香蕉抗寒分子机制研究进展

吴烁凡1,何维弟2,李春雨2,董涛2,毕方铖2,盛鸥2,邓贵明2,胡春华2,窦同心2,高慧君2,刘思文2,易干军2,姚振1*,杨乔松2*

1长江大学园艺园林学院,湖北荆州 434000;2广东省农业科学院果树研究所·农业农村部南亚热带果树生物学与遗传资源利用重点实验室·广东省热带亚热带果树研究重点实验室,广州 510640)

摘 要:香蕉是世界上最受欢迎的水果之一,也是第四大粮食作物,低温是影响其种植区域、生长发育、产量和品质的重要环境因素。近年来,随着基因组学和蛋白质组学等分子生物学技术的深入发展,香蕉抗寒分子机制的研究取得了较大进展,笔者重点从生物膜、磷酸化信号通路、基因表达、蛋白表达4个层面综述香蕉品种抗寒性差异的分子机制和调控途径,旨在为香蕉抗寒品种的培育和栽培措施的改进提供理论参考。

关键词:香蕉;抗寒;分子机制

香蕉(Musa spp.)是芭蕉科芭蕉属的多年生大型草本单子叶植物,在全球120 多个热带亚热带国家和地区均有种植[1]。香蕉不仅是世界上重要的热带水果之一,更是继水稻、小麦和玉米的第四大粮食作物,成为农业生产国重要的粮食作物和经济作物[2]。据FAO 数据统计,2019 年全球种植香蕉达515.86万hm2,中国香蕉收获面积达到35.89万hm2,产量接近1200 万t,中国已经成为世界第二大香蕉生产国(FAO,2019,http://www.fao.org/home/zh/)。我国香蕉种植区域主要位于南亚热带地区,包括广东、福建、广西、云南、海南等省(区),香蕉已成为南亚热带地区的重要经济来源,在促进农村经济发展、农业增效和农民增收等方面发挥着重要作用[3]

低温是限制植物分布、生长发育、产量和品质的主要环境因素之一,而对于热带起源的香蕉来说更是如此。主栽香蕉品种喜热不耐冷,最适生长温度为24~32 ℃,由于其耐寒性差,在10 ℃左右生长就会受阻甚至停止,降至5 ℃时香蕉的叶片就会受冷害变黄,低至2 ℃植株就会受到严重的伤害甚至枯死[4-5]。随着全球气候的变化,冬季和早春周期性的低温寒害已经严重影响了我国香蕉产业的健康发展[6],因此选育抗寒香蕉品种并改良其栽培措施对香蕉产业的可持续发展至关重要。香蕉种质资源中抗寒品种比较丰富,目前风味佳、种植面积广的香牙蕉(Musa AAA group‘Cavendish’)对低温敏感,但与香牙蕉同属于芭蕉科芭蕉属的大蕉(Musa ABB group‘Dajiao’)、粉 蕉(Musa ABB group‘Pisang Awak’)等种质抗寒性较强。2012年以来,随着基因组学、蛋白质组学等分子生物学技术的快速发展,特别是香蕉基因组序列的公布[7],国内外科研工作者广泛开展了不同香蕉品种抗寒性差异分子机制、香蕉自身抗寒基因资源挖掘的研究,笔者重点从生物膜、磷酸化信号通路、基因表达、蛋白表达4 个层面综述香蕉品种抗寒性差异的分子机制和调控途径,旨在为香蕉抗寒分子辅助育种和栽培措施的改进提供理论依据。

1 不同抗寒性香蕉品种生物膜的低温应答反应

植物对低温的应答反应是一个复杂的过程,涉及多方面调控,当外界环境温度变低时,生物膜是细胞最先感受低温变化也是最容易遭受低温伤害的部位。生物膜的主要组成成分包括脂类和蛋白质,低温能诱导膜硬化,导致膜系统的紊乱(如离子通道开放、膜相关电子传递反应和酶促反应速率下降等)。

一般认为香蕉的耐冷性与生物膜脂质组分、比例结构以及膜脂中不饱和脂肪酸的含量有关。何维弟[8]利用脂质组学分析比较了低温胁迫下冷敏感香牙蕉和耐冷大蕉各种甘油脂的变化,在香牙蕉和大蕉中共检测到了16 大类344 种脂质,香牙蕉和大蕉各脂质所占比例接近,大蕉脂质变化对低温更敏感,其研究发现大蕉主要通过膜脂成分的改变来应对低温胁迫:如溶血磷脂的增加,总磷脂酸、磷脂酰胆碱、磷脂酰乙醇胺和长链磷脂酰丝氨酸的减少有利于提高膜的自然弯曲能力;不饱和度相对较高的磷脂酸、磷脂酰胆碱和磷脂酰乙醇胺增加有利于提高膜的流动性;36-磷脂酰丝氨酸的稳定能够维持膜纳米结构域的稳定性,使膜功能正常运转;不饱和度较高的单半乳糖甘油二酯和双半乳糖甘油二酯以及磷脂酰甘油的增加提高了光合膜的稳定性。赖志宸等[9]以福建旗山野生蕉(抗寒种)和栽培品种天宝蕉(冷敏感香牙蕉品种)的叶片为材料,克隆获得了ω-3 脂肪酸去饱和酶基因Mu-FAD7-1Ma-FAD7-1,发现低温胁迫下天宝蕉Ma-FAD7-1 转录水平变化小,而野生蕉Mu-FAD7-1的表达量随温度的降低而显著升高,并在13 ℃时达到峰值,在4 ℃的低温胁迫下仍有较高水平的表达量,表明野生蕉FAD7 可能是发挥抗寒功能的基因。大蕉中去饱和酶MaFAD2-2基因在低温胁迫3 h时显著上调表达,这些可能与大蕉在磷脂酰胆碱总量减少的情况下仍保持较高不饱和度的磷脂酰胆碱显著增加有关[8]

