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        氮素在植物中的利用綜述

        2020-04-16 12:55:39白文欽胡明瑜王春萍蔣曉英雷開(kāi)榮吳紅
        江蘇農(nóng)業(yè)科學(xué) 2020年4期
        關(guān)鍵詞:吸收氮素儲(chǔ)存

        白文欽 胡明瑜 王春萍 蔣曉英 雷開(kāi)榮 吳紅

        摘要:氮素是植物生長(zhǎng)的必需營(yíng)養(yǎng)元素,世界范圍內(nèi)常施用大量氮肥以提高作物的產(chǎn)量。然而,氮肥不僅價(jià)格昂貴,還會(huì)污染環(huán)境,威脅人類身體健康。解決這一問(wèn)題的其中一個(gè)策略是開(kāi)發(fā)一些能夠固氮和高效利用氮肥的作物,這樣可以在減少氮肥施用的情況下獲得較高的作物產(chǎn)量。本文梳理了植物對(duì)氮元素從源到庫(kù)器官的吸收、重組和利用的整個(gè)過(guò)程,分析了植物中參與分配和瞬時(shí)存儲(chǔ)的無(wú)機(jī)氮和有機(jī)氮的形式,以及它們?nèi)绾斡绊懙目捎眯?、代謝和再活化。同時(shí),介紹了氮素轉(zhuǎn)運(yùn)蛋白在源和庫(kù)器官中的基本功能及其在調(diào)節(jié)氮的運(yùn)輸、氮信號(hào)傳導(dǎo)調(diào)控中的重要性。認(rèn)為氮素轉(zhuǎn)運(yùn)蛋白是提高氮素利用效率和作物產(chǎn)量有效靶標(biāo),這為如何結(jié)合當(dāng)前的研究發(fā)現(xiàn)來(lái)推動(dòng)未來(lái)的作物工程發(fā)展提供了有價(jià)值的線索。

        關(guān)鍵詞:氮素;吸收;同化;儲(chǔ)存;分配;運(yùn)輸調(diào)控;作物改良

        中圖分類號(hào): S143.1文獻(xiàn)標(biāo)志碼: A

        氮是植物生長(zhǎng)和繁殖所需的基本營(yíng)養(yǎng)元素。植物根系主要吸收無(wú)機(jī)氮(硝酸鹽和銨鹽)作為氮源,但在特定生態(tài)系統(tǒng)中生長(zhǎng)的一些植物也能夠從土壤吸收有機(jī)氮[1-4]。除了從土壤中直接獲得氮化合物之外,豆科(Leguminosae)植物能夠通過(guò)與根瘤中的細(xì)菌共生來(lái)固定大氣中的氮元素。根據(jù)植物種類和環(huán)境條件,獲得的無(wú)機(jī)氮被還原為結(jié)瘤、根或光合活性源葉中的氨基酸,在一些熱帶豆科植物的根瘤中,這些氨基酸被用來(lái)生產(chǎn)脲類化合物[5]。所產(chǎn)生的氨基酸或脲主要通過(guò)長(zhǎng)距離運(yùn)輸?shù)男问綇脑矗ɡ绺?、結(jié)節(jié)和成熟葉)運(yùn)輸?shù)綆?kù)。在營(yíng)養(yǎng)生長(zhǎng)期,根系和葉片是主要的氮庫(kù),而生殖生長(zhǎng)期的花、果實(shí)和種子是主要的氮同化物導(dǎo)入庫(kù)[6]。

        植物通過(guò)木質(zhì)部將氮從根部運(yùn)輸?shù)窖?,而從源葉到庫(kù)的氮運(yùn)輸發(fā)生在韌皮部中,同時(shí),庫(kù)器官通常也存在有少量的木質(zhì)部運(yùn)輸形式[7-8]。在氮從土壤向源到庫(kù)的運(yùn)輸過(guò)程中,定位在質(zhì)膜上的轉(zhuǎn)運(yùn)蛋白是必不可少的。它們能夠調(diào)節(jié)根系對(duì)氮的吸收、運(yùn)輸和種子中氮的存儲(chǔ)[9-10]。在植物的營(yíng)養(yǎng)生長(zhǎng)階段,氮可利用程度和利用率主要取決于植物對(duì)土壤中氮的吸收、同化和運(yùn)輸?shù)綆?kù)的效率;在植物的生殖生長(zhǎng)階段,種子中的氮利用效率不僅僅取決于土壤中氮的可利用程度和吸收效率,還取決于從源葉、莖或根中的蛋白質(zhì)和氮的臨時(shí)存儲(chǔ)庫(kù)中可獲得的氮量,以及氨基酸(或脲)遷移效率[6]。

        本文綜述了植物對(duì)氮的吸收、同化和從源到庫(kù)分配的分子機(jī)制,以及無(wú)機(jī)氮(即硝酸鹽和銨態(tài)氮)和有機(jī)氮(即氨基酸和脲)在植物中的分配機(jī)理,并闡述了它們?cè)谠春蛶?kù)中的生理作用,以及它們對(duì)植物生產(chǎn)力的重要性。

        1?氮素吸收與同化

        科學(xué)家們已經(jīng)在根中鑒定出各種具有不同底物親和性和特異性的無(wú)機(jī)和有機(jī)氮轉(zhuǎn)運(yùn)蛋白。這些高度多樣化的吸收系統(tǒng)使植物根系能夠適應(yīng)(包括養(yǎng)分脅迫條件下)不同氮成分和濃度的土壤環(huán)境。盡管這方面的研究不斷深入,但是不同的轉(zhuǎn)運(yùn)蛋白及下游因子是如何來(lái)協(xié)調(diào)氮的同化過(guò)程的,仍不完全清楚。

