乙烯的生物合成与信号及其对种子萌发和休眠的调控

宋松泉 刘 军 徐恒恒 张 琪 黄 荟 伍贤进



乙烯的生物合成与信号及其对种子萌发和休眠的调控

宋松泉1,3,*刘 军2徐恒恒2张 琪2黄 荟3伍贤进3

1中国科学院植物研究所, 北京 100093;2广东省农业科学院农业生物基因研究中心, 广东广州 510640;3怀化学院民族药用植物资源研究与利用湖南省重点实验室/ 生物与食品工程学院, 湖南怀化 418008

种子萌发是一种关键的生态和农业性状, 由调控种子休眠状态和萌发潜势的内在和外部信息所决定, 在植物随后的生长发育和产量中起着极其重要的作用。休眠是指种子在合适的条件下暂时不能萌发。乙烯是一种简单的具有多种功能的气体植物激素, 在分子、细胞和整体植物水平调节植物的代谢。在适宜和逆境条件下, 乙烯通过与其他信号分子的相互作用影响植物的行为。本文主要综述乙烯的生物合成与信号、乙烯在种子萌发和休眠释放中的作用以及乙烯与植物激素脱落酸和赤霉素的相互作用; 并提出了需要进一步研究的科学问题, 试图为解释乙烯调控种子萌发与休眠的分子机制提供新的研究思想。

脱落酸; 生物合成与信号; 交叉反应; 乙烯; 赤霉素; 种子萌发和休眠

种子是可持续农业和植物生物多样性必需的重要遗传传递系统, 种子的成功萌发和幼苗建成在农业生产和自然生态系统中起决定性作用[1-2]。休眠(dormancy)是指在合适的条件下种子暂时不萌发[3]。在许多种子植物中, 种子休眠是一种适应性特征, 使植物能够在逆境条件下存活[4]。种子休眠对植物特别是一年生植物的存活是非常重要的, 因为它能确保种子仅仅在环境条件合适时萌发[5-6]。与野生物种比较, 大多数农作物品种表现出休眠水平降低, 以及播种后高的出苗率[7-8]。种子休眠特性的不适当丧失引起新鲜成熟种子的迅速萌发, 或者甚至在收获前萌发(pre-harvest sprouting), 也称为胎萌(vivipary), 导致农业生产中产量和质量的巨大损失, 严重影响收获后的种子管理和随后的产业利用[9]。

种子萌发与休眠在对环境信号的反应中被脱落酸(abscisic acid, ABA)和赤霉素(gibberellin, GA)之间的平衡所调控; 高水平的ABA和低水平的GA引起种子深休眠和出苗率降低, 而低水平的ABA和高水平的GA诱导种子胎萌[1,6,10-12]。此外, 其他植物激素(乙烯、茉莉酸和生长素)也在种子萌发控制中起作用[13-15], 特别是乙烯通过复杂的信号网络调节许多物种的萌发与休眠[13,16-17]。

乙烯是一种简单的具有多种功能的气体植物激素, 在分子、细胞和整体植物水平调节植物的代谢[18-20]。在适宜和逆境条件下, 乙烯通过与其他信号分子的相互作用影响植物的行为[21-22]。本文主要综述乙烯的生物合成与信号, 乙烯在种子萌发和休眠释放中的作用, 以及乙烯与植物激素ABA和GA的相互作用; 试图为解释乙烯调控种子萌发与休眠释放的分子机制提供新的研究思想。

1 乙烯的生物合成与信号途径

1.1 乙烯的生物合成

Arc等[13]提出, 萌发种子中乙烯的生物合成途径与植物其他器官相同, 即甲硫氨酸→S-腺苷甲硫氨酸(S-adenosyl-methionine, S-AdoMet)→1-氨基环丙烷-1-羧酸(1-aminocyclopropane-1-carboxylic acid, ACC)→乙烯。乙烯的作用主要取决于它在细胞中的浓度以及植物组织对它的敏感性[23-24]。Lieberman等[25]最初在一个化学模式系统中发现甲硫氨酸是乙烯的前体, 乙烯来自甲硫氨酸的C3和C4; 标记的甲硫氨酸能被苹果()果实组织有效地转化成为乙烯[26]。这些发现随后被其他研究人员用苹果和其他植物组织证实[27], 然而, 更重要的进展是S-AdoMet和ACC被确定为植物中乙烯合成的前体[27]。甲硫氨酸通过3个关键的酶促反应产生乙烯: (1)甲硫氨酸被S-AdoMet合成酶转化成为S-AdoMet; (2) ACC合酶(ACC synthase, ACS)转化S-AdoMet成为ACC; (3) ACC氧化酶(ACC oxidase, ACO)分解ACC释放乙烯(图1)。ACC的形成通常被认为是乙烯生物合成途径中的限速步骤[19]。除了ACC以外, ACS也产生5’-甲硫腺苷(5’-methylthioadenosine, MTA), 它被用于新的甲硫氨酸的合成, 确保即使当甲硫氨酸库变小时, 高速率的乙烯生物合成也能被维持(图1)。ACO催化ACC转化成为乙烯是氧依赖的, 在厌氧条件下, 乙烯的形成被完全抑制; 在这个反应中, 还需要Fe2+和抗坏血酸(ascorbic acid, AsA)作为辅因子和共同底物。ACC能被转化成为丙二酰ACC(malonyl-ACC, MACC), 从而被失活; 从ACC分解形成的有毒气体氰化氢(HCN)被β-氰丙氨酸合酶(β-cyanoalanine synthase)去毒(图1)[19]。在N2下, ACC在苹果组织中积累[27]。

