Circadian Regulation of Plant Responses to Shade and UVB

  • Donald Fraser

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


Plants are sessile organisms and must adapt their development to the environment. Light quality is a primary informational signal that plants sense and respond to. Specialised photoreceptor proteins perceive changes in light quality that indicate the presence of neighbouring vegetation and the risk of shade. Shade avoiding species respond through stem elongation and elevation
of leaves. Many plant environmental responses are regulated by the circadian clock through a process called circadian gating. This thesis shows that the inhibition of hypocotyl elongation by UV-B appears to be under circadian regulation in Arabidopsis thaliana. This is likely achieved through a temporal coincidence of 1) the circadian-gated peak of UV-B-induced GA catabolism genes with 2)
the greatest UV-B-induced reductions in auxin signaling. Shade avoidance can have detrimental effects on yield in commercial growing environments, so knowledge of circadian regulation of plant responses to light quality provides a toolset for product quality improvements. This thesis shows that UV-B inhibits shade avoidance in the commercially important crop, Coriandrum sativum, though there are only marginal differences when UV-B is delivered at different times of day. Although shade avoidance can provide plants with a competitive advantage
in fast growing stands, excessive stem elongation can be detrimental to plant survival. As such, plants have evolved multiple feedback mechanisms to attenuate photoreceptor-mediated shade avoidance signalling. The combination of a very low red to far red ratio (R:FR) and low levels of photosynthetically active radiation (PAR) present in deep canopy shade can, together, trigger phytochrome A (phyA) signalling; inhibiting shade avoidance and promoting plant survival. This thesis also shows that very low R:FR in a background of low PAR increases expression of the circadian clock component TIMING OF CAB EXPRESSION1 (TOC1) in a phyA-dependent manner at dusk and that TOC1 antagonises shade avoidance in these conditions.
Date of Award23 Jan 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorKeara A Franklin (Supervisor) & Antony Dodd (Supervisor)

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