Abstract
The timing of the transition to flowering is carefully controlled by plants in order to optimize sexual reproduction and the ensuing production of seeds, grains, and fruits. The genetic networks that regulate floral induction are best characterized in the temperate eudicot Arabidopsis in which the florigen gene FT plays a major role in promoting the transition to flowering. Legumes are an important plant group, but less is known about the regulation of their flowering time. In the model legume Medicago truncatula (Medicago), a temperate annual plant like Arabidopsis, flowering is induced by prolonged cold (vernalization) followed by long day lengths (LD). Recent molecular-genetic experiments have revealed that a FT-like gene, MtFTa1, is a central regulator of flowering time in Medicago. Here, we characterize the three Medicago FRUITFULL (FUL) MADS transcription factors, MtFULa, MtFULb, and MtFULc using phylogenetic analyses, gene expression profiling through developmental time courses, and functional analyses in transgenic plants. MtFULa and MtFULb have similarity in sequence and expression profiles under inductive environmental conditions during both vegetative and reproductive development while MtFULc is only up regulated in the apex after flowering in LD conditions. Sustained up regulation of MtFULs requires functional MtFTa1 but their transcript levels are not affected during cold treatment. Overexpression of MtFULa and MtFULb promotes flowering in transgenic Arabidopsis plants with an additional terminal flower phenotype on some 35S:MtFULb plants. An increase in transcript levels of the MtFULs was also observed in Medicago plants overexpressing MtFTa1. Our results suggest that the MtFULs are targets of MtFTa1. Overall, this work highlights the conserved functions of FUL-like genes in promoting flowering and other roles in plant development and thus contributes to our understanding of the genetic control of the flowering process in Medicago.
Highlights
Flowering time is an important adaptive trait in crop plants because of its major effect on plant yield and productivity (Jung and Muller, 2009; Putterill et al, 2013)
Our study focuses on the small temperate forage legume model plant, Medicago truncatula (Medicago) as it offers a number of advantages including a sequenced genome and tagged mutant lines for forward and reverse genetic screens (Tadege et al, 2009; Young et al, 2011; Putterill et al, 2013)
Translation of the MtFULb sequence indicated that it encoded a protein of 148 amino acids that was truncated at the C-terminus compared to Arabidopsis FUL (FUL) and the other MtFULs
Summary
Flowering time is an important adaptive trait in crop plants because of its major effect on plant yield and productivity (Jung and Muller, 2009; Putterill et al, 2013). The genetic network that regulates the transition to flowering is best understood in the small Brassicaceous annual weed, Arabidopsis (Srikanth and Schmid, 2011; Andres and Coupland, 2012). In Arabidopsis, at least six pathways transduce external and internal cues and regulate flowering time by converging on floral integrator genes such as FT, SOC1, and LFY (Srikanth and Schmid, 2011; Andres and Coupland, 2012). After winter, FLC inhibition of FT and SOC1 is relieved, allowing the photoperiod pathway via the activator gene, CO to up-regulate them which promotes flowering in the long days (LDs) of spring. The commitment to flower is associated with the development of inflorescence meristem which grows indeterminately producing floral primordia on its flanks
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