水稻同化物轉(zhuǎn)運(yùn)及其對逆境脅迫響應(yīng)的機(jī)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(xué)國家重點(diǎn)實(shí)驗(yàn)室，杭州 310006）

由于溫室氣體排放增多，全球極端氣候頻發(fā)，農(nóng)作物產(chǎn)量及糧食安全受到嚴(yán)重威脅[1-4]。在某種惡劣條件下，單一脅迫可導(dǎo)致產(chǎn)量絕收[5]，加之耕地面積銳減，糧食安全問題日趨嚴(yán)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(yù)計到2035年，水稻產(chǎn)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(chǎn)方式提高作物產(chǎn)量迫在眉睫[13-14]。產(chǎn)量形成與干物質(zhì)積累、分配及轉(zhuǎn)運(yùn)密切相關(guān)，實(shí)質(zhì)上是“源庫流”協(xié)調(diào)統(tǒng)一的過程。“源”是制造或供應(yīng)光合產(chǎn)物的器官，“庫”是接受或積累光合產(chǎn)物的器官，“流”是指光合同化物從源到庫的運(yùn)輸[15]。

光合同化物主要以蔗糖的形式通過韌皮部進(jìn)行運(yùn)輸，即蔗糖裝載進(jìn)入小葉脈篩管分子后，經(jīng)長距離運(yùn)輸進(jìn)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(jié)構(gòu)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(jìn)入韌皮部，最終運(yùn)向籽粒。莖稈貯藏同化物及其向籽粒轉(zhuǎn)運(yùn)能力可能是影響作物高產(chǎn)的重要途徑。因而，近年來作物單產(chǎn)難以提高甚至減產(chǎn)，極有可能與極端溫度等逆境下同化物轉(zhuǎn)運(yùn)受抑有關(guān)。目前對“源”和“庫”的研究較多，但對“流”的關(guān)注相對較少，尤其是逆境下同化物轉(zhuǎn)運(yùn)特征及響應(yīng)機(jī)理。因此，本文在綜述同化物運(yùn)輸和分配機(jī)理的基礎(chǔ)上，明確植株對逆境脅迫的響應(yīng)機(jī)制，重點(diǎn)分析同化物轉(zhuǎn)運(yùn)對逆境脅迫的響應(yīng)機(jī)制，以期為提高水稻的穩(wěn)產(chǎn)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(zhuǎn)運(yùn)機(jī)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(zhì)外體與共質(zhì)體途徑。質(zhì)外體途徑需要大量轉(zhuǎn)運(yùn)體的參與，例如 SWEET蛋白及蔗糖轉(zhuǎn)運(yùn)蛋白（sucrose transporter，SUT）[18-20]。一般情況下，SWEET蛋白將蔗糖分子運(yùn)輸?shù)郊?xì)胞壁，再由蔗糖轉(zhuǎn)運(yùn)蛋白（sucrose transporter，SUT）轉(zhuǎn)運(yùn)至篩管-伴胞復(fù)合體[18-20]。在同化物裝載過程中，質(zhì)膜上的 SUT是重要載體，裝載的過程由質(zhì)子動力勢驅(qū)動[21]，主要負(fù)責(zé)蔗糖從“源”到“庫”的質(zhì)外體運(yùn)輸，并在蔗糖感應(yīng)、“源”器官裝載、韌皮部長距離運(yùn)輸和“庫”器官卸載等過程中發(fā)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(zhì)外體韌皮部裝載中發(fā)揮作用[22-25]。OsSUT2主要作用于蔗糖從液泡到胞質(zhì)的轉(zhuǎn)運(yùn)[26]，OsSUT3僅在花粉管中表達(dá)，在葉片裝載中的表達(dá)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(guān)[29-31]。鑒于此，OsSUT1可能是質(zhì)外體韌皮部裝載中最為重要的蔗糖轉(zhuǎn)運(yùn)體，因?yàn)橐延醒芯勘砻?，OsSUT1蛋白占主導(dǎo)地位，其表達(dá)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(yǎng)生長未發(fā)生顯著變化，但籽粒淀粉積累減少，結(jié)實(shí)率下降[33]，似表明OsSUT1蛋白表達(dá)受阻時，有另外的韌皮部裝載途徑代替或者彌補(bǔ)其功能，例如共質(zhì)體途徑[24]。

