植物花分生组织终止发育机制的研究进展*

张 科, 郭鑫鑫, 刘西岗, 郭 琳**



植物花分生组织终止发育机制的研究进展*

张 科1†, 郭鑫鑫1,2†, 刘西岗1, 郭 琳1**

(1. 中国科学院遗传与发育生物学研究所农业资源研究中心 石家庄 050022; 2. 中国科学院大学 北京 100049)

植物花的发育依赖于花分生组织(floral meristem, FM)活性的维持与分化。当FM完成各轮花器官原基的起始后, 其活性会程序性地终止(termination), 这个过程就是FM的终止发育过程(FM determinacy)。FM的终止发育是一个复杂精细且多步骤的调控过程。WUSCHEL(WUS)是一个具有同源异型结构域(homeodomain)的转录因子, 其对FM的活性维持及终止发育发挥着重要作用。越来越多的研究表明许多转录因子以及环境信号、激素信号和表观遗传相关因子通过对及其调节基因的调控来影响FM终止发育过程。本研究组首次在组织水平阐明了生长素和细胞分裂素调控FM活性的分子机制: AUXIN RESPONSE FACTOR3(ARF3)能够整合、()和生长素(auxin)的信号通过抑制细胞分裂素(cytokinin)信号系统调控FM终止发育; 还首次证实光信号对FM活性调控的分子机制: FAR-RED ELONGATED HYPOCOTYL3(FHY3)通过()信号途径介导光信号对表达的调控; 在基因水平上首次揭示了染色质构象变化对FM终止发育的调控作用: AGAMOUS(AG)可以直接结合到的调控区并招募PcG蛋白对5′-TSS和3′-CRE的结合, 以介导染色质环状结构(chromatin loop)的形成, 进而抑制的转录, 而DNA TOPOISOMERASE 1 TOP1α (TOP1α)对染色质的高级结构的重塑作用也是该染色质环形成的基础。随着3D基因组(three dimensional genomics)时代的到来为我们进一步理解FM终止发育机制提供了新的窗口, 而FM终止发育机制在农业生产上的运用也体现了其巨大的应用价值。本文首先简述了拟南芥()花发育研究的发展史, 介绍了花发育相关领域所关心的3个科学问题，并着重阐述了FM终止发育调控的研究进展与前景。

花分生组织; 终止发育;; 环境信号; 激素信号; 表观遗传

20世纪80年代初, 随着拟南芥()作为模式植物的优势逐渐被人们认可[1], 人们对植物花发育机制的研究也揭开了序幕。通过对拟南芥诱变筛选, 很多与花器官的决定以及发生位置相关的突变体被陆续发现[2-3]。1991年, Bowman和Meyerowitz[4]博士通过对这些花发育突变体进行遗传分析提出了经典的“ABC模型”: 拟南芥四轮花器官的起始分化(花萼、花瓣、雄蕊和心皮)分别被A()、B(和,/)和C()3组基因所控制; 每一类基因分别在相邻的两轮花器官中表达, 并控制所在轮次花器官的形态建成; 此外A类基因与C类基因间存在着相互拮抗作用[4-5], 这个理论为我们更好地理解花发育及其形态建成提供了基本的模型[6]。

20世纪80年代末, 拟南芥中根癌农杆菌介导的T-DNA对拟南芥进行的遗传转化体系的建立[7]、首个RFLP图谱的绘制[8]以及基于T-DNA插入和基因组图谱的基因克隆方法的建立[9-10]为2000年后分子生物学的迅猛发展和花发育分子调控机制的研究提供了基础。

花发育机制的研究主要围绕着3个科学问题展开: 1)花器官决定及形态建成机制; 2)分生组织的维持与分化机制; 3)花分生组织活性终止发育的调控机制。本文介绍了拟南芥花发育研究领域的3个主要科学问题, 并重点综述了拟南芥FM终止发育过程中所受到不同层次调控的机制, 同时展望该领域的未来研究趋势。

