Genetic Information Exchange and Variation Mechanism of Grafted Plants
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植物受物理因素等损伤后，自身会进行创伤愈合，在植物进行伤口修复过程中，植物激素促使细胞再生，同时植物激素相关转录因子进一步调控细胞分化、细胞周期激活和维管束组织重建等细胞进程[1-2]。嫁接就是利用植物受伤后会进行伤口自我修复原理，把接穗(scion)的枝或芽嫁接到砧木(stock/rootstock)的茎或根上，使砧穗的两个创面的形成层相吻合，通过细胞分裂增生，形成有功能的维管组织进而长成新植株的方法。嫁接用“+”表示，即砧木+接穗，也可以用“/”表示，即接穗/砧木[3-4]。嫁接分为3种类型：自根嫁接(autograft)，接穗和砧木来自同一植株；同源嫁接(homograft)，接穗和砧木来自同一物种的不同植株；异源嫁接(heterograft)，接穗和砧木来自两个不同物种[5]。
在现代农业中，嫁接作为一种古老的技术在果树、观赏植物、蔬菜、花卉等植物的繁殖、栽培和育种上产生了很大的经济效益[6-7]。同样嫁接在保持良种特性、改变植物的开花结果习性、矮化植株、延长接穗植株寿命、改良果实的品质、提高生物胁迫(病原物、虫害)和非生物胁迫(重金属、干旱、冷害、热害等)抗性等方面也起着重要作用[8]。
嫁接可以提高果蔬的品质，包括果实外观(大小、形状、颜色)、风味(糖、酸、芳香物质、酚类等)、有益化合物(矿物质、维生素、胡萝卜素)[9]。嫁接影响接穗果实品质的研究在茄子/茄子[10-11]、番茄/枸杞[12]均有相应的研究报道。嫁接是防治危害茄科和葫芦科植物的土壤病原微生物和害虫的一种综合防治措施。在蔬菜生产中，嫁接在提高茄科作物，如茄子[13]、番茄[14]、辣椒[15]，和葫芦科作物，如西瓜[16]、甜瓜[17]、黄瓜[18]对生物胁迫适应能力方面已被广泛应用。嫁接同样可以提高植物对非生物胁迫的适应能力，研究表明：嫁接栽培能让植株适应重金属含量较高的土壤环境，其主要原因是砧木可以限制重金属离子向接穗运输；另一种原因就是通过根系解毒作用[19]。嫁接也能增强涝害、干旱、热害等环境胁迫的适应性[4]。
传统观念认为，嫁接是一种无性繁殖方式，不会引起嫁接当代及后代的遗传物质改变。但很多研究表明植物之间的嫁接可以引起性状改变，且可以稳定遗传[20-21]。目前关于嫁接引起遗传变异的机理主要有以下4个方面的解释：第一，嫁接可以诱导砧穗间遗传物质交流，即以一种基因水平转移的方式进行[22-26]；第二，砧穗间RNA长距离单向或双向运输至相应的靶点，执行调控植物的生长发育功能[27-30]；第三，表观遗传修饰，即siRNA调控DNA甲基化，即嫁接后砧穗间发生了siRNA的转移，引起受体基因组的超甲基化或去甲基化的发生[31-33]；第四，由于“砧木—接穗—环境”三者间的相互作用，导致嫁接植株的表型变异[34]。本文将从嫁接引起遗传交流及诱导变异的可能机理进行综述。
1. 嫁接引起遗传信息交流
前人研究表明：核酸是细胞自生性(cell-autonomous)的，即存在于合成的细胞里，不能转运至相邻细胞，但越来越多的研究表明：核酸也能转运到合成以外的地方，调控植物的生长发育[35]。通过嫁接可以引起遗传物质通过韧皮部在砧穗间交流，实现对植物进行遗传改良。
1.1 嫁接引起遗传物质DNA交流
最新研究证明：嫁接可以引起质体基因组或大片段的DNA、线粒体DNA、核基因组DNA通过胞间连丝(plasmodesmata)进行胞间转移或交流(表1)。STEGEMANN等[22]研究表明：两种烟草的质体基因组发生了交流；研究者构建了两种转基因株系且分别携带了不同的标记基因和报告基因。一个株系为Nuc-kan:yfp，在核基因组中携带了卡那霉素抗性基因(nptII) 和黄色荧光蛋白基因(yfp)，另一个株系为Pt-spec:gfp，在质体(叶绿体)基因组中携带了大观霉素抗性基因(aadA)和绿色荧光蛋白基因(gfp)，然后将这两个转基因烟草株系相互嫁接，融合发生后切下嫁接接合部位在含两种抗生素的培养基中进行抗性筛选、荧光信号分析和PCR分子鉴定，发现嫁接使两种烟草大片段质体DNA间发生了交流，然而细胞核基因没有发生融合，他们认为大片段DNA能够在紧密相邻的细胞间转移，胞间连丝是遗传物质在异源细胞之间传递的通道。
