广东农业科学  2023, Vol. 50 Issue (10): 85-96   DOI: 10.16768/j.issn.1004-874X.2023.10.010.
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文章信息

引用本文
李雨欣, 王鹏, 刘雯, 周扬. 海马齿NHX基因家族的鉴定及表达分析[J]. 广东农业科学, 2023, 50(10): 85-96.   DOI: 10.16768/j.issn.1004-874X.2023.10.010
LI Yuxin, WANG Peng, LIU Wen, ZHOU Yang. Identification and Expression Analysis of NHX Gene Family of Sesuvium portulacastrum[J]. Guangdong Agricultural Sciences, 2023, 50(10): 85-96.   DOI: 10.16768/j.issn.1004-874X.2023.10.010

基金项目

海南省自然科学基金(318QN189);海南省教育厅项目(Hnky2021-19,Qhys2021-248)

作者简介

李雨欣(1999—),女,在读硕士生,研究方向为植物耐盐机理,E-mail:lyx15716360352@163.com.

通讯作者

周扬(1988—),男,博士,副教授,研究方向为植物抗逆机理,E-mail:zhouyang@haiananu.edu.cn.

文章历史

收稿日期:2023-07-23
海马齿NHX基因家族的鉴定及表达分析
李雨欣 , 王鹏 , 刘雯 , 周扬     
海南大学 热带农林学院(农业农村学院、乡村振兴学院)/海南省热带园艺作物品质调控重点实验室,海南 海口 570228
摘要:【目的】 海马齿(Sesuvium portulacastrum L.)是典型的海岸植物和红树林伴生植物,能够固沙护岸,在海水中可以正常生长,具有极强的耐盐性。Na+/H+逆向转运蛋白(Na+/H+ exchange,NHX)在植物耐盐和生长发育中起关键作用,为了解NHX在海马齿耐盐过程中的作用,对海马齿NHX基因家族进行鉴定和分析。【方法】 采用生物信息学方法从海马齿全长转录组数据中鉴定NHXs成员,对其蛋白理化性质、保守基序和进化关系进行分析,并利用荧光定量PCR技术研究盐胁迫下NHXs成员的表达模式。【结果】 从海马齿中共鉴定出12个SpNHX基因,命名为SpNHX1~SpNHX12SpNHXs基因编码的氨基酸长度为276~554 aa,分子量为31.22~61.21 kD,等电点为5.55~8.64,不稳定指数为32.14~50.54,均为疏水性蛋白。系统发育分析表明,SpNHX蛋白可分为2个亚组,同一亚组具有相似的保守基序。多序列比对结果发现,SpNHX均具有Na+/H+ exchange保守结构域。转录组数据和荧光定量PCR结果显示,SpNHXs基因在盐胁迫下均受到不同程度的诱导,其中SpNHX1SpNHX9SpNHX10SpNHX11SpNHX12在盐胁迫下显著上调表达。【结论】 本研究明确了海马齿NHX基因家族在高盐胁迫下的表达模式,表明SpNHX1SpNHX9SpNHX10SpNHX11SpNHX12可能参与海马齿盐胁迫响应,为进一步研究NHX在海马齿耐盐过程中的作用提供理论基础。
关键词海马齿    盐胁迫    NHX基因家族    基因鉴定    生物信息学分析    基因表达    
Identification and Expression Analysis of NHX Gene Family of Sesuvium portulacastrum
LI Yuxin , WANG Peng , LIU Wen , ZHOU Yang     
School of Tropical Agriculture and Forestry (School of Agricultural and Rural Aff airs, School of Rural Revitalization), Hai nan Univ ersity/Hainan Key Laboratory for Quality Regulation of Tropical Horticultural Crops, Haikou 570228, China
Abstract: 【Objective】 Sesuvium portulacastrum, a typical coastal and mangrove associate plant, is capable of retaining sand and protecting banks. It can grow normally in seawater and has a strong salt tolerance. Na+/H+ exchange (NHX) plays a key role in salt tolerance and plant growth with development. In order to understand the role of NHX in salt tolerance of S. portulacastrum, the NHX genes were identified and analyzed. 【Method】 In this study, the S. portulacastrum NHX genes were identified by using informatic method through its full length transcriptome data. Their physicochemical properties, conserved motifs and evolutionary relationships were analyzed, and the expression patterns of NHX genes under s alt stress were studied by quantative real-time PCR (qRT-PCR). 【Result】 The results showed that 12 SpNHX genes were identified from S. portulacastrum and named as SpNHX1 to SpNHX12. The number of amino acids ranged from 276 to 554, with the molecular weight ranged from 31.22 to 61.21 kD, isoelectric point from 5.55 to 8.64, and instability ind ex from 32.14 to 50.54. All the proteins were hydrophobic. Phylogenetic analysis showed that SpNHX proteins can be divided into two subgroups. Similar conserved motifs composition was found in each subgroup. Multiple sequence alignment results showed that all SpNHX proteins had Na+/H+ exchange conserved domains. RNA-seq data and qRT-PCR results showed that SpNHX genes were induced to different degrees under salt stress, and SpNHX1, SpNHX9, SpNHX10, SpNHX11 and SpNHX12 were significantly up-regulated under salt stress. 【Conclusion】 This study clarified the expression patterns of NHXs in S. portulacastrum under high salt stress, indicating that SpNHX1, SpNHX9, SpNHX10, SpNHX11 and SpNHX12 may be involved in the response of S. portulacastrum to salt stress, which provided a theoretical basis for further research on the role of NHX in the process of salt tolerance of S. portulacastrum.
Key words: Sesuvium portulacastrum    salt stress    NHX gene family    gene identification    bioinformatic analysis    gene expression    

