陳迪，潘偉槐，周哉材，嚴(yán)旭，潘建偉3*

（1.浙江師范大學(xué)化學(xué)與生命科學(xué)學(xué)院，浙江金華321004；2.紹興文理學(xué)院生命科學(xué)學(xué)院，浙江紹興312000；3.蘭州大學(xué)生命科學(xué)學(xué)院，蘭州730000）

氮（N）、磷（P）、鉀（K）和鋅（Zn）等是植物生長(zhǎng)發(fā)育的重要必需元素。N是植物所需的最重要的大量元素之一，土壤中的硝酸鹽（NO3-）和銨鹽（NH4+）是植物吸收利用氮源的主要形式。植物根的表皮和皮層細(xì)胞通過(guò)不同的轉(zhuǎn)運(yùn)蛋白從土壤中吸收NO3-，然后通過(guò)代謝還原成銨鹽，由銨直接合成氨基酸，供植物利用[1]。P是能量轉(zhuǎn)移和各類(lèi)代謝等的重要調(diào)控元素。K參與植物細(xì)胞的許多生理過(guò)程，如酶激活、氣孔運(yùn)動(dòng)和pH的穩(wěn)態(tài)等。Zn是植物生長(zhǎng)發(fā)育所必需的微量元素，參與糖類(lèi)、脂類(lèi)和核酸等各類(lèi)代謝的調(diào)控。

由于植物營(yíng)養(yǎng)是作物高產(chǎn)、穩(wěn)產(chǎn)的重要生理基礎(chǔ)，目前對(duì)植物細(xì)胞如何吸收利用N、P、K和Zn等營(yíng)養(yǎng)元素的分子機(jī)制已有較為深入的研究，尤其是對(duì)這些營(yíng)養(yǎng)元素的轉(zhuǎn)運(yùn)蛋白本身的調(diào)控研究已取得長(zhǎng)足進(jìn)展，并且是近幾年作物營(yíng)養(yǎng)研究領(lǐng)域的熱點(diǎn)之一。本文在扼要介紹N、P、K和Zn等轉(zhuǎn)運(yùn)蛋白理化特性的基礎(chǔ)上，著重介紹了最近幾年有關(guān)這些轉(zhuǎn)運(yùn)蛋白的生物學(xué)功能及其作用機(jī)制的最新進(jìn)展，為作物養(yǎng)分高效利用和品質(zhì)遺傳改良提供新的研究思路和策略。

1 植物氮轉(zhuǎn)運(yùn)蛋白及其吸收運(yùn)輸機(jī)制

已知高等植物有4類(lèi)轉(zhuǎn)運(yùn)蛋白具有NO3-的轉(zhuǎn)運(yùn)活性，包括 NRT1/PTR（nitrate transporter 1/peptide transporter，硝酸鹽轉(zhuǎn)運(yùn)蛋白1/寡肽轉(zhuǎn)運(yùn)蛋白）或NPF（NRT1/PTR family，NRT1/PTR家族）、NRT2、CLC（chloride channel family，氯離子通道家族）和SLAC/SLAH（slow anion channels，慢速陰離子通道蛋白）。高等植物為適應(yīng)不同濃度的NO3-環(huán)境，分別進(jìn)化出2類(lèi)NO3-轉(zhuǎn)運(yùn)蛋白系統(tǒng)：高親和轉(zhuǎn)運(yùn)系統(tǒng)（high-affinity transport system,HATS）和低親和轉(zhuǎn)運(yùn)系統(tǒng)（low-affinity transport system,LATS）。HATS由 NRT2/NNP（NRT2/nitrate-nitrite-porter）家族成員組成，當(dāng)外界NO3-濃度低至微摩爾水平時(shí)，其對(duì)NO3-的吸收有積極作用；相反，LATS由NPF家族成員主導(dǎo)，在毫摩爾濃度時(shí)吸收NO3-。這2類(lèi)轉(zhuǎn)運(yùn)蛋白系統(tǒng)的轉(zhuǎn)運(yùn)活性均需要細(xì)胞提供能量和胞外質(zhì)子梯度[2]。

