Towards a Systems Approach for Lignin Biosynthesis in Populus trichocarpa: Transcript Abundance and Specificity of the Monolignol Biosynthetic Genes

As a step toward a comprehensive description of lignin biosynthesis in Populus trichocarpa, we identified from the genome sequence 95 phenylpropanoid gene models in 10 protein families encoding enzymes for monolignol biosynthesis. Transcript abundance was determined for all 95 genes in xylem, leaf, shoot and phloem using quantitative real-time PCR (qRT-PCR). We identified 23 genes that most probably encode monolignol biosynthesis enzymes during wood formation. Transcripts for 18 of the 23 are abundant and specific to differentiating xylem. We found evidence suggesting functional redundancy at the transcript level for phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), 4-coumarate:CoA ligase (4CL), p-hydroxycinnamoyl-CoA:quinate shikimate p-hydroxycinnamoyltransferase (HCT), caffeoyl-CoA O-methyltransferase (CCoAOMT) and coniferyl aldehyde 5-hydroxylase (CAld5H). We carried out an enumeration-based motif identification and discriminant analysis on the promoters of all 95 genes. Five core motifs correctly discriminate the 18 xylem-specific genes from the 77 non-xylem genes. These motifs are similar to promoter elements known to regulate phenylpropanoid gene expression. This work suggests that genes in monolignol biosynthesis are regulated by multiple motifs, often related in sequence.