梨果实石细胞木质素合成调控机制研究进展

王红宝,王永博,王 晋*,李 勇,李 晓,王迎涛,王亚茹*

(河北省农林科学院石家庄果树研究所,石家庄 050061)

摘 要: 石细胞是由木质素沉积形成的厚壁组织细胞,是制约梨果实品质提升的重要因素。梨果肉石细胞的形成与木质素的生物合成、转移和沉积密切相关。因此,探究梨果实发育过程中木质素合成调控机制对调节石细胞形成及果实品质改良具有重要意义。总结近年来与梨果实木质素合成调控机制有关的研究,对转录因子、激素、糖、钙、活性氧、光质及花粉直感等调控因子在梨果实木质素合成中的作用机制进行综述,旨在为梨果实木质素的调控网络深入研究及品质改良提供参考。

关键词:梨;石细胞;木质素;调控机制

梨(Pyrus)是中国乃至世界范围内重要的水果之一,爽脆可口,香甜多汁,深受消费者的喜欢[1]。梨果实品质受多种因素的综合影响,其中一类木质化的厚壁组织细胞——石细胞便是重要的因素之一[2]。石细胞含量过高会导致果肉粗糙、口感低劣、硬度增加,同时也会影响梨果实糖分、有机酸、维生素含量等品质指标[3]。石细胞由薄壁组织细胞在其初生壁上沉积木质素等而形成。木质素在细胞壁外层开始发生,逐步向质膜发展,以分散颗粒状沿纤维素微纤丝的方向分层沉积,交替排列,直到充满整个细胞腔,形成石细胞[4]。木质素由多个木质素单体聚合而成,其生物合成过程由苯丙氨酸起始,经过一系列羟基化、甲基化、连接和还原反应生成木质素单体,木质素单体被运送到外质体氧化聚合形成愈创木基木质素(Guaiacyl lignin,G-型木质素)、对-羟基苯基木质素(Hydroxy-phenyl lignin,H-木质素)和丁香基木质素(Syringyl lignin,S-木质素)[5]。梨果实石细胞木质素属于G-S 木质素,充分发育的梨石细胞含有约40%的木质素[6]。因此,调控木质素的积累是降低梨果实石细胞含量的关键。

目前,梨果实木质素生物合成的调控机制不断被解析,其合成调控过程受到信号途径、环境因素等多重因子的调控,要实现从分子层面抑制木质素的合成需要更为完整的理论支撑。为此,笔者围绕近些年梨果实木质素生物合成调控机制研究(转录因子、激素、糖、钙、活性氧、光质、花粉直感)展开综述,旨在为深入研究梨果实木质素合成调控网络奠定理论基础,为通过生物技术手段进行梨果实品质的遗传改良提供参考。

1 梨果实木质素合成相关的酶基因

木质素的生物合成由PAL、C4H、4CL、HCT、C3H、CSE、COMT、CCoAOMT、CCR 等酶催化完成(表1)[7-17]。其中PAL和4CL是调控木质素含量的关键酶,C3H、CCoAOMT和COMT是木质素特异途径中的关键酶,CCR、CAD 和POD 是木质素单体合成及聚合的关键酶。PAL是木质素代谢途径中的关键酶和限速酶,位于苯丙烷代谢途径的入口[18]。在梨果实的不同发育阶段,PbPAL1PbPAL2 表现出与木质素含量一致的变化趋势,并与木质素生物合成的关键基因表达模式相似,在拟南芥中过表达结果显示随着木质素含量的增加,维管间纤维和木质部细胞的细胞壁增厚[7]。作为苯丙烷代谢途径中的第2 步关键酶C4H,PbC4H1PbC4H2PbC4H3 不仅可以增加维管间纤维和木质部细胞的木质素含量,还可以增加细胞壁厚度[8,19]

表1 参与梨果实木质素生物合成途径的关键酶基因
Table 1 Key enzyme genes involved in the lignin biosynthesis pathway of pear fruit

酶Enzyme苯丙氨酸解氨酶Phenylalanine ammonia lyase肉桂酸-4-羟基化酶Cinnamate 4-hydroxylase缩写Abbreviation PAL基因名称Gene name PbPAL1,PbPAL2参考文献Reference[7]C4H PbC4H1,PbC4H2,PbC4H3[8]4-香豆酸辅酶a连接酶4-coumaric acid:coenzyme a ligase羟基肉桂酰基转移酶Hydroxycinnamoyl transferase,HCT 4CL Pb4CL1,Pb4CL3[1]HCT [9]咖啡酰莽草酸酯酶Caffeoyl shikimate esterase咖啡酸-5-羟基阿魏酸-O-甲基转移酶Catechol-O-methyltransferase咖啡酰辅酶a-O-甲基转移酶Caffeoyl-coa O-methyltransferase CSE PbHCT49,PbHCT50,PbHCT2,PbHCT17,PbHCT18,PbHCT85 PbCSE1功能描述Function description促进木质素生物合成Promote lignin biosynthesis增加木质部细胞的细胞壁厚度,促进木质素生物合成Ⅰncrease the thickness of xylem cell wall and promote lignin biosynthesis调控木质素的生物合成Regulation of lignin biosynthesis参与木质素合成和石细胞形成的重要候选基因Ⅰmportant candidate genes involved in lignin synthesis and stone cell formation调控木质素的生物合成Regulation of lignin biosynthesis[10]COMT Pbr013510.1[11]CCoAOMT PbCCOMT1,PbCCOMT3[12]肉桂酰辅酶a还原酶Cinnamoyl coa reductase肉桂醇脱氢酶Cinnamyl alcohol dehydrogenase CCR CAD促进木质素代谢相关结构基因表达,促进木质素生物合成Promote the expression of structural genes related to lignin metabolism and promote lignin biosynthesis调控木质素的生物合成Regulation of lignin biosynthesis调控木质素的生物合成Regulation of lignin biosynthesis[13][14][5,15]漆酶Laccases LAC PbCCR1,PbCCR2,PbCCR3 PpCAD1,PpCAD2,PbrCAD20,PbrCAD27,PbrCAD31 PbLAC1[16]过氧化物酶Peroxidases POD PpPOD2,PpPOD3,PpPOD4增加木质部细胞的细胞壁厚度,促进木质素生物合成Ⅰncrease the thickness of xylem cell wall and promote lignin biosynthesis调控木质素的生物合成,为石细胞合成的关键基因Ⅰt regulates lignin biosynthesis and is a key gene in stone cell synthesis[17]

