Jung Young-Ho, Agrawal Ganesh Kumar, Rakwal Randeep, Kim Jung-A, Lee Mi-Ok, Choi Pil Gyu, Kim Young Jin, Kim Min-Jea, Shibato Junko, Kim Sun-Hyung, Iwahashi Hitoshi, Jwa Nam-Soo
Department of Molecular Biology, College of Natural Science, Sejong University, Seoul 143-747, Korea.
Plant Physiol Biochem. 2006 Jan;44(1):68-77. doi: 10.1016/j.plaphy.2005.12.001. Epub 2006 Feb 17.
The rice genome contains at least seven expressed Rop small GTPase genes. Of these Rops, OsRac1 is the only characterized gene that has been implicated in disease resistance as a positive regulator. To our interest in finding a negative ROP regulator of disease resistance in rice, we applied a "phylogeny of function" approach to rice Rops, and identified OsRacB based on its close genetic orthologous relationship with the barley HvRacB gene, a known negative regulator of disease resistance. To determine the function of OsRacB, we isolated the OsRacB cDNA and conducted gene expression and transgenic studies. OsRacB, a single copy gene in the genome of rice, shared 98% identity with HvRacB at the amino acid level. Its mRNA was strongly expressed in leaf sheath (LS) and in panicles, but was very weakly expressed in young and mature leaves. The basal mRNA level of OsRacB in LS of two-week-old seedlings was strongly down-regulated upon wounding by cut and treatment with jasmonic acid. A dramatic down-regulation in the OsRacB transcripts was also found in plants inoculated with the blast pathogen, Magnaporthe grisea. Interestingly, transgenic rice plants over-expressing OsRacB showed increased symptom development in response to rice blast pathogens. Additionally, fluorescence microscopy of green fluorescent protein (GFP):OsRacB-transformed onion cells and Arabidopsis protoplasts revealed OsRacB association with plasma membrane (PM), suggesting that PM localization is required for proper function of OsRacB. Based on these results, we suggest that OsRacB functions as a potential regulator for a basal disease resistance pathway in rice.
水稻基因组至少包含7个表达的Rop小GTP酶基因。在这些Rop蛋白中，OsRac1是唯一已被鉴定为抗病正向调节因子的基因。出于我们对寻找水稻抗病负向ROP调节因子的兴趣，我们对水稻Rop蛋白应用了“功能系统发育”方法，并基于其与大麦HvRacB基因（一种已知的抗病负向调节因子）的紧密遗传直系同源关系鉴定出了OsRacB。为了确定OsRacB的功能，我们分离了OsRacB cDNA并进行了基因表达和转基因研究。OsRacB是水稻基因组中的单拷贝基因，在氨基酸水平上与HvRacB有98%的同一性。其mRNA在叶鞘（LS）和穗中强烈表达，但在幼叶和成熟叶中表达非常弱。两周大幼苗叶鞘中OsRacB的基础mRNA水平在切割伤口和用茉莉酸处理后被强烈下调。在用稻瘟病菌Magnaporthe grisea接种的植物中也发现OsRacB转录本显著下调。有趣的是，过表达OsRacB的转基因水稻植株对稻瘟病菌的症状发展增加。此外，绿色荧光蛋白（GFP）：OsRacB转化的洋葱细胞和拟南芥原生质体的荧光显微镜观察显示OsRacB与质膜（PM）相关，表明质膜定位是OsRacB正常功能所必需的。基于这些结果，我们认为OsRacB作为水稻基础抗病途径的潜在调节因子发挥作用。
