邢佳毅 刘 伟
(北京市农林科学院蔬菜研究中心,农业部都市农业华北重点实验室,北京 100097)
嫁接作为一种园艺技术最早起源于2 000多年前的东亚,当时是用于解决以有限的耕地集约化种植蔬菜,20世纪末传入欧洲,之后传入北美(Kubota et al.,2008)。现今,嫁接主要用于提高作物产量、减少病虫害发生以及降低种植成本等(Rivard &Louws,2008;Mudge et al.,2009;Rivard et al.,2010;Haroldsen et al.,2012)。嫁接的优势主要体现在以下几方面:提高优良品种的产量潜力(Oztekin et al.,2009)、提高作物适应不良环境的能力(Rivard et al.,2010)、减少化学物质(药剂和肥料)的使用(Louws et al.,2010)、提高作物对水分和土壤等资源的利用率(Schwarz et al.,2012)、通过表观遗传学产生有益的基因型变异(Albacete et al.,2015)、提高果实品质等(Lee et al.,2010;S á nchez-Rod rí guez et al.,2013)。
番茄(Solanum lycopersicumL.)作为重要的园艺作物之一,随着其在设施生产中的发展,连作障碍、次生盐渍化、青枯病以及根结线虫等土传病害日益严重(Thompson et al.,2007)。选择适宜的砧木嫁接不仅能够提高番茄果实产量、改善品质(Flores et al.,2010),还能够提高植株抵御病毒、细菌、真菌和线虫等生物胁迫的能力(Kunwar et al.,2015),以及增强植株的耐热性、耐寒性、耐盐性、耐旱性、耐弱光能力等(Venema et al.,2008;Colla et al.,2010;Schwarz et al.,2010)。目前,关于嫁接番茄的研究主要集中在砧木对于接穗的影响,砧木能够通过韧皮部和木质部为接穗的生长提供水分、养分、激素、代谢产物、多肽、有机小分子以及核酸等物质(Estañ et al.,2005;Villalta et al.,2008;Dun et al.,2009)。砧木中的 mRNA能够通过韧皮部传递到接穗中,调控叶片形态的发育(Kim et al.,2001)。
虽然嫁接在番茄上应用广泛,但对于嫁接如何提高番茄抗逆性的机制仍然不太清楚。本文就近年来番茄嫁接中砧木与接穗之间有关养分吸收与运输、激素调控、基因与蛋白表达等方面的研究进行简要综述,以期为嫁接在番茄抗逆栽培和品种改良方面提供理论参考。
优良的番茄砧木往往根系发达,具有更强的水分和养分吸收能力,能够促进作物的生长发育,为作物抗逆性的提高提供生理基础(Martinez-Rodriguez et al.,2008;Flores et al.,2010)。众所周知,砧木对于养分和水分的摄取能够影响植株的表型,但是这其中的生理机制还知之甚少。
研究表明,与Florida47自根苗相比,以Beaufort和Multifort为砧木的番茄嫁接苗对水分和N素的利用率均有所提高,并且产量也分别提高了近27%和30%(Djidonou et al.,2013)。砧木LA1777能够增加接穗Moneymaker叶片中C素和N素的积累,其中C素含量增加主要是由于叶片中淀粉的累积,从而提高了番茄植株的耐寒性(Venema et al.,2008)。Khah等(2006)也证实,番茄嫁接后植株能够通过提高P素的利用率来提高产量。在干旱条件下,由于植株缺乏蒸腾作用,导致作物根系对N素的吸收和转运出现问题(Robredo et al.,2011);干旱胁迫还能抑制N代谢过程中酶的活性,从而降低植株对N的吸收(Li & Lascano,2011)。S á nchez-Rod rí guez等(2013)研究表明,以Zarina为砧木进行嫁接能够提高干旱条件下番茄植株对N的吸收和光呼吸作用,增加番茄叶片对的光合同化产物。