胡 靜，胡小柯，尉秋實(shí)，袁惠君，Yousaf Jamal(.甘肅省治沙研究所,甘肅省荒漠化與風(fēng)沙災(zāi)害防治省部共建國家重點(diǎn)實(shí)驗(yàn)室,甘肅 蘭州 730030；.蘭州理工大學(xué)生命科學(xué)與工程學(xué)院，甘肅 蘭州 730050)

植物內(nèi)整流K+通道AKT1的研究進(jìn)展

胡 靜1，胡小柯1，尉秋實(shí)1，袁惠君2，Yousaf Jamal1
(1.甘肅省治沙研究所,甘肅省荒漠化與風(fēng)沙災(zāi)害防治省部共建國家重點(diǎn)實(shí)驗(yàn)室,甘肅 蘭州 730030；2.蘭州理工大學(xué)生命科學(xué)與工程學(xué)院，甘肅 蘭州 730050)

K+是植物生長發(fā)育所必需的大量營養(yǎng)元素。內(nèi)整流K+通道(ArabidopsisK+transporter 1,AKT1)屬于Shaker家族，是介導(dǎo)K+吸收的重要通道，為質(zhì)膜的K+感應(yīng)器，參與調(diào)節(jié)細(xì)胞的生長發(fā)育、調(diào)控氣孔運(yùn)動及植物蒸騰作用，能夠提高植株的抗旱耐鹽性，因而在植物生長過程中具有重要作用。該文概述了AKT1的結(jié)構(gòu)、組織表達(dá)定位和表達(dá)調(diào)控及功能等方面的研究進(jìn)展，并提出采用蛋白組學(xué)、基因工程技術(shù)及RNAi手段深入研究K+、Na+吸收及轉(zhuǎn)運(yùn)的協(xié)同調(diào)控機(jī)制，提高作物對土壤中K+的利用效率及AKT1在植物生理代謝、抗逆性中的作用。

內(nèi)整流K+通道(AKT1)；K+吸收；Shaker家族

土壤中K+的濃度大部分在0.025～5 mmol·L-1范圍內(nèi)波動[1-2]，而細(xì)胞質(zhì)中的K+含量卻高至60～200 mmol·L-1[3-6]。K+是維持植物正常生理代謝活動所必需的大量營養(yǎng)元素[7-9]，然而當(dāng)植物受到如鹽[10-11]、K+饑餓[6]及營養(yǎng)毒害(如NH4+、Al3+等)[10,12]等環(huán)境脅迫時(shí)，會引起細(xì)胞質(zhì)中的K+濃度顯著降低。因此，在大多數(shù)土壤中，較低的K+含量難以維持正常的植物生長發(fā)育[13-14]，植物需要調(diào)節(jié)自身的生理及形態(tài)學(xué)水平來適應(yīng)K+虧缺的環(huán)境。1963年，Epstein等[15]提出了植物K+吸收的兩條不同途徑，即高親和性和低親和性組分，前者主要是植物通過K+轉(zhuǎn)運(yùn)載體蛋白逆濃度梯度從土壤中吸收K+，后者則是K+通道介導(dǎo)的K+跨膜運(yùn)輸。內(nèi)整流K+通道(ArabidopsisK+transporter 1,AKT1)屬于Shaker家族，在植物吸收K+中發(fā)揮重要作用[16-20]。

近年來，有關(guān)內(nèi)整流K+通道AKT1的研究受到學(xué)術(shù)界的廣泛關(guān)注，隨著研究的不斷深入，發(fā)現(xiàn)該K+通道的功能不僅僅局限于介導(dǎo)K+吸收，為此，本文就內(nèi)整流K+通道AKT1的結(jié)構(gòu)、組織表達(dá)定位及功能方面的研究進(jìn)展進(jìn)行概述，在此基礎(chǔ)上提出采用蛋白組學(xué)、基因工程技術(shù)及RNAi手段深入研究K+、Na+吸收及轉(zhuǎn)運(yùn)的協(xié)同調(diào)控機(jī)制，以期提高作物對土壤中K+的利用效率及AKT1在植物生理代謝、抗逆性中的作用。

1 內(nèi)整流K+通道AKT1的結(jié)構(gòu)

1.1AKT1基因的克隆

1992年，Sentenac等[21]通過酵母鉀虧缺突變體互補(bǔ)法從擬南芥中首次分離了K+通道AKT1，該基因含11個(gè)外顯子及10個(gè)內(nèi)含子，位于第2條染色體上，是第1個(gè)克隆到的植物內(nèi)向整流K+通道基因。同時(shí)分離出來的還有K+通道KAT1。繼擬南芥后，先后在番茄(Lycopersiconesculentum)[17]、玉米(Zeamays)[22]、胡蘿卜(Daucuscarota)[23]、水稻(Oryzasativa)[24]、大麥(Hordeumvulgare)[25]、小麥(Triticumaestivum)[26]、小花堿茅(Puainelliatenuiflora)[27]、霸王(Zygophyllumxanthoxylum)[28]、陸地棉(Gossypiumhirsutum)[29]及鹽地堿蓬(Suaedasalsa)[30]等中克隆得到AKT1(表1)?？梢?，AKT1已引起了研究者的廣泛關(guān)注。

如表1所示，高等植物中內(nèi)整流K+通道AKT1基因約含2 570～2 810個(gè)核苷酸的開放閱讀框(open reading frame,ORF)，編碼857～935個(gè)氨基酸。對13種植物的內(nèi)整流K+通道AKT1的氨基酸序列進(jìn)行了分析(圖1)，發(fā)現(xiàn)單子葉植物之間的同源性為76%～97%，雙子葉植物之間的同源性為68%～97%，而單子葉與雙子葉植物之間的同源性僅為63%～68%?？梢?，不同植物的內(nèi)整流K+通道AKT1具有一定的差異，特別是單子葉與雙子葉植物之間，可能是因?yàn)樗鼈冊陂L期的進(jìn)化過程中產(chǎn)生了分異。

