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Figure 1.Induction and release of dormancy in Arabidopsis seeds |
Non-dormant Arabidopsis seeds germinate in thepresence of water, light and favourable temperature but dormant seeds are unable to do so (Figure 1). Arabidopsis seeds have a non-deep physiological dormancy, which can be broken by low temperatures (stratification) or dry storage (after ripening).
Both environmental and endogenous factors play a role in the induction and release of seed dormancy. Induction occurs during the maturation of seeds in the silique and is strongly determined by abscisic acid levels.
Physiological research on seed dormancy has been extensive but the molecular knowledge of this process is still limited. Only a few genes that influence seed dormancy have been isolated and characterized. The molecular mechanisms of the induction of dormancy and the breaking by low temperatures and dry storage are still largely unknown.
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We study the molecular mechanism of seed dormancy, using several approaches:
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Figure 2. Release of dormancy by after-ripening in the Ler and Cvi accessions. |
1 Cloning and characterization of dormancy QTL and mutants
Accessions of Arabidopsis have differences in seed dormancy (Figure 2).
This has been exploited in a quantitative trait loci (QTL) analysis of the accessions Landsberg erecta (Ler) and Cape Verde Islands (Cvi), whereby different QTL for seed dormancy were obtained (Alonso-Blanco et al., 2003). One of these (DOG1) has been cloned (Bentsink, personal communication). In addition, a mutant screen has yielded several mutants with a reduced seed dormancy (Peeters et al., 2002). We are presently cloning and characterizing several of these QTL and mutants (Figure 3) with the ultimate aim to unravel the dormancy-germination pathway.
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Figure 3. Map positions of the DOG QTL and the rdo mutants |
Furthermore, we are performing novel mutagenesis screens to isolate mutants that specifically affect the stratification or after ripening process. Gene expression and DNA chip experiments will be used to identify additional genes involved in dormancy and germination.
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2 Study of chromatin modifications during dormancy
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Developmental phase transitions require the regulation and highly co-ordinated expression of many genes, which is linked with chromatin dynamics and organisation. Preliminary observations showed differences in chromatin organisation between immature, dormant and not-dormant Arabidopsis embryos (Figure 4). We would like to further study this by analysing chromatin organisation during induction and release of dormancy.
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Figure 4. Embryos with different dormancy levels show differences in chromatin structure. DAPI-stained embryo nuclei are shown of immature Ler seeds (A), dormant Cvi seeds (B), not-dormant after-ripened Cvi seeds (C) and not-dormant dog1 mutant seeds (D). Bright spots are heterochromatic chromocenters. Bar = 4_m.
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3 Seed storability
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Figure 5. Wild-type (Ler), abi3 and lec1-3 seeds |
Dormancy determines the beginning of the period in which seeds are able to germinate, whereas storability determines the end of this period. Mutants with increased seed storability were identified in a screen for longer surviving seeds in an abscisic acid insensitive3 (abi3) or leafy cotyledon1-3 (lec1-3) background (mutants without dormancy and with low storability; Figure 5). We started to clone and characterize several of these mutants.
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References
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Alonso-Blanco, C., Bentsink, L., Hanhart, C.J., Blankestijn-de Vries, H. and Koornneef, M. (2003). Analysis of natural allelic vaiation at seed dormancy loci of Arabidopsis thaliana. Genetics 164, 711-729.
Peeters, A.J., Blankestijn-de Vries, H., Hanhart, C.J., Leon-Kloosterziel, K.M., Zeevaart, J.A., and Koornneef,M. (2002). Characterization of mutants with reduced seed dormancy at two novel rdo loci and a further characterization of rdo1 and rdo2 in Arabidopsis. Physiologia Plantarum 115, 604-612.
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© 2007, Max Planck Institute for Plant Breeding Research, Cologne |
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