The unique LcFT1 gene sequence linked to early-flowering in the tropical litchi ‘Khom’

Litchi (Litchi chinensis) was a royal tribute in ancient China. Today, it is still regarded as one of the most luxurious fresh fruits in summer because of the short harvest season. Floral induction in litchi is promoted by the expression of the FLOWERING LOCUS T (FT), LcFT1, when the leaves sense cool temperatures in winter. Moreover, two alleles of the LcFT1 promoter affect the sensitivity and expression level of the gene, resulting in early-flowering or late-flowering traits in litchi of Chinese origin. Some litchi cultivars are able to flower and produce in the tropical region of central Thailand, so called “tropical litchi”, but their flowering mechanism is less understood. In this study, we cloned and investigated the expression of the LcFT homolog from the early-season cultivar ‘Sanyuehong’, the mid-season cultivar ‘Hak Ip’, the late-season cultivar ‘Rose Red’ and the tropical litchi ‘Khom’. Our results revealed a duplication of the partial promoter in the first intron of LcFT1 of tropical litchi ‘Khom’, resulting in a 676 bp insertion. The ‘Khom’-type LcFT1 produces identical transcripts as other cultivars. Although this allele seems to be derived from the late-flowering-type, its expression is earlier than the late-flowering-type. On the other hand, the sequence and expression of LcFT2 was conserved in the cultivars we examined. Today, ‘Khom’ is used in litchi breeding in Taiwan to generate new cultivars that can flower and bear fruit under increasing global warming conditions. Our first findings on the ‘Khom’-type LcFT1 identified a selective target for introgression of the flowering trait in litchi.