植物Ca2+解码机制已经被广泛研究,包括许多解码器,从钙调素(CaM),一个高度保守的Ca2+传感器,到植物特异性钙调素样(CML)蛋白;钙调磷酸酶B样蛋白(CBLs)及其相互作用的激酶(CIPKs);以及钙依赖蛋白激酶(CDPKs或CPKs)(DeFalco et al., 2010)。相比之下,我们对Ca2+编码过程所知甚少,该过程涉及Ca2+通道和转运体的功能与调节。尽管已经鉴定出几个植物基因家族编码与动物中已知的Ca2+可渗透通道相关的蛋白质,包括那些与环核苷酸门控通道(CNGCs)和谷氨酸受体(GLRs)相似的蛋白质,但这些通道如何在各种信号传导过程中协同或独立地编码特定的Ca2+信号,在很大程度上仍未被探索。本文伯小远将带着大家了解一些这方面的研究进展,与此同时,小远也会为大家简单介绍非生物应激反应中钙信号的编码机制。
未来的工作应指向探讨CNGCs和GLRs在控制花粉管Ca2+振荡中的功能关系,以及这些通道如何被多肽及其受体等其他信号调控的机制(Johnson et al., 2019)。
在共生关系建立的起始阶段,植物细胞核内Ca2+浓度会按一定幅度和频率波动(钙振荡)。这种早期的钙信号是驱动共生关系建立所必需的。共生微生物的信号分子(如丛植菌根真菌的菌根因子和根瘤菌的结瘤因子)被宿主细胞表面受体识别后,激活下游信号转导蛋白,进而激活核周内质网定位的通道蛋白和转运蛋白DMI1、CNGC15及MCA8(Ca2+-ATPase),被激活的通道蛋白和转运蛋白组成钙信号“编码器”(图3a)。因内质网腔Ca2+浓度远高于细胞核质,当钙信号编码器激活后,由DMI1和CNGC15将Ca2+快速运到核质空间(钙激增),过高的核质Ca2+立即被MCA8泵回内质网腔,使核质钙离子水平下调(钙衰减),从而为下一次钙离子流入细胞核做准备。Ca2+通过钙“编码器”周期性流进和流出细胞核,从而使细胞核内钙离子浓度产生振荡。振荡的钙信号被细胞核定位的Ca2+/钙调蛋白依赖的蛋白激酶CCaMK(解码器)识别,以磷酸化的方式激活下游转录因子,启动早期共生信号(图3b)。
在模式植物拟南芥中的研究发现,Ca2+波和慢波动电位可以作为长距离信号的载体,驱动植物系统性的信号传递。而这其中细胞膜钙离子通道蛋白GLR3.3,GLR3.6及质子泵AHA1可能参与了植物长距离Ca2+波和慢波动电位的产生和传递。GLRs作为钙离子通道,同时还是谷氨酸受体。当植物被昆虫或其它动物啃食后,维管束内大量的谷氨酸被释放到细胞外,从而激活GLR通道活性使Ca2+进入细胞质内。Ca2+及其他离子的进出引起细胞膜电位变化,形成在器官水平测到的慢波动电位。Ca2+波,慢电位和H2O2协同建立长距离信号,介导非损伤组织的响应(如茉莉酸合成),从而抵御昆虫啃食。
局部暴露于压力下会产生H2O2和Ca2+信号。Ca2+信号可以激活CPKs和CBLs-CIPKs,从而磷酸化并激活RbohD。激活的RbohD产生H2O2,H2O2通过细胞壁扩散到邻近的细胞,在那里它通过像GHR1这样的RLK诱导Ca2+信号。H2O2可以激活细胞表面的Ca2+信号,也可以通过PIP水通道进入细胞,激活细胞内的Ca2+信号。Ca2+和H2O2信号之间的相互激活产生自传播的Ca2+和ROS波,这些波可以传播到远处的组织,引起系统获得性驯化反应。
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