同化物的共質(zhì)體裝載是一個被動的過程，無需消耗能量，但需要葉肉細(xì)胞和韌皮部間有較高的濃度梯度及細(xì)胞間有發(fā)達(dá)的胞間連絲[34-35]。由于水稻葉片中有發(fā)達(dá)的胞間連絲，共質(zhì)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(shù)量呈顯著正相關(guān)關(guān)系，絕大部分具有高等或中等胞間連絲數(shù)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(zhì)外體裝載植物[38-41]。然而，還需要更多的生理和分子實(shí)驗(yàn)予以證明。近年來有學(xué)者提出一種特殊的共質(zhì)體裝載形式，即聚合物陷阱，通過消耗代謝能量將蔗糖轉(zhuǎn)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(xì)胞擴(kuò)散回葉肉細(xì)胞，但可以通過大的胞間連絲轉(zhuǎn)入篩管細(xì)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(jī)制有很強(qiáng)的可塑性，會隨著生長發(fā)育、生物和非生物脅迫以及不同基因型而發(fā)生改變[44-45]，在特定環(huán)境下采取的裝載方式還需進(jìn)一步探討。

1.2 同化物在莖鞘韌皮部的運(yùn)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(xì)胞組成[46]。同化物在韌皮部中的運(yùn)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(diào)節(jié)。此外，篩管幾何體結(jié)構(gòu)也是影響同化物在韌皮部運(yùn)輸不可忽視的因素，尤其篩板孔形態(tài)特征。當(dāng)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(jìn)一步流失。長效解決篩管受損的機(jī)制則是在篩孔處產(chǎn)生胼胝質(zhì)，即β-1，3-葡聚糖，由質(zhì)膜中的胼胝質(zhì)合酶合成并沉積于質(zhì)膜和細(xì)胞壁之間[47]。Mullendore等[48]研究表明，植物在傷害條件下胼胝質(zhì)幾分鐘內(nèi)可把小篩板孔堵塞。當(dāng)環(huán)境恢復(fù)正常后，胼胝質(zhì)可在胼胝質(zhì)水解酶介導(dǎo)下降解[47]。因此，篩管中的 P-蛋白及胼胝質(zhì)等物質(zhì)可堵塞篩板孔[49]，從而影響同化物運(yùn)輸。

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

穗頸節(jié)間是“源”器官連接穗部的唯一通道，其結(jié)構(gòu)特征與結(jié)實(shí)率、籽粒充實(shí)度和產(chǎn)量密切相關(guān)[55]。研究表明，水稻穗頸節(jié)間維管束數(shù)量對莖鞘非結(jié)構(gòu)性碳化合物的轉(zhuǎn)運(yùn)具有顯著的正向直接效應(yīng)[56]。穗頸較粗、維管束數(shù)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(zhuǎn)運(yùn)的效率，從而提高收獲指數(shù)[57]。于曉剛等[58]發(fā)現(xiàn)，穎果維管束可以通過調(diào)控同化物的運(yùn)輸與卸載影響穎果形成和稻米品質(zhì)。然而，有關(guān)穗頸節(jié)間特征與同化物轉(zhuǎn)運(yùn)關(guān)系的研究較少，有待進(jìn)一步深入研究。

1.3 同化物在穎果中的運(yùn)輸與卸載

韌皮部卸載是指韌皮部中同化物進(jìn)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(yùn)輸[59]。同化物通過葉、莖、穗軸、枝梗和小穗軸等維管系統(tǒng)向穎果輸送，最終通過穎花背部維管束進(jìn)入穎果。同化物在韌皮部中進(jìn)行長距離運(yùn)輸后可通過質(zhì)外體或共質(zhì)體途徑卸出，這兩個途徑可單獨(dú)起作用，也可同時存在，不同發(fā)育階段可采取不同的卸載途徑[60-61]。在發(fā)育的種子中，母體與胚性組織間無胞間連絲，同化物必須經(jīng)質(zhì)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(jìn)入胚乳的途徑存在爭議，但有一點(diǎn)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(zhì)體連通，但有珠心突起組織和質(zhì)外體空間，由背部維管束運(yùn)進(jìn)穎果的同化物必須經(jīng)過質(zhì)外體后才能進(jìn)入胚乳[62-63]。養(yǎng)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(zhì)外體—胚乳的路線進(jìn)行運(yùn)輸?shù)腫64]。

在質(zhì)外體卸載中，蔗糖由載體介導(dǎo)跨膜卸出到質(zhì)外空間，一方面可被蔗糖轉(zhuǎn)化酶分解為葡萄糖和果糖，然后由己糖載體吸收進(jìn)入庫細(xì)胞；另一方面可由蔗糖載體介導(dǎo)直接吸收進(jìn)庫細(xì)胞[65]。期間涉及3個過程，首先是蔗糖在轉(zhuǎn)運(yùn)體（SUTs）的作用下跨膜進(jìn)入質(zhì)外體，然后蔗糖被細(xì)胞壁轉(zhuǎn)化酶（CINs）水解，最后在單糖轉(zhuǎn)運(yùn)體（MSTs）的作用下進(jìn)入細(xì)胞[66]，表明SUTs、CINs和MSTs共同協(xié)調(diào)蔗糖的卸載過程。一般情況下，蔗糖轉(zhuǎn)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(zhuǎn)運(yùn)，而在貯藏器官中較高，有利于蔗糖水解并轉(zhuǎn)化為淀粉。