在N不足的情況下，NRT2在植物NO3-/H+同向吸收過(guò)程中起著關(guān)鍵性調(diào)控作用，但需要伴侶蛋白NAR2的參與（圖1）。水稻基因組編碼5個(gè)OsNRT2家族成員，其中OsNRT2.3a和OsNRT2.3b由OsNRT2.3通過(guò)選擇性剪接機(jī)制形成，在不同組織中發(fā)揮功能。水稻OsNRT2.1、OsNRT2.2和OsNRT2.3a介導(dǎo)NO3-的吸收，但依賴(lài)于伴侶蛋白OsNAR2.1的參與、根的發(fā)育狀態(tài)和植株體內(nèi)N的水平[8]。此外，其他物種的NRT2成員也已經(jīng)被鑒定（表1）。

第3類(lèi)轉(zhuǎn)運(yùn)體是氯離子通道蛋白CLC，具有轉(zhuǎn)運(yùn)NO3-的功能。擬南芥和水稻基因組均編碼7個(gè)CLC家族成員（表1）。有研究表明，CLCa和CLCb是定位于液泡膜上的2NO3-/1H+逆向轉(zhuǎn)運(yùn)蛋白，在液泡NO3-的積累中發(fā)揮重要的作用（圖1）。而定位于液泡膜上的CLCg的具體功能目前尚不清楚。當(dāng)前，對(duì)煙草和大豆CLC功能的研究還較落后（表1）。

第4類(lèi)NO3-轉(zhuǎn)運(yùn)體是SLAC/SLAH。擬南芥的SLAC/SLAH家族共有5個(gè)成員（表1），這些基因的蛋白產(chǎn)物均定位在質(zhì)膜上。AtSLAC1和AtSLAH3在保衛(wèi)細(xì)胞中編碼S型陰離子通道，能直接調(diào)控脫落酸（abscisic acid,ABA）信號(hào)，促使Cl-和NO3-從保衛(wèi)細(xì)胞中釋放，刺激氣孔關(guān)閉。AtSLAH2在根中柱細(xì)胞中表達(dá)，介導(dǎo)NO3-的輸出，與從根到莖中NO3-的運(yùn)輸有關(guān)。功能分析表明，SLAH2是NO3-的特異性通道，其活性受CBL1和CIPK21/23互作蛋白調(diào)控。而SLAH3的活性受CDPK21（calciumdependent protein kinase，鈣離子依賴(lài)性蛋白激酶）磷酸化激活（圖1）[25]。目前，在水稻中僅鑒定出2個(gè)SLAC基因（表 1），其中 SLAC1受蛋白激酶OsSAPK8（stress-activated protein kinase，應(yīng)激活化蛋白激酶）的正調(diào)控。這些研究結(jié)果表明，SLAC/SLAH在不同組織部位中介導(dǎo)NO3-的運(yùn)輸。

表1 不同植物物種已被鑒定的N轉(zhuǎn)運(yùn)蛋白類(lèi)型和種類(lèi)Table 1 Types and numbers of identified N transporter in different plant species

植物NH4+的運(yùn)輸主要由銨轉(zhuǎn)運(yùn)蛋白（ammonium transporters,AMT）介導(dǎo)。擬南芥共有6個(gè)AMT轉(zhuǎn)運(yùn)蛋白（表1）。最近的研究發(fā)現(xiàn)，AMT1;1和1;2受CBL1-CIPK23的磷酸化調(diào)控，從而抑制NH4+的轉(zhuǎn)運(yùn)活性[26]（圖1）。水稻共有10個(gè)AMT類(lèi)轉(zhuǎn)運(yùn)蛋白，分為 4個(gè)亞類(lèi)：OsAMT1、OsAMT2、OsAMT3和OsAMT4。其中OsAMT1是高親和轉(zhuǎn)運(yùn)蛋白，而OsAMT2～AMT4介導(dǎo)低親和NH4+的轉(zhuǎn)運(yùn)[23]，但至今有關(guān)OsAMT2～AMT4的報(bào)道較少。最近研究表明，轉(zhuǎn)錄因子OsDOF18（DNA binding with one finger 18）正調(diào)控OsAMT1;1、1;3和2;1的表達(dá)，從而促進(jìn)植物細(xì)胞對(duì)NH4+的吸收[27]。此外，在小麥和玉米中也已鑒定出若干個(gè)AMT蛋白（表1），但其具體的作用機(jī)制仍有待研究。

