氮素在植物中的利用综述

2020-04-16 12:55白文钦胡明瑜王春萍蒋晓英雷开荣吴红
江苏农业科学 2020年4期
关键词:吸收氮素储存

白文钦 胡明瑜 王春萍 蒋晓英 雷开荣 吴红

摘要:氮素是植物生长的必需营养元素,世界范围内常施用大量氮肥以提高作物的产量。然而,氮肥不仅价格昂贵,还会污染环境,威胁人类身体健康。解决这一问题的其中一个策略是开发一些能够固氮和高效利用氮肥的作物,这样可以在减少氮肥施用的情况下获得较高的作物产量。本文梳理了植物对氮元素从源到库器官的吸收、重组和利用的整个过程,分析了植物中参与分配和瞬时存储的无机氮和有机氮的形式,以及它们如何影响氮的可用性、代谢和再活化。同时,介绍了氮素转运蛋白在源和库器官中的基本功能及其在调节氮的运输、氮信号传导调控中的重要性。认为氮素转运蛋白是提高氮素利用效率和作物产量有效靶标,这为如何结合当前的研究发现来推动未来的作物工程发展提供了有价值的线索。

关键词:氮素;吸收;同化;储存;分配;运输调控;作物改良

中图分类号: S143.1文献标志码: A

氮是植物生长和繁殖所需的基本营养元素。植物根系主要吸收无机氮(硝酸盐和铵盐)作为氮源,但在特定生态系统中生长的一些植物也能够从土壤吸收有机氮[1-4]。除了从土壤中直接获得氮化合物之外,豆科(Leguminosae)植物能够通过与根瘤中的细菌共生来固定大气中的氮元素。根据植物种类和环境条件,获得的无机氮被还原为结瘤、根或光合活性源叶中的氨基酸,在一些热带豆科植物的根瘤中,这些氨基酸被用来生产脲类化合物[5]。所产生的氨基酸或脲主要通过长距离运输的形式从源(例如根、结节和成熟叶)运输到库。在营养生长期,根系和叶片是主要的氮库,而生殖生长期的花、果实和种子是主要的氮同化物导入库[6]。

植物通過木质部将氮从根部运输到芽,而从源叶到库的氮运输发生在韧皮部中,同时,库器官通常也存在有少量的木质部运输形式[7-8]。在氮从土壤向源到库的运输过程中,定位在质膜上的转运蛋白是必不可少的。它们能够调节根系对氮的吸收、运输和种子中氮的存储[9-10]。在植物的营养生长阶段,氮可利用程度和利用率主要取决于植物对土壤中氮的吸收、同化和运输到库的效率;在植物的生殖生长阶段,种子中的氮利用效率不仅仅取决于土壤中氮的可利用程度和吸收效率,还取决于从源叶、茎或根中的蛋白质和氮的临时存储库中可获得的氮量,以及氨基酸(或脲)迁移效率[6]。

本文综述了植物对氮的吸收、同化和从源到库分配的分子机制,以及无机氮(即硝酸盐和铵态氮)和有机氮(即氨基酸和脲)在植物中的分配机理,并阐述了它们在源和库中的生理作用,以及它们对植物生产力的重要性。

1 氮素吸收与同化

科学家们已经在根中鉴定出各种具有不同底物亲和性和特异性的无机和有机氮转运蛋白。这些高度多样化的吸收系统使植物根系能够适应(包括养分胁迫条件下)不同氮成分和浓度的土壤环境。尽管这方面的研究不断深入,但是不同的转运蛋白及下游因子是如何来协调氮的同化过程的,仍不完全清楚。