        1.1?根系對(duì)硝酸鹽和銨鹽的吸收

        植物根系通過(guò)硝酸鹽轉(zhuǎn)運(yùn)蛋白的介導(dǎo),從土壤中吸收硝酸鹽和硝酸銨[10]。硝酸鹽轉(zhuǎn)運(yùn)蛋白家族根據(jù)對(duì)硝酸鹽的轉(zhuǎn)運(yùn)活性,可分為低親和性和高親和性兩大類,其中硝酸鹽轉(zhuǎn)運(yùn)蛋白1(NRT1)屬于低親和力系統(tǒng),在擬南芥的53種NPF/NRT1蛋白中,迄今為止已經(jīng)有16種蛋白的功能被鑒定出。除了擬南芥中的NPF6.3/NRT1.1和水稻中的NRT 1.1B之外,它們都只具有低硝酸鹽親和力[11-14]。高親和力硝酸鹽轉(zhuǎn)運(yùn)蛋白屬于NRT2家族,在擬南芥中有7個(gè)NRT2蛋白,而在水稻中則有3個(gè)NRT2蛋白[10]。NPF/NRT1和NRT2轉(zhuǎn)運(yùn)體家族成員是質(zhì)子偶聯(lián)蛋白,除了雙重功能的NPF7.3/NRT1.5轉(zhuǎn)運(yùn)蛋白[15]和NPF2.7/硝酸輸出轉(zhuǎn)運(yùn)蛋白(NAXT1)之外,它們都能夠介導(dǎo)硝酸鹽的外流[16]。此外,研究發(fā)現(xiàn)氯化物通道蛋白(CLC)家族也具有轉(zhuǎn)運(yùn)硝酸鹽的功能[17]。

        在擬南芥中,至少有6種轉(zhuǎn)運(yùn)蛋白參與了根系對(duì)硝酸根的吸收。NPF63/NRT1.1[也稱氯酸鹽抗性蛋白1(CHL1)]和NPF4.6/NRT1.2主要在高硝酸鹽環(huán)境下發(fā)揮作用;NRT2.1、NRT2.2、NRT2.4和NRT2.5則在硝酸鹽匱乏環(huán)境中發(fā)揮作用,這4種蛋白吸收的硝酸鹽占植物吸收的硝酸鹽總量的95%,其中NRT2.1和NRT2.2起主要作用?;诨虮磉_(dá)分析和蛋白定位的結(jié)果,研究者認(rèn)為NRT2.4和NRT2.5可能主要參與了通過(guò)根毛區(qū)表皮和皮層從土壤中直接獲得硝酸鹽的過(guò)程,而NRT2.1和NRT2.2則是介導(dǎo)了硝酸鹽通過(guò)質(zhì)外體途徑進(jìn)入皮質(zhì)和內(nèi)皮層細(xì)胞的過(guò)程[18-21]。近年來(lái),研究者們?cè)诜眩⊿olanum lycopersicum)、水稻(Oryza sativa)、小麥(Triticum aestivum)和玉米(Zea mays)等植物中也鑒定出了大量硝酸鹽運(yùn)輸?shù)鞍譡10,22-23]。

        除了硝酸鹽之外,植物也能夠吸收NH+4作為氮源,但是過(guò)量的銨會(huì)對(duì)植物細(xì)胞產(chǎn)生毒害,因此植物對(duì)銨態(tài)氮的吸收和同化受到嚴(yán)格的調(diào)控。在植物中,存在一類銨轉(zhuǎn)運(yùn)蛋白(AMT)來(lái)實(shí)現(xiàn)銨鹽的吸收,根據(jù)其對(duì)銨鹽的親和性,也可以將其分為高親和轉(zhuǎn)運(yùn)蛋白和低親和轉(zhuǎn)運(yùn)蛋白。AMT在植物中廣泛存在,研究者們分別在擬南芥(6種)、水稻(10種)、楊樹(shù)(14種)和松樹(shù)(3種)中發(fā)現(xiàn)了大量的AMT編碼基因[24-27]。在擬南芥中,有4種AMT蛋白參與了根系對(duì)銨鹽的吸收,其中AMT1;1、AMT1;3、AMT1;5蛋白通過(guò)表皮直接吸收土壤中的銨,而AMT1;2蛋白則在皮層和內(nèi)皮層細(xì)胞中表達(dá),介導(dǎo)了銨鹽的質(zhì)外體吸收。進(jìn)一步研究發(fā)現(xiàn),擬南芥通過(guò)AMT1;1、AMT1;2和AMT1;3蛋白吸收的銨鹽占銨鹽吸收總量的90%~95%[28]。在水稻中,銨鹽的吸收是由OsAMT1;1、OsAMT1;2和OsAMT1;3蛋白來(lái)完成,其中OsAMT1;1和OsAMT1;2基因表達(dá)水平隨著環(huán)境銨鹽濃度提高而上調(diào);而OsAMT1;3基因則在氮匱乏狀態(tài)下表達(dá),使得水稻能夠適應(yīng)低銨鹽環(huán)境[29-30]。

        1.2?根系對(duì)氨基酸的吸收

        植物根系對(duì)有機(jī)氮的吸收的研究主要集中在氨基酸方面[1,31]。在擬南芥中,預(yù)計(jì)已鑒定出超過(guò)100種氨基酸轉(zhuǎn)運(yùn)蛋白,大多屬于氨基酸-多胺-膽堿(APC)轉(zhuǎn)運(yùn)蛋白超家族和屬于DMT超家族的多種酸轉(zhuǎn)運(yùn)進(jìn)出蛋白家族(UMAMIT)[32]。研究較多的氨基酸轉(zhuǎn)運(yùn)蛋白家族包括氨基酸通透酶家族(AAPs)、賴氨酸和組氨酸轉(zhuǎn)運(yùn)蛋白家族(LHTs)、脯氨酸轉(zhuǎn)運(yùn)蛋白家族(ProTs)、γ-氨基丁酸轉(zhuǎn)運(yùn)蛋白家族(GATs)、生長(zhǎng)素轉(zhuǎn)運(yùn)蛋白家族(AUXs)、芳香氨基酸和中性氨基酸轉(zhuǎn)運(yùn)蛋白家族(ANTs)。一般而言,底物特異性和親和力在不同轉(zhuǎn)運(yùn)子蛋白家族之間存在差異。AAPs被認(rèn)為具有廣泛底物特異性的中等親和力,擬南芥中的AAP1蛋白參與了根系吸收谷氨酸和中性氨基酸的過(guò)程[33],而AAP5蛋白則具有轉(zhuǎn)運(yùn)堿性氨基酸的能力[34]。LHTs被推測(cè)具有高親和力運(yùn)輸能力。AtLHT1定位于根表皮和葉肉細(xì)胞,能夠?qū)⒅行院退嵝园被釋?dǎo)入根部,同時(shí)能夠運(yùn)輸作為乙烯前體的1-氨基環(huán)丙烷-1-羧酸(ACC)[35-37]。擬南芥AtProt2蛋白參與了脯氨酸和甜菜堿的導(dǎo)入過(guò)程,超量表達(dá)該基因能夠提高植物的耐鹽性,這表明在缺水情況下受脅迫的植物根系細(xì)胞會(huì)提高內(nèi)部溶質(zhì)的濃度以提高滲透壓[38]。

