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University of Helsinki Institute of Biotechnology
Wood Development Group
Contact Information:
Wood Development Group Institute of Biotecnology Viikinkaari 1 (P.O.Box 65)
00014 University of Helsinki
Tel: +358-9-1911
fax +358-9-1915 9366

Yrjö Helariutta
+358-9-191 59422

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Arabidopsis as a model for wood development

We are approaching wood development in two systems. Although Arabidopsis is not a tree, all the major elements of wood development are observed during Arabidopsis ontogeny.

1. Anatomy
We are focusing on the primary development (procambium) of the vascular tissue in the Arabidopsis root, since this is anatomically a very simple developmental model.

First of all, we have determined the cell-lineage relationships of the vascular tissue of Arabidopsis root (Mähönen et al., 2000). We have shown that the vascular tissue is established through a series of asymmetric cell divisions in the procambium. The pattern of these divisions around the developing xylem provides an intriguing parallel to the pattern of cambial cell divisions between phloem and xylem, which are very prominent in wood and storage root development.

2. Mutants
Subsequently, we have been searching for and characterizing of mutations that are informative for formative and/or proliferative functions related to procambial development of Arabidopsis root.

3. Molecular control of cell proliferation of the vascular tissue
Our first research focus has been the molecular analysis of the WOODEN LEG (WOL) locus. This locus was first identified through a recessive mutation in Dr. Philip Benfey’s lab in the New York University (where Y.H. carried out his post-doctoral work during 1995-1998) and the phenotype was primarily characterized by Dr. Ben Scheres’s lab (Utrecht University). The primary effect of the wol mutation is the lack of formative cell divisions required for organization of the vascular tissue in the procambium (Scheres et al. 1995). This is already evident during embryogenesis and results in too few cell files of the root, indicating that the role of the WOL gene is to promote the proliferation of the vascular tissue during embryogenesis and root development in Arabidopsis. As a result of a positional cloning effort, we have recently identified the WOL gene (Mähönen et al. 2000). The WOL gene belongs to a novel gene family, encoding two component signal transducers with a putative extracellular receptor domain (homologous to the DhkA of Dictyostelium discoideum; Wang et al.1996) and a cytoplasmic phosphorelay motif that consists of a histidine kinase like domain and two receiver domains in an unusual tandem arrangement. Therefore, we postulate that the WOL gene product is a receptor molecule that controls the asymmetric cell divisions of the vascular initials through a specific signal transduction pathway. WOL is expressed specifically in the vasculature from the early stages of embryogenesis on, consistent with a sensory role in a signal transduction pathway controlling the root vascular morphogenesis.

Dr. Tatsuo Kakimoto’s group (Osaka University) has shown that WOL is the same gene as CRE1 and it functions as a cytokinin receptor (Inoue et al., 2001). Together our results indicate that cytokinins play an essential role in the proliferation of vascular tissue, at least in Arabidopsis root. Dr. Takeshi Mizuno’s group (Nagoya University) is also active in the analysis of CRE1/WOL like genes (Suzuki et al., 2001; Ueguchi et al., 2001). In an interaction with various labs, we are currently investigating the role of CRE1/WOL like genes in regulating vascular development in Arabidopsis. Interestingly, we have found CRE1/WOL like genes to be active also in the cambial zone of the trunk of the silver birch (Betula pendula) tree. It is therefore possible that some aspects of the regulation of cell proliferation during vascular development in Arabidopsis root and in trunk of a tree are common.

4. Molecular control of vascular tissue identity
In order to address the genetic control of pattern formation, we have identified a locus, ALTERED PHLOEM DEVELOPMENT (APL) in Arabidopsis (Bonke, Thitamadee et al. 2003). In apl mutants, there is absence of phloem accompanied with development of supernumerary cells with wood characteristic. APL encodes a transcription factor that is expressed specifically in the developing phloem tissue. Furthermore, when expressed under a heterologous WOL promoter conferring a spatially broader specificity, APL inhibits the differentiation of xylem tissue. Taken together, this suggests that APL has a dual role both in promoting phloem differentiation and in repressing wood formation in a wrong location in relation to cambium.

5. Further development of Arabidopsis root as a model for vascular development and wood formation
Through the analysis of the WOL and APL genes in the context of vascular development, we have started to broaden the concept of root as a model for organogenesis in plants (originally established by Benfey et al. 1993, Scheres et al. 1994, Dolan et al. 1993, Berleth and Jurgens, 1993). Our analysis of the cell lineage relationships in the vascular tissue of the root indicates a stereotypic pattern of asymmetric cell divisions that form the basis of the patterning of the vascular tissues in the root (Mähönen et al.). This provides a firm anatomical basis for further understanding of the cellular basis of vascular development in Arabidopsis. Consequently, we are going to continue the characterization of vascular patterning based on various genetic and cellular methods.

6. Functional Genomics of Phloem Development

Benfey et al. 1993 Development. 119: 57.
Berleth and Jurgens 1993 Development 118: 575.
Bonke, Thitamadee et al. 2003, Nature 426: 181-186
Dolan et al. 1993 Development 119: 71.
Inoue et al. 2001 Nature 409: 1060.
Mähönen et al. 2000 Genes Dev 14: 2938
Scheres et al. 1994 Development 120: 2475.
Scheres et al. 1995 Development 121: 53.
Suzuki et al. 2001 Plant Cell Physiol 42: 107.
Ueguchi et al. 2001 Plant Cell Physiol 42:231.