果蔬为人类生命活动提供丰富的营养、能量和生物活性物质,包括氨基酸、蛋白质、不饱和脂肪、膳食纤维、维生素、胡萝卜素、矿物质、多酚等[1]。然而,绝大多数的果蔬采后呼吸代谢旺盛,水分含量急剧下降,可溶性糖等营养物质严重损失,加速贮藏品质劣变,最终丧失商品价值[2]。目前,对果蔬的成熟衰老和品质劣变的生理调控的研究较多,并已开发一系列的综合保鲜技术。但是,有关延缓果蔬品质劣变的调控机制,人们仍缺乏深层次的基础理论认识,特别是贮藏品质的转录调控,极大地制约了果蔬贮藏保鲜产业技术持续发展。因此,深入开展果蔬采后贮藏品质劣变调控机制研究,可为果蔬贮藏保鲜和加工提供重要的理论基础。本文以低温贮藏为切入点,分别从果蔬采后品质劣变、糖代谢及其转录调控等方面进行阐述,以期为果蔬保鲜技术的应用及研究提供理论参考。
果蔬采后呼吸旺盛导致营养物质损失,严重破坏其食用品质。研究表明,甜玉米常温贮藏4 d后,可溶性固形物、维生素C和有机酸等重要营养成分指标的含量分别下降了58.33%、22.28%和56.85%[3];常温贮藏14 d后,其蔗糖浓度从67.1mg/gFW降低至4.4mg/gFW,总糖浓度也从125 mg/g FW降低至30 mg/g FW[4]。此外,果蔬采后蒸腾速率增强,组织水分快速损失,导致表面皱缩萎蔫、嫩度下降,严重影响其外观品质;而且由于在贮藏过程中受到病原微生物的侵染,果蔬极易发生腐败变质[5-6]。可见,有效维持果蔬贮藏过程中的外观和营养品质、抑制病害的发生,是延缓果蔬采后品质劣变的主要工作。
目前,有关果蔬采后贮藏品质研究主要集中在物理、化学和生物保鲜等方面,包括气调贮藏、低温贮藏,涂料涂膜保鲜、杀菌防腐保鲜等。其中,低温贮藏是果蔬采后保鲜技术中最常用的方法之一,可有效地维持果蔬采后外观和营养品质。研究发现,随着贮藏温度升高,果蔬蒸腾作用和呼吸作用增强,损耗大量的水分和营养物质,增加失重率,使其货架期变短[4];而低温(4℃)贮藏,可显著抑制甜玉米在贮藏期间的呼吸强度,延缓可溶性糖含量的下降,抑制淀粉的累积[7-9],较好地保持了其营养品质。与常温(30℃)贮藏相比,低温贮藏(2℃、5℃)也能显著降低椰果的呼吸速率,有效降低质量损失、外壳褐变、丙二醛(malondialdehyde,MDA)含量等,并将贮藏期从3 d延长至6周[10]。儿菜在低温贮藏下可保持良好的外观品质,避免质量、硬度、硫代葡萄糖苷和可溶性糖含量下降[11]。低温贮藏还能减缓冬枣果实硬度下降、推迟果皮转红,维持其外观品质,而且能降低呼吸速率、质量损失和腐烂率,抑制可溶性固形物、可滴定酸和抗坏血酸的降解,从而维持其营养品质[12]。
其次,低温贮藏能够提高果蔬中抗氧化物质的含量、抗氧化酶的基因表达量和酶活性,从而有助于维持其贮藏品质。研究表明,低温(4℃)贮藏下,甜玉米MDA含量较低,而活性氧清除酶系统的超氧化物歧化酶、过氧化氢酶、抗坏血酸过氧化物酶活性则保持较高[13]。在不同贮藏温度(0、10、15℃)下,0℃贮藏的鲜切洋葱能保持较好的外观品质,并具有更低的辛辣味道和呼吸速率,而且总酚、花色苷和槲皮素等生物活性物质的含量也维持在相对较高的水平[14]。低温贮藏不仅能增强冬枣果实中抗氧化酶活性及增加基因表达量[12],有助于维持西兰花中多酚等生物活性物质的含量并增强抗氧化酶活性[15],而且能使儿菜的原花青素和总酚等物质的含量保持在较稳定的水平[11]。
