Expression of the Genes in the Cactaceae Cambial Zone Suggests Their Involvement in Wood Development.

The vascular cambium is a lateral meristem that produces secondary xylem (i.e., wood) and phloem. Different Cactaceae species develop different types of secondary xylem; however, little is known about the mechanisms underlying wood formation in the Cactaceae. The gene family encodes transcription factors that regulate plant development. The role of class I genes in the regulation of the shoot apical meristem, inflorescence architecture, and secondary growth is established in a few model species, while the functions of class II genes are less well understood, although the class II KNOX protein KNAT7 is known to regulate secondary cell wall biosynthesis. To explore the involvement of the genes in the enormous variability of wood in Cactaceae, we identified orthologous genes expressed in species with fibrous ( and ), non-fibrous (), and dimorphic () wood. Both class I and class II genes were expressed in the cactus cambial zone, including one or two class I paralogs of , as well as one or two class II paralogs of --. While the gene ( and its ortholog are expressed during secondary growth in the and stem, respectively, we did not find orthologs in the Cactaceae cambial zone, which suggests possible differences in the vascular cambium genetic regulatory network in these species. Importantly, while two class II paralogs from the clade were expressed in the cambial zone of and , we did not detect ortholog expression in the cambial zone of . Differences in the transcriptional repressor activity of secondary cell wall biosynthesis by the orthologs could therefore explain the differences in wood development in the cactus species.