cDNA cloning and characterization of a plant protein that may be associated with the harpinPSS-mediated hypersensitive response.

Hypersensitive response-assisting protein (HRAP) is a novel plant protein that can intensify the harpinPSS-mediated hypersensitive response (HR) in harpinPSS-insensitive plants, such as the vegetative stage of sweet pepper. In this report, we identified a HRAP cDNA clone from sweet pepper (Capsicum annuum cv. ECW). The sequence of this cDNA clone showed no appreciable similarity to any other known sequences. However, it contained three positively charged regions, a typical signal peptide and a cAMP-dependent phosphorylation site. The hrap mRNA accumulated preferentially during the incompatible interaction of sweet pepper leaves with a pathogenic bacterium, Pseudomonas syringae pv. syringae. When the hrap gene transcription level was high, the sweet pepper leaves readily expressed the harpinPSS-mediated HR. The hrap gene transcription level in sweet pepper was also higher during the reproductive stage than during the vegetative stage. The HRAP distribution in an individual plant and different plant species was investigated. We found that all the organs of sweet pepper, except fruit, could express two different forms of HRAP. Moreover, the hrap gene was presented in many plant species including tobacco, Arabidopsis, and rice. In conclusion, our results suggest that the hrap gene is widely distributed throughout the plant world and its transcription level correlates with plant sensitivity to harpinPSS. The interaction between HRAP and harpinPSS reveals a novel way to interpret the interaction mechanism between plants and bacterial pathogens.