表1 在一些高等植物中克隆得到的AKT1基因Table 1 AKT1 genes cloned from higher plants

1.2 AKT1的結(jié)構(gòu)

植物中的Shaker通道是目前研究最為清楚的K+轉(zhuǎn)運(yùn)家族，研究表明，在序列及結(jié)構(gòu)上，其與動物中Shaker家族的K+通道相似[33]。內(nèi)整流K+通道為典型的Shaker家族成員，其具有6個(gè)跨膜區(qū)(S1～S6)，S4是通道的電壓敏感區(qū)，是一個(gè)具有Arg/Lys-Xaa-Xaa-Arg/Lys重復(fù)序列區(qū)，S5～S6間是離子通道孔徑形成區(qū)，中間在內(nèi)，兩端暴露在外，為β發(fā)夾形狀，且有一個(gè)孔道區(qū)域-P環(huán)(pore，P)，此區(qū)域在K+通道中非常保守，并含有具有高選擇性K+通道特點(diǎn)的基本序列-GYGD/E[21,34]。N端為胞質(zhì)內(nèi)一個(gè)很短的結(jié)構(gòu)域(大約60個(gè)氨基酸)；C端也同樣位于胞質(zhì)內(nèi)，具環(huán)核苷酸結(jié)合位點(diǎn)(CYCL.N.:cyclic nucleotide-binding site)和一個(gè)在大部分K+通道中存在的錨蛋白區(qū)(ANKY:ankyrin-related domain)[35](圖2)。錨蛋白可使蛋白與細(xì)胞骨架結(jié)合，或促進(jìn)蛋白與蛋白之間的相互作用[36]。已有研究認(rèn)為對Ca2+敏感的CBL(calcineurin B-like Ca2+)和CIPK(CBL-interacting protein kinase)家族同2C型蛋白磷酸酶可以形成一個(gè)蛋白磷酸化或去磷酸化的網(wǎng)絡(luò)從而來調(diào)節(jié)植物AKT1類K+通道，其中CIPK就是與AKT1的錨蛋白區(qū)域進(jìn)行相互作用的[37-38]。此外，錨蛋白區(qū)是區(qū)分AKT1類與KAT1類基因的標(biāo)志，前者有錨蛋白區(qū)，而后者沒有[39-40]。

圖1 植物的內(nèi)整流K+通道AKT1系統(tǒng)進(jìn)化分析Fig.1 Phylogenetic analysis of inward rectifying K+ channel AKT1 from different plant species

圖2 內(nèi)整流K+通道AKT1結(jié)構(gòu)Fig.2 Structure of inward rectifying K+ channel AKT1

注：在文獻(xiàn)[28,31]基礎(chǔ)上修改而成。

Note: Modified by [28,31].

2 AKT1定位及組織表達(dá)研究

Basset等[41]報(bào)道AKT1主要在擬南芥成熟根表皮和根毛中表達(dá)。Lagarde等[18]以AKT1的cDNA為探針，通過Northern印跡雜交檢測AKT1主要在擬南芥成熟根中表達(dá)，而在葉組織中表達(dá)量非常少；利用Western印跡雜交對油菜(Brassicanapus)根系進(jìn)行定位表達(dá)，證實(shí)AKT1基因的表達(dá)集中在根的細(xì)胞質(zhì)膜上；利用AKT1-GUS融合基因檢測，進(jìn)一步表明了AKT1主要在擬南芥成熟根質(zhì)膜細(xì)胞中表達(dá)。而番茄中的LKT1則主要分布在根毛[31]。PutAKT1-GFP融合蛋白轟擊洋蔥(Alliumcepa)表面，瞬間表達(dá)表明，PutAKT1主要定位在細(xì)胞質(zhì)膜上[27]。水稻的OsAKT1主要在根和胚芽鞘中表達(dá)，其中在表皮和內(nèi)皮層細(xì)胞中的表達(dá)非常強(qiáng)烈，而相比之下，脈管及外皮層等部位的表達(dá)量則很低；葉中也有少量的表達(dá)[24,42]。SKT1定位在馬鈴薯(Solanumtuberosum)根中[32]。HvAKT1也主要在大麥的根中表達(dá)，葉組織中也有較低的表達(dá)量[25]。而GhAKT1卻主要在陸地棉葉中表達(dá)，根及莖中的表達(dá)量較低，但也定位在細(xì)胞質(zhì)膜上[27]。綜上，在高等植物中，AKT1主要在植物根的細(xì)胞質(zhì)膜中表達(dá)，可能參與K+吸收。

3 AKT1的功能

3.1 調(diào)節(jié)細(xì)胞生長發(fā)育

植物腫瘤細(xì)胞在增殖過程中受AKT1介導(dǎo)的K+吸收的促進(jìn)，且在AKT1敲除的突變體中，腫瘤細(xì)胞的發(fā)育則明顯受到抑制[43]。經(jīng)農(nóng)桿菌侵染引起腫瘤的葉組織與未經(jīng)侵染中的K+通道基因的表達(dá)譜進(jìn)行了對比分析，結(jié)果發(fā)現(xiàn)二者中TPK表達(dá)豐度沒有明顯變化，與未侵染的相比，經(jīng)農(nóng)桿菌侵染后的葉組織中AKT1表達(dá)量卻顯著增加，AKT2和GORK轉(zhuǎn)錄豐度則顯著下降[43-44]。由此可見，K+通道與植物細(xì)胞的生長發(fā)育有關(guān)。與擬南芥野生型相比，akt1突變體植株在含有100 μmol·L-1或更低濃度的K+的介質(zhì)中生長受到了抑制、吸收86Rb+的量減少，同時(shí)檢測不到K+內(nèi)流[17]。當(dāng)AKT1突變體幼苗置于外界低K+濃度環(huán)境時(shí)，與野生型相比，其根毛很短[27]。因此，AKT1在調(diào)節(jié)植物生長發(fā)育方面起著非常重要的作用。

