# Biosynthetic Pathways of Hormones in Plants
## 7 Abscisic Acid
After IPP is formed, it is converted into a C\({}_{20}\) compound known as geranylgeranyl pyrophosphate (GGPP) [103]. The conversion of geranylgeranyl pyrophosphate (GGPP) into phytoene, which is a C40 carotenoid, is the initial and rate-limiting step involved in carotenoid synthesis. This reaction is catalyzed by the enzyme phytoene synthase (Figure 8). Subsequently, phytoene undergoes a series of enzymatic conversions in a sequential order, resulting in the formation of \(\zeta\)-carotene, lycopene, \(\beta\)-carotene, and zeaxanthin [104]. The subsequent step, which is specific to the ABA biosynthesis pathway and occurs within plastids, involves the epoxidation of zeaxanthin and antheraaxanthin to produce violaxanthin. This enzymatic conversion is mediated by zeaxanthin epoxidase. Following this step, the oxidative cleavage of xanthophylls, such as 9-_cis_-violaxanthin and/or 9'-_cis_-neoxanthin, occurs, leading to the formation of xanthoxin. This step is catalyzed by 9-_cis_-epoxycarotenoid dioxygenase and subsequently exported to the cytosol. Once 9-_cis_-epoxycarotenoids are cleaved, xanthoxin is further converted into ABA [96]. Three possible pathways have been proposed for the final stages of ABA biosynthesis that involves abscisic aldehyde, xanthoxic acid, or abscisic alcohol acting as intermediates. In the first proposed pathway, the enzyme encoded by the _AtABA2_ gene, which is a short-chain alcohol dehydrogenase/reductase, catalyzes the initial step and produces  
Figure 7: The methylerythnitol phosphate (MEP) and mevalonate (MVA) pathways lead to the conversion of isopentenyl diphosphate to cycloartenol and phytoene. ABA aldehyde. The final step in this biosynthetic pathway is catalyzed by ABA aldehyde oxidase. The activities of aldehyde oxidase and xanthine dehydrogenase require molybdenum cofactor sulfurase, which is responsible for the sulfurylation of a dioxo form into a sulfurylated mono-oxo form, with L-cysteine used as a sulfur donor [94; 105]. The second pathway of ABA biosynthesis that involves xanthoxic acid was discovered in ripening avocado (_Persea americana_) fruits. In these experiments, inhibition of aldehyde oxidase activity by tungstate, which is a potent inhibitor of molybdenumzymes in plants, led to the accumulation of xanthoxin. This result indicates that xanthoxin is a substrate for aldehyde oxidase. It is possible to speculate that the two ABA biosynthetic pathways may operate in an organ- and/or developmental stage-dependent manner, suggesting a potential regulatory mechanism [96; 98; 106]. Figure 8: The biosynthetic pathways of abscisic acid (phytohormone is green). The last ABA biosynthetic pathway, which involves abscisicic alcohol, can be activated in certain mutants. It was shown that in the _flacca_ and _sitiens_ mutants of tomato (_Solanum Lyopersicum_), when abscisicic aldehyde was supplied externally, it was converted into abscisicic alcohol. Subsequently, abscisic alcohol was oxidized to ABA (Figure 8) [107]. This reaction is a non-specific source of ABA in wild-type plants, though it may play a more significant role in mutants that have impaired capacity to directly convert abscisicic aldehyde into ABA. This finding suggests that the shunt pathway serves as an alternative route for ABA production in tomato mutants [96].