VDE基因在植物抗逆响应中的功能
2016-10-26宋兴舜孙丽娜张秋艳王瑞芳
宋兴舜,孙丽娜,张秋艳,王瑞芳
(东北林业大学生命科学学院,哈尔滨 150040)
VDE基因在植物抗逆响应中的功能
宋兴舜,孙丽娜,张秋艳,王瑞芳
(东北林业大学生命科学学院,哈尔滨150040)
叶黄素循环存在于高等植物和绿色藻类,是保护植物光合机构免受过剩光能破坏的重要机制。作为叶黄素循环关键酶,紫黄质脱环氧化酶(VDE)可催化双脱氧紫黄质(V)脱环氧化成单环氧玉米黄质(A)进而生成玉米黄质(Z),保护光合器官免受过剩光能破坏。VDE活性受多种因素调节,在非生物胁迫下,植物多种叶绿素荧光等指标均受影响,近期利用过表达、突变体与野生型植株对比研究进一步揭示VDE基因在植物生长中的重要作用。文章综述近年来有关VDE在植物抗逆生理中的功能研究进展。
VDE;紫黄质脱环氧化酶;非生物胁迫;光合参数;荧光参数
宋兴舜,孙丽娜,张秋艳,等.VDE基因在植物抗逆响应中的功能[J].东北农业大学学报,2016,47(9):85-90.
Song Xingshun,Sun Lina,Zhang Qiuyan,et al.Functions ofVDEin plant response to stresses[J].Journal of Northeast Agricultural University,2016,47(9):85-90.(in Chinese with English abstract)
植物生长过程受多种机制调节,光作为绿色植物生长基本能量来源对植物生长起重要作用。绿色植物通过光合作用将光能转变成化学能储存并合理利用。为提高光合利用率,植物自身利用调节保护系统适应光能强弱变化[1]。弱光下,植物尽可能将光能转变成自身可利用能量;强光下,植物吸收的光能超过自身所需能量,不能促进自身光合作用,反而破坏光合机构,即光抑制。过剩光能被叶绿素吸收引起三重态叶绿素(3Chl)产生,将能量运输至基态O2,产生高度有害的单线氧(1O2)甚至其他活性氧[2-3]。光系统受体终端过剩的光能可通过Mehler反应产生有毒害作用的活性氧[4]。早期研究表明,暴露在胁迫环境下会增加细胞内活性氧水平,抑制PSⅡ修复,这是光抑制主要原因。为避免或减轻抑制作用,植物启动多种防御机制,其中叶黄素循环是保护植物光合机构免受过剩光能破坏的重要机制[5]。叶黄素循环是1957年由Sapozhnikov等发现。该循环有三种叶黄素参与,分别是双环氧紫黄质V、单环氧玉米黄质A和玉米黄质Z。在植物吸收过剩光能时,双环氧紫黄质在紫黄质脱环氧化酶(VDE)作用下脱环氧化成单环氧玉米黄质生成玉米黄质;在暗处,玉米黄质在玉米黄质环氧化酶(ZEP)作用下重新环化成单环氧玉米黄质生成双环氧紫黄质,自然界中很多环境因子均会影响该循环(见图1)。
循环中Z含量增加可提高植株抗氧化能力[6]。研究发现Z的增加主要有三种作用:①通过热耗散过程在吸收并传递光能的色素分子PSⅡ天线色素上将过剩激发能转变为热能[7-10];②通过NPQ (Non-photochemical quenching,非光化学猝灭)独立过程建立起光氧化胁迫耐受性,将能量以热能形式放出,可保护类囊体膜脂质免受氧化损害[11-13];③降低补光叶绿素天线大小,并降低该膜流动性,以减少在类囊体内活性氧的渗透[12,14]。而VDE活性增强将导致Z增加,表明VDE在叶黄素循环中起重要作用。
图1 叶黄素循环Fig.1Xanthophyll cycle
1 VDE基因
1.1VDE基本结构特征
1996年Bugos等首先从莴苣中克隆VDE全长cDNA,编码473个氨基酸,其中转运肽长125个氨基酸[15]。相继从番茄、小麦、水稻、菠菜、毛竹、茶树、枸杞等[16-21]植物中克隆VDE基因。其分子质量约为43 ku,等电点为5.4。VDE是一种水溶性酶,存在高等植物类囊体腔内[22],光能促进跨类囊体膜pH梯度建立,由水解产生大量H+和PQ(质体醌)库所输入的H+会引起类囊体腔内pH下降,在pH<6.0时,VDE结合到类囊体膜上后被激活具有催化作用。由于VDE基因中间脂质蛋白类似区域一致性不高,但其晶体结构中均含有高度保守的折叠模式,Bugos等将VDE划分到脂质蛋白类似家族[23]。