叢枝菌根真菌與植物共生對(duì)植物水分關(guān)系的影響及機(jī)理

祝 英， 熊俊蘭， 呂廣超， Asfa Batool， 王兆濱， 李樸芳， 熊友才,*

1 草地農(nóng)業(yè)生態(tài)系統(tǒng)國家重點(diǎn)實(shí)驗(yàn)室, 干旱農(nóng)業(yè)生態(tài)研究所, 蘭州大學(xué)生命科學(xué)學(xué)院, 蘭州 730000 2 甘肅省科學(xué)院生物研究所, 蘭州 730000 3 蘭州大學(xué)信息科學(xué)與工程學(xué)院, 蘭州 730000

叢枝菌根真菌與植物共生對(duì)植物水分關(guān)系的影響及機(jī)理

祝 英1,2， 熊俊蘭1， 呂廣超1， Asfa Batool1， 王兆濱3， 李樸芳1， 熊友才1,*

1 草地農(nóng)業(yè)生態(tài)系統(tǒng)國家重點(diǎn)實(shí)驗(yàn)室, 干旱農(nóng)業(yè)生態(tài)研究所, 蘭州大學(xué)生命科學(xué)學(xué)院, 蘭州 730000 2 甘肅省科學(xué)院生物研究所, 蘭州 730000 3 蘭州大學(xué)信息科學(xué)與工程學(xué)院, 蘭州 730000

自1885年Frank首次提到菌根(mykorhiza)概念以來，大量的試驗(yàn)證實(shí)了叢枝菌根真菌(AMF)與植物根系之間形成具有一定結(jié)構(gòu)和功能的共生體，促進(jìn)植物生長(zhǎng)并提高干旱耐受能力，在干旱生態(tài)系統(tǒng)中發(fā)揮重要的作用。該研究多集中在對(duì)宿主植物生理生態(tài)的影響及其機(jī)制方面，然而菌根共生對(duì)宿主植物水分吸收和信號(hào)產(chǎn)生、傳遞的影響研究少而分散，缺少系統(tǒng)總結(jié)。綜述了最近四十多年叢枝菌根真菌與植物共生體對(duì)宿主植物干旱適應(yīng)性影響研究進(jìn)展，討論了菌根共生對(duì)植物根冠通訊的影響及機(jī)理。干旱脅迫下AMF與植物共生，通過影響宿主植物一系列生理生態(tài)過程，提高宿主植物橫向根壓和縱向蒸騰拉力。經(jīng)典的Ohm吸水模型是該方向最有代表性的研究成果，該模型揭示了菌根共生的根外菌絲具有不同于根細(xì)胞的細(xì)胞結(jié)構(gòu)和水分運(yùn)輸性能，這為宿主植物提供一種特殊的快速吸水方式，可提高植物對(duì)土壤水分的吸收和運(yùn)輸能力。研究表明，AMF會(huì)影響宿主植物根冠通訊過程，如誘發(fā)信號(hào)級(jí)聯(lián)反應(yīng)，誘導(dǎo)根系盡早感知水分脅迫并產(chǎn)生非水力根源信號(hào)，提高宿主對(duì)干旱的耐受性。討論了AMF在根冠通訊分子機(jī)制研究方面存在的問題及可能的解決途徑，展望了AMF在干旱農(nóng)業(yè)生產(chǎn)中的應(yīng)用潛力。

叢枝菌根真菌; 共生; 水分運(yùn)輸; 非水力信號(hào); 根冠通訊

叢枝菌根真菌(AMF)與大多數(shù)陸生植物關(guān)系密切，與植物形成共生體后，通過進(jìn)一步改良土壤結(jié)構(gòu)影響根際微生物群落結(jié)構(gòu)、宿主植物的生理生態(tài)功能、物質(zhì)元素生物地球化學(xué)循環(huán)、陸生生態(tài)系統(tǒng)結(jié)構(gòu)和功能等，并對(duì)氣候環(huán)境變化發(fā)揮反饋?zhàn)饔肹1-3]。菌根與植物水分代謝密切相關(guān)，1885年Frank首次提到“mykorhiza”一詞，并指出水分和營養(yǎng)物質(zhì)經(jīng)過外生菌絲運(yùn)送到植物根尖，植物與真菌之間形成互惠共生關(guān)系[4]。1971年Safir等人[5]將接種Glomusmosseae可降低大豆水運(yùn)輸?shù)淖枇Γ诓桓淖兏敌螒B(tài)的情況下，同時(shí)促進(jìn)地上部分生長(zhǎng)，其實(shí)驗(yàn)結(jié)果在Science上發(fā)表，從此開始了叢枝菌根真菌與植物水分關(guān)系的系統(tǒng)研究。

