張 鵬， 張然然， 都韶婷

(浙江工商大學(xué)環(huán)境科學(xué)與工程學(xué)院，浙江杭州 310018)

植物體對硝態(tài)氮的吸收轉(zhuǎn)運機制研究進展

張 鵬， 張然然， 都韶婷*

(浙江工商大學(xué)環(huán)境科學(xué)與工程學(xué)院，浙江杭州 310018)

硝態(tài)氮是高等植物重要的氮素營養(yǎng)，直接影響植物的生長。植物根系吸收硝態(tài)氮并向地上部轉(zhuǎn)運的機制一直是研究者十分關(guān)注的問題。近幾年的深入研究使得新的現(xiàn)象與結(jié)論被揭示，推動了我們對植物體吸收轉(zhuǎn)運硝態(tài)氮生理與分子機制的認識。本文綜述了近年來國內(nèi)外關(guān)于植物硝態(tài)氮吸收轉(zhuǎn)運的生理及分子機制的相關(guān)研究結(jié)果。通過整理歸類植物硝酸鹽吸收相關(guān)的生理學(xué)數(shù)據(jù)，介紹了影響植物吸收硝態(tài)氮的各種因素?；谀まD(zhuǎn)運體在植物硝態(tài)氮吸收轉(zhuǎn)運過程中發(fā)揮的重要作用，本文還重點介紹參與該過程的四大基因家族的成員及功能，即硝酸鹽轉(zhuǎn)運體1(NRT1)、硝酸鹽轉(zhuǎn)運體2(NRT2)、氯離子通道(CLC)和s型陰離子通道(SLAC)，以期為后續(xù)研究者提供一個較為全面的理論依據(jù)。

硝態(tài)氮； 親和力轉(zhuǎn)運系統(tǒng)； 吸收； 再分配； 轉(zhuǎn)運蛋白

1 生理機制

1.1 硝態(tài)氮的吸收轉(zhuǎn)運

圖1 擬南芥中部分硝酸鹽轉(zhuǎn)運蛋白的功能 [3,15,48]Fig.1 Function of nitrate transporters in Arabidopsis thaliana[注(Note)：HATS—高親和吸收系 High-affinity transport systems; LATS—低親和吸收系統(tǒng) Low-affinity transport systems.]

運輸?shù)街仓甑厣喜康牡爻藚⑴c代謝過程外，一部分又以氨基酸的形態(tài)通過韌皮部向根部轉(zhuǎn)運，甚至外泌[14]。由韌皮部向下運輸?shù)礁康目偟窟h遠超過根系的生長需要，大部分又進入木質(zhì)部同新吸收的氮素一起向上運輸至地上部[15]。與此同時，硝態(tài)氮也能從老葉運輸?shù)叫氯~中，進行再分配[16]。再分配是所有作物提高氮素利用效率的主要因素，也是植物對硝態(tài)氮吸收轉(zhuǎn)運的重要過程之一。

1.2 硝態(tài)氮吸收的影響因素

與其他礦質(zhì)養(yǎng)分類似，植物對硝態(tài)氮的吸收也受到許多外界因素的影響。鑒于弄清植物吸收硝態(tài)氮的影響因素對于農(nóng)業(yè)氮素利用有著重要意義，因而筆者從環(huán)境條件、養(yǎng)分元素和信號物質(zhì)等對吸收過程影響較大的幾個方面介紹它們對植物硝態(tài)氮吸收的影響(表1)。

表1 硝態(tài)氮吸收的影響因素

就供氮條件的變化而言，它也會極大地影響植物對硝態(tài)氮的吸收。例如，外源硝酸鹽能顯著促進高親和力轉(zhuǎn)運系統(tǒng)的轉(zhuǎn)運能力。以大麥作為研究材料，在培養(yǎng)液中添加硝酸鹽后，組成型高親和力轉(zhuǎn)運系統(tǒng)的活性可以增加3倍左右[23]。甚至，誘導(dǎo)型高親和力轉(zhuǎn)運系統(tǒng)具有更強的硝酸鹽誘導(dǎo)效應(yīng)[24]。介質(zhì)中的銨態(tài)氮同樣也會影響植物對硝態(tài)氮的吸收能力[25]。研究結(jié)果顯示，銨離子濃度超過 1×10-5mol/L會抑制硝酸鹽的吸收[26]。氨基酸也能直接影響植物對硝態(tài)氮的吸收，如分別給大麥和玉米幼苗供應(yīng)谷氨酸和天冬氨酸等均抑制了硝酸鹽的吸收[27-28]。綜上所述，在實際農(nóng)田使用氮肥的過程中，所選用的氮肥形態(tài)也會極大地影響植物對硝態(tài)氮的吸收效率。

