In 2012, Prof. XIA Zhengjun in Northeast Institute of Geography and Agroecology of the Chinese Academy of Sciences(IGA, CAS) and cooperators deciphered the molecular basis of the E1 locus through positional cloning and found that E1 is the key regulators modulating photoperiodic flowering in soybean.（2012, PNAS）. However, key members of the E1 pathway are largely unknown.
On 02 march 2022, this research team online presented their latest finding ‘FT genes bridge the maturity gene E1 and florigens in photoperiodic regulation of flowering in soybean’ in ‘Plant Physiology ’.

At first, the encoded E1 protein is proven to be a transcriptional repressor. Based on RNA-Sequencing (RNA-Seq) analysis, the expression of seven GmMDE genes (Glycine max MADS-box genes downregulated by E1 MADS-box genes (GmMDEs ) was suppressed when E1 was overexpressed and promoted when E1 was knocked out through CRISPR/Cas9-mediated mutagenesis. These GmMDEs exhibited similar tissue specificity and expression patterns, e.g. their response to photoperiod, E1 expression, and E1 genotype. E1 repressed GmMDE promoter activity. Taking two GmMDEs, GmMDE05 and GmMDE06 as example, E1 epigenetically silences their expression by directly binding to their promoters to increase H3K27me3 levels.

GmMDE06 was focused to explore the function of GmMDEs in flowering time. Overexpression of GmMDE06 promoted flowering and post-flowering termination of stem growth. The late flowering phenotype of E1-overexpressing soybean lines was reversed by overexpression of GmMDE06, placing GmMDE06 downstream of E1. GmMDE06 also promoted post-flowering termination of stem growth by repressing the expression of the shoot identity gene Dt1. Overexpression of GmMDE06 increased the expression of the soybean FLOWERING LOCUS T orthologs GmFT2a and GmFT5a, leading to a feedback upregulation of GmMDE, indicating that GmMDE and GmFT2a/GmFT5a form a positive regulatory feedback loop promoting flowering. The E1-GmMDEs-GmFT2a/5a-Dt1 signaling pathway illustrates how soybean responds to photoperiod by modulating flowering time and post-flowering stem termination.

           Figure 1. Proposed model for E1 regulation of the photoperiodic response of soybean (Image by ZHAI Hong et al.).

ZHAI Hong, an associate professor in IGA, CAS, is the first author of the article, and research professor, XIA Zhengjun is the corresponding author of the article. Professor BAI Xi from Northeast Agricultural University, Professor KONG Fanjiang from Guangzhou University and Professor LUO Xiao from Peking University participated in some of the work.