解析在应对环境胁迫时叶绿体作为感应器和触发器的新兴角色

Current researches on plant stress biology suggest that chloroplasts may act as a sensor that is able to distinct various stresses and translate the perceived information, and thereby releasing signal to nucleus. Over the past decades, genetic and molecular and biochemical approaches have considerably unveiled the hidden complexities of plant responses to abiotic stress, which have mainly been focused on studying various signaling cascades. However, the fundamental basis of stress perception mechanisms of plants have not been understood comprehensibly. Unlike biotic stress in which many of receptor proteins recognizing bacterial “non-self” molecules are extensively unveiled, the initial events related with stress perception of plants exposed to abiotic stresses are largely unknown. Recent research suggests that chloroplast might play important role for the stress perception as the immediate chloroplast perturbation induces coordinated gene expressions between chloroplast and nucleus to mitigate photosynthesis and to stimulate defense response. The constraint of photosynthesis has been thought as a major cause of stress perception and the stressed chloroplasts indeed activate cellular defense responses via the process known as “chloroplast-to-nucleus retrograde signaling”. .One of the earliest responses of chloroplasts to stress is the accumulation of chemically distinct reactive oxygen species (ROS) mainly generated by photosystem II (PSII) and I (PSI). Although the impact of ROS has been a subject of intense research in stress biology, relatively little is known their precise targets and any molecular components associated with specific ROS. The difficulties were reasoned as they increase almost simultaneously under stress, thus it is difficult to define the biological activity of one particular ROS and to link it to a certain cellular response. However, by using Arabidopsis flu mutant that accumulates singlet oxygen (1O2), one of ROS, within chloroplasts in non-invasive and controlled manner, we could get rid off the problem, at least in part. Because of short lifespan, a limited diffusion distance and an imminent reactivity with other macromolecules, it seems1O2 produced by PSII are not able to diffuse across chloroplast envelope, yet the nuclear gene expression changes are obvious in flu mutant. Moreover the fact that the flu phenotype is conditioned by the presence of nuclear-encoded and plastid-targeted EXECUTER1 (EX1) and EXECUTER2 (EX2) proteins guide this project towards next step in revealing the mechanism how EX1 and EX2 are able to sense 1O2 and react to activate chloroplast-to-nucleus retrograde signaling. Based on our recent data, we believe our proposed research objectives at PSC are fully feasible to open new research era of chloroplast biology in conjunction with environmental stresses. Our long-term research aims to reengineer the sensing machinery of chloroplast, which may allows us to reinforce stress tolerance of plants.

植物因其固着性而进化出复杂精细的机制以应对多变的环境。以往研究揭示了植物对非生物胁迫应答的复杂性，但植物究竟如何响应胁迫还知之甚少。近来研究表明，叶绿体能利用多种质体因子来传递感知到的胁迫信息。受胁迫的叶绿体通过“叶绿体到细胞核逆行信号”传导过程来激发细胞水平的防御反应。叶绿体对胁迫的最早期反应是光系统II和光系统I产生的ROS水平的升高。我们选择在非胁迫和受控条件下能专门产生1O2的拟南芥flu 突变体作为实验对象，利用遗传、分子和生化方法来：1）破译与光合体系II相关的1O2的感知机制；研究由1O2引起的、EXECUTER蛋白依赖的信号传导过程的分子机制2）找到激活细胞间和细胞内交流的信号传导分子；3）解析1O2触发的“叶绿体到细胞核逆行信号”传导过程，找到关键传递因子。通过该研究希望能洞察逆境胁迫中叶绿体的新角色，构建叶绿体的感应机制，为增强植物的抗胁迫能力研究奠定理论基础。

叶绿体充当环境传感器，将感知到的环境波动/压力因子转换为逆行信号的形式已成为共识。在这些确定的分子中，核编码的叶绿体蛋白EXECUTER1（EX1）被认为是单线态氧（1O2）传感器，可在感知到1O2时介导逆行信号。尽管EX1介导了明显的表型，其作用方式仍有待说明。还已知1O2会迅速改变核转录组。在对1O2介导的核基因中，一个转录共调节子，即SIGMA FACTOR-BINDING PROTEIN1（SIB1），先前被发现在核和叶绿体中都双重定位。然而，像EX1一样，SIB1的确切功能仍有待探索。..在过去的四年中，我们推出了两个关键的单线态氧（1O2）信号组件：FtsH金属蛋白酶和SIB1。对于前一种，结果显示FtsH蛋白酶介导的的EX1蛋白水解是信号启动的组成部分。该结果表明核编码的叶绿体FtsH蛋白酶通过在释放1O2时促进其蛋白水解来协调EX1介导的1O2信号传导。对于后一种情况，我们发现SIB1除1O2外还对SA敏感，并且以非偶联方式调节与光合作用相关的核和叶绿体基因，最终导致植物特异性细胞死亡程序。这些发现发表在非常著名的国际期刊上，包括PNAS和Plant Cell。此外，这些结论为研究提供了新的思路：（i）EX1的1O2依赖型修饰，将通过FtsH刺激其降解； （ii）SIB1在不同亚细胞区室中的作用方式。

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