在砀山酥梨基因组中鉴定到29 个4CL 基因和82个HCT基因,其中Pb4CL1Pb4CL3可能参与了梨果实木质素合成,PbHCT49PbHCT50PbHCT2PbHCT17PbHCT18PbHCT85的基因表达与梨果实石细胞表达模式相符,被鉴定为参与石细胞形成的重要候选基因,4CL活性抑制和HCT基因转录末端序列的缺失都会导致木质素含量降低[1,9]。Xu等[10]认为PbCSE1与木质素沉积和石细胞形成有关,过表达PbCSE1 可提高梨果实木质素含量,并且参与木质素生物合成的相关基因的表达量也有所增加。为鉴定梨OMT 基因家族,Cheng 等[12]对26 个OMT 基因进行时空表达分析,其中PbCCOMT1PbCCOMT3 的启动子区域都含有木质素合成的调控基序,并且在梨果实发育过程中PbCCOMT1PbCCOMT3 的转录水平与石细胞和木质素含量呈正相关。COMT主要参与S-木质素的合成,催化G-木质素转化为S-木质素,20世纪梨中COMT基因与木质素含量呈正相关,与石细胞形成有很大相关性[11]。

Cheng 等[13]从梨基因组中挖掘出31 个CCR 基因和26 个CAD 基因,构建了CCRs 和CADs 的系统发育树,通过实时定量聚合酶链式反应(qRT-PCR)明确了PbCAD2PbCCR1PbCCR2PbCCR3参与了梨果实中木质素的合成[14]。其中PbCCR1在砀山酥梨木质素的生物合成中起着积极的作用,过表达PpCAD2 具有更高的木质素含量和CAD 酶活性,CAD活性下降可以改变木质素结构,而对木质素总量没有显著影响[5]。此外,研究发现,PbrCAD1PbrCAD20PbrCAD27PbrCAD31 的表达模式与梨果实发育过程中石细胞含量的变化相似[15]。在Wangkumbae 梨发育过程中PpPOD1 与木质素变化趋势一致,PpPOD2PpPOD3PpPOD4 等基因可能是梨果实中石细胞合成的关键基因[17]。将PyrmiR1890的两个靶基因PbLAC1PbLAC14进行遗传转化,过表达PbLAC1 可以显著增加木质素含量和木质部细胞的细胞壁厚度,参与了木质素的生物合成[16]

2 调控梨果实木质素合成的转录因子

在石细胞形成过程中,木质素生物合成基因形成了一个复杂的网络,转录因子的调节作用在其中发挥着重要作用,如MYB(v-myb avian myeloblastosis viral oncogene homolog)、NAC(NAM、ATAF 和CUC)、bZⅠP(basic region-leucine zipper)、KNOX(Knotted1-like homebox)和锌指蛋白(zinc-finger)等。其中MYB 转录因子在木质素生物合成共表达网络中数量更多,并且与木质素通路结构基因的连接更显著(表2)[20-32]

表2 调控梨果实木质素合成的转录因子
Table 2 Transcription factors controlling lignin synthesis in pear fruit

类型Species MYB转录因子MYB transcription factor名称Name PbrMYB24参考文献Reference[20]PbrMYB169[21]PbMYB140[22]PbMYB61[23]PbMYB308[23]BZR转录因子BZR transcription factor PbBZR1[24]NAC转录因子NAC transcription factor PbrNSC [25]bZⅠP转录因子bZⅠP transcription factor PbbZIP48[26]KNOX转录因子KNOX transcription factor PbKNOX1[27]PHD-finger基因PHD-finger geneⅠDD基因ⅠDD gene TCP转录因子TCP transcription factor miRNA PbPHD10[28]PbIDD3 PbIDD5 PbTCP14 PbTCP15 PbrmiR397a[29][30][31]未知家族Unknown family PbrSTONE功能描述Function description结合顺式作用元件(AC-Ⅰ,AC-Ⅱ和MBS),激活木质素和纤维素合成基因的表达。Binding cis-acting elements (AC-Ⅰ,AC-Ⅱ, and MBS) activates the expression of lignin and cellulose synthesis genes.激活木质素代谢相关结构基因,促进木质素沉积。Activate structural genes related to lignin metabolism and promote lignin deposition.激活PbC3H和PbCCoAOMT启动子活性,促进木质素的生物合成。Activate the promoter activity of PbC3H and PbCCoAOMT to promote the biosynthesis of lignin.促进木质素的生物合成。Promote lignin biosynthesis.结合PbLAC1启动子中的AC元件,调控木质素的生物合成。Binding AC element in PbLAC1 promoter regulates lignin biosynthesis.抑制PbCOMT3和phbhct6启动子活性,并且在梨果实中短暂沉默PbBZR1增加木质素含量。Ⅰnhibition of PbCOMT3 and phbhct6 promoter activity and temporary silencing of PbBZR1 in pear fruit increased lignin content.激活PbrMYB169、Pbr4CL4和PbrLAC4转录,促进木质素和纤维素沉积。Activation of PbrMYB169,Pbr4CL4 and PbrLAC4 transcription,promote lignin and cellulose deposition.激活PbC3H1和PbCCOMT2转录,促进果核附近薄壁细胞的木质化和木质素的生物合成。Activation of PbC3H1 and PbCCOMT2 transcription promoted lignification of parenchyma cells near the fruit nucleus and lignin biosynthesis.抑制木质素合成中涉及的关键结构基因的转录,抑制木质素的生物合成。Ⅰnhibit the transcription of key structural genes involved in lignin synthesis and inhibit lignin biosynthesis.调控梨果实木质素合成。Regulation of lignin synthesis in pear fruit.参与木质素代谢和次生细胞壁的形成。Ⅰnvolved in lignin metabolism and secondary cell wall formation.参与石细胞形成过程中次生细胞壁的增厚。Ⅰnvolved in the thickening of secondary cell wall during stone cell formation.降低漆酶基因表达和木质素含量。Decreased laccase gene expression and lignin content.与PbrC3H互作关系协同调控木质素合成。The interaction with PbrC3H regulates lignin synthesis synergistically.[32]

