植物OST1基因功能研究进展

2015-04-07 13:03陈妮妮沈晓艳王增兰
湖北农业科学 2015年3期
关键词:信号转导植物

陈妮妮++沈晓艳++王增兰

摘要:植物在生长过程中会遭受各种逆境胁迫,环境胁迫会引起植物体内一系列的信号反应,其中脱落酸(ABA)信号途径是一条重要的胁迫应答途径。作为ABA信号通路中的蛋白质激酶之一,气孔开放因子1(Stomatal opening factor 1,OST1)在植物逆境应答反应中扮演重要角色。因此,研究OST1基因在植物逆境应答中的功能有助于阐述植物耐逆分子机制。从OST1的相互作用因子及其在信号通路中的调控作用等方面进行阐述,对其介导的逆境应答机制进行了系统总结。

关键词:植物;逆境胁迫;OST1基因功能;信号转导

中图分类号:Q789 文献标识码:A 文章编号:0439-8114(2015)03-0513-04

DOI:10.14088/j.cnki.issn0439-8114.2015.03.001

Advances in Function of OST1 Gene in Plants

CHEN Ni-ni, SHEN Xiao-yan, WANG Zeng-lan

(Shandong Key Lab of Plant Stress Research/College of Life Sciences,Shandong Normal University, Jinan 250014, China)

Abstract: Plants are subjected to all kinds of stresses in the process of growth. Environmental stresses can make plants to produce a series of signal transduction. Signal pathway of abscisic acid (ABA) is an important stress response pathway. Stomatal opening factor 1 (OST1), one of protein kinases in ABA signaling pathway, plays a pivotal role in plant. Studying the function of OST1 gene in plants will help to elaborate the molecular mechanisms of plant stress tolerance. Interacting factors of OST1 and its regulatory roles in the signal pathway were reviewed. The mechanisms of OST1 in stress response of plants were summarized.

Key words: plant;adversity stress;OST1 gene function;signal transduction

植物的生长发育过程会受到多种逆境胁迫的影响,而植物在进化过程中形成了应对高温、低温、高盐、干旱等环境胁迫的保护机制。蛋白激酶和蛋白磷酸酶通过催化蛋白质磷酸化和去磷酸化来参与细胞信号转导途径,如参与ABA信号途径等,从而在非生物胁迫应答中起到非常重要的作用[1,2]。

蔗糖非酵解型蛋白激酶[SNF1 (Sucrose non-fermenting-1)-related protein kinase,SnRK]是一类广泛存在于植物中的Ser/Thr类蛋白激酶,参与植物体内包括ABA信号途径在内的多种信号途径的转导,在植物的抗逆境反应中起着非常重要的作用[3,4]。气孔开放因子1(Stomatal opening factor 1,OST1,即SnRK2E或SnRK2.6)属于SnRK2激酶家族,通过对其保守区域和作用位点以及上游和下游调控因子等的分析,剖析了其在响应胁迫反应中ABA依赖及ABA不依赖途径中的作用[5-7]。近几年通过对其调控网络的研究还发现,OST1在植物碳源及能源供应[8]以及ABA调控开花周期过程[9]中发挥作用,以此来调节植物的生长和发育。

1 OST1在胁迫响应中的调控机制

1.1 ABA依赖的调控途径

ABA作为高等植物中普遍存在的一种植物激素能调控多种生理相关反应,比如开花时间、果实成熟以及对逆境胁迫如干旱、盐碱等的反应[10-14]。其中,OST1作为ABA信号途径的重要组分,参与ABA调控的逆境胁迫信号途径。

