Plant Hormones in Wood Formation -Novel Insights into the Roles of Ethylene and Gibberellins

Björklund, S. 2007. Plant hormones in wood formation –Novel insights into the roles of ethylene and gibberellins. Doctor’s dissertation, ISSN 1652-6880, ISBN 978-91-576-7380-0 The role of plant hormones in wood development has been studied for decades, and their crosstalk in many biological processes is the subject of increasing focus. In this thesis, modern biological tools have been used to provide novel insights into the roles of gibberellins and ethylene in wood formation in the model tree Populus tremula x tremuloides. A new idea describing the crosstalk between gibberellins (GAs) and indole-3-acetic acid (IAA) is presented. It is demonstrated that GAs stimulate polar IAA transport in secondary stems of Populus, a stimuli likely to be caused by the enhanced expression of the auxin efflux carrier PttPIN1. Extensive crosstalk between the two hormones is also suggested from global gene transcript analysis of GA and IAA regulated genes in Populus stem tissues. Most genes that were induced by GA were similarly induced by IAA, suggesting that both hormones can regulate similar biological processes, independently or in concert. To conclusively establish the role of endogenous ethylene in wood formation, a reverse genetics approach was used. Ethylene insensitive Populus trees were produced by transgenic expression of the mutated ethylene receptor Atetr1-1. These trees showed ethylene insensitivity to several wood related responses in pharmacological experiments. More interestingly, these trees were inhibited in their eccentric stem growth pattern in response to leaning. This is one of the characteristic phenotypes in tension wood formation, hypothesized to be ethylene mediated. This is the first conclusive demonstration of the function of endogenously produced ethylene in wood development, and the first documentation of the involvement of ethylene in meristematic activity in plants. We also use a transgenic approach to modify the expression of PttACCox, which is coding for the last enzyme in ethylene biosynthesis, and thereby provide solid evidence for its importance in regulating ethylene biosynthesis in wood forming tissue. Ethylene responses are believed to be mediated via a large number of so-called ethylene response factors (ERFs). Here all ERFs in the Populus genome were identified. Putatively important ERFs in stem tissues were found by analysing their expression pattern in response to ethylene. Further, ethylene responsive genes were identified in a global gene analysis.