The aim of my research is to understand the mechanisms of metabolic control of shoot elongation and wood formation in the model plant Populus. We want to understand how plant hormones and other metabolites are involved in the control of plant development, and how different environmental cues, such as photoperiod, affect the metabolic control of growth and development. The metabolic control of growth and development are studied by using both targeted and untargeted metabolomics approaches.

Gibberellins (GAs) are a group of 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. Our project is concerned with the role of GAs in plant development, and daylength responses, focusing on the tree hybrid aspen (Populus tremula x P. tremuloides) as a model system. Our approach for studying the role of GAs in trees is to study endogenous expression of GA biosynthetic and signalling genes and to transform Populus with genes encoding those genes.

Populus transformed with the AtGA20ox1 under the control of the 35S promoter shows elongated internodes, longer petioles and larger leaves, reduced root formation and increased shoot biomass.We have also char- acterized the GA receptor, GID1 in Populus. Four orthologs of GID1 have been identified in Populus tremula x P. tremuloides (PttGID1.1 to 1.4). When PttGID1.1 and PttGID1.3 were overexpressed in Populus with a 35S promoter, overexpressors shared several similar phenotypic traits with previously described 35S:AtGA20ox1 overexpressors, including rapid growth and increased elongation.

We are also studying the role of GAs and other signaling compounds in wood formation. The role of GAs is done by both using transgenic Populus with increased levels of GAs and signalling and by predicting where GAs are formed and perceived during wood formation. For example, we have quantified GAs and analyzed the expression of GA biosynthesis genes and genes with predicted roles in GA signalling in tangential sections across the cambial region of aspen trees (Populus tremula). The results show, for example inter alia, that the bioactive GA and GA 14 predominantly occur in the zone of expansion of xylem cells.

Studies with transgenic Populus overexpressing AtGA20ox1 or PttGID1 with 35S or a xylem-specific promoter, suggest that GAs are required for two distinct processes in wood formation with tissue-specific signalling pathways: xylogenesis, mediated by GA signalling in the cambium, and fibre elongation in developing xylem.

By using a metabolomics approach we are also studying how specific patterns of metabolites, including signalling compounds, are vary in different regions of the wood-forming zone in Populus. From the data we can conclude that cambial activity, cell expansion and secondary cell wall thickening are tightly coupled processes, Many of these patterns can be explained on the basis of the developmental processes taking place within these regions, e.g. in the cambium.

Many woody species with indeterminate growth show complete cessation of elongation growth after only a few weeks 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 have also been studying transgenic GA plants to elucidate how changes in GA levels and signalling affect photoperiodic growth. Studies in PHYA and GA biosynthesis/signalling overexpressors are used with transcriptomic and metabolomic approaches to elucidate the early signalling pathways in short-day induced growth cessation, including identification of new putative signalling compounds.

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