1 花器官决定及形态建成机制

早期研究中大量的参与花器官形态建成的突变体被发现[3], 随着这些基因被陆续的克隆定位, 它们的时空表达模式和分子调控机制也相继被解析。“ABC模型”也逐渐被完善: 除AP2外, 所有A、B和C基因都是MADS转录因子[11];被归为A类基因[12-13], 此外有4个MADS基因()被归为新的一类基因, 即E类基因, 进而发展出了“ABCE模型”[14-15]; A、B和C 3类基因均需要E类基因的参与才能发挥功能;既能够被miRNA调控, 又可以直接负调控参与3轮和4轮花器官形成建成的[16-18]。

遗传学研究同时发现这些同源异型基因除调控花器官的发生外, 还可调控FM的活性, 如不仅特异地调控雄蕊和心皮的决定(organ identity), 还能促进FM活性的终止发育[2,4,19-21];除调控萼片和花瓣的决定外, 还能增强完全缺失突变体的花中花表型, 暗示其也可以调控FM活性[22]。

2 分生组织的维持与分化机制

分生组织活性的维持与分化依赖于源自周围细胞的各种外源信号及内部基因的协同调控。其中()-(构成的反馈调节回路对干细胞活性的维持起着重要作用[23]。WUS是具有同源异型结构域的转录因子, 在顶端分生组织(shoot apical meristem, SAM)和花序分生组织(inflorescence, IM)的组织中心区(organizing center, OC)表达, 并可以通过细胞间迁移扩散到分生组织的前3层细胞中, 即中心区(central zone, CZ)[24-28]。可以促进干细胞的增殖, 维持分生组织干细胞活性。突变体丧失顶端优势, 在萌发1周后由于缺少足够的干细胞仍没有真叶产生[29]; 相反, 异位表达基因可以使体细胞转变成干细胞而发育出新的器官原基, 甚至把根细胞转化成叶原基细胞[30-31], 从而证明WUS具有诱导植物干细胞产生的作用。Rodriguez等[25]证明WUS的空间分布模式受到复杂而精细的调控; WUS蛋白C-端63个氨基酸对其维持正常的空间分布模式是必需的, 其中WUS-box负责WUS蛋白在核内驻留, 而EAR-like参与WUS蛋白的向核外输出的过程。是分生组织干细胞的标记基因, 编码一个多肽作为信号分子扩散到OC区, 并与该区域定位在细胞膜上的CLV1-CLV2所形成的受体结合, 经过信号转导最终抑制的表达[32-33]。WUS对的调控作用依赖于自身的浓度, 即高浓度抑制的转录而低浓度激活的转录[34-35]。Perales等[34]发现, WUS能与调控区内多个顺式作用元件结合, 并且对这些元件有不同的亲合力, 较低浓度的WUS以单体形式与的顺式作用元件结合, 而较高浓度的WUS以二聚体的形式与的顺式作用元件结合。

这样就形成了一个负反馈调节回路, 即的表达能够抑制的表达, 而下调表达又会导致表达的下降从而缓解CLV3对的抑制作用, 通过这样的反馈调节机制调控干细胞增殖和分化的动态平衡[23]。所以在花发育过程中,的作用至关重要, 它的表达必须在时间和空间上受到精确控制。

3 花干细胞活性的终止调节

花发育过程中, 当各轮次花器官发育起始后FM活性将适时地终止, 这个过程就是FM的终止发育过程, 该过程是花发育成熟的保证, 也是植物胚胎发育及世代交替的前提。是维持分生组织干细胞活性的关键基因, 因此植物对于FM活性的调控就主要体现在对表达模式的调控上。在花发育第6期时各轮花器官原基起始完成, 植物会通过不同层次的调控方式来终止的表达, 进而终止FM的活性[21,36-39]。FM的终止发育是一个十分复杂的调控过程, 受到不同因素的影响, 如花发育相关基因、环境信号、激素信号和表观遗传学等。

3.1 花器官决定基因对FM活性的调控

3.1.1 AG信号途径对FM活性的调控

AG可以对表达进行直接调控。在花发育的第3期,在WUS与开花起始因子LEAFY(LFY)协同作用下被激活表达; 第6期时, AG通过关闭表达进而终止FM活性, 其是FM终止发育主要的调控因子之一[21,36]。在的完全缺失突变体中由于没能适时地终止表达而导致严重的FM缺陷, 即形成重复的萼片-花瓣-花瓣的花中花表型[36,40]。

