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磷脂酸(phosphatidic acid,PA)是一种常见的细胞甘油磷脂,也是最简单的膜脂之一[1]。几乎所有生物体内都会产生PA,包括酵母[2]、动物[3]和植物[4]等。在植物中PA的合成主要包括2种方式:第1种是从头合成途径,3-磷酸甘油先后在3-磷酸甘油酰基转移酶和溶血磷脂酸脂酰基转移酶的作用下发生连续2步酰化反应生成PA,这也是PA合成的主要途径[5];第2种是磷脂的降解途径,磷脂降解又可分为2种,一种是磷脂酶D通过水解某些结构磷脂直接生成PA[6−9],另一种是磷脂酶C通过水解磷脂酰肌醇生成二脂酰甘油[10],继而经二脂酰甘油激酶磷酸化生成PA[11]。PA作为一种结构膜脂,磷酸基团头部能够以“静电/氢键开关模型”的方式与蛋白质结合[12]。在该模型中,PA结合蛋白中带正电荷的碱性氨基酸残基[13−17],可以吸引磷脂双分子层上的负电荷,并通过静电作用影响膜环境,继而与PA头部的磷酸基团形成氢键,产生一个稳定的蛋白质结合位点。PA这种携带负电荷并且在生理条件下呈圆锥体的独特分子构型[18],使得PA结合蛋白的结构域能够特异性的与其结合,保证了生物膜结构的稳定和功能的发挥。除了作为膜脂的一个结构成分以及甘油脂合成的前体物质,PA还是一个关键信号分子,在多种生物学过程中发挥重要作用。正常条件下,行使信号功能的PA含量甚微;在植物应答各种生物与非生物胁迫过程中PA水平则会迅速升高,但这种积累往往是短暂的。这说明生物体拥有一套严格控制信号分子PA含量的机制,这对调节PA信号传递强度、维持细胞物质与能量代谢的动态平衡至关重要[19−22]。
尽管PA拥有多种功能,但目前对于细胞内PA含量的动态变化仍知之甚少,这主要受PA检测手段的限制。过去主要采用诸如薄层层析、高效液相色谱和放射性同位素标记等理化手段对其含量进行测定[23]。近年来,质谱分析技术的应用使得对包括PA在内的各种脂质的测定更准确和高效[24−25]。然而,这些理化手段都只能对组织器官中PA总量与种类进行定量或定性分析,无法捕捉细胞内PA的动态与瞬时变化信息。通常认为,内质网上的PA含量相对较高,这有助于促进磷脂和三酰甘油的合成[26−27];而质膜和细胞器中行使信号功能的PA含量甚微。显然,采用理化测定会掩盖胞内PA的真实变化。因此,亟需开发一种能可视化分析细胞内PA的工具。
大量研究表明:生物体中PA结合蛋白种类繁多,这些蛋白中的PA结合域(phosphatidic acid binding domains,PABD)结构亦存在差异。其中一些PABD因与PA结合专一性强,被用来开发多种PA探针 [28−33]。酵母蛋白Spo20p中的PA结合域已被证实具有极强的专一性[33],只结合PA而不结合其他磷脂。目前基于这一PABD所开发的PA探针在医学研究中应用广泛,但在植物中的应用却有限[34]。因此,本研究旨在构建基于Spo20p中的PABD的植物PA荧光探针,为剖析植物早期应答逆境胁迫的细胞分子机制提供新工具。
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将拟南芥Arabidopsis thaliana种子置于含有湿润滤纸的培养皿中,4 ℃冰箱内避光放置3 d后将其点播在湿润的土壤基质上(V营养土∶V蛭石∶V珍珠岩=3∶1∶1)。将播种后的盆栽用透明塑料盖覆盖,置于24~25 ℃室温、50%~60%湿度、14 h光照/10 h黑暗的植物培养箱中培养,7 d后揭去塑料盖间苗。
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将拟南芥种子进行消毒,加入适量灭菌过的蒸馏水,在4 ℃冰箱内避光放置3 d,点播在1/2 MS培养基上,置于植物培养间中培养。
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酵母蛋白Spo20p中存在一个高度专一的PA结构域(PABD),该PABD由40个氨基酸残基构成,只结合PA而不结合其他磷脂[33−34],因此,本研究选择该PABD与荧光蛋白融合,构建PA荧光探针。根据PAPD核苷酸序列[33]设计引物(表1)进行PCR扩增,获得与该PABD相对应的DNA片段;同时PCR扩增获得GFP基因编码区序列。随后,利用无缝克隆试剂盒(生工),将GFP和PABD片段克隆至植物表达载体pSY06的限制性内切酶位点Kpn I和Pst I之间,获得重组质粒pSY06-GFP-PABD。PABD连接至GFP基因编码区的3′端,构成GFP-PABD融合基因,该融合基因的表达由组成型启动子UBQ10驱动。重组质粒中的插入片段经菌落PCR、Kpn I和Pst I双酶切以及测序验证正确后,采用热激法将质粒转化至农杆菌Agrobacterium tumefaciens GV3101,用于后续拟南芥遗传转化。
表 1 相关引物序列
Table 1. Sequence of related primers
用途 引物名称 引物序列(5′→3′) PABD基因合成 ZYP301 CCTGCTAATTTCAAGGCTAATTCATTGCTGTTCGCTCGAGATCTGAGTCCGGACTGCAGGTTGGATCCGGTACCGAGCT ZYP302 GAAGACGTGATAGGCTACATGTGAAGCTTAAATCCTTGAGGAATAAAATCCACAAACAACTTCACCCAAA ZYP303 TGCTTCCTGAACAATTGTCCATTCTAGATTCTGTCTTGTTGATATCAAGACCTGCTAATTTCAAGGCTAA ZYP304 GTACCCAAGCTTCACATGTAGCCTATCACGTCTTCTGCTTCCTGAACAATTGTCC ZYP305 CACAAACAACTTCACCCAAACTGTCGGTTCGATGACGCCACTAAGACTAGTTAAGGTACCGGCGTAATCATGGTC PABD基因克隆 ZYP308 TCCGGACTCAGATCTCGAGC ZYP309 AGCTATAGTTCTAGATCTAGATTAACTAGTCTTAGTGGCGTC GFP基因克隆 ZYP306 TATCGATGGCGCCAGCTGAGGATGGTGAGCAAGGGCGA ZYP307 AGATCTGAGTCCGGACTTGTACAGCTCGTCCATCCGGACTCAGATCTCGAGC 转基因拟南芥鉴定 MSY01 TGGTGTTAGTTTCTAGTTTGTGCG MSY02 TCATCATGACAGATCTGCGC 表达量分析 ZYP389 GACCACTACCAGCAGAACACC ZYP390 CTTGTACAGCTCGTCCATGC -
