Abstract
BackgroundFlowering time is an important trait for productivity in legumes, which include many food and fodder plants. Medicago truncatula (Medicago) is a model temperate legume used to study flowering time pathways. Like Arabidopsis thaliana (Arabidopsis), its flowering is promoted by extended periods of cold (vernalization, V), followed by warm long day (LD) photoperiods. However, Arabidopsis flowering-time genes such as the FLOWERING LOCUS C (FLC)/ MADS AFFECTING FLOWERING (MAF) clade are missing and CONSTANS-LIKE (CO-LIKE) genes do not appear to have a role in Medicago or Pisum sativum (pea). Another photoperiodic regulator, the red/far red photoreceptor PHYTOCHROME A (PHYA), promotes Arabidopsis flowering by stabilizing the CO protein in LD. Interestingly, despite the absence of CO-LIKE function in pea, PsPHYA plays a key role in promoting LD photoperiodic flowering and plant architecture. Medicago has one homolog of PHYA, MtPHYA, but its function is not known.ResultsGenetic analysis of two MtPHYA Tnt1 insertion mutant alleles indicates that MtPHYA has an important role in promoting Medicago flowering and primary stem elongation in VLD and LD and in perception of far-red wavelengths in seedlings. MtPHYA positively regulates the expression of MtE1-like (MtE1L), a homologue of an important legume-specific flowering time gene, E1 in soybean and other Medicago LD-regulated flowering-time gene homologues, including the three FLOWERING LOCUS T-LIKE (FT-LIKE) genes, MtFTa1, MtFTb1 and MtFTb2 and the two FRUITFULL-LIKE (FUL-LIKE) genes MtFULa and MtFULb. MtPHYA also modulates the expression of the circadian clock genes, GIGANTEA (GI) and TIMING OF CAB EXPRESSION 1a (TOC1a). Genetic analyses indicate that Mtphya-1 Mte1l double mutants flowered at the same time as the single mutants. However, Mtphya-1 Mtfta1 double mutants had a weak additive effect in delaying flowering and in reduction of primary axis lengths beyond what was conferred by either of the single mutants.ConclusionMtPHYA has an important role in LD photoperiodic control of flowering, plant architecture and seedling de-etiolation under far-red wavelengths in Medicago. It promotes the expression of LD-induced flowering time genes and modulates clock-related genes. In addition to MtFTa1, MtPHYA likely regulates other targets during LD floral induction in Medicago.
Highlights
Flowering time is an important trait for productivity in legumes, which include many food and fodder plants
MtPHYA was detected in a wide range of tissues 2 h after dawn, including cotyledons, leaves, apical buds, open flowers and roots (Fig. 1a) and was slightly more abundant in short day (SD) than in long day (LD) photoperiods (Fig. 1b)
Medicago is a long day plant, but the molecular basis of this photoperiodic regulation is still unclear because Medicago lacks a CO function, which has a central role in LD-mediated acceleration of flowering in Arabidopsis
Summary
Flowering time is an important trait for productivity in legumes, which include many food and fodder plants. Arabidopsis flowering-time genes such as the FLOWERING LOCUS C (FLC)/ MADS AFFECTING FLOWERING (MAF) clade are missing and CONSTANS-LIKE (COLIKE) genes do not appear to have a role in Medicago or Pisum sativum (pea) Another photoperiodic regulator, the red/far red photoreceptor PHYTOCHROME A (PHYA), promotes Arabidopsis flowering by stabilizing the CO protein in LD. Like the winter annual ecotypes of Arabidopsis thaliana (Arabidopsis), Medicago is a temperate-climate plant whose flowering time is promoted by long day (LD) photoperiods and by vernalization – an extended exposure to winter cold. This ensures that flowering occurs in the lengthening warm days of spring only after winter has passed [10]. The key regulatory genes known to be targets of the Arabidopsis vernalization pathway - the repressor FLOWERING LOCUS C (FLC) and associated MADS AFFECTING FLOWERING (MAF) clade are missing in Medicago [4, 8]
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