表 1 植物嫁接引起DNA转移Table 1. The horizontal gene transfer through graftingDNA 植物(接穗/砧木) plant species (scion/rootstock) 质体基因组plastid DNA 转基因烟草/转基因烟草[22] Nuc-kan:yfp/Pt-spec:gfp 核基因nuclear gene 转基因烟草/转基因烟草，粉蓝烟草/普通烟草[26] Nt-kan:yfp/Nt-hyg，Nicotiana glauca/Nicotiana tabacum 质体基因组plastid DNA 普通烟草/美花烟草[36] Nicotiana tabacum/Nicotiana sylvestris 线粒体基因mitochondrial gene 普通烟草/美花烟草[37] Nicotiana tabacum/Nicotiana sylvestris 叶绿体基因chloroplast gene 粉蓝烟草/普通烟草，粉蓝烟草/本氏烟草[24] Nicotiana tabacum/Nicotiana glauca，Nicotiana tabacum/Nicotiana benthamiana 美国罗格斯大学WAKSMAN微生物研究所为了检测含DNA的质体和线粒体两个细胞器能否在植物细胞间移动，研究者将携带庆大霉素抗性标记的N. tabacum和携带大观霉素抗性的N. sylvestris的两个不同烟草品种进行嫁接，因为嫁接后在接合部位形成新的“细胞—细胞”通道，从而可以检测两个截然不同的植物细胞器能否在细胞间进行移动。通过在双抗性培养基筛选得到具备抗大观霉素和庆大霉素抗性标记的无性系，并对其进行分子鉴定及叶绿体DNA测序分析，研究结果表明：161 kb的质体基因组通过嫁接结合部发生了移动[36]。为进一步证明线粒体DNA同样可以在植物细胞间进行移动，研究者将携带N. undulata细胞质雄性不育基因的N. tabacum (N.undulata的雄性不育基因是由线粒体基因决定)与N. sylvestris (可育)进行嫁接，结果表明：少量的线粒体DNA可以从N. sylvestris转移到N.undulata，并引起花育性恢复[37]。FUENTES等[26]用同样的研究方法证明了嫁接可以使细胞核基因组在嫁接接合部位发生基因水平转移。
1.2 嫁接引起遗传物质RNA的交流
1970年，KOLLMANN等[38]在植物韧皮部的汁液里检测到了如蛋白质、RNAs这样的大分子物质。后续的研究发现多种内源蛋白和RNA可以在植物韧皮部运输，转移的mRNA编码转录调控因子，调控细胞周期、激素反应和新陈代谢等发育过程(表2)。
表 2 韧皮部汁液中RNA鉴定Table 2. The identification of RNA in plant phloem sap基因gene 功能function 植物plant species KN1 mRNA同源结合蛋白homeodomain protein 玉米[39] Zea mays HAK1 钾转运子potassium transporter 大麦[40] Hordeum vulgare FT 诱导开花flowering signal 拟南芥[41] Arabidopsis thaliana Hd3a 编码水稻成花素flowering signal 水稻[42] Oryza sativa CmF-308 促进侧根发育positively regulate lateral roots 西瓜，番茄[43] Citrullus lanatus，Solanum lycopersicum FTL2/FT 诱导开花flowering signal 南瓜[44] Cucurbita moschata 对于嫁接而言，嫁接后形成有功能的维管组织系统，包括木质部和韧皮部，木质部主要负责运输植物根系吸收的水分和一些无机化合物。而韧皮部主要负责运输源—库之间的有机营养(植物激素、离子、氨基酸、糖类物质)，并可传递RNA分子[45]。表3中列出了目前研究发现的多种植物间同源嫁接、异源嫁接引起的RNA长距离运输及相对应的功能。
KIM等[27]将羽状复叶的野生型番茄(Xa)为接穗，叶片为鼠耳状的突变体(Me)为砧木进行嫁接，结果发现在砧木中特异表达的PFP-LeT6 mRNA长距离运输至接穗，导致接穗叶形向砧木叶形变异。KUDO等[50]在番茄和马铃薯的嫁接体的研究中也发现了相似的现象。BANERJEE等[28]研究发现：马铃薯叶片中产生的块茎发育信号BEL5 RNA能够通过嫁接接合部位向下转移到匍匐茎茎尖，调控马铃薯块茎的形成。NOTAGUCHI等[29]利用本氏烟(Nicotiana benthamiana)作接穗，拟南芥为砧木进行异源嫁接，结果发现有138个拟南芥的mRNA转录本在接穗中检测到。YANG等[30]研究表明：在葡萄间嫁接后发现超过3 000个mRNA通过嫁接结合部发生了转移，这些基因分别调控包括细胞合成、生物合成、新陈代谢活动、胁迫响应、信号传导等进程。
关于嫁接引起非编码RNA (miRNA、siRNA)交流也有很多报道(表3)，miRNA339是调控磷酸盐平衡的miRNA分子，在拟南芥、烟草等嫁接研究中都表明其可以长距离运输，调控接穗植株的磷酸盐平衡[61-62]。BHOGALE等[64]研究发现：miRNA172是控制马铃薯块茎形成的miRNA分子，可以通过嫁接接合部位长距离运输至匍匐茎，调控块茎发育靶基因StBEL5的表达，促进马铃薯块茎膨大。这些长距离运输的miRNA可以响应嫁接引起的各种生理进程，通过调控转录因子或靶基因的表达来参与调控植物的生长发育和对环境胁迫的应答。近年诸多研究发现：在嫁接植株体内，siRNA是作为一种沉默信号可在植物体内移动，介导转录水平基因沉默(TGS)[71]和转录后基因沉默(PTGS)[72]。YOO等[69]将野生型黄瓜与自发型RNA沉默的南瓜嫁接，在黄瓜接穗的韧皮液中检测到siRNA信号。ZHAO等[70]将转基因樱桃为砧木，非转基因樱桃为接穗，发现砧木中的siRNA信号能转移到接穗并增强其病毒抗性。类似的研究在烟草/转基因烟草[73]、番茄/转基因烟草[75]也有相应的报道。总之大量siRNA能够通过嫁接接合部位在维管组织中的韧皮部长距离移动，并作为系统信号对受体基因组进行表观遗传修饰。
