In many woody species the initiation of cold acclimation and dormancy are synchronised with the end of the growth season and the onset of low temperatures in the autumn. The length of the photoperiod is an important primary signal for these growth responses. The perception of daylength requires a photoreceptor to distinguish light and dark, and an internal clock to perceive time. Physiological studies involving light quality experiments, photoreceptor mutants and the use of transgenic plants have demonstrated the importance of the red/far-red light (R/FR) absorbing phytochrome photoreceptor in the transduction of these signals. The phytochrome genes encode a family of receptors. In Arabidopsis the apoprotein is encoded by five genes, designated PHYA, -B, -C, -D and E.

 

The function of phytochrome in photoperiodic perception is not well understood. However, both phyA and phyB and other light stable phytochromes appears to be involved in the photoperiodic control of flowering in Arabidopsis, and phyB has also been shown to mediate photoperiodic control of tuber formation in potato. The aim in our project is to understand the role of phytochrome in short-day induced growth cessation in trees. We are using hybrid aspen (Populus tremula x P. tremuloides) as a model system, transformed with different phytochrome genes in sense and/or antisense directions. Hybrid aspen transformed with the oat PHYA gene results in trees with shorter internodes and insensitivity to short-day induced growth cessation. This result indicates that phyA plays an important role in sensing daylength in Populus. In order to confirm the role of phyA we have isolated a PHYA cDNA from hybrid aspen and are now studying antisense PHYA plants. Phenotype of hybrid aspen transformed with the oat PHYA cDNA. Gibberellin control of shoot elongation Gibberellins (GAs) are a group of more than 100 tetracyclic diterpenes, some of which are essential endogenous regulators that influence growth and development events throughout the life cycle of a plant, e.g. shoot elongation, expansion and shape of leaves, flowering and seed germination. The best examples of the importance of GAs in control of shoot elongation are the GA-deficient mutants of Arabidopsis, maize and pea, which have reduced levels of active GA(s) compared to wild-type plants, and a dwarf phenotype attributed to a reduction in internode length. Our project is concerned with the role of GAs in plant development, and daylength responses, focusing on trees, although Arabidopsis and tobacco are used as model systems too. Our approach for studying the role of GAs in trees is to study endogenous expression of GA biosynthetic genes, e.g. GA 20-oxidase and GA 3„-hydroxylase genes, and to transform trees with genes encoding GA biosynthetic enzymes. Hybrid aspens transformed with the GA 20-oxidase cDNA from Arabidopsis (the GA5 gene) under the control of the CaMV 35S promoter show elongated internodes, longer petioles and larger leaves, reduced root formation and increased shoot biomass. As application studies have shown that both IAA and GAs are required for the differentiation of xylem fibres, we are also usingthe transgenic plants to study the role of GAs in the differentiation of xylem fibres. Many woody species with indeterminate growth show complete cessation of elongation growth after only a few weeks in short photoperiods. Studies in Salix have shown that GA levels are decreased after only a few days in short photoperiods. In hybrid aspen transformed with the oat PHYA gene the dwarf phenotype is correlated with a reduction in GA levels, but in short photoperiods there is no further reduction in GA contents, in marked contrast to the pattern in wild-type plants. These observations imply that GAs have an important role as signals in the photoperiodic regulation of shoot elongation. We are now studying transgenic GA plants, both sense and antisense, to elucidate how changes in GA levels affect photoperiodic growth.

 

Moritz, T. and Olsen, J. (). Comparison between high-resolution selected ion monitoring, selected reaction monitoring and four-sector tandem mass spectrometry in quantitative analysis of gibberellins in milligram amounts of plant tissue. Analytical Chemistry, 67: .

 

Olsen, J. E., Junttila and Moritz T. () A localised decrease of GA1 in shoot tips of Salix pentandra seedlings preceds cessation of shoot elongation under short photoperiod. Physiol. Plant. 95: 627-632.

 

Uggla, C., Moritz, T., Sandberg, G. & Sundberg, B. () Radial concentration gradients of endogenous IAA over the cambial region of Pinus sylvestris L. suggests a role in positional signalling during wood development. Proc. Natl. Acad. Sci. USA. 93:

 

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

 

Eriksson, M.E., Israelsson, M., Olsson,.O. & Moritz, T. () Increased gibberellin biosynthesis in transgenic trees promotes growth, biomass production and xylem fibre length. Nature Biotech. 18: 784-788.

 

Eriksson, M.E. & Moritz, T. () Daylength and spatial expression of a gibberellin 20-oxidase isolated from hybrid aspen (Populus tremula L. x P. tremuloides Michx.). Planta 214: 920-930.