ABA and the Floral Transition

Plants use endogenous and environmental cues to trigger flowering. While variations in day length and temperature play a major role in controlling the transition to flowering, little is known about water stress-derived signals. Drought conditions cause early flowering in various plant species. Since it is well recognized that drought conditions also stimulate abscisic acid (ABA) accumulation, ABA signalling might underpin the observed early flowering response. Experiments have shown that exogenous applications of ABA cause flowering time alterations, suggesting that ABA might be an endogenous component affecting the floral transition. Confirming a role for endogenous ABA in flowering, mutants impaired in ABA production display flowering alterations, although a consensus as to the precise mode of action of ABA in plants is lacking. ABA activates flowering in several plant species and in Arabidopsis it promotes activation of the key floral gene FLOWERING LOCUS T. However, how ABA signalling is integrated in the floral network remains poorly understood. The evidence reviewed here suggests that ABA activates a complex network of signalling components including transcription factors with contrasting effects in flowering. Lesions in these ABA-Photoperiod interaction signalling genes produce alterations in flowering but their regulation and site of action have been so far elusive. ABA is mainly known as a stress hormone and its role in flowering is only beginning to emerge. In this chapter we will review the evidence for and against ABA controlling the floral transition.