2.1 MYB

在梨中已鉴定出多个与木质素生物合成相关的MYB 转录因子。通过qRT-PCR 分析PbMYB 的表达,鉴定出PbMYB25PbMYB52 参与梨果实发育过程中木质素生物合成的调控[33]R2R3-MYB 转录因子作为MYB转录因子中最大的一类,在梨基因组中已鉴定出104 个,其中有28 个被认为是参与木质素生物合成途径的调控因子[34]。近期研究发现,在梨果肉愈伤组织和拟南芥花序茎中过表达PbrMYB24,可显著提高木质素、纤维素含量,增厚次生细胞细胞壁。双荧光素酶试验、酵母单杂交试验和EMSA 表明,PbrMYB24 通过结合不同的顺式作用元件(AC-Ⅰ,AC-Ⅱ和MBS)直接激活木质素和纤维素合成基因的表达。此外,PbrMYB24 可以调控PbrNSCPbrMYB169 的表达,同时也能被PbrNSCPbrMYB169所激活,形成的层级调控网络协同调控石细胞中木质素和纤维素合成[20]

PbrMYB169 是木质素生物合成激活型转录因子,可通过选择性识别启动子中的AC-原件(ACCTAACC)结合,显著激活木质素代谢相关8 个结 构 基 因(C3H1CCR1CCOMT2CAD4CL14CL2HCT2LAC18)启动子活性,引起木质素沉积。在转基因拟南芥中,过表达PbrMYB169可有效增强木质素基因的表达,促进木质素沉积和增加细胞壁厚度,但未改变丁香基和愈创木酰基木质素单体的比例[21]。这与PbBZR1 相反,PbBZR1 抑制木质素生物合成基因PbCOMT3phbhct6 启动子的活性,并且在梨果实中短暂沉默PbBZR1 增加木质素含量,诱导15个木质素生物合成基因的表达[24]。宋林艳[22]发现PbMYB140受上游调控因子PbMYB46和PbMYB83的激活调控,PbMYB83也可间接促进木质素生物合成。

参与次生细胞壁合成和木质素生物合成调节的MYB 转录因子是高度保守的。通过瞬时过表达和基因沉默以及稳定转化苹果愈伤组织,发现Pb-MYB61 可以促进木质素的生物合成,PbMYB308 具有相反的作用。进一步的试验结果表明,PbMYB61通过与PbLAC1 启动子中的AC 元件结合上调表达来调节石细胞中木质素的形成。然而,PbMYB308通过与PbMYB61结合形成不能激活PbLAC1表达的二聚体来负调节石细胞木质素合成。由此阐明了PbMYB308-PbMYB61-PbLAC1模块参与梨石细胞中木质素的生物合成的分子机制[23]

2.2 NAC、bZIP、KNOX

Wang 等[25]通过整合木质素纤维素共表达网络和梨果实石细胞eQTL图谱,在206个梨品种中发现了与AtNST1/2/3 相关的NAC 转录因子,鉴定出一个NAC 候选转录因子PbrNSC(NAC stone cell promoting factor)。研究证实PbrNSC 可以通过激活PbrMYB169Pbr4CL4PbrLAC4 启动子发挥调控功能,促进梨果实石细胞形成和木质素沉积。研究还发现PbrNSC蛋白C端存在两个保守的LP和WQ结构域,突变任何一个保守结构域都会影响PbrNSC转录因子对下游基因的转录调控作用,从而影响PbrNSC 转录因子正向调控次生细胞壁合成的生物学功能。Gong等[26]采用转录组学、蛋白质组学和代谢组学等手段,构建了梨果实内石细胞与维管束的分布规律模型。通过共表达网络和基因表达模式分析,发现一个关键转录因子PbbZIP48,该转录因子基因在果核附近高度表达,通过激活PbC3H1PbCCOMT2的转录,促进了果核附近薄壁细胞的木质化,导致石细胞的大量形成。同时研究显示,PbbZIP48 通过与PbC3H1PbCCOMT2 启动子区域的G-box(ACGTG)基序结合促进两者表达量上调。KNOX 基因家族中的BP(brevipedicellus,BP)在木质素代谢和细胞壁发育中起负调控作用。Cheng 等[27]在梨基因组中鉴定出18 个非重复的KNOX 基因,主要分布在STM-likeBP-likeKNAT2/6-likeKNAT7-likeKNAT3-5-like 亚家族中。系统发育树聚类和序列比对表明,PbKNOX1 与拟南芥BP基因是梨同源基因。同时研究发现PbKNOX1作为木质素代谢的负调节因子,其过表达不仅使导管细胞次生细胞壁厚度降低了约19%,木质素含量降低约13%,还可以下调木质素代谢途径中多个关键结构基因(C4HC3HHCTCCOMTCCRF5HCOMTCAD)的表达,抑制石细胞木质化。

