The vascular cambium is the wood-forming tissue in woody plants. The cambium is a uniseriate layer that gives rise to xylem and phloem (Fig. 1). Cell division and differentiation in the vascular cambium are regulated by both internal and external factors. One of the most critical factors regulating division and differentiation in the cambium is the plant growth regulator Indole-acetic acid (IAA). The main interest of my co-workers is to understand how IAA controls cell division and differentiation in the vascular cambium in conjunction with environmental factors. In my group we work on the following related projects:

- Regulation of cambial dormancy
- Control of vascular patterning
- Cell fate determination during vascular development

Regulation of cambial dormancy: The vascular cambium of woody plants displays a cyclical pattern of activity with cell division beginning in late April-May and continuing until late August-September (in the northern hemisphere). Thus, from September to April the vascular cambium is considered to be dormant, and it shows no detectable cell division. In this project we are trying to understand how the internal (e.g. IAA) and external factors (e.g. temperature and light) interact to regulate cambial activity. Dormancy of the cambium can be divided into two phases; rest and quiescence. In woody plants it has been shown that IAA can stimulate cell division in the vascular cambium. What distinguishes the rest phase of dormancy from quiescence is that during the rest phase of dormancy, IAA is unable to stimulate cambial cell division, whereas it can in the quiescent phase (Bonga and Little ). These observations imply that cambial sensitivity to IAA plays an important role in regulating cell division. We are investigating how the sensitivity of the cambium to IAA is regulated in hybrid aspen. In particular we are addressing three questions:

- What is the molecular basis of the differences between rest and quiescence?
- What are the targets of dormancy inducing signals?
- How is the cambium maintained in the dormant state?

Our approach to address these questions has been to clone important regulators of the cell cycle and investigate their regulation during the entry of cambium into dormancy. We have cloned genes that are expressed in dormant and active cambium, and are in the process of altering their expression in hybrid aspen (Poplar) and Arabidopsis to find out whether we can override the environmental control of cell division in the vascular cambium.

Control of vascular patterning: The vascular cambium gives rise to xylem mother cells, which undergo division, expansion and cell death to form the constituents of the mature xylem. There is a spatial and temporal separation of these different developmental processes giving rise to the characteristic patterning seen in the xylem tissue. We are interested in understanding how the temporal and spatial separation of molecular events occurring during xylem formation is regulated in the cambium. A hypothesis to explain patterning of the vascular cambium has been proposed based upon the distribution of IAA in it. It has been shown by Uggla et al () that IAA is distributed in a radial concentration gradient. This result indicates that such a radial distribution of IAA may serve to control the timing and cellular expression of the different regulatory genes which, in turn, would control patterning in the xylem. Thus, understanding the patterning in xylem would require understanding of the signal transduction pathways controlled by IAA. In order to understand how IAA controls vascular patterning, we are identifying genes that act as positive or negative regulators of IAA signal transduction from poplar. To date, 12 such candidate genes have been identified and cDNAs and promoters of the corresponding genes have been cloned. Future work will involve altering the expression pattern of the candidate genes (in terms of both level and tissue specificity) to investigate the effect on xylem formation.

Cell fate determination in the vascular cambium: The cambium is a uniseriate layer which gives rise to two different types of tissue, namely xylem and phloem. At the moment very little is known about the molecular mechanisms underlying the formation of xylem and phloem from the cambium. The formation of two different cell types from a single cell has been well studied in organisms such as Drosophila and C. elegans, and related regulators have been found to play a critical role in cell fate determination in these two model organisms. These results indicate that basic mechanisms and regulators of cell fate determination may be conserved between different organisms. One such regulator in Drosophila is the product of the gene Pumilio: an mRNA binding protein that regulates the translation of Hunchback and controls abdomen formation. We have identified genes related to Pumilio from the cambial region of poplar and Arabidopsis and are investigating the role of these genes in xylem and phloem formation using sense and antisense expression, as well as reverse genetics in Arabidopsis thaliana. 

 

Umeda M., Bhalerao R.P., Schell J., Uchimiya H., Koncz C. () A distinct cyclin-dependent kinase-activating kinase of Arabidopsis thaliana. PNAS 95:

 

Németh K., Salchert K., Putnoky P., Bhalerao R.P., Koncz-Kálmán Z., Stankovic-Stangeland B., Bako L., Ökrész L., Stabel S., Geigenberger P., Stitt M., Rédei G.P., Schell J., Koncz C. () Pleiotropic control of glucose and hormone responses by PRL1, an alpha-importin binding WD-protein, in Arabidopsis. Genes and Development 12:

 

Bhalerao R.P., Salchert K., Bako L., Muranaka T., Machida Y., Ökrész L., Schell J., Koncz C. () PRL1-binding regulates the activity of glucose responsive plant SNF1 protein kinase homologs. PNAS 96: .