        1.3?氮同化與氨基酸合成和運(yùn)輸

        硝酸鹽還原酶將硝酸鹽還原為亞硝酸鹽[39]。而亞硝酸鹽還原酶則將轉(zhuǎn)運(yùn)到質(zhì)體中的亞硝酸鹽進(jìn)一步還原為銨鹽[40]。擬南芥中存在2種硝酸還原酶編碼基因AtNIA1和AtNIA2,在2種基因同時(shí)突變的擬南芥突變體nia1、nia2中,其硝酸還原酶活性只有野生型擬南芥的0.5%[41]。

        谷氨酰胺合成酶(GS)是氮同化的關(guān)鍵酶,其與谷氨酸合酶(GOGAT)一起形成所謂的GS-GAGOT環(huán),將銨鹽轉(zhuǎn)化成為谷氨酰胺。根據(jù)GS定位的不同,可以將GS分為胞質(zhì)型GS(GS1)和質(zhì)體型GS(GS2)。葉綠體GS2主要參與了氨的初級(jí)同化和葉肉中的光呼吸銨的同化作用[42]。GS1參與了根中氨的初級(jí)同化,特別是在高硝酸鹽環(huán)境下對(duì)氮同化有一定的作用[43]。有研究表明,擬南芥中的GS1型蛋白GLN1;2在成熟葉片中定位于伴細(xì)胞中,能夠促進(jìn)韌皮部的氮裝載到庫(kù)中[44]。在葉片衰老過(guò)程中,GS1酶的活性被誘導(dǎo)提高,將從氨基酸分解代謝中得到的原料重新進(jìn)行氮同化[45]。根據(jù)電子供體的不同,植物中的谷氨酸合酶(GOGAT)分為鐵氧還蛋白依賴的GOGAT(Fd-GOGAT)和NADP依賴的GOGAT(NADH-GOGAT),前者主要位于葉肉細(xì)胞的葉綠體中,后者則存在于葉和根伴隨細(xì)胞的質(zhì)體中,二者均可促進(jìn)氮的同化和分配[46]。水稻中存在Fd-GOGAT抑制因子的編碼基因OsARE1,突變?cè)摶蚩梢匝泳徶参锏乃ダ?,提高低氮條件下氮的利用效率,進(jìn)而提高水稻產(chǎn)量[47]。

        葉綠體(和細(xì)胞質(zhì))是大量蛋白來(lái)源氨基酸開(kāi)始合成的部位,氨基酸的轉(zhuǎn)運(yùn)依賴轉(zhuǎn)運(yùn)蛋白來(lái)完成。研究發(fā)現(xiàn),擬南芥中存在一種谷氨酸/蘋(píng)果酸轉(zhuǎn)運(yùn)蛋白(AtDiT2),該蛋白能夠?qū)被徂D(zhuǎn)運(yùn)出葉綠體[48]。矮牽牛中則存在一種陽(yáng)離子氨基酸轉(zhuǎn)運(yùn)蛋白(PhpCAT),該蛋白能夠?qū)⒎枷阕灏被釓馁|(zhì)體中運(yùn)輸出來(lái)[49]。

        1.4?根-芽氮運(yùn)移

        木質(zhì)部參與了根-芽的硝態(tài)氮流、氮同化物、其他營(yíng)養(yǎng)元素和水分的運(yùn)輸。地面組織的運(yùn)動(dòng)是由葉片表面的蒸騰作用引起的,木質(zhì)部的靜水壓力可梯度向下延伸到根部[50]。氨基酸的運(yùn)輸往往發(fā)生在木質(zhì)部(和韌皮部)中,其中天門冬氨酸、谷氨酸、天冬酰胺和谷氨酰胺含量最為豐富[51]。在結(jié)瘤的熱帶豆科植物中,木質(zhì)部中90%以上的氮以脲的形式出現(xiàn),脲也是韌皮部中主要含氮化合物[52]。在外施不添加氮的營(yíng)養(yǎng)液時(shí),結(jié)瘤大豆木質(zhì)部中的氮主要以氨基酸和脲的形式存在,無(wú)機(jī)氮的含量極低[53]。然而,當(dāng)葉片中氮發(fā)生同化作用時(shí),木質(zhì)部硝酸鹽的濃度可能超過(guò)有機(jī)氮化合物的濃度[54]。木質(zhì)部不僅保證了植物各個(gè)組織生理功能的直接氮供應(yīng),而且還能夠沿著其運(yùn)輸途徑實(shí)現(xiàn)氮的回收,在根、莖、葉主脈以及木質(zhì)部到韌皮部的轉(zhuǎn)移中建立氮貯藏池,以用于快速供應(yīng)生長(zhǎng)需求旺盛的庫(kù)。

        2?營(yíng)養(yǎng)組織中氮的儲(chǔ)存

        在各種源器官、組織或亞細(xì)胞結(jié)構(gòu)中沉積的氮化合物的類型和數(shù)量對(duì)氮的運(yùn)輸和分配到庫(kù)具有相當(dāng)大的影響。此外,氮儲(chǔ)藏池的形成能有效地控制胞質(zhì)和質(zhì)外體的氮濃度,從而通過(guò)前饋或反饋控制影響氮的吸收、運(yùn)輸和同化。

