An overview of the working model for the photoperiodic flowering time pathway in soybean. Parallel representations of the network under long-day conditions and short-day conditions are shown as well as the genes that when mutated infer photoperiod-insensitive flowering. Under long-day conditions, Phytochrome A homologs GmPHYA3 and GmPHYA2 promote E1 expression and inhibit GmELF3 expression. E1 up-regulates GmFT4a (a change-of-function FT that suppresses flowering) and down-regulates GmFT2a and GmFT5a, all of which are FT homologs (Nan et al., 2014; Samanfar et al., 2017; Xia et al., 2012; Zhai et al., 2014). GmGIa is a GI homolog which inhibits GmFT2a (but interestingly not GmFT5a) thus delaying flowering under long-day conditions (Watanabe et al., 2011).
Under short-day conditions, GmELF3 is expressed. GmELF3 represses E1 by physically associating with the E1 promoter. This leads to the release of the E1 suppression of the GmFT genes, thus promoting flowering under short days (Lu et al., 2017; Xia et al., 2012). Natural variation in the GmFT gene family is at least partially responsible for flowering time variation in soybean, with several polymorphic sites significantly associated with flowering time variation (Jiang et al., 2019). Soybean plants which carry loss-of-function alleles for E1, GmGIa, GmPHYA3 and GmPHYA2 exhibit photoperiod insensitive flowering as higher transcript levels of the FT genes are present (Xu et al., 2013). Thus, these genes may represent strong candidates for elucidating photoperiod-insensitivity in Cannabis.