Kinetics of Phenylpropanoid gene expression in maize growing internodes: Relationships with cell wall deposition
Both for cattle nutrition and biofuel production, the improvement in maize (Zea mays L.) cell wall degradability depends on understanding the genetic mechanisms involved in the biosynthesis of phenylpropanoids. Most of the genes involved in monolignol and p-hydroxycinnamate biosynthesis are known, but many belong to multigene families. A macro-array with cell wall gene specific tags was used to characterize the different gene expression profiles in maize ear internode at four stages from 7 d before silking to 15 d after silking. Gene expression profiles were related to biochemical variation observed for lignin content, lignin structure, and esterified and etherified ferulic acid content. Most of the significantly expressed genes had a maximum at the first stages of sampling with their expression decreasing rapidly thereafter. A few genes had a second later expression peak. In each multigene family, only a restricted number of genes were expressed during maize cell wall formation in the below-ear internode. Genes for three phenylalanine ammonia-lyases, two cinnamate 4-hydroxylases, two 4-coumarate:coenzyme A ligases, three caffeoyl-CoA O-methyltransferases, but only one cinnamoyl-CoA reductase, two cinnamyl alcohol dehydrogenases, one ferulate 5-hydroxylase, the only caffeic acid O-methyltransferase, and a ZRP4-like O-methyltransferase were significantly expressed. These genes are likely the most important ones in maize stem lignification, and hence are priority targets in maize breeding.