        2.1?硝酸鹽和銨儲(chǔ)存

        有研究發(fā)現(xiàn),在葉和根的液泡/細(xì)胞質(zhì)中硝酸鹽的濃度較高[55],當(dāng)細(xì)胞質(zhì)中硝酸鹽的濃度較高時(shí),硝酸鹽會(huì)儲(chǔ)存在液泡中,當(dāng)細(xì)胞質(zhì)中硝酸鹽濃度降低,影響氮同化時(shí),硝酸鹽又會(huì)從液泡中轉(zhuǎn)移到細(xì)胞質(zhì)中。在擬南芥中,AtCLCa基因編碼的氯離子通道蛋白AtCLCA能夠通過(guò)偶聯(lián)硝酸鹽和質(zhì)子的轉(zhuǎn)運(yùn),將硝酸鹽存儲(chǔ)到液泡中,并影響植物體中的硝酸鹽濃度[56-57]。擬南芥中AtNRT2.7基因編碼液泡膜轉(zhuǎn)運(yùn)蛋白,主要是在種子特別是干種子中表達(dá),對(duì)于在種子中累積硝酸鹽具有重要的作用[58-63]。在水稻中,OsNPF7.2基因編碼硝酸鹽低親和轉(zhuǎn)運(yùn)蛋白,該蛋白定位于水稻根系伸長(zhǎng)區(qū)和成熟區(qū)細(xì)胞的液泡膜。敲除OsNPF7.2基因?qū)?dǎo)致水稻在高硝酸鹽條件下的生長(zhǎng)延遲,表明缺少該蛋白會(huì)影響硝酸鹽在根細(xì)胞中的分配,使得液泡不能起到調(diào)控細(xì)胞內(nèi)氮元素含量的作用[64]。

        過(guò)量的銨鹽會(huì)導(dǎo)致植物中毒,為避免銨鹽濃度過(guò)高帶來(lái)的毒性,大量的銨鹽被存儲(chǔ)在液泡中,其在液泡中的濃度可達(dá)1 mmol/L,以維持細(xì)胞質(zhì)內(nèi)較低的銨鹽濃度[60-61]。與成熟葉相比,老葉和幼葉中的銨鹽濃度通常更高,這是由氨基酸分解代謝和光呼吸作用造成的。與液泡膜上硝酸鹽轉(zhuǎn)運(yùn)蛋白相比,參與液泡內(nèi)銨鹽存儲(chǔ)的轉(zhuǎn)運(yùn)蛋白有待研究[62]。

        2.2?氨基酸的儲(chǔ)存

        在發(fā)育過(guò)程中或受到非生物/生物脅迫,植物葉片中游離氨基酸成分及其濃度的變化較大[63]。隨著葉片衰老,葉片中氨基酸的總濃度穩(wěn)步下降,而韌皮部汁液中的總氨基酸含量增加[64]。此外,氨基酸的種類隨著植物的年齡和氮素營(yíng)養(yǎng)的變化而變化[65]。例如,在煙葉的幼葉中,脯氨酸含量占總氨基酸含量的20%,而在成熟和衰老的葉片中只有2%。與低硝酸鹽條件相比,在高硝酸鹽條件下油菜(Brassica napus)葉片中的游離氨基酸含量更高,氨基酸組成更豐富[66]。此外,干旱和鹽脅迫等環(huán)境條件能夠引起葉片中氨基酸的積累,這可能是相關(guān)氨基酸合成能力加強(qiáng)或者蛋白質(zhì)降解釋放所造成的[67]。逆境脅迫解除后,植物在恢復(fù)過(guò)程中,暫時(shí)儲(chǔ)存的氨基酸可參與新陳代謝過(guò)程或轉(zhuǎn)運(yùn)到新的沉淀物中。葉片儲(chǔ)存氨基酸的種類隨植物種類而變化,主要是谷氨酸、天冬氨酸、谷氨酰胺、脯氨酸,而在C4植物玉米中主要為天冬酰胺[68]??傮w而言,在不同的膜轉(zhuǎn)運(yùn)系統(tǒng)、不斷變化的轉(zhuǎn)運(yùn)機(jī)制以及底物特異性和親和力的作用下,大量的氨基酸在細(xì)胞或儲(chǔ)存池中的含量、種類頻繁地發(fā)生變化。

        運(yùn)用蛋白質(zhì)組學(xué),可鑒定出擬南芥液泡膜的氨基酸轉(zhuǎn)運(yùn)蛋白,其中就包括了陽(yáng)離子氨基酸轉(zhuǎn)運(yùn)蛋白(CAT)[69]。亞細(xì)胞定位試驗(yàn)證實(shí),擬南芥CAT2和CAT4定位于液泡膜上[70],但是它們?cè)谝号葸\(yùn)輸中的直接功能尚需進(jìn)一步研究。番茄中也存在定位于液泡膜的氨基酸轉(zhuǎn)運(yùn)蛋白SlCAT9,該蛋白的編碼基因在果實(shí)成熟期大量表達(dá),進(jìn)一步研究發(fā)現(xiàn),SlCAT9蛋白介導(dǎo)了GABA(γ-氨基丁酸)與GLU/ASP的交換[71]。除了CAT蛋白,研究者在擬南芥中發(fā)現(xiàn)了負(fù)責(zé)芳香和中性氨基酸轉(zhuǎn)運(yùn)的ANT1蛋白和負(fù)責(zé)丙氨酸和脯氨酸轉(zhuǎn)運(yùn)的AtAVT3蛋白[72-73]。

        2.3?脲類儲(chǔ)藏

        在熱帶豆科植物中,氮主要以尿素的形式存儲(chǔ)在莖、葉柄或葉組織中[74]。在葉中,脲可能存儲(chǔ)在葉肉細(xì)胞的液泡中。固氮大豆有一類特化的旁側(cè)葉肉組織,研究表明,脲主要存在于這個(gè)組織之中,該組織也是營(yíng)養(yǎng)生長(zhǎng)過(guò)程中蛋白質(zhì)儲(chǔ)存的主要場(chǎng)所[75]。然而,參與上述過(guò)程的轉(zhuǎn)運(yùn)蛋白還未被鑒定出。有研究表明,尿囊素濃度上升可能會(huì)引發(fā)植物體對(duì)非生物和生物脅迫的一般反應(yīng)[76]。

