Night-Break Experiments Shed Light on the Photoperiod1-Mediated Flowering

Abstract

Plants utilize variation in day length (photoperiod) to anticipate seasonal changes. They respond by modulating their growth and development to maximize seed production, which in cereal crops is directly related to yield. In wheat (Triticum aestivum), the acceleration of flowering under long days (LD) is dependent on the light induction of PHOTOPERIOD1 (PPD1) by phytochromes. Under LD, PPD1 activates FLOWERING LOCUS T1 (FT1), a mobile signaling protein that travels from the leaves to the shoot apical meristem to promote flowering. Here, we show that the interruption of long nights by short pulses of light ("night-break" [NB]) accelerates wheat flowering, suggesting that the duration of the night is critical for wheat photoperiodic response. PPD1 transcription was rapidly upregulated by NBs, and the magnitude of this induction increased with the length of darkness preceding the NB. Cycloheximide abolished the NB up-regulation of PPD1, suggesting that this process is dependent on active protein synthesis during darkness. While one NB was sufficient to induce PPD1, more than 15 NBs were required to induce high levels of FT1 expression and a strong acceleration of flowering. Multiple NBs did not affect the expression of core circadian clock genes. The acceleration of flowering by NB disappeared in ppd1-null mutants, demonstrating that this response is mediated by PPD1. The acceleration of flowering was strongest when NBs were applied in the middle of the night, suggesting that in addition to PPD1, other circadiancontrolled factors are required for the up-regulation of FT1 expression and the acceleration of flowering.Plants can anticipate diurnal and seasonal fluctuations in their environment and adjust their growth and development to coincide with favorable conditions. In flowering plants, reproductive development must be optimally timed to minimize the risk of damage to sensitive floral organs by late frosts or early high temperatures. The correct timing of this transition is a major determinant of reproductive success and, in cereal crops such as wheat (Triticum aestivum), of grain yield. Therefore, an improved understanding of the regulation of flowering time can contribute to the development of crop varieties better adapted to diverse environments. Photoperiodic flowering responses vary in different species; short-day (SD) plants and long-day (LD) plants exhibit accelerated flowering in SD and LD, respectively, while day-neutral plants exhibit similar flowering profiles irrespective of day length Allard, 1920, 1923). Plants possess complex regulatory mechanisms to perceive and respond to changes in photoperiod, which ensure that flowering occurs only under inductive conditions. The regulation of flowering time by photoperiod is best understood in Arabidopsis (Arabidopsis thaliana), a LD plant where the photoperiodic response is mainly controlled by CONSTANS (CO). CO expression is regulated by the circadian clock and peaks in the late afternoon and evening (Suárez-López et al., 2001). The CO protein is destabili...

Effect ofphyBandphyCloss-of-function mutations on the wheat transcriptome under short and long day photoperiods

“…This PPD1 allele has a deletion in its promoter region that encompass the binding site of the ELF3 protein repressor [5], resulting in ectopic expression of PPD1 during the night [14], which is critical for the photoperiodic response as demonstrated in night-break experiments. Induction of PPD1 in the middle of a 16 h night (SD) by a 15 m pulse of light accelerates heading time almost as much as a LD photoperiod [4]. In Brachypodium , it has been proposed that PHYC activation of PPD1 is mediated by ELF3 [5], so the elimination of an ELF3 binding site in the Ppd-A1a allele in wheat may limit the transmission of the phytochrome signal to PPD1 .…”

Coincidence of potato CONSTANS (StCOL1) expression and light cannot explain night‐break repression of tuberization

“…Similar findings were made in the short‐day plants rice and Chrysanthemum, where a night break given in the middle of the night period had the strongest inhibitory effect on the flowering (Horridge and Cockshull , Ishikawa et al ). In long‐day plant wheat, several night breaks were tested and the most successful night break was the one given at the middle of the night period (Pearce et al ). The action of a night break may not be regulated through coincidence between StCOL1 and light, but through a mechanism involving the night length.…”

“…Although StCOL1 was expressed at a low level during a classic night break, StSP5G was induced and tuberization was repressed. In wheat, the greatest night‐break‐mediated flower induction was observed in the middle of the night when PPD1 expression was low (Pearce et al ). The authors suggested one or more unknown circadian clock genes gated the night‐break effect, which may be the case for potato as well.…”

“…A night break not only represses short-day tuberization, but also short-day flowering in species like Chrysanthemum (Borthwick and Cathey 1962, Horridge and Cockshull 1989, Higuchi et al 2012). Furthermore, night breaks given in the long nights of a short day can induce flowering in long-day plants like wheat (Pearce et al 2017). Although a lot of research has been done on night breaks, it is not fully understood how a night break regulates tuberization.…”

The enigma of dual reproduction in potato : casting light on tuberization and flowering

“…A night break not only inhibits short-day tuberization, but also short-day flowering in species like Chrysanthemum (Borthwick and Cathey, 1962, Horridge and Cockshull, 1989, Higuchi et al, 2012. Furthermore, night breaks given in the long nights of a short day can induce flowering in long-day plants like wheat (Pearce et al, 2017). Although a lot of research has been done on night breaks, it is not fully understood how a night break regulates tuberization.…”