已經(jīng)被鑒定出的5個蔗糖轉(zhuǎn)運(yùn)體中，OsSUT2、OsSUT3、OsSUT4和OsSUT5 mRNAs僅在開花時表達(dá)且僅維持到花后5d，而OsSUT1授粉后立即表達(dá)，并且持續(xù)到開花后25d[22,67]。此外，OsSUT1還參與了同化物跨過糊粉層運(yùn)輸?shù)桨l(fā)育中胚乳的過程[33,63,67]。研究表明，OsSUT1水稻突變體的營養(yǎng)生長過程和野生型幾乎沒有區(qū)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(jié)實(shí)率下降[33]。雖然這5種OsSUTs的表達(dá)及功能有所不同，但OsSUTs之間可相互協(xié)調(diào)，共同維持生物體的整個生命活動[22-25,26-31]。對于轉(zhuǎn)化酶基因，研究表明CINs過量表達(dá)株系籽粒淀粉含量及粒重顯著增加，目前已經(jīng)有7個CINs被鑒定出，其中OsCIN1、OsCIN2、OsCIN4和OsCIN7在未成熟種子中表達(dá)，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(jīng)鑒定出 4個單糖轉(zhuǎn)運(yùn)體（MSTs），其中OsMST2、OsMST3和OsMST5屬于糖轉(zhuǎn)運(yùn)體[70-71]。盡管水稻轉(zhuǎn)運(yùn)體表達(dá)特征已有研究，但其在水稻中表達(dá)活性與同化物卸載和產(chǎn)量形成的關(guān)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(zhì)膜H+-TP酶的活性密切相關(guān)[72]。同化物卸出到籽粒時，首先是質(zhì)膜 H+-TP酶促進(jìn)糖的逆化學(xué)勢共轉(zhuǎn)運(yùn)，然后被轉(zhuǎn)運(yùn)的蔗糖在蔗糖酶的作用下被水解，從而維持篩管末端質(zhì)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(zhì)外體空間的水勢降低，并促進(jìn)篩管中糖和水的流出[72]。

2 水稻同化物轉(zhuǎn)運(yùn)對逆境脅迫的響應(yīng)

2.1 高溫?zé)岷?/h3>

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(zhì)體裝載，葉肉細(xì)胞與篩管分子之間的胞間連絲數(shù)量、頻率、大小及生理活性均可能成為限制同化物運(yùn)輸?shù)囊蛩?。研究表明胞間連絲易受環(huán)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(guān)閉的現(xiàn)象，低溫處理28h，葉片維管束鞘和維管薄壁細(xì)胞接口處的胞間連絲因胼胝質(zhì)堵塞而關(guān)閉[73]。長時間的高溫脅迫也可能會導(dǎo)致水稻葉片胞間連絲關(guān)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(zhì)外體裝載不是水稻葉片主要的裝載方式，但在共質(zhì)體裝載受阻的情況下，質(zhì)外體裝載不失為一個很好的補(bǔ)充。這些蔗糖轉(zhuǎn)運(yùn)體基因（OsSUT1、OsSUT2、OsSUT3、OsSUT4和OsSUT5）表達(dá)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(zhì)子動力勢所驅(qū)動[21]，如果高溫導(dǎo)致植物體內(nèi)代謝紊亂，不能形成正常的質(zhì)子動力循環(huán)，勢必引起裝載紊亂。

高溫導(dǎo)致水稻產(chǎn)量降低與籽粒充實(shí)度變差關(guān)系較大，因?yàn)橥镅b載、運(yùn)輸及卸載中任意一個環(huán)節(jié)受高溫逆境脅迫均可能導(dǎo)致蔗糖轉(zhuǎn)運(yùn)受阻，造成“流”的不暢。前期研究結(jié)果表明，高溫條件下，無論是耐熱性較強(qiáng)還是耐熱性較差的品種，劍葉光合能力并未發(fā)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?yàn)槿~片表面實(shí)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(chǎn)物在莖鞘韌皮部運(yùn)輸及籽粒卸載有關(guān)。但由于胞間連絲超微結(jié)構(gòu)及生理活性方面的研究還較薄弱，還有待進(jìn)一步驗(yàn)證。