Liu等[39]发现AG能够直接结合到的转录起始位点(TSS), 即5′-TSS, 以及下游非翻译区3′-CRE, 其包含两个CArG元件。AG可能通过在这两个区域招募PcG蛋白TERMINAL FLOWER2 (TFL2)来调控组蛋白甲基化修饰状态, 最终影响的表达; 随后的研究表明拟南芥中DNA拓扑异构酶DNA TOPOISOMERASE 1 TOP1α (TOP1α)可以通过影响位点核小体的组装及密度进而影响AG对于位点的结合[41]。最新的研究证明AG和TFL2可以直接相互作用在花发育时可以促进位点染色质环的形成进而抑制基因表达[42]。

AG还可以对表达进行间接调控。虽然对于表达的关闭是必需的, 但是超表达()并不能产生明显的FM终止缺陷表型, 而的表达也只有轻微的下调[43]; 另一方面, 从第3期的表达被激活到第6期的表达关闭大约需要2 d时间, 这暗示AG对的抑制作用可能还需要其他基因的参与[37]。KNUCKLES(KNU)是一个具有C2H2锌指结构域的转录抑制因子[44]。研究表明介导AG对的抑制调控。AG通过促进基因位点H3K27me3的去甲基化来直接激活的表达, 进而抑制的表达, 这个过程大约需要2 d时间[37,45]。

此外还有许多其他的调控因子通过AG信号途径对表达进行调控LFY协同WUS启动的表达;()编码一个具有SAND结构域的转录因子[46-47], 而()编码一个bZIP转录因子, 两者均可以激活的表达[48-49]。编码具有锌指结构域的YABBY家族的转录因子, 参与协调FM终止发育与心皮发育的过程[50-51]。受到AG的诱导表达并协同KNU介导AG对的抑制作用[37,52-53]。()编码一个RNA结合蛋白, 包含6个串联的CCCH锌指结构域[54]; HUA2是一个转录因子,双重突变能够增强弱突变体的FM终止缺陷, 且的表型与一致, 暗示和均参与信号途径[55]。

除了受到转录水平的调控, 还受到转录后水平的调控。()在植物体中广泛表达, 编码具有RNA切割活性的酶, 与miRNA在组织中的积累有关[56]。miRNA是一类~22 nt的非编码RNA, 由Dicer切割具有发卡结构的转录本产生, 可以特异地介导靶基因mRNA的降解, 从而抑制该基因表达[57]。HEN1可能通过miRNA来介导其对的抑制作用[58]; HEN2作为一个推定的RNA解旋酶, 可能参与对mRNA前体的加工过程; HEN4是含有K同源异型结构域蛋白。表型与相似, 研究表明、和共同参与了对的第2个内含子加工过程[59]。

3.1.2 AP2对FM活性的调控

花器官决定基因中A类基因也参与了FM的终止调控, 并且可以拮抗的功能[4,22]。指cDNA中在与miRNA172匹配的位点处引入了6个错配的碱基, 从而消除miRNA172对表达的抑制作用[17]。在转基因植株中,mRNA丰度大幅度降低; 将其导入会导致花器官数量以及FM活性的增加, 暗示AP2不仅可以负调控的表达, 还可以通过独立于信号途径调控FM活性[22], 而有证据表明AP2对信号途径起负调控作用[60]。最近的研究表明,是AP2的靶基因, 其编码一个生长素反应因子, 介导了AP2和AG对的调控作用[61]。

3.1.3 SUP对FM活性的调控

SUPERMAN(SUP)包含一个C2H2锌指结构域且在C-端含有EAR样基序, 可以界定3轮和4轮花器官的边界, 对于雄蕊的发育具有重要作用[62-65]。中雄蕊数目增多,的表达被延长, 而且与的表型一致, 暗示可通过来调控FM的终止发育[64,66]。还能够增强的FM缺陷表型,在心皮位置产生更多轮次的花瓣, 意味着对FM活性的调控独立于信号途径[62]。分子机制的研究表明, SUP可以抑制生长素的合成, 在突变体中第3和第4轮花器官原基部位有过量的生长素合成, 导致产生额外的雄蕊原基, 而外施生长素抑制剂PCIP可以部分互补突变体多雄蕊表型[65]。

