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

一种小的g蛋白Rac被认为是动物细胞骨架和NADPH氧化酶介导的免疫系统的关键调节因子。在植物防御系统中，Rac的功能细节尚不清楚，尽管已经指出植物Rac蛋白可能调节感染组织中超氧化物的产生。为了确定植物Rac蛋白在防御反应中的功能，本研究对烟草Rac-过表达蛋白的系统获得抗性（SAR）和局部获得抗性（LAR）进行了研究。当rac -过表达物与病原菌丁香假单胞菌（Pseudomonas syringae pv tabaci, Psf）接种后，其PR基因表达量明显高于接种Pst的野生型植株，其LAR诱导速度明显快于接种Pst的野生型植株。此外，将一种非病原体丁香假单胞菌（Pseudomonas syringae pv glycinea）接种于rac -过表达者的下部叶片，可导致上部叶片中pr -基因的表达。酸性PR mRNA和水杨酸均在接种rac -过表达者的烟草上部叶片中显著积累，而在野生型SR-1中未检测到积累。上述结果表明，接种非致病性假单胞菌可诱导Rac过表达烟草产生SAR，而非致病性假单胞菌对野生型烟草不产生SAR。这些结果也表明，sa介导的防御反应可能很容易在rac -过表达者中诱导。为了证实这一假设，我们研究了烟草植物对SA的敏感性。rac -过表达基因对SA的敏感性比野生型提高了10倍。SA敏感性的增强可能是rac -过表达者防御反应增强的主要原因。综上所述，这表明Rac小g蛋白在sa介导的防御系统的信号感知中起重要作用。此外，这些结果表明Rac基因有可能成为抗病作物分子出血的靶标。

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.