Cubas等[1]年通过分析金鱼草CYC和玉米TB1保守区域,得出二者都有bHLH(basic-Helix-Loop-Helix)二级结构域,依据这个二级结构域寻找已知功能的相关蛋白,发现bHLH结构域也存在于水稻DNA结合蛋白PCF1和PCF2中,根据bHLH二级结构域定义了一个新的转录因子家族,称为TCP(TB1、CYC和PCFs)家族。通过随机结合位点选择试验和电泳迁移率转移试验(EMSA)确定了TCP家族结合区域碱基,根据结合位点碱基序列不同,将TCP家族分为两大类:Ⅰ类(GGNCCCAC)和Ⅱ类(GTGGNCC)[2]。Ⅱ类又分为CYC/TBI、CIN两大类,CIN包括TCP10、TCP3、TCP4、TCP2、TCP24、TCP13、TCP5、TCP17[3]。
TCP4是植物特有的转录因子TCP家族成员之一,调控植物的生长发育,影响多种植物激素的合成,参与植物抗逆调节。TCP4具有bHLH结构域,因此,转录因子TCP4能够与功能基因的顺式作用元件结合调控其表达,表现为转录抑制或转录激活。通过原位杂交试验发现,TCP4主要在子叶和幼叶中表达并调节这些组织器官的形态[4]。本研究综述了转录因子TCP4在植物生长发育过程和非生物胁迫的情况下参与植物激素的合成等作用。这些研究结果对植物生长调节和逆境下优良品种的选育有重要的指导意义。
转录因子TCP4参与植物生长发育过程,例如主根、侧根、叶片形态、花蕊形成、表皮毛分化、子叶舒展、下胚轴延伸、避荫反应、植物油脂合成等重要生长发育过程。
当拟南芥种子萌发时,需要经过2个阶段,首先在土壤中萌发时处于黑暗环境,其生理特征是尖端钩,黄色且闭合的子叶,快速延伸的下胚轴,这一过程称之为黄化。接着当子叶破土而出时处于光照环境,尖端钩会逐渐消失,黄色且闭合的子叶会逐渐变绿并展开,这一过程称之为去黄化。转录因子TCP4激活生长素生物合成相关的基因,促进幼苗下胚轴伸长。转录因子PIFs(PHYTOCHROME-INTERACTING FACTORs)在种子黄化过程中起着核心作用,并且在调节子叶形态方面也起主要作用[5]。SMALL AUXIN UP RNA(SAUR)基因家族最初被定义为一类生长素诱导的基因,其主要作用是促进细胞扩张,进而调节下胚轴的延伸[6]。其转录活性对光反应也显示出组织特异性[7]。拟南芥缺失突变体pifs(pif1、pif3、pif4、pif5、pifq)在黑暗中开放子叶[8]。当黄化幼苗处于光照环境下,转录因子PIFs磷酸化并降解。SAURs是PIFs的直接靶基因,但由于PIFs在子叶和下胚轴中都有表达[9]。Dong等[10]提出光介导TCP4-SAUR16和SAUR20形成转录因子复合体介导在种子去黄化过程中子叶展开。在子叶黄化过程中,PIF蛋白积累抑制TCP4结合到SAUR16/50的顺式作用元件,从而抑制其转录活性。在去黄化过程中,PIF磷酸化并降解,解除抑制,SAUR16/50转录活性提高,致使闭合子叶展开。
表皮毛是叶片表皮形态发生过程中第一类分化的细胞类型,是研究细胞分化的理想模型。参与表皮毛模式化和分化的基因大多数都是转录因子,能够精准地调控表皮细胞发育。转录因子TCP调控植物生长发育,花器官对称性、叶片、花瓣形态发生[11]。拟南芥过表达Ⅱ类TCP能够抑制叶片和花瓣中细胞增殖、促进下胚轴和子叶表皮细胞的分化[12-13]。GLABROUS INFLE SCENSEMS(GIS)独立于核内复制调节表皮毛发育,gis缺失突变体其叶片和茎上表皮毛分枝增加。KAKTUS(KAK),一种E3泛素降解酶,能够负调节核蛋白的积累,通过促进相关bHLH转录因子GLABRA3(GL3)和ENHANCER OF GL3(EGL3)降解进而调控表皮毛分枝[14]。除了这些转录因子,植物激素赤霉素(GA)也能够影响核蛋白积累来调控表皮毛分枝。BRANCHLESS TRICHOME(BLT)编码一种具有盘绕结构域的蛋白质,它的功能丧失也会导致表皮毛不分枝[15]。通过比较TCP4缺失突变体、过表达转基因株系和野生型表皮毛分枝的数量以及生物化学分析表明,转录因子TCP4结合到GIS顺式作用元件,抑制表皮毛分化,从而在器官形态发生与细胞分化之间建立联系[16]。
叶片是植物进行光合作用的主要器官,进而影响植物整个生长发育周期。细胞分裂和细胞扩张的协调是叶片大小和形状的关键。CIN-TCPs以高度冗余的方式调控侧器官发育、叶片衰老、激素合成和转导。一种是通过调节细胞分裂素途径和调控细胞周期基因来抑制叶片边缘区域的细胞增殖,另一种是通过促进茉莉酸的生物合成和调控基因WRKY53表达量上调加速叶片衰老。转录因子TCPs能够直接激活miR369b和CYCLIN DEPENDENT KINASE INHIBITOR1(ICK1),从而调节细胞增殖[17-18]。CIN-TCPs的丧失或功能降低导致锯齿叶的形成,Edgardo等通过比较tcp2 tcp3 tcp4 tcp10四突变体和野生型叶片的转录活性、细胞分裂能力、叶边缘褶皱程度,揭示了miR319-TCP转录因子在器官发育过程中的调控作用[19]。