同时,生物膜上的相关蛋白质也在香蕉耐冷性中起着重要作用。低温胁迫后会造成叶绿体内膜和线粒体内膜上的电子传递链受损,产生过多活性氧(ROS),氧化生物膜中的脂质和蛋白质。当植物体内的ROS产生过多时,会通过自身的抗氧化系统来清除,其中过氧化物酶(POD)扮演着重要的角色,这使得POD酶活性成为重要的抗寒生理指标。Zhang等[10]研究发现低温胁迫下大蕉体内的POD 酶活性显著提高。香蕉POD是一个大的蛋白家族,参与多种生理代谢过程,可根据提取液中所需的离子浓度分为可溶性POD 和离子结合型POD。Yang 等[11]在低温处理大蕉蛋白质组分析中也鉴定到了5 个POD,其中4 个为下调表达的可溶性POD,1 个为上调表达的离子结合型MaPOD52。为了厘清膜定位POD等膜蛋白在大蕉耐冷过程中的作用,He等[12]进一步利用膜蛋白质组学系统地分析比较了低温胁迫下冷敏感香牙蕉和耐冷大蕉膜蛋白表达谱变化。10 ℃低温处理6 h后,香牙蕉出现了低温胁迫表型,表现为幼苗叶片失水萎蔫、体内丙二醛含量增加、ROS积累以及相对电导率升高等,而大蕉无明显变化。随后对10 ℃低温处理0、3和6 h后的耐冷大蕉和冷敏感香牙蕉进行膜蛋白质组分析,从中分别鉴定到了137 和82 个差异表达的膜蛋白,其中在冷处理3 h 后大蕉有80 个膜蛋白呈上调表达,而香牙蕉只上调表达了11个膜蛋白。与香牙蕉相比,大蕉看似未变的表型下,无论膜蛋白还是各种脂质都发生了显著差异变化。研究发现大蕉在低温胁迫早期通过积累膜定位的水通道蛋白和过氧化物酶以平衡细胞水势和体内ROS含量,而香牙蕉响应低温相对迟缓从而出现叶片失水、ROS含量增加以及细胞膜损伤等寒害表型。该研究筛选到了冷胁迫下位于细胞膜上的4 种离子结合型POD 含量不同程度的增加,其中MaPOD52MaPOD P7 仅在冷处理3 h 后就在大蕉中表达量增加,表明大蕉更具耐冷性的原因在于低温胁迫下MaPOD52MaPOD P7 在大蕉中较早被诱导表达并迅速增加,通过对ROS的清除作用参与了膜保护功能。冷胁迫等非生物胁迫会致使植物细胞失水,而水通道蛋白在调节细胞水势、维持细胞形态和功能的完整方面发挥着重要作用,可以帮助植物抵御外界胁迫[13]。根据位置的不同和序列的相似性,可以将水通道蛋白分成7个亚类,其中质膜水通道蛋白(PIPs)和液泡水通道蛋白(TIPs)在响应多种非生物胁迫时发挥着重要作用。Xu 等[14]在拟南芥中超量表达香牙蕉PIP1;1后,发现植株自身的导水能力提高、膜损伤程度降低,增强了植株的抗盐和抗旱能力;在香牙蕉中超量表达MaPIP2;6b 能提高盐胁迫下植株的导水能力和降低膜损伤[15];超量表达MaPIP1;2 则提升了香牙蕉在多种非生物胁迫下的耐受能力[13]。He 等[12]印证和进一步发现,大蕉经冷处理3 h后,MaPIP1;1、MaPIP2;4、MaPIP2;6b和MaTIP1;3 四种水通道蛋白的丰度增加,而在香牙蕉中其表达量与0 h相比没有变化或者没有检测到,推测MaPIP1;1、MaPIP2;4、MaPIP2;6b 和MaTIP1;3这4种水通道蛋白是通过维持叶细胞水势使得大蕉更具耐冷性。

另外,结合脂质组学和膜蛋白质组学分析,何维弟[8]发现低温胁迫下大蕉中MaABCA7 含量的增加有助于脂质代谢原材料acyl-CoA向内质网转运,促进磷脂酸、二脂酰甘油和三脂酰甘油向磷脂酰胆碱转化,3 个MaTGD 表达量增加促进磷脂酸、磷脂酰胆碱和二脂酰甘油运向叶绿体,促进半乳糖甘油二酯的合成,启动原核途径促进磷脂酰甘油的增加,导致大蕉叶绿体膜稳定性提高,从而减少低温对叶绿素和光合效率的影响。此外大蕉还通过中心磷脂的减少和交替氧化酶MaAOX1的增加激活了抗氰途径,减少线粒体ROS的积累,以此来应对低温胁迫。

