Crosstalk and abscisic acid: the roles of terpenoid hormones in coordinating development

The sesquiterpenoid hormone abscisic acid (ABA) regulates many aspects of plant growth and development. It has been difficult, however, to understand how this hormone functions in a myriad of events. Genetic analysis, particularly in Arabidopsis, has identified genes that modulate ABA responsiveness, but a molecular framework has not been developed to explain how these genes direct ABA-mediated developmental events. Certainly, some of the diversity of processes influenced by ABA is a result of crosstalk with other signalling pathways. In other cases, the complex development of a multicellular organism with different cell types and growth conditions throughout its life cycle also increases the possible output signals of ABA action. In this article, we touch on some of these issues in the context of ABA signalling during embryogenesis. On a more speculative level, we propose that a developmental and molecular framework of ABA action in the embryo may be gained from two chemically related terpenoid signalling hormones in animals: juvenile hormone (JH) and retinoic acid (RA). Many of the developmental issues with regard to ABA action in plant embryos are mirrored in JH studies from invertebrates, and the molecular action of RA in vertebrates suggests that transcriptional regulation is a direct output of RA addition. Both of these systems may be useful in furthering our developmental and molecular understanding of ABA action in plants.