南湖新闻网讯（通讯员 余慧敏）近日，我校邓秀新院士团队潘志勇研究组研究成果以“Plant Lysin Motif extracellular proteins are required for arbuscular mycorrhizal symbiosis”为题在PNAS发表，研究揭示了LysMe蛋白通过抑制免疫调节菌根共生形成机理。
丛枝菌根真菌(AMF)可以与大多数陆生植物形成互惠共生关系。AMF向植物提供矿质养分和水分，植物则为AMF提供碳源。作为一种外来入侵生物，AMF可以顺利进入植物根系细胞而不遭受植物免疫排斥，但个中缘由尚不明了。课题组前期发现，AMF能分泌含有细胞溶解素基序（LysM）的效应因子来降低植物免疫促进菌根共生。有趣的是，植物也分泌LysM类蛋白，那么植物中是否也存在“植物效应因子〞的角色呢？
研究人员以柑橘为对象的研究发现，植物LysM胞外蛋白（LysMe）在菌根丛枝细胞中特导诱导表达，且系统进化分析显示LysMe仅存在于柑橘、番茄、水稻、苜蓿等菌根寄主植物中，而在拟南芥等不能形成菌根的植物中缺乏。借助苜蓿和番茄等菌根模式系统的研究表明，植物分泌LysMe蛋白于丛枝周膜与真菌细胞壁之间的间隙（PAS），即植菌界面膜空间；敲除LysMe基因显著降低AMF定殖水平，基因回补后植株的AMF定殖水平则恢复到野生型水平；系列生化实验表明，LysMe可能通过结合几丁质和壳聚糖来削弱几丁质等物质触发的免疫反应，从而促进AMF定殖和共生形成。该研究暗示，植物和它们的AMF伙伴一样，进化出了一套“植物效应因子”，可通过调节自身免疫促进菌根共生的建立。
华中农业大学果蔬园艺作物种质创新与利用全国重点实验室博士生余慧敏为论文第一作者，潘志勇教授为论文通讯作者。华中农业大学邓秀新院士和欧阳波教授、美国马里兰大学萧顺元教授、荷兰瓦赫宁根大学Erik Limpens博士和Ton Bisseling院士、湖北省农科院果茶所白福玺博士等也参与了该工作。华中农业大学蛋白质平台为该研究提供了技术支持。本研究得到了国家重点研发计划、国家自然科学基金、武汉市科技创新专项和华中农业大学自主创新基金的资助。
审核人：潘志勇
【英文摘要】
Arbuscular mycorrhizal fungi (AMF) can form a mutually beneficial symbiotic relationship with most land plants. They are known to secrete lysin motif (LysM) effectors into host root cells for successful colonization. Intriguingly, plants secrete similar types of LysM proteins; however, their role in plant–microbe interactions is unknown. Here, we show that Medicago truncatula deploys LysM extracellular (LysMe) proteins to facilitate symbiosis with AMF. Promoter analyses demonstrated that three M. truncatula LysMe genes MtLysMe1/2/3, are expressed in arbuscule-containing cells and those adjacent to intercellular hyphae. Localization studies showed that these proteins are targeted to the periarbuscular space between the periarbuscular membrane and the fungal cell wall of the branched arbuscule. M. truncatula mutants in which MtLysMe2 was knocked out via CRISPR/Cas9-targeted mutagenesis exhibited a significant reduction in AMF colonization and arbuscule formation, whereas genetically complemented transgenic plants restored wild-type level AMF colonization. In addition, knocking out the ortholog of MtLysMe2 in tomato resulted in a similar defect in AMF colonization. In vitro binding affinity precipitation assays suggested binding of MtLysMe1/2/3 with chitin and chitosan, while microscale thermophoresis (MST) assays revealed weak binding of these proteins with chitooligosaccharides. Moreover, application of purified MtLysMe proteins to root segments could suppress chitooctaose (CO8)-induced reactive oxygen species production and expression of reporter genes of the immune response without impairing chitotetraose (CO4)-triggered symbiotic responses. Taken together, our results reveal that plants, like their fungal partners, also secrete LysM proteins to facilitate symbiosis establishment.
