Abstract
BackgroundFlowers open at sunrise and close at sunset, establishing a circadian floral movement rhythm to facilitate pollination as part of reproduction. By the coordination of endogenous factors and environmental stimuli, such as circadian clock, photoperiod, light and temperature, an appropriate floral movement rhythm has been established; however, the underlying mechanisms remain unclear.ResultsIn our study, we use waterlily as a model which represents an early-diverging grade of flowering plants, and we aim to reveal the general mechanism of flower actions. We found that the intermediate segment of petal cells of waterlily are highly flexible, followed by a circadian cell expansion upon photoperiod stimuli. Auxin causes constitutively flower opening while auxin inhibitor suppresses opening event. Subsequent transcriptome profiles generated from waterlily’s intermediate segment of petals at different day-time points showed that auxin is a crucial phytohormone required for floral movement rhythm via the coordination of YUCCA-controlled auxin synthesis, GH3-mediated auxin homeostasis, PIN and ABCB-dependent auxin efflux as well as TIR/AFB-AUX/IAA- and SAUR-triggered auxin signaling. Genes involved in cell wall organization were downstream of auxin events, resulting in the output phenotypes of rapid cell expansion during flower opening and cell shrinkage at flower closure stage.ConclusionsCollectively, our data demonstrate a central regulatory role of auxin in floral movement rhythm and provide a global understanding of flower action in waterlily, which could be a conserved feature of angiosperms.
Highlights
Flowers open at sunrise and close at sunset, establishing a circadian floral movement rhythm to facilitate pollination as part of reproduction
The effects of auxin on plant development have been largely attributed to the transcriptional and translational regulations governed by the tryptophan aminotransferase (TAA) and flavin monooxygenase (YUC)-dependent synthesis pathway, Transport Inhibitor Response1/ Auxin-Related F-BOX (TIR1/AFB)-Aux/indole acetic acid (IAA)-regulated signaling pathway and PIN-FORMED (PIN)-mediated transport pathway [13,14,15,16,17,18]
Cell wall biogenesis has been implicated as a core event downstream of auxin regulations that determines anisotropic cell expansion during plant development including floral circadian movement
Summary
Flowers open at sunrise and close at sunset, establishing a circadian floral movement rhythm to facilitate pollination as part of reproduction. Many flowers open during the day and close at night, exhibiting a 24-h circadian rhythm, which is known as Linné’s floral clock [1] This circadian movement pattern serves as an environmental adaptation characteristic for creating an opportunity for pollination [1]; in Ipomoea nil, the flower-opening time is determined by the length of dark period [2]. Cell wall biogenesis has been implicated as a core event downstream of auxin regulations that determines anisotropic cell expansion during plant development including floral circadian movement. A combination of transcriptional, translational and post-translational regulations leads to coordination among various processes including the circadian clock, hormone biosynthesis, light signaling, and cell wall modification to establish a proper opening-closing period in flowers. The phenomenon of floral movements have been broadly described during the past 20 years’ research [9, 20], the underlying regulatory mechanisms are largely elusive
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