        2.4?蛋白質(zhì)的儲(chǔ)存

        理論上講,植物體中的所有蛋白質(zhì)均可認(rèn)為是氮元素的存儲(chǔ)池。許多酶可能在營(yíng)養(yǎng)組織中的氮存儲(chǔ)中發(fā)揮著作用。研究發(fā)現(xiàn),相當(dāng)大比例的核酮糖-1,5-二磷酸羧化/加氧酶是非活性的,在葉片衰老過(guò)程中,大亞基和小亞基似乎被獨(dú)立降解以用于再活化[77]。葉片中GS實(shí)際酶活性總是比根據(jù)GS2含量推測(cè)出的酶活理論值低,這暗示了GS2也可能具有氮存儲(chǔ)功能。此外,有研究發(fā)現(xiàn),核糖體蛋白可以通過(guò)自噬作用實(shí)現(xiàn)氮元素的回收和再活化[78],這意味著核糖體蛋白也是一種重要的氮源存儲(chǔ)池。營(yíng)養(yǎng)貯藏蛋白(VSPs)是植物營(yíng)養(yǎng)組織中氮存儲(chǔ)的主要形式,所有植物物種中都存在VSPs。在植物氮吸收能力下降時(shí),VSPs在氮的累積和流動(dòng)過(guò)程中具有重要的作用。也有研究發(fā)現(xiàn),VSPs在植物適應(yīng)非生物或生物脅迫中有一定的作用[79-80]。

        3?氮素的分配

        大多數(shù)的氮是以氨基酸形式從源葉中被運(yùn)輸出,而在一些物種中則是以脲類的形式被運(yùn)出。在衰老過(guò)程中,葉片成為氮/氨基酸的主要來(lái)源[81],氮源的再活化和隨后的氮在庫(kù)中的使用對(duì)于保持種子產(chǎn)量十分重要[82]。

        3.1?蛋白質(zhì)降解與氮的再活化

        在受到脅迫或者衰老時(shí),葉片中會(huì)發(fā)生有機(jī)氮的再活化,自噬相關(guān)蛋白和液泡蛋白酶參與了這個(gè)過(guò)程[64]。在營(yíng)養(yǎng)缺乏的情況下,自噬程度會(huì)加劇,這有助于對(duì)氮的再活化[83]。在植物中,一些衰老誘導(dǎo)合成的氨基酸轉(zhuǎn)運(yùn)蛋白已經(jīng)被鑒定出來(lái),但是其在氮再活化中的作用還未知[64]。有研究發(fā)現(xiàn),在葉片衰老過(guò)程中,韌皮部中天冬酰胺和谷氨酰胺的濃度增加,這意味著發(fā)生了一系列的轉(zhuǎn)氨基反應(yīng),增加了谷氨酸和酰胺類物質(zhì)的合成[84]。

        3.2?有機(jī)氮和無(wú)機(jī)氮在庫(kù)中的分配

        韌皮部中的篩分子(SEs)和伴胞(CCs)形成長(zhǎng)距離的運(yùn)輸管道,可使氮向庫(kù)運(yùn)輸。氮常以氨基酸或脲的形式從葉片向庫(kù)運(yùn)輸[85]。韌皮部的硝酸鹽濃度相對(duì)較低,硝酸鹽與氨基酸的比例通常為1 ∶(10~100)[19,54]。氮的葉外運(yùn)輸則是通過(guò)胞間連絲這種共質(zhì)體途徑[86]。

        通過(guò)被動(dòng)運(yùn)輸,氨基酸、脲類和硝酸鹽從薄壁組織或束鞘細(xì)胞中釋放,進(jìn)入葉片質(zhì)外體,然后被導(dǎo)入韌皮部[87-89]。參與硝酸鹽運(yùn)輸?shù)饺~/韌皮部質(zhì)外體的轉(zhuǎn)運(yùn)蛋白尚未被發(fā)現(xiàn),但韌皮部裝載硝酸鹽過(guò)程中發(fā)揮作用的轉(zhuǎn)運(yùn)蛋白已經(jīng)被鑒定出,包括NPF2.13/NRT1.7、NRT2.4和NRT2.5[1-3]。NPF2.13/NRT1.7 定位于葉片細(xì)脈的SEs/CCs中,促進(jìn)老葉中韌皮部的裝載。NRT2.4在葉片韌皮部(可能是韌皮部薄壁組織)或接近韌皮部中表達(dá),在缺乏氮條件下,從質(zhì)外體中回收硝酸鹽,從而使氮向SEs/CCs運(yùn)動(dòng)。NRT2.5基因在擬南芥葉片的細(xì)脈中表達(dá),并與NRT2.4一起影響葉片中硝酸鹽的再活化和其在韌皮部的運(yùn)輸。NPF1.2/NRT1.11和NPF1.1/NRT1.12基因在葉片主脈伴細(xì)胞表達(dá),它們除了在硝酸鹽從木質(zhì)部到韌皮部傳遞中發(fā)揮作用外,還在硝酸鹽向幼嫩組織的分配中起重要的作用[90]。

        對(duì)于從葉片中導(dǎo)出氨基酸的轉(zhuǎn)運(yùn)蛋白的研究相對(duì)較少[91]。有研究表明,擬南芥中的雙向氨基酸轉(zhuǎn)運(yùn)蛋白(BAT1)可能在氨基酸從韌皮部向庫(kù)組織的運(yùn)輸中發(fā)揮作用[92],AAPs可能參與了從葉質(zhì)體中攝取氨基酸到SEs/CCs復(fù)合體的過(guò)程[93]。對(duì)擬南芥的研究表明,AAP8在源葉中韌皮部表達(dá),在氨基酸韌皮部裝載過(guò)程中起著關(guān)鍵作用,并且能強(qiáng)烈地影響庫(kù)的大小和數(shù)量[94]。在菜豆中,PvUPS1基因能夠在整株植物的韌皮部位中表達(dá),其編碼的蛋白對(duì)于尿囊素在韌皮部的轉(zhuǎn)載以及運(yùn)輸?shù)秸诎l(fā)育的庫(kù)中具有重要的作用[95]。