旱地農(nóng)業(yè)系統(tǒng)主要指年平均降雨量在300—550 mm的生態(tài)系統(tǒng)和農(nóng)業(yè)區(qū)域，水分是該區(qū)域作物生長(zhǎng)的主要限制因子。大量研究表明，干旱條件下，AMF與植物共生可以調(diào)節(jié)宿主植物根源信號(hào)合成[6-7]，提高滲透調(diào)節(jié)能力[8-11]、氧化酶活性[12-14]和水分吸收利用效率[15-16]等，賦予作物優(yōu)良的耐旱性和生長(zhǎng)特性，這為旱地農(nóng)業(yè)向高產(chǎn)潛能和高耐旱性發(fā)展提供一種有效途徑。雖然已經(jīng)公認(rèn)AMF能改善宿主植物水分代謝和增強(qiáng)抗旱性的功能，但是其作用機(jī)制還尚未形成共識(shí)和系統(tǒng)總結(jié)。特別是干旱脅迫下，AMF對(duì)宿主非水力信號(hào)產(chǎn)生及根冠通訊的影響更缺乏深入系統(tǒng)認(rèn)識(shí)。本文從干旱脅迫下AMF調(diào)節(jié)宿主植物各種生理生態(tài)指標(biāo)和可能的機(jī)理入手，著重綜述并討論AMF提高土壤水分吸收、運(yùn)輸和對(duì)宿主根源信號(hào)產(chǎn)生及根冠通訊的影響，以深入了解水分脅迫下，AMF促進(jìn)水分吸收和提高宿主對(duì)非水力信號(hào)的響應(yīng)機(jī)制，發(fā)揮其在干旱農(nóng)業(yè)生產(chǎn)中的應(yīng)用潛力。

1 菌根共生對(duì)宿主干旱適應(yīng)性的影響

植物通過改變自身的生理、形態(tài)和物候特性等方式對(duì)干旱脅迫做出反應(yīng)。AMF與植物共生能改變水分運(yùn)移、生理和形態(tài)特性，進(jìn)而影響植物對(duì)干旱的適應(yīng)性。1979年Reid指出菌根有利于干旱脅迫下的植物逃避干旱損傷。大量的研究證實(shí)了這個(gè)觀點(diǎn)，接種AMF的植物與沒有接種AMF的相比，在干旱脅迫下AMF明顯提高宿主植物氮和碳的同化速率，表現(xiàn)在可溶性蛋白、氨基酸、含氮酶和組織氮素含量較高[17-18]；提高酶活性[10]；降低脯氨酸積累和其他含氮物質(zhì)引起的氧化損傷[8-11]；根系較大[19]；增加蠟質(zhì)層保護(hù)葉片，增強(qiáng)耐脫水性能[20]；增加土壤水分?jǐn)z取，降低永久萎焉點(diǎn)的相應(yīng)土壤含水量，并能迅速從干旱脅迫狀態(tài)恢復(fù)正常等[21]；并且AMF與植物共生提高宿主對(duì)干旱的耐受性與營養(yǎng)元素的吸收密切相關(guān)。80%的菌根研究表明，在干旱脅迫下接種AMF明顯促進(jìn)宿主植物生長(zhǎng)，這表現(xiàn)在AMF有利于植物對(duì)營養(yǎng)物質(zhì)的吸收，對(duì)提高耐旱性發(fā)揮重要作用；在充分供水條件下，AMF植物比沒有接種AMF的植物生長(zhǎng)快速，表明AMF能促進(jìn)營養(yǎng)元素的吸收，緩解磷元素脅迫；而葉片中磷的含量可以控制保衛(wèi)細(xì)胞對(duì)ABA的敏感性，進(jìn)而對(duì)調(diào)節(jié)氣孔行為發(fā)揮作用[22]。

