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Molecular and Biophysical Dissection of Changes in Dormancy in Seeds of Sisymbrium officinale and Arabidopsis thaliana

An Arabidopsis thaliana plant in bloom

This project is one of a series in the theme of 'Diagnosis of Viability and Germination'.

An important objective of the MSB project is the safekeeping of living seeds. However, when seeds do not germinate they can either be dead or dormant. In Sisymbrium officinale and Arabidopsis thaliana changes in seed dormancy coincide with changes in the sensitivity or responsiveness to naturally occurring factors that stimulate germination, such as light and nitrate. Hilhorst (Plant Physiol., 1990) explained this by assuming that the number of accessible receptors for phytochrome fluctuates, and that nitrate acts as a cofactor in the binding of phytochrome. When environmental factors change to more optimal conditions for germination these receptors become more accessible at the membrane surface, or are produced more abundantly. Accessibility of the phytochrome receptor has been postulated to depend on the membrane fluidity, imposing either a recessed conformation of the receptor inside the double-lipid layer or a conformation that surfaces the membrane, allowing interaction with phytochrome and nitrate. In this model membrane fluidity is thought to determine exposure of the receptor by varying the thickness of the lipid bilayer. Temperature physically changes the fluidity of membranes. Homeoviscous adaptation is the mechanism whereby unsaturated fatty acids aid in maintaining membranes in a fluid state necessary for biological functioning. Desaturases add to the buffering capacity in the maintenance of the membrane fluidity (Nishida and Murata, Ann Rev Pl Physiol Pl Mol Biol, 1996), altering the fatty acid composition of the membrane. Putatively, this mechanism would function in seeds both to maintain the fluidity of the membrane, and to enable the breaking of dormancy.

Complementary to the membrane hypothesis, availability of a phytochrome receptor may be regulated by abundance of the receptor, rather than its accessibility. Biological receptors consist of proteins encoded by genes. Transcription and translation, in combination with turnover of mRNA and protein, regulate expression/abundance of a protein. In addition, post-translational modification may influence the activity of a protein. Identification of differences in protein abundance/modification between dormant and non-dormant samples may lead to identifying proteins that constitute a receptor, which would form a molecular explanation for dormancy.

The objective of this project is to explain dormancy in terms of membrane fluidity and the molecular mechanism behind it, whether it be availability or abundance of a putative receptor. The purpose is to generate a better understanding of dormancy, which can be used to understand the behaviour of banked seeds. The project may lead to the development of a diagnostics test to assess dormancy in any accession of the MSB.

Duration: 2004-2008

Outputs: PhD thesis, peer-reviewed publications

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