Identification of Genes Differentially Expressed in Shoot Apical Meristems and in Mature Xylem of Populus tomentosa

Shoot apical meristems (SAM) produce stem cells, lateral organs, and secondary meristems (i.e., vascular cambium). Molecular mechanisms of wood formation derived from the vascular cambium have been extensively investigated; however, transcriptome characterization of SAM involved in wood formation in the axial direction remains limited. In this study, shoot apices (SAM with adjacent short shoot) and mature xylem were sampled from apical buds and main stems of Populus tomentosa seedlings, respectively, and their gene expression profiles were compared using the Affymetrix Poplar Genome GeneChip. In total, 9,823 genes (~16 %) were identified with differential expression in the two tissues. Many genes related to primary cell wall expansion and pectin biosynthesis were preferentially expressed in shoot apices, such as expansin, xyloglucan endotransglucosylase/hydrolase, pectinacetylesterase, pectin methylesterase, and plant invertase/pectin methylesterase inhibitor. In contrast, genes involved in lignin and cellulose biosynthesis of secondary cell walls were more transcribed in mature xylem, including 4-coumarate: CoA ligase, cinnamate-4-hydroxylase, cinnamyl alcohol dehydrogenase, phenylalanine ammonia-lyase, laccase, caffeoyl CoA 3-O-methyltransferase, S-adenosylmethionine synthetase, cellulose synthase (CesA), and glucan synthase-like. Interestingly, two cell cycling-related genes, cyclin-dependent kinase (CDK) and cyclin, were preferentially expressed in shoot apices that underwent active cell division, while an inhibitor of CDK (cyclin-dependent kinase inhibitor) was more transcribed in mature xylem. Furthermore, different members of the same gene families (i.e., pectin lyase-like, glycosyl hydrolase, UDP-glycosyltransferase, arabinogalactan proteins, FASCICLIN-like arabinogalactan proteins, hydroxyproline-rich glycoprotein, tubulins, NACs, CesA-like) had differential expression in the two tissues, indicating their diverse functions in either primary or secondary cell wall formation. The identified differentially expressed genes could provide important clues for the understanding of axial wood formation originated from SAM in forest trees.