3.2 环境信号对FM活性的调控

植物的生长发育需适应自身所处环境, 而分生组织活性的维持与终止是重要的发育过程, 光和温度等环境信号对这一过程有重要的调控作用[67-68]。

3.2.1 光信号对FM活性的调控

对于植物来说, 光不仅是一种能源, 也是调节其生长发育的重要的环境信号。光信号能够通过影响植物激素, 如生长素和细胞分裂素, 调控干细胞的活性。黑暗处理能改变分生组织中PIN1的亚细胞定位和生长素的浓度分布, 而生长素与细胞分裂素互作调控了分生组织活性[69]。拟南芥中,是重要的信号途径的成员, 其编码一个转录因子并拥有特异的结合位点(CACGCGC)[70-71]。Li等[67]发现, FHY3对FM的终止发育和SAM活性的维持均具有重要作用; 在SAM中, FHY3介导光信号对的抑制作用, 进而调控的表达; 在FM中, FHY3可以直接抑制而激活来促进FM的终止发育。

3.2.2 温度对FM活性的调控

温度对花发育也有着重要影响, 但是其作用机制并不十分清楚。最近, Conn等[72]发现mRNA存在着不同的剪接形式, 其中SEP3.3/SEP3相对于野生型缺少第6个外显子, 即161~174碱基。有趣的是SEP3.3/SEP3受到温度诱导, 在低温条件下SEP3.3/ SEP3的丰度升高。作为MADS结构域的转录因子, SEP3可以形成四聚体, 其K结构域负责两个二聚体间的互作, 以形成同源四聚体或异源四聚体, 而SEP3.3/SEP3中碱基的缺失使K结构域的功能丧失, 导致SEP3.3只能以二聚体的形式发挥功能[68,72]。SEP3:SEP3只能恢复的花器官决定缺陷而无法恢复其FM终止缺陷; 这是由于SEP3.3无法与AG形成的异源四聚体, 导致无法有效调控下游基因的表达, 如CRC和KNU, 最终导致FM活性不能适时终止[68]。

3.3 激素对FM活性的调控

植物激素对于植物的花发育过程同样具有重要的调节作用。例如赤霉素(gibberellin, GA)能够通过拮抗DELLA蛋白而促进花发育相关的同源异型基因表达, 如、和, 以保证正常的花形态建成[73]。

3.3.1 生长素对FM活性的调控

生长素在分生组织的维持与分化的调节过程中发挥着重要作用。在SAM中, 生长素能够促进器官原基的分化与生长[74], 报告基因显示在器官原基起始部位生长素活性最强[75]。在花发育过程中,通过对生长素合成的精细调控, 协调了3轮和4轮花器官的发育以及FM的活性, 其通过与PcG蛋白互作抑制和的表达进而影响生长素在雄蕊-心皮边界上的积累[65]。此外第6期时, 生长素还介导了对FM活性的抑制以及对心皮发育的促进作用, CRC通过抑制膜蛋白基因()的转录影响生长素的运输过程, 协调FM的终止和心皮的分化过程[57]。

ARF3是生长素响应因子, 受生长素的诱导表达[76-77]。Liu等[61]发现, ARF3可以促进FM的终止发育;突变显著增强的FM终止缺陷; ARF3不仅可以直接结合到的调控区, 还介导AG对的抑制作用; AP2也通过直接抑制的表达调控FM的活性, 所以能够整合花发育相关基因和的信号调控FM的终止发育。