1.1.1 OST1活性受ABA信号途径中的上游因子调控 研究发现,OST1在正常情况下是由ABA信号途径中的负调控因子PP2Cs(Ser/Thr protein phosphatases type 2C)抑制的[15-17]。PP2Cs家族A包括HAB1、ABI1、ABI2、PP2CA等蛋白质。Yoshida等[7]对abi1-1和aba2-1突变体株系的研究发现,只有abi1-1突变体抑制ABA依赖的OST1活性。通过酵母双杂交等试验,证明ABI1是通过与OST1 C-末端区域Ⅱ结合,使OST1激酶活性区域的Ser/Thr残基脱磷酸化失活[18]。不过通过对PP2Cs家族成员HAB1及ABI2的研究,发现两者都有调控OST1激酶活性方面的功能[15,19]。那么OST1活性是如何被ABA信号途径调控的,Park等[20]通过酵母双杂交等试验发现ABA与PYR/PYL家族结合会抑制PP2Cs。Nishimura等[21]的研究也表明PYR/PYL/RCAR与PP2Cs成员中的ABI1的互作最强,并且发现pyr1/pyl1/pyl2/pyl4 四突变体对ABA调控的气孔开闭不敏感。之后进一步的研究发现在ABA调控低湿度、高CO2等逆境胁迫过程中PYR/PYL/RCAR与ABA结合,两者构成的复合物与PP2Cs相互作用,抑制了其去磷酸化活性,从而使OST1磷酸化激活[18,22-26]。endprint

1.1.2 OST1对下游质膜蛋白的调控 OST1在响应胁迫途径中受ABA信号激活后,作用于下游的SLAC1和KAT1离子通道或NADPH氧化酶,通过调控保卫细胞内的离子含量来调节气孔的关闭,以此对逆境胁迫做出反应[27-29]。

Geiger等[30]对干旱胁迫下的ABA信号途径进行研究,通过BiFC分析OST1相互作用因子SLAC1及ABI1,并在爪蟾卵母细胞中共表达SLAC1和OST1,证明OST1磷酸化并激活SLAC1。Xue等[26]研究高CO2胁迫下OST1诱导气孔关闭的过程,并建立了CO2调控SLAC1导致气孔关闭的模型:CO2在细胞内转化成HCO3-,激活ABA信号途径,诱导PYR/PYL/RCAR激活OST1,OST1使下游SLAC1磷酸化激活,使气孔关闭。Vahisalu等[27]研究表明在臭氧胁迫下,ABA诱导ROS活性氧的产生,ROS激活的OST1使SLAC1 N-末端磷酸化,激活S-型离子通道,促进保卫细胞阴离子外排,引起质膜去极化,激活外排K+通道,使得保卫细胞K+外排,离子的流失使得保卫细胞膨压下降,导致气孔关闭。但ROS激活OST1的机制还需要进一步研究。总之,在臭氧、低湿度、高CO2等胁迫条件下OST1可以通过磷酸化而激活SLAC1,促进保卫细胞Cl-外排,引起气孔关闭[25,31,32]。

对于ABA信号通路中OST1对KAT1的调控,Sato等[28]通过LC-MS/MS分析证明KAT1的Thr306和Thr308为其磷酸化位点。通过对其单突变体的研究,并在爪蟾卵母细胞和酵母中检测KAT1的活性,证明Thr306是KAT1保持活性必需的,并且是OST1使KAT1发生磷酸化引起气孔关闭的位点。之后的进一步研究证明OST1在应对多种逆境胁迫时通过磷酸化抑制KAT1活性,从而抑制K+的转运,以此调节气孔关闭[18,25,33]。

除了对离子通道的调控,OST1还可以直接使NADPH氧化酶之一的AtRbohF磷酸化[18,34]。Sirichandra等[29]通过质谱分析,AtRbohF NADPH氧化酶的Ser174和Ser13由OST1磷酸化,并通过YFP及GFP等试验证明OST1与AtRbohF相互作用,进而可以形成OST1诱导ROS产生的通路,即OST1通过对下游AtRbohF NADPH氧化酶的激活,产生ROS。而Vahisalu等[27]研究发现ROS可以通过某种途径激活OST1,并促进OST1对SLAC1的激活,引起气孔关闭。