鉴定该酶属于脂质运载蛋白家族成员,具有结合和转运疏水小分子的特征区域。该蛋白在不同物种中显示有三个很强的同源结构域[23-24]:①N-末端有一个半胱氨酸富集区,是VDE的活性位点[25],二硫苏糖醇(DTT)的抑制位点;②中部有一个脂转运蛋白信号区,可能是双环氧紫黄质和MGDG结合位点,而MGDC是VDE蛋白发挥催化功能的必需脂类。因此,此区域是对VDE活性最有效的脂质区;③C-端为谷氨酸富集区,含有大量负电荷,是VDE与类囊体膜结合部位。与其他脂质蛋白类似,VDE具有3个高度保守被称为SCR (Structurally conserved regions)的区域,二级结构具有八个反向平行的β-折叠结构[19]。
通过植株不同组织VDE相对表达水平研究发现,在植株成熟叶片中VDE表达量最多,而根和果中表达量最低,VDE表达多集中在光合组织部位[17,25-27]。李欣、Li等通过构建CsVDE-GFP载体,利用GFP荧光标记在黄瓜原生质体中亚细胞定位,表明CsVDE蛋白表达在黄瓜原生质体的叶绿体上,用免疫胶体金方法证实该结论[19,26]。
1.2VDE活性调节
VDE活性受pH、AsA(抗坏血酸)、V的可利用性、温度、脂质结构、UV-B、DTT(二硫苏糖醇)等影响。VDE可根据类囊体腔内pH的不同结合到类囊体膜上或释放出来[28]。在体内,VDE最适pH为4.8,在体外,VDE最适pH为5.2,当pH≥7.0时,VDE将以自由态形式存在,当pH为6.6时,一般的VDE分子结合到类囊体膜上,在研究VDE酶释放与pH关系时发现,VDE定位于内囊体膜内侧,与类囊体膜结合后方可发挥作用[29]。脱环氧化作用须有AsA参与,AsA是调节VDE环化作用与活性的内源电子供应体,是VDE酶底物之一[23],VDE酶受AsA浓度调节[30],当植物遇到低温或强光胁迫时,可通过提高AsA浓度适应这种逆境[31]。V可利用性调节脱环氧化作用,一般情况下植物体内只有约60%V转化为A和Z[32],但V的转化效率可随强光处理时间而提高[33]。VDE酶活性也受温度影响,可很大程度影响V的利用性和转化效率,后来研究发现在5℃下Z形成受严重抑制。另外研究发现VDE酶活性与V和MGDG比例有关。DTT是VDE酶专一抑制剂,低浓度DTT可强烈抑制VDE酶活性[34]。UV-B辐射使VDE酶失活,可抑制PSⅡ活性,并降低V可利用性[35]。此外,VDE亚基紫黄质和单半乳糖甘油二酯(MGDG)也对VDE活性起重要作用[36]。
2 VDE基因对非生物胁迫响应
2.1VDE基因对强光胁迫的响应
植物在适宜光照条件下,随叶绿素吸收光能增加,固定CO2和光合速率均会提高,但光照过度却会抑制光合作用甚至导致光合机构氧化损伤[37-38]。在高光下,植物生长和产量会大大受损,高等植物中PSⅡ最大光化学效率降低是光抑制的重要特征。野生型CsVDE基因在不同光照下对比显示:强光下VDE基因响应更快速,光强度增加使NPQ、(A+Z)/(V+A+Z)比值增加,而Fv/Fm和Pn均降低,其趋势为:强光(1 200 μmol·m-2·s-1)>正常光(500 μmol·m-2·s-1)>低光(100 μmol·m-2·s-1)。相关研究表明,叶黄素循环色素池(V+A+Z)在强光下也会有所增加[39]。对于转基因植株来说,强光胁迫下,反义转基因植株中(V+A+Z)、(A+Z)/(A+ Z+V)和NPQ有所升高,Fv/Fm有所下降,且对反义转基因植株来说,强光下其NPQ、Fv/Fm和(A+ Z)/(V+A+Z)变化均明显低于野生型。高光胁迫下,Pn和Fv/Fm在WT中减少量要明显高于转基因植株,低温与高光胁迫下,NPQ、(A+Z)/(V+A+Z)逐渐上升,但过表达植株增加程度>野生植株>抑制表达植株[16,26]。过表达植株中高的NPQ和(A+Z)/ (V+Z+A)表明转基因植株比野生型植株耗散更多能量,对光合器官提供有效保护。对于转基因烟草中荧光参数日变化分析发现,Fv/Fm在上午随光强增加而下降,13:00达最小值,之后逐渐上升;NPQ与FO从上午到13:00呈上升趋势,13:00达最大值,之后逐渐降低。VDE表达可受温度和光照日变化调节,VDE过表达增加脱环氧化水平并有效缓解高光和低温胁迫下PSⅠ和PSⅡ的光抑制[26]。总之,胁迫下叶黄循环循环各组分及叶绿素荧光参数变化表明,VDE在避免植物因吸收过剩光能产生的损害中起重要作用。