【研究意义】土壤盐渍化已经成为全球主要环境压力之一,会导致作物减产、品质下降、农田退化和耕田面积减小[1-2]。植物NHX(Na+/H+ exchange)逆向转运蛋白广泛存在于多种生物中,主要参与植物响应盐胁迫过程中离子平衡的重建,目前已在多种植物中开展了相关研究。对植物NHX基因家族的全面鉴定和分析,不仅可以为NHX应答盐胁迫机制提供重要信息,而且为植物耐盐性遗传改良提供理论基础。【前人研究进展】盐胁迫会对植物的生长发育造成不利影响,引发离子毒害,进而引起营养和水分亏缺、代谢失衡,最终导致膜功能紊乱和细胞死亡[3-5]。为提高生存几率,在长期的演化过程中,植物会通过改变外部形态调节内部生理过程以适应高盐环境或缓解由盐胁迫带来的伤害。植物的耐盐机制主要包括渗透调节机制、离子平衡和区域化机制、活性氧清除机制、信号传导机制、植物激素调节机制等[6]。其中,维持体内离子稳态是植物适应盐胁迫的最重要的策略之一[7]。盐胁迫下,Na+-K+平衡是植物细胞耐盐的重要机制[8-10]。K+是植物中最丰富的阳离子,它对调节渗透压、膜电位和膨胀压力、气孔运动和酶活性至关重要[11-12]。Tester等[13]发现盐胁迫可能导致K+缺乏,因为Na+会抑制K+的摄取,由此产生的高Na+/K+比率可以破坏细胞稳态。为避免Na+在细胞质中积累,耐盐植物通过将多余的Na+隔离到液泡中,有利于降低胞质中Na+含量和提高胞质中K+含量[14-15]。NHX是细胞中Na+(K+)/H+的逆向转运蛋白,具有保守的Na+/H+交换结构域,属于单价阳离子/质子反转运蛋白1(Monovalent Cation: Proton Antiporter 1,CPA1)超家族,主要利用H+-ATP酶和H+-PPase形成的两个质子泵,产生H+电化学梯度,将Na+从细胞质转运到液泡或细胞外,从而维持Na+的稳定性,减轻Na+在细胞内积累产生的毒性作用[16-20]。NHX蛋白主要存在于液泡、核内体和质膜中,大多数NHX蛋白含有10~12个跨膜螺旋结构域(TMs)[21-24]。研究表明,位于植物液泡中的NHX在离子稳态和耐盐性中起重要作用[25-27]。NHX具有维持细胞膨压、Na+(K+)浓度、膜囊泡运输和调节细胞内pH稳态的功能,是与植物耐盐相关的关键因子之一[28]。根据在植物细胞中的定位,NHX家族分为3个亚类,即液泡(Vac)、质膜(PM)和内膜(Endo)[29]。研究发现,位于液泡中的NHX可以逆着Na+和K+浓度梯度运输,将细胞质内的Na+和K+外排出细胞或区隔化到液泡内,从而降低细胞质的渗透势[30]。拟南芥中位于液泡膜上的AtNHX1基因是植物中第1个被克隆的NHX基因,在酵母中异源表达该基因可弥补酵母nhx1突变菌株对高浓度Na+、K+、Li+敏感的表型[31],在拟南芥中过表达AtNHX1能够提高其耐盐性[32]。Mushke等[33]发现在向日葵(Helianthus annuus)中过表达小麦(Triticum aestivumTaNHX2基因可减轻细胞损伤,提高叶绿素和脯氨酸含量,并增强活性氧清除能力,进而增强其耐盐性。在烟草(Nicotiana tabacum)中过表达TaNHX1TaNHX3可以增强烟草对高盐的耐受性[34-35]。杨晓妮[36]将短芒野大麦(Hordeum brevisubulatum)的Na+/H+逆向转运蛋白基因HbNHX1转入小青杨(Populus pseudo-simonii Kitag),在盐胁迫下,发现其根系萌发能力可达到30%,而野生型对照无法正常生根,表明HbNHX1可显著增强小青杨的耐盐能力。【本研究的切入点】海马齿(Sesuvium portulacastrum L.)是番杏科海马齿属多年生双子叶盐生植物,在我国华南海边沙地较为常见。有研究指出,海马齿可在0~600 mmol/L NaCl下生长并完成生活史,在800 mmol/L NaCl的高盐溶液中仍能生长,具有较强的耐盐性和环境适应性[37-40]。对海马齿耐盐基因的挖掘一方面有助于了解海马齿耐盐机理,另一方面可为提高植物耐盐性提供基因资源。但海马齿NHX基因家族尚未见系统研究和报道[27]。【拟解决的关键问题】本研究对海马齿NHX基因家族成员进行鉴定,并对其蛋白理化性质、进化关系、保守基序以及盐胁迫下的表达模式进行分析,为后续NHXs的基因功能研究和进一步利用该类基因进行植物耐盐性遗传改良提供理论基础。

