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基因小传 | 不可不知的细胞信号“分子开关”ROP2
发布时间:2020-11-09 16:44:45


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(ROP2CA)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(ROP2DN)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(ROP2CA)定位在保卫细胞的质膜,而持续失活形式的ROP2(ROP2DN)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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