表 3 植物嫁接引起RNA通过韧皮部进行长距离运输Table 3. Long-distance RNA molecules transported in grafting plant phloem基因gene 功能function 植物(接穗/砧木) plant species (scion/rootstock) mRNA Cmpp16 介导mRNA长距离运输mRNA movement 黄瓜/南瓜↑[46] Cucumis sativus/Cucurbita moschata CmNACP 调控顶端分生组织发育apical meristem development 黄瓜/南瓜↑[47] Cucumis sativus/Cucurbita maxima PFP-LeT6 叶片形态发育leaf development 番茄/番茄↑[27] Solanum lycopersicum/Solanum lycopersicum Cmgaip，△ DELLA-gai 赤霉素酸转录调控因子，叶片发育TFS of GA, leaf development 南瓜/南瓜↑；拟南芥/拟南芥↑；番茄/番茄↑[48] Cucurbita moschata/Cucurbita moschata; Arabidopsis thaliana/Arabidopsis thaliana; Solanum lycopersicum/Solanum lycopersicum StBEL5 块茎发育tuber development 马铃薯/马铃薯↓[28, 49] Solanum tuberosum/Solanum tuberosum PFP-LeT6 叶片形态发育leaf development 马铃薯/番茄↑[50] Solanum tuberosum/Solanum lycopersicum FT 诱导开花flowering signal 拟南芥/拟南芥↑[51] Arabidopsis thaliana/Arabidopsis thaliana FT 诱导开花flowering signal 南瓜/印度南瓜↑[52] Cucurbita moschata/Cucurbita maxima AUX/IAA 生长素信号转导auxin signaling 南瓜/西瓜↑[53] Cucurbita moschata/Citrullus lanatus IAA18，IAA28 抑制侧根形成negatively regulate lateral root formation 烟草/拟南芥↓[54] Nicotiana tabacum/Arabidopsis thaliana Mhgai1 矮化植株dwarf phenotypes 番茄/番茄↑[55] Solanum lycopersicum/Solanum lycopersicum POTH1 块茎发育tuber development 马铃薯/马铃薯↓[56] Solanum tuberosum/Solanum tuberosum PbKN1 病毒运动结合蛋白binding protein 鸭梨/杜梨↑↓[57] Pyrus bretschneideri/Pyrus betulaefolia 2 006个mRNA 诱导开花，营养胁迫响应flowering signal, nutrient stress response 拟南芥/拟南芥↑↓[58] Arabidopsis thaliana/Arabidopsis thaliana 13个mRNA 受体蛋白激酶，细胞饰变，抵御胁迫receptor kinase, cell modification, stress defense 烟草/拟南芥↑[29] Nicotiana benthamiana/Arabidopsis thaliana 3 000个mRNA 新陈代谢，抵御环境胁迫，信号转导metabolism, responses to stress and signal transduction 葡萄/葡萄↑↓[30] Vitis girdiana/Vitis palmata 787，3 485个mRNA 初生和次生代谢，激素信号，转运蛋白primary and secondary metabolism, hormone signaling, transporters 西瓜/葫芦，西瓜/南瓜↑↓[59] Citrullus lanatus/Lagenaria siceraria, Citrullus lanatus/Cucurbita moschata miRNA miR399 控制磷酸盐平衡regulate phosphate homeostasis 拟南芥/拟南芥，烟草/烟草↑[60-61] Arabidopsis