1 材料与方法 1.1 海马齿NHX基因家族鉴定

从拟南芥基因组数据库(http://www.arabidopsis.org)和水稻基因组数据库(http://rice.plantbiology.msu.edu//)下载已发表的NHX蛋白序列。拟南芥NHX蛋白序列进一步用作查询序列,在海马齿全长转录组数据(NCBI,PRJNA930602)[41]中进行比对分析,以鉴定海马齿NHX基因家族。然后基于PROSITE(https://prosite.expasy.org/)和NCBI保守结构域数据库(CDD,https://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi)鉴定NHX蛋白的Na+/H+ exchange结构域(PF00999)[42]。最后,利用在线工具ExPASy(http://web.expasy.org/compute_pi/)对NHX蛋白的氨基酸数目、等电点(pI)、分子量(MW)、亲水性以及不稳定系数等理化性质进行分析。使用TMHMM Server v2.0(https://services.healthtech.dtu.dk/service.php?TMHMM-2.0)预测跨膜结构域(TMHs)。

1.2 海马齿NHX蛋白的系统发育分析

使用MEGA X对海马齿(Sesuvium portulacastrum,Sp)、拟南芥(Arabidopsis thaliana,At)和水稻(Oryza sativa,Os)NHX蛋白序列通过Clustal W方法进行多序列比对,采用最大似然法(Maximum Likelihood,ML)构建系统发育进化树,Bootstrap值设为1 000。