1.1 根系对硝酸盐和铵盐的吸收

植物根系通过硝酸盐转运蛋白的介导,从土壤中吸收硝酸盐和硝酸铵[10]。硝酸盐转运蛋白家族根据对硝酸盐的转运活性,可分为低亲和性和高亲和性两大类,其中硝酸盐转运蛋白1(NRT1)属于低亲和力系统,在拟南芥的53种NPF/NRT1蛋白中,迄今为止已经有16种蛋白的功能被鉴定出。除了拟南芥中的NPF6.3/NRT1.1和水稻中的NRT 1.1B之外,它们都只具有低硝酸盐亲和力[11-14]。高亲和力硝酸盐转运蛋白属于NRT2家族,在拟南芥中有7个NRT2蛋白,而在水稻中则有3个NRT2蛋白[10]。NPF/NRT1和NRT2转运体家族成员是质子偶联蛋白,除了双重功能的NPF7.3/NRT1.5转运蛋白[15]和NPF2.7/硝酸输出转运蛋白(NAXT1)之外,它们都能够介导硝酸盐的外流[16]。此外,研究发现氯化物通道蛋白(CLC)家族也具有转运硝酸盐的功能[17]。

在拟南芥中,至少有6种转运蛋白参与了根系对硝酸根的吸收。NPF63/NRT1.1[也称氯酸盐抗性蛋白1(CHL1)]和NPF4.6/NRT1.2主要在高硝酸盐环境下发挥作用;NRT2.1、NRT2.2、NRT2.4和NRT2.5则在硝酸盐匮乏环境中发挥作用,这4种蛋白吸收的硝酸盐占植物吸收的硝酸盐总量的95%,其中NRT2.1和NRT2.2起主要作用。基于基因表达分析和蛋白定位的结果,研究者认为NRT2.4和NRT2.5可能主要参与了通过根毛区表皮和皮层从土壤中直接获得硝酸盐的过程,而NRT2.1和NRT2.2则是介导了硝酸盐通过质外体途径进入皮质和内皮层细胞的过程[18-21]。近年来,研究者们在番茄(Solanum lycopersicum)、水稻(Oryza sativa)、小麦(Triticum aestivum)和玉米(Zea mays)等植物中也鉴定出了大量硝酸盐运输蛋白[10,22-23]。

除了硝酸盐之外,植物也能够吸收NH+4作为氮源,但是过量的铵会对植物细胞产生毒害,因此植物对铵态氮的吸收和同化受到严格的调控。在植物中,存在一类铵转运蛋白(AMT)来实现铵盐的吸收,根据其对铵盐的亲和性,也可以将其分为高亲和转运蛋白和低亲和转运蛋白。AMT在植物中广泛存在,研究者们分别在拟南芥(6种)、水稻(10种)、杨树(14种)和松树(3种)中发现了大量的AMT编码基因[24-27]。在拟南芥中,有4种AMT蛋白参与了根系对铵盐的吸收,其中AMT1;1、AMT1;3、AMT1;5蛋白通过表皮直接吸收土壤中的铵,而AMT1;2蛋白则在皮层和内皮层细胞中表达,介导了铵盐的质外体吸收。进一步研究发现,拟南芥通过AMT1;1、AMT1;2和AMT1;3蛋白吸收的铵盐占铵盐吸收总量的90%~95%[28]。在水稻中,铵盐的吸收是由OsAMT1;1、OsAMT1;2和OsAMT1;3蛋白来完成,其中OsAMT1;1和OsAMT1;2基因表达水平随着环境铵盐浓度提高而上调;而OsAMT1;3基因则在氮匮乏状态下表达,使得水稻能够适应低铵盐环境[29-30]。

1.2 根系对氨基酸的吸收

植物根系对有机氮的吸收的研究主要集中在氨基酸方面[1,31]。在拟南芥中,预计已鉴定出超过100种氨基酸转运蛋白,大多属于氨基酸-多胺-胆碱(APC)转运蛋白超家族和属于DMT超家族的多种酸转运进出蛋白家族(UMAMIT)[32]。研究较多的氨基酸转运蛋白家族包括氨基酸通透酶家族(AAPs)、赖氨酸和组氨酸转运蛋白家族(LHTs)、脯氨酸转运蛋白家族(ProTs)、γ-氨基丁酸转运蛋白家族(GATs)、生长素转运蛋白家族(AUXs)、芳香氨基酸和中性氨基酸转运蛋白家族(ANTs)。一般而言,底物特异性和亲和力在不同转运子蛋白家族之间存在差异。AAPs被认为具有广泛底物特异性的中等亲和力,拟南芥中的AAP1蛋白参与了根系吸收谷氨酸和中性氨基酸的过程[33],而AAP5蛋白则具有转运碱性氨基酸的能力[34]。LHTs被推测具有高亲和力运输能力。AtLHT1定位于根表皮和叶肉细胞,能够将中性和酸性氨基酸导入根部,同时能够运输作为乙烯前体的1-氨基环丙烷-1-羧酸(ACC)[35-37]。拟南芥AtProt2蛋白参与了脯氨酸和甜菜碱的导入过程,超量表达该基因能够提高植物的耐盐性,这表明在缺水情况下受胁迫的植物根系细胞会提高内部溶质的浓度以提高渗透压[38]。

