Hysteresis in PIF4 and ELF3 dynamics dominates warm daytime memory in Arabidopsis

Plants may experience large diurnal temperature fluctuations. Our knowledge of the molecular mechanisms of integration of these fluctuations and the resulting growth patterns is limited. Here we show that hypocotyl growth during the night responded not only to the current temperature but also to preceding daytime temperatures, revealing a memory of previous conditions. Daytime temperature affected the nuclear levels of PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and LONG HYPOCOTYL 5 (HY5) during the next night. These jointly accounted for the observed growth kinetics, whereas memory of prior daytime temperature was impaired in the pif4 and hy5 mutants. PIF4 promoter activity largely accounted for the temperature dependent changes in PIF4 protein levels. Noteworthy, the decrease in PIF4 promoter activity triggered by cooling required a stronger temperature shift than the increase caused by warming. This hysteretic pattern required EARLY-FLOWERING 3 (ELF3). Warm temperatures promoted the formation of nuclear condensates of ELF3 in hypocotyl cells during the afternoon but not in the morning. These nuclear speckles showed poor sensitivity to subsequent cooling. We conclude that ELF3 achieves hysteresis and drives the PIF4 promoter into the same behaviour, enabling a memory of daytime temperature conditions.