2.3 其他转录因子

植物同源结构域(plant homeodomain,PHD)是锌指结构域家族的一类转录调控因子,广泛参与植物的生长发育过程。Cao等[28]利用qRT-PCR技术鉴定了梨果实中10 个表达的PHD-finger 基因,其中PbPHD10 被鉴定为调控木质素合成的重要候选基因。ⅠDD(indeterminate domain)家族蛋白编码作为锌指蛋白转录因子之一,也参与梨果实木质素合成。如PbIDD3PbIDD5 被推断参与了梨果实木质素代谢和次生细胞壁的形成[29]。TCP(teosinte branched1,cycloidea,and proliferating cell factor family)蛋白是植物特有的转录因子,参与调节次生细胞壁的形成和程序性细胞死亡[35]。根据砀山酥梨不同发育阶段表达模式分析,表明PbTCP14PbTCP15 与果实木质素的积累模式和石细胞含量趋势一致,这两个转录因子可能参与了梨果实石细胞形成过程中次生细胞壁的增厚[30]

除转录因子调控外,许多小分子RNA(microRNA,miRNA)能够参与调节次生细胞壁合成过程中的木质化。PbrmiR397a 已被证明通过转录后调控PbrLAC抑制漆酶基因表达,从而调控梨果实木质素沉积,最终降低果实石细胞含量;研究还发现了PbrmiR397a 启动子区域与石细胞含量相关的重要SNP 位点,为开发分子检测标记和辅助育种奠定了基础[31]。Zhang等[32]通过全基因组关联研究,鉴定出一个与梨石细胞发育相关的新基因PbrSTONE,通过梨果实的瞬时转化和拟南芥的稳定转化验证,发现PbrSTONE可以调控梨果实石细胞和木质素的形成,并明确了其与木质素合成通路中关键基因PbrC3H 存在互作关系,从而协同调控石细胞组分木质素的合成机制。

3 调控梨果实木质素合成的信号途径

梨果实木质素的合成调控是一个复杂的生物学过程,不仅直接受酶基因的影响,调节基因所编码的转录因子也可通过激活或抑制结构基因的表达,间接调控木质素的积累水平。同时与激素、糖、钙、光质等信号途径和环境因素密切相关(表3)[36-48]

表3 调控梨果实木质素合成信号途径的关键基因
Table 3 Key genes regulating the signaling pathway of lignin synthesis in pear fruit

类型Species SAUR基因SAUR gene名称Name PbrSAUR13,PbrSAUR52参考文献Reference[36]ARF基因ARF gene PbrARF13[37]UGT基因UGT gene PbUGT72AJ2[38]PbUGT72AJ2-pGEX4T-1[39]BGLU 基因BGLU gene PFP基因PFP gene CML基因CML gene Dof转录因子Dof transcription factor CRYs基因CRYs gene PbBGLU1,PbBGLU16 PbPFP[40][41]PbCML3[42]PuDof2.5[43]PbCRY1a [44]PbMYB103 TCP基因TCP gene PbTCP10[30]COBRA基因COBRA gene RBOH基因RBOH gene PbCOBL1、3、12、13、14[45]PbRBOHA,PbRBOHD[46]RBOH亚型PuRBOHFRBOH subtype PuRBOHF PuRBOH [47]SOD基因SOD gene PbCSD3功能描述Function description PbrSAUR13抑制石细胞和木质素的合成积累,PbrSAUR52促进石细胞和木质素的合成积累。PbrSAUR13 inhibits the synthesis and accumulation of stone cells and lignin,PbrSAUR52 promotes the synthesis and accumulation of stone cells and lignin.与PbrNSC的启动子结合并抑制其表达,降低石细胞含量。Ⅰt binds to the promoter of PbrNSC,inhibits its expression,and reduces the content of stone cells.催化木质素单体的葡萄糖偶联介导糖基化,调控木质素沉积和石细胞发育。The glucose-coupling of lignin monomers mediates glycosylation,regulates lignin deposition and stone cell development.催化木质素单体转化为木质素单体糖苷。Catalyze the conversion of lignin monomer to lignin monomer glycoside.调控木质素和石细胞含量。Regulate lignin and stone cell content.促进木质素的生物合成。Promote lignin biosynthesis.调控木质素的生物合成。Regulation of lignin biosynthesis.与PuPRX42-like互作调控木质素的生物合成。Ⅰnteraction with PuPRX42-like regulates lignin biosynthesis.激活基于NAC-MYB的转录因子调节下游基因的表达促进木质素生物合成。Activation of the NAC-MYB transcription factor regulates the expression of downstream genes and promotes lignin biosynthesis.调控木质素的生物合成。Regulation of lignin biosynthesis.参与光周期调控,调控木质素合成和石细胞形成。Participates in photoperiodic regulation,regulates lignin synthesis and stone cell formation.调控木质素合成和石细胞形成。Regulate lignin synthesis and stone cell formation.参与石细胞形成过程中ROS代谢活动,调控木质素的生物合成。Participate in ROS metabolism during stone cell formation and regulate lignin biosynthesis.被PuMYB169 激活,产生ROS 调控木质素合成相关结构基因(PuPOD2、Pu-LAC2),调控木质素的生物合成。Activated by PuMYB169,ROS is produced to regulate lignin synthesis-related structural genes(PuPOD2,PuLAC2)and regulate lignin biosynthesis.调控石细胞木质化。Regulates the lignification of stone cells.[48]

3.1 激素

3.1.1 生长素 生长素上调小RNA(small auxin-up RNA,SAUR)是生长素快速响应的基因,促进形成层活性,导致富含木质素的维管组织发育,进而调控木质素的积累[49]。砀山酥梨中共鉴定出116 个SAUR 基因,筛选出PbrSAUR13PbrSAUR52 两个关键基因。通过对39 d砀山酥梨果实的瞬时转化和草莓的稳定转化体系,发现PbrSAUR52主要促进石细胞和木质素的合成积累,而PbrSAUR13抑制石细胞和木质素的合成积累[36]。近期研究发现,外施200 μmol·L-1萘乙酸(NAA)可降低梨果实石细胞的含量,同时降低转录调节因子PbrNSC 的表达量。PbrNSC 能够直接结合生长素响应因子PbrARF13,过表达PbrARF13 可显著降低梨果实中的石细胞含量;而利用病毒诱导的基因沉默(VⅠGS)抑制PbrARF13 的表达,表型则相反。从而明确了PbrARF13可直接与PbrNSC的启动子结合并抑制其表达,降低石细胞含量。此外,通过石细胞形成相关基因的共表达网络分析,PbrNSC 被确定为PbrMYB132的上游调控因子,PbrMYB132能够与纤维素合成酶基因(PbrCESA4b/7a/8a)和木质素合成基因(PbrLAC5)的启动子结合,激活其表达,促进纤维素和木质素的合成。由此,PbrARF13-PbrNSCPbrMYB132 调控级联能够响应生长素信号,调控梨果实石细胞中纤维素和木质素的生物合成[37]

