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

Jared B Fudge,Richard C Macknight,Kirankumar S Mysore,James L Weller,Jiangqi Wen,Rebecca E Laurie,Robyn H Lee

doi:10.3389/fpls.2018.00496

Abstract

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

Highlights

In annual plants, the transition from vegetative growth to flowering, termed floral induction, is regulated by environmental and endogenous cues to promote flowering in spring time (Srikanth and Schmid, 2011; Letswaart et al, 2012; Romera-Branchat et al, 2014)
Our study indicates that all three MtSOC1s are up-regulated by favorable seasonal cues in the shoot apex, via both MtFTa1-dependent and MtFTa1independent pathways, and likely play a role in the regulation of Medicago flowering
Since exposure of SDraised vernalized WT plants to just 3 long day (LD) is sufficient to commit Medicago to flowering (Laurie et al, 2011), we investigated whether MtSOC1 is induced in the shoot apical meristem (SAM) after this period of exposure to LD photoperiod, and the role of MtFTa1 in this activation (Figure 5 and statistical analysis of this data is shown in Supplementary Figure 9)

Summary

INTRODUCTION

The transition from vegetative growth to flowering, termed floral induction, is regulated by environmental and endogenous cues to promote flowering in spring time (Srikanth and Schmid, 2011; Letswaart et al, 2012; Romera-Branchat et al, 2014). In the Brassica Arabidopsis thaliana, the key environmental cues which promote flowering are exposure to a prolonged period of cold (vernalization), followed by long day (LD) photoperiods Endogenous signals such as carbohydrate status, gibberellin metabolism, developmental stage, and the autonomous floral promotion pathway interact to promote flowering (Andres and Coupland, 2012; McClung et al, 2016; Cheng et al, 2017). In Arabidopsis, floral induction is repressed in non-inductive conditions by the MADS-box transcription factors FLOWERING LOCUS C (FLC) and SHORT VEGETATIVE PHASE (SVP) (Andres and Coupland, 2012). Our study indicates that all three MtSOC1s are up-regulated by favorable seasonal cues in the shoot apex, via both MtFTa1-dependent and MtFTa1independent pathways, and likely play a role in the regulation of Medicago flowering

MATERIALS AND METHODS

RESULTS

DISCUSSION