BackgroundFlowering reversion can be induced in soybean (Glycine max L. Merr.), a typical short-day (SD) dicot, by switching from SD to long-day (LD) photoperiods. This process may involve florigen, putatively encoded by FLOWERING LOCUS T (FT) in Arabidopsis thaliana. However, little is known about the potential function of soybean FT homologs in flowering reversion.MethodsA photoperiod-responsive FT homologue GmFT (renamed as GmFT2a hereafter) was cloned from the photoperiod-sensitive cultivar Zigongdongdou. GmFT2a gene expression under different photoperiods was analyzed by real-time quantitative PCR. In situ hybridization showed direct evidence for its expression during flowering-related processes. GmFT2a was shown to promote flowering using transgenic studies in Arabidopsis and soybean. The effects of photoperiod and temperature on GmFT2a expression were also analyzed in two cultivars with different photoperiod-sensitivities.Results GmFT2a expression is regulated by photoperiod. Analyses of GmFT2a transcripts revealed a strong correlation between GmFT2a expression and flowering maintenance. GmFT2a transcripts were observed continuously within the vascular tissue up to the shoot apex during flowering. By contrast, transcripts decreased to undetectable levels during flowering reversion. In grafting experiments, the early-flowering, photoperiod-insensitive stock Heihe27 promotes the appearance of GmFT2a transcripts in the shoot apex of scion Zigongdongdou under noninductive LD conditions. The photothermal effects of GmFT2a expression diversity in cultivars with different photoperiod-sensitivities and a hypothesis is proposed.Conclusion GmFT2a expression is associated with flowering induction and maintenance. Therefore, GmFT2a is a potential target gene for soybean breeding, with the aim of increasing geographic adaptation of this crop.

A FRUITFULL homolog GmFULa was cloned and found to play roles in the flowering and maturation of soybean. Soybean varieties exhibit great diversity in terms of flowering and maturation due to differences in their photoperiodic responses. The underlying mechanism remains unclear despite the fact that some upstream flowering genes have been studied. FRUITFULL (FUL) genes are one group of downstream flowering genes known to have major roles in reproductive transition, floral meristem identity, and floral organ identity. However, FUL homologs and their functions are poorly understood in soybean. Here, a soybean FUL homolog was cloned from the late-maturing photoperiod-sensitive soybean variety Zigongdongdou (ZGDD) and designated GmFULa. In ZGDD, GmFULa exhibited a terminal-preferential expression pattern, with higher expression in the root and shoot apices than in the middle parts. Diurnal rhythm analysis revealed that photoperiod regulates the GmFULa expression level but does not alter its diurnal rhythm. ZGDD was maintained under different photoperiod conditions (long day, LD; short day, SD; LD after 13 short days, SD13-LD) to assess GmFULa expression in newly expanded leaves and in the shoot apex. From this analysis, GmFULa expression was detected in the floral meristem, floral organs and their primordia; trifoliate leaves; and the inflorescence meristem, with the expression levels induced by SD and inhibited by LD. GmFULa expression was also associated with maturity in seven soybean varieties with different photoperiod sensitivities. Therefore, photoperiod conditions affect the expression level of GmFULa but not its diurnal rhythm. The gene plays pleiotropic roles in reproductive transition, flowering, and leaf development and is associated with maturity in soybean.

Soybean [Glycine max (L.) Merr.] is a short-day crop that favors temperate weather. There have been a lot of studies on the responses to photoperiod, temperature or photothermal comprehensive regimes in soybean. However, little work has been conducted in the interactive effects of photoperiod and temperature on the development of soybean. To fully understand the photothermal responses of soybean and to identify the varietal variations in these traits, we designed two photoperiodic treatments of 12 h (short day or SD) and 16 h (long day or LD) integrated with two thermal conditions, i.e. high temperature (HT) (summer seeding in Beijing) and low temperature (LT) (spring seeding) in the current study. In 2007, the responses to the photoperiod, temperature, and photoperiod-temperature combinations of 10 spring sowing soybean varieties (lines) from the Northeast and 18 summer sowing varieties (lines) from Yellow-Huai-Hai River Valleys of China were identified in 4 photothermal regimes (LD+LT, LD+HT, SD+LT, SD+HT). In 2008, 50 varieties (lines) were used to test the photoperiodic responses. The results showed that SD promoted the developmental rate of soybean regardless of the temperature conditions; HT shortened the number of the days from emergence to flowering no matter whether the photoperiod was long or short. There was significant interaction between temperature and photoperiod from emergence to flowering. With the increase of temperature, the promotive effect of SD on developmental rate of soybean was enhanced, and the HT hastening effect was strengthened by SD as well. The apparent differences in photoperiod response sensitivity (PRS), temperature response sensitivity (TRS) and photothermal comprehensive response sensitivity (PTCRS) between ecotypes were observed. The above three indices of spring sowing soybean varieties from the Northeast were all lower than those of summer sowing varieties from Yellow-Huai-Hai River Valleys. However, the differences between TRSvalues under the two photoperiod treatments and that between PRS values under the two temperature conditions in spring sowing soybean varieties from the Northeast were both larger than those in summer sowing varieties from the Valleys, and it indicated that there was higher photoperiod × temperature interaction in the spring sowing varieties. The relationship between photothermal responses of soybean varieties and their ecological adaptability was discussed, and it was proposed that, in breeding program, emphasis should be paid not only on the identification of responses of soybean varieties to the individual photoperiod or temperature factor but also on the photothermal interaction.