ROP (Rho-related GTPases from plant)是植物体内特有的一类Rho家族的信号转导小G蛋白，在植物生长发育的多个方面都可以作为信号分子开关发挥功能。自1993年首次从豌豆中发现以来^[1]，ROP已在拟南芥、水稻、玉米和烟草等多个物种中获得分离。迄今为止，多项研究表明ROP在叶表皮毛、根毛和花粉管的极性生长、种子萌发、细胞骨架动态组装以及气孔运动等方面起重要作用。

基因信息

Locus: AT1G20090

Name: ROP2

Other Name: ARABIDOPSISRAC-LIKE 4, ARAC4, ATRAC4, ATROP2, RHO-RELATED PROTEIN FROM PLANTS 2

基因功能

ROP2作为模式植物拟南芥ROP家族11个成员之一，属于第一类的ROP，参与许多重要的生理过程：如根毛极性生长、气孔关闭、叶表皮拼图细胞的形成、植物响应生物及非生物胁迫等。

突变体表型

图1 rop2突变体表型^[2]

rop2 突变体的根毛明显短于野生型，在国内拟南芥突变体共享中心AraShare中有两个该基因的T-DNA插入突变体（N677858和N675195，详情请查阅http://www.arashare.cn/)。

基因表达谱

图2. ROP2基因表达图谱(引自eFP browser)

研究概况

作为小G蛋白ROP家族的成员，ROP2存在GDP结合的非活性形式和GTP结合的活性形式。GTP水解酶活化蛋白GAP将活性的ROP2转换为非活性的ROP2^[3]，而鸟苷酸交换因子GEF可以将非活性的ROP2转换为活性的ROP2^[4]。活性的ROP2通过下游效应蛋白（如ICR/RIP、RIC等）调控钙离子浓度梯度、活性氧积累、囊泡分泌、细胞骨架动态组装等细胞活动^[5][6]，如图3所示：

图3 ROP的调控因子^[7]

2002年，Claire S.Grierson和杨贞标课题组首次发现ROP2在根毛极性生长中的关键作用^[8]。随着根毛的生长，ROP2的定位不断发生变化：在根毛的命运决定阶段，ROP2首先出现点状聚集，根毛在这里起始；在根毛起始和延伸过程中，ROP2一直定位于根毛的顶端，控制根毛的极性生长；当根毛成熟后，ROP2变为均匀的质膜定位^[8]（图4）。过量表达ROP2或持续活性形式的ROP2（ROP2^CA）会使根毛生长的位置发生变化，并导致根毛变长^[8]。高量的ROP2表达会使根毛的顶端出现分叉和鼓包^[8]。持续失活形式的ROP2（ROP2^DN）过量表达会导致根毛变短，个别根毛出现弯曲^[8]，如图5所示。在影响ROP2活性或定位的突变体如：fer-4^[9]、scn1^[10][11]、gef4gef10^[12]、plp^[13]、pat4^[14]、map182、aro2/3/4^[15]和arf1^[10][16]等中，根毛的极性生长也受到严重破坏。

图4 ROP2在根毛极性生长过程中的定位^[8]

图5 ROP2调控根毛的极性生长^[8]

2008年，Youngsook Lee和杨贞标课题组发现ROP2在光诱导的气孔开放中也发挥着重要的作用^[17]。GUS组化分析显示：ROP2在保卫细胞中高量表达^[17]。融合蛋白的荧光数据表明：持续活性的ROP2（ROP2^CA）定位在保卫细胞的质膜，而持续失活形式的ROP2（ROP2^DN）定位于保卫细胞的胞质^[17]（图6）。研究结果证明：光可以诱导气孔的开放，而被光激活的ROP2以负反馈的形式调控气孔的开放^[17]。调控ROP2活性的GEF2^[18]、GEF4^[18]以及ROP2下游的效应蛋白RIC7^[19]在光诱导的气孔开放中也发挥着重要作用。

图6 ROP2及其突变形式在气孔中的定位^[17]

与光诱导的气孔开放相反，ABA可以诱导气孔的关闭^[20]。研究表明：ABA处理可导致保卫细胞质膜上的ROP2定位于胞质，从而使ROP2失活，导致气孔关闭^[20]（图7）。在种子的萌发过程中，ABA可以诱导GEF2的降解，而ROP2通过与GEF2互作，抑制GEF2的降解，进而参与ABA抑制的种子萌发过程^[21]。

图7 ABA处理导致ROP2定位于胞质^[20]

]此外，ROP2在叶表皮拼图细胞的形成^[22-24]、叶表皮毛细胞的极化^[24]、花瓣的发育^[25]、根的向重力性^[26]、茎顶端分生组织和真叶的发育^[27]以及耐受盐胁迫^[28]等方面也发挥着重要作用。在此，不再赘述。

基因网络

图8. ROP2蛋白调控网络(引自STRING)

其他物种研究情况

早在2003年，J. E. Fowler课题组在玉米中创制了ROP2突变体，研究显示：玉米中的ROP2突变体，雄配子体的传代效率明显低于野生型^[29]。近年来，关于ROP2在其他物种的研究相对较少。考虑到ROP2的重要功能，其同源蛋白在其它物种中的生物学功能值得我们深入研究。

展望

ROP2作为分子开关，在细胞的极性生长^[8][30]、气孔的运动^[17][20]、植物激素的响应^[20][21]以及植物耐受非生物胁迫方面^[28]均发挥着重要的作用。尤其是在根毛的极性生长方面，ROP2的定位及活性调控已经成为根毛极性生长研究的热点^[30]。然而，在根毛的极性生长过程中，ROP2的表达调控、蛋白质稳定性、蛋白翻译后修饰等方面研究较少，有待进一步探究。

参考文献

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