3.2 參與根系K+吸收及提高植株的抗旱耐鹽性

內(nèi)整流K+通道AKT1主要在根中表達(dá)，說明其可能參與K+吸收[24-25,27,42]。對akt1突變體的研究表明，AKT1在低濃度K+(10 μmol·L-1)條件下介導(dǎo)植物根細(xì)胞K+吸收[17]。通過電壓鉗與膜片鉗技術(shù)發(fā)現(xiàn)CIPK23和CBL1或CBL9協(xié)同調(diào)節(jié)可以激活A(yù)KT1，并進(jìn)一步證明AKT1在植物吸收K+方面起著非常重要作用，特別是在外界高K+條件下[38]。在K+饑餓條件下，小麥根部的TaAKT1的表達(dá)豐度顯著上調(diào)，進(jìn)一步通過膜片鉗分析，發(fā)現(xiàn)小麥根中[K+]in電流受K+饑餓的刺激，可見TaAKT1介導(dǎo)根中K+的內(nèi)向電流[26]。通過吸收動力學(xué)對突變體athak5和akt1-1的研究表明，AtHAK5介導(dǎo)低濃度K+下K+吸收，AKT1在高濃度范圍內(nèi)(0.9 mmol·L-1)介導(dǎo)K+的吸收[16]，AtHAK5與AKT1在K+吸收中所起的作用如下：植物根細(xì)胞質(zhì)膜中存在著負(fù)責(zé)K+吸收的兩個(gè)主要系統(tǒng)，高親和性和低親和性系統(tǒng)，前者即系統(tǒng)Ⅰ在外部K+濃度較低時(shí)起作用；后者即系統(tǒng)Ⅱ在外部K+濃度較高時(shí)起作用，其臨界點(diǎn)是0.2 mmol·L-1，大約84%與78%的低親和性及高親和K+的吸收由AKT1與AtHAK5介導(dǎo)，系統(tǒng)Ⅰ與Ⅱ約20%的是未鑒定的組分；AtHAK5與AKT1分別主要介導(dǎo)高親和性及低親和性K+的吸收[45]。然而atakt1突變體在含NH4+時(shí)其生長受到抑制，而NH4+是KT/HAK/KUP轉(zhuǎn)運(yùn)蛋白的抑制劑[46]，K+轉(zhuǎn)運(yùn)載體受到抑制時(shí)，AKT1可介導(dǎo)高親和性K+吸收[20]?？梢姡谥参镏蠥KT1參與介導(dǎo)對NH4+不敏感的K+吸收[19]。外界K+濃度沒有影響擬南芥AtAKT1在mRNA水平上的表達(dá)[18,47]，說明AtAKT1可介導(dǎo)微摩爾及毫摩爾級的K+吸收。而電壓鉗技術(shù)研究表明番茄中的LKT1介導(dǎo)低親和性K+吸收[31]。

Ahmad等[48]發(fā)現(xiàn)，5%及10%PEG(polyethylene glycol)處理下，與野生型植株(WT)相比，超表達(dá)OsAKT1的水稻OX株系地上部及根中K+濃度及生長速率明顯增加；且10%PEG明顯提高了OX株系的氣孔導(dǎo)度，同時(shí)OX株系體內(nèi)的含水量顯著高于AKT1缺失體osakt1；控水的干旱處理下，OX株系的K+含量及生長速率明顯高于Osakt1及WT株，可見，OsAKT1可通過提高K+含量而增強(qiáng)植株的抗旱及滲透脅迫能力。酵母菌株G19由于介導(dǎo)Na+外排的ATP酶ENA1～ENA4的缺失使其對Na+具有較高的敏感性[49]。而已有研究表明，擬南芥AtHKT1;1被該酵母表達(dá)后，可通過介導(dǎo)細(xì)胞Na+的吸收從而提高G19的Na+敏感性，抑制NaCl條件下G19的生長[50]。而K+(1 mmol·L-1)添加不同濃度Na+(0、10、30及50 mmol·L-1)的AP(arginine-phosphate)培養(yǎng)基中，與轉(zhuǎn)化空載體的對照相比，轉(zhuǎn)化AtHKT1;1的G19生長顯著受抑，而轉(zhuǎn)化ZxAKT1和AtAKT1的G19長勢均顯著好于空載體對照，推測在NaCl處理下，轉(zhuǎn)化霸王ZxAKT1和擬南芥AtAKT1的G19的K+吸收能力顯著增強(qiáng)，使得酵母細(xì)胞中積累較多的K+以顯著緩解Na+對細(xì)胞的毒害，從而提高耐鹽性[30]。與此相似，在缺失高親和性K+吸收體系的酵母菌株10A(trk1, ura3)中也表現(xiàn)為耐鹽性提高[51]。

3.3 鹽條件下可能參與根系K+的外排

鹽處理下，atakt1-2株系與WT的K+吸收速率沒有顯著差異，因此AtAKT1對K+積累不起作用[52]。NaCl處理可能引起質(zhì)膜去極化而阻礙通道介導(dǎo)K+吸收[53]，實(shí)際上，低K+加鹽處理下，atakt1-2株系中的外排速率要低于野生及athak5-3株系，因此AtAKT1可能是一個(gè)K+外排的途徑。因?yàn)樵诘蚄+下，質(zhì)膜可能被超級化，從而使通道打開。K+饑餓附加NaCl處理降低了atakt1-2株系K+外排，使根和地上部K+濃度比其它株系的高；無鹽條件下沒有發(fā)現(xiàn)同樣的現(xiàn)象(質(zhì)膜超極化，高的內(nèi)部K+，低的外部K+)，可能是因?yàn)闆]有NaCl的情況下，膜勢為負(fù)值而避免了K+外排[52]。值得注意的是，在這些條件下，AtAKT1的調(diào)節(jié)亞基AtKC1，不能阻止K+通過AtAKT1的外排，而在非極端環(huán)境條件下[54-55]，僅缺失AtAKT1降低了根中K+的外排[52]。Nieves-Cordones[52]指出，10 μmol·L-1K+下，鹽的出現(xiàn)(30 mmol·L-1NaCl)降低了AtHAK5的表達(dá)并引起K+通過AtAKT1外排，可能是因?yàn)镹a+引起膜勢去極化，因此K+的凈吸收受到抑制。

