Ethylene suppresses vessel element formation during the tension wood response of hybrid aspen

The functional specialization of xylem fibers and vessels for mechanical support and water transport, respectively, is part of the success which allowed Angiosperms to colonise more diverse habitats. Fibers and vessel elements are derived from the so-called fusiform cambial initials present within the vascular cambium, but the factors that specify the decision to form the one type over the other, is not clear. One case where fiber formation is favoured at the expense of vessel element formation, is during tension wood (TW) formation. Tension wood forms relative to the gravity vector after displacement of the stem of a tree, due to environmental factors like wind or erosion, and is characterized by stimulation of cambial growth on the upper side of stem. This asymmetric growth response requires functional ethylene signaling, and we set out to study whether ethylene is also involved in determining the fate of the cambial derivatives during TW formation. Application of 1-aminocyclopropane-1-carboxylic acid (ACC) as a soil drench to horizontally inclined hybrid aspen (Populus tremula L. x P. tremuloides Michx) trees had a dramatic effect on vessel formation during tension wood formation. Exogenous ACC initially blocked vessel formation but finally resulted in simultaneous formation of numerous vessels around the whole circumference of gravitationally stimulated stems. Quantification of vessel element density in transgenic, ethylene insensitive hybrid aspen trees as well as in newly generated transgenic trees heterologously overexpressing the Arabidopsis thaliana ETHYLENE-INSENSITIVE2 C-terminal region (EIN2Cend) supported the role of ethylene in suppressing vessel formation during tension wood formation. The two types of transgenic trees also displayed opposite uprighting responses of the stem. While the ethylene insensitive trees had a severely impaired ability to upright after horizontal inclination, an EIN2C overexpressing line showed a tendency towards a faster response. In conclusion, our results reveal an important role for ethylene in determining the fate of the cambial initials, determining the ratio between fibers and vessels in gravitationally stimulated stems. Furthermore, the results indicate that vessel density might contribute functionally in the response to gravitational stimulus.