1.1.1 ACC合酶 在种子萌发过程中, 乙烯的增加与和转录本的逐渐积累有关[14,28]。ACS定位于细胞质, 是依赖吡哆醛-5’-磷酸(pyridoxal-5’- phosphate, PLP)酶的成员之一, 它利用维生素b6作为酶功能的辅助因子[19]。在拟南芥()中, ACS由12个成员组成的多基因家族编码, 其中8个编码功能性ACC合酶(ACS2、ACS4~ACS9、ACS11),是一个失活的异构体,是一个假基因,和编码氨基转移酶[29]。三维结构测定表明, ACS形成功能二聚体; 异源二聚体的形成增加了ACS蛋白家族的结构和功能复杂性[30]。在拟南芥中, 大的基因家族表现出一种组织专一的和差异的表达模式; 利用单个和多个敲除突变体, 证明基因家族的个别成员具有特定的发育和生理作用, 而且它们之间也存在着复杂的组合相互作用[30]。在许多物种中, 不同的内外信号调节乙烯生物合成的水平, 在基因表达的水平起作用; 这些诱导因子包括生长素、细胞分裂素、油菜素甾体、乙烯、铜、机械刺激、臭氧、病原体和伤害[19,31]。

根据C端结构, ACS蛋白分成三种类型。类型I ACS蛋白在它们的C端结构域含有一个假定的钙依赖蛋白激酶(calcium-dependent protein kinase, CDPK)磷酸化靶位点和3个促分裂原激活的蛋白激酶(mitogen-activated protein kinase, MAPK), 类型II ACS蛋白仅仅含有MAPK磷酸化位点, 而类型Ⅲ ACS蛋白不含任何磷酸化位点[32]。研究表明, 在拟南芥[33-34]和番茄()[35]中, 一些ACS成员的差异磷酸化引起蛋白质通过蛋白酶体(proteasome)降解; 一些ACS成员的蛋白稳定性进一步被蛋白磷酸酶2A(protein phosphatase 2A, PP2A)和PP2C所调节[36-37]; 这些结果表明磷酸化和去磷酸化之间的复杂平衡确保蛋白质的活性和稳定性。

图1 乙烯生物合成途径

S-腺苷甲硫氨酸(S-AdoMet)合成酶催化从甲硫氨酸形成S-AdoMet, 合成1分子的S-AdoMet消耗1分子的ATP(1)。ACC合酶催化S-AdoMet转化成为ACC是乙烯合成的限速步骤(2)。随着ACC的合成, 甲硫腺苷(MTA)是ACC合酶产生的副产物。MTA回到甲硫氨酸的再循环保存了甲硫基, 能够维持细胞中恒定的甲硫氨酸浓度。ACC丙二酰化作用成为丙二酰-ACC 使ACC库枯竭并减少乙烯的产生。ACC氧化酶利用ACC作为底物, 催化乙烯合成的最后步骤, 同时产生二氧化碳和氰化物(3)。氰化物被β-氰丙氨酸合酶代谢产生无毒的物质。ACC合酶和ACC氧化酶被同源异构蛋白、发育和环境信息的转录调节用虚线箭头表示。引自Lin等[19]。

The formation of S-adenosyl methionine (S-AdoMet) from methionine is catalysed by S-AdoMet synthetase at the expense of one molecule of ATP per molecule of S-AdoMet synthesized (1). A rate-limiting step of ethylene synthesis is the conversion of S-AdoMet to ACC by ACC synthase (2). Methylthioadenosine (MTA) is the by-product generated, along with ACC, by ACC synthase. Recycling of MTA back to methionine conserves the methylthio group and is able to maintain a constant concentration of methionine in cells. Malonylation of ACC to malonyl-ACC depletes the ACC pool and reduces ethylene production. ACC oxidase catalyses the final step of ethylene synthesis using ACC as substrate and generates carbon dioxide and cyanide (3). Cyanide is metabolized by β-cyanoalanine synthase to produce non-toxic substances. Transcriptional regulation of both ACC synthase and ACC oxidase by homeotic proteins and developmental and environmental cues is indicated by dashed arrows. From Lin et al.[19]

1.1.2 ACC氧化酶 尽管ACS被认为是大多数植物对非生物和生物胁迫反应中产生乙烯的一个关键调节酶[38], 但是ACO活性已经被证明在种子萌发过程中起重要作用[14,39]。有趣的是, 分离ACO的关键环节是在提取介质中加入AsA[40]。虽然AsA对蛋白质稳定性/活性的确切作用还不清楚, 但已经证实AsA通过向活性位点提供一个单电子参与ACC环的打开[41]。这一催化反应释放乙烯和氰甲酸根离子(NCCO2)−, 后者被分解成为CO2和氰化物(CN−)[41]。ACO属于双加氧酶(dioxygenase)超家族, 需要Fe2+作为辅因子, 重碳酸氢盐作为激活剂[42-44]。ACO的亚细胞定位目前还不清楚, 一些研究将ACO定位于细胞质; 而另一些研究则将ACO定位于质膜[45-46]。尽管ACO蛋白序列不包含任何预测的跨膜结构域, 但该蛋白仍有可能通过直接(或者间接)相互作用与质膜结合[31]。在拟南芥中, ACO也由一个含有5个成员的多基因家族编码[ACO1、ACO2、ACO4、At1g12010 (ACO3)和At1g77330 (ACO5)]。拟南芥中的3个基因也是由乙烯自动调控的[47]。Van de Poel 等[48]通过数学模型研究推测, ACO存在转录后和/或者翻译后调节机制。

