Chromatin remodelling in Arabidopsis embryos during induction and release of seed dormancy
Final Report Abstract
Germination timing is determined by seed dormancy, which prevents a viable seed from germinating under favourable conditions. Dormancy induction in Arabidopsis occurs during maturation and desiccation of seeds in the silique. During seed maturation, the embryo shifts from an active growing phase into a quiescent phase with very low metabolic activity. When dormancy is released and conditions are favourable, the seed germinates and the embryo resumes growth. The role of chromatin changes in this process is not understood. This project was aimed to study the role of chromatin remodelling during seed maturation, dormancy and germination. An analysis of chromatin organisation at the microscopic level showed that nuclei strongly decrease in size during the first half of the seed maturation phase. Nuclei increase in size again after germination of non-dormant seeds. The decrease in nuclear size is regulated by the seed maturation control gene ABI3 and is not affected by the seed dormancy level. Decreased nuclear size might represent a protection mechanism against desiccation. Recent work in my lab revealed a role for the chromatin-remodelling factor HISTONE MONOUBIQUITINATION1 (HUB1) in seed dormancy. Histone H2B monoubiquitination is required for transcription initiation and elongation. Transcriptome analysis of matured siliques of the hub1 mutant revealed an overlap in differentially regulated genes with a mutant in the TFIIS transcription elongation factor, which also shows reduced dormancy. In addition, we identified several other transcription elongation genes that are required for the induction of seed dormancy. Our present hypothesis is that transcription elongation factors are required to facilitate gene transcription, which is hampered by the reduction in nuclear volume. This hypothesis is presently being tested. In an unbiased screen for additional chromatin remodelling factors that are required for seed dormancy, several mutants were identified. The most interesting of these is histone deacetylase9, which shows strongly reduced seed dormancy levels. This gene will be further characterised in a follow-up project that has been granted. In this project, major changes in nuclear size during seed maturation and germination were identified. The reduction in nuclear size during seed maturation correlates with a requirement for transcription elongation factors at the end of the seed maturation phase. Mutations in transcription elongation factors cause reduced seed dormancy.
Publications
- (2008). Molecular networks regulating Arabidopsis seed maturation, after-ripening, dormancy and germination. New Phytologist 179: 33-54
Michael J. Holdsworth, Leónie Bentsink and Wim J.J. Soppe
