SummaryWe have performed transcript and metabolite profiling of isolated cambial meristem cells of the model tree aspen during the course of their activity-dormancy cycle to better understand the environmental and hormonal regulation of this process in perennial plants. Considerable modulation of cambial transcriptome and metabolome occurs throughout the activity-dormancy cycle. However, in addition to transcription, posttranscriptional control is also an important regulatory mechanism as exemplified by the regulation of cell-cycle genes during the reactivation of cambial cell division in the spring. Genes related to cold hardiness display temporally distinct induction patterns in the autumn which could explain the step-wise development of cold hardiness. Factors other than low temperature regulate the induction of early cold hardiness-related genes whereas abscisic acid (ABA) could potentially regulate the induction of late cold hardiness-related genes in the autumn. Starch breakdown in the autumn appears to be regulated by the 'short day' signal and plays a key role in providing substrates for the production of energy, fatty acids and cryoprotectants. Catabolism of sucrose and fats provides energy during the early stages of reactivation in the spring, whereas the reducing equivalents are generated through activation of the pentose phosphate shunt. Modulation of gibberellin (GA) signaling and biosynthesis could play a key role in the regulation of cambial activity during the activity-dormancy cycle as suggested by the induction of PttRGA which encodes a negative regulator of growth in the autumn and that of a GA-20 oxidase, a key gibberellin biosynthesis gene during reactivation in spring. In summary, our data reveal the dynamics of transcriptional and metabolic networks and identify potential targets of environmental and hormonal signals in the regulation of the activity-dormancy cycle in cambial meristem.

We report the first comprehensive analysis of the effects of elevated [CO2] on gene expression in source leaf and stem sink tissues in woody plants. We have taken advantage of coppiced Populus deltoides (Bartr.) stands grown for 3 years under three different and constant elevated [CO2] in the agriforest mesocosms of Biosphere 2. Leaf area per tree was doubled by elevated [CO2] but although growth at 800 v. 400 µmol mol–1 CO2 resulted in a significant increase in stem biomass, growth was not stimulated at 1200 µmol mol–1 CO2. Growth under elevated [CO2] also resulted in significant increases in stem wood density. Analysis of expression data for the 13 490 clones present on POP1 microarrays revealed 95 and 277 [CO2]-responsive clones in leaves and stems respectively, with the response being stronger at 1200 µmol mol–1. When these [CO2]-responsive genes were assigned to functional categories, metabolism-related genes were the most responsive to elevated [CO2]. However within this category, expression of genes relating to bioenergetic processes was unchanged in leaves whereas the expression of genes for storage proteins and of those involved in control of wall expansion was enhanced. In contrast to leaves, the genes up-regulated in stems under elevated [CO2] were primarily enzymes responsible for lignin formation and polymerisation or ethylene response factors, also known to induce lignin biosynthesis. Concomitant with this enhancement of lignin biosynthesis in stems, there was a pronounced repression of genes related to cell wall formation and cell growth. These changes in gene expression have clear consequences for long-term carbon sequestration, reducing the carbon-fertilisation effect, and the potential for increased lignification may negatively impact on future wood quality for timber and paper production.