1.2 乙烯信号

在曝露于乙烯气体下, 暗生长的拟南芥幼苗的“三重反应(triple response)”表型能够使我们容易识别乙烯不敏感的(ethylene-insensitive)和组成性反应(constitutive-response)的突变体, 这些突变体的克隆和鉴定导致了乙烯信号转导途径的线性模型的提出[49-50], 即植物中存在一条复杂的乙烯信号途径包括正、负反馈调控环, 并特别强调植物如何精细调控的机制(图2)。

图2 拟南芥中乙烯信号途径的最近模型

乙烯由受体蛋白ETR1、ERS1、ETR2、ERS2和EIN4 (绿色表示)感受, 受体是乙烯信号的负调控因子。受体通过它们的GAF结构域(在受体的细胞质区域用五边形表示)与其他的受体相互作用, 并在ER膜中形成更高层次的复合物。铜(一种乙烯结合的辅因子, 红色圆圈)由铜转运体RAN1 (橙色表示)传递给受体。RTE1 (粉红色)与ETR1相联系, 介导受体信号输出。(A)在乙烯缺乏时, 受体激活CTR1 (黄色)。CTR1通过直接磷酸化EIN2的C-末端(蓝色圆圈)使其失活(紫色)。EIN2能够直接与受体的激酶结构域(在受体的细胞质区域在五边形下较大的椭圆)相互作用。EIN2的水平通过26S蛋白酶体(灰色)被F-box蛋白ETP1和ETP2(绿色星状物)负调控。在细胞核中, 转录因子EIN3/EIL1 (红色)通过蛋白酶体被另外2个F-box蛋白EBF1/2(蓝色星状物)降解。在EIN3/EIL1缺乏时, 乙烯反应基因的转录被关闭。(B)在乙烯存在时, 受体与激素结合并失去活性, 依次关闭CTR1。这种失活阻止正调控因子EIN2的磷酸化。EIN2的C-末端被一种未知的机制所剪切, 并移动到细胞核, 在细胞核中使EIN3/EIL1稳定和诱导EBF1/2的降解。转录因子EIN3/EIL1形成二聚体, 激活乙烯靶基因的表达, 包括F-box基因(深蓝色卷曲线, 它产生抑制乙烯途径活性的负反馈环)或者转录因子基因(淡蓝色线, 它依次始启一个转录级联, 导致数百个乙烯调控基因的活化和抑制)。在乙烯反应基因中有受体基因(绿色线), 它的mRNA被乙烯上调, 以及被翻译成为一批新的没有与乙烯结合的受体分子; 这些受体分子然后活化负调控因子CTR1, 从而提供了在不添加乙烯的情况下向下调节乙烯信号的手段。途径中的其他调控节点是核糖核酸外切酶EIN5 (淡橙色, 它控制的mRNA水平)以及F-box蛋白ETP1和ETP2 (绿色星状物, 在乙烯存在时, 它们被降解, 导致EIN2的稳定)。正箭头和负箭头分别表示激活和下调这个过程。颜色变浅表示的分子(在‘没有乙烯’中的EIN3/EIL1, 或者在‘乙烯’中的ETP1/2和EBF1/2)相应于蛋白酶体介导降解的被标记的不稳定蛋白。卷曲线表示特定的mRNA, 它们的颜色与相应的蛋白质颜色相一致。引自Merchante等[51]。

Ethylene is perceived by the receptor proteins ETR1, ERS1, ETR2, ERS2, and EIN4 (represented in green), the receptors are negative regulators of ethylene signaling. The receptors interact with other receptors and form higher order complexes in the ER membrane through their GAF domains (represented as pentagons in the receptors’ cytosolic domain). Copper (a cofactor for ethylene binding, red circles) is delivered to the receptors by the copper transporter RAN1 (represented in orange). RTE1 (in pink) is associated with ETR1 and mediates the receptor signal output. (A) In the absence of ethylene, the receptors activate CTR1 (in yellow). CTR1 inactivates EIN2 (in purple) by directly phosphorylating (blue circles) its C-terminal end. EIN2 can directly interact with the kinase domain of the receptors (represented as the larger ovals under the pentagons in the cytosolic domain of the receptors). The levels of EIN2 are negatively regulated by the F-box proteins ETP1 and ETP2 (green star)the 26S proteasome (gray). In the nucleus, the transcription factors EIN3/EIL1 (in red) are being degraded by two other F-box proteins, EBF1/2 (blue star), through the proteasome. In the absence of EIN3/EIL1, transcription of the ethylene response genes is shut off. (B) In the presence of ethylene, the receptors bind the hormone and become inactivated, which in turn, switches off CTR1. This inactivation prevents the phosphorylation of the positive regulator EIN2. The C-terminal end of EIN2 is cleaved off by an unknown mechanism and moves to the nucleus where it stabilizes EIN3/EIL1 and induces degradation of EBF1/2. The transcription factors EIN3/EIL1 dimerize and activate the expression of ethylene target genes, including the F-box gene(dark blue curly line) [which generates a negative feedback loop dampening the activity of the ethylene pathway] or the transcription factor gene(light blue line) [which, in turn, initiates a transcriptional cascade resulting in the activation and repression of hundreds of ethylene-regulated genes]. Among the ethylene responsive genes the receptor gene is(green line), whose mRNA is up-regulated by ethylene and translated into the new batch of ethylene-free receptor molecules which then activate the negative regulator CTR1, thus providing the means of tuning down ethylene signaling in the absence of additional ethylene. Other regulatory nodes in the pathway are the exoribonuclease EIN5 (light orange), which controls the levels ofmRNA, and the F-box proteins ETP1 and ETP2 (green star) that are degraded in the presence of ethylene leading to the stabilization of EIN2. Positive and negative arrows represent activation and down-regulation processes, respectively. Molecules shown in fading colors (EIN3/EIL1 in ‘no ethylene’, or ETP1/2 and EBF1/2 in ‘ethylene’) correspond to unstable proteins targeted to proteasome-mediated degradation. Curly lines indicate specific mRNAs, with their colors matching that of the corresponding proteins. From Merchante et al.[51]