AMF通過改變植物系統(tǒng)耐旱性和細(xì)胞耐旱性兩方面來提高宿主對(duì)干旱的耐受性。系統(tǒng)耐旱性主要是指在干旱脅迫下，AMF有利于宿主植物改變形態(tài)結(jié)構(gòu)，例如株型變小、根系變大、調(diào)整生物量分配、關(guān)閉氣孔、分泌球囊霉素團(tuán)聚土壤水分等方式以達(dá)到防止植物脫水的功能；細(xì)胞耐旱性是指干旱脅迫下，AMF有利于宿主植物快速調(diào)節(jié)細(xì)胞內(nèi)的生理生化物質(zhì)，以表現(xiàn)出抵御干旱脅迫的能力，例如提高氧化酶活性、積累蛋白質(zhì)、脯氨酸、甜菜堿等滲透調(diào)節(jié)物質(zhì)，提高植物耐干旱的耐受性。據(jù)此，吳強(qiáng)盛總結(jié)了AMF提高宿主干旱適應(yīng)性的可能機(jī)制模式框架[23]，為了更直觀的了解菌根共生過程中AMF發(fā)揮的作用，在此基礎(chǔ)上對(duì)機(jī)制模式圖進(jìn)一步修改(圖1)。該圖全面概括了40多年來AMF提高宿主植物抗旱性的生理生態(tài)機(jī)制研究成果。但是，所有生理生態(tài)反應(yīng)最基礎(chǔ)的應(yīng)該是信號(hào)物質(zhì)和信號(hào)傳遞方式與速度，沒有信號(hào)物質(zhì)積累和傳遞，宿主不會(huì)作出相應(yīng)的生理生態(tài)改變，因此信號(hào)物質(zhì)產(chǎn)生和傳遞，是宿主對(duì)干旱脅迫作出生理生態(tài)響應(yīng)最原始的推動(dòng)力。下面將從AMF有利于宿主植物水分吸收、轉(zhuǎn)運(yùn)和非水力信號(hào)產(chǎn)生及傳遞入手，討論干旱脅迫下，AMF影響宿主植物信號(hào)調(diào)節(jié)和傳遞的可能機(jī)制。

圖1 AMF提高宿主植物干旱適應(yīng)性的機(jī)制模式圖(參照吳強(qiáng)盛總結(jié)的模式圖[23]，作修改)Fig.1 Possible mechanisms of AMF improving drought adaptability of host plants

2 菌根共生的水分運(yùn)輸機(jī)制

根系吸水主要依靠?jī)煞N動(dòng)力：橫向根壓和縱向蒸騰拉力。三種吸水途徑：質(zhì)外體途徑(指水分通過細(xì)胞壁、細(xì)胞間隙等沒有細(xì)胞質(zhì)部分的移動(dòng)，阻力小，所以這種移動(dòng)方式速度快)，跨膜途徑(指水分從一個(gè)細(xì)胞移動(dòng)到另一個(gè)細(xì)胞，要兩次通過質(zhì)膜，還要通過液泡膜，故稱跨膜途徑)和共生體途徑(指水分從一個(gè)細(xì)胞的細(xì)胞質(zhì)經(jīng)過胞間連絲，移動(dòng)到另一個(gè)細(xì)胞的細(xì)胞質(zhì)，形成一個(gè)細(xì)胞質(zhì)的連續(xù)體，移動(dòng)速度較慢)。1971年Safir首次發(fā)表接種Glomusmosseae可降低大豆水運(yùn)輸?shù)淖枇Φ挠^點(diǎn)[5]，而Sands和Theodorou認(rèn)為應(yīng)將水分運(yùn)輸分成兩部分，一部分是水分從土壤運(yùn)輸?shù)礁?，即橫向根壓；另一部分是植物體內(nèi)的水分運(yùn)輸，即縱向蒸騰拉力。他們?cè)趯?duì)松樹苗的研究發(fā)現(xiàn)，干旱脅迫時(shí)，AMF植物根系土壤的阻力更大，這可能與AMF植物根系結(jié)構(gòu)有關(guān)[24]。利用示蹤技術(shù)研究松樹苗菌根根系和非菌根根系水分的吸收轉(zhuǎn)運(yùn)差異，結(jié)果菌根根系的水分運(yùn)移速度明顯高于非菌根根系，為菌根根系促進(jìn)水分運(yùn)移提供直接證據(jù)，接著又對(duì)菌根和非菌根根系結(jié)構(gòu)觀察，為菌根根系降低水分運(yùn)輸阻力提供證據(jù)[25-28]。Khalvati等人在干旱脅迫下，利用大麥的分根實(shí)驗(yàn)來衡量菌根菌絲對(duì)宿主吸水的貢獻(xiàn)，結(jié)果有4%的水分通過AMF菌絲運(yùn)送到宿主根部[29]。Ruth等人利用高分辨率在線水含量傳感器定量分析菌根真菌菌絲對(duì)水分的吸收效率，結(jié)果表明，菌絲提高總水分吸收率的20%[30]。有關(guān)菌絲對(duì)水分的運(yùn)輸能力報(bào)道不一，一方面是由于研究菌絲對(duì)水分運(yùn)輸能力的方法不一致，另一方面可能因宿主植物和AMF的不同而存在差異。

