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

Rac是一种小分子G蛋白，是动物免疫系统的重要调节因子，由细胞骨架和NADPH氧化酶介导。在植物防御系统中，Rac蛋白的功能尚不清楚，但已指出植物Rac蛋白可能调节受感染组织中超氧化物的产生。本研究通过对烟草Rac超表达基因的系统获得抗性（SAR）和局部获得抗性（LAR）的研究，探讨Rac蛋白在烟草防御反应中的作用。当Rac-过表达体接种病原细菌烟草致病菌（Psf）时，PR基因的表达水平更高，LAR的诱导速度也比接种Pst的野生型快得多。此外，接种的非病原体，假单胞菌pv glycoprotein，Rac-过表达的下部叶片引起PR基因在上部叶片的表达。酸性PR mRNA和水杨酸都显著积累在接种的Rac-过表达的上部叶片中，而在野生型SR-1烟草中没有检测到积累。这些结果表明，在Rac过表达的烟草中，通过接种非病原假单胞菌诱导SAR，尽管非病原假单胞菌在野生型烟草中不诱导SAR。这些结果也表明，SA介导的防御反应可能很容易在Rac过表达诱导。为了证实这一假设，我们调查了烟草植株对水杨酸的敏感性。Rac-过表达烟草对水杨酸的敏感性比野生型烟草提高了10倍。SA敏感性的增强可能是Rac-过表达细胞防御反应增强的主要原因。两者合计，这表明，Rac小G蛋白在SA介导的防御系统的信号感知中发挥重要作用。此外，这些结果表明，Rac基因有希望作为抗病作物的分子出血的目标。

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.