A Circadian Rhythm Set by Dusk Determines the Expression of FT Homologs and the Short-Day Photoperiodic Flowering Response in Pharbitis

Abstract

Seasonal control of flowering through responsiveness to daylength shows extreme variation. Different species flower in response to long days or short days (SDs), and this difference evolved several times. The molecular mechanisms conferring these responses have been compared in detail only in Arabidopsis thaliana and rice (Oryza sativa) and suggest that a conserved pathway confers daylength responses through regulation of FLOWERING LOCUS T (FT) transcription by CONSTANS (CO). We studied Pharbitis (Ipomoea nil; formerly, Pharbitis nil), a widely used SD model species and a member of the Convolvulaceae, and showed using transgenic plants together with detailed expression analysis that two putative orthologs of FT (Pn FT1 and Pn FT2) promote flowering specifically under SDs. These genes are expressed only under SDs, and light flashes given during the night reduce their expression and prevent flowering. We demonstrate that in Pharbitis a circadian rhythm set by the light-to-dark transition at dusk regulates Pn FT expression, which rises only when the night is longer than 11 h. Furthermore, Pharbitis accessions that differ in their critical night-length responses express Pn FT at different times after dusk, demonstrating that natural genetic variation influencing the clock regulating Pn FT expression alters the flowering response. In these assays, Pn FT mRNA abundance was not related to Pn CO expression, suggesting that Pn FT may be regulated by a different transcription factor in Pharbitis. We conclude that SD response in Pharbitis is controlled by a dedicated light sensitive clock, set by dusk, that activates Pn FT transcription in darkness, a different mechanism for measuring daylength than described for Arabidopsis and rice.

Differential effects of light-to-dark transitions on phase setting in circadian expression among clock-controlled genes in Pharbitis nil

“…17 Here, we strengthen and extend this idea by further illustrating effects of light-to-dark transitions on circadian expression of Pharbitis homologs of a series of Arabidopsis clock and clock-controlled genes. Circadian phases in expression of these genes are set by the dark-to-light transition at dawn without being affected by dusk transitions, further supporting the concept that Pharbitis retains a clock system similar to that in Arabidopsis as well as the system a differently regulated system dedicated to control of photoperiodic flowering.…”

“…13 On the other hand, Pharbitis nil , a member of the Convolvulaceae and widely used as a model short day plants (SDP), measures the duration of the night. 17 In Pharbitis, darkness is required for inducing expression of the florigen-related gene FT1 ( P. nil FT1 or Pn FT1 ), the functional ortholog of Arabidopsis FT . 17 Moreover, the circadian phase of Pn FT1 expression in Pharbitis is strongly set by the light-to-dark transition at dusk or light-off in day/night cycles, which allows its expression to proceed only when plants are exposed to sufficiently long nights and eventually to accumulate specifically under SDs.…”

“…17 In Pharbitis, darkness is required for inducing expression of the florigen-related gene FT1 ( P. nil FT1 or Pn FT1 ), the functional ortholog of Arabidopsis FT . 17 Moreover, the circadian phase of Pn FT1 expression in Pharbitis is strongly set by the light-to-dark transition at dusk or light-off in day/night cycles, which allows its expression to proceed only when plants are exposed to sufficiently long nights and eventually to accumulate specifically under SDs. 17 Such a phase setting manner in Pn FT1 expression indicates presence of a dusk-set photoperiodic time-keeping rhythm and a clock system reset by the light-to-dark transition.…”

“…17 Moreover, the circadian phase of Pn FT1 expression in Pharbitis is strongly set by the light-to-dark transition at dusk or light-off in day/night cycles, which allows its expression to proceed only when plants are exposed to sufficiently long nights and eventually to accumulate specifically under SDs. 17 Such a phase setting manner in Pn FT1 expression indicates presence of a dusk-set photoperiodic time-keeping rhythm and a clock system reset by the light-to-dark transition. However, the clock in Arabidopsis appears to be rather reset by the light-on at dawn without being affected by the light-to-dark transition at dusk.…”

“…18,19 Therefore, Pharbitis is likely to contain a clock system that controls photoperiodic flowering whose manner of light resetting is inconsistent with that of Arabidopsis. 17 …”