RXLR effectors, a class of secreted proteins that are transferred into host cells to manipulate host immunity, have been reported to widely exist in oomycetes, including those from genera Phytophthora, Hyaloperonospora, Albugo, and Saprolegnia. However, in Pythium species, no RXLR effector has yet been characterized, and the origin and evolution of such virulent effectors are still unknown. Here, we developed a modified regular expression method for de novo identification of RXLRs and characterized 359 putative RXLR effectors in nine Pythium species. Phylogenetic analysis revealed that all oomycetous RXLRs formed a single superfamily, suggesting that they might have a common ancestor. RXLR effectors from Pythium and Phytophthora species exhibited similar sequence features, protein structures, and genome locations. In particular, there were significantly more RXLR proteins in the mosquito biological control agent P. guiyangense than in the other eight Pythium species, and P. guiyangense RXLRs might be the result of gene duplication and genome rearrangement events, as indicated by synteny analysis. Expression pattern analysis of RXLR-encoding genes in the plant pathogen P. ultimum detected transcripts of the majority of the predicted RXLR genes, with some RXLR effectors induced in infection stages and one RXLR showing necrosis-inducing activity. Furthermore, all predicted RXLR genes were cloned from two biocontrol agents, P. oligandrum and P. periplocum, and three of the RXLR genes were found to induce a defense response in Nicotiana benthamiana. Taken together, our findings represent the first evidence of RXLR effectors in Pythium species, providing valuable information on their evolutionary patterns and the mechanisms of their interactions with diverse hosts.

RXLR效应器，一类被转移到宿主细胞中操纵宿主免疫分泌蛋白，已报道广泛存在于卵菌，包括那些从属疫，Hyaloperonospora，白锈病，和水霉。但是，在腐霉属物种中，尚未鉴定出RXLR效应子，而且这种毒性效应子的起源和进化仍然未知。在这里，我们开发了一种改良的正则表达式方法，用于从头鉴定RXLR，并在九个腐霉菌中鉴定了359个推定的RXLR效应子种类。系统发育分析表明，所有卵菌的RXLR都形成一个单一的超家族，这表明它们可能具有相同的祖先。腐霉属和疫霉属的RXLR效应子表现出相似的序列特征，蛋白质结构和基因组位置。特别是，有在蚊生物控制剂显著更RXLR蛋白P.贵阳比其他八个腐霉种和P.贵阳RXLRs可能是基因复制和基因组重排事件的结果，由同线性分析所指示的。植物病原体P. ultimum中RXLR编码基因的表达模式分析检测到大多数预测的RXLR基因的转录本，其中一些RXLR效应子在感染阶段被诱导，而一个RXLR显示出坏死诱导活性。此外，所有的预测RXLR基因从两个生物防治剂，克隆寡雄腐霉和P. periplocum，和三个的RXLR基因被发现在诱导防御响应本塞姆氏烟草。综上所述，我们的发现代表了腐霉属物种中RXLR效应子的第一个证据，提供了有关其进化模式以及与多种宿主相互作用的机制的有价值的信息。