Molecular Breeding of Disease Resistant Plants with Regulation of Small G-protein, Rac.

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A small G-protein, Rac, has been known as a key regulator of immune system mediated by the cytoskeleton and NADPH oxidase in animals. In plant defense system, details of Rac functions are still unclear, although a possibility that plant Rac proteins regulate superoxide generation in infected tissues has been pointed out. In this research, to determine the function of plant Rac proteins in defense responses, systemic aquired resistance (SAR) and local aquired resistance (LAR) of tobacco Rac-overexpressor were investigated. When Rac-overexpressor was inoculated with a pathogenic bacterium, Pseudomonas syringae pv tabaci (Psf), higher level of PR gene was expressed and LAR was induced much quicker than wild type plant inoculated with Pst. Moreover, inoculation of a nonpathogen, Pseudomonas syringae pv glycinea, to lower leaves of Rac-overexpressor cause PR-gene expression in upper leaves. Both acidic PR mRNA and salicylic acid prominently accumulated in the upper leaves of inoculated Rac-overexpressor, whereas no accumulation was detected in a wild type, SR-1, tobacco. These results indicate that SAR was induced in Rac overexpressed tobacco by the inoculation with nonpathogenic Pseudomonas, although the nonpathogen doesn't induce SAR in wild type tobacco. These results also suggest that SA-mediated defense reactions might be easily induced in Rac-overexpressor. To confirm the hypothesis, we investigated SA sensitivity of tobacco plants. SA sensitivity of Rac-overexpressor was enhanced to 10 times higher than wild type tobacco. The enhacement of SA sensitivity should be the main reason of the stronger defense reactions in Rac-overexpressor. Taken together, it is suggested that Rac small G-proteins play an important role in the signal perception of the SA-mediated defense system. Moreover, these results suggest the possibility that Rac genes have promise as a target of molecular bleeding of disease resistant crops.