Wood has evolved to transport water, give mechanical strength and to withstand abiotic and biotic stresses during the long life span common to many forest trees. To fulfil these needs wood is composed of a variety of cell types, each of which exhibits a large degree of phenotypic plasticity both in morphology and chemistry (Fig. 1). All components of the wood originate from the vascular cambium, the lateral meristem between the xylem and phloem. Important fibre and wood properties such as annual ring width, fibre morphology and cell wall structure and chemistry are determined during the development of cambial derivatives. Knowledge about the physiological and genetic mechanisms controlling cell division, expansion and secondary wall formation is required to understand wood variations and wood cell properties. We are exploiting the advantages provided by poplar and Arabidopsis as model plants, and combining physiological and molecular techniques and expertise for our research in this field.

 

The role of plant hormones
Auxin (IAA) is a key regulator of cambial growth. Polarly transported auxin maintains the cambial meristem and stimulates cambial cell division and expansion of derivatives. We have used mass spectrometry in combination with tangential cryosectioning to demonstrate that IAA is distributed in a steep radial concentration gradient across developing xylem and phloem elements (Fig. 2). 

 

From this observation we suggest that the IAA gradient provides positional information responsible for pattern formation in cambial growth. This idea was further supported by studies in which Agrobacterium tumefaciens IAA biosynthetic genes were expressed in transgenic hybrid aspen under the control of different promoters. From the visualisation of auxin in cambial region tissues under different growth conditions, we have proposed a model for its role in controlling seasonal and spatial growth patterns of wood formation in tree stems.

 

The roles of growth regulators other than auxin in cambial growth are not so well characterised. We have initiated research on ethylene, and its possible interaction with auxin, in cambial growth. Ethylene is particularly strongly induced during mechanical stress and bending, which cause eccentric growth and reaction wood to be formed in tree stems. The reaction wood in angiosperms (tension wood) is of particular interest due to its accumulation of cellulose. We have developed a micro-scale GC-MS technique for analysing ACC in cambial tissues, and cloned a novel ACC oxidase from poplar that is expressed in developing xylem cells. Bending poplar stems induces increases in ACC levels and ACC oxidase transcripts, and alters the IAA balance. An EST library from tension wood and transgenic poplar plants will be used to further study this response.

 

 

Arabidopsis as a model system for secondary xylem development
We have exploited Arabidopsis as a useful model for secondary growth. Arabidopsis produces large amounts of secondary xylem when grown in appropriate conditions. We have demonstrated that the ethylene receptor mutant ein4 is associated with deficiency in xylem fibres, suggesting that ethylene plays a role in fibre development. We are currently using other mutants to investigate the function of hormones in cambial growth. Other advantages of Arabidospis, such as the ease of generating Arabidopsis transgenics and screening for novel mutants of the species, can be used in cambial research.

 

Uggla C., Moritz T., Sandberg G & Sundberg B () Auxin as a positional signal in pattern formation in plants. Proc. Natl. Acad. Sci. USA 93: .

 

Moritz, T. & Sundberg, B. () Identification and quantification of cytokinins in active and dormant cambial region tissues from Pinus sylvestris (L.) stems. Physiol. Plant. 98: 693-698.

 

Tuominnen, H., Puech, L., Fink, S. & Sundberg, B. () A radial concentration gradient of indole-3-acetic acid is related to secondary xylem development in hybrid aspen. Plant Physiol. 115: 577-585

 

Sterky, F., Regan, S., Karlsson, J., Hertzberg, M., Rohde, A., Holmberg, A., Amini, B., Bhalerao, R., Larsson, M., Villarroel, R., Van Montagu, M., Sandberg, G., Olsson, O., Teeri, T., Boerjan, W., Gustafsson, P., Uhlén, M., Sundberg, B. , & Lundeberg, J. () Gene discovery in the wood-forming tissues of poplar: analysis of expressed sequence tags. Proc. Natl. Acad. Sci. USA 95: .

 

Uggla, C., Mellerowicz, E., & Sundberg B. () Endogenous indole-3-acetic acid controls cambial growth by positional signalling in Pinus sylvestris (L.). Plant Physiol. 117: 113-121.

 

Sundberg, B. & Uggla., () Origin and dynamics of indoleacetic acid under polar transport in Pinus sylvestris. Physiol. Plant. 104:22-29.

 

Regan, S., Bourquin, V. Tuominen H & Sundberg, B () Accurate and high resolution in situ hybridisation analysis of gene expression in secondary stem tissues. Plant Journal 19:363-368.

 

Tuominen, H., Olsson, O. & Sundberg. B () Genetic engineering of wood formation; Expression of bacterial IAA-biosynthetic genes in hybrid aspen (Populus tremula x P. tremuloides. In: Molecular Biology of Woody Plants. (eds. Jain, S.M., Minocha, S.C.) Klüwer Publishers, The Netherland.

 

Sundberg, B. , Uggla, C. & Tuominen, H. () Cambial growth and auxin gradients. In: Cambium: the Biology of Wood Formation. (eds. Savidge, R., Barnett, J., Napier, R.). BIOS Scientific Publishers, UK. (In press).

 

Tuominen, H., Peuch, L., Regan, S., Fink, S., Olsson, O. & Sundberg, B. () Cambial-region specific expression of the Agrobacterium iaa genes in transgenic Populus visualized by a linked uidA reporter gene. Plant Physiol. (in press).