3.3.2 细胞分裂素对FM活性的调控

细胞分裂素对维持细胞增殖和分生组织活性都起着至关重要的作用[78-82]。报告基因显示, 在SAM的OC区和FM的中心区域细胞分裂素活性最强, 这与和细胞分裂素受体基因()的表达区域一致[77,83]。Zhang等[77]发现, 用细胞分裂素类似物6-BA外源处理拟南芥花序分生组织能显著增强的FM终止缺陷, 说明细胞分裂素也参与了FM的终止调控; ARF3通过直接抑制细胞分裂素的合成基因()以及受体基因, 而间接抑制()的表达促进FM的终止发育; ARF3还介导了生长素和信号通路对细胞分裂素信号系统的抑制作用; 原位杂交显示细胞分裂素能够延迟FM中的关闭时间。此外细胞分裂素能通过维持WUS的稳定性, 来保持WUS正常的空间分布[35]。

3.4 表观遗传对FM活性的调控

2000年以来, 人们逐渐意识到表观遗传学对生长发育的重要性[84-86], 而随着高通量测序技术等多种实验技术的突破, 使表观遗传学研究迅速成为一个生物学新兴领域[87]。表观遗传学的调控机制涉及多个层面: 组蛋白修饰(histone modification)、染色质重塑(chromatin remodeling)、非编码RNAs (noncoding RNAs, ncRNA)以及DNA甲基化(DNA methylation)等, 而越来越多的证据表明表观遗传学对FM终止发育有着重要的调控作用[88-89]。

3.4.1 组蛋白的修饰

核小体是染色质的基本单元, 它由4种组蛋白(H3、H4、H2A和H2B)和缠绕其上的约146 bp的DNA组成[88]。组蛋白的氨基酸残基存在着各种修饰, 如甲基化、乙酰化、磷酸化以及泛素化等。这些修饰的改变能够影响染色质的高级结构, 进而影响基因的表达[90-92]。

核小体H3第27位赖氨酸的3甲基化, 即H3K27m3, 是一类重要的起负调控作用的组蛋白修饰, 该修饰的建立与维持依赖于PcG(polycomb group)蛋白复合体的调控, 其包括PRC2(polycomb repressive complex 2)和PRC1[93], 而TrxG(trithorax group)具有甲基转移酶的活性, 可调控H3K27m3的甲基化。是PcG的靶基因, 其基因上的组蛋白存在着H3K27m3修饰; CURLY LEAF(CLF)是PCR2的成员之一,中的表达升高[94-98]。ULT1包含SAND结构域, 具有trxG的功能, 其能够独立于而促进的表达[47,99]。

的基因位点也存在着高水平的H3K27m3, 其可以介导对的抑制作用, AG能直接结合到的启动子区且结合位点与PcG蛋白重合, 其通过促进基因位点的去甲基化激活该基因的表达[100]。

同样受到PcG蛋白的调控,能剧烈地增强的FM的终止缺陷表型; 在及其同源基因()组成的双突中基因位点的H3K27m3水平显著下降[39]; 此外PRC1的成员的突变也能增强的缺陷, 该基因能结合到的调控区, 即5-TSS和3-CRE, 与AG的结合位点重合, 而且TFL2对的结合依赖于AG, 因此AG可能通过募集PcG蛋白引起基因位点H3K27m3状态的变化来实现对表达的调控[39]。

3.4.2 染色质重塑

染色质重塑涉及染色质包装状态的改变及核小体组装与去组装过程, 这需要一系列ATP依赖的酶参与, 它们可以通过打乱原有的组蛋白与DNA间的联系, 进而达到重塑染色质结构的目的[101-102]。SPLAYED (SYD)属于SWITCH/SNF家族, 能直接促进的表达[103]。突变体中的表达下降, 且存在FM终止缺陷[103-104]。此外, 研究表明SYD和BRAHMA (BRM)还介导了LFY对的激活作用[105]。

FASCIATA1(FAS1)和FAS2是CAF-1复合体(chromatin assembly factor1complex)的亚基, 负责核小体的装配。在和突变体中,被异位表达在CZ区且SAM形态异常[106]。TOP1α/ MGOUN1(MGO1)编码Ⅰ型DNA拓扑异构酶, 其参与染色质重塑、DNA复制和修复等过程[107]。能增强以及和的FM缺陷表型, 暗示其对FM活性的调控作用; 研究表明TOP1α可以通过影响核小体密度来促进AG和PcG蛋白与的结合[41,108]。

