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随着工业化的迅速发展,土壤的重金属污染已经成为了全球关注的问题。中国农田土壤镉污染面积已经超过2×105 hm2,生产镉含量超标的农产品达1.46×106 t·a-1[1]。镉通过食物链进入人体,引起了如“痛痛病”等慢性疾病。世界各国的土壤都存在重金属污染问题[2]。如何治理和修复污染土壤已经成为国际热点问题。20世纪80年代之后植物修复(phytoremediation)逐步发展起来,因其操作简便、成本低廉、环境友好等优点而日益受到人们的关注[3-4]。学术界公认的超富集植物需要满足2个条件:①植物地上部富集的重金属达到一定临界值,不同的重金属其富集界限不同;②地上部分富集的重金属含量高于地下部分。据报道,已鉴定的重金属超积累植物有500多种[5],但镉超积累的植物较少见,已发现的仅有遏蓝菜属Thlaspi的几种植物[6]。何冰等[7]发现了一种新型的锌、镉、铅超积累植物——矿山型东南景天Sedium alfredii。矿山型东南景天对土壤中的镉、锌、镍等重金属具有极强的超积累富集能力[8],更重要的是适量的重金属含量已成为其维持更好的生长状态所必需[9]。因此,查清东南景天体内涉及重金属离子吸收、转运、隔离、耐受等相关基因的分子机制,对林木重金属高抗新品种的培育具有重要理论指导意义。捕光蛋白复合体(LHC)能把捕获到的光能量迅速传导到光化学反应中心,光系统Ⅰ和光系统Ⅱ都有各自的LHC(LHC Ⅰ和LHCⅡ)。自LHCⅡ被发现以来[10],人们对其结构和功能的研究取得了较大的进展[11-12]。LHC Ⅱ含有Lhcb1-6等6种蛋白质,它们在类囊体膜中进行光能的吸收和传递,此外在维持类囊体膜的结构,调节激发能量在2个光系统之间的分配,光保护以及对各种环境的适应等过程中都起着重要的作用。林江波等[13]克隆和分析了中国水仙Narcissus tazetta var. chinensis叶绿素a/b结合蛋白,推测其可能属于Lhcb1类蛋白;高志民等[14]克隆了毛竹Phyllostachys edulis的a/b结合蛋白cab-PhE1,证明该基因在叶绿体的类囊体中表达;向太和等[15]分离出了水稻Oryza sativa捕光叶绿素a/b结合蛋白,分析显示该基因在叶片和茎中表达无差异,但是光对其表达有明显促进作用;Simon等[16]研究证明:叶绿素结合蛋白中的LI181家族和胁迫响应相关;张敏等[17]研究了2种生态型东南景天Lhcb2基因在镉和锌胁迫时的表达变化,并发现镉、锌胁迫处理能够提高转基因烟草Nicotiana tabacum中该基因的表达,且转基因烟草能够积累更多的镉。此外,菊花Chrysanthemum morifolium等植物中的Lhcb蛋白也已被克隆[18]。但这些研究主要集中于LHCB基因在不同光处理后的表达差异以及蛋白的结构和起源进化,对于Lhcb蛋白与植物重金属胁迫相关方面的研究还相对较少。本研究初步探讨了该基因在镉、铜、铅胁迫后的表达差异,结合前人研究推测东南景天LHCB基因可能是通过调节光合作用为重金属胁迫后的植物解毒机制提供能量,从而缓解植物体中毒的现象。
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矿山型东南景天无性系采集于浙江省衢州市的一个铅/锌矿区,取回后在人工气候箱中水培处理。胁迫处理时在水中添加设定浓度的重金属离子,然后分别提取根、茎、叶的核糖核酸(RNA),用于后续研究。
RNA提取使用总RNA提取试剂盒(加拿大,NORGEN, Thorold),DNA凝胶回收试剂盒购自AXYGEN公司(中国上海),2×Goldstar Taq Master Mix购自北京康为世纪生物技术公司(中国北京),反转录试剂盒(Prime ScriptTM RT reagent Kit),大肠埃希菌Escherichia coli DH5α感受态细胞和荧光定量试剂盒SYBR Prime ScriptTM RT-PCR Kit均购自Takara公司(中国大连)。
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以本实验室前期构建的cDNA文库为模板,利用转录组测序结果设计特异引物LHCB-F和LHCB-R,ORF-F和ORF-R,分别以LHCB-F和M13-以及LHCB-R和M13+从基因中间向两端扩增,以ORF-F和ORF-R扩增ORF进行验证(表 1)。20.0 μL扩增体系为:模板0.6 μL,引物1.0 μL,2×Goldstar Taq Master Mix10 μL,双蒸水(ddH2O)7.4 μL。聚合酶链式反应(PCR)程序为:94 ℃预变性10 min;94 ℃变性30 s,60 ℃ 30 s,72 ℃延伸1 min,32个循环;最后72 ℃延伸10 min。
表 1 引物名称及对应序列
Table 1. Name and sequence of primers
引物名称 引物序列(5′→3′) LHCB-F GTGGATCTTTTGACCCACTT M13R CAG GAA ACA GCT ATG ACC M13F TGT AAA ACG ACG GCC AGT LHCB-R TTCTCAACAGGTCCTTTTCC ORF-F ATGGCCACATCTGCTATCCAATC ORF-R TTATTTGCCGGGGACAAAGTTT LHCB-RT-F GAGGCGCACTGTGAAAAGCA LHCB-RT-R TTTCAGGGAAGACGCAGCCT UBC-F TGGCGTCGAAAAGGATTCTGA UBC-R CCTTCGGTGGCTTGAATGGAT T7 TAATACGACTCACTATAGGG SP6 ATTTAGGTGACACTATAG T7ter TGCTAGTTATTGCTCAGCGG LHCB-F-BamH Ⅰ CGCGGATCCTCATCCGCCTTTGCTGGC LHCB-R-Xho Ⅰ CCGCTCGAGTTATTTGCCGGGGACAAAGTTT -