2 植物磷轉(zhuǎn)運(yùn)蛋白及其吸收運(yùn)輸機(jī)制

已知植物磷轉(zhuǎn)運(yùn)蛋白（phosphate transporter,PHT）主要分為5個(gè)家族：PHT1、PHT2、PHT3、PHT4和PHT5。

圖1 植物N、P、K和Zn轉(zhuǎn)運(yùn)蛋白及其調(diào)控機(jī)制[25-26,28-31]Fig.1 Transporters and their action mechanisms of N,P,K and Zn in plants[25-26,28-31]

PHT1主要定位于質(zhì)膜上（圖1）。從表2可知，擬南芥共有9個(gè)PHT1成員，在其他物種中也已經(jīng)被鑒定到多個(gè)PHT1成員。已知擬南芥RING類(lèi)型 E3泛素化連接酶NLA（nitrogen limitation adaptation）能介導(dǎo)PHT1的降解，從而降低PHT1的質(zhì)膜豐富度和Pi的吸收[28]（圖1）。水稻共有13個(gè)PHT1成員，多數(shù)功能已經(jīng)被鑒定（表2）。有研究顯示，水稻OsNLA通過(guò)與OsPHT1.2和OsPHT1.8互作，促進(jìn)其降解，從而抑制Pi的過(guò)度積累。在Pi充足的條件下，水稻酪蛋白激酶ⅡCK2磷酸化OsPHT，抑制OsPHT與磷酸鹽轉(zhuǎn)運(yùn)蛋白運(yùn)輸促進(jìn)因子（phosphate transporter traffic facilitator 1,PHF1）的互作，從而阻止OsPHT從內(nèi)質(zhì)網(wǎng)到質(zhì)膜的運(yùn)輸，導(dǎo)致其在內(nèi)質(zhì)網(wǎng)（endoplasmic reticulum,ER）積累[32]。有研究表明，低Pi能促進(jìn)小麥MYB轉(zhuǎn)錄因子TaPHR1（phosphate starvation response 1）與 TaPHT1基因啟動(dòng)子結(jié)合，從而上調(diào)TaPHT1的轉(zhuǎn)錄[33]。相反，擬南芥轉(zhuǎn)錄因子MYB62經(jīng)低Pi誘導(dǎo)下調(diào)AtPHT1;1和AtPHT1;4的轉(zhuǎn)錄[34]，并且抑制WRKY75表達(dá)也會(huì)導(dǎo)致AtPHT1;1和AtPHT1;4轉(zhuǎn)錄下調(diào)[35]。同樣，大麥轉(zhuǎn)錄因子TabHLH1上調(diào)促進(jìn)煙草NtPHT1表達(dá)，從而提高植株對(duì)低Pi的耐受性[36]。此外，其他多個(gè)物種的PHT1功能也陸續(xù)被鑒定（表1）。以上研究結(jié)果表明，PHT1轉(zhuǎn)錄調(diào)控和轉(zhuǎn)錄后調(diào)控均為介導(dǎo)磷吸收的重要調(diào)控機(jī)制。

PHT2轉(zhuǎn)運(yùn)蛋白定位于葉綠體內(nèi)膜上（圖1）。擬南芥和水稻均只有1個(gè)PHT2成員（表2），均在葉中表達(dá)且受低Pi誘導(dǎo)。目前，PHT2的具體作用機(jī)制仍有待進(jìn)一步研究。

PHT3轉(zhuǎn)運(yùn)蛋白定位于線(xiàn)粒體內(nèi)膜上（圖1）。從表2中可以看出，擬南芥有3個(gè)AtPHT3成員，在水稻、小麥等物種中也存在多個(gè)PHT3同源蛋白。PHT4轉(zhuǎn)運(yùn)蛋白位于高爾基體上（圖1），參與調(diào)控葉的形成、植物防御、耐鹽性等過(guò)程。擬南芥共有6個(gè)AtPHT4成員，主要在根和葉中表達(dá)，其他物種也存在多個(gè)PHT4轉(zhuǎn)運(yùn)蛋白（表2）。但PHT3和PHT4的具體功能至今仍知之甚少。