根據(jù)土壤根系吸水模型Ohm′s law，公式計(jì)算：

式中，R是水分的運(yùn)輸阻力；Ψsoil是土壤水勢(shì)；Ψroot surface是根系表面水勢(shì)；Ψroot xylem是根木質(zhì)部水勢(shì)。AMF外延菌絲的水分運(yùn)輸提高宿主根部的橫向壓力，也就是提高了土壤和根系表面的水勢(shì)差，降低水分在土壤中的運(yùn)輸阻力，因此AMF有利于宿主植物的水分吸收。

同時(shí)，AMF與植物共生通過增加蒸騰速率或(和)氣孔導(dǎo)度等方式，提高植物的縱向蒸騰拉力，有利于土壤水分的吸收。早在1980年Levy和Krikum在柑橘根部接種G.fasciculatus的實(shí)驗(yàn)發(fā)現(xiàn)，在水分脅迫和脅迫解除期間，菌根增強(qiáng)宿主植物的蒸騰速率和氣孔導(dǎo)度，有利于水分更通暢快速的運(yùn)輸[16]。接著Allen研究表明，接種和不接種AMF植物的葉面積和根長(zhǎng)沒有顯著差別，在根和葉水勢(shì)不變的情況下，菌根植物葉片蒸騰速率提高100%，整個(gè)植株體內(nèi)水分運(yùn)輸阻力減少50%，AMF根外菌絲到根部水分運(yùn)輸速率每個(gè)侵入位點(diǎn)約為2.8×10-5mg/s[31]。之后大量的研究證實(shí)菌根植物與非接菌植物相比，宿主植物的葉片氣孔導(dǎo)度、蒸騰速率和光合作用均有顯著提高[15,32-34]，很大程度上提高植物的縱向蒸騰拉力，對(duì)土壤水分吸收有益。還有大量研究表明，菌根共生在不改變總根系生物量的情況下，能影響根系分支、根直徑和根系密度等[35-37]，這些特點(diǎn)均有利于說明菌根共生對(duì)宿主植物水分吸收有貢獻(xiàn)。

由于AMF是多核無隔膜或隔膜非常稀少的菌絲體，水分可直接通過菌絲到達(dá)叢枝，菌絲內(nèi)的水分運(yùn)輸幾乎沒有阻力，到達(dá)菌絲頂端叢枝后，水分滲出到宿主根內(nèi)細(xì)胞，縮短水分在根內(nèi)運(yùn)輸路徑的同時(shí)，可能提供一種特殊的吸水途徑。Alexopolis等[38]和Allen[3]分別指出水分在菌根根外菌絲的運(yùn)輸方式(圖2)。菌絲頂端細(xì)胞壁具有彈性和親水性，因此頂端細(xì)胞壁有時(shí)可以張開小口，使水分滲出[39]。Querejeta等用染料在干旱脅迫下研究水分的運(yùn)移情況時(shí)發(fā)現(xiàn)，白天葉片氣孔打開，水分由F向A運(yùn)輸；然而在夜間，由于土壤嚴(yán)重干旱土壤水勢(shì)較低，水分由A向F運(yùn)輸，甚至水分能從菌絲頂端滲出到土壤的現(xiàn)象[40]。Bárzana等的研究也表明在充分供水和干旱脅迫下，AMF確實(shí)具有調(diào)節(jié)宿主質(zhì)外體和細(xì)胞間水分運(yùn)輸?shù)耐緩?，AMF這種靈活調(diào)節(jié)宿主水分的運(yùn)輸能力可能會(huì)根據(jù)植物水分的需求，更靈活的響應(yīng)干旱脅迫，并利用示蹤染料抑制水通道蛋白活性實(shí)驗(yàn)，發(fā)現(xiàn)水分虧缺條件下，根據(jù)宿主植物水的儲(chǔ)存和地上部分需求，AMF共生能更靈活的調(diào)節(jié)水分運(yùn)輸[41]。Li等人從根內(nèi)球囊霉中首次克隆、鑒定兩個(gè)功能水通道蛋白基因GintAQPF1和GintAQPF2，為AMF向宿主植物提供水分運(yùn)輸從而提高植物耐旱性提供強(qiáng)有力的證據(jù)[42]。AMF菌絲提供的特殊吸水功能，有別于植物本身具有的3種吸水方式，在對(duì)宿主植物響應(yīng)干旱脅迫方面發(fā)揮重要作用。

