The plant hormone abscisic acid (ABA) plays a critical role in enhancing plant survival during water deficit. While there is no doubt that ABA is a carotenoid derivative and that carotenoid cleavage occurs in the chloroplast, uncertainty remains about which tissues are responsible for synthesizing ABA. McAdam and Brodribb (10.1104/pp.17.01829) review and critique recent molecular studies that have suggested that ABA is synthesized in the phloem companion cells or guard cells and find them unconvincing. An alternative to these recent models is the classical hypothesis that ABA biosynthesis likely occurs in the mesophyll cells of seed plants. To test this hypothesis, the authors took advantage of the unique foliar anatomies of an angiosperm and four conifer species in which the mesophyll can be isolated from the vascular tissue. In all five species tested, considerable ABA biosynthesis occurred in mesophyll tissue that had been separated from vascular tissue. In addition, the removal of the epidermis from the mesophyll in two conifer species had no impact on the observed increase in ABA levels under water deficit. These results suggest that mesophyll cells are the predominant location of water deficit-triggered ABA biosynthesis in the leaf.
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