3.1.2 其他激素 脱落酸(abscisic acid,ABA)、茉莉酸甲酯(methyl jasmonate,MeJA)和水杨酸(salicylic acid,SA)等外源激素可通过影响木质素生物合成途径关键基因表达和转录因子,显著抑制梨果肉石细胞的木质化。Li等[7]PbPALs家族成员的上游调控序列中发现了多个响应激素的元件,其中PbPAL1 只含有ABA 反应元件(ABA response element,ABRE),PbPAL3 含有ABA 响应元件ABRE、MeJA 响应元件(MeJA response element,JARE)和SA响应元件(SA response,TCA),并且在PbPAL1PbPAL3上游2000 bp启动子序列中发现了AC元件,故推测ABA、SA 和MeJA 可以通过调节PbPAL1PbPAL3基因表达,进而影响梨果实石细胞的形成。

PbKNOX 启 动 子 含 有TCA、CGTCA 基 序 和ABRE 等激素响应元件,SA 通过诱导microRNA 的表达可以调节梨果实木质素合成和石细胞发育[31]。由此推断,SA、ABA 和MeJA 可能直接或间接调节PbKNOXs的表达,从而影响石细胞的形成。大多数PbRBOH 启动子含有大量与激素相关的元件,研究表明3种激素(SA、ABA和MeJA)处理梨果实后,大多数PbRBOHs 在短时间内被转录诱导,对PbRBOHs 的表达有显著影响。值得注意的是,10 种PbRBOH启动子中并未发现SA应答元件。然而,在SA 处理的果实中,PbRBOHI 的表达水平仍然发生了变化。这可能是各种植物激素之间相互作用、相互诱导的结果[50]

此外,研究发现ABA 处理后梨果实中PbC4Hs的表达量先升高后降低,而MeJA处理后PbC4Hs的表达水平显著升高。经SA 处理后,梨果实中PbC4H1PbC4H2 表达水平升高,PbC4H3 表达水平下降[8]。因此,适当的生长素和细胞分裂素配比可以抑制木质素的合成。这些结果为梨果生产提供了重要的指导,可以通过喷施外源激素来调节梨果实木质素代谢的强度,从而不同程度地抑制石细胞的形成,改善果实品质。

3.2 糖

木质素的代谢过程主要包括木质素单体的生物合成、运输和聚合。木质素单体合成过程中通常会发生糖基化修饰,糖基化可以提升木质素单体的溶解度和稳定性,有利于木质素单体的运输和储存[51]。从木质素单体到木质素单体糖苷的转化需要尿苷二磷酸糖基转移酶(uridine diphosphate glycosyltransferase,UGT)的参与。PbUGT72AJ2基因参与木质素单体糖基化[38],重组蛋白PbUGT72AJ2-pGEX4T-1 能够催化梨果实中木质素单体转化为木质素单体糖苷。Wang等[39]研究了PbUGT72A2转录在梨果实中的定位,并分析了PbUGT72AJ2 的酶促反应动力。通过梨果实瞬时转化验证,在梨果实中过表达PbUGT72AJ2,木质素和石细胞含量几乎没有变化,而沉默该基因则显著增加木质素和石细胞含量。由此,PbUGT72AJ2 通过催化木质素单体的葡萄糖偶联介导糖基化,影响下游基因的表达以及木质素单体的含量,从而影响梨果实木质素沉积和石细胞发育。

木质素单体糖苷被运送到细胞壁的特定部位,之后木质素开始去糖基化,在β-葡萄糖苷酶的作用下水解为木质素单木质素醇,最终形成木质素[51]β-葡萄糖苷酶(β-glucosidases,BGLU)去糖基化功能在木质素单体转运中起着重要作用。Wang 等[40]筛选到3 个可能参与木质素合成的候选基因PbBGLU1PbBGLU15PbBGLU16,其中PbBGLU1PbBGLU16转录主要位于梨果实木质素沉积区和石细胞区,过表达PbBGLU1PbBGLU16 后,梨果实中木质素和石细胞含量显著增加,而沉默PbBGLU1PbBGLU16 后,与对照组之间木质素和石细胞的含量没有显著差异。

葡萄糖作为木质素生物合成的重要碳源,其代谢与苯丙烷代谢途径密切相关。外源葡萄糖可以激活葡萄糖信号通路并调节其下游代谢活动,通过上调苯丙烷通路相关基因(PbPALPbHCTPbCOMTPbPRX)的表达诱导苯丙烷化合物的产生[52]。Jiao等[41]研究发现,砀山酥梨果实发育早期果糖-6-磷酸-1-磷酸转移酶(fructose-6-phosphate-1-phosphoric acid,PFP)基因的表达模式与木质素含量的变化趋势一致,通过验证,PbPFP 正调控梨果实木质素生物合成。此外还观察到葡萄糖信号与激素信号之间的广泛串扰,特别是与ABA、GA 和SA 信号的串接。Pbr016851.1Pbr002006.1Pbr035515.1 作 为At-MYB52的同源基因,在葡萄糖处理的愈伤组织中表达上调,说明葡萄糖反应性转录因子的调控在葡萄糖介导木质素生物合成中发挥着重要作用。因此,研究葡萄糖反应转录因子如何感知葡萄糖信号并调控下游苯丙烷通路基因是非常有前景和意义的。