2 分子機制

早在30年前，研究者就利用擬南芥這一模式植物開展了硝酸鹽轉(zhuǎn)運體編碼基因的相關(guān)研究。目前，已經(jīng)克隆了4類硝酸鹽轉(zhuǎn)運蛋白基因家族，主要包括NRT1、NRT2、CLC和SLAC[15]。它們均編碼了受硝酸鹽誘導(dǎo)的共運體，但差異較大，基因序列無相似性[47]。我們將上述擬南芥的硝酸鹽轉(zhuǎn)運體基因家族的概況作如下介紹，其命名及功能[48]見圖1和表2。

2.1 NRT1轉(zhuǎn)運蛋白

表2 擬南芥中硝酸鹽轉(zhuǎn)運蛋白的命名及功能

續(xù)表2 Table 2 continuous

常用名Commonname別名Othername基因序列[79]Agicode功能Function硝酸鹽響應(yīng)/表達位置Nitrateresponse/Expressionlocation參考文獻ReferenceNRT2.5ATNRT2.5AT1G12940高親和吸收系統(tǒng)High-affinitytransportsystem莖Stem[70]NRT2.6ATNRT2.6AT3G45060高親和吸收系統(tǒng)High-affinitytransportsystem根、莖Root,stem[53]NRT2.7ATNRT2.7AT5G14570硝態(tài)氮儲存Nitratestorage組成型/液泡膜Constitutive/Tonoplast[69]CLCa-AT5G40890CLCb-AT3G27170CLCc-AT5G49890CLCd-AT5G26240CLCe-AT4G35440CLCf-AT1G55620CLCg-AT5G33280參與硝酸鹽動態(tài)平衡Involvedinnitratehomeostasis液泡膜Tonoplast[15][71][74]液泡膜Tonoplast[72]液泡膜Tonoplast[75]高爾基體Dictyosome[75]葉綠體Chloroplast[75]高爾基體Dictyosome[75]液泡膜Tonoplast[75]SLAC1-AT1G12480陰離子通道AnionchannelSLAH1-AT1G62280-SLAH2-AT4G27970-SLAH3-AT5G24030-SLAH4-AT1G62262-質(zhì)膜Plasmamembrane[78][15][76][15][76][78][15][76]

注(Note): “-”表示未知 Unkown.

2.2 NRT2轉(zhuǎn)運蛋白

目前，在擬南芥中已報道發(fā)現(xiàn)7種NRT2轉(zhuǎn)運蛋白，該家族中大部分成員均能在根內(nèi)表達，并且都是高親和轉(zhuǎn)運蛋白。其中，NRT2.1和NRT2.2主要通過在根中表達后促使植物根部從土壤中吸收硝酸鹽[64]。更有反向遺傳學(xué)的研究表明，NRT2.2只有在NRT2.1缺失的情況下才顯著表達[65]。因此，一般認為相較于NRT2.2，NRT2.1調(diào)控植物根系吸收硝酸鹽的過程更為重要。此外，研究還發(fā)現(xiàn)硝酸鹽濃度較低時，它們的表達量都明顯減少[66]。甚至，其他氮源(銨鹽和谷氨酸鹽)供應(yīng)不足，也會導(dǎo)致NRT2.1的表達量受到抑制[67]。NRT2.1的表達量還受光合作用產(chǎn)物的影響[66]。除了上述兩個轉(zhuǎn)運蛋白，NRT2.4也是備受研究者關(guān)注的對象。它是一種高親和轉(zhuǎn)運蛋白，在側(cè)根的表皮部分和莖的韌皮部表達，在氮素(硝酸鹽)缺乏的情況下，nrt2.4突變體對硝酸鹽的吸收及其根韌皮部硝酸鹽的外泌均減少，表明NRT2.4在低氮情況下對硝酸鹽的運輸起著很大的作用[68]。與NRT2.1、NRT2.2和NRT2.4不同，NRT2.7在種子細胞的液泡膜上表達，負責(zé)將硝酸鹽運輸?shù)椒N子的液泡內(nèi)儲存，其表達量在種子成熟后達到一個最高峰[69]。目前為止，關(guān)于NRT2家族的其他成員如NRT2.3、NRT2.5和NRT2.6的功能研究僅有少量報道。如NRT2.5在沒有外源氮素(硝酸鹽和銨鹽)供應(yīng)的情況下，其表達量增加，當(dāng)供氮恢復(fù)時表達受到抑制[53,70]。不同的是，研究發(fā)現(xiàn)NRT2.3和NRT2.6在根和莖中的表達，并且不受供氮濃度的影響[53]。