1.3 海马齿NHX蛋白的保守基序分析

利用MEME在线工具(http://meme-suite.org/tools/meme)分析海马齿NHX蛋白的保守基序,设置输出值为10,其余参数采用默认值,使用TBtools软件[43]对其可视化。

1.4 基于转录组数据的基因表达分析

为了解SpNHXs基因响应盐胁迫的表达模式,从海马齿转录组数据(NCBI,PRJNA930581)[41]中查找SpNHXs基因的FPKM值,使用TBtools软件绘制基因表达热图。

1.5 实时荧光定量PCR检测

海马齿种植在海南大学农科实验基地,采用扦插繁殖的方式进行水培。选取生长旺盛、大小一致的海马齿(从上往下数4个茎节,去掉下面2个茎节的叶片),洗净后用定植棉固定在水培箱种植槽内,用霍格兰营养液[44]在自然条件下水培海马齿10 d,通过制氧器制氧,每隔5 d换1次营养液。选取生长一致的海马齿进行800 mmol/L NaCl胁迫处理,于0(CK)、6、12 h后取其根系,液氮速冻,并保存在-80 ℃冰箱中以备提取RNA,每个处理3次重复。

用植物总RNA提取试剂盒〔天根生化科技(北京)有限公司,DP437〕提取RNA,反转录试剂盒〔宝生物工程(大连)有限公司,RR047A〕合成cDNA第一链。用Primer Premier 5.0设计引物(表 1),并交由生工生物工程(上海)股份有限公司合成。采用ChamQTM Universal SYBR qPCR Master Mix(诺唯赞生物科技有限公司,7E280C8)进行PCR扩增,PCR反应体系为:cDNA模板1 μL,上下游引物(10 μmol/L)各0.5 μL;2×SYBR qPCR Mix 5 μL,ddH2O补足至10 μL。PCR反应程序:95℃预变性30 s;95 ℃ 15 s,58℃ 30 s,72 ℃ 60 s,40个循环;65℃ 5 s;95℃变性DNA产物。每基因设3次重复。以SpGAPDH作为内参基因[45]。基因的相对表达量采用2-ΔΔCT方法计算,将SpNHX1在0 h的表达量定为1,其他基因及不同处理时间的表达量均与其比较进行定量。采用Excel 2010处理数据并绘制柱状图。

表 1 荧光定量PCR引物序列 Table 1 Primer sequences for qPCR

2 结果与分析 2.1 海马齿NHX基因家族鉴定

以拟南芥的NHX蛋白序列为靶序列,通过Blast搜索,在海马齿全长转录组数据中筛选出12个NHX基因,并将它们命名为SpNHX1~SpNHX12表 2)。通过ExPASy网站对其蛋白质特性进行分析,发现12个SpNHX的氨基酸序列长度在276(SpNHX2)~554 aa(SpNHX1);蛋白分子量在31.22~61.21 kD,理论等电点pI在5.55~8.64。SpNHXs蛋白不稳定指数介于32.14~ 50.54,其中只有SpNHX5、SpNHX6、SpNHX9、SpNHX10和SpNHX12的不稳定指数大于40,表明海马齿SpNHX蛋白中稳定蛋白居多。SpNHX蛋白的亲水指数在0.293~0.718之间,表明海马齿SpNHX蛋白均为疏水性蛋白。

表 2 海马齿NHX蛋白的理化性质 Table 2 Physicochemical properties of NHX proteins in Sesuvium portulacastrum