此外,低温不仅有助于延缓果蔬呼吸作用等生理代谢过程,降低新陈代谢水平,而且能抑制病原微生物的侵染[16-17]。但是,不同果蔬贮藏的耐冷性存在差异。不适宜的低温贮藏可能会导致果蔬出现冷害症状,加速其品质劣变。杨桃果实在5℃贮运过程中,虽然出现轻微冷害症状,但总体能维持果实品质,有效延长其贮藏期和货架期;而10℃贮运则加速了果实的转黄和腐烂[18]。3℃~5℃低温贮藏有助于延长荔枝果实的贮藏期和货架期;但贮藏温度跃升后,诱导了荔枝果皮中脂肪酶、磷脂酶D、脂氧化酶的活性,从而破坏果皮组织细胞膜的完整性,最终导致果皮褐变的发生[19]。因此,在果蔬保鲜实践过程中,低温贮藏通常结合其他物理、化学和生物保鲜技术,从而进一步提高果蔬的贮藏性。
糖代谢是果蔬采后品质劣变的重要过程。糖为果蔬的生命活动提供能量,是果蔬代谢活动的保证,是果蔬品质的重要决定因素。除了作为能量物质外,糖还可作为信号分子参与果蔬成熟、衰老和应激反应[20]。果蔬采后糖含量取决于糖降解和合成的平衡,这是由蔗糖合成酶(sucrose synthase,SUS)、蔗糖磷酸合酶(sucrose phosphate synthase,SPS)、蔗糖转化酶(sucrose invertase,IN)、酸性转化酶(acidic invertase,AI)和中性转化酶(neutral invertase,NI)等几个关键酶催化的[21]。其中,蔗糖是糖积累的主要形式。研究表明,高SUS和SPS活性,以及较低水平的AI、NI和IN活性,可促进蔗糖的积累。采后苹果外施用腺嘌呤核苷三磷酸(adenosine triphosphate,ATP),可通过增强苹果中琥珀酸脱氢酶、细胞色素氧化酶、SUS和SPS的合成活性,并降低NI、AI和IN的活性来延缓蔗糖降解,从而改善苹果的内在品质[22]。硝普酸钠处理苹果也可通过增强SUS 和 SPS 活性,并抑制 IN、NI和 AI的活性[23],延缓蔗糖降解,从而延缓苹果品质下降。但是,乙烯可诱发还原糖积累,促进果蔬采后蔗糖含量的下降。研究表明,外源乙烯处理采后蓝莓,能抑制SPS基因的表达,诱导AI和NI基因的表达,从而促进蔗糖降解和加速果实软化[24]。可见,通过调控蔗糖代谢维持果蔬采后糖积累水平,与其采后品质密切相关。此外,果蔬中的淀粉在成熟和衰老过程中水解转化为可溶性糖,使得果蔬甜度增加。α-淀粉酶(α-amylase,Amy)作为淀粉转化的关键酶,催化淀粉水解生成葡萄糖、蔗糖和果糖等可溶性糖,在植物结构和代谢中起着至关重要的作用[25]。猕猴桃是一种典型的以淀粉形式积累碳水化合物的水果,在采后贮藏过程中,淀粉被转化为可溶性糖;使用2,4-表油菜素内酯处理猕猴桃,可抑制Amy、AI、NI、SPS、SUS、己糖激酶(hexokinase,HXK)和磷酸果糖激酶(phosphofructokinase,PFK)的活性,延缓淀粉水解生成可溶性糖[26],进而影响猕猴桃甜度和柔软质地。由此可见,果蔬采后糖代谢水平直接影响其成熟衰老进程及贮藏品质。
