Thomas Moritz - Hormonal Control of Shoot Elongation and Wood Formation

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The aim in my research project is to understand the mechanisms of hormonal control of shoot elongation and wood formation in the model plant Populus. We want to understand how plant hormones are involved in the control of plant development, and how different environmental cues, such as photoperiod, affect the hormonal control of growth and development. In parallel, the metabolic changes in response to hormonal and photoperiodic changes are studied by using a metabolomics approach.
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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 day length 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.

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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. In contrast, Populus expressing the AtGA3ox1 gene under the 35S promoter exhibits no major changes in morphology. We have concluded that 20-oxida- tion is the limiting step, rather than 3β-hydroxylation, in the formation of GA1 and GA4 in elongating shoots of hybrid aspen, and that ectopic GA3ox expression alone cannot increase the flux towards bioactive GAs.
Recently, we have also characterized 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 in wood formation. This 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 GA1 and GA4 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.
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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 photo periods. In hybrid aspen transformed with the oat PHYA gene, the dwarf phenotype is correlated with a reduction in GA levels, but in short photo-periods 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 photo-periodic 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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Key publications

Eriksson M, Israelsson M, Olsson O, Moritz T 2000. Increased gibberellin biosynthesis in transgenic trees promotes growth, biomass production and xylem fibre length. Nature Biotech. 784-788.

Israelsson M, Mellerowicz E, Chono M, Gullberg J, Moritz T 2004 Cloning and overproduction of GA 3-oxidase in hybrid aspen trees: effects on GA homeostasis and development. Plant Physiol. 135: 221-230

Jonsson P, Johansson A, Gullberg J, Trygg J, A J, Grung B, Marklund S, Sjöström M, Antti H, Moritz T 2005 High through-put data analysis for detecting and identifying differences between samples in GC/MS-based metabolomic analyses Anal. Chem. 77: 5635-5642.

Israelsson M, Sundberg B, Moritz T 2005 Tissue-specific localisation of gibberellins in wood-forming tissues in aspen. Plant J. 44: 494-504.

Mauriat M, Moritz T 2009 Analyses of GA20ox- and GID1-overexpressing Populus suggest gibberellins play two distinct roles in wood formation. Plant J. 58: 989-1003.

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July 2010

Friday, July 30, 2010
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