Accumulation and Resistance to Phytophthora infestans

Active oxygen species (AOS) are responsible for triggering defense responses in plants. Respiratory burst oxidase homologs ( rboh genes) have been implicated in AOS generation. We have isolated two rboh cDNAs, NbrbohA and NbrbohB , from Nicotiana benthamiana leaves. NbrbohA was expressed constitutively at a low level and the transcripts were increased after mechanical stress of control leaf infiltration, whereas NbrbohB was induced specifically by the protein elicitor INF1 from the potato pathogen Phytophthora infestans . We examined the function of the Nbrboh genes in AOS generation and in the hypersensitive response (HR) using virus-induced gene silencing (VIGS). VIGS indicated that both genes are required for H 2 O 2 accumulation and for resistance to Phytophthora. VIGS of Nbrboh genes also led to a reduction and delay of HR cell death caused by INF1. We further demonstrate that the induction of HR-like cell death by overexpression of a constitutively active mutant of a mitogen-activated protein kinase kinase, MEK DD , is compromised by VIGS of NbrbohB . We found that MEK DD induced NbrbohB but not NbrbohA . This work provides genetic evidence for the involvement of a mitogen-activated protein kinase cascade in the regulation of rboh genes.