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)國家重點實驗室, 干旱農(nóng)業(yè)生態(tài)研究所, 蘭州大學生命科學學院, 蘭州 730000 2 甘肅省科學院生物研究所, 蘭州 730000 3 蘭州大學信息科學與工程學院, 蘭州 730000

叢枝菌根真菌與植物共生對植物水分關系的影響及機理

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)國家重點實驗室, 干旱農(nóng)業(yè)生態(tài)研究所, 蘭州大學生命科學學院, 蘭州 730000 2 甘肅省科學院生物研究所, 蘭州 730000 3 蘭州大學信息科學與工程學院, 蘭州 730000

自1885年Frank首次提到菌根(mykorhiza)概念以來，大量的試驗證實了叢枝菌根真菌(AMF)與植物根系之間形成具有一定結構和功能的共生體，促進植物生長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。該研究多集中在對宿主植物生理生態(tài)的影響及其機制方面，然而菌根共生對宿主植物水分吸收和信號產(chǎn)生、傳遞的影響研究少而分散，缺少系統(tǒng)總結。綜述了最近四十多年叢枝菌根真菌與植物共生體對宿主植物干旱適應性影響研究進展，討論了菌根共生對植物根冠通訊的影響及機理。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胞的細胞結構和水分運輸性能，這為宿主植物提供一種特殊的快速吸水方式，可提高植物對土壤水分的吸收和運輸能力。研究表明，AMF會影響宿主植物根冠通訊過程，如誘發(fā)信號級聯(lián)反應，誘導根系盡早感知水分脅迫并產(chǎn)生非水力根源信號，提高宿主對干旱的耐受性。討論了AMF在根冠通訊分子機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)中的應用潛力。

叢枝菌根真菌; 共生; 水分運輸; 非水力信號; 根冠通訊

叢枝菌根真菌(AMF)與大多數(shù)陸生植物關系密切，與植物形成共生體后，通過進一步改良土壤結構影響根際微生物群落結構、宿主植物的生理生態(tài)功能、物質元素生物地球化學循環(huán)、陸生生態(tài)系統(tǒng)結構和功能等，并對氣候環(huán)境變化發(fā)揮反饋作用[1-3]。菌根與植物水分代謝密切相關，1885年Frank首次提到“mykorhiza”一詞，并指出水分和營養(yǎng)物質經(jīng)過外生菌絲運送到植物根尖，植物與真菌之間形成互惠共生關系[4]。1971年Safir等人[5]將接種Glomusmosseae可降低大豆水運輸?shù)淖枇?，在不改變根系形態(tài)的情況下，同時促進地上部分生長，其實驗結果在Science上發(fā)表，從此開始了叢枝菌根真菌與植物水分關系的系統(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與植物共生可以調節(jié)宿主植物根源信號合成[6-7]，提高滲透調節(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AMF能改善宿主植物水分代謝和增強抗旱性的功能，但是其作用機制還尚未形成共識和系統(tǒng)總結。特別是干旱脅迫下，AMF對宿主非水力信號產(chǎn)生及根冠通訊的影響更缺乏深入系統(tǒng)認識。本文從干旱脅迫下AMF調節(jié)宿主植物各種生理生態(tài)指標和可能的機理入手，著重綜述并討論AMF提高土壤水分吸收、運輸和對宿主根源信號產(chǎn)生及根冠通訊的影響，以深入了解水分脅迫下，AMF促進水分吸收和提高宿主對非水力信號的響應機制，發(fā)揮其在干旱農(nóng)業(yè)生產(chǎn)中的應用潛力。

1 菌根共生對宿主干旱適應性的影響

植物通過改變自身的生理、形態(tài)和物候特性等方式對干旱脅迫做出反應。AMF與植物共生能改變水分運移、生理和形態(tài)特性，進而影響植物對干旱的適應性。1979年Reid指出菌根有利于干旱脅迫下的植物逃避干旱損傷。大量的研究證實了這個觀點，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引起的氧化損傷[8-11]；根系較大[19]；增加蠟質層保護葉片，增強耐脫水性能[20]；增加土壤水分攝取，降低永久萎焉點的相應土壤含水量，并能迅速從干旱脅迫狀態(tài)恢復正常等[21]；并且AMF與植物共生提高宿主對干旱的耐受性與營養(yǎng)元素的吸收密切相關。80%的菌根研究表明，在干旱脅迫下接種AMF明顯促進宿主植物生長，這表現(xiàn)在AMF有利于植物對營養(yǎng)物質的吸收，對提高耐旱性發(fā)揮重要作用；在充分供水條件下，AMF植物比沒有接種AMF的植物生長快速，表明AMF能促進營養(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)細胞對ABA的敏感性，進而對調節(jié)氣孔行為發(fā)揮作用[22]。

