Activation of hypersensitive response genes in the absence of pathogens in transgenic tobacco plants expressing a rice small GTPase.

Transgenic tobacco ( Nicotiana tabacum L.) plants constitutively expressing a rice ( Oryza sativa L.) gene encoding a small GTPase, rgp1, showed marked resistance to tobacco mosaic virus (TMV) infection compared with the wild type [H. Sano et al. (1994) Proc Natl Acad Sci USA 91:10556-10560]. In order to examine the gene expression profile, the temperature-shift method was adopted to hyper-activate the N-gene inducing the hypersensitive response (HR), and transcripts of 11 representative HR genes were analyzed. In transgenic and wild-type plants, transcripts of 10 genes were induced during the HR; however, in most cases, their expression level was higher in the former than in the latter. Mock treatment of transgenic plants also efficiently induced transcripts of 8 out of 11 genes after temperature shift, indicating that their activation is mediated by the N-gene. Salicylic acid and its glucoside-conjugates were induced in both transgenic and wild-type plants, but their quantity in the former was unusually higher than in the latter. These results suggest that expression of rgp1 positively influenced the signaling pathway of the HR, resulting in higher induction of salicylates. This possibly caused a "priming effect" that hyper-activates the HR genes through the N-gene without TMV infection. It was thus conceivable that, despite a structural similarity to the Rab-family of GTPases, which function in membrane trafficking, rgp1 might participate in the signal transduction pathway of the HR.