3.4 參與Na+吸收

早期學(xué)者認(rèn)為，K+通道在鹽脅迫下可以介導(dǎo)低親和性Na+的吸收[47,56-58]，但首先介導(dǎo)低親和性K+的吸收[21,59-60]。Amtmann和Sanders[61]研究表明，在低Na+濃度條件下沒有明顯的Na+流通過內(nèi)整流K+通道。然而，這些通道在高鹽條件下可介導(dǎo)相對量Na+和K+的吸收[61]。TEA+(tetraethylammonium chloride)被認(rèn)為K+通道的專一性抑制劑[32,62-63]。在水稻中發(fā)現(xiàn)，耐鹽性品種對Cs+和TEA+(K+通道抑制劑)并不敏感，而Cs+和TEA+的存在條件下，鹽敏感品種細(xì)胞質(zhì)中Na+濃度則均顯著降低[64]。對2個(gè)不同耐鹽性的水稻品種進(jìn)行比較分析[24]發(fā)現(xiàn),150 mmol·L-1NaCl處理48 h后，IR29中Na+濃度約為1 400 μmol·g-1，而K+濃度與對照相比增加了50%；Pokkali中Na+濃度約為200 μmol·g-1，K+濃度則約為對照的50%；有趣的是，鹽處理下OsAKT1在鹽敏感品種IR29中表達(dá)豐度顯著高于耐鹽品種Pokkali，由此表明，OsAKT1在鹽敏感型品種中可能參與Na+吸收。此外，Voigt等[65]認(rèn)為TEA+和Cs+能夠抑制豇豆(Vignasinensis)體內(nèi)Na+的積累，這表明K+通道在豇豆低親和性Na+吸收方面起作用。Wang等[66]研究發(fā)現(xiàn)，高鹽(150 mmol·L-1NaCl)下，抑制劑Cs+和TEA+能夠顯著抑制鹽生植物鹽地堿蓬根部的22Na+內(nèi)流，由此推測AKT1可能具有介導(dǎo)高鹽條件下鹽地堿蓬Na+的吸收的功能。Mori等[67]對堿蓬研究發(fā)現(xiàn)，培養(yǎng)基中含有5 mmol·L-1K+時(shí)，Na+的吸收會顯著降低，K+濃度高于5 mmol·L-1時(shí)，Na+的吸收維持不變，當(dāng)K+濃度為50 mmol·L-1時(shí)，表現(xiàn)為小幅度降低，因此認(rèn)為，堿蓬中至少存在兩種Na+的吸收途徑，即對K+敏感及不敏感的兩條途徑。其中途徑2介導(dǎo)對外界K+不敏感的Na+的吸收途徑，且20及100 mmol·L-1NaCl處理下，TEA+的添加顯著降低了各組織中的Na+濃度，因此推測其可能由K+通道介導(dǎo)。小麥根中的TaAKT1 mRNA水平在缺K+的條件下上調(diào)，與此同時(shí)，K+饑餓引起瞬間的Na+電流，說明TaAKT1在K+饑餓條件下介導(dǎo)Na+吸收[26]。然而，擬南芥中提高細(xì)胞質(zhì)中Na+的濃度抑制了AKT1對K+的吸收[68]；在中度鹽脅迫(50 mmol·L-1NaCl)下，擬南芥atakt1-2突變體[16]與野生型植株相比，其Na+濃度及凈吸收速率均沒有顯著性差異[52]，且一些AKT1通道的基因(MKT1，OsAKT1及AKT1)表達(dá)在鹽脅迫下會下調(diào)[42,69-70]。AKT1類通道由于對K+具有高的特異選擇性不可能介導(dǎo)其它陽離子如Na+的吸收[71]。Ardie等[27]研究表明，小花堿茅PutAKT1在擬南芥中超表達(dá)后，鹽脅迫下轉(zhuǎn)基因植株的Na+含量下降的同時(shí)K+含量卻明顯提高。由此可見，內(nèi)整流K+通道是否參與Na+的吸收仍存在爭議。

3.5 質(zhì)膜的K+感應(yīng)器

外界K+濃度降低時(shí)，內(nèi)整流K+通道可能具有K+傳感的作用[72]。擬南芥根部的AKT1可能為K+傳感器[73]。首先，AKT1是主要的內(nèi)整流K+通道，優(yōu)先在擬南芥的根部表達(dá)且定位在表皮細(xì)胞的質(zhì)膜上[18,21]。其次，AKT1介導(dǎo)K+的吸收，表現(xiàn)為典型的雙親和性特性[18,20]。AKT1能感知[K+]ext的波動并以依賴[K+]ext的方式轉(zhuǎn)變其吸收動力學(xué)位點(diǎn)，位點(diǎn)的轉(zhuǎn)變與AKT1的磷酸化水平有關(guān)，而磷酸化水平又由CIPK23調(diào)控[38,74]。同樣，NO3-感應(yīng)也依賴CHL1蛋白的磷酸化水平。

此外，還有一個(gè)證據(jù)支持內(nèi)整流K+通道AKT1具有K+傳感器作用。有研究表明，植物應(yīng)對K+虧缺的反應(yīng)是皮層細(xì)胞的膜超極化，但是在akt1的突變體中這種超極化會完全消失[18,20]。進(jìn)一步通過異源表達(dá)發(fā)現(xiàn)，當(dāng)[K+]ext降低至微摩爾濃度時(shí)卵母細(xì)胞表達(dá)AKT1通道會顯著地轉(zhuǎn)變?yōu)槌瑯O化[13]。這些結(jié)果說明，AKT1可感應(yīng)[K+]ext的變化進(jìn)而影響定位在質(zhì)膜上由H+-ATPases調(diào)控的膜電勢[75-76]?？梢娫贏KT1與H+-ATPases間可能存在著一個(gè)通訊機(jī)制。近期的研究表明，K+可能結(jié)合H+-ATPases的C-末端并作為H+-ATPases解偶聯(lián)ATP水解活性和質(zhì)子泵活性一種內(nèi)在的負(fù)調(diào)節(jié)器[77]。進(jìn)一步的假設(shè)認(rèn)為K+營養(yǎng)虧缺影響了AKT1介導(dǎo)的K+吸收而引起[K+]cyt的減少，進(jìn)而質(zhì)膜附近減少的[K+]cyt會激活H+-ATPases的活性，因而引起質(zhì)膜超極化而介導(dǎo)的K+吸收的K+通道或載體被激活[73]。

