LcCOL7 and LcCOL8 Negatively Regulate Plant Flowering Independent of Day Length.

CONSTANS-LIKE (COL) genes are pivotal regulatory components in the photoperiodic flowering pathway of plants. These genes can be regulated by both photoreceptors and the circadian clock, modulating plant flowering responses under specific day lengths by regulating florigen levels. However, the COL gene family in Litchi chinensis Sonn. has not yet been characterized. In this study, we identified eight COL family members in litchi and classified them into three subgroups based on phylogenetic analysis. The analysis of cis-regulatory elements within the promoters of LcCOLs revealed a wide distribution of elements associated with light, hormone, and stress responses. Transcript expression profiling indicated that most LcCOLs exhibited relatively high expression levels in leaf buds, leaves, and young fruits. Diurnal expression analysis under natural photoperiod conditions revealed that the expression peaks of all LcCOLs, with the exception of LcCOL1, occurred during the nighttime. The heterologous overexpression of LcCOL7 and LcCOL8, the closest homologs to AtCO, in Arabidopsis thaliana significantly delayed the flowering time under both long-day (LD) and short-day (SD) conditions, indicating that these genes act as repressors of flowering. This study provides a foundational basis for elucidating the molecular mechanisms underlying litchi flowering regulation and identifies promising candidate genes for the molecular breeding of litchi flowering-related agronomic traits.