圖2 菌根共生根部示意圖(a)菌根外生菌絲吸水模式圖(b)Fig.2 Water transport within a hypha (a) Schematic of root mycorrhizal symbiosis (b)圖2表明水分在菌絲內(nèi)的運(yùn)輸方式：A和B是宿主植物與菌絲之間水分運(yùn)移，E和F是菌絲與土壤之間水分運(yùn)移。這兩者之間的水分運(yùn)輸必須穿過菌絲膜，從而限制水分運(yùn)輸，但是菌絲內(nèi)部沒有或很少有隔膜，水分能快速實(shí)現(xiàn)B—C—D—E之間的運(yùn)移(參照1996年Alexopolis等提出的AMF吸水模型[38])

3 菌根共生對(duì)宿主根冠通訊的影響機(jī)制

1985年英國的Blackman和Davies[43]對(duì)玉米盆栽分根控水實(shí)驗(yàn)研究表明：受到干旱脅迫的那部分根能夠引發(fā)地上部分葉片的氣孔關(guān)閉，但葉水勢(shì)、膨壓和脫落酸的含量與充分供水時(shí)沒發(fā)生顯著變化，據(jù)此而提出根冠通訊的理論。也就是說，氣孔導(dǎo)度的下降不是由于水分虧缺直接引起的，而是根系首先感受水分脅迫而產(chǎn)生根信號(hào)，根信號(hào)傳遞到葉片來控制氣孔行為，這種根信號(hào)被稱為非水力信號(hào)(non-hydraulic root-sourced signal, nHRS)，并且在長(zhǎng)期進(jìn)化過程中這種信號(hào)向拉伸或弱化的方向演變[44-46]。Graham[47], Levy[48]和Sands[49]等研究不同植物菌根在干旱脅迫下對(duì)水力導(dǎo)度的影響，結(jié)果發(fā)現(xiàn)AMF均能影響宿主的水力導(dǎo)度。由于AMF植物根系有利于水分吸收和利用，因此土壤要比非菌根植物干旱的快，所以根系非水力信號(hào)的產(chǎn)生也早于非菌根植物。

1991年Augé等在玫瑰根圍接種根內(nèi)球囊霉(GlomusintraradicesSchench & Smith,Gi)和不接種Gi的控水分根實(shí)驗(yàn)研究顯示，兩種處理的氣孔導(dǎo)度有顯著差別，接種Gi干旱/不接種Gi供水的處理較早降低氣孔導(dǎo)度，而兩種處理葉水勢(shì)和葉片含水量沒有顯著變化，這意味著干旱脅迫下，菌根植物對(duì)土壤水分的吸收速度較快，而較早產(chǎn)生非水力信號(hào)，使氣孔導(dǎo)度下降，減少蒸騰，達(dá)到節(jié)水保水的目的[50]。干旱脅迫條件下，葉片氣孔行為是最直觀的表現(xiàn)，菌根脫水較早而首先引發(fā)非水力信號(hào)產(chǎn)生，通過縱向蒸騰拉力將這些化學(xué)信號(hào)通過木質(zhì)部運(yùn)輸?shù)饺~片的保衛(wèi)細(xì)胞，保衛(wèi)細(xì)胞通過失去膨壓而關(guān)閉氣孔，從而達(dá)到非水力信號(hào)通過根冠通訊而使氣孔關(guān)閉目的，來平衡宿主植物的水分利用。

根冠通訊信號(hào)主要包括脫落酸(ABA)、細(xì)胞分裂素(CTK)、生長(zhǎng)素、木質(zhì)部pH 值和鈣離子(Ca2+)等[51]。ABA是感受干旱脅迫的重要非水力化學(xué)信號(hào)分子之一，是調(diào)節(jié)氣孔關(guān)閉的信號(hào)分子[52]。研究表明AMF不但能調(diào)節(jié)宿主植物ABA的含量[53-56]，而且它本身也能產(chǎn)生ABA[57]。干旱脅迫下，AMF共生影響宿主植物的碳源分配，減少非必要類異戊二烯(單萜和倍半萜烯)合成，提高必要類異戊二烯(脫落酸，葉綠素和類胡蘿卜素)含量[7]，有利于ABA的快速合成和積累，提高宿主植物對(duì)干旱的反應(yīng)速度，進(jìn)而提高耐旱能力。而且Aroca等通過對(duì)野生型和 ABA 基因突變性西紅柿研究，發(fā)現(xiàn)菌根形成和對(duì)干旱耐受性能力，均受植物 ABA 顯性基因調(diào)節(jié)[6]。干旱脅迫下，AMF與宿主植物共生影響ABA控制氣孔行為的機(jī)制有兩種猜測(cè)：1)AMF與植物共生影響ABA在根冠的運(yùn)輸；2)AMF與植物共生影響葉片保衛(wèi)細(xì)胞對(duì)ABA的敏感性。Ebel等研究表明，菌根共生影響木質(zhì)部ABA含量[58]；Goicoechea等人研究表明叢枝菌根的ABA含量低于沒有AMF共生的紫花苜蓿根系[55]；Estrada-Luna和Davies的研究也表明接種AMF辣椒葉片ABA含量低于未接種植物[56]。而且Duan等對(duì)豇豆的研究表明，菌根共生并不影響氣孔對(duì)ABA的敏感性[59]?？梢?，AMF共生通過影響ABA含量和根冠運(yùn)輸來調(diào)節(jié)葉片氣孔行為的。菌根共生植物根、木質(zhì)部和葉片ABA含量均低于非菌根植物，表明AMF與植物共生對(duì)宿主非水力信號(hào)的影響可能也存在弱化的作用。

