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. 2019 Sep 27;10:1168. doi: 10.3389/fpls.2019.01168

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Copyright © 2019 Felemban, Braguy, Zurbriggen and Al-Babili

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Abscisic acid (ABA) biosynthesis pathway. De novo ABA biosynthesis begins in plastid, catalyzed by several enzymes (ZEP, zeaxanthin epoxidase; VDE, violaxanthin de-epoxidase; ABA4, ABA-deficient 4; NCED, nine-cis-apoxicarotenoid-dioxygenase ) that convert zeaxanthin into xanthoxin. After transport into the cytosol, xanthoxin is subjected to oxidation steps mediated by three enzymes (ABA2, ABA-deficient 2; AAO3, abscisic aldehyde oxidase and ABA3, ABA-deficient 3) giving rise to ABA. Abscisic acid can be modified by CYP707A enzymes that yield phaseic acid (PA). The latter is the substrate of the PA reductase (PAR) that forms dihydrophaseic acid. Alternatively, ABA is glucosylated by ABA glucosyltransferase (GT) into ABA-GT that can be transported into the vacuole and the endoplasmic reticulum (ER). Abscisic acid–GT can be converted back into ABA by the glycosyl hydrolase BG1. Created with ‘Biorender’.