3.3 钙

钙离子(Ca2+)作为细胞内信号转导的第二信使,在调节细胞壁形成、木质素合成和活性氧代谢等方面发挥着重要的生理作用[53],这暗示木质素代谢和石细胞的形成与钙有关。

外源钙通过促进钙信号转导和上调转录因子调控木质素生物合成,从而增加梨果实中石细胞含量[54]。研究发现,0.1%的硝酸钙能够减少木质素含量,0.5%的硝酸钙处理后,梨愈伤组织中木质素含量和次生代谢物的积累显著增加,愈伤组织呈现褐色和暗沉。同时研究证明,大量差异表达基因(differentially expressed genes,DEGs)的次级代谢途径基因(CMLCAMCDPKCBL 和CIPK)和转录因子可提供外源钙调控网络,引起代谢途径的改变,导致梨愈伤组织中木质素的积累。其中类钙调蛋白(calmodulin-like protein,CML)作为Ca2+的主要感受器,可通过调节下游靶蛋白的活性来调节多种细胞功能[55]。根据转录组数据结合qRT-PCR 分析发现,PbrCML表达状况与钙处理后的愈伤组织变化趋势一致,过表达PbCML3 可以促进梨果实中木质素代谢,显著增加其石细胞含量,验证发现PbCML3基因极可能作为转运蛋白参与调控某些转录因子从而间接调控木质素的生物合成[42],但CML对木质素含量的具体调控机制有待进一步研究。果面喷施5%CaCl2可以降低新高梨果实中PRX 的活性,从而抑制果实木质素的合成[56]。外施CaCl2可显著抑制黄金梨中PAL、CAD 和PRX 的活性,且PpCAD1PpCAD2基因的表达下调,影响石细胞形成[57]

Dof(DNA Binding With One Zinc Finger)转录因子在木质素沉积中是一个正向调节因子[58]。研究发现,CaCl2处理南果梨果实后,其内源Ca2+水平升高,PuDof2.5 表达量下降。PuDof2.5 作为转录激活因子,与木质素生物合成基因PuPRX42-like 启动子结合并抑制其转录;因此,PRX活性降低,木质素的生物合成受到抑制,石细胞含量和密度降低。而过表达PuDof2.5 可上调PuPRX42-like 表达水平,增强PRX 酶活性,显著加速木质素积累和石细胞形成[43]。梨果实发育早期叶面喷施2%的CaCl2,不仅可以提高果实贮藏期间Ca/N 和Ca/K 的比值,而且可以降低梨果实的硬度和木质素含量,抑制 “铁头病” 的发生[59]

3.4 活性氧

活性氧(reactive oxygen species,ROS)作为信号分子在植物细胞木质化过程中发挥着不可或缺的作用。ROS 迸发主要由呼吸爆发氧化酶(respiratory burst oxidase homologue,RBOH,又称NADPH 氧化酶)产生,参与梨果实石细胞的木质化过程[60]。前人研究表明,PbRBOHAPbRBOHBPbRBOHD 在梨果实中转录本丰度较高,其中PbRBOHAPbRBOHD 的表达趋势与梨果实石细胞含量一致,且亚细胞定位显示PbRBOHAPbRBOHD分布在质膜上,结合超氧化物含量的变化和时空表达分析,推定PbRBOHAPbRBOHD 参与了石细胞形成过程中ROS 代谢活动[46]。梨果实中的木质素单体由PbUGT72AJ1PbUGT72AJ2PbUGT72AK1 催 化形成木质素糖苷,然后通过膜转运到细胞壁沉积,在PbDIR4 的催化下形成木质素低聚物。最后,POD和LAC 利 用RBOH(PbRBOHAPbRBOHD)和SOD 产生的ROS 和O2 催化木质素聚合物链的伸长[16]

随着研究的深入,人们发现RBOH 亚型PuRBOHF 在梨果实石细胞次生壁形成中起重要作用。Wang 等[47]利用共聚焦显微镜和透射电镜观察发现ROS与细胞壁木质化之间的空间一致性。此外,石细胞木质化可被RBOH抑制剂二苯基氯化碘盐(diphenyleneiodonium chloride,DPⅠ)所抑制,在生理水平上证明了RBOH 介导的ROS 对梨果木质素生物合成的关键作用。PuRBOHF 过表达的梨果实和愈伤组织的木质素含量显著高于对照,而PuRBOHF沉默则抑制了木质素的积累。在此基础上,PuRBOHF 可被PuMYB169 激活,并通过产生ROS 调控木质素合成相关结构基因(PuPOD2PuLAC2)的转录。这是首次揭示PuRBOHF基因参与梨果实木质素代谢的分子机制。

Li 等[48]研究了11 种PbSODs 在梨不同发育时期、不同品种和不同激素处理下的表达模式,发现PbCSD3 的转录水平与木质素含量变化一致,其表达量在成熟期先升高后降低,结合时空表达结果表明,PbCSD3 是梨果实石细胞木质化过程中活性氧代谢的候选基因。研究证明,石细胞形成是一个活性氧诱导的程序性细胞死亡(programmed cell death,PCD)的过程,石细胞分化时期、ROS 的积累与PCD重叠[61]。高表达的细胞死亡相关基因和蛋白(AED3-likeMC1-likeACD11-like)表达模式与ROS相关基因一致,其早期表达更丰富,后期下调[62]

3.5 光质

光质对木质素沉积具有重要影响,作为木质素生物合成途径中的第一个限速酶,PAL受蓝光、紫外线和远红光调控[63]。蓝光对梨果实木质化具有正向调控作用,有研究表明其主要通过调控MYB、NAC等转录因子发挥作用。在蓝光处理下PbMYB103在不同梨果实发育时期、不同梨品种呈现高表达,Pb-MYB103可能是响应蓝光的转录因子[44]