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PHT5蛋白，又被稱(chēng)為SPX-MFS蛋白（SYG1/PHO81/XPR1），定位于液泡膜上（圖1）。表2顯示：擬南芥PHT5家族共由3個(gè)成員組成，過(guò)表達(dá)AtPHT5均能引起植株生長(zhǎng)緩慢和液泡Pi的積累，表明PHT5成員具有功能豐余性；水稻SPX-MFS家族有4個(gè)成員，Pi饑餓會(huì)抑制OsSPX-MFS1的表達(dá)，但促進(jìn)OsSPX-MFS2的表達(dá)，不過(guò)它們均受OsmiR827的負(fù)調(diào)控[49]。這些研究結(jié)果證實(shí)了PHT5能調(diào)控胞內(nèi)Pi的穩(wěn)態(tài)，對(duì)維持植物細(xì)胞的正常生長(zhǎng)分化具有重要的生物學(xué)功能。

表2 不同物種已被鑒定的P轉(zhuǎn)運(yùn)蛋白的類(lèi)型和數(shù)量Table 2 Types and numbers of identified P transporters in different plant species

3 植物鉀轉(zhuǎn)運(yùn)蛋白及其吸收運(yùn)輸機(jī)制

與硝態(tài)氮吸收機(jī)制相似，植物細(xì)胞內(nèi)也同樣存在高親和與低親和的K+吸收系統(tǒng)，它們分別應(yīng)對(duì)低濃度和高濃度的K+環(huán)境。關(guān)于K+通道蛋白家族的研究，近幾年國(guó)內(nèi)已經(jīng)有較好的綜述[50]，本節(jié)著重介紹K+轉(zhuǎn)運(yùn)蛋白的功能及其作用機(jī)制。

植物細(xì)胞共有4類(lèi)K+轉(zhuǎn)運(yùn)蛋白家族：KT/HAK/KUP（K+transporter/high-affinity K+transporter/K+uptake permease，K+轉(zhuǎn)運(yùn)蛋白/高親和K+轉(zhuǎn)運(yùn)蛋白/K+吸收通透酶）、HKT（high-affinity K+transporter，高親和 K+轉(zhuǎn)運(yùn)蛋白）、CHX（cation/hydrogen exchanger，陽(yáng)離子/H+反向轉(zhuǎn)運(yùn)蛋白）和KEA（K+/H+efflux antiporters，K+/H+反向轉(zhuǎn)運(yùn)蛋白）。從表3中可以看出，擬南芥KT/HAK/KUP轉(zhuǎn)運(yùn)蛋白共有13個(gè)成員，水稻OsHAK家族共有27個(gè)成員。此外，在其他物種中也存在多個(gè)同源蛋白。KT/HAK/KUP基因幾乎在所有植物組織或器官中均有表達(dá)，表明該轉(zhuǎn)運(yùn)蛋白家族對(duì)植物器官的生長(zhǎng)發(fā)育和維持細(xì)胞中K+的平衡具有非常重要的作用。

HKT轉(zhuǎn)運(yùn)蛋白是K+/Na+同向轉(zhuǎn)運(yùn)體。HKT可分為亞族Ⅰ和Ⅱ，其中亞族Ⅰ存在于單子葉和雙子葉植物中，主要由Na+轉(zhuǎn)運(yùn)蛋白組成，亞族Ⅱ似乎只存在于單子葉植物中，由Na+和K+轉(zhuǎn)運(yùn)蛋白組成。由表3可知，擬南芥HKT家族僅有1個(gè)成員，而水稻HKT家族由9個(gè)成員組成，主要介導(dǎo)Na+的轉(zhuǎn)運(yùn)。