AMF通過改變植物系統(tǒng)耐旱性和細胞耐旱性兩方面來提高宿主對干旱的耐受性。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)結構，例如株型變小、根系變大、調整生物量分配、關閉氣孔、分泌球囊霉素團聚土壤水分等方式以達到防止植物脫水的功能；細胞耐旱性是指干旱脅迫下，AMF有利于宿主植物快速調節(jié)細胞內(nèi)的生理生化物質，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、脯氨酸、甜菜堿等滲透調節(jié)物質，提高植物耐干旱的耐受性。據(jù)此，吳強盛總結了AMF提高宿主干旱適應性的可能機制模式框架[23]，為了更直觀的了解菌根共生過程中AMF發(fā)揮的作用，在此基礎上對機制模式圖進一步修改(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)機制研究成果。但是，所有生理生態(tài)反應最基礎的應該是信號物質和信號傳遞方式與速度，沒有信號物質積累和傳遞，宿主不會作出相應的生理生態(tài)改變，因此信號物質產(chǎn)生和傳遞，是宿主對干旱脅迫作出生理生態(tài)響應最原始的推動力。下面將從AMF有利于宿主植物水分吸收、轉運和非水力信號產(chǎn)生及傳遞入手，討論干旱脅迫下，AMF影響宿主植物信號調節(jié)和傳遞的可能機制。

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

2 菌根共生的水分運輸機制

根系吸水主要依靠兩種動力：橫向根壓和縱向蒸騰拉力。三種吸水途徑：質外體途徑(指水分通過細胞壁、細胞間隙等沒有細胞質部分的移動，阻力小，所以這種移動方式速度快)，跨膜途徑(指水分從一個細胞移動到另一個細胞，要兩次通過質膜，還要通過液泡膜，故稱跨膜途徑)和共生體途徑(指水分從一個細胞的細胞質經(jīng)過胞間連絲，移動到另一個細胞的細胞質，形成一個細胞質的連續(xù)體，移動速度較慢)。1971年Safir首次發(fā)表接種Glomusmosseae可降低大豆水運輸?shù)淖枇Φ挠^點[5]，而Sands和Theodorou認為應將水分運輸分成兩部分，一部分是水分從土壤運輸?shù)礁?，即橫向根壓；另一部分是植物體內(nèi)的水分運輸，即縱向蒸騰拉力。他們在對松樹苗的研究發(fā)現(xiàn)，干旱脅迫時，AMF植物根系土壤的阻力更大，這可能與AMF植物根系結構有關[24]。利用示蹤技術研究松樹苗菌根根系和非菌根根系水分的吸收轉運差異，結果菌根根系的水分運移速度明顯高于非菌根根系，為菌根根系促進水分運移提供直接證據(jù)，接著又對菌根和非菌根根系結構觀察，為菌根根系降低水分運輸阻力提供證據(jù)[25-28]。Khalvati等人在干旱脅迫下，利用大麥的分根實驗來衡量菌根菌絲對宿主吸水的貢獻，結果有4%的水分通過AMF菌絲運送到宿主根部[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水分的吸收效率，結果表明，菌絲提高總水分吸收率的20%[30]。有關菌絲對水分的運輸能力報道不一，一方面是由于研究菌絲對水分運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，公式計算：

式中，R是水分的運輸阻力；Ψsoil是土壤水勢；Ψroot surface是根系表面水勢；Ψroot xylem是根木質部水勢。AMF外延菌絲的水分運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差，降低水分在土壤中的運輸阻力，因此AMF有利于宿主植物的水分吸收。

