Hannele Tuominen leaning against a tree trunkFoto: Fredrik Larsson

Xylem elements mature in plant vascular tissues by depositing cellulose-rich secondary cell walls until they die through programmed cell death. Cell death therefore influences the duration of xylem cell differentiation and the lifetime of the cells. I have investigated the process of xylem cell death both in the water-transporting vessels and the physically-supporting fibres of the stem in Populus trees. The current aim is to identify factors that initiate and execute xylem cell death. One of the focus areas is the signalling and functional characterisation of the metacaspase gene family using reverse genetic, forward genetic and biochemical methods in both Arabidopsis thaliana and Populus.

We have earlier shown that cell death also influences lignification of xylem elements. Work done in the Zinnia elegans tracheary element differentiation system revealed that lignin biosynthesis continues even post mortem (after cell death). Further work in Arabidopsis xylem tissues showed that the post mortem lignification of xylem vessels is assisted by their long-living neighbour cells, the so called “good neighbours” of lignification. This sequence of events needs to be strictly controlled in time and place, and we have identified members of the PIRIN gene family that, from within the good neighbours, mediate at least partially the post mortem lignification of vessel elements. Strikingly, Arabidopsis PIRIN2 influences also chemical composition of lignin, and the current work focuses on the physiological significance of this finding.

The figure consists of four individual photos showing the plant model systems in Hannele Tuominen's research group: the left photo is a microscope picture of tracheary element cell cultures of Arabidopsis thaliana, on the second left photo, a cutting of a primary root of Arabidopsis thaliana enlarged with a microscope is seen, the second right photo shows a cutting of a hybrid aspen stem and the right photo a stem cutting of aspen.

We are very much interested in how xylem maturation influences the chemical and mechanical properties and the functioning of the wood. A long-standing question has been whether it would be possible to enhance biomass production in woody tissues by extending the lifetime of the individual xylem elements. Another burning question is how xylem maturation influences responses to environmental factors such as nutrient abundance and drought. We have taken two different approaches to investigate the relationship between xylem maturation and properties of wood. The first approach aims to modify xylem maturation by transgenic technology in hybrid aspen (Populus tremula X P. tremuloides) trees using cell-specific promoters and the newest DNA editing technologies, followed by careful characterisation of the transgenic trees for growth, wood chemical and physical properties, water transport capacity and drought resistance. The second approach takes advantage of the natural variation within a Swedish aspen (Populus tremula) population with the aim to identify natural variation in the secondary cell wall and wood properties as well as in tree responses to environmental effectors, followed by elucidation of the underlying molecular mechanisms by genome-wide association mapping.