3.6 AKT1參與調(diào)控氣孔運(yùn)動及植物蒸騰作用

近期的研究表明，CIPK23-CBL1/9復(fù)合物可激活根中AKT1從而增加K+吸收[38]。此外，CIPK-CBL復(fù)合物具有調(diào)控氣孔運(yùn)動及植物蒸騰作用[78]。AKT1的突變使植物對水分脅迫的響應(yīng)增強(qiáng)，水培實(shí)驗(yàn)表明，當(dāng)添加PEG時(shí)水勢降低，akt1成株與野生型相比其水分損失較少，并表現(xiàn)為蒸騰作用降低，水分消耗減小，最終，在響應(yīng)脫落酸(ABA)時(shí)其akt1植株氣孔迅速關(guān)閉；在cipk23植株中也發(fā)現(xiàn)類似的現(xiàn)象[79]。cipk23和akt1植株對水分脅迫反應(yīng)的相似性表明受CIPK23調(diào)控的AKT1也可能參與氣孔關(guān)閉，并且在水分脅迫下對植株的生長具有抑制作用[79]。

4 展望

K+是植物生長所需的大量營養(yǎng)元素，約占植物干重的10%[71,80]。K+是構(gòu)成滲透勢的重要成分，在維系細(xì)胞正常活動所必需的跨膜電位中有不可替代的作用，并作為多種酶的激活劑參與植株體內(nèi)重要代謝[81]。內(nèi)整流K+通道AKT1具有介導(dǎo)K+吸收、K+傳感器及參與調(diào)控氣孔運(yùn)動及植物蒸騰作用的功能，此外，鹽脅迫條件下AKT1還可能參與Na+吸收。因此，基于目前的研究現(xiàn)狀，今后對AKT1的研究可從以下幾個(gè)方面展開:1)一種生理現(xiàn)象的出現(xiàn)，往往是多種蛋白互作的結(jié)果，選取典型植物，采用蛋白組學(xué)等方法深入研究K+、Na+吸收及轉(zhuǎn)運(yùn)的協(xié)同調(diào)控機(jī)制。2)采用基因工程技術(shù)，將已發(fā)掘的具有同時(shí)可介導(dǎo)低及高親和性K+吸收、可明顯提高植株耐逆性的AKT1編碼基因轉(zhuǎn)入經(jīng)濟(jì)作物、糧食作物及飼料作物中，提高作物對土壤中K+的利用效率及作物的產(chǎn)量、品質(zhì)及抗性。3)在具有代表性的植物中克隆AKT1編碼基因，同時(shí)結(jié)合RNA干擾等方法揭示其在不同類型植物中的生理代謝、K+及Na+吸收及轉(zhuǎn)運(yùn)調(diào)控、抗逆性中的功能。

References:

[1] Barber S A.Potassium.in:Soil Nutrient Bioavailability:A Mechanistic Approach.New York:Wiley,1995:231-232.

[2] Maathuis F J.Physiological functions of mineral macronutrients.Current Opinion in Plant Biology,2009,12:250-258.

[3] Britto D T,Kronzucker H J.Cellular mechanisms of potassium transport in plants.Physiologia Plantarum,2008,133:637-650.

[4] Halperin S J,Lynch J P.Effects of salinity on cytosolic Na+and K+in root hairs ofArabidopsisthaliana:Invivomeasurements using the fluorescent dyes SBFI and PBFI.Journal of Experimental Botany,2003,54:2035-2043.

[5] Kronzucker H J,Szczerba M W,Britto D T.Cytosolic potassium homeostasis revisited:42K-tracer analysis inHordeumvulgareL. reveals set-point variations in [K+].Planta,2003,217:540-546.

[6] Walker D J,Leigh R A,Miller A J.Potassium homeostasis in vacuolate plant cells.Proceedings of the National Academy of Sciences,1996,93:10510-10514.

[7] 王茜,王沛,王鎖民.鹽生植物小花堿茅外整流K+通道SKOR基因片段的克隆及序列分析.草業(yè)科學(xué),2012,29(8):1218-1223. Wang Q,Wang P,Wang S M.Clone and sequence analysis of outward-rectifying potassium channelSKORgene fragment from halophytePuccinelliatenuiflora.Pratacultural Science,2012,29(8):1218-1223.(in Chinese)

[8] 夏曾潤,王沛東,賈文,未麗,米莉,王鎖民.K+對鹽脅迫下羅布麻生長及離子吸收分配的效應(yīng).草業(yè)科學(xué),2014,31(11):2088-2094. Xia Z R,Wang P D,Jia W,Wei L,Mi L,Wang S M.Effect of K+on the growth,ion absorptionand distribution ofApocynumvenetumunder ssalt stress.Pratacultural Science,2014,31(11):2088-2094.(in Chinese)

[9] 袁惠君,劉軻,王春梅,謝輝燦,李虎軍,賈鴻震.兩個(gè)寧夏枸杞品種的耐滲透脅迫和耐鹽特征比較.草業(yè)科學(xué),2016,33(4):681-690. Yuan H J,Liu K,Wang C M,Xie H C,Li H J,Jia H Z.The differences between two cultivars ofLyciumbarbarumin osmotic stress tolerance and salt tolerance.Pratacultural Science,2016,33(4):681-690.(in Chinese)

[10] Kronzucker H,Szczerba M W,Moazami-Goudarzi M,Britto D T.The cytosolic Na+∶K+ratio does not explain salinity-induced growth impairment in barley:A dual-tracer study using42K+and24Na+.Plant,Cell and Environment,2006,29:2228-2237.

[11] Shabala S,Demidchik V,Shabala L,Cuin T A,Smith S J,Miller A J,Davies J M,Newman I A.Extracellular Ca2+ameliorates NaCl-induced K+loss fromArabidopsisroot and leaf cells by controlling plasma membrane K+-permeable channels.Plant Physiology,2006,141:1653-1665.

[12] Lindberg S,Strid H.Aluminium induces rapid changes in cytosolic pH and free calcium and potassium concentrations in root protoplasts of wheat (Triticumaestivum).Physiologia Plantarum,1997,99:405-414.

[13] Wang Y,He L,Li H D,Xu J,Wu W H.Potassium channel α-subunit AtKC1 negatively regulates AKT1-mediated K+uptake inArabidopsisroots under low-K+stress.Cell Research,2010,20:826-837.

