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绿色植物吸收光能主要依靠捕光色素蛋白复合体(light harvesting complex,LHC)完成,该复合体由LHC基因家族编码的蛋白结合叶绿素和类胡萝卜素组成,而这些LHC基因的表达是受光照调控的。强光可以抑制LHC基因的转录表达,同时能诱导一些本身具有光保护功能的LHC-Like基因表达,例如强光下OHPs(one helix proteins)、SEPs(stress enhanced proteins)和ELIPs(early light induced proteins)基因的表达增加[1-2]。ELIPs是核基因编码的受光诱导的叶绿体类囊体膜蛋白,最早发现于豌豆Pisum sativum黄化苗转绿过程。在此过程中ELIPs比其他的光诱导蛋白出现的更早,并在光合细胞器叶绿体发育完全后迅速消失[3-4]。ELIPs在蛋白结构上高度保守,目前已知的所有ELIPs都有3个α-螺旋跨膜结构,其中螺旋Ⅰ和螺旋Ⅲ与光系统中所有捕光叶绿素a/b结合蛋白相应部位有很高的序列同源性,因此ELIP蛋白被归入叶绿素a/b结合蛋白超家族,并根据其跨膜螺旋数量分为3个亚组[2,5]。目前,已经在拟南芥Arabidopsis thaliana[6]、葡萄Vitis vinifera[7]、紫花苜蓿Medicago sativa[8]、银杏Ginkgo biloba[9]等多种植物中克隆鉴定获得了ELIP基因。有关ELIP蛋白基因功能的研究也越来越得到研究者的重视。ELIPs是在有光的情况下产生的,在类囊体色素-蛋白质复合物的光保护或组装中起作用,也可在叶绿体到有色体的转换中发挥作用,对光合作用系统起到光修复作用[10-12]。一些ELIP基因在山墙藓Tortula ruralis中被干旱激活[13],在豌豆中能被紫外线B波段(UV-B)激活[14],在小麦Triticum aestivum中被低温激活[15]。推测ELIPs除了具有光保护功能外,可能具有适应与光抑制有关的非生物胁迫环境的功能[16]。此外,在拟南芥中发现,AtELIP1和AtELIP2基因的缺失会使其种子萌发率下降,且ELIP蛋白的抑制因子DAG1突变后,拟南芥种子萌发率则高于野生型,表明ELIP蛋白具有促进种子萌发的作用[17]。可见ELIPs在多种生理过程中可能具有重要作用。毛竹Phyllostachys edulis是重要的森林资源,生长迅速,材质优良,其光合作用一直倍受关注。人们已从竹子光合作用生理特性[18-19]、分子机理[20-25]等方面进行了大量研究。然而,对于毛竹的ELIPs尚缺乏研究。本研究以毛竹为研究对象,克隆ELIP基因,全面分析它的分子特征及表达模式,构建了PeELIP3表达载体并转化模式植物拟南芥,对基因功能进行了初步分析,以期为深入研究ELIP基因在竹子光保护中的生物学功能提供参考。
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以实验室培养的毛竹实生苗为材料。培养条件:温度为18~25 ℃,光周期为16 h/8 h,光照强度为250~350 µmol·m−2·s−1,待长至6个月时进行光照处理。光强处理:将毛竹实生苗在黑暗适应24 h后分别移至于不同的光照强度(0、300、600、900、1 200和1 500 µmol·m−2·s−1)下,处理2 h后取样。强光处理:将毛竹实生苗置于1 200 µmol·m−2·s−1的强光下,分别在处理后0、0.5、1.0、2.0、4.0、8.0和12.0 h时取样。黄化苗处理:在黑暗条件下培养毛竹实生黄化苗,待长至1个月时,将毛竹黄化苗放置在250~350 µmol·m−2·s−1的光照下进行处理,分别在处理后0、0.5、1.0、4.0和8.0 h取样,并以室内正常光照(250~350 µmol·m−2·s−1)条件下生长的毛竹实生苗为对照(ck)。3次重复,每次重复的每个处理4~6株毛竹实生苗,所有处理均取苗顶端第3片叶为样本,液氮速冻后置于−80 ℃保存备用。
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从前期毛竹基因组测序和RNA-Seq结果[22, 26]中筛选出与拟南芥ELIPs相似的基因,暂时将其认定为毛竹ELIP基因,并根据其序列设计特异性扩增引物(表1),进行目的基因扩增。利用Trizol法提取毛竹叶片的总RNA,并使用反转录试剂盒(Promage,美国)合成cDNA作为模板。PCR产物经琼脂糖凝胶DNA回收试剂盒完成回收,然后连接到pGEM-T easy Vector,转化大肠埃希菌Escherichia coli DH5α感受态细胞,筛选阳性克隆,在生工生物工程(上海)股份有限公司进行基因测序。
表 1 引物序列
Table 1. Sequence of primers
用途 基因名称 正向引物序列(5′→3′) 反向引物序列(5′→3′) 基因克隆 PeELIP1 ATGGCGACCAAGGTGGCCTT CTAGACGTTGACGAGCGGGGC PeELIP2 ATGGCGACGACCATGATGGC TTACACTACTAGTTTTAGACGTTGAC PeELIP3 ATGGCGACGACCATGATGAC TTAGATGTTGACGAACGGCGC 表达分析 PeELIP1 ATCATGTCCGCTGACGCCGA CTTTGTGCTAGACGTTGACGAGC PeELIP2 ACGACCATGATGGCCTCGAG TTGGGCGTCTCCGTTGGATC PeELIP3 GCGCATCTAGCCTGTGCAAT TTGTTCTGGGCCCTCACGAC 对克隆获得的基因及其编码的氨基酸序列进行分析,其中编码蛋白的基本理化性质通过ExPasy(http://www.expasy.org/)在线工具获取,相应功能结构域分析使用美国国家生物信息中心(NCBI)链接的在线数据库CD Search (http://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi);利用Plant-mPLoc (http://www.csbio.sjtu.edu.cn/bioinf/plant/#)和WoLF PSORT (https://www.genscript.com/psort/wolf_psort.html)预测亚细胞定位。此外,利用MEGA 7.0[27]提供的Clustal W工具对毛竹ELIP基因编码的氨基酸序列与其他物种ELIPs序列进行比对分析,且使用邻接法(neighbor-joining)构建系统进化树,重复次数1 000次,其他参数使用系统默认值。