        3.3?參與分配的氮素來(lái)源

        植物從土壤中吸收氮元素,而氮的含量則取決于作物種類、基因型和環(huán)境條件[45]。例如,在油菜中,硝酸根轉(zhuǎn)運(yùn)蛋白的表達(dá)和活性在生殖生長(zhǎng)階段降低,硝酸鹽吸收較營(yíng)養(yǎng)階段少[96-97]。此外,氮的吸收取決于土壤水分的有效性,多雨條件下,研究者發(fā)現(xiàn)田間生長(zhǎng)的油菜植株的根系在生長(zhǎng)發(fā)育過(guò)程中能夠大量地吸收氮元素[98]。在某些情況下有些地區(qū)夏季降水量不足,在這種情況下,植物需要將氮從蛋白質(zhì)和氮儲(chǔ)存池中再活化,以確保生殖庫(kù)的氮營(yíng)養(yǎng)需求。在擬南芥中,相較于高氮環(huán)境,植株氮利用效率(NUE)和氮活化效率(NRE)在低氮條件下更高。同時(shí),在低氮條件下,同位素標(biāo)記的14NO3 主要集中在植物的種子當(dāng)中[99]。對(duì)玉米而言,籽粒中有62%的氮來(lái)自氮的再活化作用,其余38%的氮來(lái)源于根的吸收[1-3]。有研究表明,在土壤氮含量充足的情況下,植物在整個(gè)生長(zhǎng)周期中,根系都能夠持續(xù)吸收氮元素[100],于是,由于常在植物生長(zhǎng)早期施用硝酸鹽肥料,在植物生育階段土壤中的硝態(tài)氮可能會(huì)被耗盡。

        4?氮進(jìn)入庫(kù)

        種子是主要的氮庫(kù),而在營(yíng)養(yǎng)生長(zhǎng)和多年生植物中,根、發(fā)育的葉和莖或樹(shù)干是主要的氮庫(kù)。有研究表明,溶質(zhì)和大分子物質(zhì)在根和莖中韌皮部的卸載方式有所不同[101]。現(xiàn)有研究表明,來(lái)自韌皮部的氨基酸的釋放發(fā)生在維管結(jié)構(gòu)的末端,由UmamiT轉(zhuǎn)運(yùn)蛋白家族來(lái)完成。其中UMAMIT11和UMAMIT14定位于與原生木質(zhì)部和韌皮部相鄰的細(xì)胞質(zhì)膜,能夠促進(jìn)氮卸載和運(yùn)輸?shù)椒N子。SIAR1(UMAMIT18)蛋白定位于細(xì)胞膜上,其編碼基因在能夠維管組織中表達(dá),也具有將氮源卸載并運(yùn)輸?shù)椒N子中的作用[102-103]。進(jìn)一步研究發(fā)現(xiàn),UMAMIT14和UMAMIT18不僅僅在種子發(fā)育中起作用,它們對(duì)氨基酸在根韌皮部的卸載也具有重要的作用[104]。在種皮中,氨基酸和硝酸鹽通過(guò)被動(dòng)運(yùn)輸輸出到種子質(zhì)外體,然后被一些轉(zhuǎn)運(yùn)蛋白導(dǎo)入到正在發(fā)育的胚中,并影響種子中蛋白的累積[105-106]。研究者發(fā)現(xiàn),在豌豆中,氨基酸通透酶編碼基因PvAAP1在種皮和子葉表皮轉(zhuǎn)移細(xì)胞以及貯藏薄壁細(xì)胞中表達(dá),對(duì)于種子中儲(chǔ)存蛋白的累積具有重要的作用[107]。在水稻中,OSAAP6蛋白定位于細(xì)胞內(nèi)質(zhì)網(wǎng)上,調(diào)控種子內(nèi)蛋白質(zhì)的積累,OsAAP6基因表達(dá)水平高的植株的種子內(nèi)蛋白質(zhì)含量也相應(yīng)增加[108]。

        硝酸鹽的運(yùn)輸過(guò)程會(huì)影響種子的發(fā)育和休眠過(guò)程[109]。擬南芥NPF2.12/NRT1.6存在于角果和珠柄的微管組織中,可促進(jìn)硝酸鹽向胚的運(yùn)輸,從而影響種子發(fā)育和種子內(nèi)硝酸鹽的積累[110]。此外,NPF5.5基因在擬南芥的胚中表達(dá),該基因突變會(huì)減少胚中氮的含量,但似乎對(duì)種子發(fā)育沒(méi)有影響[111]。NRT2.7定位于胚細(xì)胞液泡膜,調(diào)節(jié)硝酸鹽在液泡中的積累,從而影響種子萌發(fā)[58]。

        5?源到庫(kù)中的氮運(yùn)輸和代謝的關(guān)系

        氮從源到庫(kù)分配受源器官中氮的吸收和代謝、源輸出和庫(kù)輸入氮的能力的影響。在擬南芥和豆科植物中的研究已經(jīng)證明,芽中氨基酸運(yùn)輸能夠調(diào)控根中氮的吸收、源的代謝和庫(kù)的分配。在豌豆中,增加氨基酸在韌皮部的累積和裝載可以正向調(diào)控根中氮的吸收,進(jìn)而影響源和庫(kù)中可利用氮的同化和使用[37,112-113]。此外,參與韌皮部裝載過(guò)程的氨基酸轉(zhuǎn)運(yùn)蛋白能影響氮從源到庫(kù)的量,并影響種子發(fā)育[94]。相反,韌皮部氨基酸的濃度似乎對(duì)胚對(duì)氮的吸收沒(méi)有影響。例如,AAP8基因突變的擬南芥植株韌皮部中,降低氨基酸濃度對(duì)種子的氮含量沒(méi)有影響,但種子數(shù)量會(huì)顯著減少[94,114]。一般來(lái)說(shuō),定位于種子的氨基酸和硝酸鹽轉(zhuǎn)運(yùn)蛋白可能調(diào)控了導(dǎo)入胚中的氮含量,轉(zhuǎn)運(yùn)蛋白編碼基因表達(dá)水平的改變會(huì)影響種子中貯藏蛋白的積累[113,115-116]。然而,與單個(gè)種子所吸收的氮量相比,分配給許多庫(kù)(即水果和種子)的氮量,不僅取決于源和庫(kù)中轉(zhuǎn)運(yùn)蛋白的活性,而且受植物中可用的總氮量以及在生殖生長(zhǎng)前期建立的總庫(kù)數(shù)的影響。

