Lifting response of hybrid aspen: Time-lapse video showing 28 days of the tension wood response of a wild type hybrid aspen (Populus tremula x P. tremuloides). Video created by Bernard Wessels.

When a tree is placed horizontally, it starts to bend to lift itself back upright towards the light. A special type of wood, called tension wood, is formed in the bending region and works like a contracting muscle to lift the tree up. Bernard Wessels showed in his PhD thesis that the plant hormone ethylene is required for the up-lifting response in hybrid aspen. He also identified new components that are involved in the regulation of this process. He defended his PhD thesis at Umeå University on the 5th of October. 

“We analysed genetically modified hybrid aspen trees that were altered in their response to ethylene”, says Bernard Wessels. “These trees were placed horizontally to induce the up-lifting response and so the formation of tension wood. Over a period of four weeks, we took pictures in regular intervals to monitor how the trees lift themselves up. We combined all pictures into a time-lapse video and compared the up-lifting response of the mutants with a non-modified control. Mutants that lost the ability to sense the ethylene signal struggled to lift themselves up from the surface of the table while mutants that were more sensitive to ethylene lifted themselves up faster.”

Bernard Wessels also found that ethylene influences the frequencies of the different xylem cell types in the tension wood. Hybrid aspen wood consists mainly of two cell types; the vessel elements and the fibers, and ethylene was shown here to favour the formation of the fibers over the vessel elements. “We think that the mechanical strength of the fibres is needed to lift the tree up”, explains Bernard Wessels.”

In another approach, Bernard Wessels and his colleagues used a large dataset that contains information about which genes are active during different stages of wood formation. They screened for genes that are involved in the communication of the ethylene signal. In all different stages of wood formation, they found genes that were related to ethylene, and they identified and characterized new genes that are involved in the communication of the ethylene signal.

Bernard Wessels has performed his graduate studies at the Umeå Plant Science Centre, Department of Plant Physiology, Umeå University. His results help to understand how wood develops in trees that are displaced from their original position due to mechanical stimulus such as wind. This knowledge is very interesting for the forestry industry and might be needed to select for trees that can deal better with extreme variations in weather.

Link to the thesis: urn:nbn:se:umu:diva-151724

What is ethylene?
Did you ever experience that your green bananas ripened faster when you placed them next to an apple? This is caused by the plant hormone ethylene that is produced by the ripening apple. Ethylene is a colourless gas with a faint sweet odour that acts as a hormone in plants. It stimulates fruits to ripe but it is also involved in many other aspects of plant development, e.g. like germination of seeds, senescence, reaction to environmental stresses or mechanical wounding. Ethylene is of high commercial interest because it fastens the ripening process of fruits and vegetables and the senescence of cut flowers.


About the thesis defence:

On Friday, the 5th of October, Bernard Wessels, Department of Plant Physiology, Umeå University, defended his thesis, entitled ’The significance of ethylene and ETHYLENE RESPONSE FACTORS in wood formation of hybrid aspen’. The public defence took place at 9:00 am in Lilla hörsalen ( KB.E3.01) in the KBC building, Umeå University. The faculty opponent was Prof. Kurt Fagerstedt, Faculty of Biological and Environmental Sciences, University of Helsinki, Finland. Supervisor of the PhD thesis was Hannele Tuominen.

For more information, please contact:

Bernard Wessels, Department of Plant Physiology, Umeå University
Telephone +4670 0130923
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.