水稻同化物轉運及其對逆境脅迫響應的機理?

張彩霞，符冠富，奉保華，陳婷婷，陶龍興

（中國水稻研究所/水稻生物學國家重點實驗室，杭州 310006）

由于溫室氣體排放增多，全球極端氣候頻發，農作物產量及糧食安全受到嚴重威脅[1-4]。在某種惡劣條件下，單一脅迫可導致產量絕收[5]，加之耕地面積銳減，糧食安全問題日趨嚴峻[6-7]。水稻是世界最重要的糧食作物之一[8-11]，為滿足人類對糧食日益增長的需求，預計到2035年，水稻產量與2010年相比需增加25%[12]。鑒于糧食安全隱患加劇及需求的增加，通過有效生產方式提高作物產量迫在眉睫[13-14]。產量形成與干物質積累、分配及轉運密切相關，實質上是“源庫流”協調統一的過程?！霸础笔侵圃旎蚬夂袭a物的器官，“庫”是接受或積累光合產物的器官，“流”是指光合同化物從源到庫的運輸[15]。

光合同化物主要以蔗糖的形式通過韌皮部進行運輸，即蔗糖裝載進入小葉脈篩管分子后，經長距離運輸進入庫器官。適宜生長條件下，光合作用形成的碳水化合物大部分以非結構性碳水化合物形式暫時貯存在莖鞘中[16]，籽粒灌漿開啟后，貯存于莖鞘中的同化物重新活化、裝載進入韌皮部，最終運向籽粒。莖稈貯藏同化物及其向籽粒轉運能力可能是影響作物高產的重要途徑。因而，近年來作物單產難以提高甚至減產，極有可能與極端溫度等逆境下同化物轉運受抑有關。目前對“源”和“庫”的研究較多，但對“流”的關注相對較少，尤其是逆境下同化物轉運特征及響應機理。因此，本文在綜述同化物運輸和分配機理的基礎上，明確植株對逆境脅迫的響應機制，重點分析同化物轉運對逆境脅迫的響應機制，以期為提高水稻的穩產性和抗逆性提供理論參考，為未來糧食安全提供保障[17]。

1 水稻韌皮部同化物轉運機理

1.1 同化物在葉片的裝載

同化物在葉片的裝載主要有兩種途徑，即質外體與共質體途徑。質外體途徑需要大量轉運體的參與，例如 SWEET蛋白及蔗糖轉運蛋白（sucrose transporter，SUT）[18-20]。一般情況下，SWEET蛋白將蔗糖分子運輸到細胞壁，再由蔗糖轉運蛋白（sucrose transporter，SUT）轉運至篩管-伴胞復合體[18-20]。在同化物裝載過程中，質膜上的 SUT是重要載體，裝載的過程由質子動力勢驅動[21]，主要負責蔗糖從“源”到“庫”的質外體運輸，并在蔗糖感應、“源”器官裝載、韌皮部長距離運輸和“庫”器官卸載等過程中發揮重要作用[21]。目前水稻上已鑒定出 5個 SUTs基因，即 OsSUT1、OsSUT2、OsSUT3、OsSUT4和OsSUT5，僅OsSUT1和OsSUT3可能在質外體韌皮部裝載中發揮作用[22-25]。OsSUT2主要作用于蔗糖從液泡到胞質的轉運[26]，OsSUT3僅在花粉管中表達，在葉片裝載中的表達很少[27-28]，而OsSUT4是一類低親和性載體，主要與蔗糖在次生維管組織韌皮部中的裝載有關[29-31]。鑒于此，OsSUT1可能是質外體韌皮部裝載中最為重要的蔗糖轉運體，因為已有研究表明，OsSUT1蛋白占主導地位，其表達主要集中在葉片和葉鞘韌皮部篩管和伴胞中[32]。然而OsSUT1基因敲除后，水稻營養生長未發生顯著變化，但籽粒淀粉積累減少，結實率下降[33]，似表明OsSUT1蛋白表達受阻時，有另外的韌皮部裝載途徑代替或者彌補其功能，例如共質體途徑[24]。