大量研究表明，植物激素水平例如細(xì)胞分裂素、生長(zhǎng)素、生長(zhǎng)素相關(guān)物質(zhì)、脫落酸以及茉莉酸等由于與AMF共生而改變[52,60-63]。Murakami-Mizukami 等研究發(fā)現(xiàn)AMF可以改變宿主根系A(chǔ)BA和IAA的含量，影響葉片氣孔行為，從而有利于宿主逃避干旱脅迫[54]；ABA與細(xì)胞分裂素之間的平衡對(duì)葉片氣孔的調(diào)節(jié)效果比單獨(dú)ABA或細(xì)胞分裂素效果要好[55]。菌根共生增加宿主植物IAA、GA和CTK的含量，降低ABA和乙烯含量，對(duì)宿主植物平衡水分代謝應(yīng)對(duì)干旱脅迫有非常重要的作用[64]。但是目前有關(guān)AMF與植物共生的信號(hào)交互作用對(duì)干旱響應(yīng)機(jī)制的研究報(bào)道還不多。因此，AMF與植物共生信號(hào)的交互效應(yīng)對(duì)干旱脅迫的響應(yīng)是目前需要研究的熱點(diǎn)和難點(diǎn)問題[65]。

AMF共生植物與非共生植物對(duì)土壤營養(yǎng)物質(zhì)的吸收不同，AMF共生植物有利于磷和鈣元素向地上部分運(yùn)輸。研究發(fā)現(xiàn)，氣孔導(dǎo)度與向日葵木質(zhì)部磷濃度和陰陽離子密切相關(guān)，而接種AMF能顯著提高木質(zhì)部磷元素含量[66]。葉片細(xì)胞質(zhì)和質(zhì)外體的鈣離子濃度參與ABA對(duì)氣孔的調(diào)控，AMF與植物共生影響葉片鈣離子的濃度，對(duì)氣孔行為也發(fā)揮一定的調(diào)節(jié)作用[67-68]。菌根共生影響木質(zhì)部pH值，可能存在其它信號(hào)物質(zhì)而影響干旱條件下非水力信號(hào)的根冠通訊[69]。少量氫離子從木質(zhì)部運(yùn)輸?shù)饺~片，卻能對(duì)葉片質(zhì)外體pH值有很大影響；葉片pH值升高有利于ABA向保衛(wèi)細(xì)胞轉(zhuǎn)移，進(jìn)而調(diào)控氣孔行為[70]。隨著土壤干旱的加劇，葉水勢(shì)下降，水力信號(hào)產(chǎn)生(hydraulic root sourced signal, HRS)，并由nHRS和HRS共同調(diào)節(jié)葉片的水分代謝和氣體交換，在整個(gè)植株水平上對(duì)干旱刺激作出生理生態(tài)響應(yīng)，例如干旱脅迫下的滲透調(diào)節(jié)作用和活性氧代謝調(diào)節(jié)。大量研究表明，AMF能提高宿主植物的滲透調(diào)節(jié)能力[11,71-73]，降低活性氧對(duì)宿主植物造成的損傷[12-14,74]。

總之，菌根共生有利于宿主對(duì)水分和營養(yǎng)物質(zhì)的吸收。干旱脅迫下，由于菌根共生特殊的水分吸收方式，使根部較早的產(chǎn)生非水力信號(hào)，由于菌根植物自身的養(yǎng)分積累和AMF菌絲對(duì)宿主植物激素含量有調(diào)節(jié)功能，有利于根源信號(hào)的傳遞和宿主植物對(duì)信號(hào)的快速應(yīng)答，最終提高了宿主植物對(duì)干旱的耐受性。