2.1.2 高溫脅迫對同化物在莖鞘中運(yùn)輸?shù)挠绊?/h4>

目前，籽粒開始灌漿后光合產(chǎn)物從莖鞘薄壁細(xì)胞重新裝載回韌皮部的機(jī)理還不清楚，但不論是共質(zhì)體還是質(zhì)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(zhì)大量產(chǎn)生并在篩孔中沉積，幾分鐘內(nèi)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(dǎo)致同化產(chǎn)物儲藏于莖鞘薄壁細(xì)胞中[43]。逆境解除后，胼胝質(zhì)在胼胝質(zhì)水解酶的作用下降解，同化物可從薄壁細(xì)胞重新裝載進(jìn)入韌皮部篩管分子，隨后運(yùn)輸至其它“庫”組織[43]。然而，高溫脅迫時間過長，脅迫強(qiáng)度超過臨界值，胼胝質(zhì)氧化酶可能受高溫影響而導(dǎo)致活性降低甚至失活，阻礙胼胝質(zhì)的降解，導(dǎo)致胼胝質(zhì)始終沉積于篩板上，使莖鞘干物質(zhì)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(zhuǎn)運(yùn)相關(guān)酶活性及相關(guān)基因表達(dá)受抑也可能是同化物轉(zhuǎn)運(yùn)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(zhuǎn)運(yùn)相關(guān)酶及基因的表達(dá)。然而，莖鞘中的這些酶及基因?qū)Ω邷仨憫?yīng)的研究仍未見報道[75]。雖然灌漿結(jié)實(shí)期莖鞘貯藏同化物向籽粒轉(zhuǎn)運(yùn)是提高作物產(chǎn)量的有效手段，但有關(guān)莖鞘韌皮部裝載與卸載方面的研究報道還較少，需要進(jìn)一步深入探討。

2.1.3 高溫脅迫對同化物在穎果中運(yùn)輸及卸載的影響

水稻韌皮部同化物進(jìn)入穎果需要以共質(zhì)體途徑經(jīng)過穎果背部韌皮部，維管束薄壁細(xì)胞及珠心突起，然后再以質(zhì)外體途徑進(jìn)入穎果背部糊粉層，最終進(jìn)入胚乳細(xì)胞。研究表明，水稻受精后6d穎果背部維管束才能發(fā)育完整，因而此期發(fā)生高溫脅迫不僅會影響穎果背部韌皮部、木質(zhì)部、維管束薄壁細(xì)胞間胞間連絲的發(fā)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(chǎn)物及其它營養(yǎng)物質(zhì)進(jìn)入籽粒。更為嚴(yán)重的是高溫還會導(dǎo)致穎果背部維管束早衰，嚴(yán)重阻礙同化物運(yùn)輸。高溫不僅抑制背部維管束的發(fā)育，還可能影響穎果珠心及珠心突起的發(fā)育，不利于光合產(chǎn)物的卸載[62]。已有研究表明，珠心及珠心突起異常會導(dǎo)致灌漿速率降低，產(chǎn)量顯著下降[76]。在質(zhì)外體卸載階段，同化物受SUTS、CINs和MSTs轉(zhuǎn)運(yùn)體共同調(diào)控。若高溫抑制其活性及表達(dá)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(zhuǎn)化及胚乳中淀粉的合成[77]。此外，無論高溫還是適溫環(huán)境中，耐熱性較強(qiáng)的植株幼嫩果實(shí)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?dú)庀鬄?zāi)害[83]，是制約糧食作物產(chǎn)量及地域分配的主要限制因素[84-85]。盡管全國大部分地區(qū)冷害的頻率和強(qiáng)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(guān)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(yǎng)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(fù)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(guān)閉的現(xiàn)象，28h葉片維管束鞘和維管薄壁細(xì)胞接口處的胞間連絲因胼胝質(zhì)堵塞而關(guān)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(jìn)行局部低溫處理，觀察到葉片光合作用受抑，暗呼吸增強(qiáng)，韌皮部阻力增大，同化物轉(zhuǎn)運(yùn)受抑制[92]。通過對植物進(jìn)行局部低溫處理，研究結(jié)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(cè)漏增加，同時向下運(yùn)輸?shù)奶妓衔餃p少，向根部運(yùn)輸?shù)奶妓衔锖繙p少[93]。灌漿過程中低溫逆境不僅導(dǎo)致植株凈光合生產(chǎn)能力下降，同時也抑制了碳水化合物向韌皮部轉(zhuǎn)運(yùn)[94]，導(dǎo)致稻谷的充實(shí)度變差及品質(zhì)變劣[95]。

