Abstract

BackgroundLignin 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.ResultsThe 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.ConclusionThe 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.

Highlights

  • Lignin is a phenolic heteropolymer in secondary cell walls that plays a major role in the development of plants and their defense against pathogens

  • cinnamyl alcohol dehydrogenase (CAD) gene family organization In model species for which the genome is completely sequenced, 71 CAD genes have been identified to date: 9 in Arabidopsis [36], 12 in Oryza [30], 15 in Populus, 18 in Vitis, and 17 in Medicago

  • Additional file 1 includes the list of these CAD gene names based on the standard established by the International Populus Genome Consortium (IPGC)[35] with the names of species (Poptr for Populus trichocarpa for example), the protein name (CAD), and a designation of family and clade memberships derived from this study

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Summary

Introduction

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 cinnamyl alcohol dehydrogenase (CAD) is a key enzyme in lignin biosynthesis as it catalyzes the final step in the synthesis of monolignols. Lignin is a phenolic heteropolymer that provides plant cells with structural rigidity, a barrier against insects and other pestilent species, and hydrophobicity [1,2,3,4]. The composition of lignin consists of various phenylpropanoids, predominantly the monolignols p-coumaryl, coniferyl, and sinapyl alcohols. Lignin contains guaiacyl subunits (G units) and p-hydroxyphenyl units (H units) polymerized from coniferyl alcohol and from p-coumaryl alcohol respectively. In addition to Gunits and some H-units [7], syringyl units (or S-units) polymerized from sinapyl alcohol. There are exceptions found within each group [7] and variation in lignin composition can even occur between cell types within the same plant

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