thaliana/Arabidopsis thaliana Nicotiana benthamiana/Nicotiana benthamiana miR399，miR395 调控矿质营养平衡regulate mineral homeostasis 拟南芥/拟南芥；甘蓝型油菜/甘蓝型油菜↑[62] Arabidopsis thaliana/Arabidopsis thaliana; Brassica napus/Brassica napus miR172 块茎发育tuber development 马铃薯/马铃薯↓[63] Solanum tuberosum/Solanum tuberosum miR156 调控植物株型和块茎形成tuber development 马铃薯/马铃薯↓[64] Solanum tuberosum ssp.Andigena/Solanum tuberosum ssp. Andigena 47个miRNA 生长发育和代谢进程growth and metabolism processes 西瓜/西葫芦↑[65] Citrullus lanatus/Lagenaria siceraria 17个miRNA 盐胁迫响应salt stress response 黄瓜/南瓜↑[66] Cucumis sativus/Cucurbita maxima 17个miRNA 干旱胁迫响应drought stress response 黄瓜/南瓜↑[67] Cucumis sativus/Cucurbita maxima 27个miRNA 干旱胁迫响应drought stress response 葡萄/葡萄↑↓[68] Vitis vinifera/Vitis berlandieri siRNA CmPSRP1 RNA结合蛋白RNA binding protein 黄瓜/南瓜↑[69] Cucumis sativus/Cucurbita maxima PNRSV-hpRNA 增强坏死斑病毒抗性PNRSV resistance 甜樱桃/转基因樱桃↑[70] Prunus mahaleb/Transgenic Prunus mahaleb CoYMV:35SIR 叶片和根系发育leaf and root development 烟草/烟草↑↓[71] Nicotiana benthamiana/Nicotiana benthamiana GSA 叶绿素合成chlorophyll synthesis 烟草/转基因烟草↑[72] Nicotiana benthamiana/Transgenic N. benthamiana NtTOM1，NtTOM3 增强烟草花叶病毒抗性TMV resistance 烟草/转基因烟草↑[73] Nicotiana benthamiana/Transgenic N. benthamiana DMC1 花粉育性pollen fertility 烟草/转基因烟草↑[74] Nicotiana benthamiana/Transgenic N. benthamiana NtTOM1，NtTOM3 增强番茄花叶病毒和烟草花叶病毒抗性TMV resistance 番茄/转基因烟草，烟草/转基因烟草↑[75] Solanum lycopersicum/Transgenic N. benthamiana, Nicotiana benthamiana/Transgenic N. benthamiana StGBSS1 块茎形成tuber development 烟草/马铃薯↓[76] Nicotiana benthamiana/Solanum tuberosum 注：“↑”表示RNA由砧木向接穗运输；“↓”表示RNA由接穗向砧木运输；“↑↓”表示双向运输。
Note: “↑” indicates mRNA movement through the graft union from the stock into the scion; “↓” indicates mRNA movement through the graft union from the scion into the stock; “↑↓” indicates bidirectional movement.以上研究为不同物种间通过同源或异源嫁接发生遗传信息交流提供了新的视野，也为远缘嫁接提供了参考。
2. 嫁接引起遗传变异的可能机制
2.1 基因的水平转移
越来越多的研究发现：不同物种的细胞紧密接合时会诱发遗传物质水平转移(HGT)，即受体生物通过无性方式从供体生物获得遗传物质的过程，有别于世代繁衍过程中的有性传递方式。水平基因转移又称横向基因转移(lateral gene transfer，LGT)；相反，遗传物质由亲代到子代称为垂直基因传递[23]。水平基因转移能跨越种间隔离，在亲缘关系远或近的生物有机体间都可以进行遗传信息转移。可以使受体生物绕过点突变和重组快速形成新物种，加速基因组的革新和进化，提高生物遗传多样性[77]。在嫁接实验中，很多研究者证明了嫁接会引起遗传物质水平基因转移。其根本机制有两方面的解释。THYSSEN等[36]和GURDON等[37]提出嫁接结合部融合后砧穗的细胞间形成新的胞间连丝，导致砧穗其中一方细胞被有功能的细胞器(如：质体)填充后组合重建，然后转移的DNA分子会对线粒体等基因组修饰。STEGEMANN等[24]认为是系统发育的结果，即在没有基因间重组的情况下，外源基因组取代了原始基因组，相当于是植物细胞器捕获，这与RICE等[25]提出的“融合—兼并(Fusion-Compatibility)”机理相一致。
