关键词:小麦;耐盐突变体RH8706-49;TaSC基因启动子;TAIL-PCR;表达活性 Abstract High salinity is one of the major abiotic stress factors in wheat. Exploring stress related genes from salt-tolerant wheat varieties and analyzing their regulatory mechanism are helpful for elucidating the salt tolerance mechanism in wheat. In this study, the promoter sequence of a salt-tolerant related gene TaSC, designated ProTaSC, was cloned from salt-tolerant wheat mutant RH8706-49 by TAIL-PCR and silicon cloning method. A series of cis-acting elements including abscisic acid response element (ABRE), MYB protein binding site (MBS), TATA-box and CAAT-box were predicted in the promoter region. Among them ABRE and MBS are involved in abiotic stress responses. Beta-glucuronidase gene was used as reporter to study the expression characteristic of ProTaSC, showing that the full-length fragment and two 5'-progressive deletion fragments (681 bp and 1096 bp) were able to trigger GUS expression. However, GUS expression was undetectable when the fragment was less than 343 bp. These results suggest that the full-length promoter has promoting activity and the sequence between -681 to -343 nucleotides is the basic core region of ProTaSC. ProTaSC is a tissue-specific promoter because GUS gene driven by full-length ProTaSC was expressed in root, leaf, anther, sepals, and mature pods, but not in stem, petal, young fruit, and seed of Arabidopsis harboring ProTaSC:GUS. Quantification of GUS activity assay showed that ProTaSC was induced significantly by NaCl (200 mmol L-1) and ABA (10 μmol L-1) in the transgenic Arabidopsis seedlings, indicating ProTaSC is a functional sequence induced by NaCl or ABA treatment.
以RH8706-49基因组DNA (图2-A)为模板进行TAIL-PCR表明, 基因特异引物和随机引物AD4组合的第3轮扩增得到一条322 bp的片段(图2-B, 箭头所示)。该序列与第2轮扩增产物的分子量差异接近162 bp, SP3单引物扩增没有条带出现(图2-B), 符合目的条带的特征。对该序列克隆、测序后与TaSC基因比对, 得到ATG上游313 bp的序列, 其DNA序列见图3中蓝色斜体所示部分。以这313 bp序列为基础拼接得到ATG上游1419 bp的逻辑序列, 根据逻辑序列设计引物, 扩增RH8706-49基因组DNA, 得到约1400 bp的片段(图2-C, 箭头所示), 对其克隆和测序显示, 该序列长度为1419 bp, 与逻辑序列的同源性达90%, 命名为ProTaSC。 显示原图|下载原图ZIP|生成PPT 图2 TaSC基因启动子的克隆 A: RH8706-49的基因组DNA; B: TaSC基因启动子的TAIL-PCR扩增; C: TaSC基因的启动子扩增; M: DL2000 ladder, 条带自上而下的分子量依次外2000、1000、750、500、200和100 bp; 1: SP3和AD4的三扩结果(箭头所示为目的片段); 2: SP2和AD4的二扩结果; 3: SP3引物自扩结果; 4: TaSC启动子的扩增结果, 箭头示启动子ProTaSC扩增片段。 -->Fig. 2Cloning of TaSC promoter A: electrophoretic pattern of genomic DNA of RH8706-49; B: TAIL-PCR amplification of TaSC promoter; C: amplification of TaSC promoter from RH8706-49; M: DL-2000 DNA ladder in 2000, 1000, 750, 500, 200, and 100 bp (from the top to the bottom); 1: profile of the tertiary TAIL-PCR amplification with SP3 and AD4 ( target band shown by the arrow); 2: profile of the secondary TAIL-PCR amplification with SP2 and AD4; 3: the amplification profile with SP3 primer only; 4: Amplification of TaSC promoter ProTaSC (target band shown by the arrow). -->
