拟南芥MAPK级联信号系统调控植物响应低磷的机制研究

Phosphorus is an essential macronutrient for plant growth, development, and reproduction. Despite the abundance of phosphorus in soil, very little of it is available as phosphate (Pi) for plants. Therefore, plants grown in soil often experience low Pi (LP) stress. Pi starvation is one of the most severe stresses that affect the yield of crops. To deal with the limited availability of Pi, plants have evolved a variety of adaptive strategies. Understanding these strategies will facilitate us to breed crops with improved ability to acquire and utilize Pi. Over the past decades, many downstream components and a few secondary signaling moleculars in plant LP responses networks have been identified and extensively studied. However, we currently have limited information on the Pi receptor (or sensor) that plant sense the external Pi and signal cascade that plant accept and transduce the signal from receptor (or sensor). Our prelimilary data show that, MKK9-MPK3/MPK6 mediated MAPK cascade is one of this kind of signal cascades. The transcription of a series of Pi-responsive genes, the uptake and accumulation of Pi are regulated by the MAPK cascade. WRKY75 is genetically downstream of MKK9-MPK3/MPK6 during the regulation of Pi responses. Since WRKY75 is not the substrate of MAPK but transcription of WRKY75 can be induced by MKK9-MPK3/MPK6, we propose that MKK9-MPK3/MPK6 may phosphorylate an unrevealed transcription factor TFx, subsequently promote the transcription of WRKY75, and thereby regulate Pi responses. In this project, we proposed to identify and analyze the upstream MKKKx and downstream substrate TFx. Study on the mechanism of MKKKx-MPK9-MPK3/MPK6 and TFx in regulating the plant Pi responses will improve our knowledge on the signaling pathway of the plant Pi responses.

磷是植物必需的大量营养元素之一，对植物的生长、发育及繁殖起着重要作用。土壤中可被植物直接吸收的磷的含量很低，严重威胁农作物生产。植物在长期适应过程中进化产生了响应低磷胁迫的机制。研究植物对低磷响应的机制将为培育可高效吸收及利用磷的作物提供重要的理论基础。多年来，研究者揭示了植物低磷响应的大量下游组分和一些次级响应信号，但该研究领域中，有关磷受体(或感受器)、接受受体来源信号的信号转导系统的研究很缺乏。项目组前期的研究结果发现：MKK9-MPK3/MPK6参与的MAPK级联信号系统调控多种磷响应基因的转录、植株对磷的吸收和积累；该级联信号系统对低磷响应的调控可能是通过磷酸化转录因子TFx来调控WRKY75的表达,进而调控下游响应基因。本项目拟研究植物低磷响应过程中MKK9-MPK3/MPK6的上游MKKKx和下游TFx,阐明一条接受受体来源信号的信号转导系统，揭示其调控植物对低磷响应的机制

磷是植物必需的大量营养元素之一，对植物的生长、发育及繁殖起着重要作用。土壤中可被植物直接吸收的磷的含量很低，严重威胁农作物生产。植物在长期适应过程中进化产生了响应低磷胁迫的机制。研究植物对低磷响应的机制将为培育可高效吸收及利用磷的作物提供重要的理论基础。多年来，研究者揭示了植物低磷响应的大量下游组分和一些次级响应信号，但该研究领域中，有关磷受体(或感受器)、接受受体来源信号的信号转导系统的研究很缺乏。项目组前期的研究结果发现：MKK9-MPK3/MPK6 参与的MAPK 级联信号系统调控多种磷响应基因的转录、植株对磷的吸收和积累；该级联信号系统对低磷响应的调控可能是通过磷酸化特定转录因子来调控WRKY75 的表达,进而调控下游响应基因。项目通过酵母互作方法，从拟南芥中的MKKK基因中筛选到MKKK15、RAF22和ZIK4能与MKK9互作。利用Pull down、Co-IP和磷酸化方法证明，RAF22和ZIK4能与MKK9在体外和体内互作并磷酸化MKK9。但突变体和互补植株磷含量及吸收测定证明，ZIK4是磷响应中MKK9的上游MKKK。利用WRKY75启动子删除及酵母双杂交的筛选，得到MYC3和MYC4可能是ZIK4-MKK9-MPK3/MPK6磷酸化底物，磷酸化后转录因子结合启动子的能力增强；对这两个基因的突变体及过表达模拟植株中的磷含量、磷吸收及下游磷响应基因的分析，证明MYC3和MYC4确实是该级联调控WRKY75基因响应磷表达的转录因子。根据项目的研究结果，我们提出MAPK级联介导的磷响应模型：该MAPK级联可以被磷信号激活，进而磷酸化两个MYC增强对WRKY75启动子的结合，增加WRKY75的表达，提高植株磷吸收能和植株磷含量。项目研究的完成对全面理解磷响应的信号转导机制提供了新的理论依据。

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