桑树叶片中不同环氧鲨烯环化酶类基因的表达与三萜类化合物组成和含量的变化

Expression of Different Mulberry Oxidosqualene Cyclase(OCS)Genes and Composition and Content Variation of Triterpenes in Mulberry Leaves

由环氧鲨烯环化酶(OCS)催化的环化反应是氧化鲨烯合成三萜类化合物的第一步反应。通过对野生桑种川桑(Morus notabilis)、栽培桑种广东桑(Morus atropurpurea)桑叶中不同OCS基因的表达分析及桑叶中三萜类化合物组成和含量检测,探究桑树中三萜类化合物合成的关键酶。通过与拟南芥(Arabidopis thaliana)的OCS氨基酸序列进行比对,在川桑基因组中共鉴定得到12个OCS基因。半定量RT-PCR检测发现有9个OCS基因在川桑与广东桑的叶片中均有表达,其中MnPEN1、MnPEN2、MnPEN4基因的表达水平在2个桑种的叶片中差异较明显。通过气相色谱-质谱(GC/MS)分析发现川桑与广东桑叶片中的三萜类化合物组成有着较大差异,川桑的叶片中含有较多的α-香树素,广东桑叶片中的羊毛甾醇含量明显较高;而与这2种化合物合成相关的MnLAS、MnPEN1、MnPEN2、MnPEN4基因的表达水平在2个桑种之间也存在差异。推测在2个供试桑种的桑叶中,由于OCS基因表达水平的差异使相关三萜类化合物合成酶的活性不同,这是造成2个桑种桑叶中三萜类化合物组成和含量差异的原因之一。

The cyclization of oxidosqualene catalyzed by oxidosqualene cyclase（ OCS） is the first committed step in the synthesis of triterpenes. In this study,the expression profiles of different OCS genes and the composition and content of triterpene compounds in leaves of wild mulberry species Morus notabilis and mulberry cultivar Morus atropurpurea were analyzed to investigate the key enzymes involve in the synthesis of triterpene in mulberry. After alignment with amino acid sequences of Arabidopsis thaliana OCSs,12 OCS genes were identified from the genome of M. notabilis. Semi-quantitativeRT-PCR showed that 9 OCS genes were all expressed in leaves of M. notabilis and M. atropurpurea,among which the expression levels of MnPEN1,MnPEN2,and MnPEN4 genes were obviously different between two mulberry species.GC/MS analysis indicated that the composition of triterpenes was greatly varied in leaves of M. notabilis and M.atropurpurea. The content of α-amyrin was higher in leaves of M. notabilis,while the content of lanosterol was higherin leaves of M. atropurpureu. In addition,the expression levels of MnLAS,MnPEN1,MnPEN2 and MnPEN4 which were closely realted to the synthesis of above two compounds were different between two mulberry species. It is suggested that the different expression levels of OCS genes led to the various activities of triterpene synthase,which was one of the explanations for different composition and contents of triterpenes in leaves of two tested mulberry species.