1.1.3 OST1对转录因子的调控 OST1在逆境应答过程中,除了对膜蛋白的调控外,还有其他调节途径,如对转录因子b-ZIP及SNAC1等的调控。

转录因子b-ZIP包括ABF2、ABF3等,Sirichandra等[35]研究OST1及ABF的相互作用,发现保卫细胞的细胞核中的OST1可以直接使ABF3磷酸化。Fujii等[36]通过in-gel激酶活性分析,证明SnRK2蛋白激酶(包括OST1)具有ABA依赖的激活AREB/ABF的作用,从而在ABA响应逆境胁迫及调控植物生长发育的过程中具有一定影响[37-40]。

SNAC1在植物中被广泛研究,受干旱、盐渍等逆境胁迫诱导,并受多种胁迫相关基因的调控[41-43]。Vilela等[44]在拟南芥ost1突变体中异位表达玉米中的拟南芥OST1同源基因ZmOST1,植株恢复对干旱响应的气孔关闭的表型。Vilela等[44]进一步的研究发现ZmOST1使ZmSNAC1转录因子磷酸化,并且对其定位和稳定性具有一定的影响,同时ZmSNAC1结合在ZmOST1的ABA-box区域,与PP2Cs形成竞争,参与ABA信号途径,证明玉米中ABA信号途径下游ZmOST1激活ZmSNAC1转录因子,启动渗透胁迫相关基因表达,进而对逆境胁迫进行调控。

1.2 ABA不依赖的调控途径

经过多年的研究发现,OST1同时还存在不依赖ABA的调控机制,比如渗透胁迫。Yoshida等[7]对abi1-1、abi2-1和aba2-1拟南芥突变体株系进行渗透胁迫处理,发现OST1-GFP依然表达,证明了在渗透胁迫条件下OST1的激活可以不需要ABA的参与。另外,OST1的两个区域是被激活的位点:区域Ⅱ和区域Ⅰ。区域Ⅰ是渗透胁迫条件下不依赖ABA的调控机制必需的,该区域促进OST1磷酸化,进而调控气孔关闭。

2 OST1在植物生长发育中的作用

ABA不仅在植物胁迫应答中发挥作用,还具有调控植物生长发育(比如种子成熟、根茎生长等)的重要作用[10]。近几年的研究发现,OST1作为ABA信号通路中的重要成员参与植物生长发育的调控。比如OST1通过对转录因子b-ZIP的调控而启动种子成熟和休眠中相关基因的表达来影响植物生长发育[37-40]。Wang等[9]在拟南芥snrk2.2/2.3/2.6突变体中表达SnRK2.6而恢复FLC的表达,而ABFs同样促进FLC的表达。Wang等[9]认为OST1通过对b-ZIP的调控来促进下游FLC的表达,以此参与ABA对开花周期的调控过程。

Zheng等[8]对拟南芥OST1的研究发现OST1具有调控植物生长和种子生成等过程中碳源供应的功能。对拟南芥snrk2.6突变体研究发现OST1基因的失活导致拟南芥种子中油料的合成下降7%~25%,种子干重的下降幅度大于24%[8]。而对过表达OST1拟南芥植株的研究发现,生长22 d的过表达植株比野生型植株叶中可溶性糖的含量增加34.7%。OST1可能的作用机理:OST1在叶的维管组织中表达,通过调控蔗糖-6-磷酸合酶的活性来调控蔗糖代谢,进而调控光合作用和碳的固定[8]。固定的碳源可以用于种子中不饱和脂肪酸等的合成,进而提高种子干重。Zheng等[8]的研究还发现,在种子形成和幼苗生长中OST1增强ABA的敏感性,由此得出OST1在植物生长发育中的重要作用。endprint

3 展望

目前对于OST1结构的研究已经相对全面,对其功能上的研究也颇有进展。从最初的只对干旱胁迫应答的研究,到目前的对低光照、O3和高浓度CO2等逆境应答的研究中,均反映出OST1在胁迫应答中的重要作用。PYR/PYL/RCAR及PP2Cs对于OST1的调控以及OST1对下游离子通道及转录因子等的调控揭示了其在逆境应答及调控植物生长发育中的分子调节机制。但OST1的调控网络非常复杂,其调控机制以及其是否与胁迫应答中的其他蛋白激酶相互影响还有待进一步研究。

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植物罢工啦?
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