目前,糖代谢作用是果蔬采后低温贮藏保鲜研究的热点问题之一[27]。研究表明,冷锻炼有助于延缓苹果长期低温贮藏后果肉硬度下降、细胞膜功能障碍、粉化程度降低等品质劣变[28]。其中,苹果体内异淀粉酶、淀粉分支酶和淀粉合酶(starch synthase,SS)等基因表达水平显著下调,而葡聚糖水双激酶(alpha-glucan water dikinase,GWD)、Amy等基因表达水平显著上调,最终促进淀粉逐渐分解并转化为蔗糖、果糖和葡萄糖;此外,PFK,1,6-二磷酸果糖酶和HXK等基因表达水平显著下调,而SUS、SPS、IN、AI和NI等基因表达上调,进一步促进可溶性糖的积累。Wang等[29]研究发现,低温(0℃)贮藏下梨果实的硬度和总可溶性糖含量均高于常温贮藏;随着贮藏时间的延长,果实SUS和SPS基因表达水平和酶活性均下降,而IN基因表达水平和酶活性则上升;低温贮藏60 d后,梨果实未出现冷害症状,且果实蔗糖和山梨醇含量降低,而果糖和葡萄糖含量升高。在桃果实中,外源一氧化氮结合低温(4℃)处理,也能促进SPS基因表达水平和酶活性上升,而抑制蔗糖裂解酶基因表达和酶活性,从而提高果实中蔗糖含量,降低葡萄糖和果糖含量[30]。鲜切甜瓜在常温(15℃)贮藏过程中,硬度、肉色等感官品质会迅速恶化,呼吸速率加快,乙烯释放增多;而低温(5℃)贮藏可显著延长其货架期,并保持较高的蔗糖和有机酸含量[31]。其中,低温能促进磷酸烯醇式丙酮酸羧化酶基因的表达,而抑制AI、NI、柠檬酸合酶、乌头酸酶和苹果酸酶等基因的表达。此外,低温能诱导番薯Amy、SUS和PFK等基因表达上调,而下调β-葡萄糖苷酶、葡萄糖-1-磷酸腺苷转移酶和SS等基因表达,从而有助于块根中可溶性糖含量的增加[32]。Xie等[33]研究表明,4℃贮藏可减少采后马铃薯发芽率和腐烂率,并促进可溶性糖积累;赤霉素处理可在马铃薯块茎低温贮藏过程中上调β-淀粉酶(β-amylase,BAM)、脲苷二磷酸葡萄糖焦磷酸化酶的表达,下调腺苷二磷酸葡萄糖焦磷酸化酶、颗粒结合淀粉合成酶以及酸性转化酶抑制子的表达,从而降低淀粉合成速率,并加快淀粉及蔗糖水解速率,促进低温糖化;而S-香芹酮处理,则减缓低温糖化现象的发生。因此,低温贮藏有助于维持果蔬中可溶性糖等营养物质的含量,延长贮藏期;而低温诱导果蔬的可溶性糖积累,与其糖代谢相关酶的基因表达水平和酶活性变化密切相关。
植物转录因子广泛参与抗逆境胁迫过程,CBF/DREB、WRKY、bHLH和MYB等还参与果蔬生长发育和成熟衰老过程中转录调控,特别是糖代谢[34]。早期研究表明,甜玉米中的转录因子ZmERF3不仅能受到低温条件的诱导,而且还参与其他生物胁迫过程,如干旱、高盐及茉莉酸、水杨酸和脱落酸(abscisic acid,ABA)等激素信号途径[35]。玉米中的转录因子ZmCBF3基因在水稻中超表达后,能够诱导胁迫基因的表达,提高水稻抗旱、抗盐和抗低温的能力[36]。在香蕉果实中,低温能诱导MaICE1转录因子的基因表达,进而促进MaNAC1转录因子的基因表达,后者与MaCBF1转录因子相互作用后,共同启动冷诱导信号转导途径,从而提高香蕉果实贮藏耐冷性[37]。Li等[38]研究发现,甘薯冷锻炼(10°C)5 d后转入4°C冷藏,能有效控制MDA含量增加,减少活性氧的产生,并能促进SUS基因表达提高蔗糖含量,进而减缓发生冷害;其中,低温可能通过诱导甘薯中的转录因子SwDREB1的基因表达,进而促进BAM基因的表达,加快淀粉水解。此外,WRKY转录因子也参与果蔬成熟过程中蔗糖合成的调节,SPS基因的表达可促使成熟南瓜蔗糖的积累[39]。Huang等[40]研究表明,外源施用果糖和ABA可诱导葡萄中的转录因子WRKY22基因表达,进而诱导下游靶基因蔗糖非发酵相关蛋白激酶1、HXK、细胞壁转化酶、SPS和海藻糖-6-磷酸磷酸酯酶等糖代谢相关酶的表达,从而降低葡萄成熟过程中蔗糖、葡萄糖和果糖的含量;而葡萄中的转录因子WRKY22基因沉默后则显著提高了葡萄可溶性糖含量。