Wang Lei, Jiang Wendi, Zhang Yuanbao, Wang Shanzhi, Ma Rongcai, Sun Wenxian
China Agricultural University, College of Biological Sciences, Beijing, China ;
Phytopathology. 2013 May 8. doi: 10.1094/PHYTO-12-12-0333-R.
XA21, as a pattern recognition receptor in rice, senses the quorum-sensing (QS) signal molecule Ax21 secreted by Xanthomonas oryzae pv. oryzae (Xoo) and mediates hypersensitive response-like immunity against the pathogen. However, for the infection of another pathovar, X. o. pv. oryzicola (Xoc), relatively weak defense responses are observed in XA21-containing rice cultivars. In the present study, we demonstrated that the Xoc Δax21 mutant caused more severe disease symptoms than the wild type in XA21 rice cv. IRBB21, but not in non-Xa21 rice cv. Taipei 309. The substance(s) secreted by the wild-type Xoc strain, but not those by the ax21 mutant triggered host immunity against Xoo PXO99 Δax21 in Xa21 rice. Immunoblot analysis corroborated that Xoc, like Xoo, synthesizes and secretes Ax21. Furthermore, the membrane fusion protein RaxA was demonstrated to be required, but the ATP-binding cassette transporter RaxB was dispensable for Ax21 secretion in Xoc. In addition, we demonstrated that Ax21 functions as a QS signal molecule that regulates biofilm formation in Xoc. However, Ax21 signaling is dispensable for bacterial motility, the production of extracellular polysaccharide and protease secretion in Xoc. Interestingly, the two-component system RaxR/H was involved in the regulation of bacterial motility and the regulation was likely independent on Ax21 signaling in Xoc. Taken together, the results indicated that Ax21 secreted by Xoc might induce plant immunity that plays a significant role in rice defense against the pathogen infection.
XA21作为水稻中的一种模式识别受体，可感知由水稻白叶枯病菌（Xanthomonas oryzae pv. oryzae，简称Xoo）分泌的群体感应（QS）信号分子Ax21，并介导对该病原体的类过敏反应免疫。然而，对于另一个致病变种水稻条斑病菌（X. o. pv. oryzicola，简称Xoc）的感染，在含有XA21的水稻品种中观察到相对较弱的防御反应。在本研究中，我们证明Xoc Δax21突变体在XA21水稻品种IRBB21中比野生型引发更严重的病害症状，但在非Xa21水稻品种台北309中并非如此。野生型Xoc菌株分泌的物质而非ax21突变体分泌的物质，在Xa21水稻中触发了宿主对Xoo PXO99 Δax21的免疫。免疫印迹分析证实，Xoc与Xoo一样，合成并分泌Ax21。此外，膜融合蛋白RaxA被证明是Ax21在Xoc中分泌所必需的，但ATP结合盒转运蛋白RaxB对于Ax21分泌是可有可无的。此外，我们证明Ax21作为一种QS信号分子，调节Xoc中的生物膜形成。然而，Ax21信号传导对于Xoc中的细菌运动性、胞外多糖的产生和蛋白酶分泌是可有可无的。有趣的是，双组分系统RaxR/H参与了细菌运动性的调节，且这种调节可能独立于Xoc中的Ax21信号传导。综上所述，结果表明Xoc分泌的Ax21可能诱导植物免疫，这在水稻抵御病原体感染中发挥重要作用。