隐花色素(cryptochromes,CRYs)介导的蓝光信号已被证明参与了次生细胞壁增厚与光周期调控开花[64-65]。Wang等[44]研究发现蓝光增强了梨愈伤组织中4CL14CL2COMTCESLAC7 等木质素合成相关基因的表达,促进了木质素的积累。此外,在白梨中鉴定出4 种蓝光受体CRY,分别为PbCRY1aPb-CRY1bPbCRY2aPbCRY2b,其中PbCRY1aPb-CRY1bAtCRY1 密切相关。同时证实了PbCRY1a作为木质素沉积的上游信号,通过激活基于NACMYB 的转录因子调节下游基因的表达促进石细胞木质化。由此,CRY 介导的蓝光信号在梨果实细胞壁木质化过程中起重要作用,并通过调控下游基因促进梨果实石细胞的形成。

迄今为止,关于光调控梨果实中木质素生物合成作用的研究主要集中在生理方面,如不同颜色果袋对果实品质的影响。研究表明光质可以通过影响PAL、C4H、4CL 等光诱导酶调控木质素的生物合成,绿色透光果袋抑制PbNAC83a 的表达,促进Pb-NAC83fPbNAC91a 的表达。橙色透光果袋促进PbNAC83aPbNAC83fPbNAC91a 的表达。Pb-NAC83a 表达量受光质影响与木质素含量的变化趋势一致,其基因表达可能是受红光促进、蓝光抑制[66]。近期研究发现,TCP基因和COBRA基因参与了次生细胞壁的形成,其中PbTCP10PbCOBL13121314在果实中高表达,其启动子的顺式作用元件含有光响应元件,参与了光周期调控,进而调节石细胞的形成[30,45]。此外,光响应元件如Box4、GATAmotif、GT1-motif 也存在于RBOH 启动子中,说明套袋处理改变梨果实木质素和石细胞含量可能与RBOH的转录有关[50]。Wang等[67]研究发现无纺布套袋梨果实,通过下调苯丙烷生物合成相关DEGs 抑制茌梨梨果皮木质素合成,进而减小果实皮孔直径,并推断果皮木质素含量可能与套袋透光性、光合速率及波长有关,而造成这些影响的具体机制需要进一步探索。

4 花粉直感

不同品种授粉后,梨树花粉能直接影响当年内母本果实品质,果实硬度、可溶性固形物含量、可滴定酸含量和石细胞数量等与木质素含量相关的性状,也表现出明显的花粉直感现象[68]。Yan等[69]以鸭梨和圆黄对砀山酥梨授粉,果实石细胞含量表现出显著的花粉直感现象,以鸭梨授粉的果实石细胞团的数量和聚合度较高,细胞壁的致密性更强,但两者授粉的梨果实石细胞团形成过程是一致的,说明梨果实的父本花粉直感现象表现具有阶段性。此外,一些初级代谢物和木质素合成中间代谢物也参与了花粉调控木质素代谢和石细胞形成,使用OPⅠS-DA模型分析授粉后47 d和64 d的砀山酥梨果实的初级代谢物和木质素合成中间代谢物,发现授粉通过影响糖、氨基酸、脂肪酸和香豆酸含量调控梨果实木质素合成[70]。由此可推断,花粉影响了梨果实中苯丙氨酸的合成,进而调控了木质素的合成。不同授粉品种对梨果实发育过程中果肉、种子内源激素含量及相关酶活性都具有重要影响[71]

microRNA 通过糖酸代谢和激素信号传导等途径广泛参与梨果实发育和果实品质的调控,其中microRNA 受花粉直感现象的影响,参与调控梨果实木质素代谢和石细胞形成[72]。已有研究报道,不同授粉品种显著影响梨果实木质素代谢相关基因和蛋白质的表达,例如授粉影响果实漆酶基因microRNA 的表达[73],过氧化物酶47(PER47)、β-葡萄糖苷酶(BGLU15)以及漆酶4(LAC4)的表达[74],间接调控木质素沉积,最终影响梨果实石细胞的形成,但花粉直感以何种方式调节microRNA表达进而影响果实石细胞形成需进一步探索。

5 总结与展望

梨果实木质素生物合成的调控过程复杂,涉及多层面、多因子的协同作用。目前,MYB、bZⅠP、NAC 及KNOX 类转录因子都参与了梨果实木质素生物合成的调控,Trihelix[75]、LⅠM[76]和LTF1[77]类转录因子同样在木质素合成调控中起到了关键作用。单独研究某个或少数几个转录因子不足以揭示木质素生物合成的巨大调控网络,基于NAC-MYB 的基因调控网络(NAC-MYB-GRN)模型被广泛认为是支撑木质素生物合成的基础[78],并且已被证明参与梨石细胞的木质化过程。另外,研究发现,PbMYB308-PbMYB61-PbLAC1模块参与梨石细胞中木质素的生物合成[23]PbrARF13-PbrNSC-PbrMYB132 级联能够响应生长素信号,调控梨果实石细胞中的纤维素和木质素生物合成[37],其他复合调控家族参与木质素合成的报道较少。

当前对梨果实木质素合成的单一信号途径的调控研究较多,多信号调控的交叉调控网络的研究较少。激素、糖、钙和活性氧等是影响木质素合成的重要信号调控因子。此外,可能存在其他信号因子参与调控梨果实木质素的生物合成,譬如短肽信号[79]、受体激酶[80]、泛素化和其他响应对木质素的沉积等。这些信号因子是否存在时空特异性与相互影响,对梨果实木质素生物合成的调控作用及分子机制,仍有待进一步阐明。据报道,区域生态、树形等外部因素及梨树的脱萼生物学功能与木质素沉积也具有相关性[81]。随着DNA/RNA 甲基化测序、MicroRNA 测序和CHⅠP-Seq 等前沿技术的引入,以及多组学联合应用和CRⅠSPR-Cas9 技术的不断优化[82],得以构建完整的木质素合成调控网络来阐明调控梨果实中木质素生物合成的分子机制,实现梨果实木质素合成和石细胞形成的精准调控,提高梨果实品质。

