Rice WRKY45 Plays a Crucial Role in Benzothiadiazole-Inducible Blast Resistance

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Abstract
Benzothiadiazole (BTH) is a so-called plant activator and protects plants from diseases by activating the salicylic acid (SA) signaling pathway. By microarray screening, we identified BTH- and SA-inducible WRKY transcription factor (TF) genes that were upregulated within 3 h after BTH treatment. Overexpression of one of them, WRKY45, in rice (Oryza sativa) markedly enhanced resistance to rice blast fungus. RNA interference–mediated knockdown of WRKY45 compromised BTH-inducible resistance to blast disease, indicating that it is essential for BTH-induced defense responses. In a transient expression system, WRKY45 activated reporter gene transcription through W-boxes. Epistasis analysis suggested that WRKY45 acts in the SA signaling pathway independently of NH1, a rice ortholog of Arabidopsis thaliana NPR1, which distinguishes WRKY45 from known Arabidopsis WRKY TFs. Two defense-related genes, encoding a glutathione S-transferase and a cytochrome P450, were found to be regulated downstream of WRKY45 but were not regulated by NH1, consistent with the apparent independence of the WRKY45- and NH1-dependent pathways. Defense gene expression in WRKY45-overexpressed rice plants varied with growth conditions, suggesting that some environmental factor(s) acts downstream of WRKY45 transcription. We propose a role for WRKY45 in BTH-induced and SA-mediated defense signaling in rice and its potential utility in improving disease resistance of rice, an importance food resource worldwide.