Toward a Molecular Understanding of Abscisic Acid Actions in Floral Transition

The transition from the vegetative growth phase to flowering is a crucial checkpoint for plant reproduction and survival, especially under environmental stress conditions. Numerous factors regulate flowering time, including exogenous environmental cues such as day length and temperature, as well as salt and drought stresses, and endogenous phytohormone signaling cascades. Gibberellins and ABA are one classic combination of phytohormones which antagonistically regulate several biological processes, including seed dormancy and germination, primary root growth and seedling development. As regards control of flowering time, gibberellin exhibits a positive role, and represents an important pathway in the regulation of floral transition. However, over the past decades, numerous investigations have demonstrated that the contribution of the stress hormone ABA to floral transition is still controversial, as both positive and negative effects have been documented. It is important to determine why and how ABA shows this contradictory effect on flowering time. In this up to date review, primarily based on recent publications and emerging data, we summarize the distinct and contrasting roles of ABA on floral transition, while the detailed molecular mechanisms underlying these roles are discussed. Finally, the remaining challenges and open questions in this topic are presented.