2 不同抗寒性香蕉品种磷酸化信号通路的低温应答反应

在低温胁迫下细胞膜的流动性会减弱,定位于细胞膜或核被膜上Ca2+转运蛋白会引起Ca2+信号的变化,导致膜内外形成钙离子浓度低,然后诱导冷响应基因表达[16]。丝裂原活化蛋白激酶MAPK级联反应引起的磷酸化修饰在植物抗寒性中的作用已有较多的报道,特别是在模式植物中。例如,在拟南芥中发现MKK2 途径介导了冷和盐胁迫信号[17],拟南芥MEKK1通过Ca2+信号磷酸化参与冷胁迫反应[18],在冷驯化过程中形成MEKK1-MKK2-MPK4调控通路级联上游的膜硬化功能[19]。然而,在香蕉中报道的信息还很少。高洁[20]采用比较磷酸化蛋白质组学,比较冷敏感香牙蕉和耐冷大蕉在低温胁迫后的磷酸化蛋白的表达谱变化,从香牙蕉中共鉴定到352 个有定量数据的磷酸化蛋白,这些蛋白包括了438 个磷酸化多肽和483个磷酸化位点。从大蕉中共鉴定到144 个有定量数据的磷酸化蛋白,这些蛋白包括了167个磷酸化多肽和188个磷酸化位点,其中5个磷酸化蛋白包括了6 条显著上调表达和3 条显著下调表达的磷酸化多肽。相反,在香牙蕉中并不包含这些差异性变化的多肽。将这些差异磷酸化蛋白进行互作分析发现,MKK2处于网络中心,表明MKK2互作网络可能是大蕉响应低温胁迫的关键磷酸化网络。通过Western Blot 发现随着低温胁迫时间的增加,大蕉MKK2 T31 磷酸化程度随之提高,说明MKK2 T31的磷酸化在响应低温胁迫过程中具有特异性,而在香牙蕉中并没有检测到其磷酸化过程。转录因子HY5 是一类bZIP 类的转录因子(大蕉中HY5的S133磷酸化程度在冷胁迫3 h后达到对照的3.44倍,而在香牙蕉中HY5的S133 并没有差异变化),其在光信号以及介导光感受器响应进而促进光形态建成方面具有重要的作用,此外有研究发现HY5 具有调控植物响应UV-B 和不同植物激素(如生长素、赤霉素、细胞分裂素以及脱落酸等)的功能[21-22]。在模式植物拟南芥中,低温影响了HY5的转录表达水平,但是其调控是除CBF和ABA途径外独立的调控途径。有研究表明HY5通过Z-box和其他作用元件正向调控冷诱导基因的表达,以确保冷驯化的完成[23]。在MKK2 互作网络中,HY5 被预测与香蕉MAPK6互作,MAPK6在拟南芥中已被证实由MKK2 磷酸化后参与调控低温以及盐胁迫信号通路[17]。另外,低温胁迫下大蕉和香牙蕉体内海藻糖的含量测定结果说明大蕉遭受低温胁迫3 h后,其海藻糖磷酸合成酶磷酸化的显著上升可能对其或者相关酶活性产生了影响,确保在大蕉遭受低温胁迫早期就能通过增加海藻糖的含量以应对逆境。Gao等[24]认为磷酸化信号通路中的MKK2 互作网络,以及其他特异性响应冷胁迫的磷酸化蛋白在大蕉抗寒中发挥了非常重要的作用。Gao 等[25]通过RNAi 将耐冷大蕉中的MAPK3基因进行了沉默,发现降低了大蕉的耐冷性。双荧光素酶互补和酵母双杂实验发现,大蕉MKK2-MAPK3-ICE1 能发生互作。最近Tak 等[26]在香蕉品种Rasthali 中(AAB 基因型)超表达MusaMAPK5(命名不同,与上述大蕉MAPK3 相似性达到99%),发现转基因植株的耐冷能力较对照组提高,这与脯氨酸水平的提高和丙二醛含量的降低有关,另外还发现纯化的MusaMAPK5 可以磷酸化香蕉的NAC042 和SNAC67 转录因子,这两个转录因子都是与香蕉植株抗逆相关的重要调节因子[27-28]

3 不同抗寒性香蕉品种低温胁迫下的基因差异表达

最近在模式植物[29]和一些作物[30]中发现了长片段非编码RNA(lncRNA),并证实了lncRNAs 是植物面对生物胁迫[31]和非生物胁迫[32](包括冷胁迫[33])反应中的关键调节因子。lncRNAs是一类参与调控的非编码RNA,通过碱基配对(对于RNA/DNA 序列)或是作为支架(对于蛋白质)在转录、翻译和表观遗传水平上发挥作用。Liu 等[34]以福建三明野生蕉(抗寒种)为材料,首次利用RNAseq 技术分析了其在低温胁迫下的mRNAs 和lncRNAs,鉴定出了12 462个lncRNA,而mRNA在冷胁迫条件下比正常生长条件下发生了更多的选择性剪接事件。在三明野生蕉应对冷胁迫的反应中,许多蛋白激酶基因是lncRNA的靶标,研究发现lncRNA 主要通过调节丝氨酸/苏氨酸或双特异性蛋白激酶,在翻译后修饰、信号转导途径和转录因子激活过程中改变酶活性、细胞定位或与其他蛋白的结合来影响三明野生蕉对低温的反应。