采用十六烷基三甲基溴化铵(CTAB)法[19]提取东南景天叶片的基因组DNA,以其为模板,引物和扩增程序同cDNA序列克隆。10 g·kg-1琼脂糖凝胶电泳后回收目的片段,分光光度计测定其比例,将该片段连接到pGEM-T-easy载体上,热激转化大肠埃希菌DH5α感受态细胞,37 ℃过夜培养,挑取单克隆至800 μL LB液体培养基(LB/ampicillim),37 ℃,180 r·min-1,过夜培养,用通用引物T7和SP6进行菌液聚合酶链式反应(PCR)检测,阳性结果送测序。测序和引物合成由生工生物工程(上海)股份有限公司完成。
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参考金晓芬[20]和桑健等[21]的研究数据,分别以400 μmol· L-1的氯化镉(CdCl2),硫酸铜(CuSO4)和硝酸铅[Pb(NO3)2]的水处理东南景天无性系植株0.5,6.0,12.0,24.0,48.0,72.0和96.0 h,以未处理的东南景天无性系为对照,按照NORGEN的RNA提取试剂盒说明,分别提取不同处理时间东南景天根、茎、叶的RNA,设置3个生物学重复。用NanoDrop 2000分光光度计(Thermo, Massachusetts, 美国)测定浓度,用10.0 g·kg-1的琼脂糖电泳分析完整性。反转录根据RNA的含量取1.5~4.0 μg RNA,使用主要scripttm逆转录试剂盒(Prime ScriptTM RT reagent Kit)反转合成第1链,稀释3倍后利用ABI 7300实时荧光定量PCR仪(Applied Biosystems, Foster City, 美国)进行qRT-PCR分析。根据LHCB基因cDNA序列用Primer 3设计实时荧光定量引物LHCB-RT-F和LHCB-RT-R。以东南景天泛素结合酶基因(ubiquitin conjugating enzyme 9)为内参,其特异性引物为UBC-F和UBC-R。实时荧光定量PCR扩增体系(10.0 μL)为:模板2.0 μL,特异引物(10 μmol· L-1)0.4 μL,2×SYBR Premix Ex TaqTM 10.0 μL,50×ROX Reference Dye 0.4 μL,双蒸水(ddH2O)6.8 μL。扩增条件:95 ℃预变性10 s,95 ℃变性5 s,60 ℃复性31 s,40个循环。做3个重复·样品-1。数据分析采用ΔΔCT法计算相对定量,目标基因相对定量=2-ΔΔCT[22]。
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根据LHCB基因序列设计引物LHCB-F-BamH Ⅰ和LHCB-R-Xho Ⅰ,在其两端引入酶切位点BamH Ⅰ和Xho Ⅰ。以东南景天cDNA文库为模板进行扩增,利用限制性内切酶BamH Ⅰ和Xho Ⅰ处理PCR产物和pET-28a,回收目的片段后进行连接,获得重组原核表达载体pET-28a-LHCB。将pET-28a-LHCB基因重组质粒用热激转化的方法转入E. coli BL21(DE3)感受态细胞,挑选单克隆过夜培养后用载体上的通用引物T7和T7ter进行PCR检测插入片段的长度,阳性质粒送测序并用BamH Ⅰ和Xho Ⅰ 37 ℃处理3 h后电泳分析。在含有Kanamycin的LB液体培养基中培养细菌,当D(600)为0.6~0.8时,加入异丙基硫代半乳糖苷(IPTG,0.1 mmol· L-1)诱导培养,为了研究蛋白的具体定位,分离上清和包涵体,收集菌体进行蛋白质变性聚丙烯酰胺凝胶电泳(PAGE)分析。
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用DNAMAN软件,DNAtools,SignalP 4.1 Server(http://www.cbs.dtu.dk/services/SignalP/)和ExPASy(http://web.expasy.org/cgi-bin/compute_pi/pi_tool)等软件分析测定的DNA,cDNA及其编码的蛋白质结构特点,利用美国国家生物技术信息中心(NCBI)数据库信息进行比对(表 1)。
Characterization of a light-harvesting chlorophyll a/b binding protein (LHCB) gene, SaLhcb2, in Sedum alfredii
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摘要: LHCB基因对植物适应各种环境胁迫的过程中起着重要作用。序列分析显示:东南景天Sedum alfredii的SaLhcb2基因全长929 bp, 其中开放阅读框(ORF)为798 bp, 含有1个74 bp的内含子。通过蛋白序列比对, SaLhcb2基因编码的蛋白与多种植物的蛋白序列同源性都很高(92%以上)。分析400 μmol·L-1镉离子(Cd2+), 铜离子(Cu2+)和铅离子(Pb2+)胁迫处理后的东南景天, 结果显示:镉处理后SaLhcb2基因在茎、叶中表达量快速上升(12.0 h内), 根中表达量到48.0 h后才上调。铜处理后0.5 h根中表达显著上调, 胁迫6.0 h后茎中表达量显著上调, 随后一直降低, 叶片中该基因表达量一直较低。铅处理后, 根中表达量降低, 96.0 h左右比对照略微上调, 而茎中96.0 h内表达量相比对照都上调, 叶片中96.0 h内都降低。研究结果表明:SaLhcb2与东南景天的镉、铜、铅胁迫抗性有密切的相关性。
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关键词:
- 植物学 /
- 镉、铜、铅 /
- 叶绿素a/b结合蛋白(LHCB) /
- 荧光定量PCR /
- 东南景天
