Barakat Abdelali, Bagniewska-Zadworna Agnieszka, Choi Alex, Plakkat Urmila, DiLoreto Denis S, Yellanki Priyadarshini, Carlson John E
The School of Forest Resources, The Huck Institutes of the Life Sciences, Pennsylvania State University, 324 Forest Resources Building, University Park, PA 16802, USA.
BMC Plant Biol. 2009 Mar 6;9:26. doi: 10.1186/1471-2229-9-26.
Lignin is a phenolic heteropolymer in secondary cell walls that plays a major role in the development of plants and their defense against pathogens. The biosynthesis of monolignols, which represent the main component of lignin involves many enzymes. The cinnamyl alcohol dehydrogenase (CAD) is a key enzyme in lignin biosynthesis as it catalyzes the final step in the synthesis of monolignols. The CAD gene family has been studied in Arabidopsis thaliana, Oryza sativa and partially in Populus. This is the first comprehensive study on the CAD gene family in woody plants including genome organization, gene structure, phylogeny across land plant lineages, and expression profiling in Populus.
The phylogenetic analyses showed that CAD genes fall into three main classes (clades), one of which is represented by CAD sequences from gymnosperms and angiosperms. The other two clades are represented by sequences only from angiosperms. All Populus CAD genes, except PoptrCAD 4 are distributed in Class II and Class III. CAD genes associated with xylem development (PoptrCAD 4 and PoptrCAD 10) belong to Class I and Class II. Most of the CAD genes are physically distributed on duplicated blocks and are still in conserved locations on the homeologous duplicated blocks. Promoter analysis of CAD genes revealed several motifs involved in gene expression modulation under various biological and physiological processes. The CAD genes showed different expression patterns in poplar with only two genes preferentially expressed in xylem tissues during lignin biosynthesis.
The phylogeny of CAD genes suggests that the radiation of this gene family may have occurred in the early ancestry of angiosperms. Gene distribution on the chromosomes of Populus showed that both large scale and tandem duplications contributed significantly to the CAD gene family expansion. The duplication of several CAD genes seems to be associated with a genome duplication event that happened in the ancestor of Salicaceae. Phylogenetic analyses associated with expression profiling and results from previous studies suggest that CAD genes involved in wood development belong to Class I and Class II. The other CAD genes from Class II and Class III may function in plant tissues under biotic stresses. The conservation of most duplicated CAD genes, the differential distribution of motifs in their promoter regions, and the divergence of their expression profiles in various tissues of Populus plants indicate that genes in the CAD family have evolved tissue-specialized expression profiles and may have divergent functions.
木质素是次生细胞壁中的一种酚类杂聚物，在植物发育及其对病原体的防御中起主要作用。构成木质素主要成分的单木质醇的生物合成涉及多种酶。肉桂醇脱氢酶（CAD）是木质素生物合成中的关键酶，因为它催化单木质醇合成的最后一步。已在拟南芥、水稻中对CAD基因家族进行了研究，在杨树中也有部分研究。这是首次对包括基因组组织、基因结构、陆地植物谱系间的系统发育以及杨树中的表达谱分析在内的木本植物CAD基因家族进行全面研究。
系统发育分析表明，CAD基因分为三个主要类别（分支），其中一个类别由裸子植物和被子植物的CAD序列代表。另外两个分支仅由被子植物的序列代表。除PoptrCAD 4外，所有杨树CAD基因都分布在类别II和类别III中。与木质部发育相关的CAD基因（PoptrCAD 4和PoptrCAD 10）属于类别I和类别II。大多数CAD基因在物理上分布在重复块上，并且在同源重复块上仍处于保守位置。对CAD基因的启动子分析揭示了在各种生物和生理过程中参与基因表达调控的几个基序。CAD基因在杨树中表现出不同的表达模式，在木质素生物合成过程中只有两个基因在木质部组织中优先表达。
CAD基因的系统发育表明，该基因家族的辐射可能发生在被子植物的早期祖先中。杨树染色体上的基因分布表明，大规模和串联重复对CAD基因家族的扩展有显著贡献。几个CAD基因的重复似乎与杨柳科祖先中发生的一次基因组重复事件有关。与表达谱分析相关的系统发育分析以及先前研究的结果表明，参与木材发育的CAD基因属于类别I和类别II。来自类别II和类别III的其他CAD基因可能在生物胁迫下的植物组织中发挥作用。大多数重复的CAD基因的保守性、其启动子区域基序的差异分布以及它们在杨树植物各种组织中的表达谱差异表明，CAD家族中的基因已经进化出组织特异性表达谱，并且可能具有不同的功能。