2.2 海马齿NHX蛋白的系统发育分析

通过MEGA X软件对海马齿(Sp)、拟南芥(At)、水稻(Os)、高粱(Sorghum bicolor,Sb)、小麦(Triticum aestivum,Ta)、柳枝稷(Panicum virgatum,Pv)、青稞(Hordeum vulgare,Hv)、金银花(Lonicera japonica,Lj)、毛果杨(Populus trichocarpa,Pt)、葡萄(Vitis vinifera,Vv)、番茄(Solanum lycopersicum,Sl)、笋瓜(Cucurbita maxima,Cm)、陆地棉(Gossypium hirsutum,Gh)、甜菜(Beta vulgaris,Bv)、菠菜(Spinacia oleracea,So)和马铃薯(Solanum tuberosum,St)的NHX蛋白序列进行比对,并构建系统发育树,结果如图 1所示。91条NHX蛋白序列被分为2个亚家族(Vac和Endo亚家族),其中,SpNHX5和SpNHX 6为Endo亚家族成员,其他10个成员(SpNHX1/2/3/4/7/8/9/10/11/12)均属Vac亚家族。

图 1 NHX蛋白的系统发育关系 Fig. 1 Phylogenetic relationship of NHX proteins

2.3 海马齿NHX蛋白的保守基序分析

通过在线软件MEME分析SpNHXs蛋白序列保守基序(图 2)。从图 2可以看出,不同SpNHXs的Motif数量相差较大,为4~10个。除SpNHX2外,其他SpNHXs均含有Motif 1、Motif 2、Motif 3、Motif 5、Motif 6和Motif 7,说明这6个基序在SpNHXs蛋白高度保守。此外,Motif 4和Motif 9仅存在于Vac亚家族成员中。综上,属于同一亚群的NHXs成员具有非常相似的基序类型和数量,但同一亚组的成员之间也存在基序模式的差异。

图 2 海马齿NHX蛋白保守基序分析 Fig. 2 Conserved motif analysis of NHX proteins in Sesuvium portulacastrum

2.4 海马齿NHX蛋白的多序列比对

利用DNAMAN对海马齿NHX蛋白进行多序列比对分析,12个SpNHX蛋白的N端均含有Na+/H+交换结构域(图 3)。且SpNHX1、SpNHX2、SpNHX4、SpNHX5、SpNHX7、SpNHX8和SpNHX9蛋白中该结构域内均存在1个氨氯吡嗪脒基序,该基序是植物中Na+离子抑制剂氨氯吡嗪脒的结合位点。

红色框:假定的阳离子结合位点;黄色框:氨氯吡嗪脒结合位点(FFI/LY/FLLPPI);黑线:Na+/H+交换结构域 Red box: Putative cation-binding site; Yellow box: Amiloride-binding site (FFI/LY/FLLPPI); Black line: Na+/H+ exchange domain 图 3 海马齿NHX蛋白多序列比对分析 Fig. 3 Multiple sequence alignment analysis of NHX proteins in Sesuvium portulacastrum

2.5 海马齿NHX蛋白跨膜结构分析

利用TMHMM在线网站对海马齿NHX蛋白跨膜结构域进行预测,结果显示,海马齿12个NHX蛋白均存在多个跨膜结构域,皆为跨膜蛋白。但每个SpNHX蛋白跨膜结构域的数量和分布有所不同,SpNHX8含有12个跨膜结构域,SpNHX1和SpNHX4含有11个跨膜结构域,SpNHX5、SpNHX7和SpNHX9含有10个跨膜结构域,SpNHX12含有9个跨膜结构域,SpNHX3含有8个跨膜结构域,SpNHX11和SpNHX6含有7个跨膜结构域,SpNHX10含有6个跨膜结构域,SpNHX2仅含有5个跨膜结构域(图 4)。

图 4 海马齿NHX蛋白跨膜结构预测 Fig. 4 Transmembrane structure predictions of NHX proteins in Sesuvium portulacastrum

2.6 基于盐胁迫下转录组数据的海马齿NHX家族基因表达分析

为研究盐胁迫下海马齿NHX基因家族表达的变化趋势,根据800 mmol/L NaCl胁迫下海马齿根系的RNA-seq数据(图 5)对其NHXs基因的转录水平进行分析。结果发现,盐胁迫下,海马齿NHX家族基因均受到不同程度的诱导。其中,SpNHX1SpNHX 2SpNHX3SpNHX9SpNHX10SpNHX11SpNHX12在盐胁迫下的表达趋势均是先上升后下降;SpNHX4SpNHX5SpNHX6SpNHX7SpNHX8在盐胁迫下的表达趋势则是直线上升。