已知HAK5和AKT1是擬南芥中2大主要的K+吸收系統(tǒng)。從圖1中可以看出：在低K+條件下，擬南芥生長(zhǎng)素響應(yīng)因子（auxin response factor 2,ARF2）能與HAK5啟動(dòng)子結(jié)合從而抑制HAK5的表達(dá)[31]；CBL1與CIPK9互作能促進(jìn)HAK5的磷酸化和K+的吸收；擬南芥AKT1介導(dǎo)的K+吸收表現(xiàn)為雙親和特性，在K+不足的情況下，CBL1/9能分別與CIPK23互作，并招募CIPK23至細(xì)胞質(zhì)膜，進(jìn)而磷酸化AKT1，導(dǎo)致由低親和吸收轉(zhuǎn)變?yōu)楦哂H和；而CBL10作為負(fù)調(diào)控因子直接與AKT1互作，抑制AKT1介導(dǎo)的K+胞質(zhì)轉(zhuǎn)運(yùn)[51]；水稻OsAKT1是鹽敏感型K+吸收的主要通道，OsCBL1-OsCIPK23復(fù)合物能提高OsAKT1介導(dǎo)的K+吸收[52]，表明OsAKT1介導(dǎo)的K+轉(zhuǎn)運(yùn)過(guò)程受OsCBL1-OsCIPK23的正調(diào)控。

CHX屬于CPA（cation/proton antiporters，陽(yáng)離子/質(zhì)子反向轉(zhuǎn)運(yùn)蛋白）超家族。表3顯示，擬南芥共有28個(gè)成員，其中研究最廣泛的CHX17介導(dǎo)質(zhì)膜K+的轉(zhuǎn)運(yùn)[53]（圖1）。關(guān)于CHX的功能在水稻和野生大豆中也有報(bào)道，但其具體作用機(jī)制有待進(jìn)一步研究。

植物KEA家族是K+/H+轉(zhuǎn)運(yùn)體。擬南芥有6個(gè)KEA成員，它們均在維管組織中表達(dá)（表3）。葉綠體有3個(gè)K+輸出反向轉(zhuǎn)運(yùn)體KEA1～KEA3，其中KEA1和KEA2位于葉綠體內(nèi)膜上，調(diào)控葉綠體K+/H+的穩(wěn)態(tài)（圖1），均與光合作用相關(guān)[54]。有關(guān)水稻、玉米和高粱等作物的KEA作用機(jī)制仍需進(jìn)行深入研究。

表3 不同物種已被鑒定的K轉(zhuǎn)運(yùn)蛋白的類(lèi)型和數(shù)量Table 3 Types and numbers of identified K transporters in different plant species

4 植物鋅轉(zhuǎn)運(yùn)蛋白及其吸收運(yùn)輸機(jī)制

Zn轉(zhuǎn)運(yùn)蛋白可分為ZIP、CDF（cation diffusion facilitator，陽(yáng)離子擴(kuò)散促進(jìn)子家族）和P1B-ATP酶型家族HMA等蛋白家族（表4）。ZIP轉(zhuǎn)運(yùn)蛋白在植物和動(dòng)物中功能相當(dāng)保守，在Zn2+的吸收和轉(zhuǎn)運(yùn)過(guò)程中不可或缺。擬南芥ZIP家族共有16個(gè)成員，其中研究最清楚的是AtZIP1和AtZIP2，分別定位在液泡膜和質(zhì)膜上，介導(dǎo)液泡Zn2+的輸出和質(zhì)膜Zn2+/Mn2+的吸收[71]。水稻ZIP家族有17個(gè)成員，主要參與 Zn2+轉(zhuǎn)運(yùn)[72]。

表4 不同物種已被鑒定的Zn轉(zhuǎn)運(yùn)蛋白的類(lèi)型和數(shù)量Table 4 Types and numbers of identified Zn transporters in different plant species

