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A Novel Role for Phytochrome in Dormancy Release Inhibition

Diagrammatic representation of the influence of light quality (R, red; FR, far-red; B, blue) on dormancy release, dormancy induction, and germination of hydrated annual ryegrass seeds

This project is one of a series in the theme of Diagnosis of Viability. Phytochrome regulates seed germination of light-sensitive species, with red (R) light generally stimulating and far-red (FR) inhibiting germination. Seed dormancy controls the quantity of light required to stimulate germination, thus changes in sensitivity to light can occur over time as a result of changes in dormancy status. However it is only recently that the light environment during dormancy release has been the primary subject of investigation. Following the discovery of a new dormancy release mechanism for Lolium rigidum seeds involving hydrated storage, which circumvents the need for months of dry after-ripening, the light environment has been found to play a pivotal role and a novel role for phytochrome in the inhibition of dormancy release has been described.

Along with seed water content, the light environment can regulate whether or not dormancy release will occur in dormant L. rigidum. Temperature controls the rate at which dormancy release takes place when light quality and water levels are permissive, being faster at warmer temperatures. Dormant seeds (seeds that are unable to germinate in light or darkness) lose their dormancy and become light-sensitive during incubation of hydrated seeds in darkness, thus becoming able to germinate in light (R or white) but not darkness. Incubation in FR light also results in dormancy release, but dormant seeds that are incubated in R or white light remain dormant. Thus, in addition to its role in stimulating germination, phytochrome now has a role in modulating dormancy release.

Membranes have been suggested to be the primary site of temperature perception at the cellular level in many systems, including seeds. Temperature can alter phospholipid headgroup spacing directly, with a rise in temperature resulting in an increase in membrane fluidity and cross-sectional area and phase transitions occurring at key temperatures. Along with inherent temperature-activity relationships of enzymes, the change in membrane fluidity also allows temperature to control adjustments to membrane lipid composition. A gradual alteration in level of membrane fatty acid desaturation occurs during temperature acclimation in many organisms. The activities of membrane-associated proteins, such as ion channels, receptors, transporter proteins and enzymes, may change dramatically as alterations occur in dynamic properties of the lipid fraction of membranes. There is some evidence for temperature-controlled membrane lipid alterations in seeds. Furthermore, membrane function is pivotal to the activity of phytochromes, which move through the nuclear membrane to directly influence gene expression, a process that is temperature sensitive. This project will focus on the relationship between seed dormancy status, as modulated by temperature and light quality, with membrane fluidity, lipid composition and ODS expression.

Using a well defined physiological response, this project will investigate a novel role for light in modulating dormancy release with two primary aims: 1) To identify biochemical and molecular changes in seeds during dormancy release induced by dark/far-red, and 2) to investigate how red, blue and white light inhibit dormancy release.

Duration: 2006-2010

Outputs: peer-reviewed publications

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