Leaf dehydration is needed to induce abscisic acid accumulation in roots of citrus plants

The root system is the main organ involved in the water uptake and it is assumed that it perceives soil dehydration, triggering water stress responses at the entire plant level, including abscisic acid (ABA) accumulation. However, several lines of evidence demonstrate that leaves directly adjust their water status (and induce increases in ABA levels) in response to changes in the evaporative demand. To evaluate the influence of leaf water status on ABA accumulation in roots, the canopy of citrus plants was kept at high humidity while roots were dehydrated. Leaves and roots of completely dehydrated plants increased levels of ABA whereas plants with turgid leaves did not accumulate ABA either in leaves or in roots. Furthermore, in dehydrated roots, the expression of several key genes involved in carotenoid and ABA biosynthetic pathways such as CsβCHX1 and CsNCED1 was strongly affected by the water status of leaves whereas water availability in roots showed a marginal influence. Overall, results confirm that leaves are a relevant source of ABA for roots during dehydration and also demonstrate that leaf-dehydration is required to trigger ABA-induced responses in roots.