植物已经进化出生理机制来适应低温等不利的环境条件,钙和钙传感器已被发现在这些生理机制中发挥关键作用。其中钙调素类蛋白(calmodulinlike proteins,CMLs)、钙调蛋白(calmodulin proteins,CAMs)、钙调磷酸酶B 类蛋白(calcineurin B-like proteins,CBLs)和钙依赖蛋白激酶(calcium-dependent protein kinases,CDPKs)等信号转导分子可以感知细胞内钙离子浓度的变化,并通过蛋白磷酸化介导下游信号,从而影响相关靶点的活性和基因表达[35-36]。在水稻中COLD1 和OsCIPK7 被认为能感应冷信号并调节Ca2+离子流,但它们如何在低温下调节Ca2+离子流的机制并不清楚[37]。在依赖钙信号的通路中,CDPK由于其钙敏感和蛋白激酶活性,在参与植物响应逆境的信号转导过程中发挥着重要作用[38]。如转OsCDPK13基因水稻品系在低温处理后表现出比野生型更好的恢复率[39],而通过对Os-CPK17 基因敲除、沉默和过表达品系的分析,发现OsCPK17是抵御冷胁迫所必需的[40]。Li等[41]从香蕉中筛选出44 个CDPK 基因,并比较了粉蕉(较耐冷香蕉品种)和冷敏感香牙蕉(品种名:巴西蕉)低温处理下的MaCDPK基因的表达量,发现粉蕉在低温处理下诱导表达的MaCDPK基因数量远多于香牙蕉,其中粉蕉中的5 个MaCDPK 基因(MaCDPK211123940)的表达量显著上调并高于香牙蕉。低温条件下,植物细胞内钙离子浓度的变化可以作为第二信使调控下游一系列抗寒基因的转录。Yang等[42]通过比较冷敏感香牙蕉和耐冷大蕉基因表达谱变化,发现众多与信号转导相关的差异基因都与依赖钙信号的通路有关,包括CMLKIC(Calciumbinding protein KIC)以及CIPK(CBL-interacting protein kinase)的同系物。作为一个蛋白激酶,CIPK在植物响应冷胁迫过程中起着重要的作用,在水稻中过表达OsCIPK3OsCIPK7 基因能提高其抗寒性[43-44]。低温胁迫下OsMSR2的同源基因Ma06_g37000在大蕉中的表达量远高于香牙蕉,而水稻类钙调素基因OsMSR2能在提高植株抗旱和抗盐能力的同时,对ABA的敏感度也有所增加[45],表明大蕉更具抗寒性的原因可能在于存在一条ABA 途径的钙信号转导通路,使其更快的调控下游信号以及相关抗寒基因的表达以应对低温胁迫。另外,低温胁迫3 h 后发现大蕉中EIL1(Ethylene insensitive 3-like 1)基因的表达量达到峰值,并随着时间的延长其表达量逐步下降,而香牙蕉中EIL1基因的表达量则持续上升且显著低于大蕉。EIL1 基因作为乙烯信号转导的初级转录因子,由此推测大蕉能快速应对低温胁迫的原因在于对乙烯的敏感响应。Hu等[46]比较了粉蕉(较耐冷香蕉品种)和冷敏感香牙蕉(品种名:巴西蕉)在低温胁迫下的不同耐受情况,发现粉蕉比香牙蕉更耐低温胁迫的原因在于粉蕉的ABA和ROS信号网络比香牙蕉更活跃,其中经冷处理后,相对于香牙蕉,在粉蕉中发现上调表达的39个和56个耐冷基因分别参与了ABA依赖的信号网络和ROS 信号网络调控,这些基因分别属于PYL、PP2C、SnRK2、MYB、NAC、DREB、CML、CBL、CDPK、CTA、MAPKC、WRKY 和RAV 家族。而PYLPP2C-SnPK2基因介导的ABA信号在香蕉对非生物胁迫(如冷、盐和渗透压力)的响应中发挥着重要作用。研究发现,在低温处理下,粉蕉中的PYL-PP2CSnPK2基因较香牙蕉表达量显著上调,表明ABA信号的核心成分在粉蕉中比在香牙蕉中更活跃,以应对冷胁迫[47]。类束状阿拉伯半乳糖蛋白基因(fasciclin-like arabinogalactan protein,FLAs)是位于细胞壁或质膜中的一类与生长发育有关的基因,最新的研究表明其在香蕉冷胁迫反应中也发挥着重要作用。Meng等[48]研究了低温胁迫下香蕉FLAs在冷敏感香牙蕉(品种名:巴西蕉)和耐冷大蕉2 个品种中的表达差异,发现大多数的MaFLAs 在低温胁迫下表达下调,而鉴定到的3个MaFLAs基因MaFLA14MaFLA18MaFLA29在2个品种中均表达上调,并且在大蕉品种中的表达水平更高,另外MaFLA27基因只在大蕉中表达上调,说明低温处理下大蕉体内的这4 个FLA 基因的积累促进了冷信号通路,进而促进植物细胞壁成分的生物合成,从而提高了大蕉的抗寒性。