同化物的共質體裝載是一個被動的過程，無需消耗能量，但需要葉肉細胞和韌皮部間有較高的濃度梯度及細胞間有發達的胞間連絲[34-35]。由于水稻葉片中有發達的胞間連絲，共質體裝載途徑可能起主要作用[36]。已有研究表明，蔗糖濃度與胞間連絲數量呈顯著正相關關系，絕大部分具有高等或中等胞間連絲數量的植物均具有較高的蔗糖濃度[37]，而水稻葉片蔗糖含量明顯高于擬南芥等質外體裝載植物[38-41]。然而，還需要更多的生理和分子實驗予以證明。近年來有學者提出一種特殊的共質體裝載形式，即聚合物陷阱，通過消耗代謝能量將蔗糖轉化成棉子糖、水蘇四糖等低聚糖。低聚糖不能通過胞間連絲從篩管細胞擴散回葉肉細胞，但可以通過大的胞間連絲轉入篩管細胞，從而維持韌皮部中高的糖濃度[19,36-37,42-43]?？傊?，植物同化物裝載機制有很強的可塑性，會隨著生長發育、生物和非生物脅迫以及不同基因型而發生改變[44-45]，在特定環境下采取的裝載方式還需進一步探討。

1.2 同化物在莖鞘韌皮部的運輸

高等植物的韌皮部由一系列縱向排布的篩管分子、伴胞和薄壁細胞組成[46]。同化物在韌皮部中的運輸速率不僅受到“源”端和“庫”端壓力梯度控制，還受到“源”中韌皮部裝載速率和“庫”中韌皮部卸載速率的調節。此外，篩管幾何體結構也是影響同化物在韌皮部運輸不可忽視的因素，尤其篩板孔形態特征。當篩管受損有汁液外流時，P蛋白就被定位于篩板孔，封堵篩分子以阻止汁液的進一步流失。長效解決篩管受損的機制則是在篩孔處產生胼胝質，即β-1，3-葡聚糖，由質膜中的胼胝質合酶合成并沉積于質膜和細胞壁之間[47]。Mullendore等[48]研究表明，植物在傷害條件下胼胝質幾分鐘內可把小篩板孔堵塞。當環境恢復正常后，胼胝質可在胼胝質水解酶介導下降解[47]。因此，篩管中的 P-蛋白及胼胝質等物質可堵塞篩板孔[49]，從而影響同化物運輸。

植物營養生長階段，葉片等同化器官產生的光合同化物主要貯存于莖鞘中，通過質外體或共質體途徑卸載進入細胞后，以葡萄糖和果糖形式貯存于液泡中，當籽粒開始灌漿后，重新合成蔗糖裝載進入篩管運往庫器官。從莖鞘薄壁細胞重新裝載回韌皮部的機理尚不清楚，既可能是共質體途徑，也可能是質外體途徑[43]。共質體途徑同薄壁細胞與韌皮部篩管分子之間胞間連絲的數量、大小及是否通暢有關，而質外體途徑主要受蔗糖轉運體（SUTs）的影響[43]。Scofield等認為[33,50-52]，OsSUT1參與葉鞘儲藏淀粉重新動員的同化物轉運，而SUT2作為蔗糖信號感應器，在蔗糖轉運等分子調控過程中發揮重要作用[53-54]。然而，有關莖鞘中同化物貯藏及轉運的研究相對較少，還需要進一步探討。

穗頸節間是“源”器官連接穗部的唯一通道，其結構特征與結實率、籽粒充實度和產量密切相關[55]。研究表明，水稻穗頸節間維管束數量對莖鞘非結構性碳化合物的轉運具有顯著的正向直接效應[56]。穗頸較粗、維管束數目多的品種，能夠保持“流”的通暢，有利于提高同化物向籽粒轉運的效率，從而提高收獲指數[57]。于曉剛等[58]發現，穎果維管束可以通過調控同化物的運輸與卸載影響穎果形成和稻米品質。然而，有關穗頸節間特征與同化物轉運關系的研究較少，有待進一步深入研究。

