Scandinavian research institute: Wood Development Group, Helsinki

Ykä Helariutta (third from the left in front) and his Wood Development Group. From www.biocenter.helsinki.fi

Within reach of the vast Finnish woodlands, the Wood Development Group resides at the Institute of Biotechnology at Helsinki University. The group is headed by Ykä Helariutta and comprises currently 5 postdocs, 7 graduate students and 2 technicians as well as a number of under-graduate students and occasional visitors. Their common interest is the genetic and molecular basis of wood development, and although the research is of fundamental character it contributes with valuable knowledge for the forest industry on how trees can be grown and their wood processed.

Trees successfully colonized the barren soils of Earth after the first plants had emerged from the water, and the development of a wooden, vascular tissue was key to their success. It allowed them to transport water to the upper parts and to grow not only upwards reaching for the light, but thanks to secondary growth they also developed thick trunks that could sustain their huge weight.

Arabidopsis serves as a model for wood development. From www.biocenter.helsinki.fi

Though wood is typical for trees and shrubs it shares many genetic and morphological similarities with the vascular tissue of weedy plants. This has allowed Ykä Helariutta and his colleagues to take advantage of the small model plant Arabidopsis thaliana in their studies. In 2003, the group identified the WOODEN LEG gene, WOL, which is required for the asymmetric cell divisions in the root meristem that lead to the phloem and procambium cells (see Genes and Development 14: 2938). This development is paralleled by the cambial cell divisions between the phloem and xylem in the trunks of trees, and the group is now searching for WOL-like genes that are expressed in the cambial zone of silver birch (Betula pendula).

Using the Arabidopsis model system they were later able to identify another gene, ALTERED PHLOEM DEVELOPMENT (APL), which is crucial for phloem-xylem patterning. The discovery was published in Nature and can turn out to be crucial for understanding wood formation in trees, since APL seems to repress wood formation. In order to identify homologues of these genes in trees, Ykä Helariutta has initiated a functional genomics approach in birch in collaboration with Tapio Palva, Jaakko Kangasjärvi and Lars Paulin (see also former article in SPPS Newsletter).

AHP6 is expressed in protoxylem and adjacent pericycle cells. From Science 311: 94

WOL is a two-component receptor, and the Japanese group of Tatsuo Kakimoto has shown it to be activated by cytokinin to initiate phosphorelay signalling. The Finnish and Japanese groups have since joined their efforts in order to identify the genetic and hormonal components of vascular development, and in 2006 they published a common paper in Science. It demonstrates how the AHP6 gene counteracts cytokinin signaling by inhibiting the phosphotransfer process. This enables protoxylem specification and accordingly the gene is expressed in the protoxylem and adjacent pericycle cell lineages.

Recently, Ykä Helariutta and his colleagues have taken the regulatory role of cytokinin further and studied how this phytohormone control wood formation in trees. In a study published 2008 in PNAS, they produced transgenic poplar (P. tremula x tremuloides) trees expressing a cytokinin catabolic gene, CKO2. The transgenic trees had reduced concentration of biologically active cytokinin and were phenotypically distinct with a thinner stem diameter due to reduced radial growth.

The Wood Development Group is now trying to reverse these results in order to construct transgenic poplar trees with increased cytokinin signalling and consequently also enhanced wood biomass production. Such trees would be ideally suited as a carbon source for large scale biofuel production and could turn out to be a valuable contribution in the global efforts to fight the climate crisis.

You can find more information about the Wood Development Group at their official homepage.