Photoperiodic Regulation of Tuber Enlargement in Water Yam

In tuberous crops, tuber enlargement is one of the most important target traits for yield formation. It has long been known that tuber growth in yams is enhanced by short-day (SD) conditions, but the mechanism of tuber enlargement remains unknown. Here, we analyzed the photoperiodic regulation of tuber enlargement in water yam (Dioscorea alata L.). The photoperiod experiments in seedlings showed that tuber enlargement is initiated under SD conditions (<= 10 h daylength) within 20 days of treatment. DaFT2, a FLOWERING LOCUS T (FT)-like gene, was upregulated in SD and downregulated in long-day (LD) conditions in tubers, suggesting that DaFT2 promotes tuber enlargement. DaFT1, the other FT-like gene, was significantly upregulated only in the leaves under LD, and its expression pattern was opposite to that of DaFT2 in the tubers. A night-break experiment showed that tuber growth was inhibited by red light in the dark period. These results suggest that the tuber enlargement of water yam is completely dependent on the photoperiod and that it involves an FT gene-mediated mechanism in response to the SD condition by red light sensing.