[14] Z?rb C,Senbayram M,Peiter E.Potassium in agriculture-status and perspectives.Journal of Plant Physiology,2014,171:656-669.

[15] Epstein E,Rains D,Elzam O.Resolution of dual mechanisms of potassium absorption by barley roots.Proceedings of the National Academy of Sciences,1963,49:684.

[16] Gierth M,M?ser P,Schroeder J I.The potassium transporter AtHAK5 functions in K+deprivation-induced high-affinity K+uptake and AKT1 K+channel contribution to K+uptake kinetics inArabidopsisroots.Plant Physiology,2005,137:1105-1114.

[17] Hirsch R E,Lewis B D,Spalding E P,Sussman M R.A role for the AKT1 potassium channel in plant nutrition.Science,1998,280:918-921.

[18] Lagarde D,Basset M,Lepetit M,Conejero G,Gaymard F,Astruc S,Grignon C.Tissue-specific expression ofArabidopsisAKT1 gene is consistent with a role in K+nutrition.The Plant Journal,1996,9:195-203.

[19] Rubio F,Nieves-Cordones M,Alemán F,Martínez V.Relative contribution of AtHAK5 and AtAKT1 to K+uptake in the high-affinity range of concentrations.Physiologia Plantarum,2008,134:598-608.

[20] Spalding E P,Hirsch R E,Lewis D R,Qi Z,Sussman M R,Lewis B D.Potassium uptake supporting plant growth in the absence of AKT1 channel activity inhibition by ammonium and stimulation by sodium.The Journal of General Physiology,1999,113:909-918.

[21] Sentenac H,Bonneaud N,Minet M,Lacroute F,Salmon J M,Gaymard F,Grignon C.Cloning and expression in yeast of a plant potassium ion transport system.Science,1992,256:663-665.

[22] Bauer C S,Hoth S,Haga K,Philippar K,Aoki N,Hedrich R.Differential expression and regulation of K+channels in the maize coleoptile:Molecular and biophysical analysis of cells isolated from cortex and vasculature.The Plant Journal,2000,24:139-145.

[23] Formentin E,Varottob S,Costaa A,Downeyc P,Breganted M,Nasod A,Piccod C,Gambaled F,Schiavo F L.DKT1,a novel K+channel from carrot,forms functional heteromeric channels with KDC1.FEBS Letters,2004,573:61-67.

[24] Golldack D,Quigley F,Michalowski C B,Kamasani U R,Bohnert H J.Salinity stress-tolerant and-sensitive rice(OryzasativaL.) regulate AKT1-type potassium channel transcripts differently.Plant Molecular Biology,2003,51:71-81.

[25] Boscari A,Mathilde C,Volkov V,Golldack D,Hybiak J,Miller A J,Amtmann A,Fricke W.Potassium channels in barley:Cloning,functional characterization and expression analyses in relation to leaf growth and development.Plant,Cell and Environment,2009,32:1761-1777.

[26] Buschmann P H,Vaidyanathan R,Gassmann W,Schroeder J I.Enhancement of Na+uptake currents,time-dependent inward-rectifying K+channel currents,and K+channel transcripts by K+starvation in wheat root cells.Plant Physiology,2000,122:1387-1398.

[27] Ardie S W,Liu S,Takano T.Expression of the AKT1-type K+channel gene fromPuccinelliatenuiflora,PutAKT1,enhances salt tolerance inArabidopsis.Plant Cell Reports,2010,29:865-874.

[28] 周向睿.K+通道基因(ZxAKT1)編碼蛋白在多漿旱生植物霸王Na+吸收中的作用機(jī)制研究.蘭州:蘭州大學(xué)博士學(xué)位論文,2011. Zhou X R.ZxAKT1 gene encoding AKT1-type K+channel plays important roles in Na+uptake in the succulent xerophyteZygophyllumxanthoxylum.PhD Thesis.Lanzhou:Lanzhou Univerty,2011.(in Chinese).

[29] Xu J,Tian X,Egrinya E A,Li Z.Functional characterization ofGhAKT1,a novel Shaker-like K+channel gene involved in K+uptake from cotton (Gossypiumhirsutum).Gene,2014,545:61-71.

[30] Duan H R,Ma Q,Zhang J L,Hu J,Bao A K,Wei L,Wang Q,Luan S.Wang S M.The inward-rectifying K+channel SsAKT1 is a candidate involved in K+uptake in the halophyteSuaedasalsaunder saline condition.Plant and Soil,2015,395(1-2):173-187.

[31] Hartje S,Zimmermann S,Klonus D,Mueller-Roeber B.Functional characterisation of LKT1, a K+uptake channel from tomato root hairs,and comparison with the closely related potato inwardly rectifying K+channel SKT1 after expression inXenopusoocytes.Planta,2000,210:723-731.

[32] Zimmermann S,Talke I,Ehrhardt T,Nast G,Müller-R?ber B.Characterization of SKT1,an inwardly rectifying potassium channel from potato,by heterologous expression in insect cells.Plant Physiology,1998,116:879-890.

[33] Jan L Y,Jan Y N.Cloned potassium channels from eukaryotes and prokaryotes.Annual Review of Neuroscience,1997,20:91-123.

[34] Sussman M R.ShakingArabidopsisthaliana.Science,1992,256:619.

[35] Wang P,Guo Q,Wang Q,Zhou X R,Wang S M.PtAKT1 maintains selective absorption capacity for K+,over Na+,in halophytePuccinelliatenuiflora,under salt stress.Acta Physiologiae Plantarum,2015,37(5):1-10.

[36] Véry A A,Sentenac H.Molecular mechanisms and regulation of K+transport in higher plants.Annual Review of Plant Biology,2003,54:575-603.

[37] Lee S C,Lan W Z,Kim B G,Li L,Cheong Y H,Pandey G K,Lu G,Buchanan B B,Luan S.A protein phosphorylation/dephosphorylation network regulates a plant potassium channel.Proceedings of the National Academy of Sciences,2007,104:15959-15964.

[38] Xu J,Li H D,Chen L Q,Wang Y,Liu L L,He L,Wu W H.A protein kinase,interacting with two calcineurin B-like proteins,regulates K transporter AKT1 inArabidopsis.Cell,2006,125:1347-1360.