4 菌根共生對(duì)根冠通訊研究存在的問題及可能解決方法

根冠通訊理論是植物水分關(guān)系研究領(lǐng)域最重要的進(jìn)展之一。該理論基于水分脅迫下植物的生理生態(tài)特征變化而提出的，該理論對(duì)植物抗干旱脅迫機(jī)制研究具有重要的意義。我們將根冠通訊分子機(jī)制研究存在的難題分解成三個(gè)問題：1)根細(xì)胞如何感知水分脅迫？2)早期抗旱基因的表達(dá)與nHRS 的關(guān)系？3)晚期抗旱基因的表達(dá)與HRS的關(guān)系？研究不同水分梯度下根系分泌物和根成分差異，對(duì)揭示植物根細(xì)胞感知水分脅迫的信號(hào)組件具有潛力。ABF (ABA-binding factor)是早期響應(yīng)干旱脅迫的重要轉(zhuǎn)錄子，而Ca2+作為第二信使，在HRS產(chǎn)生前，協(xié)同ABA調(diào)控保衛(wèi)細(xì)胞的關(guān)閉發(fā)揮重要作用。因此，研究不同水分梯度下植物各器官ABA、Ca2+等含量與分布，有望在揭示早期抗旱基因的表達(dá)與nHRS 的關(guān)系方面有所突破。HRS產(chǎn)生，一系列晚期抗旱功能基因表達(dá)，如RD系列脫水響應(yīng)基因、水離子通道蛋白基因和滲透調(diào)節(jié)蛋白基因等，晚期抗旱基因的表達(dá)對(duì)整個(gè)植物株型、生物量分配和細(xì)胞生理生化等性能產(chǎn)生影響，所以通過研究不同水分梯度下植物的系統(tǒng)耐旱性(指標(biāo)包括株型、生物量分配等)和細(xì)胞耐旱性(抗氧化酶系和滲透物質(zhì)等)，可望揭示晚期抗旱基因的表達(dá)與HRS的關(guān)系。菌根共生對(duì)以上3個(gè)環(huán)節(jié)均有影響，系統(tǒng)展開以上3個(gè)問題的研究，以期對(duì)揭示AMF影響根冠通訊分子機(jī)制研究有所啟示。

5 結(jié)語

AMF與植物共生對(duì)宿主植物生理生態(tài)影響和提高植物對(duì)非生物脅迫(重金屬、鹽堿和干旱等)的耐受性方面已經(jīng)取得很多成果。AMF與植物共生一方面通過根系分泌物團(tuán)聚土壤來保留土壤水分和外延菌絲促進(jìn)水分吸收，另一方面菌根共生對(duì)根源化學(xué)信號(hào)產(chǎn)生和傳遞有一定的調(diào)節(jié)作用，將干旱信息迅速傳遞到地上部分，使宿主植物迅速對(duì)干旱響應(yīng)，減少氣孔導(dǎo)度、降低水的蒸騰損耗，積累滲透物質(zhì)、提高氧化酶活性，以合理分配和利用現(xiàn)有水分資源，度過干旱脅迫。正是由于菌根能更迅速的感受干旱脅迫，并積極對(duì)水分脅迫做出響應(yīng)，所以AMF賦予宿主植物更強(qiáng)的干旱耐受性和干旱脅迫下繼續(xù)生長(zhǎng)的特性。目前，AMF與植物共生對(duì)干旱耐受性機(jī)理的研究，已經(jīng)逐漸由生理生態(tài)研究，進(jìn)一步轉(zhuǎn)向以基因組、蛋白質(zhì)組學(xué)和生理生態(tài)為基礎(chǔ)的研究。AMF與植物共生提高宿主植物干旱耐受性的信號(hào)產(chǎn)生和傳遞機(jī)制以及生理生態(tài)響應(yīng)機(jī)理研究，為現(xiàn)代旱地農(nóng)業(yè)育種和發(fā)展開辟新的途徑。

致謝：西澳大學(xué)植物生物學(xué)院何新華博士、蘭州大學(xué)生命科學(xué)學(xué)院趙旭哲博士幫助寫作，特此致力。

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Effects of arbuscular mycorrhizal fungi and plant symbiosis on plant water relation and its mechanism

ZHU Ying1,2, XIONG Junlan1, Lü Guangchao1, ASFA Batool1, WANG Zhaobin3, LI Pufang1, XIONG Youcai1,*

1StateKeyLaboratoryofGrasslandandAgro-Ecosystems,InstituteofAridAgroecology,SchoolofLifeSciences,LanzhouUniversity,Lanzhou730000,China2InstituteofBiology,GansuAcademyofSciences,Lanzhou730000,China3SchoolofInformationScienceandEngineering,LanzhouUniversity,Lanzhou730000,China