此外,贺立红等[49]通过克隆香蕉的HSP70 cDNA,得到了Ma-HSP70-1Ma-HSP70-2 2个序列不同的基因片段,低温胁迫能使2 个基因的表达上调。匡云波[50]克隆了香蕉栽培品种天宝蕉(冷敏感香牙蕉品种)叶片中11 个糖代谢关键酶基因,并对其在低温胁迫下的表达进行了研究。张俊芳等[51]从冷敏感香牙蕉(品种名:巴西蕉)中克隆获得了乙烯应答因子结合蛋白基因MaERF,发现低温胁迫条件下该基因在香牙蕉中的表达量上调,推测其在香牙蕉应对低温胁迫过程中可能起到重要作用。Wang等[52]从冷敏感香牙蕉(品种名:巴西蕉)根系cDNA文库中获得了5 个谷胱甘肽硫转移酶(Glutathione S-transferases,GSTs)基因MaGSTU1~3MaGSTF1MaGSTL1,定量分析发现,MaGSTU1~3 基因受冷、盐、干旱诱导表达上调,而MaGSTF1MaGSTL1 基因受信号分子影响表达上调,表明MaGSTs 基因在香牙蕉应对低温等非生物胁迫过程中可能起到重要作用[53]

4 不同抗寒性香蕉品种低温胁迫下的蛋白差异表达

目前,已有大量研究分析了香蕉响应低温胁迫的分子机制,也鉴定到了许多受低温诱导或抑制的基因,但是这些研究得到的结果只是一些定量的mRNA 数据,由于转录后的调节导致基因在转录水平和其对应的蛋白质相关性不大,并不能直接通过基因的表达变化推测低温胁迫对蛋白质表达的影响。而蛋白质组学研究聚焦在基因组编码的所有蛋白质,不仅能补充转录组学的研究,还能更直接地反映与逆境胁迫相关的信号和代谢途径。蛋白质组学为从蛋白质水平阐明植物发育和胁迫反应的分子机制提供了重要途径,近年来,人们利用蛋白质组学方法鉴定了香蕉对低温胁迫的大量响应蛋白。Yang等[11]首次使用基于iTRAQ的比较蛋白质组学技术对大蕉幼苗在8 ℃低温胁迫0、6、24 h 以及在28 ℃环境恢复24 h这4个时期发生变化的蛋白质进行了深入分析,共鉴定出3477个蛋白质,其中809个蛋白质表现出差异表达,通过Western blot和酶活性分析比较了冷敏感香牙蕉和耐冷大蕉之间3个关键代谢途径中的7 个候选蛋白,结果显示大蕉更具抗寒性的原因在于:通过上调谷氨酸乙醛酸转氨酶(GGAT)、乙醛酸丝氨酸转氨酶(SGAT)和丝氨酸羟甲基转移酶(SHMT)等蛋白的表达,提高光呼吸途径,减少活性氧的产生;通过上调SOD 和CAT的表达,清除过多的活性氧;通过下调脂氧合酶(LOX)的表达,减少膜脂过氧化以及通过降低己糖胺酶(β-Hex)等细胞壁降解酶的表达来维持细胞壁结构,这些代谢途径在大蕉抗寒过程中起重要作用。孙勇等[54]同样应用比较蛋白质组学技术对7 ℃低温处理0、3、5和7 d的冷敏感香牙蕉(品种名:巴西蕉)幼苗叶片总蛋白进行分析,发现经低温胁迫后有85个蛋白质点的表达丰度出现明显变化,利用质谱成功地鉴定到其中的82 个蛋白主要参与了蛋白质的加工及翻译后修饰、光合作用、糖与能量代谢等生理学过程。

另有研究发现,富羟脯氨酸糖蛋白(HRGPs)作为植物细胞壁的一种结构蛋白,其重要成员之一的阿拉伯聚半乳糖蛋白(AGPs)在植物抗冷害机制中也发挥着重要作用[55]。王莹莹[56]以冷敏感香牙蕉(品种名:巴西蕉)和耐冷大蕉(品种名:东莞大蕉)为试验材料,运用数字基因表达谱技术和qPCR 技术研究香蕉AGPs基因对低温胁迫在转录水平上的响应,在低温胁迫下共鉴定到了21个与AGP相关的差异表达基因,其中8个为高表达冷响应基因,另外发现低温胁迫下大蕉中的4 个AGPs 基因的表达量随着温度的降低而升高;此外,还利用9种不同的AGP单克隆抗体,运用免疫荧光技术从蛋白水平上分析了低温胁迫下冷敏感香牙蕉和耐冷大蕉AGPs的分布及表达差异,发现低温胁迫下大蕉幼苗叶片中抗体JIM14、JIM15 和PN16.4B 所识别的AGPs 含量有所增加,且它们所识别的AGPs 主要分布于叶片表皮、后壁细胞中,说明在大蕉中主要通过积累与植物细胞壁成分相关的蛋白质来维持细胞壁结构以提高抗寒性。