总而言之，砧木和接穗嫁接后，在愈伤组织、细胞壁、维管组织的重建过程中伴随着含DNA的细胞器在砧穗间发生水平转移并整合到对方的生殖细胞和胚胎细胞染色体组中，或者取代原本的基因组，从而引起遗传变异。嫁接引起基因水平转移是一种创造新物种的无性途径，同时也为得到新的异源多倍体提供了一种新方法。
2.2 RNA长距离运输引起遗传变异
在生物体内，RNA可以在细胞之间传递遗传信息，即传递给细胞相应的行动计划，编码出特定的蛋白质调控生理活动和新陈代谢[78]。近年来的研究发现：某些RNA分子作为非细胞自生(non-cell-autonomous)的信号分子，可进行胞间或长距离转运控制相关器官的发育或参与防御反应。关于RNA在植物体内的长距离转运，胞间连丝是RNA的胞间转运通道，而维管束组织中的韧皮部是其长距离转运的路径[79]。植物韧皮部汁液中有多种mRNA[45]、miRNA[60-61]和siRNA[69]。RNA分子是大分子物质，特别是mRNA，理论上在没有任何辅助的情况下是无法通过胞间连丝到达韧皮部汁液里的。有研究表明mRNA的长距离移动是一个选择性的过程[48, 78]。特异的核糖体蛋白能与mRNA特异性或非特异性结合，从而形成核糖体蛋白复合物。然后通过胞间连丝从韧皮部的伴胞进入筛管后进行长距离转运[27, 79]。另外TURNBULL等[80]在拟南芥的微嫁接研究发现：RNA也可以随着韧皮部液流进行被动扩散。长距离转运的mRNA作用于目标靶点，调控植物的生长发育及响应各种生理进程，从而引起相应的表型变异。这表明RNA长距离运输也是诱导嫁接遗传变异的主要原因之一。
综上所述，嫁接使砧木和接穗形成完善的维管组织。在生长过程中，韧皮部汁液中存在的mRNA、miRNA、蛋白质等内源大分子物质选择性地从伴胞经胞间连丝进入筛管，并在砧穗间进行长距离运输，到达需要的位置后调控植物相关器官的发育或参与防御反应，进而引起砧木或接穗的表型变异。此外，刘用生等[81]认为砧木中的mRNA转移到接穗中后，可以被反转录转座子反转录成cDNA，进一步整合到接穗细胞的染色体组中，导致遗传物质变异。
2.3 嫁接引起的表观遗传调控
表观遗传调控是指DNA序列不发生改变，而基因表达却发生了可遗传改变，主要有DNA甲基化、组蛋白修饰、染色质重塑和非编码RNA等调控方式，其中DNA甲基化是表观遗传调控的主要方式[82]。DNA甲基化在植物中广泛存在，在植物生长发育过程中，DNA甲基化可以导致植物育性、叶形及其他生理形态变异[83]。嫁接和其他的环境胁迫一样，可以导致一定频率的DNA甲基化模式的变化[84]。WU等[85]研究表明：近缘物种间的异源嫁接会引起可遗传的甲基化改变。LEWSEY等[33]在拟南芥的嫁接试验中发现24-nt siRNA由接穗转移到砧木，并介导受体细胞的DNA甲基化。CAO等[32]通过榨菜与紫甘蓝嫁接嵌合体进行DNA甲基化测序分析，结果表明：嫁接诱导DNA甲基化改变是可遗传的，并且是由于siRNA改变直接引起的，从而引起表型变异。KASAI等[76]在烟草与马铃薯异源嫁接研究中发现，接穗烟草的siRNA引起砧木马铃薯相关结薯靶基因的转录基因沉默(TGS)，并且可以在马铃薯中稳定遗传。
目前关于嫁接引起的表观遗传调控多数是DNA甲基化引起的，主要分为转录水平的基因沉默和转录后水平的基因沉默，前者往往和启动子的甲基化有关，后者则与编码区的甲基化有关，启动子和编码区的过度甲基化能引起基因沉默；而去甲基化则有利于基因表达。对于嫁接引起甲基化模式及水平发生改变的机理，主要有两种观点：一是嫁接作为一种胁迫，在一定程度上对基因组造成冲击，是嫁接环境诱导了表观修饰的发生；二是嫁接引起遗传物质(siRNA)发生交流，通过RNA指导的DNA途径来调控受体基因组的超甲基化或是去甲基化的发生。
2.4 “砧木—接穗—环境”的相互作用
植物的表型变异一般是基因与环境相互作用的结果。嫁接引起的表型变异多数由“砧木−接穗−环境”间的相互作用引起，砧穗间互作模式主要为嫁接后引起蛋白质、mRNA、植物激素通过韧皮部进行相互转运，加上环境信号的输入，进而引起性状改变[86]，所以探索其变异机理对于品种改良和提高产量具有很重要的意义。MARTÍNEZ-BALLESTA等[87]综述了嫁接后砧穗间互作是嫁接亲和性、生长、抗性、代谢等生理及生物学性状变化的主要原因。GONÇALVES等[88]研究发现：砧穗互作会引起嫁接樱桃植株的果实品质及光合参数等生理指标变化。在苹果的嫁接研究中发现砧木可以调控接穗基因的表达模式，从而提高对火疫病的抗性[89]。ALONI等[90]认为嫁接后砧穗间的激素信号转导及环境调控是引起植物生长发育、果实品质改变的主要因素。PAOLO等[91]分析葡萄嫁接后砧穗和环境的相互作用，结果表明接穗产量、品质及抗性既与砧木有关，也和自然环境因子密切相关。植物嫁接后砧穗间的分子因子(如：RNA、蛋白质、质体)和自然环境因子(如：水分、植物激素、营养)协同调控接穗的生长、产量、品质、抗性等各项生长及生理进程[92-93]。砧木和接穗间存在着显著的相互作用，这种互作进而影响植物的生长、形态、生理和抗性[94]。总之，就嫁接植物而言，嫁接植株本身结合了两种不同的基因型，砧木和接穗建立了新的联系，就有了砧木、接穗和环境三者间的相互作用[34]。此外，在嫁接接合部位还存在着砧木与接穗二者的相互作用，这些相互作用决定了砧木和接穗在生长发育过程中的表型改变。