以哥伦比亚野生型拟南芥(WT)为植物材料,利用农杆菌花序侵染法转化拟南芥。将获得的T1代拟南芥种子进行表面消毒,均匀铺在含有草铵膦(10 mg·L−1)和特美汀(50 mg·L−1)的1/2 MS筛选培养基上进行筛选。将具有除草剂抗性的阳性苗移栽到土壤中继续培养,3周后提取叶片DNA并设计引物(表1)进行PCR鉴定,对T2和T3代转基因株系进行除草剂抗性的遗传分析,最终筛选得到纯合单插入位点转基因株系。
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以在1/2 MS培养基中生长7 d的转基因拟南芥根系作为测定材料,使用高纯度RNA提取试剂盒(全式金)提取RNA并对其进行纯度检测和浓度测定。取等量RNA,使用反转录试剂盒(翌圣)进行单链cDNA的合成。选择拟南芥肌动蛋白基因Actin为内参基因,设计引物(表1)进行实时荧光定量PCR (RT-qPCR),并对数据采用2−∆∆Ct法进行分析。
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将野生型和转基因株系种子点播在1/2 MS方形培养基上,竖直培养6 d后,挑选长势均匀一致的幼苗(每个基因型,20株),分别转移至含有0和50 mmol·L−1 NaCl的1/2 MS固体培养基中继续竖直培养,7 d后观察表型。
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将在1/2 MS方形培养基上竖直培养6 d的幼苗分别浸泡在含有0、2和10 μmol·L−1 PA的1/2 MS液体培养基中,依次处理5、10、15和20 min后,利用荧光显微镜(ZEISS,Axio Imager 2)对拟南芥根尖分生区PA的分布进行观察并拍照。
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将含PA荧光探针的拟南芥转基因株系种子点播在1/2 MS方形培养基上,竖直培养6 d,选择均匀一致的幼苗分别浸泡在含有100 mmol·L−1 NaCl、10 mmol·L−1 NaHCO3和兼含两者的1/2 MS液体培养基中,处理5、10和15 min后用荧光显微镜观察根尖分生区PA的分布。
Generation and application of a fluorescent probe for phosphatidic acid in Arabidopsis thaliana
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摘要:
目的 磷脂酸(PA)是甘油脂生物合成的前体,又是参与植物生长发育调节和各种逆境响应的重要信使物质,然而目前对植物细胞中PA含量动态变化的了解十分有限。本研究试图构建一种能有效监测植物细胞PA含量变化的荧光探针,并用之测定盐碱胁迫过程中胞内PA含量的变化。 方法 将Spo20p蛋白中高度专一的PA结合域相对应的核苷酸序列与绿色荧光蛋白基因融合,经遗传转化获得携带该融合基因的转基因拟南芥Arabidopsis thaliana株系,其表达受组成型启动子UBQ10驱动,产生的融合蛋白成为专一结合PA的荧光探针。随后,运用该荧光探针监测盐碱胁迫下胞内PA含量的变化。 结果 构建获得7个不同的纯合、单插入位点转基因拟南芥株系。实时荧光定量PCR (RT-qPCR)分析显示:不同株系中融合基因的表达量存在差异。不同浓度外源PA处理试验显示:随着表达量的升高,PA探针可有效监测到2 μmol·L−1 PA处理根尖10 min后细胞中PA含量的变化;而当PA探针表达量较低时,对PA监测灵敏度显著下降,表明在一定程度上荧光探针对PA监测的灵敏度与其表达量相关联。运用PA荧光探针发现:盐碱胁迫处理根尖5 min即可诱导质膜上或胞内PA的积累,暗示PA在植物早期盐碱胁迫响应中可能产生重要作用。 结论 本研究构建了一种可对细胞内PA含量进行有效监测的荧光探针,该探针可用于监测植物盐碱胁迫早期响应过程中胞内PA水平的变化,从而为早期逆境响应研究提供新工具。图7表1参36 Abstract:Objective Phosphatidic acid (PA) serves as an important signal molecule involved in the regulation of plant growth and development and different responses to various stresses as well as a general precursor for glycerolipid biosynthesis. However, little is known thus far about the dynamic changes of PA in plant cells. This study attempted to construct a fluorescent probe that can effectively monitor the changes of PA in plant cells and use it to measure the changes of intracellular PA under saline-alkaline stresses. Method The corresponding nucleotide sequence for PA-specific binding domain within the Spo20p protein was fused to the green fluorescent