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

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

JOURNAL=Frontiers in Plant Science

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.634091

DOI=10.3389/fpls.2021.634091

ISSN=1664-462X

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