developed: the number of S. hermonthica seedlings attaching to roots of WRKY45-kd plants was almost 3-fold higher compared with wild-type cv Nipponbare (Fig.…”

“…However, BTH application can induce resistance even in WRKY45-kd plants, suggesting that downstream components in the WRKY45-independent SA/BTH pathway are also able to contribute to resistance against S. hermonthica in rice. Such a pathway is likely to be the NH1/OsNPR1 pathway, as shown previously (Shimono et al, 2007;Nakayama et al, 2013). The involvement of JA in the NH1/OsNPR1 pathway is unlikely, because foliar BTH application onto the hebiba mutants is able to induce resistance (Supplemental Fig.…”

“…Consistently, BTH application induces JA accumulation (Supplemental Fig. S8) and JA pathway genes such as LIPOXYGENASE, 12-Oxophytodienoate Reductase1 (OPR1), and OPR4 (Shimono et al, 2007;Nakayama et al, 2013). However, BTH application can induce resistance even in WRKY45-kd plants, suggesting that downstream components in the WRKY45-independent SA/BTH pathway are also able to contribute to resistance against S. hermonthica in rice.…”

“…OsWRKY45 is a key regulator of the SA/BTH signaling pathway and is involved in BTH-induced resistance against Magnaporthe grisea (Shimono et al, 2007). OsWRKY62 is one of the genes downstream of the SA/ BTH signaling pathway that is regulated by OsWRKY45 (Nakayama et al, 2013). NH1/OsNPR1 is a rice ortholog of Arabidopsis NPR1, which regulates an independent SA signaling pathway in rice (Shimono et al, 2007).…”

CC-NBS-LRR-Type R Proteins for Rice Blast Commonly Interact with Specific WRKY Transcription Factors

“…However, Pb1 encodes an R protein-like CC-NB-LRR protein with atypical NB domain . Our recent studies showed that Pb1 specifically interacts with rice WRKY45 (Shimono et al 2007(Shimono et al , 2012Takatsuji et al 2010;Nakayama et al 2013;Akagi et al 2014;Takatsuji 2014), a TF that plays a major role in the salicylic acid signaling pathway, and that the blast resistance due to Pb1 is dependent on WRKY45 . Rice WRKY45 is regulated by ubiquitin-proteasome degradation in the nuclei to suppress untimely defense activation in the absence of pathogen infection .…”