1.2.1 乙烯信号途径 乙烯信号级联从乙烯与受体结合开始, 到转录调节结束。乙烯受体是一个多成员家族, 在拟南芥中由ETR1 (ethylene resistant 1)、ERS1 (ethylene response sensor 1)、ETR2、ERS2和EIN4 (ethylene insensitive 4)组成, 它们与乙烯的结合都具有高的亲和力。根据接受区域的存在(ETR1、ETR2和EIN4)或者缺乏(ERS1和ERS2), 受体可分为2种类型[51]。这些受体以同源二聚体的形式起作用, 是信号途径的负调控因子, 在乙烯缺乏时主动抑制乙烯反应[21,51-52]。已经证明, 这些受体在乙烯反应的控制中是大量冗余的, 但不同异构体之间具有一些功能特异性[51]。受体主要存在于内质网(endoplasmic reticulum, ER)膜中, 由于乙烯能够在细胞的水环境和脂质环境中自由扩散, 受体的ER定位可能促进与其他细胞成分的相互作用和/或者使信号能够与其他途径整合[53]。

根据系统发育分析和共有的结构特征, 所有的乙烯受体都有一个模块化结构, 包括一个负责与乙烯结合的N端跨膜结构域, 一个不同受体类型之间与蛋白质-蛋白质相互作用有关的GAF结构域, 以及一个与途径下游组分相互作用所需的C端结构域[49,54]。尽管受体的C端具有细菌双组分组氨酸激酶(two- component histidine kinases)的结构相似性, 但受体的自体激酶活性(autokinase activity)在乙烯反应中仅仅起较小的作用[53]。乙烯受体的基本功能单元是能够与乙烯结合的同源二聚体。在通过GAF结构域相互作用的同源二聚体中, 能够发生更高层级的联系, 从而在膜中产生受体簇[55-56]。

由细胞内铜转运体RAN1 (copper transporter RAN1)提供的铜是乙烯结合和受体功能都需要的[57]。功能丧失突变的植株缺乏乙烯结合活性, 表现出类似于受体功能丧失的突变体的表型; 此外, 用铜螯合剂处理的弱等位基因表现出类似于乙烯处理的野生型植株的表型[58], 以及添加Cu2+到这些植株能部分抑制的表型[57]。这些结果提出, RAN1在乙烯受体的生物发生中起必不可少的作用。

乙烯敏感性逆转1 (reversion-to-ethylene sensitivity 1, RTE1)是乙烯反应的一种负调控因子[59], 与受体一起共定位在ER, 但在高尔基体(Golgi apparatus)的膜中也被检测到[60]。RTE1通过促进ETR1从失活(在乙烯存在下)信号状态转变成为活化(无乙烯)信号状态专一地激活ETR1[61]。

尽管受体的确切输出功能仍然不清楚, 但遗传学研究表明, 在乙烯缺乏时, 受体激活了途径中的一个负调控因子CTR1 (constitutive triple response 1)。CTR1是一种丝氨酸/苏氨酸(Ser/Thr)蛋白激酶, 当它被活化时形成同源二聚体。活化的CTR1激酶二聚体参与乙烯受体簇之间的交叉相互作用[62]。CTR1的下游是EIN2, 乙烯信号级联中的一个关键分子。EIN2蛋白包含一个由12个预测的跨膜结构域组成的N端疏水区域和一个含有保守的核定位序列的亲水C端区域[63-64]。疏水区域与金属离子转运体的NRAMP家族类似, 但EIN2没有表现出转运体的活性[65]。EIN2存在于ER膜中, 与乙烯受体的激酶结构域相互作用[66]。当用乙烯处理时, EIN2积累, 以及对于下游途径组分EIN3的稳定是绝对需要的[67]。EIN2作为关键组分在乙烯信号中起作用, 但是花了13年多的时间才确定这个有趣的分子怎样从ER中的受体把乙烯信号传递到核内转录因子EIN3/EIL1, 从而调控下游基因的表达。已经表明, EIN2的C端从ER膜物理运动到细胞核, 允许乙烯信号到达下游组分EIN3和EILs[64,68-69]。Chen等[70]的研究表明, 在乙烯存在下, EIN2在多个丝氨酸和苏氨酸残基上缺乏磷酸化。Ju等[68]随后证明, EIN2与CTR1之间存在物理相互作用, 在乙烯缺乏时CTR1直接磷酸化EIN2 C端的蛋白激酶, 从而阻止C端向下游组分EIN3及其同系物EILs传递信号。然而, 去磷酸化是否直接促进EIN2的裂解或者增强EIN2 C端的稳定性目前还不清楚[71]。Ju等[68]的研究结果显示, 对于CTR1和EIN2之间的信号转导不需要MAPKK或者MAPK活性。EIN2的C端一旦进入细胞核将使EIN3稳定和引起EIN3/EILs依赖的转录级联的活化[64,68-69]。

