Tree production as means for timber, pulp, paper, and energy production are of significant economic value. In the future the demand for wood is likely to be increasing as it is a good renewable material, and also as a direct effect of the increasing human population.

The active growth rate, and the period of growth decide how much wood is produced. Timing traits, i.e. when the tree starts and stops to grow have a direct effect on wood yield. An early induction of dormancy (or late bud flush) leads to loss of biomass production each growth cycle. The tree hybrid aspen, Populus tremula L. x P. tremuloides Michx., is a good model system for deciduous species. Bud set and dormancy of this tree is dependent upon autumnal short days. Accurate timing of seasonal events in trees such as bud dormancy are crucial since any failures are probable to lead death, at least at high latitudes where the associated cold is likely to be strong.

In order to be able to anticipate the diurnal cycle of light and dark, most organisms have developed a molecular time measuring system called a circadian (circadian lat.= about a day) oscillator or clock. It is an adaptive strategy that will enable synchronisation of the organismís cellular and physiological events to its most favourable time of the day, and under natural conditions needs to be reset to local time (entrained) on a daily basis.

It will also synchronise seasonal events such as flowering and bud dormancy in order for it to occur at a time of the year when it is the most favourable. The clock integrates both daily and seasonal changes in light as well as temperature.

 

To investigate the unique biology of quiescence i.e. environmentally induced meristem arrest (dormancy) in apical buds of hybrid aspen, recent progress in clock biology from the plant model species Arabidopsis thaliana will be used. Expression of clock genes with an effect on photoperiodical time keeping will be modulated in aspen by transgenic techniques. I am interested in the induction of bud dormancy in hybrid aspen, and combine these studies with characterisation of vegetative dormancy in Arabidopsis. Arabidopsis ecotypes found to develop vegetative quiescence will be mutagenised, and affected genes in mutants having aberrant induction of quiescence will be cloned and analysed. Some ecotypes of A. thaliana have the competence to over winter at low temperatures (winter annuals) under natural conditions, and hence develop some sort of quiescence. This area of research has not been addressed in Arabidopsis and it has strong potential to be very rewarding, giving new information of importance to bud dormancy in trees.

The objectives of the suggested project are:

• How does the biological timing work in hybrid aspen?
• What role does the circadian clock play in timing of bud dormancy?
• Which genes control vegetative quiescence in Arabidopsis?

In order to characterise the function of the circadian oscillator in aspen and Arabidopsis, circadian regulated genes such as chlorophyll a/b binding protein 2 (CAB2/LHCB1) or CIRCADIAN CLOCK REGULATED 2 (CCR2) will be fused to the reporter gene luciferase (LUC). Luciferase use the substrate luciferin which enable photon emission from the plants which carry such constructs.

I am currently in the process of setting up an high quality imaging facility in order to be able to accurately follow the output rhythms of weak light emission from plants that carry these promoter:LUC constructs in real time.

Circadian clock associated genes, which have been characterised in Arabidopsis, are well conserved in aspen. Currently the central clock myb-genes CIRCADIAN CLOCK ASSOCIATED 1(CCA1) /LATE ELONGATED HYPOCOTYL (LHY), and the pseudo-response regulator gene family member TIMING OF CAB 1 (TOC1) are targeted for analysis in aspen.

A number of newly isolated Arabidopsis clock mutants that are likely to pinpoint novel genes involved in the complex circadian timing system are currently being analysed in my lab.

Shortly, new screens to search for mutants involved in vegetative quiescence will be set-up.

 

 

Eriksson, M.E., Hanano, S., Southern, M.M., Hall, A., and Millar, A.J. () Response regulator homologues have complementary, light-dependent functions in the Arabidopsis circadian clock. Planta. On line. 4 September.

 

Eriksson, M.E., and Millar A.J. () The circadian clock: a plantís best friend in a spinning world. Plant Physiol. 132, (2) 732-738.
 
Eriksson, M.E., and Moritz, T. () Daylength and spatial expression of a gibberellin 20-oxidase isolated from hybrid aspen (Populus tremula L. x P. tremuloides Michx.). Planta. 214, (6) 920-930.

 

Eriksson, M.E., Israelsson, M., Olsson, O. and Moritz, T. () Increased gibberellin biosynthesis in transgenic trees promotes growth, biomass production and xylem fiber length. Nat. Biotechnol. 18, (7) 784-788.

 

Olsen, J.E., Juntilla, O., Nilsen, J., Eriksson, M.E., Martinussen, I., Olsson, O., Sandberg, G. and Moritz, T. () Ectopic expression of oat phytochrome A in hybrid aspen changes critical daylength for growth and prevents cold acclimatization, Plant J. 12, (6) .