The transcriptomic response to a short day to long day shift in leaves of the reference legume Medicago truncatula.

Geoffrey Thomson,Joanna Putterill,James Taylor

doi:10.7717/peerj.6626

Abstract

Photoperiodic flowering aligns plant reproduction to favourable seasons of the year to maximise successful production of seeds and grains. However understanding of this process in the temperate legumes of the Fabaceae family, which are important both agriculturally and ecologically, is incomplete. Previous work in the reference legume Medicago truncatula has shown that the FT-like gene MtFTa1 is a potent floral activator. While MtFTa1 is upregulated by long-day photoperiods (LD) and vernalisation, the molecular basis of this is unknown as functional homologues of key regulatory genes present in other species, notably CONSTANS in A. thaliana, have not been identified. In LD MtFTa1 maintains a near constant diurnal pattern of expression unlike its homologue FT in A. thaliana, which has a notable peak in expression at dusk. This suggests a different manner of regulation. Furthermore, M. truncatula possesses other FT-like genes such as two LD induced MtFTb genes which may also act in the regulation of flowering time. MtFTb genes have a diurnal pattern of expression with peaks at both four and sixteen hours after dawn. This study utilises RNA-Seq to analyse the transcriptome of M. truncatula leaves to identify genes which may regulate or be co-expressed with these FT-like genes following a shift from short-day photoperiods to inductive long-days. Specifically this study focuses on the first four hours of the day in the young leaves, which coincides with the first diurnal peak of the FTb genes. Following differential expression analysis at each timepoint, genes which alter their pattern of expression are distinguished from those which just alter their magnitude of expression (and those that do neither). It goes on to categorise these genes into groups with similar patterns of expression using c-means clustering and identifies a number of potential candidate photoperiod flowering time genes for future studies to consider.

Highlights

The regulation of flowering controls the important developmental shift between the vegetative and reproductive growth phases of the plant, aligning plant sexual reproduction with favourable seasonal environmental variation
Two similar experiments are analysed where plants were grown in SD conditions until they were 10 days old and were shifted at ZT8 into LD conditions to describe the transcriptomic changes which occur in the M. truncatula leaves following such shifts, alongside the FLOWERING LOCUS T (FT)-like genes
This study further elucidates the photoperiodic acceleration of flowering in the reference legume species M. truncatula which interestingly appears to lack a CO-like regulator

Summary

Introduction

The regulation of flowering controls the important developmental shift between the vegetative and reproductive growth phases of the plant, aligning plant sexual reproduction with favourable seasonal environmental variation. FT is activated by circadian and light signals aligning which facilitates the formation of the GIGANTEA-FLAVIN-BINDING KELCH REPEAT, F-BOX 1 (GI-FKF1) complex This complex degrades CYCLING DOF FACTOR (CDF) transcription factors which otherwise form a complex with a TOPLESS (TPL) protein to repress the expression of the transcription factor CONSTANS (CO). FT protein is the principal mobile floral signal (florigen) which is transported to the shoot apical meristem via the phloem to induce flowering via activation of a second floral integrator gene SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and the floral meristem identity gene APETALA1 This results in a state of floral commitment and the development of flowers (Turck, Fornara & Coupland, 2008)

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