Immediate induction of APETALA1-like gene expression following a single short-day in Pharbitis nil

Pharbitis [Ipomoea] nil cv. Violet is an excellent model plant for the study of photoperiodic induction of flowering because it can be induced flowering by a single short-day. However there are few molecular-level studies of the induction of flowering in Pharbitis. To gain insight into the photoperiodic induction of flowering, we isolated an APETALA1-like gene (PnAP1), which showed high similarity to SQUAMOSA (SQUA) and AP1. PnAP1 expression at the shoot apex was induced only under flowering-inductive conditions, and PnAP1 expression was induced from 24 h after the start of the inductive dark period. Both night-break and low ambient temperature during the dark period effectively repressed PnAP1 expression and flowering. Timing of PnAP1 expression assessed through induced cotyledon removal indicated that the floral stimulus began to move from cotyledons 14 h after the start of the inductive dark period. The results indicate that floral transitions begin immediately after the inductive dark period and that PnAP1 is a good molecular marker of floral transition.