显示原图|下载原图ZIP|生成PPT 图3小麦耐盐突变体RH8706-49的TaSC启动子的DNA序列 蓝色斜体字母表示TAIL-PCR的扩增结果; 下画虚线表示TATA盒; 下画实线表示CAAT盒; 阴影表示其他功能顺式作用元件, 元件名称备注在序列下方, 其中ABRE元件(CACGTG )和MBS元件(CAACTG)与高盐等逆境胁迫响应有关。 -->Fig. 3DNA Sequences of TaSC promoter in salt-tolerant wheat mutant RH8706-49 The amplification fragment of TAIL-PCR is shown by blue italic letters. TATA box and CAAT box are underlined with dotted and solid lines respectively. Shadow shows other functional cis-acting elements whose name remarks under the sequence. Among them, ABRE element (CACGTG) and MBS element (CAACTG) are related to stress response. -->
对ProTaSC启动子及其不同5'末端缺失片段驱动GUS的转基因拟南芥植株进行GUS染色, 结果显示全长序列(1419 bp)具有启动功能, 而且在幼苗的根、茎、叶中均有表达(图4-A); 5′末端部分缺失的1096 bp 片段和681 bp片段也具有启动活性, 但启动活性较全长序列弱, 而且仅在叶片中检测到表达(图4-B, C); 转≤343 bp启动子片段的拟南芥没有GUS显色反应(图4-D, E)。上述结果表明, ProTaSC的基本启动活性中心位于起始密码子ATG上游-681至-343位核苷酸区间, 然而该区间仅具有较弱的启动功能, 表达量增强还需要-681位核苷酸上游的序列,在根中表达则需要-1096位核苷酸上游的序列。 显示原图|下载原图ZIP|生成PPT 图4转不同长度ProTaSC片段的拟南芥GUS组织化学染色结果 A: 转1419 bp启动子; B: 转1096 bp片段启动子; C: 转681 bp片段启动子; D: 转343 bp片段启动子; E: 转152 bp片段启动子。 -->Fig. 4GUS assay results of transgenic Arabidopsis harboring different lengths of ProTaSC promotor fragments A: transferred with the 1419 bp promotor; B: transferred with the 1096 bp promotor fragment; C: transferred with the 681 bp promotor fragment; D: transferred with the 343 bp promotor fragment; E: transferred with the 152 bp promotor fragment. -->
2.4 TaSC启动子的组织表达特异性
在转全长ProTaSC的拟南芥中, 主要在根(图5-A)、叶片(图5-B, C)、花药和萼片(图5-D, E)及成熟果荚壳(图5-F)中检测到GUS表达, 且以根尖(图5-A)和叶脉(图5-B, C)中表达量较高, 但在主茎、花瓣、幼果和种子中没有检测到。可见, 该启动子有明显的组织表达特异性。 显示原图|下载原图ZIP|生成PPT 图5转全长启动子ProTaSC拟南芥不同组织的GUS染色结果 A: 根; B: 茎上叶; C: 莲座叶; D: 花、幼嫩果荚和茎; E: 盛开的花; F: 成熟的角果。 -->Fig. 5GUS assay results of different tissues of Arabidopsis harboring full-length promoter ProTaSC A: root; B: stem leaf; C: rosette leaf; D: floral, young pot and stem; E: blooming flower; F: mature pod. -->
2.5 ProTaSC启动子受盐和ABA诱导表达
对NaCl和ABA分别处理不同时间的转全长ProTaSC拟南芥进行GUS酶活定量检测表明, 相对于0 h对照, NaCl处理24 h时GUS表达量上升近1倍, 表达量随后下降至初始水平, 在胁迫72 h时表达量增长至620 pmol 4-MU mg-1 protein·min-1, 是对照的3倍以上(图6); ABA处理24 h时GUS表达量上调很明显, 至36 h时表达量达到466 pmol 4-MU mg-1 protein min-1, 到72h时表达量下降至接近初始水平(图6)。可见, ProTaSC启动子的表达活性受NaCl和ABA诱导, 但是响应时间不同。 显示原图|下载原图ZIP|生成PPT 图6不同胁迫条件下转基因拟南芥的GUS定量分析 **和 ***分别表示胁迫处理与对照(0 h)在0.01和0.001概率水平的差异显著性(t检验)。 -->Fig. 6Quantification of GUS activity in transgenic Arabidopsis under different stresses ** and *** indicate significant between the stress treatment and the control (0 h) at the 0.01 and 0.001 probability level, respectively (t-test). -->