2.2VDE基因对干旱胁迫的响应
干旱是制约农作物生长发育主要因素之一,我国约有1/3可耕地处在干旱或半干旱地区[40]。对于植株自然条件失水干旱下叶片中VDE基因表达量分析显示,VDE转录水平在干旱条件下逐渐升高,推测植物在干旱胁迫下自身机能受损,随干旱时间延长及程度加重,自身启动由VDE催化的叶黄素循环途径抵御不适的外界环境。轻度干旱胁迫下,对叶片光化学效率影响小,随干旱时间延长程度加重,光化学效率明显下降,相反,NPQ明显上升,因为这种胁迫下,产生过多光能,只能通过热耗散形式释放多余能量,才可减缓自身光抑制。检测到脱环氧化速率(A+Z)/(V+A+Z)增加;早期研究表明,干旱会引起叶黄素循环色素池容量增加,干旱胁迫会使A和Z含量增加[41-43]。植物生长调节剂可通过响应胁迫在一定程度上缓解环境变化带给植物的各种损害,脱落酸(ABA)是其中之一[44-45],干旱下能引起ABA积累,导致气孔关闭,离子向木质部运输增强。Guan等通过干旱诱导内源ABA研究枸杞VDE基因正反馈调节[17]。数据表明,分别对枸杞幼苗进行abamineSG(ABA积累抑制剂)、干旱及abamineSG+干旱处理下,单独abam⁃ineSG对ABA和叶绿素影响小,而在二者双重处理下ABA增加量与叶绿素减少量明显高于单独干旱处理,相应VDE表达量与ABA变化趋势基本一致。相同处理下监测野生型与转基因拟南芥最大光合速率、脱环氧化速率及NPQ,研究发现干旱下VDE基因缺失使拟南芥抽薹和开花时间提前,气孔导度及蒸腾速率降低,呼吸作用受抑制,非光化学猝灭、最大光化学效率降低[46],突变体植株萎蔫程度更高,成活率更低。
2.3VDE基因对低温胁迫响应
低温是限制对光敏感植物活性重要因素。低温胁迫下植物体内发生系列适应性变化,包括组织结构和生理生化变化[47]。低温对叶绿体和PSⅡ反应中心产生影响,与低温下植物光合活性下降有关。胁迫下引发明显PSⅡ和PSⅠ光抑制,PSⅠ相对PSⅡ敏感性要高[48-49],说明叶黄素循环色素存在于PSⅡ和PSⅠ[50-51]。低温胁迫下(4℃),可氧化的P7100显著下降[25],因为低温下玉米黄质能影响膜的流动性并减少质体醌再次氧化[52-54],对于转基因型植株而言,可氧化P7100比野生型降低的更缓慢[25],由于在转基因植物中玉米黄质增加能保护脂质过氧化并增加PSⅠ稳定性,或是转基因植株中玉米黄质增加可终止更多活性氧。胁迫下,植物NPQ、(A+Z)/(V+A+Z)显著上升,而Pn、Fv/Fm显著下降,表明叶黄素循环和NPQ在保护光合器官免受损害中起关键作用。对于转基因植株而言,Fv/Fm下降幅度明显低于野生型植株,重新放回室温后,所有植株Fv/Fm均可恢复原有水平,VDE过表达促进叶黄素循环脱氧化作用,脱环氧化状态(Z+A)/(V+Z+A)升高。而VDE缺失突变体,NPQ和(Z+A)/(V+Z+A)比值升高程度低于野生型植株,Pn、Fv/Fm下降幅度均大于野生型植株。分析VDE低温表达量,发现CsVDE在低温条件下表达量先升后降。这些结果表明,VDE在减缓光抑制中起重要作用。
2.4VDE基因对盐胁迫的响应
盐胁迫下VDE响应研究较少,研究表明,随盐浓度增加,酶活逐渐升高,严重盐胁迫下,VDE活性降低,(A+Z)/(V+A+Z)不断上升,NPQ和qE增加表明叶黄素循环色素脱环氧化程度增加有利于能量耗散。在与转基因植株对比中发现,盐胁迫下,野生型中光抑制程度更严重,丙二醛(MDA)积累明显增加,与野生型相比,转基因植株增加程度略小。
3 结论与展望
随着对叶黄素循环研究加深,其作用机制已明确。近年来,已克隆有关叶黄素类物质合成相关酶的编码基因,为深入认识热耗散及抗性关系分子机制提供基础。Gao等在烟草中过量表达拟南芥VDE显著提高高光强下叶黄素循环活性和NPQ[16]。拟南芥中真核翻译起始因子elFiso4G为调节紫黄质脱环氧化酶表达必需[55]。VDE活性受多因素调节,如pH、抗坏血酸、紫黄质的可利用性、温度、脂质结构、UV-B、DTT等。非生物胁迫下,VDE表达量、荧光参数与光合作用参数均有显著变化。获得过表达与突变体,转基因植株与野生型植株内在与外在出现明显差异,而对非生物胁迫下各指标测定显示VDE基因作用,为深入研究VDE作用机制提供基础。