5 香蕉抗寒途径的研究进展

低温胁迫是植物面对的最常见的非生物胁迫之一,研究植物对低温的响应机制和如何提高植物抗寒性一直以来都是植物学研究领域的重点和热点,在许多冷信号通路中,ICE1-CBF-COR 冷信号通路是被研究的最为广泛且被认为是最关键的调节通路[57]COR 基因的启动子含有一个或多个C-repeat(CRT)/Dehydration responsive element(DRE)顺式作用元件,这些顺式作用元件可以响应低温等非生物胁迫[58]。而C-repeat/dehydration-responsive element Binding Factor(CBF)/Drought Responsive Element Binding(DREB)属于APETALA2(AP2)/ethylene-responsive element binding factor(ERF)型转录因子家族,可以与CRT/DRE顺式作用元件结合并对其进行调节[59]CBF/DREB1 基因本身可以在低温条件下被诱导表达,同时CBF/DREB1 还可以与COR 基因的启动子上的CRT/DRE 顺式作用元件相结合,形成CBF-COR 调节通路,共同诱导冷相关基因的表达[60]。在拟南芥中已发现3 个CBF/DREB1s基因参与调控冷响应基因的表达[60],由于CBF/DREB1基因本身可被低温诱导表达,说明存在其他的转录因子控制其在低温下的表达。有研究发现CBF/DREB1(主要是CBF3/DREB1A)通路是由一个MYC 型的转录因子ICE1控制的,ICE1作为CBF基因的上游转录因子,诱导CBF 基因的表达,调控其转录,从而形成ICE1-CBF-COR 冷信号调节通路[61-62]。接近40%的冷响应基因和46%的冷调节转录因子都受到ICE1的调控,这表明ICE1 在调节CBF3/DREB1A 以及其他冷响应基因过程中扮演着主调节器的作用[63]

已有大量研究表明ICE1 在高等植物冷响应下的功能是保守的,在拟南芥[64]、小麦[65]、水稻[66]和番茄[67]等作物中超表达ICE1ICEs 类同系物基因能够有效增强转基因作物的耐冷性。ICE1 由泛素化[68]、SUMO 化[69]以及磷酸化[70]调控。但是关于MAPK 级联-ICE1的抗寒调控机制直到最近几年才被深入报道。ICE1 受MAPK 级联磷酸化调控,Li等[71]和Zhao等[72]研究发现MPK3/6能够磷酸化ICE1促进其降解,而MKK2/MPK4 通过降低MPK3/6的表达维持ICE1的稳定,从而增强植株的抗寒能力。Yang 等[73]和Teige 等[17]发现拟南芥经低温胁迫后可以诱导MKK2的表达,并激活MAPK4/MAPK5进而抑制MAPK3/MAPK6的活性,减少ICE1 磷酸化导致的降解,达到提高植株抗寒性的目的,说明MKK2-MAPK4/MAPK5-ICE1调控途径在拟南芥抗寒方面起重要调控作用。研究表明MaMKK2MaMAPK3MaICE1 在大蕉低温胁迫中起重要作用[8]。对大蕉MaMAPK3 基因进行干涉后发现其表达量受到了严重抑制,并导致植株抗寒性下降;而对香牙蕉和大蕉进行低温胁迫处理后,MaMKK2基因通过调控细胞功能进而对大蕉更具抗寒性作出了贡献。最新研究表明MaMKK2-MaMAPK3-MaICE1-MaPOD P7 通路是大蕉耐冷性的正向调节因子[25]。Liu 等[74]从福建抗寒野生香蕉Huanxi 中成功克隆了胁迫相关基因KIN10 和冷驯化相关基因HOS1ICE1,表达分析表明,这些基因的表达和蔗糖合成受到低温的显著影响:在冷胁迫刺激下,HOS1的表达受到抑制进而影响ICE1的表达,而KIN10受低温影响后可通过调节蔗糖的合成,改变渗透压从而参与冷反应。但值得注意的是,在拟南芥中,与AtICE1 有相互作用的是AtMAPK4/6 和AtMAPK3,而AtMAPK3 在冷胁迫中起负调节作用[72-73],这与香蕉中MaMAPK3 发挥的作用不同,说明拟南芥和香蕉在MAPK3介导的调控机制上存在较大差异。虽然MAPK 级联-ICE1 信号转导途径的多个成员已经被证明在香蕉抗寒中起重要作用,但是对于相关基因间的互作调控机制尚不明确,MAPK 级联-ICE1 抗寒途径调控网络还有待进一步研究。