        6?氮運(yùn)輸調(diào)控

        為了實(shí)現(xiàn)氮的有效吸收和利用,植物在不同層面上發(fā)展出了一整套氮運(yùn)輸?shù)恼{(diào)控機(jī)制[117-118]。

        6.1?細(xì)胞水平調(diào)節(jié)

        硝酸鹽能夠調(diào)節(jié)包括NPF63/NRT1.1、NPF7.3/NRT1.5、NPF7.2/NRT1.8、NRT2.1和NRT2.2在內(nèi)的硝酸鹽轉(zhuǎn)運(yùn)蛋白的轉(zhuǎn)錄水平,從而影響氮在植物中的運(yùn)輸[119]。在擬南芥中,NPF63/NRT1.1不僅僅參與硝酸鹽的吸收和轉(zhuǎn)運(yùn),也能夠作為外界硝酸鹽信號(hào)的感受器,通過(guò)調(diào)控多種生理學(xué)和形態(tài)學(xué)上的變化(如種子休眠和形成側(cè)根)對(duì)外界硝酸鹽的變化作出應(yīng)答[120]。包括NLP7在內(nèi)的大量轉(zhuǎn)錄因子參與了硝酸鹽的信號(hào)傳導(dǎo)和轉(zhuǎn)運(yùn)[121-124]。NLP7可結(jié)合與硝酸鹽信號(hào)傳導(dǎo)和硝酸鹽同化相關(guān)的基因。有趣的是,硝酸鹽并不能影響NLP7基因的表達(dá)水平,而是通過(guò)調(diào)節(jié)NLP7蛋白在細(xì)胞核中的留存實(shí)現(xiàn)NLP7蛋白的大量累積[124]。此外,植物中存在一種CPSF蛋白,這種蛋白有2種剪切形式:CPSF30-S和CPSF30-L,其中CPSL30-L能夠通過(guò)調(diào)節(jié)NPF63/NRT1.1參與植物硝酸鹽信號(hào)的調(diào)控。此外,轉(zhuǎn)錄組分析發(fā)現(xiàn),在cpsf30突變體中,許多氮轉(zhuǎn)運(yùn)及同化相關(guān)基因的表達(dá)水平發(fā)生了明顯的變化,說(shuō)明該蛋白對(duì)于調(diào)控硝酸鹽信號(hào)通路具有重要的作用[125]。

        研究表明,銨鹽可以通過(guò)依賴時(shí)間或者濃度變化的形式調(diào)控AMT蛋白的磷酸化,從而影響其功能。進(jìn)一步研究表明,擬南芥中的AtAMT1;1和AtAMT1;2能夠被CIPK23磷酸化,從而抑制銨鹽的吸收和轉(zhuǎn)運(yùn),以避免植物細(xì)胞的銨中毒[126-127]。大量研究表明,硝酸鹽和銨鹽會(huì)對(duì)彼此的吸收、同化、分配產(chǎn)生影響,其分子機(jī)理在一定程度上得到了的解釋[67,128]。

        除了硝酸鹽和銨鹽之外,氮轉(zhuǎn)運(yùn)蛋白編碼基因的表達(dá)還受細(xì)胞和組織發(fā)育狀態(tài)的調(diào)節(jié)。同時(shí),非生物或生物因素的影響,如光、鹽和干旱脅迫,線蟲(chóng)或病原體攻擊等都會(huì)對(duì)轉(zhuǎn)運(yùn)蛋白在基因或者蛋白水平上產(chǎn)生影響[32]。

        氮轉(zhuǎn)運(yùn)蛋白在植物氮素吸收和代謝中扮演著重要角色,然而,它們?cè)谵D(zhuǎn)錄或翻譯后如何相互影響,如何共同影響氮同化、儲(chǔ)存和反饋或前反饋路徑,現(xiàn)在尚未完全清楚,已有研究結(jié)果暗示可能存在核心調(diào)控能協(xié)調(diào)氮元素的分配。此外,研究表明,硝酸鹽和氨基酸轉(zhuǎn)運(yùn)蛋白可能參與了植物激素在植物體內(nèi)的運(yùn)輸[129-130],暗示氮素轉(zhuǎn)運(yùn)蛋白介導(dǎo)的植物激素轉(zhuǎn)運(yùn)可能也參與了氮素在運(yùn)輸或分配中的調(diào)控。

        6.2?整個(gè)植株水平上的氮元素遠(yuǎn)距離信號(hào)系統(tǒng)

        由于氮素在土壤中分布不均,因此植物具一套系統(tǒng)的長(zhǎng)距離運(yùn)輸機(jī)制,即當(dāng)根系一側(cè)的氮缺乏時(shí),氮元素豐富的另一側(cè)則會(huì)補(bǔ)償性地吸收更多的氮元素。由氮元素缺乏的根部產(chǎn)生的由根向莖移動(dòng)的肽激素CEP(C-末端編碼肽)誘發(fā)了這種運(yùn)輸機(jī)制。CEP能夠調(diào)控韌皮部特異性肽CEPD1和CEPD2作為長(zhǎng)距離移動(dòng)信號(hào)轉(zhuǎn)移到根部,并上調(diào)NRT2.1的表達(dá)水平,促進(jìn)氮的吸收[131]。