Abstract: The light-harvesting chlorophyll a/b binding protein (LHCB) gene plays an important role in plants adapting to various environments. Sedum alfredii is a new Zn/Cd hyper-accumulator and whether LHCB in this interesting species was related to the heavy-metal tolerance interested us a lot. In the present study, we isolated a cDNA using homologous cloning and designated it as SaLhcb2 which encoded a light-harvesting chlorophyll a/b binding protein in Sedum alfredii. A prelimary sequence analysis was conducted and homologous comparison was also performed using MegAlign. In order to identify the gene expression profiles of SaLhcb2 response to heavy-metal stress, seedlings of Sedum alfredii were treated by 400 μmol·L-1 Cd2+, Cu2+, and Pb2+ stresses with those cultured in water as a control. Roots, stems and leaves were dissected and promptly frozen in liquid nitrogen for RNA extraction and the followed real-time PCR. Sequence analysis showed that the coding sequence of SaLhcb2 was 929 bp and the open reading frame was 798 bp. The deduced protein, SaLHCB2, consisted of 266 amino acids and had a high homology (above 92%) with LHCB2 of other plants via homologous comparison. For the Cd2+, Cu2+, and Pb2+ stress treatments, the level of SaLhcb2 was elevated dramatically in stems and leaves within 12.0 h. However, with the Cd2+ treatment, the expression of SaLhcb2 in roots did not display a tendency of up-regulation until 48.0 h. For the Cu2+ treatment, in roots, the expression of SaLhcb2 showed a prompt response of up-regulation at 0.5 h and then decreased weirdly. With the Pb2+ treatment, the expression of SaLhcb2 in roots displayed a profile of down-regulation except the point of 96.0 h. Based on the above findings, it could be concluded that the expression of SaLhcb2 in Sedum alfredi was influenced by heavy-metal treatment and may function in the process of plants combating heavy-metal stress. -
表 1 引物名称及对应序列
Table 1. Name and sequence of primers
引物名称 引物序列(5′→3′) LHCB-F GTGGATCTTTTGACCCACTT M13R CAG GAA ACA GCT ATG ACC M13F TGT AAA ACG ACG GCC AGT LHCB-R TTCTCAACAGGTCCTTTTCC ORF-F ATGGCCACATCTGCTATCCAATC ORF-R TTATTTGCCGGGGACAAAGTTT LHCB-RT-F GAGGCGCACTGTGAAAAGCA LHCB-RT-R TTTCAGGGAAGACGCAGCCT UBC-F TGGCGTCGAAAAGGATTCTGA UBC-R CCTTCGGTGGCTTGAATGGAT T7 TAATACGACTCACTATAGGG SP6 ATTTAGGTGACACTATAG T7ter TGCTAGTTATTGCTCAGCGG LHCB-F-BamH Ⅰ CGCGGATCCTCATCCGCCTTTGCTGGC LHCB-R-Xho Ⅰ CCGCTCGAGTTATTTGCCGGGGACAAAGTTT -
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https://zlxb.zafu.edu.cn/article/doi/10.11833/j.issn.2095-0756.2014.06.003