[Intellectual contribution]

Rice WRKY45 plays a crucial role in benzothiadiazole-inducible resistance to fungal blast and bacterial blight diseases

Hiroshi TAKATSUJI, Masaki SHIMONO, Shoji SUGANO, Akira NAKAYAMA, Chang-Jie JIANG, Nagao HAYASHI
Disease Resistance Research Unit

   Benzothiadiazole (BTH) is a so-called eplant activatorf and protects plants from diseases by eprimingf the salicylic-acid (SA) signalling pathway. Because of its characteristic action mechanism ofe primingf, BTH exerts protective effects against a broad spectrum of diseases without major adverse effects on plant growth when applied at appropriate dosages. Assuming that a transcription factor plays a key role in the BTH-induced eprimingf, we screened for BTH-inducible genes for transcription factors by using rice 44K oligoarray and identified WRKY45 that upregulated within 3 h after BTH treatment. Overexpression of WRKY45 in transgenic rice (WRKY45-ox) driven by the constitutive maize ubiquitin promoter dramatically enhanced resistance to blast disease (Fig. 1A), the devastating fungal disease worldwide. The WRKY45-ox rice was even more blast resistant thane Senshof, which is known as a very highly blast resistant rice variety (Fig. 1C). WRKY45-ox rice plants were also highly resistant to bacterial blight diseases (Fig. 1B). Thus, WRKY45-ox rice is resistant to both fungal and bacterial diseases (emultidisease resistancef).
   RNA interference|mediated knockdown of WRKY45 (WRKY45-kd) compromised BTH inducible resistance to blast disease, indicating that WRKY45 is essential for BTH-induced defence responses. In a transient expression system, WRKY45 activated reporter-gene transcription through W-boxes, indicating that WRKY45 is a transcriptional activator acting through W-boxes. Epistasis analysis suggested that WRKY45 acts in the SA signalling pathway independently of OsNPR1, a rice ortholog of Arabidopsis NPR1 (Fig. 2), which distinguishes WRKY45 from any known Arabidopsis WRKY transcription factors. Examination of BTH inducibility in WRKY45- and OsNPR1-kd plants revealed that two defence-related genes, encoding a glutathione-S-transferase and a cytochrome P450, are regulated by WRKY45, but not by OsNPR1. On the other hand, PR1b and PR1a genes were regulated by OsNPR1, but not by WRKY45. These results are consistent with the apparent independence of the WRKY45- and OsNPR1-dependent pathways.
   Thus, rice has a SA-signalling pathway that is substantially different from the Arabidopsis counterpart. WRKY45-ox rice plants grew without severe growth defects in our greenhouse (Fig. 3). In these plants, defence reactions were not activated as revealed by the absence of PR-gene expression (Fig. 3). Thus, WRKY45-ox rice appears to be eprimedf for defence responses. However, WRKY45-ox rice showed substantial growth retardation accompanied by PR-gene expression when grown in a growth chamber (Fig. 3), suggesting that some environmental factor (s) triggered defence reactions by acting downstream of WRKY45 transcription (Fig. 2). The high degree of multi-disease resistance accompanied by minor growth retardation due to overexpression of WRKY45 makes this gene a promising tool for developing practically useful multi-disease resistant rice by a transgenic approach. To achieve this goal, however, the environment-dependent growth retardation has to be overcome by optimizing the transgene expression.

Fig. 1 Disease resistance of rice by
overexpression of WRKY45
Fig. 1  Disease resistance of rice by overexpression of WRKY45
(A and B) Disease resistance of WRKY45-ox rice to fungal blast (A) and bacterial blight (B) diseases.
(C) WRKY45-ox rice is even more blast resistant than the highly blast resistant rice variety ‘Sensho’.




Fig. 2 SA signalling pathways in Arabidopsis and rice
Fig. 2  SA signalling pathways in Arabidopsis and rice
In Arabidopsis, downstream of SA is largely controlled by NPR1. In rice, the SA pathway separate into WRKY45- and OsNPR1-dependent pathways. An environmental factor (s) acts downstream of WRKY45 and OsNPR1 to trigger defence-gene expression.




Fig. 3 Growth and PR-gene expression in WRKY45-ox rice plants under different growth conditions
Fig. 3  Growth and PR-gene expression in WRKY45-ox rice plants under different growth conditions
Greenhouse-grown WRKY45-ox rice showed only minor growth retardation, whereas, it was much more severe
in growth-chamber-grown WRKY45-ox rice. Expression of PR1b and PR1a, as markers of defence reactions,
was undetectable in greenhouse-grown but highly detected in growth-chamber-grown WRKY45-ox rice.

 

 

[Reference]
Shimono M, Sugano S, Nakayama A, Jiang C-J, Ono K, Toki S, Takatsuji H (2007) Rice WRKY45 plays a crucial role in benzothiadiazole-inducible blast resistance. Plant Cell, 19 (6) :2064-2076.


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