Ethylene and Auxin in the Control of Wood Formation

Hellgren, J.M. 2003. Ethylene and auxin in the control of wood formation. Doctoral thesis. Silvestria 268. ISSN 1401-6230, ISBN 91-576-6502-8 This thesis considers aspects of the regulation of growth rate and fibre properties in forest trees. These properties are both genetically determined and influenced by environmental stimuli. Induction of reaction wood is an environmentally induced process involving changes in growth rate and fibre properties that can be readily studied. Plant hormones are signalling agents that play important roles in the initiation and coordination of wood formation; in this thesis the plant hormones auxin and ethylene were investigated using gas chromatography/mass spectrometry (GC/MS). A novel MS technique for measuring the ethylene precursor 1-aminocyclopropane-1carboxylic acid (ACC) in minute amounts of plant tissue was developed. Ethylene is often connected to stress responses in plants, and ethylene evolution is increased when reaction wood is formed. Here it is demonstrated that this increase is regulated by ACC oxidase, the enzyme catalysing the last step in the ethylene biosynthetic pathway. This is in contrast to most of the earlier findings that tended to indicate that ethylene production directly reflects the availability of ACC. Although ethylene is strongly up-regulated during reaction wood formation, its role in modulating the growth rate and fibre properties remains unknown. Further, it is demonstrated that reaction wood in both poplar (Populus tremula L.) and pine (Pinus sylvestris L.) is formed without changes in auxin concentration in the cambial tissues. This suggests that the previously held assumption that the difference in auxin concentration is key factor in the induction of reaction wood is unsound. Further, auxin concentrations were compared in hybrid aspen trees (Populus tremula L. x tremuloides Michx.) growing vertically at different growth rates. These trees showed good correlations between auxin levels and growth rates. The growth rate was mediated by increases in the cell cycling rate rather than in the width of the cell division zone. Thus, the growth rate in poplar was correlated to auxin levels in normal wood formation, but not during reaction wood formation.