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Soybean is a photoperiod-sensitive short-day crop whose reproductive period and yield are markedly affected by day-length changes. Seed weight is one of the key traits determining the soybean yield; however, the prominent genes that control the final seed weight of soybean and the mechanisms underlying the photoperiod's effect on this trait remain poorly understood. In this study, we identify SW19 as a major locus controlling soybean seed weight by QTL mapping and determine Dt1 , an orthologous gene of Arabidopsis TFL1 that is known to govern the soybean growth habit, as the causal gene of the SW19 locus. We showed that Dt1 is highly expressed in developing seeds and regulates photoperiod-dependent seed weight in soybean. Further analyses revealed that the Dt1 protein physically interacts with the sucrose transporter GmSWEET10a to negatively regulate the import of sucrose from seed coat to the embryo, thus modulating seed weight under long days. However, Dt1 does not function in seed development under short days due to its very low expression. Importantly, we discovered a novel natural allelic variant of Dt1 (H4 haplotype) that decouples its pleiotropic effects on seed size and growth habit; i.e., this variant remains functional in seed development but fails to regulate the stem growth habit of soybean. Collectively, our findings provide new insights into how soybean seed development responds to photoperiod at different latitudes, offering an ideal genetic component for improving soybean's yield by manipulating its seed weight and growth habit. Soybean plants can perceive photoperiod signals to modulate their seed development. Under long-day conditions, expression of Dt1 , a key gene controlling the growth habit of soybean, is markedly increased in developing seeds. The Dt1 protein physically interacts with the sucrose transporter GmSWEET10a to negatively regulate the transport of sucrose from seed coat to the embryo, thus controlling seed size and weight. [ABSTRACT FROM AUTHOR]

Flowering time and stem growth habit determine inflorescence architecture in soybean, which in turn influences seed yield. Dt1, a homolog of Arabidopsis TERMINAL FLOWER 1 (TFL1), is a major controller of stem growth habit, but its underlying molecular mechanisms remain unclear. Here, we demonstrate that Dt1 affects node number and plant height, as well as flowering time, in soybean under long‐day conditions. The bZIP transcription factor FDc1 physically interacts with Dt1, and the FDc1‐Dt1 complex directly represses the expression of APETALA1 (AP1). We propose that FT5a inhibits Dt1 activity via a competitive interaction with FDc1 and directly upregulates AP1. Moreover, AP1 represses Dt1 expression by directly binding to the Dt1 promoter, suggesting that AP1 and Dt1 form a suppressive regulatory feedback loop to determine the fate of the shoot apical meristem. These findings provide novel insights into the roles of Dt1 and FT5a in controlling the stem growth habit and flowering time in soybean, which determine the adaptability and grain yield of this important crop. [ABSTRACT FROM AUTHOR]

The classical soybean (Glycine max) trait long juvenile (LJ) is essentially a reduction in sensitivity to short-day (SD) conditions for induction and completion of flowering, and has been introduced into soybean cultivars to improve yield in tropical environments. However, only one locus, J , is known to confer LJ in low-latitude varieties. Here, we defined two quantitative trait loci contributing to the LJ trait, LJ16.1 and LJ16.2 , and identified them as the florigen (FT) homologs FT2a and FT5a , respectively. The two selected florigen variations both delay flowering time under SD conditions by repressing the floral meristem identity gene GmAPETALA1. Single mutants have a relatively subtle effect on flowering time and displayed a substantial genetic compensation response, but this was absent in ft2a ft5a double mutants, which showed an enhanced LJ phenotype that translated to higher yields under SD conditions. A survey of sequence diversity suggests that FT2a and FT5a variants have diverse origins and have played distinct roles as soybean spread to lower latitudes. Our results show that integration of variants in the florigen genes offers a strategy for customizing flowering time to adjust adaptation and improve crop productivity in tropical regions. [Display omitted] • Natural variants of FT2a and FT5a confer the long-juvenile (LJ) trait in soybean • Selected FT variants repress AP1 and interact to enhance late maturity and yield • Single- ft mutants display genetic compensation, which is absent in double mutants • FT allelic diversification played distinct roles as soybean spread to the tropics Little is known about the genetics of soybean varieties adapted to low latitudes. Li et al. show variation in two florigen genes contributes to local adaptation and improved soybean yield. By exploiting combinations of alleles to overcome the genetic compensation response, this study offers a strategy to improve adaptation and crop productivity in tropical regions. [ABSTRACT FROM AUTHOR]