        7?作物改良應(yīng)用

        氮元素吸收、分配的遺傳調(diào)控和氮代謝的調(diào)控是提高作物產(chǎn)量和氮利用效率(NUE)的主要途徑[132]。研究者們希望通過(guò)超量表達(dá)相關(guān)基因以提高植物的氮利用效率,目前已取得了一定進(jìn)展。例如,在水稻中超量表達(dá)高親和力的硝酸鹽轉(zhuǎn)運(yùn)蛋白編碼基因OsNRT2.3b,能夠提高植物的氮利用效率,從而增加籽粒產(chǎn)量[133]。根據(jù)作物種類、當(dāng)?shù)卦耘喹h(huán)境、最終產(chǎn)品(例如根、葉、果實(shí)或種子)及其用途(例如人類營(yíng)養(yǎng)、動(dòng)物飼料或生物燃料)的不同,制定有針對(duì)性的改進(jìn)策略可能更有效。例如,利用OsNAR2.1基因的特異性啟動(dòng)子控制的OsNRT2.1的過(guò)量表達(dá)會(huì)提高水稻地上部分生物量和籽粒產(chǎn)量[134]。在根中過(guò)量表達(dá)的大麥丙氨酸氨基轉(zhuǎn)移酶(AlaAT)編碼基因的油菜與非轉(zhuǎn)基因油菜相比,在氮源不充沛的條件下,可以提高植物對(duì)氮的利用效率,從而提高產(chǎn)量[135]。研究者們將豌豆來(lái)源的PvAAP1基因置于其啟動(dòng)子控制下轉(zhuǎn)入豌豆,與野生型對(duì)照相比,轉(zhuǎn)基因豌豆的氮元素運(yùn)輸能力得到了顯著提高,從而增加了其種子的產(chǎn)量[113]。也有研究者嘗試在豌豆種子中特異表達(dá)氨基酸轉(zhuǎn)運(yùn)蛋白編碼基因VfAAP1,結(jié)果發(fā)現(xiàn)轉(zhuǎn)基因植株種子的大小以及氮含量明顯提高,但是單株植物的種子產(chǎn)量卻沒(méi)有明顯變化[136]。值得一提的是,研究者們也試圖通過(guò)提高銨鹽轉(zhuǎn)運(yùn)蛋白編碼基因的表達(dá)來(lái)促進(jìn)植物的生長(zhǎng)。例如,超量表達(dá)OsAMT1;1能夠在適量銨鹽或者低銨鹽的條件下促進(jìn)水稻的生長(zhǎng),但是在銨鹽濃度過(guò)高的環(huán)境中卻因?yàn)樵谒靖抵写罅坷鄯e銨鹽從而影響其生長(zhǎng)發(fā)育。此外,超量表達(dá)OsAMT1;3的水稻中氮吸收轉(zhuǎn)運(yùn)能力呈下降趨勢(shì)[137-139]。

        8?總結(jié)

        氮是植物體內(nèi)重要的營(yíng)養(yǎng)因子和信號(hào)物質(zhì),對(duì)于植物生長(zhǎng)發(fā)育具有重要的作用。為適應(yīng)外界氮元素含量的變化,植物演化出了一套復(fù)雜的調(diào)控網(wǎng)絡(luò)。近年來(lái),無(wú)機(jī)氮的運(yùn)輸、代謝及其調(diào)控的研究取得了長(zhǎng)足的進(jìn)展。盡管氨基酸和脲是氮元素長(zhǎng)距離運(yùn)輸?shù)闹饕问?,但是?duì)于有機(jī)氮的相關(guān)研究卻相對(duì)較少。不同類型的氮素轉(zhuǎn)運(yùn)蛋白在氨基酸和脲的源-庫(kù)分配中的轉(zhuǎn)運(yùn)對(duì)于氮的獲取、源的新陳代謝和庫(kù)的強(qiáng)度具有非常重要的作用。比如,氨基酸(或脲)的積累對(duì)于無(wú)機(jī)氮轉(zhuǎn)運(yùn)蛋白的表達(dá)具有負(fù)向反饋?zhàn)饔茫@種負(fù)反饋可以通過(guò)增加有機(jī)氮向庫(kù)運(yùn)輸?shù)耐縼?lái)避免,但是這種負(fù)反饋?zhàn)饔萌杂性S多細(xì)節(jié)需要進(jìn)一步完善。

        氮素轉(zhuǎn)運(yùn)蛋白作為氮吸收、轉(zhuǎn)運(yùn)、同化等過(guò)程的重要參與者,一直是研究的熱點(diǎn),其功能已經(jīng)在許多植物中得到鑒定。目前,筆者所知道的大量氮素轉(zhuǎn)運(yùn)蛋白主要承擔(dān)了植物根部氮素的吸收和轉(zhuǎn)運(yùn),對(duì)植物地上部分參與氮素分配和再活化、利用,特別是在生殖生長(zhǎng)階段促進(jìn)源-庫(kù)互作的轉(zhuǎn)運(yùn)蛋白了解很少。這部分知識(shí)的缺失,使得我們尚不能構(gòu)建出完整的氮素吸收和利用的調(diào)控網(wǎng)絡(luò),但這卻為未來(lái)的研究指出了方向。

        植物的氮利用效率受到作物物種(亞種)、植株發(fā)育水平、環(huán)境等多種復(fù)雜因素的影響,這對(duì)研究者進(jìn)行作物的氮高效利用遺傳改良提出了挑戰(zhàn)。進(jìn)行作物改良時(shí),需要盡可能兼顧無(wú)機(jī)氮和有機(jī)氮轉(zhuǎn)運(yùn)系統(tǒng),如它們表達(dá)的時(shí)空性和相互關(guān)系,以精細(xì)調(diào)節(jié)氮吸收、代謝、瞬時(shí)儲(chǔ)存和整株植物分配?,F(xiàn)代生物技術(shù)的發(fā)展,包括全基因組關(guān)聯(lián)分析、(GWAS)高通量測(cè)序、代謝組學(xué)、蛋白組學(xué)和表型組學(xué),將有助于研究者進(jìn)一步理解和整合氮分配過(guò)程中源和庫(kù)的關(guān)系,并選擇最有希望的候選者,通過(guò)基因聚合育種的方式培育出能夠高效利用氮元素的“完美”作物。

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