Abstract
The zinc finger transcription factor CONSTANS has a well-established central role in the mechanism for photoperiod sensing in Arabidopsis, integrating light and circadian clock signals to upregulate the florigen gene FT under long-day but not short-day conditions. Although CONSTANS-LIKE (COL) genes in other species have also been shown to regulate flowering time, it is not clear how widely this central role in photoperiod sensing is conserved. Legumes are a major plant group and various legume species show significant natural variation for photoperiod responsive flowering. Orthologs of several Arabidopsis genes have been shown to participate in photoperiodic flowering in legumes, but the possible function of COL genes as integrators of the photoperiod response has not yet been examined in detail. Here we characterize the COL family in the temperate long-day legume Medicago truncatula, using expression analyses, reverse genetics, transient activation assays and Arabidopsis transformation. Our results provide several lines of evidence suggesting that COL genes are unlikely to have a central role in the photoperiod response mechanism in this species.
Highlights
The length of the daily photoperiod is an important environmental variable that influences plant development
DEFINING THE CONSTANS-LIKE (COL) GENE FAMILY IN LEGUMES We previously reported a partial characterization of the COL gene family in legumes (Hecht et al, 2005) focusing on the socalled Group I COL genes (Griffiths et al, 2003)
The CONSTANS protein has been a central feature of models explaining the molecular basis for plant responses to photoperiod, and the potential conservation of CO function across flowering plants has been a topic of considerable interest
Summary
The length of the daily photoperiod is an important environmental variable that influences plant development. The study of induced mutants and natural variants affecting photoperiod responsiveness in both species has identified genes in the FT florigen family as the major target of photoperiod regulation, and have highlighted the general importance of light signaling pathways and the circadian clock for photoperiod measurement (Andres and Coupland, 2012; Brambilla and Fornara, 2013; Song et al, 2013; Tsuji et al, 2013). In Arabidopsis, one gene in particular, CONSTANS (CO), has a central role in the mechanism of photoperiod measurement, integrating clock and light signals to provide photoperiod-specific induction of FT expression (Andres and Coupland, 2012; Song et al, 2013). It has subsequently been shown that FT is an early transcriptional target of CO (Samach et al, 2000), and that the CO protein binds to the FT promoter (Tiwari et al, 2010)
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