Resistance (R) genes encode intracellular nucleotide-binding/leucine-rich repeat-containing (NLR) family proteins that serve as critical plant immune receptors to induce effector-triggered immunity (ETI). NLR proteins possess a tripartite domain architecture consisting of an N-terminal variable region, a central nucleotide-binding domain, and a C-terminal leucine-rich repeat. N-terminal coiled-coil (CC) or Toll-interleukin 1 receptor (TIR) domains of R proteins appear to serve as platforms to trigger immune responses, because overexpression of the CC or TIR domain of some R proteins is sufficient to induce an immune response. Because direct downstream signaling molecules of R proteins remain obscure, the molecular mechanisms by which R proteins regulate downstream signaling are largely unknown. We reported previously that a rice R protein named Pit triggers ETI through a small GTPase, OsRac1, although how Pit activates OsRac1 is unclear. Here, we identified OsSPK1, a DOCK family guanine nucleotide exchange factor, as an interactor of Pit and activator for OsRac1. OsSPK1 contributes to signaling by two disease-resistance genes, Pit and Pia, against the rice blast fungus Magnaporthe oryzae and facilitates OsRac1 activation in vitro and in vivo. The CC domain of Pit is required for its binding to OsSPK1, OsRac1 activation, and the induction of cell death. Overall, we conclude that OsSPK1 is a direct and key signaling target of Pit-mediated immunity. Our results shed light on how R proteins trigger ETI through direct downstream molecules.

电阻（R）基因编码细胞内核苷酸结合/富含亮氨酸的重复序列（NLR）家族蛋白，这些蛋白起着关键的植物免疫受体的作用，以诱导效应触发的免疫（ETI）。NLR蛋白具有由N端可变区，中央核苷酸结合结构域和C端富含亮氨酸的重复序列组成的三方结构。R蛋白的N末端卷曲螺旋（CC）或Toll-白介素1受体（TIR）结构域似乎充当了触发免疫反应的平台，因为某些R蛋白的CC或TIR结构域的过表达足以诱导免疫回复。由于R蛋白的直接下游信号分子仍然不清楚，因此R蛋白调节下游信号的分子机制尚不清楚。我们之前曾报道过，一种水稻R蛋白Pit通过一个小的GTP酶OsRac1触发ETI，尽管Pit如何激活OsRac1尚不清楚。在这里，我们确定OsSPK1，DOCK家族鸟嘌呤核苷酸交换因子，作为Pit的相互作用因子和OsRac1的激活剂。OsSPK1通过两个抗病基因促进信号转导，Pit和Pia对抗稻瘟病菌Magnaporthe oryzae，并在体内和体外促进OsRac1活化。Pit的CC结构域是其与OsSPK1，OsRac1激活结合以及诱导细胞死亡所必需的。总体而言，我们得出结论，OsSPK1是Pit介导的免疫的直接和关键信号转导靶标。我们的结果揭示了R蛋白如何通过直接下游分子触发ETI。