1.3 氮同化与氨基酸合成和运输

硝酸盐还原酶将硝酸盐还原为亚硝酸盐[39]。而亚硝酸盐还原酶则将转运到质体中的亚硝酸盐进一步还原为铵盐[40]。拟南芥中存在2种硝酸还原酶编码基因AtNIA1和AtNIA2,在2种基因同时突变的拟南芥突变体nia1、nia2中,其硝酸还原酶活性只有野生型拟南芥的0.5%[41]。

谷氨酰胺合成酶(GS)是氮同化的关键酶,其与谷氨酸合酶(GOGAT)一起形成所谓的GS-GAGOT环,将铵盐转化成为谷氨酰胺。根据GS定位的不同,可以将GS分为胞质型GS(GS1)和质体型GS(GS2)。叶绿体GS2主要参与了氨的初级同化和叶肉中的光呼吸铵的同化作用[42]。GS1参与了根中氨的初级同化,特别是在高硝酸盐环境下对氮同化有一定的作用[43]。有研究表明,拟南芥中的GS1型蛋白GLN1;2在成熟叶片中定位于伴细胞中,能够促进韧皮部的氮装载到库中[44]。在叶片衰老过程中,GS1酶的活性被诱导提高,将从氨基酸分解代谢中得到的原料重新进行氮同化[45]。根据电子供体的不同,植物中的谷氨酸合酶(GOGAT)分为铁氧还蛋白依赖的GOGAT(Fd-GOGAT)和NADP依賴的GOGAT(NADH-GOGAT),前者主要位于叶肉细胞的叶绿体中,后者则存在于叶和根伴随细胞的质体中,二者均可促进氮的同化和分配[46]。水稻中存在Fd-GOGAT抑制因子的编码基因OsARE1,突变该基因可以延缓植物的衰老,提高低氮条件下氮的利用效率,进而提高水稻产量[47]。

叶绿体(和细胞质)是大量蛋白来源氨基酸开始合成的部位,氨基酸的转运依赖转运蛋白来完成。研究发现,拟南芥中存在一种谷氨酸/苹果酸转运蛋白(AtDiT2),该蛋白能够将氨基酸转运出叶绿体[48]。矮牵牛中则存在一种阳离子氨基酸转运蛋白(PhpCAT),该蛋白能够将芳香族氨基酸从质体中运输出来[49]。

1.4 根-芽氮运移

木质部参与了根-芽的硝态氮流、氮同化物、其他营养元素和水分的运输。地面组织的运动是由叶片表面的蒸腾作用引起的,木质部的静水压力可梯度向下延伸到根部[50]。氨基酸的运输往往发生在木质部(和韧皮部)中,其中天门冬氨酸、谷氨酸、天冬酰胺和谷氨酰胺含量最为丰富[51]。在结瘤的热带豆科植物中,木质部中90%以上的氮以脲的形式出现,脲也是韧皮部中主要含氮化合物[52]。在外施不添加氮的营养液时,结瘤大豆木质部中的氮主要以氨基酸和脲的形式存在,无机氮的含量极低[53]。然而,当叶片中氮发生同化作用时,木质部硝酸盐的浓度可能超过有机氮化合物的浓度[54]。木质部不仅保证了植物各个组织生理功能的直接氮供应,而且还能够沿着其运输途径实现氮的回收,在根、茎、叶主脉以及木质部到韧皮部的转移中建立氮贮藏池,以用于快速供应生长需求旺盛的库。

