The critical role of the Arabidopsis circadian clock at high temperature

Abstract

The circadian clock is an internal mechanism found in most organisms generating a 24h rhythm, evolved to anticipate predictable environmental changes thus making best use of resources. Intensely studied in the model plant Arabidopsis thaliana (L.) Heynh, the circadian clock was found to control a large number of physiological traits and the expression of more than a third of the plant genes. Clocks therefore play a central role in the life of organisms, in fact plants lacking clock functionality lose their ability to anticipate the dawn showing a reduced fitness. In a perspective of raising global temperatures, impact on crops production of plants with a non optimised clock could be of major importance. The present study therefore investigated the importance for Arabidopsis plants of having a functional clock at high temperatures. Arrhythmic plants over-expressing the CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) gene were compared to wild-type Col-0 plants: while at 17°C wild-type plants showed more leaf area, biomass and chlorophyll content than CCA1ox, at 27°C the difference was even greater. This increased difference at high temperature was also confirmed in successive transcript and metabolite profiling of the two lines. Not only a functional but also an accurate clock was previously found to be important for a good plant performance. For this reason investigation on the impact of high temperature was extended to plants with an internal clock period not matched to the external light/dark cycle. Period mutant lines ztl (30h) and toc1 (20h) along with their wild-types (24h) were grown in 12LD, 15LD or 10LD cycles. ztl line showed no difference from Col-0 at 17°C but a marked difference at 27°C, where the lines with a period matching the external light/dark cycle performed better. Similarly the toc1-1 line performed better at high temperature when its clock period matched the environment. Another key feature of circadian clocks is temperature compensation, a mechanism able to maintain an accurate and robust rhythm with a period close to 24h over a broad range of temperatures. To quantify the importance of temperature compensation at high temperatures, the mutant line gi-11, defective for the temperature compensation mechanism, was used. Little differences were found at both 17°C and 27°C between the gi-11line and its wild-type WS. Finally, a screen was performed to identify new components of the clock specifically required for function at high temperatures. At the end of the screen pipeline, four putative circadian mutants were identified, which will need to be further characterised to confirm their altered rhythmicity and eventual position in the current clock model.