Photoperiod modifies the diurnal expression profile of VvPHYA and VvPHYB transcripts in field-grown grapevine leaves

Despite the crucial role that phytochromes (Phys) play in light perception and in the entrainment of the circadian clock to local time, the photoperiodic regulation of PHYA and PHYB gene expression has been poorly studied, especially in woody perennials. Here the dynamic of Vitis vinifera PHYA (VvPHYA) and PHYB (VvPHYB) transcript accumulation was studied in field-grown grapevine leaves throughout daily cycles under decreasing natural photoperiods. Given that in grapevine the entrance of buds into endodormancy (ED) is a photoperiod-driven process, increases in BR₅₀ values, a parameter that measures the depth of dormancy in single bud cuttings assays was used to determine the critical daylength at which grapevine discriminates between long day (LD) and short day (SD) photoperiod. Therefore, we monitored the daily expression profile of VvPHYA and VvPHYB transcripts before, during and after the defined critical daylength. Results showed that under LD photoperiod (21 December, daylength 14 h 40 min) the abundance of both transcripts oscillated with diurnal rhythms, attaining maximum and minimum levels before dawn and after dusk, respectively. However, under SD photoperiod (12 April, daylength 11 h 40 min) the rhythmic expression disappeared, and both transcripts expressed uniformly at high levels. Our results showing that photoperiod regulates VvPHYA and VvPHYB gene expression, contrast with those reported in Arabidopsis and in other herbaceous plants in which PHYA and PHYB gene expression is regulated by the circadian clock.