The Candidate Photoperiod Gene MtFE Promotes Growth and Flowering in Medicago truncatula

Geoffrey Thomson,Joanna Putterill,Kirankumar S Mysore,Jiangqi Wen,Lulu Zhang

doi:10.3389/fpls.2021.634091

Geoffrey Thomson, Joanna Putterill + Show 3 more

Open Access

https://doi.org/10.3389/fpls.2021.634091

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Abstract

Flowering time influences the yield and productivity of legume crops. Medicago truncatula is a reference temperate legume that, like the winter annual Arabidopsis thaliana, shows accelerated flowering in response to vernalization (extended cold) and long-day (LD) photoperiods (VLD). However, unlike A. thaliana, M. truncatula appears to lack functional homologs of core flowering time regulators CONSTANS (CO) and FLOWERING LOCUS C (FLC) which act upstream of the mobile florigen FLOWERING LOCUS T (FT). Medicago truncatula has three LD-induced FT-like genes (MtFTa1, MtFTb1, and MtFTb2) with MtFTa1 promoting M. truncatula flowering in response to VLD. Another photoperiodic regulator in A. thaliana, FE, acts to induce FT expression. It also regulates the FT transport pathway and is required for phloem development. Our study identifies a M. truncatula FE homolog Medtr6g444980 (MtFE) which complements the late flowering fe-1 mutant when expressed from the phloem-specific SUCROSE-PROTON SYMPORTER 2 (SUC2) promoter. Analysis of two M. truncatula Tnt1 insertional mutants indicate that MtFE promotes flowering in LD and VLD and growth in all conditions tested. Expression of MtFTa1, MtFTb1, and MtFTb2 are reduced in Mtfe mutant (NF5076), correlating with its delayed flowering. The NF5076 mutant plants are much smaller than wild type indicating that MtFE is important for normal plant growth. The second mutant (NF18291) displays seedling lethality, like strong fe mutants. We searched for mutants in MtFTb1 and MtFTb2 identifying a Mtftb2 knock out Tnt1 mutant (NF20803). However, it did not flower significantly later than wild type. Previously, yeast-two-hybrid assays (Y2H) suggested that Arabidopsis FE interacted with CO and NUCLEAR FACTOR-Y (NF-Y)-like proteins to regulate FT. We found that MtFE interacts with CO and also M. truncatula NF-Y-like proteins in Y2H experiments. Our study indicates that despite the apparent absence of a functional MtCO-like gene, M. truncatula FE likely influences photoperiodic FT expression and flowering time in M. truncatula via a partially conserved mechanism with A. thaliana.

Highlights

Flowering time plays a central role in optimizing the productivity and overall yield of many crops and was frequently selected for in crop domestication (Kantar et al, 2017)
In order to investigate whether a FE homolog in M. truncatula might play a role in flowering time, reciprocal BLAST searches using FE/ALTERED PHLOEM DEVELOPMENT (APL) as a query (Altschul et al, 1990) were used to identify Medtr6g444980
In a neighbor-joining tree of the 31 SHAQKYF-class MYB-like putative transcription factors identified in the M. truncatula genome, Medtr6g444980 clades closest to FE with strong bootstrap support (Figure 1A)

Summary

Introduction

Flowering time plays a central role in optimizing the productivity and overall yield of many crops and was frequently selected for in crop domestication (Kantar et al, 2017). An understanding of the regulation of flowering time in this family is critical to help realize this potential and facilitate the breeding of more productive varieties. Core components of the pathways described in other species, such as CONSTANS (CO) and FLOWERING LOCUS C, appear to be missing in temperate legumes (Kim et al, 2013; Wong et al, 2014). Progress to date has demonstrated a conserved role for phytochrome and circadian clock genes acting upstream of FLOWERING LOCUS T (FT)-like genes in the regulation of flowering time in legumes (Weller and Ortega, 2015; Jaudal et al, 2020; Weller and Macknight, 2019)

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