protein gene. Transgenic Arabidopsis thaliana lines bearing the fusion gene under the control of the constitutive promoter UBQ10 was then generated via genetic transformation. The resulting fusion protein constituted a fluorescent probe specifically binding to PA. Subsequently, this probe was employed to monitor the changes of cellular PA under saline-alkaline stresses. Result Seven transgenic A. thaliana lines homozygous for single insertion of the fusion gene was generated. Real time quantitative PCR (RT-qPCR) analysis showed that expression levels of the fusion gene varied among different lines. Experiments with various exogeneous PA concentrations revealed that as the expression level of the PA probe increased, it could effectively monitor the changes of cellular PA in the root tips treated with 2 μmol·L−1 exogenous PA for 10 min, whereas this was not the case when the expression level of the probe was low, indicating that the sensitivity of the probe for PA detection is, to a certain degree, associated with its expression level. Based on this fluorescent PA probe, PA accumulation at the plasma membrane or in the intracellular space was evident in the root tips under saline-alkaline stresses for 5 min, implying that PA may play important roles in early plant responses to saline-alkaline stresses. Conclusion A fluorescence probe for effective monitoring of cellular PA was developed in this study. This probe can monitor the alterations in cellular PA level during early plant responses to saline-alkaline stresses, thereby providing a new tool to study early responses to various stresses. [Ch, 7 fig. 1 tab. 36 ref.] -
Key words:
- phosphatidic acid /
- fluorescent probe /
- transgene /
- saline-alkaline stress /
- Arabidopsis thaliana
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表 1 相关引物序列
Table 1. Sequence of related primers
用途 引物名称 引物序列(5′→3′) PABD基因合成 ZYP301 CCTGCTAATTTCAAGGCTAATTCATTGCTGTTCGCTCGAGATCTGAGTCCGGACTGCAGGTTGGATCCGGTACCGAGCT ZYP302 GAAGACGTGATAGGCTACATGTGAAGCTTAAATCCTTGAGGAATAAAATCCACAAACAACTTCACCCAAA ZYP303 TGCTTCCTGAACAATTGTCCATTCTAGATTCTGTCTTGTTGATATCAAGACCTGCTAATTTCAAGGCTAA ZYP304 GTACCCAAGCTTCACATGTAGCCTATCACGTCTTCTGCTTCCTGAACAATTGTCC ZYP305 CACAAACAACTTCACCCAAACTGTCGGTTCGATGACGCCACTAAGACTAGTTAAGGTACCGGCGTAATCATGGTC PABD基因克隆 ZYP308 TCCGGACTCAGATCTCGAGC ZYP309 AGCTATAGTTCTAGATCTAGATTAACTAGTCTTAGTGGCGTC GFP基因克隆 ZYP306 TATCGATGGCGCCAGCTGAGGATGGTGAGCAAGGGCGA ZYP307 AGATCTGAGTCCGGACTTGTACAGCTCGTCCATCCGGACTCAGATCTCGAGC 转基因拟南芥鉴定 MSY01 TGGTGTTAGTTTCTAGTTTGTGCG MSY02 TCATCATGACAGATCTGCGC 表达量分析 ZYP389 GACCACTACCAGCAGAACACC ZYP390 CTTGTACAGCTCGTCCATGC -
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