3. 展望
砧木能提高接穗对虫害和病原物的抗性，包括昆虫和土传性病害，同时也能提高非生物胁迫的能力，如热害、寒害、干旱和盐胁迫等[34]。砧木主要通过以下几个方面实现其作用：提高优良品种的产量潜力；减小最优条件下的产量差异；降低土壤中的化学(杀虫剂、肥料)污染；提高自然资源(水分、光、土壤)的利用效率；得到新的可利用的变异植株；生产出品质更好的产品。HAROLDSEN等[35]提出了一种新的嫁接栽培模式−“Transgrafting”，即对砧木进行基因改良，接穗则用野生型，对于“Transgrafting”株系，接穗为野生型植株，转基因只在砧木中表达，同时可以提高对环境的抵抗能力。这样的栽培方式不仅可以打消消费者对转基因的顾虑，而且也是环境友好型可持续发展的生产模式。再者，很多研究表明：将果蔬作物嫁接于抗性砧木上可以显著提高接穗的光合、产量及胁迫抗性[95-96]。所以选育具备抗性的砧木，对实现生物防治目的和发展有机农业具有重要的作用。
诸多研究已经证明嫁接可以引起遗传变异。TALLER等[21]和TSABALLA等[97]研究发现：嫁接能引起辣椒果实形状变异，而且这种变异是可遗传的。王成玉等[98]将枸杞植株为砧木，以辣椒接穗，选择嫁接成功的植株作为杂交母本，将枸杞为父本进行远缘杂交，发现杂交后代的辣椒素和果实形状发生了变异，培育得到了枸杞辣椒新品种。苏联果树育种专家米丘林通过嫁接蒙导法改良和培育了很多果树新种质材料，也被称之为嫁接杂种或蒙导杂种[81]。而由砧木和接穗接合部位诱导愈伤组织产生的不定芽则被称之为嫁接嵌合体[99-100]。1868年，达尔文首次提出了嫁接杂交(graft hybridization)概念并提出泛生论，但一直没有被公认[99,101]。直到2009年STEGEMANN和BOCK首次证明了嫁接可以引起遗传物质交流后，嫁接引起遗传变异被深入研究。LIU等[101]再次提出了嫁接杂交可以作为一种简单、便于应用和有潜力的植物育种和改良方法，它能够克服远缘或近缘间植物有性育种障碍，获得优良的品种[102]。因此，合理利用嫁接杂交诱导产生的遗传变异及利用蒙导法对植物进行改良在植物育种上具有广阔的发展前景。
远缘有性杂交很难得到稳定遗传的后代，因为远缘杂交结实率较低。离体嫁接结合组培技术是实现远缘杂交的一种突破性无性杂交手段。即利用嫁接接合部位诱导产生嫁接嵌合体[103]，其主要原因是砧木和接穗的苗龄和发育状况能够影响性状遗传改变的程度，幼嫩植物组织的体细胞处于感受态，容易接受外源遗传信号物质的转化[104]。而离体嫁接中砧木和接穗组织很幼嫩，容易引起遗传信息的大量交换。所以离体嫁接技术在植物新物种培育上有很大的利用价值与空间。
植物嫁接引起遗传信息交流的研究可以应用于植物远缘有性杂交和无性杂交，嫁接诱导遗传变异在植物育种上具有很大的应用价值。此外，目前对于嫁接诱导遗传物质交流机制的研究处在初级阶段，再者基因在砧穗间长或短距离运输至作用靶点后的功能及作用机理有待深入研究。因此，进一步研究植物嫁接诱导基因交流的机理对于有目的地改良植物遗传性状具有重要的理论指导意义。
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表 1 植物嫁接引起DNA转移
Table 1 The horizontal gene transfer through grafting
DNA 植物(接穗/砧木) plant species (scion/rootstock) 质体基因组plastid DNA 转基因烟草/转基因烟草[22] Nuc-kan:yfp/Pt-spec:gfp 核基因nuclear gene 转基因烟草/转基因烟草，粉蓝烟草/普通烟草[26] Nt-kan:yfp/Nt-hyg，Nicotiana glauca/Nicotiana tabacum 质体基因组plastid DNA 普通烟草/美花烟草[36] Nicotiana tabacum/Nicotiana sylvestris 线粒体基因mitochondrial gene 普通烟草/美花烟草[37] Nicotiana tabacum/Nicotiana sylvestris 叶绿体基因chloroplast gene 粉蓝烟草/普通烟草，粉蓝烟草/本氏烟草[24] Nicotiana tabacum/Nicotiana glauca，Nicotiana tabacum/Nicotiana benthamiana 
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表 2 韧皮部汁液中RNA鉴定
