Light at night resynchronizes the evening-phased rhythms of TOC1 and ELF4 in Phaseolus vulgaris.

Circadian clocks regulate the adaptation of the organisms' physiology to the environmental light-dark cycles. Photic resetting of the clock differs among plant species. In Arabidopsis thaliana, morning-phased genes are not responsive to light signals at night, while in Phaseolus vulgaris, morning-phased genes are responsive to light at trough phases that are reached during the night. In order to explore this further, in this work we investigated the light-responsiveness at night of two P. vulgaris evening phased genes, the orthologs of TOC1 and ELF4. Our results demonstrate that the oscillation of their expression is symphasic under all applied photic conditions. Thus, under photoperiod peak phases are obtained in the evening (LD 12:12) or early at night (LD 6:18). Light application at the beginning of the night under LD 6:18 results in a phase shift of the PvTOC1 and PvELF4 oscillation, while at the end of the night the phase remains unchanged. Moreover, when light is applied at the narrow time window of the peak phase, a significant transient increase in the expression of both PvTOC1 and PvELF4 is caused. These results indicate that, depending on the plant species, evening-phased genes may also participate in the resetting of the circadian clock machinery by light.