[39] Langer K,Levchenko V,Fromm J,Geiger D,Steinmeyer R,Lautner S,Ache P,Hedrich R.The poplar K+channel KPT1 is associated with K+uptake during stomatal opening and bud development.The Plant Journal,2004,37:828-838.

[40] Pratelli R,Lacombe B,Torregrosa L,Gaymard F,Romieu C,Thibaud J B,Sentenac H.A grapevine gene encoding a guard cell K+channel displays developmental regulation in the grapevine berry.Plant Physiology,2002,128:564-577.

[41] Basset M,Conejero G,Lepetit M,Fourcroy P,Sentenac H.Organization and expression of the gene coding for the potassium transport system AKT1 ofArabidopsisthaliana.Plant Molecular Biology,1995,29:947-958.

[42] Fuchs I,St?lzle S,Ivashikina N,Hedrich R.Rice K+uptake channel OsAKT1 is sensitive to salt stress.Planta,2005,221:212-221.

[43] Deeken R,Ivashikina N,Czirjak T,Philippar K,Becker D,Ache P,Hedrich R.Tumour development inArabidopsisthalianainvolves the Shaker-like K+channels AKT1 and AKT2/3.The Plant Journal,2003,34:778-787.

[44] Becker D,Geiger D,Dunkel M,Roller A,Bertl A,Latz A,Carpaneto A,Dietrich P,Roelfsema M R G,Voelker C,Schmidt D,Mueller-Roeber B,Czempinski K,Hedrich R.AtTPK4,anArabidopsistandem-pore K+channel,poised to control the pollen membrane voltage in a pH and Ca2+dependent manner.Proceedings of the National Academy of Sciences,2004,101(44):15621-15626.

[45] Maathuis F J,Sanders D.Mechanisms of potassium absorption by higher plant roots.Physiologia Plantarum,1996,96:158-168.

[46] Santa-María G E,Rubio F,Dubcovsky J,Rodríguez-Navarro A.TheHAK1 gene of barley is a member of a large gene family and encodes a high-affinity potassium transporter.The Plant Cell,1997,9:2281-2289.

[47] Pilot G,Gaymard F,Mouline K,Chérel I,Sentenac H.Regulated expression ofArabidopsisShaker K+channel genes involved in K+uptake and distribution in the plant.Plant Molecular Biology,2003,51:773-787.

[48] Ahmad I,Mian A,Maathuis F J M.Overexpression of the rice AKT1 potassium channel affects potassium nutrition and rice drought tolerance.Journal of Experimental Botany,2016,doi:10.1093/jxb/erw103.

[49] Amtmann A,Fischer M,Marsh E L,Stefanovic A,Sanders D,Schachtman D P.The wheat cDNALCT1 generates hypersensitivity to sodium in a salt-sensitive yeast strain.Plant Physiology,2001,126(3):1061-1071.

[50] M?ser P,Eckelman B,Vaidyanathan R,Horie T,Fairbairn D J,Kubo M,Yamagami M,Yamaguchi K,Nishimura M,Uozumi N,Robertson W,Sussman M R,Schroeder J I.Altered shoot/root Na+distribution and bifurcating salt sensitivity inArabidopsisby genetic disruption of the Na+transporterAtHKT1.FEBS Letters,2002,531:157-161.

[51] Ros R,Lemaillet G,Fonrouge A,Daram P,Enjuto M,Salmon J,Thibaud J,Sentenac H.Molecular determinants of theArabidopsisAKT1 K+channel ionic selectivity investigated by expression in yeast of randomly mutated channels.Physiologia Plantarum,1999,105:459-468.

[52] Nieves-Cordones M,Alemán F,Martínez V,Rubio F.TheArabidopsisthalianaHAK5 K+transporter is required for plant growth and K+acquisition from low K+solutions under saline conditions.Molecular Plant,2010,3(2):326-333.

[53] Shabala S,Cuin T A.Potassium transport and plant salt tolerance.Physiologia Plantarum,2008,133:651-669.

[54] Duby G,Hosy E,Fizames C,Alcon C,Costa A,Sentenac H,Thibaud J B.AtKC1,a conditionally targeted Shaker-type subunit,regulates the activity of plant K+channels.The Plant Journal,2008,53:115-123.

[55] Geiger D,Becker D,Vosloh D,Gambale F,Palme K,Rehers M,Anschuetz U,Gierth M,M?ser P.Potassium transporters in plants-Involvement in K+acquisition,redistribution and homeostasis.FEBS Letters,2007,581(12):2348-2356.

[56] Fu H H,Luan S.AtKUP1:A dual-affinity K+transporter fromArabidopsis.The Plant Cell,1998,10:63-73.

[57] Gassmann W,Rubio F,Schroeder J I.Alkali cation selectivity of the wheat root high-affinity potassium transporter HKT1.The Plant Journal,1996,10:869-882.

[58] Rubio F,Gassmann W,Schroeder J I.Sodium-driven potassium uptake by the plant potassium transporter HKT1 and mutations conferring salt tolerance.Science,1995,270:1660-1663.

[59] Czempinski K,Gaedeke N,Zimmermann S,Müller-R?ber B.Molecular mechanisms and regulation of plant ion channels.Journal of Experimental Botany,1999,50:955-966.

[60] Fox T C,Guerinot M L.Molecular biology of cation transport in plants.Annual Review of Plant Biology,1998,49:669-696.

[61] Amtmann A,Sanders D.Mechanisms of Na+uptake by plant cells.Advances in Botanical Research,1998,29:75-112.

[62] Roberts S K,Tester M.Inward and outward K+-selective currents in the plasma membrane of protoplasts from maize root cortex and stele.The Plant Journal,1995,8:811-825.

[63] Schachtman D P,Schroeder J I,Lucas W J,Anderson J A,Gaber R F.Expression of an inward-rectifying potassium channel by theArabidopsisKAT1 cDNA.Science,1992,258:1654-1658.

[64] Kader M A,Lindberg S.Uptake of sodium in protoplasts of salt-sensitive and salt-tolerant cultivars of rice,OryzasativaL. determined by the fluorescent dye SBFI.Journal of Experimental Botany,2005,56:3149-3158.