EIN3和EILs (拟南芥中的EIL1)是短寿命蛋白, 它们作为乙烯信号途径的正调控因子起作用。EIN3和EIL1是产生乙烯反应的主要输出的2个关键转录因子, 对于乙烯反应基因的表达是必需的和足够的。EIN3/EILs以二聚体的形式起作用, 至少在番茄EIL1中一个保守的磷酸化位点的突变扰乱了烟草()双分子荧光互补(Bimolecular fluorescence complementation, BiFC)系统中的荧光信号, 以及消除了番茄植株中相应的转基因活性[72]。当被EIN3/EILs转录激活时, 乙烯靶基因介导了植物对乙烯的一系列反应[50]。使用染色质免疫沉淀测序(chromatin immunoprecipitation sequence, ChIP-seq), Chang等[73]发现EIN3以四波的方式(four-wave manner)调控下游基因的转录, 每波包含一组唯一的EIN3靶子, 它们逐渐增加地调节许多下游的转录级联。重要的是, 一些下游的EIN3靶子相当于其他激素信号途径的关键组分, 从而强化了不同植物激素之间存在复杂的相互作用网络的思想。在拟南芥中鉴定的上述所有乙烯信号组分在进化上距离较远的物种中都是保守的, 表明植物中的乙烯信号机制是普遍的[51]。

1.2.2 信号组分的转换与反馈调节 随着研究进展, 已经发现乙烯线性信号途径实际上是一条更为复杂的路线, 包括反馈调控的转录网络, 以及mRNA和蛋白质转换调控模块[50]。蛋白酶体介导的蛋白质降解在乙烯信号级联的调控中起主要作用。在受体水平, 乙烯通过26S蛋白酶体诱导ETR2降解; 同时, 乙烯转录激活、和[74]。

EIN2和EIN3/EIL1的蛋白水平也被专一的F-box蛋白严格调控, 在乙烯缺乏时, F-box蛋白使它们发生蛋白酶体介导的降解[75-76]。在对乙烯信号反应中, ETP1 (EIN1-targeting protein 1)和ETP2控制EIN2的水平[76], 而EBF1 (EIN3 binding F-box 1)特别是EBF2调控EIN3的水平[75,77]。为了进一步增加这个调控模块的复杂性, EBF1/2和ETP1/2的蛋白水平被乙烯下调, 至少在EBF1/2中这一过程被蛋白酶体介导[67,76]。(被乙烯转录诱导)本身就是EIN3的一个靶子, 这能够解释乙烯反应中每个EBF的不同作用[78], 从而建立一个复杂的反馈调控机制。作为这些调控回路的最终输出, 细胞核中EIN3/EIL1的蛋白水平被精细地调控, 以协调一组乙烯反应的活化。换句话说,转录的乙烯依赖性的增加与EBF1和EBF2蛋白稳定性的减少之间的平衡被认为是调节EIN3/EIL1的转换, 为调节植物对乙烯的反应提供了一种动态机制。5'–3'核糖核酸外切酶XRN4/EIN5提供了另一个层次的调控, 该酶通过一个未知的机制下调和mRNA的水平。由于EIN5的分子性质, 提出了一种控制乙烯反应的RNA降解模块[50]。与上述其他调控环不同, EIN2和ETPs都不受乙烯的转录调控[79]。

2 乙烯在种子萌发与休眠释放中的作用

2.1 乙烯对种子萌发的促进作用

乙烯的产生在种子吸胀开始后立即发生, 并随着萌发时间的延长而增加; 乙烯释放高峰与胚根突破种皮一致[2,80-81]。种子中乙烯的产生是物种依赖的, 但在吸胀过程中乙烯的释放量常常低于用气相色谱可检测到的水平。利用高灵敏度的激光光声光谱(laser photoacoustic spectroscopy), El-Maarouf- bouteau等[2]已经证实在向日葵()种子萌发结束时乙烯出现高峰。乙烯对种子萌发的促进作用是剂量依赖的, 当应用的浓度为0.1~200 μL L-1时是有效的, 这取决于物种、休眠深度和环境条件。尽管乙烯促进许多光敏种子的萌发, 但是它不能克服反枝苋()、芹菜()、莴苣()和大爪草()种子萌发对光的需要[16]。

