Molecular regulation of vascular cambium identity and activity

Sammanfattning: In plants, secondary development and wood formation originates from the cell divisions within the vascular meristem, where the vascular stem cells are located. This thesis work presents my results on the molecular regulation of vascular cambium stem cell identity and activity. I have investigated the role of the receptor-like kinase PXC1 during vascular development in Arabidopsis thaliana. Mutant analysis revealed that in the absence of PXC1, plants display a pendant phenotype and reduced secondary cell wall thickening and lignification in the inflorescence stems, showing that PXC1 is an important regulator of secondary cell wall formation in Arabidopsis. I also participated in the characterization of members of the TDIF/CLE41/CLE44-TDR/PXY-WOX4 signaling module in hybrid aspen. Functional studies showed that knock down of PtWOX4 paralogs inhibits vascular cambium activity and secondary xylem formation in transgenic trees. Moreover, over-expression of PtCLE41A and related genes induces vascular patterning defects, highly associated with ectopic cambial activity. Results from transcriptional analysis suggested that PtCLE41A and related genes positively regulate PtWOX4 during the regulation of vascular cambium activity. By analysing gene expression patterns in Norway spruce, I provided evidence for the existence of conserved mechanisms in angiosperm and gymnosperm tree species with regard to the regulation of cambium function through CLE41, TDR/PXY and WOX4-like genes. Finally, I also identified different PtLCLE genes as candidate regulators of vascular cambium activity and tree growth. I studied functions of these genes by employing transgenic approaches in hybrid aspen and showed that down regulation or up regulation of these genes affect many different phenotypic properties in hybrid aspen such as cambium activity, internode elongation, leaf size, and adventitious rooting. In conclusion, results of these projects provide new insights into the regulation of vascular cambium activity and wood formation.