[65] Voigt E,Caitano R,Maia J,Ferreira-Silva S,de Macêdo C,Silveira J.Involvement of cation channels and NH4+-sensitive K+transporters in Na+uptake by cowpea roots under salinity.Biologia Plantarum,2009,53:764-768.

[66] Wang S M,Zhang J L,Flowers T J.Low-affinity Na+uptake in the halophyte Suaeda maritima.Plant Physiology,2007,145:559-571.

[67] Mori S,Suzuki K,Oda R,Higuchi K,Maeda Y,Yoshiba M,Tadano T.Characteristics of Na+and K+absorption inSuaedasalsa(L.).Soil Science and Plant Nutrition,2011,57:377-386.

[68] Qi Z,Spalding E P.Protection of plasma membrane K+transport by the salt overly sensitive1 Na+-H+antiporter during salinity stress.Plant Physiology,2004,136:2548-2555.

[69] Kaddour R,Nasri N,M’rah S,Berthomieu P,Lachal M.Comparative effect of potassium on K and Na uptake and transport in two accessions ofArabidopsisthalianaduring salinity stress.Comptes Rendus Biologies,2009,332(9):784-794.

[70] Su H,Golldack D,Katsuhara M,Zhao C,Bohnert H J.Expression and stress-dependent induction of potassium channel transcripts in the common ice plant.Plant Physiology,2001,125:604-614.

[71] Gambale F,Uozumi N.Properties of Shaker-type potassium channels in higher plants.The Journal of Membrane Biology,2006,210:1-19.

[72] Schroeder J I,Fang H H.Inward-rectifying K+channels in guard cells provide a mechanism for low-affinity K+uptake.Proceedings of the National Academy of Sciences,1991,88:11583-11587.

[73] Wang Y,Wu W H.Potassium transport and signaling in higher.Annual Review of Plant Biology,2013,64:451-476.

[74] Li L,Kim B G,Cheong Y H,Pandey G K,Luan S.A Ca2+signaling pathway regulates a K+channel for low-K response inArabidopsis.Proceedings of the National Academy of Sciences,2006,103:12625-12630.

[75] de Witt N D,Hong B,Sussman M R,Harper J F.Targeting of twoArabidopsisH+-ATPase isoforms to the plasma membrane.Plant Physiology,1996,112:833-844.

[76] Harper J F,Surowy T K,Sussman M R.Molecular cloning and sequence of cDNA encoding the plasma membrane proton pump (H+-ATPase) ofArabidopsisthaliana.Proceedings of the National Academy of Sciences,1989,86:1234-1238.

[77] Buch-Pedersen M J,Rudashevskaya E L,Berner T S,Venema K,Palmgren M G.Potassium as an intrinsic uncoupler of the plasma membrane H+-ATPase.The Journal of Biological Chemistry,2006,281:38285-38292.

[78] Cheong Y H,Pandey G K,Grant J J,Batistic O,Li L,Kim B G,Lee S C,Kudla J,Luan S.Two calcineurin B-like calcium sensors,interacting with protein kinase CIPK23,regulate leaf transpiration and root potassium uptake inArabidopsis.The Plant Journal,2007,52:223-239.

[79] Nieves-Cordones M,Caballero F,Martínez V,Rubio F.Disruption of theArabidopsisthalianainward-rectifier K+channel AKT1 improves plant responses to water stress.Plant and Cell Physiology,2012,53:423-432.

[80] Leigh R,Wyn J R.A hypothesis relating critical potassium concentrations for growth to the distribution and functions of this ion in the plant cell.New Phytologist,1984,97:1-13.

[81] Clarkson D T,Hanson J B.The mineral nutrition of higher plants.Annual Review of Physiology,1980,31:239-298.

(責(zé)任編輯 王芳)

Study advances of plant inward rectifying K+channel AKT1

Hu Jing1, Hu Xiao-ke1, Yu Qiu-shi1, Yuan Hui-jun2, Yousaf Jamal1
(1.State Key Laboratory of Desertification and Aeolian Sand Disaster Combating, Gansu Desert Control Research Institute, Lanzhou 730070, China; 2.School of Life Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China)

Potassium (K+) is one of the major nutrients, essential for plant growth and development. Inward rectifying K+channel AKT1 belongs to Shaker-type K+channels, and a K+sensor in the plasma membrane has been shown to play an important role in mediating K+uptake. AKT1 is also essential for normal growth and development, stomatal action and transpiration, and improving the drought and salt tolerance of plants. In this review, we consider recent advances in molecular structure, localization, expression, regulation and function of AKT1. Finally, research directions for the future are proposed, including the use of proteomics, gene engineering technology, and posttranscriptional gene silencing, in order to further investigate the role of AKT1 in plant physiological metabolism.

inward rectifying K+channel AKT1; K+uptake; Shaker-type K+channel

Hu Jing E-mail:hujingvip@163.com

2016-06-02 接受日期：2016-09-13

甘肅省治沙研究所甘肅省荒漠化與風(fēng)沙災(zāi)害防治國家重點(diǎn)實(shí)驗(yàn)室(培育基地)開放基金項(xiàng)目——鹽及干旱條件下K+、Na+主要滲透調(diào)節(jié)物質(zhì)在沙拐棗中積累及分配的研究；石羊河下游退耕地植物群落優(yōu)勢種的生理生態(tài)特征研究；國家自然科學(xué)基金(31360089)；中國科學(xué)院西部之光人才計(jì)劃項(xiàng)目；國家自然科學(xué)基金(31460629)

胡靜(1982-)，女，遼寧建昌人，助理研究員，博士，主要從事植物逆境生理與分子生物學(xué)的研究。E-mail:hujingvip@163.com

10.11829/j.issn.1001-0629.2016-0295

S143.3;Q945.12

A

1001-0629(2017)04-0813-10

胡靜,胡小柯,尉秋實(shí),袁惠君,Yousaf Jamal.植物內(nèi)整流K+通道AKT1的研究進(jìn)展.草業(yè)科學(xué),2017,34(4)：813-822.

Hu J,Hu X K,Yu Q S,Yuan H J,Yousaf J.Study advances of plant inward rectifying K+channel AKT1.Pratacultural Science，2017,34(4)：813-822.