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Research progress in the regulation mechanism of lignin synthesis in pear stone cells

WANG Hongbao,WANG Yongbo,WANG Jin*,LⅠYong,LⅠXiao,WANG Yingtao,WANG Yaru*
(Shijiazhuang Institute of Fruit Trees,Hebei Academy of Agriculture and Forestry Sciences,Shijiazhuang 050061,Hebei,China)

Abstract: Stone cells are sclerenchyma cells formed by deposition of lignin, which is the most significant factor limiting the quality of pears.Therefore, it is of great significance to explore the mechanism of lignin synthesis regulation in pear fruit development for the genetic regulatory network of stone cell traits.Ⅰn this paper,the mechanisms of transcriptional factors,hormones,sugars,calcium,reactive oxygen species,light quality and pollen sensitivity in lignin synthesis in pear fruits are reviewed,aiming to provide reference for the regulatory network of lignin in pear fruits and genetic improvement of fruit quality. Several transcription factors involved in lignin biosynthesis have been identified in pears, such as MYB, NAC, bZⅠP, KNOX and zinc finger protein, among which MYB and NAC transcription factors play key regulatory roles in lignin synthesis. Study has showed that PbrMYB169, an R2R3MYB transcription factor of Pyrus bretschneideri, positively regulates lignification of stone cells in pear fruits.On the one hand,PbrMYB24 activates the transcription of lignin and cellulose biosynthesis genes by binding to different cis-element. On the other hand, PbrMYB24 binds directly to the promoters of PbrMYB169 and PbrNSC,activating the gene expression.Moreover,both PbrMYB169 and PbrNSC activate the promoter of PbrMYB24, enhancing gene expression. Research has identified PbMYB61 and PbMYB308 as candidate transcriptional regulators of stone cell formation,revealing that PbMYB61 regulates stone cell lignin formation by binding to the AC element in the PbLAC1 promoter to upregulate expression.Exogenous application of 200 μmol·L-1 NAA can reduce stone cell content and also significantly decrease the expression level of PbrNSC encoding a transcriptional regulator. Ⅰn addition,PbrARF13-PbrNSC-PbrMYB132 regulatory cascade mediates the biosynthesis of lignin and cellulose in stone cells of pear fruit in response to auxin signals.Research has found several hormone-responsive elements in the upstream regulatory sequences of PbPALs family members. ABA, SA and MeJA could regulate the expression of PbPAL1 and PbPAL3 genes,and affect the formation of fruit stone cells.A series of experiments have proved that PbUGT72AJ2 mediates glycosylation by catalyzing the glucose conjugation of monolignols and may affect the expression of downstream genes as well as the content of monolignols to affect the lignin deposition and stone cell development in pear fruit.The treatment of exogenous glucose has significantly enhanced the accumulation of lignin in pear calli. Expression of structural genes(PbPAL,PbHCT,PbCOMT and PbPRX)in lignin biosynthesis is up-regulated after glucose treatment.Transien expression of PbPFP has resulted in a significant increase of lignin content in Dangshansuli fruits on 35th day after full bloom (DAB) and in tobacco leaves, indicating that PbPFP might be associated with the enhancement of lignin biosynthesis in response to glucose treatment. Ca2+is known to inhibit stone cells in pear fruits.Ⅰn order to further explore how calcium-nitrate treatment affects lignin synthesis,the PbCML3 has been identified in pears and relevant experiments have been conducted to find that the overexpression of PbCML3 would increase the content of pear stone cell.Further analysis has identified a transcription factor,PuDof2.5,and its targets gene PuPRX42-like(lignin polymerase gene)expression has decreased in CaCl2-treated samples,which are involved in suppressing lignin biosynthesis in pear fruit.ROS is closely associated with lignin deposition and stone cell formation.Research has showed that PuRBOHF,an RBOH isoform,plays an important role in secondary wall formation in pear stone cells.Ⅰnhibitors of RBOH activity suppress ROS accumulation and stone cell lignification in pear fruit. Moreover, it has been showed that PuMYB169 regulates PuRBOHF expression,while PuRBOHF-derived ROS induces the transcription of PuPOD2 and PuLAC2. Research has showed that secondary cell wall thickening and lignin accumulation in pears may regulate by different wavelengths of light. Ⅰt is reported that CRY-mediated blue-light signal plays an important role in cell wall lignification and promotes the formation of stone cells in pears by regulating downstream genes.Results have showed that blue light induces the expression of lignin structure genes and promotes lignin accumulation. Furthermore, four blue light receptors cryptochromes have been identified in white pear,named PbCRY1a, PbCRY1b, PbCRY2a and PbCRY2b. Previous studies have reported that pollination affects the expression of laccase gene microRNA in pear fruits, and the expression of peroxidase 47(PER47), β-glucosidase (BGLU15) and laccase-4 (LAC4), thus affecting lignin synthesis. This finding demonstrates that pollination with different sources of pollens affects the synthesis of lignin in pear fruit on the levels of gene and protein expression.

Key words: Pear;Stone cell;Lignin;Regulation mechanism

中图分类号:S661.2

文献标志码:A

文章编号:1009-9980(2024)04-0750-14

DOI: 10.13925/j.cnki.gsxb.20230473

收稿日期:2023-11-13

接受日期:2024-02-01

基金项目:国家现代农业产业技术体系项目(CARS-28-04);河北省现代种业科技创新专项(21326308D);河北省农林科学院基本科研业务费(2023100102)

作者简介:王红宝,助理研究员,硕士,研究方向为梨育种及果树生理。Tel:0311-87659930,E-mail:1252146220@qq.com

*通信作者 Author for correspondence.Tel:0311-87659930,E-mail:wangjin305@foxmail.com;Tel:0311-87659930,E-mail:wyr927@126.com