2 营养组织中氮的储存

在各种源器官、组织或亚细胞结构中沉积的氮化合物的类型和数量对氮的运输和分配到库具有相当大的影响。此外,氮储藏池的形成能有效地控制胞质和质外体的氮浓度,从而通过前馈或反馈控制影响氮的吸收、运输和同化。

2.1 硝酸盐和铵储存

有研究发现,在叶和根的液泡/细胞质中硝酸盐的浓度较高[55],当细胞质中硝酸盐的浓度较高时,硝酸盐会储存在液泡中,当细胞质中硝酸盐浓度降低,影响氮同化时,硝酸盐又会从液泡中转移到细胞质中。在拟南芥中,AtCLCa基因编码的氯离子通道蛋白AtCLCA能够通过偶联硝酸盐和质子的转运,将硝酸盐存储到液泡中,并影响植物体中的硝酸盐浓度[56-57]。拟南芥中AtNRT2.7基因编码液泡膜转运蛋白,主要是在种子特别是干种子中表达,对于在种子中累积硝酸盐具有重要的作用[58-63]。在水稻中,OsNPF7.2基因编码硝酸盐低亲和转运蛋白,该蛋白定位于水稻根系伸长区和成熟区细胞的液泡膜。敲除OsNPF7.2基因将导致水稻在高硝酸盐条件下的生长延迟,表明缺少该蛋白会影响硝酸盐在根细胞中的分配,使得液泡不能起到调控细胞内氮元素含量的作用[64]。

过量的铵盐会导致植物中毒,为避免铵盐浓度过高带来的毒性,大量的铵盐被存储在液泡中,其在液泡中的浓度可达1 mmol/L,以维持细胞质内较低的铵盐浓度[60-61]。与成熟叶相比,老叶和幼叶中的铵盐浓度通常更高,这是由氨基酸分解代谢和光呼吸作用造成的。与液泡膜上硝酸盐转运蛋白相比,参与液泡内铵盐存储的转运蛋白有待研究[62]。

2.2 氨基酸的储存

在发育过程中或受到非生物/生物胁迫,植物叶片中游离氨基酸成分及其浓度的变化较大[63]。随着叶片衰老,叶片中氨基酸的总浓度稳步下降,而韧皮部汁液中的总氨基酸含量增加[64]。此外,氨基酸的种类随着植物的年龄和氮素营养的变化而变化[65]。例如,在烟叶的幼叶中,脯氨酸含量占总氨基酸含量的20%,而在成熟和衰老的叶片中只有2%。与低硝酸盐条件相比,在高硝酸盐条件下油菜(Brassica napus)叶片中的游离氨基酸含量更高,氨基酸组成更丰富[66]。此外,干旱和盐胁迫等环境条件能够引起叶片中氨基酸的积累,这可能是相关氨基酸合成能力加强或者蛋白质降解释放所造成的[67]。逆境胁迫解除后,植物在恢复过程中,暂时储存的氨基酸可参与新陈代谢过程或转运到新的沉淀物中。叶片储存氨基酸的种类随植物种类而变化,主要是谷氨酸、天冬氨酸、谷氨酰胺、脯氨酸,而在C4植物玉米中主要为天冬酰胺[68]。总体而言,在不同的膜转运系统、不断变化的转运机制以及底物特异性和亲和力的作用下,大量的氨基酸在细胞或储存池中的含量、种类频繁地发生变化。

运用蛋白质组学,可鉴定出拟南芥液泡膜的氨基酸转运蛋白,其中就包括了阳离子氨基酸转运蛋白(CAT)[69]。亚细胞定位试验证实,拟南芥CAT2和CAT4定位于液泡膜上[70],但是它们在液泡运输中的直接功能尚需进一步研究。番茄中也存在定位于液泡膜的氨基酸转运蛋白SlCAT9,该蛋白的编码基因在果实成熟期大量表达,进一步研究发现,SlCAT9蛋白介导了GABA(γ-氨基丁酸)与GLU/ASP的交换[71]。除了CAT蛋白,研究者在拟南芥中发现了负责芳香和中性氨基酸转运的ANT1蛋白和负责丙氨酸和脯氨酸转运的AtAVT3蛋白[72-73]。