2.2 外源乙烯打破种子休眠

外源乙烯或者乙烯利(一种释放乙烯的化合物)能打破一些种子的初生和次生休眠[16,82](表1)。在表现出种皮强制休眠的一些物种中, 乙烯也能打破休眠和促进萌发, 例如苍耳()、地三叶()、皱叶酸模()和拟南芥[16]。特别是在莴苣、向日葵、尾穗苋()和繁穗苋()种子中, 乙烯也能打破由高温诱导的次生休眠[16,83]。ACC能促进松果菊属()植物种子的萌发[84], 也能促进寄生植物例如独脚金()[16]种子的萌发。乙烯增加非休眠种子在非最适环境条件例如高温、渗透胁迫、缺氧和盐胁迫下的萌发[16,85-86]。

表1 乙烯、乙烯利或者1-氨基环丙烷-1-羧酸打破种子休眠的物种(引自Corbuneau et al.[16])

在苹果种子冷处理[87], 或者向日葵[88]、反枝苋[82]和柱花草()[89]种子干藏过程中, 休眠的打破都与乙烯敏感性的增加有关。在刚收获时, 休眠的向日葵种子在15℃下需要50 μL L–1乙烯才能发芽; 但在5℃下分别干藏8周和15周后, 仅仅需要10 μL L-1和3 μL L-1乙烯[88]。相反, 在诱导次生休眠的环境条件下, 乙烯的反应性在种子萌发过程中逐渐下降[88]。

2.3 乙烯生物合成与信号对种子萌发与休眠的调控

许多研究表明, 萌发能力与乙烯的产生有关, 表明乙烯调控种子萌发与休眠[13,39,90]。例如, 鹰嘴豆()、向日葵和莴苣种子在高温下热休眠的诱导与乙烯产生的降低有关[16]。乙烯产生的下降可能导致ACC-丙二酰转移酶活性的增加, ACC含量下降; ACO活性被抑制, 或者和表达降低[16,91]。相反, 一些处理(如低温、GA、一氧化氮、HCN)打破种子休眠, 导致乙烯产生增加[13]。

利用乙烯生物合成途径的抑制剂或者改变乙烯生物合成和信号途径的突变体获得的数据表明, 内源乙烯在种子萌发和休眠的调控中起关键作用。种子在氨基乙氧基乙烯甘氨酸(aminoethoxyvinyl glycine)和氨基氧乙酸(aminooxyacetic acid, AOA)(ACS活性抑制剂), CoCl2和α-氨基异丁酸(α-aminoisobutyric acid)(ACO活性抑制剂)或者2,5-降冰片二烯(2,5- norbornadiene)和硫代硫酸银(silver thiosulfate)(乙烯作用抑制剂)中的萌发表明, 内源乙烯参与萌发和打破休眠[16,92]。相反, 乙烯的直接前体ACC促进许多物种的种子萌发, 例如莴苣、向日葵、苍耳、苋属植物(sp.)、鹰嘴豆和甜菜()[16]。值得注意的是, ACC氧化的一种副产物HCN也能打破苹果[93]、向日葵[94]和反枝苋[82]种子的休眠。

利用在乙烯生物合成和信号中发生改变的拟南芥品系允许表征乙烯对休眠的调控作用。与野生型比较,和突变体的种子表现出初生休眠增加, 可能是由于ABA敏感性提高; 而突变体轻微地提高萌发速率[95-96]。EIN2的功能丧失导致拟南芥种子在萌发和早期幼苗发育过程中对盐和渗透胁迫过敏[97]。在休眠的水青冈()胚中()的表达极少, 但在湿冷过程中增加, 从而打破休眠[98]。在向日葵中,在非休眠胚中的表达是休眠胚的5倍, 以及的表达被HCN显著地促进[94]。在番茄萌发种子中转录本的丰度比非萌发种子高, 在转基因系中的过表达导致种子成熟前萌发[99]。

休眠和后熟的拟南芥种子的转录组数据表明, 在休眠状态下基因的表达上调, 在萌发状态下的表达上调[100]。在莴苣中, Argyris等[91]表明乙烯反应基因被热抑制调控; 在高温下,和的基因表达下降, 而、和的表达增加。这些结果提示激素代谢与信号调控在基因表达水平上存在差距。在小麦()种子中, 注释为乙烯代谢和信号基因的78个探针组在休眠和后熟种子之间被差异表达。果胶裂解酶1、扩展蛋白A2、β-1,3-葡聚糖酶和几丁质酶β被认为是假定的乙烯反应的下游基因, 这些基因在独行菜()种子萌发过程中在胚乳弱化和/或者胚根生长中起重要的作用[101]。

3 乙烯与植物激素脱落酸和赤霉素的相互作用

种子植物中种子休眠的激素调控可能是一种高度保守的机制。在许多物种中观察到种子休眠被ABA诱导和维持, 被GA释放[102]。利用ABA和GA生物合成和信号突变体的大量遗传研究表明, 这2种激素在种子休眠和萌发中具有重要的作用和互相拮抗的作用[10,103]。在莴苣种子萌发中, ABA抑制种子萌发, GA促进种子萌发且具有拮抗ABA的作用[104]。下面主要讨论乙烯通过抑制ABA的代谢与信号和增强GA的代谢与信号调控种子的萌发与休眠。