Cristina等(2017)采用4种不同的嫁接组合来研究低P胁迫对番茄生长发育的影响,通过分析韧皮部汁液和叶片中的离子种类以及各激素含量变化,发现低P胁迫能够影响番茄植株对P、Ca、S和Mn元素的吸收并引起乙烯和细胞分裂素的含量变化,最后筛选出在低P环境下能够稳定生长的砧木Hp-type。此外,砧木还能增强番茄接穗对于 K+、Ca2+和 Mg2+的吸收(Savvas et al.,2010)。虽然养分吸收和同化看起来对于砧木提高植株的活力非常重要,但是除了Albacete等(2009)关于75 mmol·L-1NaCl处理下嫁接能够通过提高番茄植株韧皮部汁液中的K+/Na+比值来提高植株的抗盐性以及增加产量的报道外,鲜有关于嫁接植株体内养分浓度与产量的相关性研究。
砧木不仅能够增强番茄植株对养分的吸收,在有些情况下还能减少植株对有毒离子的吸收和转运(Savvas et al.,2010)。例如,在硼(B)元素过量的环境下,以Arnold为砧木的番茄嫁接苗中编码B转运蛋白的基因表达量下降,从而减少根系对B的吸收以及该元素在茎部的积累(Gioia et al.,2017)。
砧木不但可以调节番茄植株对营养元素的吸收和运输,而且能够通过提高植株体内化合物的含量和活性来调节生长发育(Fernaândez-Garciâa et al.,2004),并因此增强植株的抗逆性(Ghanem et al.,2008;S á nchez-Rod rí guez et al.,2012b)。在盐分胁迫条件下,嫁接能够提高番茄过氧化氢酶(CAT)、抗坏血酸过氧化物酶(APX)、脱氢抗坏血酸还原酶(DHAR)和谷胱甘肽还原酶(GR)等抗氧化酶的活性(He et al.,2009)。而在高温胁迫条件下,嫁接番茄植株体内谷胱甘肽过氧化物酶(GPX)和CAT的活性增强,导致各组织中的H2O2含量增加,从而提高番茄的耐热性(Rosa et al.,2003)。Angelika和Dietmar(2013)研究表明,嫁接能够缓解弱光对于番茄生长发育的抑制,与自根苗相比,以Piccolino和Classy为砧木的嫁接番茄果实中类胡萝卜素(包括番茄红素和β-胡萝卜素)以及糖、酸的含量更高。在干旱胁迫条件下,采用Zarina作砧木能够提高植株的抗坏血酸、酚类、黄酮类、番茄红素和β-胡萝卜素等抗氧化物质的含量,从而提高樱桃番茄的产量和果实品质(S á nchez-Rod rí guez et al.,2012a)。
到目前为止,关于番茄嫁接的大部分研究还是集中在筛选养分利用率高的砧木,或者与嫁接苗栽培配套的灌溉与施肥措施上,而从生理水平和分子水平上揭示这种高养分利用率的发生机制的研究还有待加强。
在嫁接番茄中,砧木可以通过激素调节来影响接穗的生长(Holbrook et al.,2002;P é rez-Alfocea et al.,2010)。例如,以耐盐野生品种为砧木嫁接栽培品种,可以提高番茄的产量和品质,原因在于砧木能够增强调节叶面积与叶片衰老的根源性离子和激素因子的供应能力(Santa et al.,2002;Fern á ndez-Gar cí a et al.,2004)。在 75 mmol·L-1NaCl处理下,与对照相比,番茄嫁接苗叶片木质部中反玉米素(t-Z)、脱落酸(ABA)和吲哚乙酸(IAA)的浓度均发生变化(Albacete et al.,2008,2010)。此外,在盐分胁迫条件下,将野生型番茄嫁接到一个能够持续表达异戊烯基腺苷转移酶(IPT)基因(调控细胞分裂素合成)的砧木(35S∶∶IPT)上,结果发现与野生型自根苗相比,嫁接苗的产量提高了30%,这种产量上的差异很可能是由于嫁接促进了番茄植株茎部的发育并减少了花的败育,因为嫁接苗中t-Z的浓度比对照高1.5~2.0倍,而这种激素能够促进发育的果实中细胞的分化和膨大,从而提高单果质量(Ghanem et al.,2011)。