3.4.3 非编码RNAs

ncRNA作为一种不编码蛋白的RNA, 主要参与转录后水平的调控, 其主要包括两类: 一类为long ncRNA, 其长度大于200 bp; 另一类为short RNA, 包括micro RNA(miRNA)和small interfering RNA (siRNA), 其长度为20~24 bp[109]。

如上文提到的miR172可能通过阻止的转录或对mRNA切割来抑制其表达。miR172与mRNA的3′端附近区域高度互补[110], 其主要分布在花的3、4轮, 使只能表达在外侧两轮[17]。

POWERDRESS(PWR)含有一个非典型的SANT结构域, 它既可以激活的表达, 也可以通过促进miRNA172的积累来调控FM活性。和中miRNA172水平明显下降; 此外Yumul等[111]证明,只对5个的成员中的3个(,和)有促进作用。

ARGONAUTE(AGO)家族有10个成员, 是RNAi的效应因子[112]。AGO可以结合miRNA并具有切割活性[113-116]。在植物中, AGO1分布广泛, 而AGO10则主要分布在分生组织、维管束和器官原基的近轴端[117]。和可能通过抑制信号途径参与FM发育调控, 两者均对miRNA172所介导的RNA沉默过程有调节作用[116]。

3.4.4 DNA甲基化

植物中DNA甲基化发生在富含CG、CHG和CHH (H包括A、T或C)的C上, 是重要的表观遗传的修饰方式, 其可以改变染色质的结构、DNA的构象及稳定性[118]。DNA的甲基化需要甲基转移酶, 即DOMAINS REARRANGED METHYLTRANSFERASE2(DRM2), 而甲基化状态的维持则需DNA METHYLTRANSFE- RASE1(MET1)和CHROMOMETHYLASE3(CMT3)的作用[117,119-120]。的表达受到DNA甲基化的调控;的表型与一致, 就是由于基因位点DNA的高度甲基化程度升高, 使其表达下降所致[62-63]。

4 农业资源研究中心在相关方面的研究进展

多年来, 中国科学院遗传与发育生物学研究所农业资源研究中心刘西岗研究组一直致力于对拟南芥FM终止发育分子机制的解析。该组利用分子及细胞生物学、遗传学和生化分析结合生物信息学, 系统研究了不同层次因素对FM终止发育的调控机制, 并取得多项创新突破及研究成果。本研究组发现, ARF3能够整合、和生长素的信号通过直接抑制以及表达, 而间接地抑制来促进FM的终止发育[61,77]。FHY3可以通过介导光信号直接抑制的表达, 而激活的转录来调控FM的活性[67]。AG能够直接结合到的调控区, 并可以招募PcG蛋白TFL2对的结合, 进而导致在这两个调控区域形成染色杂环[39,42]; 而TOP1α可以通过调控染色质的高级结构来促进AG和PcG蛋白与WUS的结合[41]。在未来的研究中, 本研究组试图在3D基因组水平上探究拟南芥FM终止发育的机制, 并偿试解析小麦穗发育的分子机制, 探索小麦增质增产的有效途径。

5 展望

5.1 染色质间的互作对FM活性的调控

随着染色质精细结构的解析[121-123], 其复杂的空间结构对基因功能的影响逐渐引起了人们的兴趣, 随后用以检测两个特异位点染色质互作状况的3C技术便应运而生[124-125]。Guo等[42]利用3C技术证明,的5′-TSS和3′-CRE两个调控区域能够相互作用, 从而形成一个染色质环状结构抑制的转录; AG可以通过招募TFL2结合5′-TSS和3′-CRE区域, 并促进染色质环状结构的形成, 进而调控FM的终止发育。

伴随着高通量测序技术以及生物信息分析技术的发展, 由3C技术发展而来的4C(circular chromosome conformation capture)可以检测某一特定位点与全基因组染色质的互作情况; 5C(3C-carbon copy)可以检测多个位点与多个位点间的染色质互作, 两者可以用以检测染色质之间长距离互作[126-127]; 而Hi-C(high-throughput/resolution chromosome conformation capture)和ChIA-PET (chromatin interaction analysis by paired-end tag sequencing)技术则能以de novo的方式在全基因组范围内研究染色质构象以及蛋白与染色质互作的关系[128-129], 这些技术的发展为进一步在3D基因组水平研究FM终止发育机制提供了基础, 我们必定会对这一发育过程有更深刻的了解。