CDF轉(zhuǎn)運(yùn)蛋白，又稱(chēng)金屬耐受性蛋白（metal tolerance proteins,MTP）。大多數(shù)CDF是Mn2+/H+（Zn2+）逆向運(yùn)輸?shù)鞍?，介?dǎo)過(guò)度金屬陽(yáng)離子從胞質(zhì)到胞外或細(xì)胞器的轉(zhuǎn)運(yùn)。CDF通常定位于質(zhì)膜或細(xì)胞器膜上，參與金屬解毒、金屬蛋白的組裝和分泌囊泡的包裝。擬南芥CDF家族有12個(gè)成員，而水稻有10個(gè)成員，按功能可分為3類(lèi)：Zn-CDF、Zn/Fe-CDF和Mn-CDF。由圖1可知，AtMTP1和AtMTP3定位于液泡膜上，介導(dǎo)Zn2+從胞質(zhì)到液泡的轉(zhuǎn)運(yùn)，以維持胞質(zhì)低Zn2+濃度；AtMTP5和AtMTP12位于高爾基體，介導(dǎo)Zn2+從胞質(zhì)到高爾基體的轉(zhuǎn)運(yùn)[77]。

P1B-ATP酶型家族HMA在重金屬離子的胞內(nèi)分配與解毒過(guò)程中起重要作用，可分為2個(gè)亞類(lèi)：Ⅰ型（Cu+/Ag+）和Ⅱ型（Zn2+/Cd2+/Pb2+/Co2+）。擬南芥HMA亞族有8個(gè)HMA成員，其中AtHMA1～AtHMA4屬于Ⅱ型，分別介導(dǎo)不同組織中Zn2+的轉(zhuǎn)運(yùn)，而其余屬于Ⅰ型。水稻共有9個(gè)HMA成員，其中OsHMA1～OsHMA3屬于Ⅱ型，其余屬于Ⅰ型。大麥和小麥等作物中的HMA蛋白也有報(bào)道。

5 植物營(yíng)養(yǎng)元素轉(zhuǎn)運(yùn)蛋白的內(nèi)吞調(diào)控機(jī)制

盡管營(yíng)養(yǎng)元素對(duì)植物生長(zhǎng)發(fā)育是必需的，但過(guò)多的吸收與積累也會(huì)對(duì)植物細(xì)胞造成毒害；因此，對(duì)營(yíng)養(yǎng)元素吸收和轉(zhuǎn)運(yùn)進(jìn)行精確調(diào)控是植物生長(zhǎng)發(fā)育的重要調(diào)控機(jī)制之一。質(zhì)膜內(nèi)吞（endocytosis）是真核細(xì)胞吸收胞外營(yíng)養(yǎng)物質(zhì)和傳遞胞內(nèi)外信號(hào)的重要途徑，也是調(diào)控脂質(zhì)、受體和轉(zhuǎn)運(yùn)蛋白質(zhì)膜豐度與降解的重要手段。因此，深入剖析質(zhì)膜內(nèi)吞調(diào)控機(jī)制對(duì)理解植物生長(zhǎng)發(fā)育調(diào)控機(jī)制具有重要的科學(xué)意義。

已知硼和鐵轉(zhuǎn)運(yùn)蛋白的質(zhì)膜內(nèi)吞是植物細(xì)胞吸收硼和鐵及其分布的重要機(jī)制[78-81]。在低硼條件下，擬南芥硼輸入載體NIP5;1（nodulin 26-like intrinsic protein 5;1，類(lèi)Nod26膜內(nèi)在蛋白）和硼輸出載體BOR1（boron transporter 1，硼轉(zhuǎn)運(yùn)蛋白）在根尖細(xì)胞中分別定位于細(xì)胞外側(cè)質(zhì)膜和內(nèi)側(cè)質(zhì)膜上，這種亞細(xì)胞定位對(duì)硼的吸收與轉(zhuǎn)運(yùn)極為重要。硼外源處理能快速誘導(dǎo)BOR1質(zhì)膜內(nèi)吞和液泡降解，但不影響NIP5;1亞細(xì)胞定位[81]；網(wǎng)格蛋白接頭蛋白復(fù)合體AP-2亞基AP-2μ功能缺失將抑制NIP5;1質(zhì)膜內(nèi)吞和側(cè)向極性定位[80]。植物細(xì)胞鐵的吸收在很大程度上依賴(lài)于鐵輸入載體IRT1（iron-regulated transporter 1，鐵離子轉(zhuǎn)運(yùn)蛋白），而化學(xué)或遺傳損害網(wǎng)格蛋白或網(wǎng)格蛋白介導(dǎo)的內(nèi)吞（clathrin-mediated endocytosis,CME）功能促進(jìn)了IRT1的質(zhì)膜定位，尤其在質(zhì)膜外側(cè)定位[78-79]。這些研究結(jié)果表明，網(wǎng)格蛋白及其介導(dǎo)的內(nèi)吞對(duì)維持轉(zhuǎn)運(yùn)蛋白的質(zhì)膜極性定位具有重要功能。

