Masao Ishimoto¹, Toshimasa Yamazaki²

¹Soybean Applied Genomics Research Unit, ²Biomolecular Research Unit

［Abstract］

We isolated E1 gene, which has the prominent effect on soybean flowering time, by positional cloning and elucidated that E1 gene controls flowering time through the expression of soybean florigen genes.

［Keywords］

environment adaptability, florigen, flowering time, photoperiod, soybean

［Background］

Soybean is a typical short-day plant that flowers only when the photoperiod is shorter than a critical length. The cultivation area of each soybean variety is restricted to a very narrow range of latitudes according to its photoperiod sensitivity. A number of genetic factors are associated with the photoperiodic sensitivity of soybean. Understanding and identification of these genetic factors would facilitate the development of new varieties by cross breeding to combine flowering genes with different photoperiod sensitivity and contribute to the expansion of soybean cultivation area and stable production. Among the many flowering gene loci, we characterized the function and expression of the soybean E1 gene which is largely responsible for the variation in flowering time among soybean varieties.

［Results and Discussion］

1. The isolation of E1 gene is challenging because it is mapped at the centromeric region of chromosome 6 where genetic recombination is highly suppressed. Precise mapping of the E1 locus was conducted using a genetic population with more than 13,000 individuals. The E1 locus was subsequently restricted within a 17 kbp physical region.
2. Early-flowering soybean variety ‘Kariyutaka’ overexpressing exogenous E1 gene showed low expression of soybean florigen gene and resulted in the delay of flowering (Fig. 1).
3. Predicted amino acid sequence of E1 gene suggested that this gene encodes a novel transcription factor. Long-day condition was essential to induce the expression of E1 gene causing suppression of the florigen gene, whereas short-day condition inhibited the expression of E1 gene which led to the expression of florigen gene and acceleration of flowering (Fig. 2).
4. Three different recessive alleles for the E1 locus were identified from soybean resources including one allele with an amino acid substitution (e1-as), and another allele with a nucleotide deletion in the coding region (e1-fs) or deletion of large genomic region including E1 locus (e1-nl). The application of these recessive alleles in soybean breeding program enables us to shift the flowering time within 20 days.

1. The allelic information of the E1 locus will facilitate marker assisted selection for efficient breeding of new varieties with suitable photoperiodic response for each cultivation area.
2. Further study is necessary to disclose the precise mechanism of soybean flowering time. In particular, other flowering time QTLs should be identified and the interaction among these flowering genes should be clarified to advance the breeding program for new varieties of soybean.

Fig.1. Analysis of transgenic plant revealed the relationship between E1 gene and soybean florigen genes. A Japanese early flowering soybean variety ‘Kariyutaka’ showed low expression of E1 gene. Exogenous expression of E1 gene in transgenic experiment conferred suppression of florigen gene and the delay of flowering.

Fig.2. The model of diurnal expression profile of E1 gene and soybean florigen gene under different photoperiodic conditions. Long-day condition induced E1 gene expression and suppressed the expression of florigen gene. On the other hand, short-day condition changes the expression profile of these genes in opposite directions. The photoperiodic change becomes the cue for soybean flowering induction through E1 and florigen genes.

 

［Collaborators］

S. Watanabe (Saga Univ.), Z. Xia (Northeast Inst. Geography and Agroecology, China), Y. Tsubokura (Snow Brand Seed Co., Ltd.), K. Harada (NBRP Lotus and Glycine), T. Yamada (Hokkaido Univ.), T. Anai (Saga Univ.), S. Sato and S. Tabata (Kazusa DNA Res. Inst.)

[Reference]

1. Xia Z, Watanabe S, Yamada T, Tsubokura Y, Nakashima H, Zhai H, Anai T, Sato S, Yamazaki T, Lu S, Wu H, Tabata S, Harada K (2012) Positional cloning and characterization reveal the molecular basis for soybean maturity locus E1 that regulates photoperiodic flowering. Proceedings of the National Academy of Sciences USA 109(32):E2155-E2164