1.3 同化物在穎果中的運輸與卸載

韌皮部卸載是指韌皮部中同化物進入生長或貯藏組織的過程，包括同化物從篩管分子卸載和韌皮部后運輸[59]。同化物通過葉、莖、穗軸、枝梗和小穗軸等維管系統向穎果輸送，最終通過穎花背部維管束進入穎果。同化物在韌皮部中進行長距離運輸后可通過質外體或共質體途徑卸出，這兩個途徑可單獨起作用，也可同時存在，不同發育階段可采取不同的卸載途徑[60-61]。在發育的種子中，母體與胚性組織間無胞間連絲，同化物必須經質外體途徑卸載。盡管水稻光合同化物從背部維管束卸載后進入胚乳的途徑存在爭議，但有一點可以肯定，背部維管束和胚乳組織不直接接觸，二者之間無共質體連通，但有珠心突起組織和質外體空間，由背部維管束運進穎果的同化物必須經過質外體后才能進入胚乳[62-63]。養分基本是按照小穗軸中央維管束—子房背部維管束—珠心突起—珠心層質外體—胚乳的路線進行運輸的[64]。

在質外體卸載中，蔗糖由載體介導跨膜卸出到質外空間，一方面可被蔗糖轉化酶分解為葡萄糖和果糖，然后由己糖載體吸收進入庫細胞；另一方面可由蔗糖載體介導直接吸收進庫細胞[65]。期間涉及3個過程，首先是蔗糖在轉運體（SUTs）的作用下跨膜進入質外體，然后蔗糖被細胞壁轉化酶（CINs）水解，最后在單糖轉運體（MSTs）的作用下進入細胞[66]，表明SUTs、CINs和MSTs共同協調蔗糖的卸載過程。一般情況下，蔗糖轉化酶活性在源（葉）中較低，有利于形成較高的蔗糖濃度以向貯藏器官轉運，而在貯藏器官中較高，有利于蔗糖水解并轉化為淀粉。

已經被鑒定出的5個蔗糖轉運體中，OsSUT2、OsSUT3、OsSUT4和OsSUT5 mRNAs僅在開花時表達且僅維持到花后5d，而OsSUT1授粉后立即表達，并且持續到開花后25d[22,67]。此外，OsSUT1還參與了同化物跨過糊粉層運輸到發育中胚乳的過程[33,63,67]。研究表明，OsSUT1水稻突變體的營養生長過程和野生型幾乎沒有區別[33,68]，敲除基因OsSUT1后葉片碳水化合物積累無變化，但水稻籽粒淀粉積累減少，結實率下降[33]。雖然這5種OsSUTs的表達及功能有所不同，但OsSUTs之間可相互協調，共同維持生物體的整個生命活動[22-25,26-31]。對于轉化酶基因，研究表明CINs過量表達株系籽粒淀粉含量及粒重顯著增加，目前已經有7個CINs被鑒定出，其中OsCIN1、OsCIN2、OsCIN4和OsCIN7在未成熟種子中表達，但作用時期不一致，表明這4個CINs基因都可能參與蔗糖卸載[69]。水稻中已經鑒定出 4個單糖轉運體（MSTs），其中OsMST2、OsMST3和OsMST5屬于糖轉運體[70-71]。盡管水稻轉運體表達特征已有研究，但其在水稻中表達活性與同化物卸載和產量形成的關系報道還較少，其具體功能還有待深入探討。

另外還有研究表明，同化物從韌皮部的卸出與庫端蔗糖酶及質膜H+-TP酶的活性密切相關[72]。同化物卸出到籽粒時，首先是質膜 H+-TP酶促進糖的逆化學勢共轉運，然后被轉運的蔗糖在蔗糖酶的作用下被水解，從而維持篩管末端質外體空間較低的蔗糖濃度，防止其重新裝載，同時蔗糖水解使質外體空間的水勢降低，并促進篩管中糖和水的流出[72]。