随着冷信号调控网络的不断完善,显而易见冷反应是一个高度复杂的过程,受多条信号通路调节[75]。除了ICE1-CBF-COR 以及MAPK 级联-ICE1途径外,还存在其他有助于抗寒的调节通路,如MYB调节通路。MYB类转录因子首先是在动物癌细胞中发现的,主要参与动物细胞的新陈代谢,但也有研究表明MYB转录因子参与植物的逆境调控,如拟南芥中发现MYB 转录因子参与包括盐碱[76]、干旱[77]和低温[78]等多种非生物胁迫。张露月[79]发现SlMYB15 介导了番茄低温耐受能力,并在番茄低温抗性的CBF 途径中起到了正调控作用。在水稻中发现了2个参与水稻冷胁迫的MYB类转录因子Os-MYBS3和OsMYB4。超表达OsMYBS3能够提高水稻植株的抗寒性,而OsMYBS3 RNAi 植株则表现为对冷胁迫敏感。但是研究发现MYBS3抑制了CBF/DREB1 冷信号传导通路,表明水稻中除了ICECBF-COR通路外,还存在MYBS3介导的调节通路,这2条途径共同调控了水稻对短期和长期冷胁迫的适应性[80]。高洁[20]通过转录组学分析证实低温胁迫下大蕉中MpMYBS3 基因的快速激活和选择性诱导表达是其抗寒性优于香牙蕉的主要原因。Dou等[80]从耐冷大蕉中克隆并鉴定了MpMYBS3 基因,证实了MpMYBS3 在香蕉中异源过表达提高了转基因植株的耐冷性。有意思的是,MYBS3在香蕉中表达后抑制了众所周知的ICE1-CBF 冷信号通路,冷处理后MaCBF1MaCBF2 基因在转录水平受到抑制,然而,在低温胁迫下,高表达MpMYBS3的转基因香蕉显著诱导了MaMKRY46 基因的表达,表明MYBS3 介导的冷信号在香蕉的冷适应中起着关键作用。而超表达OsMYB4 显著提高了水稻的抗寒性,并诱导下游一系列COR 基因的表达[81-82]。亚油酸(LA)和α-亚麻酸(ALA)是低温胁迫下维持细胞膜完整性不可缺少的两种必需不饱和脂肪酸,而ω-3脂肪酸去饱和酶(FADs)负责将亚油酸转化为α-亚麻酸。研究表明,低温处理(3 ℃处理6 h)能降低香蕉果肉中膜脂饱和脂肪酸的含量,并抑制脂肪酸不饱和指数、不饱和脂肪酸含量和脂肪酸不饱和度的下降,从而提高香蕉的抗寒性[83]。另有研究发现,香蕉一个MYB转录因子MaMYB4可以靶向抑制几个ω-3 MaFADs的转录,包括MaFAD3-1MaFAD3-3MaFAD3-4MaFAD3-7。在冷胁迫条件下,ω-3 MaFADs 启动子中组蛋白H3 和H4的乙酰化水平升高,从而导致ω-3 MaFADs转录水平和ALA/LA比值升高。此外,组蛋白去乙酰化酶MaHDA2 与Ma-MYB4 发生物理上的相互作用,从而导致MaMYB4介导的ω-3 MaFADs 转录抑制增强,表明MaMYB4结合MaHDA2通过影响ω-3 MaFADs的乙酰化水平来抑制ω-3 MaFADs的转录,形成MaMYB4-MaHDA2-ω-3 MaFADs通路调控香蕉在低温胁迫下的脂肪酸代谢以增强香蕉的抗寒性[84]

6 展 望

近年来,随着人们不断对香蕉抗寒分子研究的深入,解析香蕉响应低温胁迫的调控通路将有望成为现实,为今后香蕉抗寒分子育种提供可能。但植物响应低温胁迫是一个十分复杂的调控过程,是由多个基因共同控制的数量性状,受许多因素和条件影响。目前,香蕉抗寒机制的研究主要侧重于非ABA 依赖的MAPK 级联-ICE1 信号转导途径,虽然已有多个像MKK2、MAPK3和ICE1等MAPK级联-ICE1 信号转导途径的成员被分别证明在香蕉抗寒上发挥着重要作用,但大都是单基因功能的分析和探讨,对于各成员之间的相互关系以及调控网络并不清晰。另外,以蛋白磷酸化为代表的蛋白可逆修饰在响应冷胁迫过程中也是十分重要且复杂的蛋白调控过程,虽然明确了MAPK 级联(MKK2-MPK4,MPK3/6)能够通过磷酸化ICE1调控其稳定性,但是哪些蛋白直接参与这一复杂调控过程和具体机制尚不清楚。针对这些问题,可以尝试利用交联质谱结合免疫共沉淀技术,高通量和高灵敏度地解析低温处理下与MKK2、MAPK3 和ICE1 直接或间接互作的蛋白网络,深入探讨多个抗寒基因间的互作关系,明确其调控网络,对于阐明香蕉抗寒的分子机制至关重要,进而有利于今后对香蕉抗寒品种的改良。

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Research progress in molecular mechanism of cold resistance in banana

WU Shuofan1,HE Weidi2,LI Chunyu2,DONG Tao2,BI Fangcheng2,SHENG Ou2,DENG Guiming2,HU Chunhua2,DOU Tongxin2,GAO Huijun2,LIU Siwen2,YI Ganjun2,YAO Zhen1*,YANG Qiaosong2*

(1College of Horticulture and Gardening,Yangtze University,Jingzhou 434000,Hubei,China;2Institute of Fruit Tree Research,Guangdong Academy of Agricultural Sciences/ Key Laboratory of South Subtropical Fruit Biology and Genetic Resource Utilization,Ministry of Agriculture and Rural Affairs/Guangdong Key Laboratory of Tropical and Subtropical Fruit Tree Research,Guangzhou 510640,Guangdong,China)