目前，對(duì)N、P、K和Zn等轉(zhuǎn)運(yùn)蛋白的內(nèi)吞研究相對(duì)較少。布雷菲德菌素A（brefeldin A,BFA）是植物細(xì)胞外吐（exocytosis）或再回收（recycling）途徑的抑制劑，是植物細(xì)胞質(zhì)膜內(nèi)吞分析的重要工具。已知K+轉(zhuǎn)運(yùn)蛋白CHX17定位于質(zhì)膜上，其表達(dá)受K+饑餓誘導(dǎo)。BFA處理會(huì)引起AtCHX17質(zhì)膜信號(hào)下降，但在BFA小體中的信號(hào)上升[82]。AtPHT1;1是質(zhì)膜和內(nèi)質(zhì)網(wǎng)定位的Pi轉(zhuǎn)運(yùn)蛋白，BFA處理同樣誘導(dǎo)PHT1;1質(zhì)膜信號(hào)下降而B(niǎo)FA小體信號(hào)上升[83]。這些研究結(jié)果表明，CHX17和PHT1;1質(zhì)膜豐度受內(nèi)吞途徑的調(diào)控。

6 展望

植物營(yíng)養(yǎng)元素的吸收與轉(zhuǎn)運(yùn)機(jī)制是近幾年植物營(yíng)養(yǎng)生理學(xué)研究領(lǐng)域的重要熱點(diǎn)之一。至今對(duì)植物N、P、K和Zn等轉(zhuǎn)運(yùn)蛋白的功能研究已經(jīng)取得了一定進(jìn)展，尤其對(duì)這些蛋白家族基因在不同生理?xiàng)l件下的響應(yīng)機(jī)制已經(jīng)有了較好的了解。然而，關(guān)于這些轉(zhuǎn)運(yùn)蛋白的具體分子作用機(jī)制或調(diào)控機(jī)制仍有待于進(jìn)一步探索。目前亟待解決的科學(xué)問(wèn)題有：1）對(duì)這些轉(zhuǎn)運(yùn)蛋白晶體結(jié)構(gòu)的解析將是理解轉(zhuǎn)運(yùn)蛋白分子作用機(jī)制的核心科學(xué)問(wèn)題，其晶體結(jié)構(gòu)能否解析成功將成為本研究領(lǐng)域的一個(gè)突破/瓶頸；2）弄清這些轉(zhuǎn)運(yùn)蛋白的分泌、膜定位和液泡降解等過(guò)程的細(xì)節(jié)問(wèn)題將是理解轉(zhuǎn)運(yùn)蛋白功能調(diào)控的核心內(nèi)容；3）將植物響應(yīng)外界的信號(hào)通路整合成全面的信號(hào)網(wǎng)通路將成為研究轉(zhuǎn)運(yùn)蛋白功能機(jī)制的突破性進(jìn)展；4）剖析不同轉(zhuǎn)運(yùn)蛋白之間的協(xié)調(diào)作用；5）分析這些轉(zhuǎn)運(yùn)蛋白在植物逆境脅迫過(guò)程中的生物學(xué)功能，尤其是在逆境條件下，對(duì)作物產(chǎn)量性狀的貢獻(xiàn)。這些科學(xué)問(wèn)題的解決將對(duì)作物產(chǎn)量生產(chǎn)（包括作物高產(chǎn)、穩(wěn)產(chǎn)和品質(zhì)育種等）具有重要的理論和現(xiàn)實(shí)指導(dǎo)意義。

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