图 5 基于海马齿RNA-seq的NHX家族基因表达 Fig. 5 Expression analysis of NHX family genes in Sesuvium portulacastrum based on RNA-seq data

2.7 盐胁迫下海马齿NHX家族的表达验证

为验证转录组数据,通过实时荧光定量PCR对800 mmol/L NaCl胁迫下海马齿根系中NHXs成员基因表达进行分析。实时荧光定量PCR结果显示,SpNHXs基因的表达模式与RNA-seq中基因的表达趋势相同(图 6)。与SpNHX1相比,SpNHX2SpNHX3SpNHX4SpNHX5SpNHX6SpNHX7SpNHX8在盐胁迫下的表达量处于较低水平。而与未处理条件下的SpNHX1基因相比,SpNHX1SpNHX9SpNHX10SpNHX11SpNHX12在800 mmol /L NaCl胁迫下显著上调表达,SpNHX1在胁迫12 h后,表达量较0 h上升了2.58倍;SpNHX9SpNHX11SpNHX12在处理6 h后的表达量分别是0 h的12.05、13.52和15.54倍;SpNHX10在盐胁迫下的表达呈先降低后升高趋势,并在胁迫12 h后其表达量达到峰值,是0 h的2.95倍。

*表示显著相关(P < 0.05),**表示极显著相关(P < 0.01) *represents significant correlation(P < 0.05), **represents extremely significant correlation(P < 0.01) 图 6 海马齿根系中NHX基因对800 mmol/L NaCl胁迫的响应 Fig. 6 Response of NHX genes in the roots of Sesuvium portulacastrum to 800 mmol/L NaCl stress

3 讨论

Na+/H+逆向转运蛋白NHX是植物体内一类重要的反向跨膜转运蛋白,其主要作用是将Na+或K+或H+进行跨膜反向转运,维持细胞内pH和Na+的动态平衡,在植物响应盐胁迫、细胞生长发育、离子稳态、渗透调节等生理生化过程中发挥着关键作用[46]NHX基因已经在拟南芥、水稻、小麦、甜菜、棉花等多种植物中被鉴定和功能表征[6, 22, 47-49]。海马齿是一种常见的海南滨海红树林伴生盐生植物,对盐离子有较强的耐受性,具有区隔化富集盐离子和重金属离子的特性[50]。前人已经从海马齿中克隆了耐盐相关基因如质膜Na+/H+逆转运蛋白基因SpSOS1[51]、质膜H+-ATPase基因SpAHA1[52]、甜菜碱醛脱氢酶基因SpBADH [53]、钙结合蛋白基因SpCBL10和蛋白激酶基因SpCIPK8[54],并且从海马齿中发现了1个NHX基因SpNHX1,可以提高转基因酵母耐盐性[55]。本研究基于海马齿全长转录组数据[43],对海马齿NHX基因家族进行鉴定,并对其保守基序、系统发育关系和基因表达进行分析。