2.3 脲类储藏

在热带豆科植物中,氮主要以尿素的形式存储在茎、叶柄或叶组织中[74]。在叶中,脲可能存储在叶肉细胞的液泡中。固氮大豆有一类特化的旁侧叶肉组织,研究表明,脲主要存在于这个组织之中,该组织也是营养生长过程中蛋白质储存的主要场所[75]。然而,参与上述过程的转运蛋白还未被鉴定出。有研究表明,尿囊素浓度上升可能会引发植物体对非生物和生物胁迫的一般反应[76]。

2.4 蛋白质的储存

理论上讲,植物体中的所有蛋白质均可认为是氮元素的存储池。许多酶可能在营养组织中的氮存储中发挥着作用。研究发现,相當大比例的核酮糖-1,5-二磷酸羧化/加氧酶是非活性的,在叶片衰老过程中,大亚基和小亚基似乎被独立降解以用于再活化[77]。叶片中GS实际酶活性总是比根据GS2含量推测出的酶活理论值低,这暗示了GS2也可能具有氮存储功能。此外,有研究发现,核糖体蛋白可以通过自噬作用实现氮元素的回收和再活化[78],这意味着核糖体蛋白也是一种重要的氮源存储池。营养贮藏蛋白(VSPs)是植物营养组织中氮存储的主要形式,所有植物物种中都存在VSPs。在植物氮吸收能力下降时,VSPs在氮的累积和流动过程中具有重要的作用。也有研究发现,VSPs在植物适应非生物或生物胁迫中有一定的作用[79-80]。

3 氮素的分配

大多数的氮是以氨基酸形式从源叶中被运输出,而在一些物种中则是以脲类的形式被运出。在衰老过程中,叶片成为氮/氨基酸的主要来源[81],氮源的再活化和随后的氮在库中的使用对于保持种子产量十分重要[82]。

3.1 蛋白质降解与氮的再活化

在受到胁迫或者衰老时,叶片中会发生有机氮的再活化,自噬相关蛋白和液泡蛋白酶参与了这个过程[64]。在营养缺乏的情况下,自噬程度会加剧,这有助于对氮的再活化[83]。在植物中,一些衰老诱导合成的氨基酸转运蛋白已经被鉴定出来,但是其在氮再活化中的作用还未知[64]。有研究发现,在叶片衰老过程中,韧皮部中天冬酰胺和谷氨酰胺的浓度增加,这意味着发生了一系列的转氨基反应,增加了谷氨酸和酰胺类物质的合成[84]。

3.2 有机氮和无机氮在库中的分配

韧皮部中的筛分子(SEs)和伴胞(CCs)形成长距离的运输管道,可使氮向库运输。氮常以氨基酸或脲的形式从叶片向库运输[85]。韧皮部的硝酸盐浓度相对较低,硝酸盐与氨基酸的比例通常为1 ∶(10~100)[19,54]。氮的叶外运输则是通过胞间连丝这种共质体途径[86]。

通过被动运输,氨基酸、脲类和硝酸盐从薄壁组织或束鞘细胞中释放,进入叶片质外体,然后被导入韧皮部[87-89]。参与硝酸盐运输到叶/韧皮部质外体的转运蛋白尚未被发现,但韧皮部装载硝酸盐过程中发挥作用的转运蛋白已经被鉴定出,包括NPF2.13/NRT1.7、NRT2.4和NRT2.5[1-3]。NPF2.13/NRT1.7 定位于叶片细脉的SEs/CCs中,促进老叶中韧皮部的装载。NRT2.4在叶片韧皮部(可能是韧皮部薄壁组织)或接近韧皮部中表达,在缺乏氮条件下,从质外体中回收硝酸盐,从而使氮向SEs/CCs运动。NRT2.5基因在拟南芥叶片的细脉中表达,并与NRT2.4一起影响叶片中硝酸盐的再活化和其在韧皮部的运输。NPF1.2/NRT1.11和NPF1.1/NRT1.12基因在叶片主脉伴细胞表达,它们除了在硝酸盐从木质部到韧皮部传递中发挥作用外,还在硝酸盐向幼嫩组织的分配中起重要的作用[90]。

