PI: Brian Jones
Staff: Ioana Gaboreanu, postdoc; Veronica Bourquin, technician
As the vascular cambium (VC) is the ultimate source of the majority of forest-derived biomass, it is increasingly imperative that we understand how it functions. Surprisingly, despite a long tradition of anatomical and physiological analysis, almost nothing is known of the molecular bases of its establishment and function.
In contrast to animal cells, most plant cells are pluripotent. However, in practice, during normal plant growth and development, cells follow a path to differentiation, with the type and degree of differentiation dependent on both short and long-distance positional cues. Continued plant growth and organ initiation depends on the supply of pluripotent cells in the apical and lateral meristems. Meristems are analogous to animal stem cell niches. The VC is a circumferential, secondary, lateral meristem that provides cells for the growth in girth of woody species.
Our understanding of the structure and molecular functioning of the primary, apical meristems has advanced significantly in recent years, largely through the use of the model herbaceous plant, Arabidopsis thaliana. Similarly to animal stem cell niches, in the apical meristems, relatively undifferentiated and inactive stem cells periodically activate and divide. Asymmetric stem cell divisions produce one cell that retains stem cell characteristics and another that has a limited proliferation and differentiation potential, initially acting as a rapid-cycling transit-amplifying cell that provides the bulk of cells for tissue initiation and growth. Several important intercellular signalling mechanisms have been identified in the apical meristems. For example, the organising centre (OC) in the shoot and the quiescent centre (QC) in the root are groups of cells that abut and communicate with the stem cells via secreted signalling molecules to maintain a stable stem cell population and niche. Ablation of OC cells leads to a collapse of the niche and the terminal differentiation of stem cells.
Just as intercellular signalling is critical in the apical meristems, so too it is expected to be in the VC. The VC most likely consists of a single layer of stem cells. In rapidly growing stems, these cells activate and divide both anticlinally (for circumferential expansion) and periclinally (for an increase in stem diameter). Periclinal divisions would be asymmetric, producing a daughter with stem cell characteristics and another with a transit-amplifying role as a xylem or phloem mother cell. Intercellular signalling is predicted to be required for maintenance and controlled functioning of this stem cell niche. It should be necessary for example, for the maintenance of a stable stem cell population, to determine the rate and direction of cell division, and to determine which of the daughter cells of an asymmetric division will take on a transit-amplifying role.
How far do the analogies with the apical meristems go? What cell types constitute the VC stem cell niche? Are there cells which fulfil the role of an organising centre in the VC? Where do the signals that determine the rate and plane of division originate? What roles do niche components play in determining the outcome of the terminal differentiation of stem cell daughters? How do short and long distance signals combine to determine developmental and environmental response outcomes?
An important first step in the controlled optimization of the VC is to understand its structure and function. We are taking an integrated approach to the task. In mammalian systems, many of the genes important for the function of one stem cell niche are critical in the functioning of at least one other. Several apical meristem regulator homologues have been shown to be differentially expressed across the VC. These are being used to identify components of the VC through a reverse genetics approach in Poplar and Arabidopsis. To date, we have concentrated on genes from three families where suppression of at least one member of the family leads to an alteration in secondary growth in glasshouse-grown Poplars.
In addition, we are using Poplar and Arabidopsis lines that have cell-specific GFP expression. We have adapted the FACS technique for the isolation of fluorescent-labelled protoplasts from these lines and will analyse their transcriptomes, proteomes and metabolomes. This data will be combined with the Poplar expression data in the BASE database to identify cell-specific genes and the optimal candidates for further analysis of VC structure and signalling components in both Poplar and Arabidopsis.