同時，AMF與植物共生通過增加蒸騰速率或(和)氣孔導度等方式，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的實驗發(fā)現(xiàn)，在水分脅迫和脅迫解除期間，菌根增強宿主植物的蒸騰速率和氣孔導度，有利于水分更通暢快速的運輸[16]。接著Allen研究表明，接種和不接種AMF植物的葉面積和根長沒有顯著差別，在根和葉水勢不變的情況下，菌根植物葉片蒸騰速率提高100%，整個植株體內(nèi)水分運輸阻力減少50%，AMF根外菌絲到根部水分運輸速率每個侵入位點約為2.8×10-5mg/s[31]。之后大量的研究證實菌根植物與非接菌植物相比，宿主植物的葉片氣孔導度、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]，這些特點均有利于說明菌根共生對宿主植物水分吸收有貢獻。

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叢枝，菌絲內(nèi)的水分運輸幾乎沒有阻力，到達菌絲頂端叢枝后，水分滲出到宿主根內(nèi)細胞，縮短水分在根內(nèi)運輸路徑的同時，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輸方式(圖2)。菌絲頂端細胞壁具有彈性和親水性，因此頂端細胞壁有時可以張開小口，使水分滲出[39]。Querejeta等用染料在干旱脅迫下研究水分的運移情況時發(fā)現(xiàn)，白天葉片氣孔打開，水分由F向A運輸；然而在夜間，由于土壤嚴重干旱土壤水勢較低，水分由A向F運輸，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確實具有調節(jié)宿主質外體和細胞間水分運輸?shù)耐緩?，AMF這種靈活調節(jié)宿主水分的運輸能力可能會根據(jù)植物水分的需求，更靈活的響應干旱脅迫，并利用示蹤染料抑制水通道蛋白活性實驗，發(fā)現(xiàn)水分虧缺條件下，根據(jù)宿主植物水的儲存和地上部分需求，AMF共生能更靈活的調節(jié)水分運輸[41]。Li等人從根內(nèi)球囊霉中首次克隆、鑒定兩個功能水通道蛋白基因GintAQPF1和GintAQPF2，為AMF向宿主植物提供水分運輸從而提高植物耐旱性提供強有力的證據(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，在對宿主植物響應干旱脅迫方面發(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)的運輸方式：A和B是宿主植物與菌絲之間水分運移，E和F是菌絲與土壤之間水分運移。這兩者之間的水分運輸必須穿過菌絲膜，從而限制水分運輸，但是菌絲內(nèi)部沒有或很少有隔膜，水分能快速實現(xiàn)B—C—D—E之間的運移(參照1996年Alexopolis等提出的AMF吸水模型[38])

3 菌根共生對宿主根冠通訊的影響機制

1985年英國的Blackman和Davies[43]對玉米盆栽分根控水實驗研究表明：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閉，但葉水勢、膨壓和脫落酸的含量與充分供水時沒發(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，根信號傳遞到葉片來控制氣孔行為，這種根信號被稱為非水力信號(non-hydraulic root-sourced signal, nHRS)，并且在長期進化過程中這種信號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水力導度的影響，結果發(fā)現(xiàn)AMF均能影響宿主的水力導度。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的控水分根實驗研究顯示，兩種處理的氣孔導度有顯著差別，接種Gi干旱/不接種Gi供水的處理較早降低氣孔導度，而兩種處理葉水勢和葉片含水量沒有顯著變化，這意味著干旱脅迫下，菌根植物對土壤水分的吸收速度較快，而較早產(chǎn)生非水力信號，使氣孔導度下降，減少蒸騰，達到節(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)非水力信號產(chǎn)生，通過縱向蒸騰拉力將這些化學信號通過木質部運輸?shù)饺~片的保衛(wèi)細胞，保衛(wèi)細胞通過失去膨壓而關閉氣孔，從而達到非水力信號通過根冠通訊而使氣孔關閉目的，來平衡宿主植物的水分利用。