对于从叶片中导出氨基酸的转运蛋白的研究相对较少[91]。有研究表明,拟南芥中的双向氨基酸转运蛋白(BAT1)可能在氨基酸从韧皮部向库组织的运输中发挥作用[92],AAPs可能参与了从叶质体中摄取氨基酸到SEs/CCs复合体的过程[93]。对拟南芥的研究表明,AAP8在源叶中韧皮部表达,在氨基酸韧皮部装载过程中起着关键作用,并且能强烈地影响库的大小和数量[94]。在菜豆中,PvUPS1基因能够在整株植物的韧皮部位中表达,其编码的蛋白对于尿囊素在韧皮部的转载以及运输到正在发育的库中具有重要的作用[95]。

3.3 参与分配的氮素来源

植物从土壤中吸收氮元素,而氮的含量则取决于作物种类、基因型和环境条件[45]。例如,在油菜中,硝酸根转运蛋白的表达和活性在生殖生长阶段降低,硝酸盐吸收较营养阶段少[96-97]。此外,氮的吸收取决于土壤水分的有效性,多雨条件下,研究者发现田间生长的油菜植株的根系在生长发育过程中能够大量地吸收氮元素[98]。在某些情况下有些地区夏季降水量不足,在这种情况下,植物需要将氮从蛋白质和氮储存池中再活化,以确保生殖库的氮营养需求。在拟南芥中,相较于高氮环境,植株氮利用效率(NUE)和氮活化效率(NRE)在低氮条件下更高。同时,在低氮条件下,同位素标记的14NO3 主要集中在植物的种子当中[99]。对玉米而言,籽粒中有62%的氮来自氮的再活化作用,其余38%的氮来源于根的吸收[1-3]。有研究表明,在土壤氮含量充足的情况下,植物在整个生长周期中,根系都能够持续吸收氮元素[100],于是,由于常在植物生长早期施用硝酸盐肥料,在植物生育阶段土壤中的硝态氮可能会被耗尽。

4 氮进入库

种子是主要的氮库,而在营养生长和多年生植物中,根、发育的叶和茎或树干是主要的氮库。有研究表明,溶质和大分子物质在根和茎中韧皮部的卸载方式有所不同[101]。现有研究表明,来自韧皮部的氨基酸的释放发生在维管结构的末端,由UmamiT转运蛋白家族来完成。其中UMAMIT11和UMAMIT14定位于与原生木质部和韧皮部相邻的细胞质膜,能够促进氮卸载和运输到种子。SIAR1(UMAMIT18)蛋白定位于细胞膜上,其编码基因在能够维管组织中表达,也具有将氮源卸载并运输到种子中的作用[102-103]。进一步研究发现,UMAMIT14和UMAMIT18不仅仅在种子发育中起作用,它们对氨基酸在根韧皮部的卸载也具有重要的作用[104]。在种皮中,氨基酸和硝酸盐通过被动运输输出到种子质外体,然后被一些转运蛋白导入到正在发育的胚中,并影响种子中蛋白的累积[105-106]。研究者发现,在豌豆中,氨基酸通透酶编码基因PvAAP1在种皮和子叶表皮转移细胞以及贮藏薄壁细胞中表达,对于种子中储存蛋白的累积具有重要的作用[107]。在水稻中,OSAAP6蛋白定位于细胞内质网上,调控种子内蛋白质的积累,OsAAP6基因表达水平高的植株的种子内蛋白质含量也相应增加[108]。

硝酸盐的运输过程会影响种子的发育和休眠过程[109]。拟南芥NPF2.12/NRT1.6存在于角果和珠柄的微管组织中,可促进硝酸盐向胚的运输,从而影响种子发育和种子内硝酸盐的积累[110]。此外,NPF5.5基因在拟南芥的胚中表达,该基因突变会减少胚中氮的含量,但似乎对种子发育没有影响[111]。NRT2.7定位于胚细胞液泡膜,调节硝酸盐在液泡中的积累,从而影响种子萌发[58]。