开花是植物生长发育中的一个重要过程,因为植物开花时机会影响其繁殖后代是否成功。CONSTANS(CO)的转录调控是诱导光周期开花的第一个关键步骤[20-21]。目前,已经确定2种直接调节CO表达的转录因子,一种是直接结合到DNA启动子序列的CYCLING DOFACTOR(CDF)转录因子,另一种是bHLH家族中的 FLOWERING BHLH(FBH)[22-23]。TCP蛋白具有bHLH结构域,因此,能够与基因的顺式作用元件结合或与蛋白相互作用[24],tcp4突变体以及相关CIN-TCP基因表达水平降低导致了晚开花表型。除此之外,抗miRNA剪切的TCP4过表达植株能够缩短花期,提前开花。通过凝胶电泳迁移试验(EMSA)以及酵母单杂交证实TCP4能够与CO的顺式作用元件结合并调控其表达,TCP4还可以与转录因子GI相互作用,从而调控CO基因的表达[25]。
植物油脂占世界油脂总产量的70%左右[26],植物油脂在食用、医药、纺织、化妆品、化工等方面具有广泛的用途[27]。在种子中合成的三酰甘油(TAG),为种子贮藏、萌发和幼苗发育提供营养物质[28]。转录因子AP2(APETALA2AP2)、WRI1(WRINKLED1)能够调节植物油生物的合成。拟南芥缺失突变体wri1-1与野生型相比,种子油脂含量约降低80%[29]。微阵列分析表明,wri1-1突变体中表达量下调的基因主要编码脂肪酸生物合成和糖酵解酶。通过酵母双杂交和双分子荧光互补试验验证转录因子WRI1与TCP4蛋白相互作用,负调节脂肪酸的生物合成。其缺失突变体油脂含量较野生型高[30]。
植物在生长发育过程中会遇到盐害、干旱、低温、高温及真菌、病毒、细菌感染等各种不利环境,从而对其造成逆境胁迫。植物体内的激素将发生变化,引发一系列的生理生化改变来适应外界环境。仅仅只有一种激素应对外界环境刺激是不可能的,而是形成相互交织的网络,甚至可能是几种激素共同调节同一过程或共用相同的信号物质。
在植物生长发育过程中,miRNA不仅能够影响根的结构,而且参与非生物胁迫,例如水稻和胡杨的干旱胁迫、白菜的热胁迫以及铜、铝、镉等重金属胁迫[31-33]。有研究表明,当拟南芥的根受到根结线虫(RNK)的侵染后,miRNA的丰度与其靶基因的表达量呈负相关的关系[34]。miR319s及其靶基因参与高盐、干旱等非生物胁迫[35]。植物激素茉莉酸(JA)作为信号分子,能够有效地抵制番茄根结线虫的侵染。茉莉酸通过十八烷类途径在叶片中合成,然而茉莉酸介导的根结线虫的免疫反应是在根中发生的。2015年有研究对根结线虫侵染植物根部鉴定出263个已知的和441个新的差异表达的miRNAs,阐述了miR319/TCP4介导的茉莉酸合成在RNK防御反应中的作用,为深入了解miR319/TCP4参与非生物胁迫调节提供了新见解[36]。
植物的生长发育过程受到寒冷和干旱等环境胁迫的强烈影响。为了应对和适应这些环境变化,植物进化出了各种生理机制应对外界刺激[37]。AP2/ERF、MYB和bZIP参与高等植物响应各种胁迫的生理过程和调控网络。miRNA319介导TCP2、TCP3、TCP4、TCP10、TCP24调控叶片发育、花瓣生长、细胞壁合成和JA合成。对水杨柳进行低温、盐(NaCl)以及干旱(PEG6000)非生物胁迫处理,发现FmTCP4能够感知非生物胁迫,调节下游基因表达,为该基因在水杨柳中调控生长发育以及逆境胁迫响应功能的研究奠定基础,并提供了水杨柳TCP4和不同物种中的同源基因及蛋白多序列比对分析,有助于揭示FmTCP4基因的潜在功能以及它们在介导激素合成和非生物胁迫中的可能作用[38]。
转录因子TCP4作为一种转录因子既能够与功能基因的顺式作用元件结合,还能够与其他转录因子相互作用。TCP4在植物生长发育各个过程中与植物激素信号转导途径中的多种蛋白(如MYB,SAP11)[39-44] 等相互作用,表明TCP4可能在植物抵抗生物和非生物胁迫等方面发挥着重要作用。在植物发育过程中TCP4还具有时空限制的表达模式,这些表达模式提高了TCP4在局部触发或拮抗激素信号传导的可能性。
近十多年来的研究揭示了部分TCP4的功能和分子机制,但这些研究仍处于初级阶段。运用生物化学、遗传学、分子生物学的方法研究揭示转录因子TCP4如何影响植物激素信号转导或其生物合成,以及与植物激素相互作用机制具有重要的意义。TCP4的同源和异源二聚体以及其他蛋白质伴侣的鉴定,并进一步确定这些蛋白复合体的靶基因,将能进一步了解TCP4在植物生长发育和抵抗逆境的遗传途径。此外,随着对TCP4基因功能和调节机制的深入研究,将对植物生长调节和逆境下优良品种的选育有重要指导意义。
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