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根据获得的毛竹ELIP基因序列的非保守区域设计定量引物(表1)。利用qRT-PCR技术分析毛竹ELIP基因在不同光照处理条件下的表达模式,反应在qTower荧光定量PCR仪上进行,体系为10.0 μL:2 × SYBR Ⅱ Green 1 Master 5.0 μL;正/反向引物各0.3 μL (10 μmol·L−1);cDNA模板1.0 μL;H2O 3.4 μL。两步法进行PCR扩增:95 ℃ 6 min;95 ℃ 10 s,62 ℃ 10 s,40个循环。选择PeNTB为内参基因[28],基因的表达变化情况采用2–ΔΔCt法分析[29]。
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通过PCR扩增得到两端分别带有BamHⅠ和XbaⅠ酶切位点的PeELIP3基因片段,酶切回收纯化后,将目的片段连接到pCAMBIA1301载体的多克隆位点,得到重组质粒pCAMBIA1301-PeELIP3。经测序验证正确后,将植物表达载体质粒利用电击法导入农杆菌Agrobacterium tumefaciens EHA105菌株,并经PCR验证正确后用于转化拟南芥。
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利用农杆菌介导的浸花法[30]转化拟南芥,获得的T0代拟南芥种子经消毒后播种在含50 mg·L−1潮霉素的1/2 MS培养基中培养,初步挑选出具有抗性的转基因植株,并移栽到营养土中。提取T1代候选转基因植株的DNA,经PCR扩增PeELIP3基因序列,进一步鉴定转基因植株。在此基础上,筛选转基因植株至T3代,进行后续分析。同时,利用半定量RT-PCR检测PeELIP3在转基因拟南芥中的表达水平[23]。
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参照韩志国等[31]方法,以3周的拟南芥植株为材料,利用IMAGING-PAM叶绿素荧光仪进行叶绿素荧光参数的测定,包括光系统Ⅱ最大光化学效率(Fv/Fm)、非光化学猝灭(NPQ)诱导曲线等。同时获取拟南芥的叶绿素荧光成像,分析野生型与转基因拟南芥植株之间的非光化学猝灭差异。
Cloning and functional analysis of early light induced protein genes of Phyllostachys edulis
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摘要:
目的 探究早期光诱导蛋白(ELIP)基因在竹子光保护中的作用,为进一步阐述竹子光保护机制提供参考依据。 方法 以毛竹Phyllostachys edulis实生苗为材料,在前期研究的基础上克隆毛竹ELIP基因,利用qRT-PCR技术研究其在不同光照诱导下的表达谱,同时通过在拟南芥Arabidopsis thaliana中异位表达对1个基因的功能进行初步鉴定。 结果 克隆获得了3个毛竹ELIP基因(PeELIP1、PeELIP2和PeELIP3),分别编码165、179和182个氨基酸。蛋白结构分析表明:3个PeELIPs蛋白均具有典型的捕光叶绿素a/b结合蛋白功能域,含3个α-螺旋跨膜结构,属于叶绿素a/b结合蛋白超家族。序列比对及进化分析表明:PeELIPs与水稻Oryza sativa、玉米Zea mays等单子叶植物的ELIPs相似性较高,同源性达72%以上,聚类在同一分支。qRT-PCR分析表明:3个PeELIPs基因在毛竹黄化苗中仅检测到微弱表达,光照处理使3个基因的表达量均显著上调;同时在正常毛竹实生苗叶片中,随着光照强度的增强和强光胁迫处理时间的延长,3个PeELIPs基因的表达量都显著上调。过表达PeELIP3可减缓转基因拟南芥在强光下Fv/Fm的下降幅度,但未影响转基因植株的非光化学猝灭系数。 结论 毛竹中至少存在3个PeELIPs,且其表达均受光照的诱导。过量表达PeELIP3能够减缓转基因拟南芥受光抑制的程度,具有一定的光保护作用。图8表1参40 Abstract:Objective With an exploration of the role of early light induced proteins (ELIPs) in photoprotection of bamboo, this study is aimed at providing reference for the further elucidation of the photoprotective mechanism in bamboo. Method The ELIP genes were isolated from moso bamboo (Phyllostachys edulis) leaves using qRT-PCR before an analysis was conducted of their expression profiles under different light conditions employing qRT-PCR and the function of one ELIP gene was