5 源到庫中的氮运输和代谢的关系

氮从源到库分配受源器官中氮的吸收和代谢、源输出和库输入氮的能力的影响。在拟南芥和豆科植物中的研究已经证明,芽中氨基酸运输能够调控根中氮的吸收、源的代谢和库的分配。在豌豆中,增加氨基酸在韧皮部的累积和装载可以正向调控根中氮的吸收,进而影响源和库中可利用氮的同化和使用[37,112-113]。此外,参与韧皮部装载过程的氨基酸转运蛋白能影响氮从源到库的量,并影响种子发育[94]。相反,韧皮部氨基酸的浓度似乎对胚对氮的吸收没有影响。例如,AAP8基因突变的拟南芥植株韧皮部中,降低氨基酸浓度对种子的氮含量没有影响,但种子数量会显著减少[94,114]。一般来说,定位于种子的氨基酸和硝酸盐转运蛋白可能调控了导入胚中的氮含量,转运蛋白编码基因表达水平的改变会影响种子中贮藏蛋白的积累[113,115-116]。然而,与单个种子所吸收的氮量相比,分配给许多库(即水果和种子)的氮量,不仅取决于源和库中转运蛋白的活性,而且受植物中可用的总氮量以及在生殖生长前期建立的总库数的影响。

6 氮运输调控

为了实现氮的有效吸收和利用,植物在不同层面上发展出了一整套氮运输的调控机制[117-118]。

6.1 细胞水平调节

硝酸盐能够调节包括NPF63/NRT1.1、NPF7.3/NRT1.5、NPF7.2/NRT1.8、NRT2.1和NRT2.2在内的硝酸盐转运蛋白的转录水平,从而影响氮在植物中的运输[119]。在拟南芥中,NPF63/NRT1.1不仅仅参与硝酸盐的吸收和转运,也能够作为外界硝酸盐信号的感受器,通过调控多种生理学和形态学上的变化(如种子休眠和形成侧根)对外界硝酸盐的变化作出应答[120]。包括NLP7在内的大量转录因子参与了硝酸盐的信号传导和转运[121-124]。NLP7可结合与硝酸盐信号传导和硝酸盐同化相关的基因。有趣的是,硝酸盐并不能影响NLP7基因的表达水平,而是通过调节NLP7蛋白在细胞核中的留存实现NLP7蛋白的大量累积[124]。此外,植物中存在一种CPSF蛋白,这种蛋白有2种剪切形式:CPSF30-S和CPSF30-L,其中CPSL30-L能够通过调节NPF63/NRT1.1参与植物硝酸盐信号的调控。此外,转录组分析发现,在cpsf30突变体中,许多氮转运及同化相关基因的表达水平发生了明显的变化,说明该蛋白对于调控硝酸盐信号通路具有重要的作用[125]。

研究表明,铵盐可以通过依赖时间或者浓度变化的形式调控AMT蛋白的磷酸化,从而影响其功能。进一步研究表明,拟南芥中的AtAMT1;1和AtAMT1;2能够被CIPK23磷酸化,从而抑制铵盐的吸收和转运,以避免植物细胞的铵中毒[126-127]。大量研究表明,硝酸盐和铵盐会对彼此的吸收、同化、分配产生影响,其分子机理在一定程度上得到了的解释[67,128]。

除了硝酸盐和铵盐之外,氮转运蛋白编码基因的表达还受细胞和组织发育状态的调节。同时,非生物或生物因素的影响,如光、盐和干旱胁迫,线虫或病原体攻击等都会对转运蛋白在基因或者蛋白水平上产生影响[32]。

氮转运蛋白在植物氮素吸收和代谢中扮演着重要角色,然而,它们在转录或翻译后如何相互影响,如何共同影响氮同化、储存和反馈或前反馈路径,现在尚未完全清楚,已有研究结果暗示可能存在核心调控能协调氮元素的分配。此外,研究表明,硝酸盐和氨基酸转运蛋白可能参与了植物激素在植物体内的运输[129-130],暗示氮素转运蛋白介导的植物激素转运可能也参与了氮素在运输或分配中的调控。

6.2 整个植株水平上的氮元素远距离信号系统

由于氮素在土壤中分布不均,因此植物具一套系统的长距离运输机制,即当根系一侧的氮缺乏时,氮元素丰富的另一侧则会补偿性地吸收更多的氮元素。由氮元素缺乏的根部產生的由根向茎移动的肽激素CEP(C-末端编码肽)诱发了这种运输机制。CEP能够调控韧皮部特异性肽CEPD1和CEPD2作为长距离移动信号转移到根部,并上调NRT2.1的表达水平,促进氮的吸收[131]。