Abstract: Banana(Musa ssp.)is one of the most popular fruits in the world and the fourth largest food crop after rice,wheat and corn.It has become an important food and economic crop in sub-Saharan Africa,South and Central America,and Asia.Bananas are originated from tropical regions,the main cultivated bananas prefer high but do not low temperature,and are sensitive to cold stress.The optimum growth temperature for bananas is 24-32 ℃,and the growth will be blocked or even stagnated under 10-12 ℃.When the temperature drops to 1-2 ℃,the plants will suffer serious damage or even die.Therefore,low temperature is an important environmental factor affecting its planting area,growth development,yield and quality.Banana planting areas in China are mainly located in the southern subtropical regions,including Guangdong,Fujian,Guangxi,Yunnan,Hainan and other provinces.Banana has become an important source of economy in our country’s southern subtropical regions and has played a key role in promoting rural economic development,agricultural efficiency,and farmers’income.However,with global climate change,cold spells often occur during winter or early spring,which can have a serious impact on banana production,and restrict the healthy development of the banana industry.Cultivating new cold-tolerant bananas has become one of the urgent problems in the development of banana industry in China.For a long time,triploids and extremely low fertility of most banana cultivars limit the improvement of cold-tolerant bananas through traditional breeding.Banana molecular breeding pro-vides an effective method for creating new banana germplasm with enhanced cold tolerance.The cold tolerance of banana germplasms is quite different.The main cultivated Cavendish banana (Musa AAA Group),which has a good flavor and taste,is cold-sensitive,but the relative germplasms such as Dajiao(Musa ABB Group)and Pisang Awak banana(Musa ABB group)are more tolerant to low temperatures of 0-4 ℃.These cold-tolerant banana varieties have become ideal genetic resources for digging out cold-tolerant genes of bananas themselves,improving cold-tolerant traits of bananas and cultivating new cold-tolerant germplasms.Therefore,studying the molecular mechanisms of different cold-tolerant banana varieties under cold stress is essential for digging out cold-tolerant genes for genetic manipulation.In recent years,with the further development of molecular biology technologies such as genomics and proteomics,especially the announcement of the banana genome sequence in 2012,the research on the molecular mechanism of banana cold tolerance has made great progress.In this review,we describe the different molecular mechanisms and regulatory networks of banana varieties from four different levels: biomembrane,phosphorylation signaling pathway,gene expression and protein expression.When the environment temperature becomes low,the biomembrane is the first part of cells to feel the changes of low temperature and is the most vulnerable to cold stress.Previous studies have shown that the cold tolerance of bananas is related to the lipid composition,ratio structure and membrane lipids of the biomembrane.At the same time,related proteins on the membrane also play an important role in banana cold tolerance.For example,peroxidases MaPOD52 and MaPOD P7 participate in the membrane protection by removing reactive oxygen species(ROS)to improve the cold tolerance of Dajiao;water channels proteins MaPIP1;1,MaPIP2;4,MaPIP2;6b and MaTIP1;3 make Dajiao more cold-tolerant by maintaining leaf cell water potential.After the membrane is subjected to cold stress,the membrane fluidity will decrease.Ca2+transporters located on the cell membrane or nuclear envelope will cause changes in the Ca2+signal,resulting in the formation of a difference in calcium ion concentration inside and outside the membrane,which in turn induces cold-responsive gene expression.So far,reports on the mining and functional research on banana cold tolerance genes have been increasing year by year,and many functional genes related to banana cold tolerance have been identified.In this paper,according to the differences in gene expression of different cold-tolerant banana varieties under cold stress,a summary of banana cold-tolerance related genes are made.In addition,due to post-transcriptional regulation,there is little correlation between the transcription level of genes and their corresponding proteins.It is not possible to directly infer the effect of cold stress on protein expression through changes in gene expression.Proteomics research focuses on all the genes encoded by the genome.Protein can not only supplement transcriptomics research,but also more directly reflect the signals and metabolic pathways related to adversity stress.But so far,there are relatively few studies on the difference in protein expression of bananas under cold stress.We have summarized the results of previous studies.The information provided may help interested researchers cultivate cold-resistant bananas and improve cultivation measures.In addition,since plant response to cold stress is a very complex regulatory process,it is a quantitative trait controlled by multiple genes and is affected by many factors and conditions.In the future,we recommend using advanced scientific methods such as mass spectrometry and co-immunoprecipitation to deeply explore the interaction between multiple banana cold-tolerance genes and clarify their regulatory network,which will be crucial for analyzing the regulatory pathways of bananas in response to cold stress.It is very beneficial to the molecular breeding of banana cold tolerance in the future.

Key words: Banana;Cold resistance;Molecular mechanism

中图分类号:S668.1

文献标志码:A

文章编号:1009-9980(2022)03-0483-12

DOI:10.13925/j.cnki.gsxb.20210468

收稿日期:2021-09-23

接受日期:2021-11-12

基金项目:国家重点研发专项(2018YFD1000302);国家自然科学基金(31872939);广东省省级科技计划项目—农业种业共性关键技术创新团队(2021KJ106,2020KJ106,2019KJ106)

作者简介:吴烁凡,男,在读硕士研究生,研究方向为园艺植物资源与利用。Tel:18671347959,E-mail:2489196739@qq.com

*通信作者Author for correspondence.E-mail:cjdxyaozhen@aliyun.com,E-mail:soyang@hotmail.com