3.1 乙烯和ABA之间的交叉作用

3.1.1 ABA对乙烯代谢的影响 在种子萌发过程中, ABA和乙烯之间的拮抗作用已经在许多物种中被阐明[13-14,105]。在拟南芥和家独行菜()中, 乙烯拮抗ABA对胚乳帽弱化和胚乳破裂的抑制作用[101]。ABA也增加打破初生和次生休眠的乙烯需要量; ABA对萌发的抑制与乙烯产生的减少有关; ABA明显抑制体内ACO的活性, 这种抑制作用与减少的转录本的积累有关[13,16,101](图3)。在拟南芥中, ABA通过ABI4介导的和的转录抑制拮抗乙烯的产生[105]。在拟南芥种子萌发过程中, 胚和胚乳中转录本的积累被ABA抑制; ABA不敏感突变体中高水平的转录本表明,的表达被ABA调节[101,106]; 在胚中,转录本的积累也被ABA抑制[106]。在家独行菜中, ABA对和的抑制作用被限于胚乳帽[101]。同样, 拟南芥突变体的芯片分析发现了转录本积累的上调[107]。

图3 乙烯、脱落酸和赤霉素在种子萌发和休眠调控中的相互作用

该方案是基于正文中引证的种子对乙烯、脱落酸或者GA响应的遗传分析、芯片数据和生理研究。乙烯通过抑制ABA的合成和促进它的失活或者分解代谢下调ABA的积累, 也负调控ABA信号。ABA通过ACS和ACO的活性抑制乙烯的生物合成。乙烯也增强GA的代谢和信号, 反过来也一样。“→”和“┤”分别表示信号级联的不同元素之间的正、负相互作用。根据Corbineau等[16]重绘。

This scheme is based on genetic analyses, microarray data, and physiological studies on seed responsiveness to ethylene, ABA or GA cited in the text. Ethylene down-regulates ABA accumulation by both inhibiting its synthesis and promoting its inactivation or catabolism, and also negatively regulates ABA signaling. ABA inhibits ethylene biosynthesis through ACS and ACO activities. Ethylene also improves the GA metabolism and signaling, and. “→” and“ ┤” indicate positive and negative interactions between the different elements of the signaling cascade, respectively. Redrew from Corbineau et al.[16].

3.1.2 乙烯对ABA代谢和信号的影响和的种子表现出比野生型更高的ABA含量, 以及萌发更缓慢[95,97]。ABA-葡萄糖酯(ABA-glucose ester)的水平在种子中减少, 因此, 增加的ABA积累可能是由于ABA结合的减少[95]。然而, 乙烯也可能调节其他酶促步骤, 芯片分析表明在中上调, 在中下调[107]。中的高水平ABA也与的上调有关[97]。

一些研究表明, 在种子萌发过程中乙烯不仅对ABA代谢起作用以降低ABA水平, 而且负调节ABA信号(图3)。实际上, 降低乙烯敏感性的突变(如、和)导致ABA敏感性的增加, 而在和(ethylene overproduction 1)中增加的乙烯敏感性降低了ABA的敏感性[96,101,108]。例如,的突变增强了种子的ABA不敏感性, 乙烯不敏感突变体如降低了ABA不敏感性[109]。然而, 在和中, 没有观察到ABA敏感性的显著性差异[96]。

此外, 水青冈种子酪氨酸磷酸酶在拟南芥种子中的过表达通过ABA信号下调和上调减少休眠, 提出在ABA信号中的负作用可能是由乙烯信号调控的结果[110]。尽管ABA和乙烯信号途径之间存在相互作用, 但遗传证据表明它们可能平行地起作用, 因为通过乙烯突变体(、、和)与突变体杂交获得的双突变体表现出ABA缺乏和乙烯敏感性改变所引起的表型[107]。

3.2 乙烯和GA之间的相互作用

在许多物种中, GA促进休眠种子的萌发, 这种休眠也被乙烯、乙烯利或者ACC打破(表1)。在拟南芥中, 乙烯恢复GA缺乏突变体的萌发, 而对番茄GA缺乏突变体的萌发没有促进效应; 但GA3促进突变体的萌发。这些数据表明乙烯和GA途径相互作用[15,39]。

在水青冈种子中, 胚在GA3中培养导致ACC的积累、ACC氧化酶活性和乙烯产生增加, 与的表达增加一致[111]。同样, GA4对拟南芥突变体种子萌发的促进作用与的增加有关[112]。在GA生物合成抑制剂多效唑存在下,的表达减少证实GA激活乙烯生物合成途径[111,113]。然而, 在钻果大蒜芥()中, Iglesias- Fernandez等[28]表明,和在萌发过程中的表达被多效唑抑制, 但不受乙烯利或者GA4+7的影响。此外, 在GA4存在时拟南芥中乙烯反应感受器1 (,)(编码一个乙烯受体家族成员)[112], 以及在GA3存在时水青冈中类[114]的上调表明了GA对乙烯反应的影响。

许多数据表明, 乙烯通过影响GA的生物合成或者信号途径来促进种子萌发。与野生型比较, GA1、GA4和GA7在拟南芥突变体的干燥成熟种子中大量地积累, 在吸胀的前2 d, GA4和GA7的含量依然比野生型高[95]。萌发过程中GA含量的变化表明, ETR1即乙烯信号途径的缺陷导致(1)GA生物合成途径的改变; (2)促进萌发需要比野生型更高水平的GA[95]。在水青冈种子中, 与活性GA合成有关的的表达在层积种子(即非休眠种子)和用GA3或者乙烯利处理的种子中仍然较低, 但由AOA所引起的乙烯生物合成的抑制导致这个转录本的增加, 表明乙烯参与了GA生物合成的调控[113]。在GA4+7、乙烯以及GA合成或者乙烯合成与信号抑制剂存在时, 钻果大蒜芥种子吸胀过程中与GA合成(和)和降解()有关的基因表达研究表明, GA生物合成被GA和乙烯显著地调控[28]。