2 水稻同化物轉運對逆境脅迫的響應

2.1 高溫熱害

2.1.1 高溫脅迫對水稻葉片裝載的影響

如前所述，水稻光合同化物葉片韌皮部裝載主要為共質體裝載，葉肉細胞與篩管分子之間的胞間連絲數量、頻率、大小及生理活性均可能成為限制同化物運輸的因素。研究表明胞間連絲易受環境因素的影響，低溫脅迫4h可觀察到玉米葉片有胞間連絲關閉的現象，低溫處理28h，葉片維管束鞘和維管薄壁細胞接口處的胞間連絲因胼胝質堵塞而關閉[73]。長時間的高溫脅迫也可能會導致水稻葉片胞間連絲關閉，從而影響水稻同化物的裝載，但類似的研究仍未見報道。雖然目前的研究表明質外體裝載不是水稻葉片主要的裝載方式，但在共質體裝載受阻的情況下，質外體裝載不失為一個很好的補充。這些蔗糖轉運體基因（OsSUT1、OsSUT2、OsSUT3、OsSUT4和OsSUT5）表達可能會受到高溫的抑制從而影響水稻同化物的裝載。此外，裝載過程由質子動力勢所驅動[21]，如果高溫導致植物體內代謝紊亂，不能形成正常的質子動力循環，勢必引起裝載紊亂。

高溫導致水稻產量降低與籽粒充實度變差關系較大，因為同化物裝載、運輸及卸載中任意一個環節受高溫逆境脅迫均可能導致蔗糖轉運受阻，造成“流”的不暢。前期研究結果表明，高溫條件下，無論是耐熱性較強還是耐熱性較差的品種，劍葉光合能力并未發生顯著下降[74]，說明其光合同化物的合成在高溫逆境中并未受顯著傷害，因為葉片表面實際溫度只有35℃左右[74]。因此推測高溫對水稻同化物在葉片裝載的影響較小，“流”的不暢可能主要與同化產物在莖鞘韌皮部運輸及籽粒卸載有關。但由于胞間連絲超微結構及生理活性方面的研究還較薄弱，還有待進一步驗證。

2.1.2 高溫脅迫對同化物在莖鞘中運輸的影響

目前，籽粒開始灌漿后光合產物從莖鞘薄壁細胞重新裝載回韌皮部的機理還不清楚，但不論是共質體還是質外體途徑均會受高溫影響[43]。與葉片相比，高溫下莖鞘和籽粒的散熱能力較低，極易受高溫傷害，氣溫40℃時，莖鞘和籽粒溫度在38℃以上[74]。研究表明，植物受到高溫脅迫時，胼胝質大量產生并在篩孔中沉積，幾分鐘內可以將篩板孔堵塞[48]。因此，高溫逆境下篩板孔堵塞而引起“流”不暢的可能性較大，導致同化產物儲藏于莖鞘薄壁細胞中[43]。逆境解除后，胼胝質在胼胝質水解酶的作用下降解，同化物可從薄壁細胞重新裝載進入韌皮部篩管分子，隨后運輸至其它“庫”組織[43]。然而，高溫脅迫時間過長，脅迫強度超過臨界值，胼胝質氧化酶可能受高溫影響而導致活性降低甚至失活，阻礙胼胝質的降解，導致胼胝質始終沉積于篩板上，使莖鞘干物質量和可溶性碳水化合物含量在高溫脅迫解除后仍呈增加趨勢[43]。另外，高溫下與蔗糖代謝、轉運相關酶活性及相關基因表達受抑也可能是同化物轉運受阻的主要原因。已有研究表明，高溫等逆境可顯著降低葉片及籽粒中蔗糖代謝及轉運相關酶及基因的表達。然而，莖鞘中的這些酶及基因對高溫響應的研究仍未見報道[75]。雖然灌漿結實期莖鞘貯藏同化物向籽粒轉運是提高作物產量的有效手段，但有關莖鞘韌皮部裝載與卸載方面的研究報道還較少，需要進一步深入探討。