2.3 CLC和SLAC轉(zhuǎn)運蛋白

擬南芥中有5種SLAC轉(zhuǎn)運蛋白：SLAC1、SLAH1、SLAH2、SLAH3和SLAH4，其中SLAH1、SLAH2、SLAH3和SLAH4都是SLAC1的同系物。SLAC1是第一個被發(fā)現(xiàn)的s-型陰離子通道上的轉(zhuǎn)運蛋白位于質(zhì)膜[76]。其后，也有學(xué)者報道了有關(guān)SLAC1的調(diào)控機制[77]。與SLAC1一樣，它的同系物SLAHs也都位于質(zhì)膜。研究發(fā)現(xiàn)當(dāng)SLAC1主導(dǎo)氣孔關(guān)閉時SLAH1和SLAH3會表現(xiàn)出與SLAC1相似的功能[76]。但是，SLAH2是否與SLAC1家族中其它轉(zhuǎn)運蛋白功能類似尚未明確。近期一項研究通過GUS染色法在保衛(wèi)細胞中僅觀察到了SLAH3以及少量的SLAH2，SLAH1和SLAH4則未觀察到[78]。雖然，有關(guān)SLAC轉(zhuǎn)運蛋白的研究仍處起步階段，但我們相信識別這類陰離子通道轉(zhuǎn)運蛋白有助于弄清楚氣孔關(guān)閉過程，并對干旱、信號傳導(dǎo)和硝酸鹽代謝有著深刻影響。

3 展望

植物對硝態(tài)氮的吸收是植物生理過程中極為關(guān)鍵的一個部分。近六七十年來的關(guān)于植物吸收硝態(tài)氮的生理機制很好地幫助并解決了較多硝態(tài)氮施用的農(nóng)業(yè)問題。隨著近一二十年的分子機制的研究，更推進了我們對植物吸收硝態(tài)氮過程的理解。尤其是負責(zé)硝酸鹽吸收的轉(zhuǎn)運蛋白，它們被認為在這個過程中發(fā)揮了重要的作用。蔡宜芳教授曾評價說這些轉(zhuǎn)運蛋白既是硝態(tài)氮的搬運工，也是負責(zé)探測土壤硝態(tài)氮含量的守門員，它們將土壤中硝態(tài)氮含量這一信息傳遞到細胞的指揮中心。因此，有關(guān)硝態(tài)氮轉(zhuǎn)運蛋白的研究仍應(yīng)給予極大的重視。此外，除了本文介紹的擬南芥中發(fā)現(xiàn)的轉(zhuǎn)運蛋白外，也有很多關(guān)于各種植物硝態(tài)氮轉(zhuǎn)運蛋白的報道，如水稻的 OsNTR1.1、大豆的 GmNRT2、番茄的 LeNRT1、大麥的HvNRT2A、煙草的NpNRT2.1及苜蓿的MtNRT1.3[3,69,80-81]。不同物種硝態(tài)氮轉(zhuǎn)運蛋白之間的差異也值得我們探究。我們相信，雖然硝態(tài)氮轉(zhuǎn)運體的功能及特點尚未完全弄清，但是隨著人們對植物體硝酸鹽吸收途徑認識的日益加深，以及分子生物學(xué)技術(shù)的發(fā)展，必將取得研究上的突破。

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Research advances in nitrate uptake and transport in plants

ZHANG Peng, ZHANG Ran-ran, DU Shao-ting*

(CollegeofEnvironmentalScienceandEngineering,ZhejiangGongshangUniversity,Hangzhou310018,China)

Nitrate is one of the major nitrogen sources for higher plants, which most frequently limits the growth of plants. Recently, the mechanisms of nitrate uptake and transport to the aboveground parts in plants have

widespread attention. New phenomenons and conclusions have been revealed and thereby give us a new sight into the physiological and molecular mechanisms of nitrate uptake and transport in plants. In this review, the current worldwide research conducted on nitrate uptake and transport in plants was summarized in the physiological and molecular levels. Besides, the influence factors responsible for nitrate uptake in plants were also discussed by integrating the large amount of physiological data. Membrane bound transporters are required for nitrate uptake, allocation and storage inside the plants. Therefore, we also summarized the members and the functions of four gene families that encode the nitrate transporters, including nitrate transporter 1(NRT1), nitrate transporter 2(NRT2), chloride channel(CLC) and slow anion channel-associated(SLAC). We hope that the recent advances in molecular biology of nitrate uptake and transport can provide a more comprehensive theoretical basis for further research.

nitrate; affinity transport system; uptake; remobilize; transporter

2014-06-05 接受日期： 2014-10-13

浙江省自然科學(xué)基金項目(LY14C130001，R13C130001); 浙江省教育廳項目(Y201432146)資助。

張鵬(1989—)，男，湖北黃石人，碩士研究生，主要從事污染環(huán)境的生理生態(tài)方面的研究。E-mail: zp2231989@163.com * 通信作者 Tel: 0571-28008209， E-mail: dushaoting@zjgsu.edu.cn

Q945.1

A

1008-505X(2015)03-0752-11