进一步研究表明,WRKY22能与SnRK1互作调节葡萄果实糖代谢;其中,SPS的活性受到SnRK1磷酸化抑制,而WRKY22基因表达沉默后可下调SnRK1基因的表达,进而诱导SPS活性增强,提高蔗糖合成速率。在火龙果果实成熟过程中,葡萄糖和果糖等可溶性糖显著积累。其中,火龙果WRKY3能直接与IN基因(HpINV2)和SUS基因(HpSUSPY1)的启动子结合,进而激活它们的转录,而HpINV2和HpSUSPY1的表达与火龙果成熟过程中葡萄糖和果糖积累呈正相关[41]。由此说明,WRKY3能激活蔗糖合成代谢基因转录,进而促进火龙果的糖积累。转录因子bZIP家族也与果蔬的糖代谢密切相关,番茄果实中异源过表达草莓中的转录因子FabZIP11能够诱导其糖分积累[42];野生型与FabZIP11过表达番茄植株在生长发育过程中总可溶性固形物和可溶性糖含量均呈上升趋势,而FabZIP11过表达番茄在果实成熟过程中总可溶性固形物和可溶性糖含量均显著提高。Sagor等[43]研究发现,番茄中的转录因子SlbZIP1和SlbZIP2可通过激活天冬酰胺合成酶和脯氨酸脱氢酶的活性,也正向调控可溶性糖的合成,进而提高果实蔗糖、葡萄糖和果糖含量。此外,香蕉果实在后熟过程中,淀粉含量不断降低,可溶性糖(麦芽糖和葡萄糖)含量逐渐增加。其中,香蕉MabHLH6转录因子能与MaGWD1启动子结合,进而诱导淀粉降解相关基因(GWD、葡聚糖磷酸酶、BAM、Amy、淀粉脱支酶、α-葡聚糖磷酸化酶和质体葡萄糖转运蛋白等)的表达,最终促进可溶性总糖含量增加[44]。在MYB转录因子家族中,Li等[45]研究表明,苹果通过介导糖转运蛋白STP13a吸收花柱周围组织中质外体的蔗糖、己糖等碳水化合物合成山梨醇,从而有助于促进花粉管的生长。其中,MdMYB39L转录因子与MdSTP13a的启动子互作,激活MdSTP13a表达;而MdSTP13a的反义表达则抑制山梨醇的生成。此外,Jiang等[46]研究发现,香蕉MaMYB16转录因子通过可变剪接分别产生MYB16L和MYB16S;其中,MaMYB16L能结合MaDREB2启动子,进而抑制Amy、BAM、GWD、磷酸葡聚糖水双激酶、AI、NI、碱性转化酶等淀粉降解相关酶基因的表达;而MaMBY16S虽无转录活性,但能与MaMYB16L竞争性结合形成非功能性异源二聚体,从而降低MaMYB16L对靶基因启动子的结合能力,解除转录抑制。进一步研究发现,MaMYB16选择性剪接随果实成熟进程而逐渐加强,引发MaMYB16L表达下调而MaMYB16S表达上调,进而促进淀粉降解基因及MaDREB2的表达,最终导致果实的成熟软化;而MaMYB16L瞬时过表达也能抑制淀粉降解及成熟相关基因的表达。在甜瓜中,蔗糖是果实风味品质的重要成分。Gao等[47]研究表明,外源乙烯处理能够诱导成熟期高蔗糖含量甜瓜果实内源乙烯的合成,并提高SPS活性促进果实中蔗糖积累,从而提高果实风味品质;而低蔗糖含量甜瓜果实对外源乙烯则无响应。其中,外源乙烯是通过诱导甜瓜中CmMYB113转录因子过表达,进而激活1-氨基环丙烷-1-羧酸氧化酶基因CmACO1和SPS基因CmSPS1的表达。由此可见,转录因子介导的糖代谢转录调控作用,在果蔬生长发育、抗逆胁迫、成熟衰老等过程中发挥重要的调控作用。