本研究根据Na+/H+交换结构域,在海马齿中共鉴定出12个NHX基因,这与拟南芥[47]和水稻[56]中的NHX基因数量存在差异。在进化过程中,不同物种之间NHX基因数量的差异是由于进化过程中基因重复和丢失造成的[29]。植物NHX蛋白家族的结构具有很高的相似性,均具有1个跨膜次数不等的功能结构域和C端调控结构域,而C端调控结构域在调节NHX蛋白活性方面起着重要作用[57]。李茹霞等[58]对菠菜(Spinacia oleracea)SpoNHX氨基酸序列进行保守基序分析,发现同一亚组的SpoNHX和AtNHX其保守基序基本一致,如Vac类中的SpoNHX1、SpoNHX2和拟南芥中的AtNHX1~4含有11个相同的保守基序,不同亚组中的NHX也有很多相同的保守基序,表明成员间保守性较高。本研究利用MEME软件对海马齿NHX蛋白的保守基序进行分析,发现同一亚组的NHX蛋白所含保守基序基本一致,如Vac亚组中的NHX蛋白,除SpNHX2外,其他NHX蛋白均含有8个相同的保守基序,其中,Motif 8基序中包含FFI/LY/FLLPPI结合位点,是真核细胞NHX抑制剂氨氯吡嗪脒结合位点(Amiloride binding-site)[59],该位点位于Na+/H+交换结构域内,其功能与液泡膜上介导Na+/H+的交换密切相关[21]。虽然Vac亚组中的SpNHX2保守性较差,但与Vac亚组中其他成员间仍存有3个共同的保守基序。植物NHXs成员的N末端是保守的,C末端是高度分化的,因为C末端可以调节逆向转运蛋白的活性,因此认为C末端序列的不同可能是不同成员功能差异的主要原因[46]。另有报道指出,拟南芥AtNHX1的C末端缺失有助于NHX增强自身的离子交换活性,包括Na+/H+和K+/H+离子的交换活性均会提高[21]。本研究通过对海马齿NHX蛋白序列进行比对,发现海马齿NHX蛋白序列N端高度保守,而C端保守性较差。SpNHX2的C端缺失,预示着其抗耐机制可能与其他成员不同。

将Na+区隔化到液泡中维持Na+的稳态,降低其对细胞质的毒性,是缓解植物盐胁迫的重要方法之一[60-61]。Xu等[62]发现,TaNHX2主要作为细胞内K+/H+反转运蛋白,在盐胁迫条件下,使酵母细胞维持较高的K+浓度。TaNHX2可以将K+和Na+转运到囊泡中,但其K+/H+交换能力高于Na+/H+交换能力。在盐胁迫条件下,TaNHX2转基因苜蓿叶片中Na+含量显著降低,K+含量和[K+]/[Na+] 比值高于野生型植株,表明TaNHX2可能同时参与了Na+和K+的转运[63]。在烟草中过表达TaNHX1TaNHX3基因可以提高烟草的耐盐性[34-35]。本课题组前期从海马齿中克隆出SpNHX1基因,并转入酵母盐敏感突变体AXT3K中,发现SpNHX1可以提高酵母的耐盐性,并且转基因酵母中Na+含量高于对照[55]。本研究利用转录组数据和荧光定量PCR技术对盐胁迫下海马齿根系中NHXs成员表达情况进行分析。转录组数据显示,NHX家族在盐胁迫下均受到不同程度的诱导,12个SpNHX基因的表达呈现两种模式:SpNHX1SpNHX2SpNHX3SpNHX9SpNHX10SpNHX11SpNHX12在盐胁迫下呈现先上升后下降的模式;SpNHX4SpN HX5SpNHX6SpNHX7SpNHX8在高盐胁迫处理后呈上升模式。荧光定量PCR结果与转录组数据基本一致,但与SpNHX1基因相比,SpNHX2SpNHX3SpNHX4SpNHX5SpNHX6SpNHX7SpNHX8在盐胁迫下的表达量处于较低水平,其他基因受盐胁迫诱导上调表达。综上,海马齿NHX在盐胁迫下响应机制存在差异,但它们的耐盐功能还需要进一步研究。

4 结论

本研究从海马齿中鉴定到12个NHX基因,系统进化分析将其分为Vac和Endo 2个亚家族。多序列比对结果显示,SpNHX蛋白均含有Na+/H+交换结构域,其中SpNHX1、SpNHX2、SpNHX4、SpNHX5、SpNHX7、SpNHX8和SpNHX9蛋白的此结构域内均存在1个Na+离子抑制剂氨氯吡嗪脒的结合位点。RNA-Seq和荧光定量结果显示,在盐胁迫条件下,SpNHXs基因均受到不同程度的诱导,SpNHX1SpNHX9SpNHX10SpNHX11SpNHX12受盐胁迫诱导显著上调表达,推测它们在响应盐胁迫中发挥着较为重要的作用。本研究结果为海马齿NHX家族基因的克隆及功能预测提供了一定的理论参考,并为海马齿耐盐功能基因的深入研究奠定了理论基础。

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