Abstract
FLOWERING LOCUS T (FT) is a key flowering integrator in Arabidopsis (Arabidopsis thaliana), with homologs that encode florigens in many plant species regardless of the type of photoperiodic response. We identified 10 FT homologs, which were arranged as five pairs of linked genes in different homoeologous chromosomal regions, in soybean (Glycine max), a paleopolyploid species. Two of the FT homologs, GmFT2a and GmFT5a, were highly up-regulated under short-day (SD) conditions (inductive for flowering in soybean) and had diurnal expression patterns with the highest expression 4 h after dawn. Under long-day (LD) conditions, expression of GmFT2a and GmFT5a was down-regulated and did not follow a diurnal pattern. Flowering took much longer to initiate under LD than under SD, and only the GmFT5a transcript accumulated late in development under LD. Ectopic expression analysis in Arabidopsis confirmed that both GmFT2a and GmFT5a had the same function as Arabidopsis FT, but the effect of GmFT5a was more prominent. A double-mutant soybean line for two PHYTOCHROME A (PHYA) genes expressed high levels of GmFT2a and GmFT5a under LD, and it flowered slightly earlier under LD than the wild type grown under SD. The expression levels of GmFT2a and GmFT5a were regulated by the PHYA-mediated photoperiodic regulation system, and the GmFT5a expression was also regulated by a photoperiod-independent system in LD. Taken together, our results suggest that GmFT2a and GmFT5a coordinately control flowering and enable the adaptation of soybean to a wide range of photoperiodic environments.
Highlights
FLOWERING LOCUS T (FT) is a key flowering integrator in Arabidopsis (Arabidopsis thaliana), with homologs that encode florigens in many plant species regardless of the type of photoperiodic response
We searched the databases with the cDNA sequence and found a bacterial artificial chromosome (BAC) clone (AC121763) containing two tandemly linked FT homologs, which were originally identified as TERMINAL FLOWER1 (TFL1) homologs (Cannon et al, 2003)
FT plays a central role as a florigen in floral induction, and its function is conserved across different plant species
Summary
FLOWERING LOCUS T (FT) is a key flowering integrator in Arabidopsis (Arabidopsis thaliana), with homologs that encode florigens in many plant species regardless of the type of photoperiodic response. Two of the FT homologs, GmFT2a and GmFT5a, were highly up-regulated under short-day (SD) conditions (inductive for flowering in soybean) and had diurnal expression patterns with the highest expression 4 h after dawn. In Arabidopsis, CONSTANS (CO) is a key protein in photoperiod sensing; it directly induces the expression of FT and the closely related gene TWIN SISTER OF FT (TSF) under long-day (LD) conditions (Samach et al, 2000; Suarez-Lopez et al, 2001; Yamaguchi et al, 2005; for review, see Lagercrantz, 2009). Under LD conditions, PHYBpromoted degradation of the CO protein is antagonized toward the end of the day by the action of PHYA and CRY2, resulting in the stabilization of CO, which in turn induces the expression of FT (Valverde et al, 2004). Night-break experiments further demonstrated that light signal transduction by phytochromes is the primary determinant of Hd3a transcription, because circadian-regulated Hd1 expression is not affected (Ishikawa et al, 2005, 2009)
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