initially validated by ectopic expression in Arabidopsis thaliana. Result All the three ELIP genes (PeELIP1, PeELIP2 and PeELIP3) isolated from moso bamboo, with 165, 179 and 182 amino acids encoded respectively have the light-harvesting chlorophyll a/b binding protein domain that consists of three α-helices transmembrane domains, indicating that they belonged with the chlorophyll a/b binding protein superfamily. As was shown in the phylogenetic analysis, PeELIPs were closely related to the ELIPs from monocotyledonous plants including Oryza sativa and Zea mays, which had a high homology of more than 72% and clustered in a same branch. With the employment of qRT-PCR, it was found that the three PeELIPs were weakly expressed in etiolated bamboo seedlings, but their expression was dramatically increased upon light treatment. Meanwhile, with the increase of light intensity and the duration of treatment with strong light stress, they were all upregulated significantly in the normal bamboo leaves. In addition, the over-expression of PeELIP3 in Arabidopsis thaliana inhibited the decline of Fv/Fm under strong light treatment, but it had no effect on non-photochemical quenching coefficient (NPQ). Conclusion In conclusion, at least three homologous genes of ELIPs can be identified from moso bamboo, all with light-inducible expression. On the other hand, the over-expression of PeELIP3 could alleviate the photoinhibition in transgenic Arabidopsis thaliana, implying that PeELIP3 might play a positive role in photoprotection. [Ch, 8 fig. 1 tab. 40 ref.] -
图 2 基于ELIPs氨基酸序列构建的系统进化树
At. 拟南芥Arabidopsis thaliana;Br. 芜青Brassica rapa;Cr. 莱茵衣藻Chlamydomonas reinhardtii;Ds. 杜氏盐藻Dunaliella salina;Gb. 银杏Ginkgo biloba;Gm. 大豆Glycine max;Hv. 大麦Hordeum vulgare;Mf. 黄花苜蓿Medicago falcate;Ms. 紫花苜蓿Medicago sativa;Os. 水稻Oryza sativa;Pe. 毛竹Phyllostachys edulis;Pp. 小立碗藓Physcomitrella patens;Ps. 豌豆Pisum sativum;Pt. 毛果杨Populus trichocarpa;Sc. 齿肋赤藓Syntrichia caninervis;Si. 番茄Solanum lycopersicum;Sr. 山齿藓Syntrichia ruralis;Ta. 普通小麦Triticum aestivum;Tp. 红车轴草Trifolium pretense;Zm. 玉米Zea mays
Figure 2 Phylogenetic tree based on the amino acid sequences of ELIPs
表 1 引物序列
Table 1. Sequence of primers
用途 基因名称 正向引物序列(5′→3′) 反向引物序列(5′→3′) 基因克隆 PeELIP1 ATGGCGACCAAGGTGGCCTT CTAGACGTTGACGAGCGGGGC PeELIP2 ATGGCGACGACCATGATGGC TTACACTACTAGTTTTAGACGTTGAC PeELIP3 ATGGCGACGACCATGATGAC TTAGATGTTGACGAACGGCGC 表达分析 PeELIP1 ATCATGTCCGCTGACGCCGA CTTTGTGCTAGACGTTGACGAGC PeELIP2 ACGACCATGATGGCCTCGAG TTGGGCGTCTCCGTTGGATC PeELIP3 GCGCATCTAGCCTGTGCAAT TTGTTCTGGGCCCTCACGAC -
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