Ghanem等(2011)还指出,盐分胁迫能够通过降低番茄茎部细胞分裂素(CK)的浓度来减少果实产量,而采用35S∶∶IPT砧木嫁接后,嫁接苗根部产生的CK能够通过韧皮部传递到植株地上部分,从而缓解盐分胁迫带来的危害。还有报道指出,在嫁接番茄中CK等激素能够通过调节库源活力来提高植株的抗逆性(Albacete et al.,2014),而 Dodd等(2009)将ABA缺失突变体(fl acca)嫁接到正常的野生型番茄砧木上,发现根部产生的ABA能够调控叶片气孔的发育。
目前在豌豆(Pisum sativum)和拟南芥(Arabidopsis thaliana)中已经证实,激素信号的传递往往与类胡萝卜素等化合物的代谢有关(Dun et al.,2009;Sieburth & Lee,2010),鉴别出这些分子及其信号元件能够更好地提高嫁接番茄适应逆境的能力。
番茄作为模式植物已经完成了全基因组测序,并得到了许多性状鲜明的突变体,这为在嫁接中筛选优质砧木,以及研究在番茄根部特异表达的基因如何影响地上部的形态发育提供了有利条件(Asins et al.,2010;Harada,2010)。
转录组学和蛋白组学的方法是研究嫁接番茄砧木与接穗之间基因和蛋白表达变化的重要方法(Vitale et al.,2014)。Turhan等(2016) 通 过SDS-PAGE和免疫印迹的方法发现,与自根苗相比,以Beril、Logure和Valiant为砧木的番茄嫁接苗能够通过提高叶片中热激蛋白HSP23和HSP60的含量来抵抗高温胁迫。Muneer等(2016)对以B-blocking为砧木的番茄嫁接苗分别进行低温(15℃)和高温(30 ℃)处理,发现温度胁迫下植株能够产生H2O2和两种形式的活性氧,并且嫁接苗中超氧化物歧化酶(SOD)、CAT和APX的含量也更高,通过2-DE技术共鉴定出87个差异表达蛋白,这些蛋白参与了防御、应激反应,离子结合、运输,光合作用和蛋白质合成等过程。Georgia等(2017)采用营养液膜技术(NFT)研究发现,低温胁迫条件下耐低温砧木LA1777能够提高番茄植株茎部的干、鲜质量,叶面积,根干质量,气孔导度,胞间CO2浓度和过氧化物酶活性等;对番茄植株叶片进行转录组分析,共筛选出361个差异表达基因,对这些基因进行功能分类发现纤维素合成可能是植株在低温下的响应机制之一。
近年来的研究发现,一些特异的RNA分子能够通过韧皮部传递来调控作物的器官发育(Harada,2010),而在模式植物烟草(Nicotiana benthamiana)中已经证实,利用转录后基因沉默技术(PTGS)对砧木中的特定基因进行沉默后,这种siRNA同样能够传递到接穗中(Kasai et al.,2011)。在嫁接番茄中开展类似试验,有利于更好地了解嫁接番茄的抗逆机制。
虽然嫁接番茄在提高植株抗逆性方面的作用已经被证实,但由于具有多重抗性的优良砧木较少,目前在生产上嫁接番茄的应用面积并不是很大。鉴于此,一方面要加强对于野生番茄资源的挖掘和选育,培育出更多的优良砧木品种;另一方面,要加深嫁接番茄砧木与接穗之间关于信号传递和基因表达等方面的研究,阐明嫁接番茄的抗逆机制,更好地利用嫁接这一手段来鉴定、开发和创造新的遗传多样性,从而提高番茄适应逆境的能力。
另外,Velez-Ramirez等(2014,2105)在研究嫁接番茄耐持续光照的试验中,将敏感型的A131接穗和耐受型的CLT接穗同时嫁接到A131上,结果发现与根部相比,提高植株茎部的耐光照性更能提高番茄的产量,因此培育耐光照的接穗比耐光照的砧木更加重要,这一结果为研究嫁接番茄植株响应抗逆境的信号发生部位提供了新思路。
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