Final Report Summary - S6KDIURNALGROWTHARAB (Integrating cell growth and cell division with light signals and circadian clock function – the role of Arabidopsis S6 kinase in the regulation of the diurnal pattern of growth) The 40S ribosomal protein S6 kinase (S6K) is an evolutionary conserved component of the TOR growth signalling pathway in eukaryotes. S6K functions as a regulator of protein synthesis due to its ability to promote mRNA translation through the direct phosphorylation of the ribosomal protein S6 in a TOR-dependent manner. Arabidopsis thaliana S6K is regulated by nutrient availability, osmotic stress, phytohormone signalling (e.g. auxin) as well as light and sugar. S6K is encoded by two genes, S6K1 and S6K2, organized in close proximity in chromosome 3. We have previously shown that S6K associates with the E2FB/RBR1 pathway to inhibit cell proliferation, an indication that S6K could constitute a molecular switch between cell growth and proliferation. Interestingly, S6K1 is co-expressed with CCA1 (CIRCADIAN CLOCK ASSOCIATED 1), a crucial component of the circadian clock core oscillator. This finding suggests that S6K transcription could be regulated by the clock.The circadian clock is an internal timekeeping mechanism allowing plants to adjust fundamental biological processes to environmental conditions. The combined action of light perception and the clock constitutes a coincidence mechanism regulating the diurnal pattern of growth especially in young seedlings. In addition the circadian clock is paramount in the regulation of plant fitness and biomass. However, very little is known on the downstream circadian-regulated outputs that account for these extremely relevant functions. Therefore, in this project we investigated the circadian regulation of Arabidopsis S6K1 and S6K2 both at the transcriptional, translational and post-translational levels. We monitored S6K1 and S6K2 expression patterns in different plant organs with specific developmental programs, focusing specially in hypocotyls, cotyledons and roots. In addition, we did a comprehensive analysis of light, circadian and growth-related phenotypes associated either with depletion or increase of S6K levels. We propose that the circadian clock provides additional transcriptional and post-translational regulation of S6Ks. This regulation is integrated with organ-specific responses, suggesting that S6K protein function could be adjusted to specific plant developmental programs. Taken together our results identify and characterize new circadian-regulated outputs. They widen our understanding of the molecular mechanisms underlying circadian clock function in plants and their relevance in adjusting plant growth responses to environmental conditions.