2.3 干旱脅迫

水稻生長季節(jié)發(fā)生干旱脅迫將嚴(yán)重影響產(chǎn)量的形成[96-100]，尤其生殖生長期[101]。碳水化合物的轉(zhuǎn)運(yùn)和分配受水分影響，適度干旱脅迫能有效促進(jìn)莖鞘中儲藏的非結(jié)構(gòu)性碳水化合物向穗部轉(zhuǎn)運(yùn)[102]。正常條件下莖鞘同化物對產(chǎn)量的貢獻(xiàn)約為 20%，干旱脅迫下可提高至50%。該現(xiàn)象與氣孔關(guān)系密切[103]，因?yàn)楦珊迪職饪讓?dǎo)度降低，CO2吸收減少，作物光合產(chǎn)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(jìn)程，調(diào)用積累在莖鞘中的 NSC，促進(jìn)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(zhuǎn)運(yùn)，提高旱作小麥的產(chǎn)量。其原因主要是ABA提高細(xì)胞壁轉(zhuǎn)化酶活性[110]，促進(jìn)篩管分子運(yùn)來的蔗糖分解為己糖，從而實(shí)現(xiàn)通過調(diào)節(jié)細(xì)胞壁轉(zhuǎn)化酶的活性使“源”器官合成的同化物進(jìn)入細(xì)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(yǎng)之一，氮的供應(yīng)對水稻植株源庫流關(guān)系影響甚大。氮供應(yīng)不足時，葉面積指數(shù)過早下降，在籽粒灌漿時易產(chǎn)生源限制；長期低氮，植株的生長受到抑制，如分蘗發(fā)生停止，植株矮小。非結(jié)構(gòu)性碳水化合物在植物體內(nèi)積累是植物適應(yīng)低氮條件的一種重要機(jī)制[111]。蔗糖在莖鞘韌皮部薄壁細(xì)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(bǔ)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(jìn)莖鞘碳、氮同化物的轉(zhuǎn)運(yùn)，與氮低效品種相比，氮高效品種抽穗后物質(zhì)轉(zhuǎn)運(yùn)能力更強(qiáng)，氮素轉(zhuǎn)運(yùn)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(zhuǎn)運(yùn)量增加，但莖鞘碳氮同化物向籽粒的轉(zhuǎn)運(yùn)減弱[112]，主要是由于儲存的碳為氮代謝提供碳骨架，從而構(gòu)成很難再被釋放的蛋白質(zhì)或者氨基酸[114]。當(dāng)?shù)?yīng)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(yǎng)器官過度生長，莖稈伸長，根冠比下降，植株易倒伏，導(dǎo)致產(chǎn)量下降[115]。

3 研究展望

綜上所述，無論正?；蚰婢硹l件下，同化物在葉片的裝載、莖鞘中的運(yùn)輸及穎果的卸載均為影響水稻產(chǎn)量及品質(zhì)形成的關(guān)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(zhuǎn)運(yùn)蛋白相關(guān)的研究相對匱乏[116-119]。面對生態(tài)環(huán)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(nèi)同化物代謝及轉(zhuǎn)運(yùn)的研究，并進(jìn)行調(diào)控以實(shí)現(xiàn)逆境條件下穩(wěn)產(chǎn)甚至增產(chǎn)的意義重大。雖然近年來，越來越多研究者認(rèn)識到，“流”已經(jīng)成為大穗型水稻進(jìn)一步發(fā)展的限制因素，“流”的不暢不僅會限制產(chǎn)量潛力的發(fā)揮，還會影響稻米品質(zhì)。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(shù)量差異及遺傳等方面，對流的質(zhì)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(zhì)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(yùn)輸及卸載特征更是少有報道。目前有關(guān)逆境脅迫對同化物轉(zhuǎn)運(yùn)影響的研究還有待完善，本文認(rèn)為最主要的原因是受研究手段及技術(shù)所限。因此，新技術(shù)的開發(fā)應(yīng)用可為該研究領(lǐng)域提供新的發(fā)展機(jī)遇，例如正電子發(fā)射動態(tài)放射性示蹤技術(shù)（dynamic radiotracer imaging techniques using positron emission tomography），預(yù)測該技術(shù)的成功應(yīng)用可彌補(bǔ)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(zhuǎn)運(yùn)的強(qiáng)有力手段[120-122]?？茖W(xué)技術(shù)的進(jìn)步，科研成果的日臻完善及科研水平的提高，可為該領(lǐng)域的研究提供更好的機(jī)遇，從而更好地造福人類。

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