Genome-wide identification of CONSTANS-like genes in Dimocarpus longan and functional characterization of DlCOL9 revealing its role in floral induction

Longan (Dimocarpus longan L.), an economically important tropical/subtropical fruit species, faces production constraints due to unreliable floral induction processes. CONSTANS-like (COL) genes play an important role in the photoperiodic flowering pathway. In this study, a total of 10, 15 and 15 DlCOL genes were identified in longan cultivars HHZ, SX and JDB, respectively. Phylogenetic analysis showed that DlCOL genes were divided into three subgroups, and the members of each subgroup had conserved B-box and CCT domains, indicating potential binding interactions between DlCOL genes and flowering-related genes. Analysis of tissue-specific expression showed that all DlCOL genes were widely expressed in various organs of longan, and were preferentially expressed in the leaves. Notably, DlCOL9 exhibited preferential expression in apical buds during the physiological differentiation stage of floral bud induction. Transgenic Arabidopsis plants overexpressing DlCOL9 displayed a significantly shortened flowering time accompanied by the increasing expression of flowering genes AtFT, AtSOC1 and AtCO, indicating that DlCOL9 has the function of promoting flowering. Furthermore, DlCOL9 was co-expressed with key flowering regulators in longan, DlFKF1, DlGI and DlSOC1. Yeast one-hybrid combined with dual-luciferase assays demonstrated that both DlGI and DlFKF1 could bind to the promoter of DlCOL9 to enhance its expression, whereas DlCOL9 transcriptionally regulated DlSOC1 directly. Our results reveal a conserved DlGI/DlFKF1-DlCOL9-DlSOC1 regulatory module governing floral induction in longan which advance the functional understanding of the DlCOL gene family and provide a molecular basis for optimizing flowering regulation in longan production.