Photoperiod response of flowering determines plant adaptation to different latitudes. Soybean, a short-day plant, has gained the ability to flower under long-day conditions during the growing season at higher latitudes, mainly through dysfunction of phytochrome A genes (E3 and E4) and the floral repressor E1. In this study, we identified a novel molecular genetic basis of photoperiod insensitivity in Far-Eastern Russian soybean cultivars. By testcrossing these cultivars with a Canadian cultivar Harosoy near-isogenic line for a recessive e3 allele, followed by association tests and fine mapping, we determined that the insensitivity was inherited as a single recessive gene located in an 842-kb interval in the pericentromeric region of chromosome 4, where E1-Like b (E1Lb), a homoeolog of E1, is located. Sequencing analysis detected a single-nucleotide deletion in the coding sequence of the gene in insensitive cultivars, which generated a premature stop codon. Near-isogenic lines (NILs) for the loss-of-function allele (designated e1lb) exhibited upregulated expression of soybean FLOWERING LOCUS T (FT) orthologs, FT2a and FT5a, and flowered earlier than those for E1Lb under long-day conditions in both the e3/E4 and E3/E4 genetic backgrounds. These NILs further lacked the inhibitory effect on flowering by far-red light-enriched long-day conditions, which is mediated by E4, but not that of red-light-enriched long-day conditions, which is mediated by E3. These findings suggest that E1Lb retards flowering under long-day conditions by repressing the expression of FT2a and FT5a independently of E1. This loss-of-function allele can be used as a new resource in breeding of photoperiod-insensitive cultivars, and may improve our understanding of the function of the E1 family genes in the photoperiod responses of flowering in soybean.
Keywords: E1Lb; Glycine max; adaptation; flowering; photoperiodism; soybean.