赤霉素信号途径依赖于DELLA蛋白, 包括GA不敏感(GAI)、抑制因子(RGA)、类RGA1 (RGL1)、RGL2和RGL3[115]。GA使DELLA蛋白不稳定, DELLA蛋白通过与GA结合发生泛素化和降解, 作为生长抑制因子起作用[116]。Achard等[117]报道, 在拟南芥中乙烯对下胚轴生长和花转变(floral transition)的一部分作用是通过它对DELLA蛋白的影响介导的。在控制种子萌发中也可能是这样, 因为DELLA蛋白似乎在种子萌发的调控中起关键作用[118-119]。因此, 种子中GA的含量和响应性可能是由乙烯对DELLA蛋白积累的调节所引起的。

4 结语

乙烯促进种子萌发和休眠释放, 通过影响ABA和GA的生物合成与信号起重要作用。在ACC的代谢中, 除了被ACO氧化成为乙烯外, 也能被ACC- N-丙二酰转移酶转化成为它的主要衍生物1-丙二酰-ACC (1-malonyl-ACC); ACC的第二个衍生物是由γ-谷氨酰转肽酶催化形成的γ-谷酰基-ACC (γ-glu­tamyl-ACC); 第三个衍生物是茉莉酯-ACC (jasmonyl- ACC); ACC也能由ACC脱氨酶代谢成为铵和α-酮戊二酸[31]。这些衍生物都是精细的生化分流器, 能够调节可用于产生乙烯的ACC池, 但这些ACC衍生物的确切生物学作用以及它们之间怎样维持平衡的机制还不清楚。

Zhang等[120]表明在缺乏的突变体中乙烯受体的N端部分可以有条件地介导受体信号的输出, 提出了一条不涉及CTR1的交替乙烯受体信号途径(alternate route of ethylene receptor signaling); 但交替信号转导途径所涉及的组分, 以及与线性信号转导途径(图2)在负调控乙烯信号中的动态协调是不清楚的。在对宽范围乙烯浓度的反应中, 这2条途径可能促进乙烯信号的动态微调。

乙烯与ABA和GA相互作用, 后2种激素都是种子萌发与休眠的重要调节因子[15,102,121]。因此, 乙烯促进种子萌发的作用可能是通过参与C2H4-GA- ABA的交互作用而产生, 但其作用是直接的还是间接的需要证明; ABA和GA对种子中乙烯生物合成和信号途径的影响也需要进一步的研究。活性氧(reactive oxygen species)也通过激素网络特别是与ABA和GA一起调控种子萌发[2,13,16], 因此, 区分不同信号途径的等级及其作为环境信号感受器的作用将是重要的。

组学(-omics)技术已应用于种子萌发与休眠释放的研究[122-124], 结合种子乙烯生物合成和信号突变体, 以及利用相应的抑制剂实验, 建立新的乙烯对种子萌发与休眠释放的组学研究体系, 包括转录组、翻译组、蛋白质组、代谢组和环境组等将有助于探明种子萌发与休眠释放过程的调控网络。

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Biosynthesis and signaling of ethylene and their regulation on seed germination and dormancy

SONG Song-Quan1,3,*, LIU Jun2, XU Heng-Heng2, ZHANG Qi2, HUANG Hui3, and WU Xian-Jin3

1Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China;2Agro-Biological Gene Research Center, Guangdong Academy of Agricultural Sciences, Guangzhou 510640, Guangdong, China;3Key Laboratory of Research and Utilization of Ethnomedicinal Plant Resources of Hunan Province / College of Biological and Food Engineering, Huaihua University, Huaihua 418008, Hunan, China

Seed germination, a key ecological and agronomic trait, is determined by both internal and external cues that regulate the dormancy status and the potential for germination in seeds, and plays a critical role during the subsequent growth, development and production of plants. Dormancy is the temporary failure of seed germination under favorable conditions. Ethylene is a simple gaseous phytohormone with multiple roles in regulation of metabolism at molecular, cellular, and whole plant levels. It influences performance of plants under optimal and stressful environments by interacting with other signaling molecules. In the present paper, we mainly summarize ethylene biosynthesis and signaling, the role of ethylene in seed germination and dormancy release, and the interaction of ethylene with phytohormone abscisic acid and gibberellin, and propose some scientific problems to be required to investigate further in order to provide an idea for explaining the molecular mechanism of seed germination and dormancy regulated by ethylene.

abscisic acid; biosynthesis and signaling; crosstalk; ethylene; gibberellin; seed germination and dormancy

2018-11-22;

2019-01-19;

2019-04-09.

10.3724/SP.J.1006.2019.84175

宋松泉, E-mail: sqsong@ibcas.ac.cn

本研究由国家科技支撑计划项目(2012BAC01B05), 国家自然科学基金项目(31371715, 31640059)和广东省科技计划项目(2016B030303007)资助。

This study was supported by the National Science and Technology Support Program (2012BAC01B05), the National Natural Science Foundation of China (31371715, 31640059), and the Guangdong Science and Technology Program (2016B0303007).

URL: http://kns.cnki.net/kcms/detail/11.1809.S.20190404.1439.002.html