5.2 作物中FM终止发育机制的研究

在作物生长过程中, FM的产生及维持是作物花器官生成及发育的前提, 而FM活性的程序性终止也是后续的生殖生长及世代交替的保证, 更是多数作物产量的保证。

在玉米()中,反馈调节回路对维持分生组织活性有重要作用。Je等[130]报道,()编码一个具有富亮氨酸重复序列的受体蛋白, 可以响应CLV3/ESR-related (CLE)多肽分子。FEA3主要表达在SAM的OC区,的SAM增大且呈扁平状。虽不能拮抗CLV3的作用, 但对Zm FON2-LIKE CLE PROTEIN 1 (ZmFCP1)不敏感, 而ZmFCP1与水稻()中发现的CLV3类似物FCP1同源[130-131]。此外含有等位基因的杂交种, 如和, 表现出更优质的农艺性状, 如其拥有增加的穗长和穗重以及更多的行数和行粒数[130]。

番茄(,)是重要的蔬菜作物, 经过多年的驯化人们逐渐选择了具有更多花序分枝和能结出更大果实的品种[132]。番茄中SAM活性的维持同样依赖于反馈回路,途径性状相关的突变体表现为多花序分枝、多花、花器官数目增多, 果实变大等[133]。Xu等[134]通过诱变筛选发现()的突变导致过渡分生组织(transition meristem, TM)显著增大;是的同源基因(Solyc04g081590); 同时发现(, Solyc11g064850)编码一种羟脯氨酸O-阿拉伯糖基转移酶(hydroxyproline O-arabinosyltransferase, HPAT), 而编码一种阿拉伯糖基转移酶(arabinosyltransferase), 属于GT77家族成员,和的突变体均表现为TM增大的表型, 此外通过CRISPR/Cas9技术突变同属于GT77家族的阿拉伯糖基转移酶基因的(, Solyc04g080080), 也能产生与和相似的表型; 这些发现暗示CLV3的阿拉伯糖基化对调节信号途径有着重要作用，而生理实验证明阿拉伯糖基化的SICLV3 ([Ara3]SICLV3)确实能够恢复的突变表型。

在番茄中, 相对于祖先种()的小的两腔室的果实, 驯化种果实腔室数目以及果实尺寸的增加, 这主要是由于()和()两个位点贡献的[135]。其中在基因下游推定的3′-CRE区域CArG元件处存在两SNP位点[39]。在近等基因系S.pim-lc中11%的果实发育出3~4个腔室; 而利用CRISPR/ Cas9技术使CArG元件缺失4个碱基, 即S.pim-lc, 同样可以导致10%的果实发育出3个腔室。位点在基因中存在着一个294 Kb的颠倒变异, 因而导致3~4腔室果实的比例接近43%, 而S.pim-lcfas和S.lyc-lc果实中3~4腔室的比例可高达70%以上[136]。Rodriguez-Leal等[136]还利用CRISPR/Cas9技术在的启动子区快速地产生多个等位突变, 从而得到控制果实腔室数这一数量性状的多个变异位点, 为作物育种提供了新思路。由此, 基于对作物中干细胞及分生组织活性维持的研究结合基因编辑等现代分子生物学技术将极大促进作物高产稳产的机理及应用的研究。

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Advances in research on floral meristem determinacy mechanisms in plants*

ZHANG Ke1†, GUO Xinxin1,2†, LIU Xigang1, GUO Lin1**

(1. Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Shijiazhuang 050022, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China)

