植物磺肽素在豆科植物根瘤发育过程中的功能及作用机制研究

Phytosulfokine (PSK), a newly identified plant peptide hormone, has been shown to regulate plant growth, development and innate immunity. Root nodule, the special organ of legume species, provides an excellent environment for symbiotic nitrogen fixation. The early developmental mechanism of nodule formation has been extensively investigated and revealed. However, the mechanism underlying nodule maturation is still not clear. In the previous study, we identified two Lotus japonicus PSK precursor genes specifically expressed in the nodules undergoing maturation. Both PSK treatment and overexpression in hairy roots significantly increased the number of mature nodules. A possible mechanism is suggested as following: PSK positively regulates the endoreduplication process of infected nodule cells by repressing JA signaling, and promotes the nodule cell enlargement, thus accelerates nodule maturation. To test this possibility, L. japonicus with enhanced or depressed PSK signaling will be used to investigate the biological function of PSK during nodule maturation at cellular and molecular levels. On the other hand, PSK receptor and its interacting proteins in L. japonicus will be studied by performing genetic and biochemical assays. Last, we will investigate the PSK-regulated pathways and key genes through RNA-seq analysis. These work will facilitate the illustrating of mechanism underlying PSK's function during nodule development. This project is important for us to exploit the regulating mechanism for legume root nodule development.

植物磺肽素是一种新兴的肽类激素，调节植物生长发育及免疫反应。根瘤是豆科植物发育的特殊器官，为根瘤菌共生固氮提供了理想的场所。目前对根瘤的早期发育机制已有一定了解，但根瘤的成熟调控机制还不清楚。我们前期研究鉴定到两个磺肽素前体基因在模式豆科植物百脉根根瘤成熟期间特异表达，外源施加或发根过表达磺肽素均显著增加成熟根瘤的数量。其可能的机制是磺肽素通过抑制茉莉酸信号途径正调控根瘤细胞内复制过程，同时促进根瘤细胞生长，从而调控根瘤成熟。为了验证这一假设，本项目拟采用磺肽素信号增强和缺失的百脉根遗传材料，在细胞和分子水平上解析磺肽素在根瘤成熟过程中的生物学功能；同时通过遗传和生化实验鉴定磺肽素受体蛋白及其互作信号分子，并运用RNA-seq技术分析下游响应路径及关键调节基因，揭示磺肽素调控根瘤发育的作用机制。本项目对于阐明豆科植物根瘤的发育调控机理具有重要的理论意义。

植物磺肽素（PSK）是一种双磺酸化修饰的五肽分子（YIYTQ），在植物生长发育及抗病反应中发挥重要的调控作用。通过本项目的实施，我们在豆科模式植物蒺藜苜蓿中发现一种新型的磺肽素——YIYTN，并将其命名为PSKδ。研究表明PSKδ特异性的存在于苜蓿根瘤中，且其前体基因MtPSKδ在根瘤发育的多个阶段（从根瘤原基到成熟根瘤的分生区和侵染区）均高丰度表达。由于磺肽素家族成员功能冗余，MtPSKδ缺失突变未出现根瘤发育缺陷表型，但过表达MtPSKδ或外源施加PSKδ小肽均能显著增加根瘤数量，表明PSKδ信号正调控根瘤形成。进一步研究发现PSKδ可能通过与乙烯信号途径互作从而调控根瘤形成。本项目还鉴定到苜蓿磺肽素的受体基因MtPSKR，功能分析MtPSKR缺失突变及过表达材料均表明该受体介导的PSK信号正调控根瘤形成及发育。本研究的价值：1）深入解析了磺肽素PSKδ调控豆科植物根瘤形成的分子机制，2）为豆科作物高产育种提供了新的基因资源。