Since Frank proposed “mykorhiza” for the first time in 1885, extensive studies have demonstrated the formation of mycorrhizae between arbuscular mycorrhizal fungi (AMF) and plant roots, and the functioning of mycorrhizae in improving plant gowth and drought adaptability under drought stress particularly in semiarid and arid ecosystems. However, information is limited on the mechanisms how AMF could affect the host plant water uptake, root signal generation and transfer, while most studies have focused on effects of AMF on their physiological and ecological changes in host plants. In this review, progresses in how AMF could balance water relations and affect root to shoot communications are summarized from studies in the last four decades, and possibly related mechanisms are also concluded. These mechanisms include enhanced water uptake, root hydraulic conductance, antioxidant activity, altered hormone relations, osmotic adjustment, aquaporin expression and nutrition absorption. Studies have showed that AMF associated symbioses have usually altered eco-physiological characteristics, e.g. stomatal conductance, plant size and abscisic acid (ABA) content, and thus enhancing the lateral root pressure and vertical transpiration to benefit for host plant′s water absorption. The Ohm′s law model, which is the most representatively traditional progress in water uptake mechanisms, could further reveal how AMF is able to improve soil water absorption and transport. This mode reveals that mycorrhizal hyphae, which are different from plant root cells, having aseptate or coenocytic and elastic hyphal wall at the tip, and only infrequent, adventitious septa, can contribute to transport water rapidly in host plants under drought stress. Thus, AMF in plant root may be able to feel drier soil more quickly and produce non-hydraulic root-sourced signals earlier. AMF can also affect root to shoot communications, such as inducing signaling cascades for root-sourced signal generation and the improvement of drought tolerance from cellular to whole plant level. Nevertheless, the composition of root exudates are complex, and the mechanisms of root to shoot communications still need to be solved: 1) how AMF help root cells to perceive root water stress; 2) relationships between early drought-gene expression and non-hydraulic root-sourced signal (nHRS); and 3) relationships between late drought-gene expression and hydraulic root-sourced signal (HRS). Possible pathways may further reveal the unknown mechanisms in root to shoot communications that are affected by AMF: 1) the differences in their composition among root exudates and root ingredient under moisture gradients, which may have potential in indicating the perception of water stress signal component; 2) the ABA-binding factor (ABF), which may be as one of the important transcripts to respond to the early drought stress, and Ca2+as a second messenger collaborating ABA to regulates the open and close of guard cells. Therefore, studies on their distribution of ABA and Ca2+in root, stem and leaf under moisture gradients may provide insight into relationships between early drought-gene expression and nHRS; and 3) relationships between the whole plant drought tolerance (e.g. plant type and biomass allocation) and the cell drought tolerance (e.g. antioxidant enzymes and penetration substances), which may address mechanisms involving in the late drought-gene expression and HRS. With the further progresses are made on the contribution of AMF symbiosis to plant water uptake and drought tolerance, we believe that AMF will have potential application in semi-arid and arid agricultural production.

arbuscular mycorrhizal fungi (AMF); symbiosis; water transport; non-hydraulic root signal; root-to-shoot communication

國家自然科學(xué)基金(31070372)；國家科技支撐計(jì)劃(2012BAD14B10); 科技部國際合作項(xiàng)目(2013DFA30950); 國家星火計(jì)劃項(xiàng)目(2012GA860003); 國家自然科學(xué)基金青年科學(xué)基金(61201421); 甘肅省青年科技基金計(jì)(1208RJYA058); 甘肅省科學(xué)院開發(fā)與應(yīng)用基金項(xiàng)目(2012JK-03); 國家公益性行業(yè)(氣象)科研專項(xiàng)(GYHY201106029-2); 高等學(xué)校博士學(xué)科點(diǎn)專項(xiàng)科研基金(20110211110022)

2013-06-06;

日期:2014-05-16

10.5846/stxb201306091556

*通訊作者Corresponding author.E-mail: xiongyc@lzu.edu.cn

祝英， 熊俊蘭， 呂廣超， Asfa Batool， 王兆濱， 李樸芳， 熊友才.叢枝菌根真菌與植物共生對(duì)植物水分關(guān)系的影響及機(jī)理.生態(tài)學(xué)報(bào),2015,35(8):2419-2427.

Zhu Y, Xiong J L, Lü G C, Asfa Batool, Wang Z B, Li P F, Xiong Y C.Effects of arbuscular mycorrhizal fungi and plant symbiosis on plant water relation and its mechanism.Acta Ecologica Sinica,2015,35(8):2419-2427.