Medicago truncatula SOC1 Genes Are Up-regulated by Environmental Cues That Promote Flowering

Abstract

Like <i>Arabidopsis thaliana</i>, the flowering of the legume <i>Medicago truncatula</i> is promoted by long day (LD) photoperiod and vernalization. However, there are differences in the molecular mechanisms involved, with orthologs of two key <i>Arabidopsis thaliana</i> regulators, <i>FLOWERING LOCUS C</i> (<i>FLC</i>) and <i>CONSTANS</i> (<i>CO</i>), being absent or not having a role in flowering time function in <i>Medicago</i>. In <i>Arabidopsis</i>, the MADS-box transcription factor gene, <i>SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1</i> (<i>AtSOC1</i>), plays a key role in integrating the photoperiodic and vernalization pathways. In this study, we set out to investigate whether the <i>Medicago SOC1</i> genes play a role in regulating flowering time. Three <i>Medicago SOC1</i> genes were identified and characterized (<i>MtSOC1a-MtSOC1c</i>). All three <i>MtSOC1</i> genes, when heterologously expressed, were able to promote earlier flowering of the late-flowering <i>Arabidopsis soc1-2</i> mutant. The three <i>MtSOC1</i> genes have different patterns of expression. However, consistent with a potential role in flowering time regulation, all three <i>MtSOC1</i> genes are expressed in the shoot apex and are up-regulated in the shoot apex of plants in response to LD photoperiods and vernalization. The up-regulation of <i>MtSOC1</i> genes was reduced in <i>Medicago fta1-1</i> mutants, indicating that they are downstream of <i>MtFTa1.</i> Insertion mutant alleles of <i>Medicago soc1b</i> do not flower late, suggestive of functional redundancy among <i>Medicago SOC1</i> genes in promoting flowering.