Effector gene birth in plant parasitic nematodes: Neofunctionalization of a housekeeping glutathione synthetase gene

Plant pathogens and parasites are a major threat to global food security. Plant parasitism has arisen four times independently within the phylum Nematoda, resulting in at least one parasite of every major food crop in the world. Some species within the most economically important order (Tylenchida) secrete proteins termed effectors into their host during infection to re-programme host development and immunity. The precise detail of how nematodes evolve new effectors is not clear. Here we reconstruct the evolutionary history of a novel effector gene family. We show that during the evolution of plant parasitism in the Tylenchida, the housekeeping glutathione synthetase (GS) gene was extensively replicated. New GS paralogues acquired multiple dorsal gland promoter elements, altered spatial expression to the secretory dorsal gland, altered temporal expression to primarily parasitic stages, and gained a signal peptide for secretion. The gene products are delivered into the host plant cell during infection, giving rise to “GS-like effectors”. Remarkably, by solving the structure of GS-like effectors we show that during this process they have also diversified in biochemical activity, and likely represent the founding members of a novel class of GS-like enzyme. Our results demonstrate the re-purposing of an endogenous housekeeping gene to form a family of effectors with modified functions. We anticipate that our discovery will be a blueprint to understand the evolution of other plant-parasitic nematode effectors, and the foundation to uncover a novel enzymatic function.植物病原体与寄生物是威胁全球粮食安全的重大因素。植物寄生习性在线虫门（Nematoda）中独立起源了四次，致使全球每一种主要粮食作物均至少存在一种对应的寄生性线虫。在经济重要性最高的垫刃目（Tylenchida）中，部分物种会在侵染宿主期间分泌被称为效应因子（effector）的蛋白质，以重编程宿主的发育与免疫进程。目前学界对线虫如何演化出新型效应因子的具体机制仍未完全明晰。本研究重构了一个新型效应因子基因家族的演化历史。研究发现，在垫刃目（Tylenchida）植物寄生线虫的演化历程中，管家基因谷胱甘肽合成酶（glutathione synthetase，GS）发生了广泛的复制事件。新产生的GS旁系同源基因获得了多个背腺启动子元件，其空间表达模式转变为在分泌型背腺中表达，时间表达模式则调整为主要在寄生阶段表达，同时还获得了用于蛋白分泌的信号肽。该基因的编码产物会在侵染过程中被输送至宿主植物细胞内，由此形成了类谷胱甘肽合成酶效应因子（GS-like effector）。值得注意的是，通过解析类GS效应因子的三维结构，本研究发现该类蛋白在演化过程中其生化活性也发生了分化，且很可能代表了一类新型类谷胱甘肽合成酶的创始成员。本研究结果证实，内源管家基因可通过功能重定向形成一类功能发生改变的效应因子家族。我们预计，该发现将为理解其他植物寄生线虫效应因子的演化过程提供研究范式，并为揭示新型酶功能奠定重要基础。