Two RxLR Avirulence Genes in Phytophthora sojae Determine Soybean Rps1k-Mediated Disease Resistance

Resistance to Phytophthora sojae (Rps) genes have been widely used in soybean against root and stem rot diseases caused by this oomycete. Among 15 known soybean Rps genes, Rps1k has been the most widely used in the past four decades. Here, we show that the products of two distinct but closely linked RxLR effector genes are detected by Rps1k-containing plants, resulting in disease resistance. One of the genes is Avr1b-1, that confers avirulence in the presence of Rps1b. Three lines of evidence, including overexpression and gene silencing of Avr1b-1 in stable P. sojae transformants, as well as transient expression of this gene in soybean, indicated that Avr1b could trigger an Rps1k-mediated defense response. Some isolates of P. sojae that do not express Avr1b are nevertheless unable to infect Rps1k plants. In those isolates, we identified a second RxLR effector gene (designated Ayr1k), located 5 kb away from Avr1b-1. Silencing or overexpression of Avr1k in P. sojae stable transformants resulted in the loss or gain, respectively, of the avirulence phenotype in the presence of Rps1k. Only isolates of P. sojae with mutant alleles of both Avr1b-1 and Avr1k could evade perception by the soybean plants carrying Rps1k.

大豆对疫霉（Phytophthora sojae）的抗性（Rps）基因已广泛应用于大豆，以抵御由这种卵菌引起的根腐和茎腐病。在已知的15个大豆Rps基因中，Rps1k在过去40年中应用最为广泛。在此，我们表明含有Rps1k的植株能够检测到两个不同但紧密连锁的RxLR效应基因的产物，从而产生抗病性。其中一个基因是Avr1b - 1，它在Rps1b存在时赋予无毒性。三条证据，包括在稳定的大豆疫霉转化体中Avr1b - 1的过表达和基因沉默，以及该基因在大豆中的瞬时表达，都表明Avr1b能够触发由Rps1k介导的防御反应。一些不表达Avr1b的大豆疫霉菌株仍然无法感染Rps1k植株。在这些菌株中，我们鉴定出第二个RxLR效应基因（命名为Ayr1k），它位于距Avr1b - 1 5kb处。在大豆疫霉稳定转化体中Avr1k的沉默或过表达分别导致在Rps1k存在时无毒性表型的丧失或获得。只有Avr1b - 1和Avr1k两个基因都具有突变等位基因的大豆疫霉菌株才能逃避携带Rps1k的大豆植株的识别。