Investigation of the role of the homologous proteins PCH1 and PCHL in phytochrome B-mediated light signalling

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Phytochromes are photoreceptors in plants that regulate growth and development in response to changes in the environment. Light quality and intensity control the conversion from the inactive Pr to the active Pfr state as well as from Pfr back to Pr. In addition, reversion of Pfr to Pr also occurs in a light-independent process called dark reversion. Phytochrome B (phyB) is of particular importance in light-grown and adult plants and plays a role in detection of light in the red light range of the visible spectrum. We recently identified PCH1 and PCHL as phyB binding proteins. Plants overexpressing PCH1 or PCHL are strongly hypersensitive to red light, while lack of functional PCH1 results in reduced responses to red light. Initial experiments show that PCH1 and PCHL decrease phyB dark reversion and thereby maintain high Pfr levels. In the proposed project, we want to investigate molecular mechanisms underlying PCH1/PCHL-mediated regulation of phyB activity, with particular emphasis on PCH1/PCHL-dependent control of dark reversion. We also want to investigate a potential role of PCH1/PCHL in cross-talk between phyB and other signalling pathways, which could affect the activity of phyB by regulation of PCH1/PCHL. Our own data and data in the literature suggest that PCH1/PCHL link other light signalling pathways to phyB-mediated red light signalling. Moreover, PCH1/PCHL might play a role during seed germination by increasing the light sensitivity of imbibed seeds and provide a molecular link between phyB and karrikins, which are chemical compounds in smoke that promote germination and seedling development. PhyB dark reversion is temperature-dependent and therefore phyB might also have a function as temperature sensor. Given that PCH1/PCHL affect dark reversion, they might play a role in temperature sensing, which we also want to investigate.