Department of Plant Pathology, Michigan State University, East Lansing, Michigan, United States of America.
PLoS One. 2012;7(4):e34701. doi: 10.1371/journal.pone.0034701. Epub 2012 Apr 5.
Pseudoperonospora cubensis, an obligate oomycete pathogen, is the causal agent of cucurbit downy mildew, a foliar disease of global economic importance. Similar to other oomycete plant pathogens, Ps. cubensis has a suite of RXLR and RXLR-like effector proteins, which likely function as virulence or avirulence determinants during the course of host infection. Using in silico analyses, we identified 271 candidate effector proteins within the Ps. cubensis genome with variable RXLR motifs. In extending this analysis, we present the functional characterization of one Ps. cubensis effector protein, RXLR protein 1 (PscRXLR1), and its closest Phytophthora infestans ortholog, PITG_17484, a member of the Drug/Metabolite Transporter (DMT) superfamily. To assess if such effector-non-effector pairs are common among oomycete plant pathogens, we examined the relationship(s) among putative ortholog pairs in Ps. cubensis and P. infestans. Of 271 predicted Ps. cubensis effector proteins, only 109 (41%) had a putative ortholog in P. infestans and evolutionary rate analysis of these orthologs shows that they are evolving significantly faster than most other genes. We found that PscRXLR1 was up-regulated during the early stages of infection of plants, and, moreover, that heterologous expression of PscRXLR1 in Nicotiana benthamiana elicits a rapid necrosis. More interestingly, we also demonstrate that PscRXLR1 arises as a product of alternative splicing, making this the first example of an alternative splicing event in plant pathogenic oomycetes transforming a non-effector gene to a functional effector protein. Taken together, these data suggest a role for PscRXLR1 in pathogenicity, and, in total, our data provide a basis for comparative analysis of candidate effector proteins and their non-effector orthologs as a means of understanding function and evolutionary history of pathogen effectors.
古巴假霜霉，一种专性卵菌病原体，是瓜类霜霉病的致病因子，这是一种具有全球经济重要性的叶部病害。与其他卵菌植物病原体相似，古巴假霜霉具有一系列 RXLR 和 RXLR 样效应蛋白，这些蛋白在宿主感染过程中可能作为毒力或无毒力决定因素发挥作用。通过计算机分析，我们在古巴假霜霉基因组中鉴定出 271 种具有可变 RXLR 基序的候选效应蛋白。在扩展这项分析时，我们介绍了古巴假霜霉效应蛋白 RXLR 蛋白 1（PscRXLR1）及其最接近的 Phytophthora infestans 同源物 PITG_17484 的功能特征，后者是药物/代谢物转运蛋白（DMT）超家族的成员。为了评估此类效应蛋白-非效应蛋白对在卵菌植物病原体中是否常见，我们研究了古巴假霜霉和 P. infestans 中假定的同源物对之间的关系。在 271 种预测的古巴假霜霉效应蛋白中，只有 109 种（41%）在 P. infestans 中有假定的同源物，对这些同源物的进化率分析表明，它们的进化速度明显快于大多数其他基因。我们发现 PscRXLR1 在植物感染的早期阶段上调表达，而且，异源表达 PscRXLR1 在 Nicotiana benthamiana 中引发快速坏死。更有趣的是，我们还证明 PscRXLR1 是由选择性剪接产生的，这是植物病原卵菌中第一个将非效应基因转化为功能性效应蛋白的选择性剪接事件的例子。综上所述，这些数据表明 PscRXLR1 在致病性中起作用，总的来说，我们的数据为候选效应蛋白及其非效应同源物的比较分析提供了基础，作为理解病原体效应蛋白功能和进化历史的一种手段。
