The discovery and the study of phytohormones constitute a fascinating chapter of plant biology, spanning nearly two centuries of research from the initial hypothesis of plant chemical messengers that originated in the 19th century to the ongoing identification of novel phytohormones in the 21st century. The diversity of phytohormones, the complexity of their mechanisms of action, and the versatility of their effects are essential to understanding how plants grow, function, interact with their environment, and face climate change. Remarkable progress has been made in characterizing the sensing and signaling mechanisms of all the different kinds of plant hormones. In parallel, the fundamental issue of the dynamic distribution of plant hormones within the plant has gained momentum with increasing precision at the cell level. This is the case for abscisic acid (ABA), which plays essential roles in reproductive development, vegetative development, and growth, stress-development interactions, physiological responses to abiotic and biotic stresses, and climate change resilience. Several transporters of ABA have been characterized and structure-function studies have recently unveiled key features of the mechanisms and functional dynamics of ABA-transporter interactions. These transport mechanisms can determine source-to-target transmission, differential distribution throughout the cell populations of source tissues and target tissues, and signal shape, intensity, and duration in target cells. It is therefore timely to emphasize the pivotal role of transport discoveries in the history of ABA research and show how recent studies of transport mechanisms yield a more integrative vision of ABA regulation and adaptive responses, open new avenues of research on signaling crosstalks, and constitute opportunity.
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