Plant Cell http://doi.org/8c7 (2015)
Trees live for decades, centuries or even longer, providing ample time for successful reproduction. However trees must contend with problems that shorter lived plants do not face, such as recovering from falling over. Gerttula et al. show that in poplars, a modular control network working through the transcription factor ARBORKNOX2 (ARK2) achieves the response needed to return trees to vertical growth.
When growing trees are laid horizontally they re-orientate by producing different types of wood on their upper and lower sides. While the lower sides of the trunk continues to make conventional wood, the upper produce specialized ‘tension’ wood capable of exerting additional forces to pull the stem upright. Trees with reduced levels of ARK2 recovered more slowly from being laid down than wild type and much slower than plants overexpressing ARK2. This was not due to any change in the number of tension fibres in the wood, instead those fibres matured more slowly in the reduced ARK2 lines.
By comparing these three lines Gerttula et al. were able to uncover some of the details of the trees self-righting behaviour. Gravity perception takes place in specialized amyloplast-containing cells, expressing the auxin transport protein PIN3. The resulting asymmetric distribution of auxin, in conjunction with the gibberellic acid from phloem tissue, leads to ARK2-dependent maturation of tension fibres and so bending of the trees' shoots upwards.