Involvement of mitogen-activated protein kinase in the induction of StrbohC and StrbohD genes in response to pathogen signals in potato

The respiratory burst oxidase homolog (rboh), a plant homolog of the catalytic subunit of phagocyte NADPH oxidase (gp91 phox ), has been implicated in O2 − generation. In the previous study, we reported that StrbohA is constitutively expressed at a low level, whereas StrbohB is upregulated after pathogen signals are perceived in potato tubers. To gain a better understanding about the role of rboh in the defense response in potato leaves, we investigated the regulatory mechanisms of the gene. StrbohB gene was activated in response to an avirulent race of Phytophthora infestans or to hyphal wall components (HWC) from the pathogen; however, levels of expression were very low. We isolated two novel rboh cDNAs, StrbohC and StrbohD, by virtue of the sequence information from the tomato whitefly-induced gene 1 (LeWfi1) and tomato TC121349, respectively. The expression of StrbohC and StrbohD was drastically upregulated in the potato leaves inoculated with P. infestans or treated with HWC elicitor. Mitogen-activated protein kinase (MAPK) is known to play a pivotal role in the oxidative burst. The transcript accumulation induced by HWC treatment was blocked by U0126, which is a specific inhibitor of MAPK kinase (MAPKK). Conversely, the StrbohC and StrbohD genes were upregulated, driven by a pathogen-inducible promoter, in transgenic potato leaves that carry a constitutively active mutant of the MAPKK, StMEK1DD, that activates defense-related MAPKs. These results suggest that a MAPK cascade is involved in the process leading to the expression of the StrbohC and StrbohD genes.