Table 2 The identification of RNA in plant phloem sap
基因gene 功能function 植物plant species KN1 mRNA同源结合蛋白homeodomain protein 玉米[39] Zea mays HAK1 钾转运子potassium transporter 大麦[40] Hordeum vulgare FT 诱导开花flowering signal 拟南芥[41] Arabidopsis thaliana Hd3a 编码水稻成花素flowering signal 水稻[42] Oryza sativa CmF-308 促进侧根发育positively regulate lateral roots 西瓜，番茄[43] Citrullus lanatus，Solanum lycopersicum FTL2/FT 诱导开花flowering signal 南瓜[44] Cucurbita moschata
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表 3 植物嫁接引起RNA通过韧皮部进行长距离运输
Table 3 Long-distance RNA molecules transported in grafting plant phloem
基因gene 功能function 植物(接穗/砧木) plant species (scion/rootstock) mRNA Cmpp16 介导mRNA长距离运输mRNA movement 黄瓜/南瓜↑[46] Cucumis sativus/Cucurbita moschata CmNACP 调控顶端分生组织发育apical meristem development 黄瓜/南瓜↑[47] Cucumis sativus/Cucurbita maxima PFP-LeT6 叶片形态发育leaf development 番茄/番茄↑[27] Solanum lycopersicum/Solanum lycopersicum Cmgaip，△ DELLA-gai 赤霉素酸转录调控因子，叶片发育TFS of GA, leaf development 南瓜/南瓜↑；拟南芥/拟南芥↑；番茄/番茄↑[48] Cucurbita moschata/Cucurbita moschata; Arabidopsis thaliana/Arabidopsis thaliana; Solanum lycopersicum/Solanum lycopersicum StBEL5 块茎发育tuber development 马铃薯/马铃薯↓[28, 49] Solanum tuberosum/Solanum tuberosum PFP-LeT6 叶片形态发育leaf development 马铃薯/番茄↑[50] Solanum tuberosum/Solanum lycopersicum FT 诱导开花flowering signal 拟南芥/拟南芥↑[51] Arabidopsis thaliana/Arabidopsis thaliana FT 诱导开花flowering signal 南瓜/印度南瓜↑[52] Cucurbita moschata/Cucurbita maxima AUX/IAA 生长素信号转导auxin signaling 南瓜/西瓜↑[53] Cucurbita moschata/Citrullus lanatus IAA18，IAA28 抑制侧根形成negatively regulate lateral root formation 烟草/拟南芥↓[54] Nicotiana tabacum/Arabidopsis thaliana Mhgai1 矮化植株dwarf phenotypes 番茄/番茄↑[55] Solanum lycopersicum/Solanum lycopersicum POTH1 块茎发育tuber development 马铃薯/马铃薯↓[56] Solanum tuberosum/Solanum tuberosum PbKN1 病毒运动结合蛋白binding protein 鸭梨/杜梨↑↓[57] Pyrus bretschneideri/Pyrus betulaefolia 2 006个mRNA 诱导开花，营养胁迫响应flowering signal, nutrient stress response 拟南芥/拟南芥↑↓[58] Arabidopsis thaliana/Arabidopsis thaliana 13个mRNA 受体蛋白激酶，细胞饰变，抵御胁迫receptor kinase, cell modification, stress defense 烟草/拟南芥↑[29] Nicotiana benthamiana/Arabidopsis thaliana 3 000个mRNA 新陈代谢，抵御环境胁迫，信号转导metabolism, responses to stress and