Demming等发现玉米黄质存在与过剩光能的耗散有关以来[56],研究发现,玉米黄质参与非辐射能量耗散[57],但其内部分子机理尚不清楚。目前对于叶黄素循环研究较多,但是对干旱胁迫下叶黄素循环组分对植物抗逆境影响研究较少;研究大多集中在简单生理指标与酶活性水平上,内部作用机制研究不够透彻;试验材料大多集中在草本等小型植物,很少选取林木与果树,研究局限性很大。同时,ABA介导植物体对多种环境胁迫应答,干旱、低温、高盐等环境胁迫均可导致植物体内ABA积累。高等植物ABA主要是以类胡萝卜素为前体,经间接途径合成。ABA合成发生在质体和细胞质中,在拟南芥中编码ZEP,ZEP催化玉米黄素和环氧玉米黄素环氧化作用,产生全反构象紫黄素。与野生型相比,拟南芥ABA缺陷突变体中新黄素含量降低而紫黄素含量增加[58],这表明AtABA4参与紫黄素到新黄素的转化。ZEP与VDE同为叶黄素循环关键酶,ZEP变化将引起VDE改变,因而推测ABA与VDE可能存在间接甚至直接联系。VDE与ABA代谢途径中相关蛋白相互作用有待深入研究。
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Functions ofVDEin plant response to stresses
SONG Xingshun,SUN Lina,ZHANG Qiuyan,WANG Ruifang
(School of Life Sciences,Northeast Forestry University,Harbin 150040,China)
The xanthophyll cycle exists in higher plants and green algae,which is considered to function as protecting plant photosynthetic apparatus from the damage of excessive light.Violaxanthin deepoxidase(VDE),a key enzyme in xanthophyll cycle,could catalyze the conversion of violaxanthin(V)via antheraxanthin(A)and then to zeaxalnthin(Z),protecting photosynthesis apparatus from photo damage. VDE activity was regulated by a variety of factors.It had been suggested that some factors,such as cholophyll fluorescence,were influenced by abiotic stresses.The recent investigations revealed the important role ofVDEin plant growth by overexpression,mutations of genes as well as wild type plants.In this review,we reviewed the recent research progress on functions of VDE in plant response to stresses.
VDE;violaxanthin de-epoxidase;abiotic stress;photosynthesis parameter;fluorescence parameter
Q943.2
A
1005-9369(2016)09-0085-06
2016-03-23
国家自然科学基金项目(30800876,31170569);中央高校基本科研业务费专项资金项目(2572015DA02,2572014EA04)
宋兴舜(1978-),男,教授,博士,博士生导师,研究方向为植物逆境生理与分子机制。E-mail:sfandi@163.com