In higher plants, plant tissues and organs are generated from meristems. Shoot apical meristem (SAM) gives rise to all of the aboveground parts for the entire life of plant through continuous production of new organ primordial, including floral meristem (FM) which finally develops as flowers. Floral development is based on the balance between FM meristem maintenance and termination. At the initial stage, floral stem cells proliferate and produce defined number of floral organs based on the “ABC model” rules. At this stage, FM activity is maintained mainly by()-() feedback loop.encodes a homeodomain containing protein. It promotes stem cell marker geneexpression whenexpression is low. It also inhibitsexpression whenexpression is high. Thus FM activity is maintained and can promote initiation of floral organs. However, after two carpels primordia initiation, FM activity is terminated in a process called FM determinacy. FM determinacy is a dynamic and multi-step process in whichplays a central role.expression is regulated by many transcription factors related to floral organ identity [(),() and], environmental signals (light, temperature, etc.), plant hormones (auxin, cytokinin, gibberellin, etc.) and epigenetic-related factors (histone modification, chromatin remodeling, non-coding RNA, DNA methylation, etc.). Using model plant, our study noted that AG terminates FM maintenance by directly repressingthroughchromatin higher structure (chromatin loop), formed by AG and one of Polycomb Group components TERMINAL FLOWER2/LIKE HETEROCHROMATIN PROTEIN1 (TFL2/LHP1); binding to5′TSS (transcription start site) and3′CRE(cis-regulatory element). DNA TOPOISOMERASE 1 (TOP1α) inhibitedexpression by modulatingnucleosome density to inhibit DNA accessibility, which also participated in the progress. AUXIN RESPONSE FACTOR3 (ARF3) induced by auxin regulated FM determinacy by repressing cytokinin biosynthesis [inhibiting cytokinin synthesis genes() and()] and signaling [inhibiting cytokinin receptor gene()], which clarified how auxin and cytokinin integrated to regulate FM activity; FAR-RED ELONGATED HYPOCOTYL 3(FHY3) activated, but inhibitedto regulate meristem determinacy and maintenance, which shed light on how light affected meristem activity. As 3D (3-dimentional) genome organization technology developed, the importance of the impact of chromatin structure on gene expression was realized and more techniques were developed and improved. Using the newly reported methods, FM determinacy mechanism required further in-depth studies. What was more was that since plant FM determinacy was regulated precisely and accurately, any defects in FM determinacy affected seed development. Exploitation of FM determinacy mechanism had the potential to importantly contribute to agricultural production, which was helpful for ensuring reproductive success, seed development and yield of agricultural crops (maize, tomato, etc.). In this review, we gave a short introduction on floral organ identity inand the mechanism of meristem maintenance and differentiation. Then we mainly focused on FM determinacy, including some recent studies by our group. Finally, we advanced the application of fundamental studies in crop yields and further prospects for research.

Floral meristem; Determinacy;; Environmental signal; Plant hormone; Epigenetics

Supported by the National Natural Science Foundation of China (31701423), the Training Program of Key Direction of Hebei Province (ZZKT201601) and the State Key Laboratory of Plant Cell and Chromosome Engineering, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences (PCCE-KF-2018-04)

Corresponding author, E-mail: guolin@sjziam.ac.cn

Jul. 3, 2018;

Jul. 16, 2018

10.13930/j.cnki.cjea.180653

Q756

A

1671-3990(2018)10-1573-12

通信作者:郭琳, 主要研究方向为植物遗传与表观遗传学。E-mail: guolin@sjziam.ac.cn

†同等贡献者: 张科, 主要研究方向为植物遗传发育与作物栽培, E-mail: zhangke0126@163.com; 郭鑫鑫, 主要研究方向为植物遗传学, E-mail: guoxinxin16@mails.ucas.ac.cn

2018-07-03

2018-07-16

† Equal contributors

*国家自然科学基金项目(31701423)、河北省重点方向培育项目(ZZKT201601)和中国科学院遗传与发育生物学研究所植物细胞与染色体工程国家重点实验室项目(PCCE-KF-2018-04)资助

张科, 郭鑫鑫, 刘西岗, 郭琳. 植物花分生组织终止发育机制的研究进展[J]. 中国生态农业学报, 2018, 26(10): 1573-1584

ZHANG K, GUO X X, LIU X G, GUO L. Advances in research on floral meristem determinacy mechanisms in plants[J]. Chinese Journal of Eco-Agriculture, 2018, 26(10): 1573-1584