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Elucidation of light signaling pathways that control the photoperiodic induction of flowering

Final Activity Report Summary - LIGHT AND FLOWERING (Elucidation of light signaling pathways that control the photoperiodic induction of flowering)

Elucidation of light signalling pathways that control the photoperiodic induction of flowering plants are able to monitor and respond to different environmental signals, such as day length, to adapt to seasonal changes and different latitudes. Day length-dependent floral initiation is mediated by the interaction between environmental light signals and the internal timekeeper mechanism called the circadian clock. In the model plant Arabidopsis thaliana the key genes in inducing the floral transition in response to long days are gigantea (gi), constans (co) and flowering locus t (ft). They act in the functional hierarchy GI-CO-FT. GI gene expression oscillates with an evening phase, and this increases the amplitude of the rhythmic transcription of CO. At the end of the long light period CO protein is stabilized by blue and far-red light and this leads to activation of FT gene expression. Unlike CO and FT, GI is involved not only in the floral transition but also in other light signalling pathways and control of the circadian clock.

The goal of the project was to gain further information on the molecular mechanisms used by plants to detect day length and trigger the floral transition. I focused on how different light qualities could regulate the activity of the transcription factor CO, a major coordinator of photoperiod detection. In particular, I studied how transcriptional regulation of the upstream regulator GI is regulated by light, and how light influences the post-transcriptional regulation of CO through stabilisation of the protein.

The temporal regulation of the CO stabilisation by different light qualities was characterised in plants constitutively expressing the CO protein. The results support the previous findings about the highly redundant functions of red light receptor phytochrome A and blue light sensing cryptochromes. Furthermore phytochrome B, that is mainly responsible for red-light triggered CO destabilsation, has a modest positive effect on the blue-light induced CO activation particularly in the evening.

I found that GI transcription is induced by blue, red or far-red light. The light sensitivity shows daily oscillations with a maximum coinciding with the circadian peak time of GI. In daily light-dark cycles, the phase of GI expression responds to the length of the photoperiod so that the peak in GI mRNA expression occurs approximately at dusk in a wide range of day lengths. The acute light response also shows photoperiod-dependent differences in the time of maximum sensitivity. Mutation in the GI gene sensitises some circadian regulated processes to be reset by the light to dark transition at dusk. My results suggest that the regulation of GI expression by light is unlikely to strongly contribute to light regulation of floral initiation in inductive day lengths, but that GI appears to be a light-responsive component of the circadian clock that acts during the late day to contribute to day length measurement. In conclusion, xenobiotic degradation under anaerobic conditions, thought to be difficult or impossible, has been proven to be possible and efficient.

Our results open new opportunities for the characterisation of microbial relationships and metabolic pathways in low redox environment, for the introduction of practical de-novo degradation abilities in existing reactors, and for the enzyme/gene analysis for further functional community engineering.