根冠通訊信號主要包括脫落酸(ABA)、細胞分裂素(CTK)、生長素、木質部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信號分子之一，是調節(jié)氣孔關閉的信號分子[52]。研究表明AMF不但能調節(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，提高宿主植物對干旱的反應速度，進而提高耐旱能力。而且Aroca等通過對野生型和 ABA 基因突變性西紅柿研究，發(fā)現(xiàn)菌根形成和對干旱耐受性能力，均受植物 ABA 顯性基因調節(jié)[6]。干旱脅迫下，AMF與宿主植物共生影響ABA控制氣孔行為的機制有兩種猜測：1)AMF與植物共生影響ABA在根冠的運輸；2)AMF與植物共生影響葉片保衛(wèi)細胞對ABA的敏感性。Ebel等研究表明，菌根共生影響木質部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等對豇豆的研究表明，菌根共生并不影響氣孔對ABA的敏感性[59]?？梢姡珹MF共生通過影響ABA含量和根冠運輸來調節(jié)葉片氣孔行為的。菌根共生植物根、木質部和葉片ABA含量均低于非菌根植物，表明AMF與植物共生對宿主非水力信號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胞分裂素、生長素、生長素相關物質、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ABA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與細胞分裂素之間的平衡對葉片氣孔的調節(jié)效果比單獨ABA或細胞分裂素效果要好[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，對宿主植物平衡水分代謝應對干旱脅迫有非常重要的作用[64]。但是目前有關AMF與植物共生的信號交互作用對干旱響應機制的研究報道還不多。因此，AMF與植物共生信號的交互效應對干旱脅迫的響應是目前需要研究的熱點和難點問題[65]。

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

總之，菌根共生有利于宿主對水分和營養(yǎng)物質的吸收。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，由于菌根植物自身的養(yǎng)分積累和AMF菌絲對宿主植物激素含量有調節(jié)功能，有利于根源信號的傳遞和宿主植物對信號的快速應答，最終提高了宿主植物對干旱的耐受性。

4 菌根共生對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系研究領域最重要的進展之一。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，該理論對植物抗干旱脅迫機制研究具有重要的意義。我們將根冠通訊分子機制研究存在的難題分解成三個問題：1)根細胞如何感知水分脅迫？2)早期抗旱基因的表達與nHRS 的關系？3)晚期抗旱基因的表達與HRS的關系？研究不同水分梯度下根系分泌物和根成分差異，對揭示植物根細胞感知水分脅迫的信號組件具有潛力。ABF (ABA-binding factor)是早期響應干旱脅迫的重要轉錄子，而Ca2+作為第二信使，在HRS產(chǎn)生前，協(xié)同ABA調控保衛(wèi)細胞的關閉發(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與nHRS 的關系方面有所突破。HRS產(chǎn)生，一系列晚期抗旱功能基因表達，如RD系列脫水響應基因、水離子通道蛋白基因和滲透調節(jié)蛋白基因等，晚期抗旱基因的表達對整個植物株型、生物量分配和細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(指標包括株型、生物量分配等)和細胞耐旱性(抗氧化酶系和滲透物質等)，可望揭示晚期抗旱基因的表達與HRS的關系。菌根共生對以上3個環(huán)節(jié)均有影響，系統(tǒng)展開以上3個問題的研究，以期對揭示AMF影響根冠通訊分子機制研究有所啟示。

5 結語

AMF與植物共生對宿主植物生理生態(tài)影響和提高植物對非生物脅迫(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聚土壤來保留土壤水分和外延菌絲促進水分吸收，另一方面菌根共生對根源化學信號產(chǎn)生和傳遞有一定的調節(jié)作用，將干旱信息迅速傳遞到地上部分，使宿主植物迅速對干旱響應，減少氣孔導度、降低水的蒸騰損耗，積累滲透物質、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，并積極對水分脅迫做出響應，所以AMF賦予宿主植物更強的干旱耐受性和干旱脅迫下繼續(xù)生長的特性。目前，AMF與植物共生對干旱耐受性機理的研究，已經(jīng)逐漸由生理生態(tài)研究，進一步轉向以基因組、蛋白質組學和生理生態(tài)為基礎的研究。AMF與植物共生提高宿主植物干旱耐受性的信號產(chǎn)生和傳遞機制以及生理生態(tài)響應機理研究，為現(xiàn)代旱地農(nóng)業(yè)育種和發(fā)展開辟新的途徑。

致謝：西澳大學植物生物學院何新華博士、蘭州大學生命科學學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

國家自然科學基金(31070372)；國家科技支撐計劃(2012BAD14B10); 科技部國際合作項目(2013DFA30950); 國家星火計劃項目(2012GA860003); 國家自然科學基金青年科學基金(61201421); 甘肅省青年科技基金計(1208RJYA058); 甘肅省科學院開發(fā)與應用基金項目(2012JK-03); 國家公益性行業(yè)(氣象)科研專項(GYHY201106029-2); 高等學校博士學科點專項科研基金(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植物水分關系的影響及機理.生態(tài)學報,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.