signal transduction 葡萄/葡萄↑↓[30] Vitis girdiana/Vitis palmata 787，3 485个mRNA 初生和次生代谢，激素信号，转运蛋白primary and secondary metabolism, hormone signaling, transporters 西瓜/葫芦，西瓜/南瓜↑↓[59] Citrullus lanatus/Lagenaria siceraria, Citrullus lanatus/Cucurbita moschata miRNA miR399 控制磷酸盐平衡regulate phosphate homeostasis 拟南芥/拟南芥，烟草/烟草↑[60-61] Arabidopsis thaliana/Arabidopsis thaliana Nicotiana benthamiana/Nicotiana benthamiana miR399，miR395 调控矿质营养平衡regulate mineral homeostasis 拟南芥/拟南芥；甘蓝型油菜/甘蓝型油菜↑[62] Arabidopsis thaliana/Arabidopsis thaliana; Brassica napus/Brassica napus miR172 块茎发育tuber development 马铃薯/马铃薯↓[63] Solanum tuberosum/Solanum tuberosum miR156 调控植物株型和块茎形成tuber development 马铃薯/马铃薯↓[64] Solanum tuberosum ssp.Andigena/Solanum tuberosum ssp. Andigena 47个miRNA 生长发育和代谢进程growth and metabolism processes 西瓜/西葫芦↑[65] Citrullus lanatus/Lagenaria siceraria 17个miRNA 盐胁迫响应salt stress response 黄瓜/南瓜↑[66] Cucumis sativus/Cucurbita maxima 17个miRNA 干旱胁迫响应drought stress response 黄瓜/南瓜↑[67] Cucumis sativus/Cucurbita maxima 27个miRNA 干旱胁迫响应drought stress response 葡萄/葡萄↑↓[68] Vitis vinifera/Vitis berlandieri siRNA CmPSRP1 RNA结合蛋白RNA binding protein 黄瓜/南瓜↑[69] Cucumis sativus/Cucurbita maxima PNRSV-hpRNA 增强坏死斑病毒抗性PNRSV resistance 甜樱桃/转基因樱桃↑[70] Prunus mahaleb/Transgenic Prunus mahaleb CoYMV:35SIR 叶片和根系发育leaf and root development 烟草/烟草↑↓[71] Nicotiana benthamiana/Nicotiana benthamiana GSA 叶绿素合成chlorophyll synthesis 烟草/转基因烟草↑[72] Nicotiana benthamiana/Transgenic N. benthamiana NtTOM1，NtTOM3 增强烟草花叶病毒抗性TMV resistance 烟草/转基因烟草↑[73] Nicotiana benthamiana/Transgenic N. benthamiana DMC1 花粉育性pollen fertility 烟草/转基因烟草↑[74] Nicotiana benthamiana/Transgenic N. benthamiana NtTOM1，NtTOM3 增强番茄花叶病毒和烟草花叶病毒抗性TMV resistance 番茄/转基因烟草，烟草/转基因烟草↑[75] Solanum lycopersicum/Transgenic N. benthamiana, Nicotiana benthamiana/Transgenic N. benthamiana StGBSS1 块茎形成tuber development 烟草/马铃薯↓[76] Nicotiana benthamiana/Solanum tuberosum 注：“↑”表示RNA由砧木向接穗运输；“↓”表示RNA由接穗向砧木运输；“↑↓”表示双向运输。
Note: “↑” indicates mRNA movement through the graft union from the stock into the scion; “↓” indicates mRNA movement through the graft union from the scion into the stock; “↑↓” indicates bidirectional movement.
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