7 作物改良应用

氮元素吸收、分配的遗传调控和氮代谢的调控是提高作物产量和氮利用效率(NUE)的主要途径[132]。研究者们希望通过超量表达相关基因以提高植物的氮利用效率,目前已取得了一定进展。例如,在水稻中超量表达高亲和力的硝酸盐转运蛋白编码基因OsNRT2.3b,能够提高植物的氮利用效率,从而增加籽粒产量[133]。根据作物种类、当地栽培环境、最终产品(例如根、叶、果实或种子)及其用途(例如人类营养、动物饲料或生物燃料)的不同,制定有针对性的改进策略可能更有效。例如,利用OsNAR2.1基因的特异性启动子控制的OsNRT2.1的过量表达会提高水稻地上部分生物量和籽粒产量[134]。在根中过量表达的大麦丙氨酸氨基转移酶(AlaAT)编码基因的油菜与非转基因油菜相比,在氮源不充沛的条件下,可以提高植物对氮的利用效率,从而提高产量[135]。研究者们将豌豆来源的PvAAP1基因置于其启动子控制下转入豌豆,与野生型对照相比,转基因豌豆的氮元素运输能力得到了显著提高,从而增加了其种子的产量[113]。也有研究者尝试在豌豆种子中特异表达氨基酸转运蛋白编码基因VfAAP1,结果发现转基因植株种子的大小以及氮含量明显提高,但是单株植物的种子产量却没有明显变化[136]。值得一提的是,研究者们也试图通过提高铵盐转运蛋白编码基因的表达来促进植物的生长。例如,超量表达OsAMT1;1能够在适量铵盐或者低铵盐的条件下促进水稻的生长,但是在铵盐浓度过高的环境中却因为在水稻根系中大量累积铵盐从而影响其生长发育。此外,超量表达OsAMT1;3的水稻中氮吸收转运能力呈下降趋势[137-139]。

8 总结

氮是植物体内重要的营养因子和信号物质,对于植物生长发育具有重要的作用。为适应外界氮元素含量的变化,植物演化出了一套复杂的调控网络。近年来,无机氮的运输、代谢及其调控的研究取得了长足的进展。尽管氨基酸和脲是氮元素长距离运输的主要形式,但是对于有机氮的相关研究却相对较少。不同类型的氮素转运蛋白在氨基酸和脲的源-库分配中的转运对于氮的获取、源的新陈代谢和库的强度具有非常重要的作用。比如,氨基酸(或脲)的积累对于无机氮转运蛋白的表达具有负向反馈作用,这种负反馈可以通过增加有机氮向库运输的通量来避免,但是这种负反馈作用仍有许多细节需要进一步完善。

氮素转运蛋白作为氮吸收、转运、同化等过程的重要参与者,一直是研究的热点,其功能已经在许多植物中得到鉴定。目前,笔者所知道的大量氮素转运蛋白主要承担了植物根部氮素的吸收和转运,对植物地上部分参与氮素分配和再活化、利用,特别是在生殖生长阶段促进源-库互作的转运蛋白了解很少。这部分知识的缺失,使得我们尚不能构建出完整的氮素吸收和利用的调控网络,但这却为未来的研究指出了方向。

植物的氮利用效率受到作物物种(亚种)、植株发育水平、环境等多种复杂因素的影响,这对研究者进行作物的氮高效利用遗传改良提出了挑战。进行作物改良时,需要尽可能兼顾无机氮和有机氮转运系统,如它们表达的时空性和相互关系,以精细调节氮吸收、代谢、瞬时储存和整株植物分配。现代生物技术的发展,包括全基因组关联分析、(GWAS)高通量测序、代谢组学、蛋白组学和表型组学,将有助于研究者进一步理解和整合氮分配过程中源和库的关系,并选择最有希望的候选者,通过基因聚合育种的方式培育出能够高效利用氮元素的“完美”作物。

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