2.1.3 高溫脅迫對同化物在穎果中運輸及卸載的影響

水稻韌皮部同化物進入穎果需要以共質體途徑經過穎果背部韌皮部，維管束薄壁細胞及珠心突起，然后再以質外體途徑進入穎果背部糊粉層，最終進入胚乳細胞。研究表明，水稻受精后6d穎果背部維管束才能發育完整，因而此期發生高溫脅迫不僅會影響穎果背部韌皮部、木質部、維管束薄壁細胞間胞間連絲的發育，還會影響背部維管束及胞間連絲的生理活性，從而阻礙光合產物及其它營養物質進入籽粒。更為嚴重的是高溫還會導致穎果背部維管束早衰，嚴重阻礙同化物運輸。高溫不僅抑制背部維管束的發育，還可能影響穎果珠心及珠心突起的發育，不利于光合產物的卸載[62]。已有研究表明，珠心及珠心突起異常會導致灌漿速率降低，產量顯著下降[76]。在質外體卸載階段，同化物受SUTS、CINs和MSTs轉運體共同調控。若高溫抑制其活性及表達量，同化物卸載將受阻。研究表明，高溫脅迫可顯著降低籽粒中VIN活性，從而減少蔗糖向己糖的轉化及胚乳中淀粉的合成[77]。此外，無論高溫還是適溫環境中，耐熱性較強的植株幼嫩果實中 CWIN活性均顯著高于耐熱性差的品種[65,78-82]。

2.2 低溫脅迫

眾多非生物脅迫中，冷熱害已成為水稻面臨的主要氣象災害[83]，是制約糧食作物產量及地域分配的主要限制因素[84-85]。盡管全國大部分地區冷害的頻率和強度有所下降，但階段性和局地性的冷害仍有加重的趨勢[86]。植物的起源地與其耐受低溫冷害的能力關系密切[87-88]，水稻作為熱帶亞熱帶作物，對低溫較敏感，通常氣溫降到4℃植株便會死亡[89]。將培養3周的水稻植株置于6℃條件下低溫處理6h后葉片卷曲，恢復正常溫度 24h后葉片展開[85]。研究表明，低溫脅迫4h葉片有胞間連絲關閉的現象，28h葉片維管束鞘和維管薄壁細胞接口處的胞間連絲因胼胝質堵塞而關閉[90]。冷害脅迫韌皮部的瞬間堵塞可能是由于鈣依賴封閉蛋白暫時性堵塞篩管引起的[91]。前人通過對植物莖進行局部低溫處理，觀察到葉片光合作用受抑，暗呼吸增強，韌皮部阻力增大，同化物轉運受抑制[92]。通過對植物進行局部低溫處理，研究結果表明低溫處理上部組織碳水化合物積累，下部組織碳水化合物含量減少；低溫處理部位上部韌皮部糖分側漏增加，同時向下運輸的碳水化合物減少，向根部運輸的碳水化合物含量減少[93]。灌漿過程中低溫逆境不僅導致植株凈光合生產能力下降，同時也抑制了碳水化合物向韌皮部轉運[94]，導致稻谷的充實度變差及品質變劣[95]。