MYB转录因子参与植物次生代谢、逆境胁迫、激素信号转导以及生长发育等过程[48]。目前,有关MYB在果蔬耐冷性和花青素合成的作用已进行深入研究。其中,花青素具有抗氧化和清除活性氧的能力,有助于提高果蔬耐冷性。An等[49]研究表明,低温能诱导苹果中转录因子MdMYB308L与MdbHLH33互作,增强MdCBF2和MdDFR的表达,从而正向调控苹果耐冷性和花青素合成;而MdMIEL1(MYB30互作E3连接酶)则通过MdMYB308L的泛素化降解起负向调控作用。Xie等[50]研究表明,低温诱导苹果中转录因子MdMYB88和MdMYB124能直接结合MdCCA1(circadian clock associated)启动子,促进MdCBFs转录因子的表达,进而激活下游耐冷相关基因COR(cold-regulated)的表达;同时,苹果花青素的积累可同时受到MdMYB88/MdMYB124和MdCBFs的正向调控,进而有助于过氧化氢的清除,提高苹果耐冷性。此外,MdMYB88/MdMYB124还可直接诱导MdCSP3(cold shock domain protein)的表达而提高苹果耐冷性。在蔬菜的研究中,低温条件下生长的紫甘蓝总花青素含量比温室条件下高约50倍;研究发现,紫甘蓝MYB转录因子家族成员BoPAP1也能受低温诱导,进而增强花青素生物合成基因 C4H、F3H、DFR、ANS和 UFGT的表达,促进花青素的积累[51]。此外,MYB转录因子还与果实低温贮藏品质密切相关。Xu等[52]研究表明,在低温(0℃)贮藏中时,EjMYB2能与EjAP2-1和EjbHLH1结合形成三元复合物,进而抑制木质素生物合成基因Ej4CL1基因的表达,减缓果实果肉木质化进程。但是,近期研究表明,MYB转录因子家族可能在香蕉果实低温(7℃)贮藏3 d后的成熟阶段中发挥复杂的调控作用。其中,MaMYB151/156/37、MaMYB3/4和 MaMYB3R1受低温诱导,在果实成熟过程中表达上调;而MaMYB75/281/219在香蕉果实中的调控作用仍不清楚[53]。此外,MaMYB4能招募组蛋白去乙酰化酶MaHDA2,通过降低ω-3脂肪酸去饱和酶的乙酰化水平,从而提高香蕉果实的耐冷性[54]。在草莓品质形成和成熟过程中,FaMYB44.2能抑制蔗糖磷酸合成酶FaSPS3的表达,而FaMYB10则起到正调控的作用[55]。由此可见,MYB转录因子家族在果实维持贮藏品质中存在复杂的调控功能;在果蔬低温贮藏过程中,糖代谢是否受到MYB的调控及其具体作用机制,有待进一步深入研究。
综上所述,果蔬低温贮藏过程中可溶性糖的积累,有助于延缓采后品质劣变;糖代谢转录调控作用会参与果蔬生长发育、抗逆胁迫、成熟衰老等过程,但目前有关果蔬低温贮藏过程中糖代谢调控作用的研究仍有待进一步探究。根据现有研究结果推测,低温可能通过MYB等转录因子,进而调控糖代谢相关酶基因的表达,维持果蔬低温贮藏过程中的糖代谢水平。因此,今后开展果蔬低温贮藏糖代谢调控机制研究的重点内容:一是筛选低温响应MYB转录因子,二是筛选调控果蔬低温贮藏过程中糖代谢水平的关键酶基因,三是揭示低温响应MYB转录因子对糖代谢关键酶基因的转录调控方式及其调控通路。开展果蔬低温贮藏过程中的分子生物学研究,阐明低温响应MYB转录因子对果蔬贮藏过程中糖代谢的调控机制,可为果蔬保鲜技术提供理论基础。
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