3 讨论
启动子是调控基因表达的顺式作用元件, 提供特定的转录因子结合位点以调控基因的表达, 是决定基因的表达特异性的重要元件。因此, 启动子特性与基因功能密切相关。例如, 胁迫诱导型启动子可以在胁迫条件下通过提高基因的表达量来增强植物的耐逆性。拟南芥rd29A启动子受低温、干旱、ABA和高盐等诱导, 遭受胁迫时显著上调其下游基因的表达, 提高了转基因植株的耐逆性[28]; rd29A驱动GmDREB3的转基因拟南芥比CaMV 35S驱动GmDREB3的转基因拟南芥有更强的耐逆性[29]。Nakashima等[30]发现, LIP9是受高盐等非生物胁迫诱导的启动子, LIP9驱动OsNAC6转基因水稻不仅耐旱性得到提高, 而且对产量没有负面影响。Jeong等[31]报道, 在根特异表达启动子RCc3的调控下, 水稻耐逆相关基因OsNAC10显著增强了转基因水稻对干旱、高盐和低温的耐受性, 改善了转基因水稻在干旱条件下的田间农艺性状, 其田间产量增加25%~42%。检测发现, RCc3:OsNAC10转基因植株的根部直径是组成型过表达植株的1.25倍, 推测增粗的根系有利于植株吸收水分和养料, 从而提高了转基因植物的耐旱性和干旱条件下的产量。可见, 启动子类型与转基因植物的胁迫条件下的生长密切相关, 而胁迫诱导表达型启动子有利于改善植物耐逆性及提高产量。 小麦盐诱导基因TaSC具有耐盐功能, 受盐诱导上调表达[20]。本研究利用TAIL-PCR和电子克隆的方法, 从耐盐小麦RH8706-49基因组DNA中克隆到TaSC的ATG上游1419 bp的启动子序列, 命名为ProTaSC (图2-C, 图3)。鉴于中国春小麦的基因组序列已经公布, 应用phytozome网站(https://phytozome.jgi.doe.gov/pz/portal.html)查询到Rh8706-49中的TaSCcDNA连同启动子ProTaSC序列(命名为ProTaSC+TaSC)在中国春基因组中的同源序列(命名为Traes_5DL_50BA3A)。对二者的比对分析表明, ProTaSC+TaSC序列与中国春基因组中的4266 bp的连续序列能够匹配, 其中启动子区域的同源性达到91.16%; 二者外显子部分的序列同源性达到95.14%, 其中的编码框(coding sequence, CDS)序列的同源性达到97.65% (附图1), 说明Traes_5DL_50BA3A与ProTaSC+TaSC的确是同源序列, 进一步证明了ProTaSC启动子序列与TaSC基因是连锁的。比对结果还表明, ProTaSC与中国春的序列存在一定差异, 有其序列特异性, 值得进一步研究其表达调控功能。 显示原图|下载原图ZIP|生成PPT 附图1耐盐小麦RH8706-49中ProTaSC+TaSC序列与中国春(Triticum aestivum L.)基因组序列的比对 ProTaSC+TaSC: 小麦RH8706-49中TaSC基因的cDNA序列(557 bp)及其启动子(1419 bp), 红色线条标注TaSC的起始密码子(ATG)和终止密码子(TAA); Traes_5DL_50BA3A: 中国春的4266 bp的基因组DNA序列, 其中含内含子2290 bp。 -->Supplementary Fig. 6Alignment of ProTaSC+TaSC sequence from salt-tolerant wheat RH8706-49 and genomic DNA sequence from Chinese spring (Triticum aestivum L.) ProTaSC+TaSC: the cDNA sequence (557 bp) and its promoter sequence (1419 bp) of TaSC gene in RH8706-49. Red bars show the start (ATG) and stop codon (TAA) of TaSC. Traes_5DL_50BA3A: the 4266 bp contiguous genomic DNA sequence of Chinese spring, containing 2290 bp of introns. -->
利用5'末端逐渐缺失实验对ProTaSC进行表达活性分析表明, ProTaSC的全长序列具有启动功能和组织表达特异性, 基本启动活性中心位于起始密码子ATG上游-681位到-343位核苷酸区间内(图4和图5)。该启动子中包含2个非生物胁迫响应元件——ABRE和MBS, 分别是与ABA和MYB蛋白相关的顺式作用元件。ABA和MYB类转录因子家族参与调控植物的多种耐盐、耐旱过程[32,33,34], 推测ProTaSC启动子具有非生物胁迫应答功能。本研究发现在盐和ABA处理的不同时间点, GUS表达量均有显著上调(图6), 表明ProTaSC是受NaCl和ABA显著诱导表达的功能序列。本研究结果为深入解析TaSC基因调控小麦耐盐性机制提供了依据。 The authors have declared that no competing interests exist. 作者已声明无竞争性利益关系。
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