2.3 干旱脅迫

水稻生長季節發生干旱脅迫將嚴重影響產量的形成[96-100]，尤其生殖生長期[101]。碳水化合物的轉運和分配受水分影響，適度干旱脅迫能有效促進莖鞘中儲藏的非結構性碳水化合物向穗部轉運[102]。正常條件下莖鞘同化物對產量的貢獻約為 20%，干旱脅迫下可提高至50%。該現象與氣孔關系密切[103]，因為干旱下氣孔導度降低，CO2吸收減少，作物光合產物生成受到限制，因而灌漿前莖鞘中貯藏的同化物成為籽粒灌漿的主要來源[104-106]。研究表明，ABA可加速灌漿進程，調用積累在莖鞘中的 NSC，促進積累同化物的再分配[107-108]。Travaglia等[109]研究表明，外源ABA可增加花期小麥碳水化合物積累并向籽粒轉運，提高旱作小麥的產量。其原因主要是ABA提高細胞壁轉化酶活性[110]，促進篩管分子運來的蔗糖分解為己糖，從而實現通過調節細胞壁轉化酶的活性使“源”器官合成的同化物進入細胞壁空間，顯著緩解源器官的壓力[110]。

2.4 氮元素

作為水稻生長過程最重要的營養之一，氮的供應對水稻植株源庫流關系影響甚大。氮供應不足時，葉面積指數過早下降，在籽粒灌漿時易產生源限制；長期低氮，植株的生長受到抑制，如分蘗發生停止，植株矮小。非結構性碳水化合物在植物體內積累是植物適應低氮條件的一種重要機制[111]。蔗糖在莖鞘韌皮部薄壁細胞合成果聚糖，維持蔗糖在源端與臨時庫端的濃度梯度，在籽粒灌漿期重新將積累的同化物輸送至籽粒，從而補償由于光合作用減弱造成的同化物積累不足[111]。然而，低氮能促進莖鞘碳、氮同化物的轉運，與氮低效品種相比，氮高效品種抽穗后物質轉運能力更強，氮素轉運和氮素利用率更高[112-113]。增施氮肥可明顯影響植株的蔗糖代謝，使蔗糖含量及合成能力提高，葉片碳氮同化物的轉運量增加，但莖鞘碳氮同化物向籽粒的轉運減弱[112]，主要是由于儲存的碳為氮代謝提供碳骨架，從而構成很難再被釋放的蛋白質或者氨基酸[114]。當氮供應過大時，莖稈生物量過量增加，葉片的正常衰老受到抑制，造成營養器官過度生長，莖稈伸長，根冠比下降，植株易倒伏，導致產量下降[115]。

3 研究展望

綜上所述，無論正常或逆境條件下，同化物在葉片的裝載、莖鞘中的運輸及穎果的卸載均為影響水稻產量及品質形成的關鍵因素，尤其是同化物（蔗糖）在莖鞘中的儲藏及再分配，然而高溫等逆境條件下與糖轉運蛋白相關的研究相對匱乏[116-119]。面對生態環境日益惡化及糧食需求急劇增加的困境[1-7]，加大對作物體內同化物代謝及轉運的研究，并進行調控以實現逆境條件下穩產甚至增產的意義重大。雖然近年來，越來越多研究者認識到，“流”已經成為大穗型水稻進一步發展的限制因素，“流”的不暢不僅會限制產量潛力的發揮，還會影響稻米品質。然而，以往的研究多集中在以維管束為主的承載“流”組織的解剖、亞種間數量差異及遺傳等方面，對流的質量研究較少，即對維管束的流量面積和“流”的生理活性，如胼胝質對流的影響、同化物的源端裝載與庫端卸出的酶和動力的研究較少，尤其在高溫、低溫及干旱等脅迫條件下，同化物在韌皮部的裝載、運輸及卸載特征更是少有報道。目前有關逆境脅迫對同化物轉運影響的研究還有待完善，本文認為最主要的原因是受研究手段及技術所限。因此，新技術的開發應用可為該研究領域提供新的發展機遇，例如正電子發射動態放射性示蹤技術（dynamic radiotracer imaging techniques using positron emission tomography），預測該技術的成功應用可彌補掃描電子顯微鏡、透射電子顯微鏡以及13C同位素示蹤等研究方法的不足，成為探索逆境脅迫下同化物轉運的強有力手段[120-122]?？茖W技術的進步，科研成果的日臻完善及科研水平的提高，可為該領域的研究提供更好的機遇，從而更好地造福人類。

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