Although the cell wall is a vital structural component of plants regulating cell volume and shape, little is known about its biosynthesis. The plant cell wall consists of a network of cellulose microfibrils cross-linked by glycans. This cellulose-glycan network is embedded in a matrix made up predominantly of polysaccharides and proteoglycans. Pectin forms a major component of this matrix and is also found in the middle lamella between cells where it has a role in regulating cell-cell adhesion. The cell wall is important in human nutrition and health as well as forming the raw material for a variety of economic processes such as paper and textile manufacture. Additionally, polysaccharides extracted from cell walls form the basis for a variety of gels, thickeners and adhesives. A detailed understanding of processes controlling plant cell wall formation is vital in order to improve current products and to develop more environmentally friendly processing methods.

 

The importance of the cell wall to plants is highlighted by the proportion of genes dedicated to the synthesis and breakdown of the wall components. An analysis of the Arabidopsis genome has predicted that over 700 genes (around 2.5% of the total genome) encode carbohydrate-active enzyme classes of glycoside hydrolases and glycosyltransferases (Henrissat et al., ). Traditionally, identification of cell wall biosynthetic enzymes has relied on time consuming biochemical purification of the enzyme followed by protein sequence determination and gene isolation. However, bioinformatics coupled with reverse genetics provides a powerful alternative strategy. We are using 2 model systems to study cell wall biosynthesis – Arabidopsis thaliana and the hybrid aspen (Populus tremula x P. tremuloides). Transcript profiling of the Poplar cambium has identified a number of genes upregulated in the zone of cell division and elongation (Hertzberg et al., ) that represent candidates to be involved in primary cell wall formation. Comparative bioinformatics has been used to identify Arabidopsis orthologues of a number of these upregulated Poplar transcripts and knockouts obtained using a reverse genetics approach. A number of these mutants show a visible phenotype and work is currently under way to characterise them and to understand the role that they play in the synthesis of the cell wall. Of particular interest is the role that the cell wall plays in signalling and providing positional information mediating patterns of development in root and shoot tissues.

The auxin indole-3-acetic acid (IAA) is an important regulator of many aspects of plant development including cell division and elongation, apical dominance, vascular development, lateral root formation and root gravitropism. An understanding of the formation, transport, perception and signal transduction of auxin represents a key factor in our understanding of plant development.

 

In recent years a number of laboratories have studied the cellular processes governing the movement of auxin from sites of synthesis to sites of action. This has resulted in the identification of putative auxin influx (AUX1/LAX family) and efflux (PIN family) carrier proteins which facilitate the entry and exit of auxin from the cell respectively (see Bennett et al., ; Gälweiler et al., ). Within the protophloem cell files of the Arabidopsis root it has been found that AUX1 localises to the basal end of the cells and PIN1 to the apical end, thus providing a directional movement of auxin towards the root tip (Swarup et al., ). Further studies have provided a functional role for AUX1 in regulating root gravitropism (Marchant et al., ) and lateral root formation (Casimiro et al., ).

 

The axr4 mutant of Arabidopsis was first described as having reduced root gravitropism and a 50% reduction in the number of lateral roots (Hobbie & Estelle ). Interestingly the axr4/aux1 double mutant displays an additive effect having only 10% of the number of wild type lateral roots. AXR4 therefore represents an important component in our understanding of auxin regulated growth. Efforts in the laboratory at Umeå are currently focusing on a map based strategy to clone the AXR4 gene. This will allow detailed studies on mechanism of AXR4 action to be carried out. Strategies will be adopted to identify the role of AXR4 in auxin signalling. This will include a proteomics based approach to identify any other protein components which interact directly with AXR4 to further elucidate the auxin signalling machinery. A reverse genetics approach will be adopted to study the role of any closely related Arabidopsis orthologues.

1. Bennett, M. J., Marchant, A., Green, H. G., May, S. T., Ward, S. P., Millner, P. A., Walker, A. R., Schultz, B. & Feldmann, K. A. () Arabidopsis AUX1 gene: A permease-like regulator of root gravitropism. Science, 273: 948-950.

 

2. Casimiro, I*., Marchant, A*., Bhalerao, R.P., Beeckman, T., Dhooge, S., Swarup, R., Graham, N., Inzé, D., Sandberg, G., Casero, P. & Bennett, M. () Plant Cell 13: 843-852. *Joint first authors.

 

3. Hobbie, L. & Estelle, M. (). The axr4 auxin resistant mutants of Arabidopsis thaliana define a gene important for root gravitropism and lateral root initiation. Plant J. 7: 211-220.

 

4. Marchant, A., Kargul, J., May, S. T., Muller, P., Delbarre, A., Perrot-Rechenmann, C. & Bennett, M. J. () AUX1 regulates root gravitropism in Arabidopsis by facilitating auxin uptake within root apical tissues. EMBO J., 18: .

 

5. Müller, A., Guan, C., Gälweiler, L., Tänzler, P., Huijser, P., Marchant, A., Parry, G., Bennett, M. J., Wisman, E. & Palme, K. () AtPIN2 defines a locus of Arabidopsis for root gravitropism control. EMBO J., 17: .

 

6. Gälweiler, L., Guan, C., Müller, A., Wisman, E., Mendgen, K., Yephremov, A. & Palme, K. () Regulation of polar auxin transport by AtPIN1 in Arabidopsis vascular tissue. Science 282: .

 

7. Henrissat, B., Coutinho, P. & Davies, G. () A census of carbohydrate-active enzymes in the genome of Arabidopsis thaliana. Plant Mol. Biol. 47: 55-72.

 

8. Swarup, R., Friml, J., Marchant, A., Ljung, K., Sandberg, G., Palme, K. & Bennett, M. () Localization of the auxin permease AUX1 suggests two functionally distinct hormone transport pathways operate in the Arabidopsis root apex Genes & Dev. 15: .

 

Other selected publications

9. Barlier, I*., Kowalczyk, M*., Marchant, A*., Ljung, K., Bhalerao, R., Bennett, M., Sandberg, G. & Bellini, C. () SUR2 gene of Arabidopsis thaliana, encodes the cytochrome P450 CYP83B1 – a modulator of auxin homeostasis. Proc. Natl. Acad. Sci. U.S.A. 97: . * joint first authors.

 

10. Marchant, A*, Bhalerao, R*., Casimiro, I., Eklöf, J., Casero, P., Bennett, M. & Sandberg, G. () AUX1 promotes lateral root formation by facilitating indole-3-acetic acid distribution between sink and source tissues in the Arabidopsis seedling. Plant Cell 14: 589-597. *joint first authors

 

11. Bhalerao, R.P., Eklöf, J. Ljung, K., Marchant, A., Bennett, M. & Sandberg, G. () Shoot derived auxin is essential for early lateral root emergence in Arabidopsis seedlings. Plant J. 29: 325-332.