# Biosynthetic Pathways of Hormones in Plants
## 7 Abscisic Acid  
Abscisic acid (ABA) also plays an important role in many cellular processes, including seed development, dormancy, germination, vegetative growth, and responses to environmental stresses [92]. ABA is ubiquitous in lower and higher plants. It was reported that young tissues contain high levels of ABA [93]. The biosynthesis of ABA predominantly takes place in vascular tissues, and it is subsequently transported into target tissues. This transportation process occurs through both the xylem and phloem, enabling bidirectional transport between the roots and shoots. The naturally occurring form of ABA is \(S\)-(+)-ABA, which has a side chain defined as 2-_cis_, 4-_trans_. The _trans_-ABA is biologically inactive, whereas \(R\)-(-)-ABA, which can potentially result from racemization through the catabolite ABA-_trans_-diol, exhibits biological activity [94; 95]. The initial stages of ABA biosynthesis occur within plastids. In plants, two distinct ABA biosynthesis pathways have been identified. The first pathway is a direct route, while the second pathway is indirect and involves the conversion of a C\({}_{15}\) compound (i.e., farnesyl pyrophosphate) and a C\({}_{40}\) carotenoid [96]. Through the characterization of ABA-deficient mutants and the isolation of relevant mutated genes, it has been determined that the indirect pathway is the primary route involved in ABA biosynthesis in plants [96; 97]. Investigations into the biosynthesis of carotenoids, as well as their pathways and mechanisms, have indicated that mevalonate (MVA) plays a minor role as a precursor of carotenoid biosynthesis in chloroplasts [98; 99]. In plants, there are two distinct pathways involved in isoprenoid production (Figure 7). The first pathway, which is known as the MVA pathway, is shared by animals, fungi, and a few bacteria. It operates in both the cytosol and mitochondria, and it is responsible for generating precursor molecules used in the synthesis of sterols, certain sesquiterpenes, and the side chain of ubiquinone. On the other hand, the second pathway, which is called the methylerythnitol phosphate (MEP) pathway, is localized within plastids [100]. The MVA pathway initiates in tandem with the condensation of three acetyl-CoA molecules (Figure 7). Subsequently, a reduction process leads to the formation of MVA, which undergoes two phosphorylation reactions to produce mevalonate-5-diphosphate. Further phosphorylation, which is followed by decarboxylation-driven dephosphorylation, yields isopentenyl pyrophosphate (IPP). This compound can be isomerized to create dimethylallyl pyrophosphate (DMAPP), which serves as a precursor of the biosynthesis of various isoprenoid-derived phytohormones [101]. On the other hand, the MEP pathway initiates in tandem with the condensation of pyruvate and glyceraldehyde 3-phosphate (Figure 7). This condensation reaction gives rise to 1-deoxy-D-xylulose-5-phosphate (DOXP). Subsequently, DOXP is converted into methylerythnitol phosphate. Methylerythnitol phosphate then undergoes conjugation with the cytidyl phosphate moiety,followed by another phosphorylation step, i.e., the release of CMP, and cyclization. The resulting product is subjected to a reduction process, leading to the formation of (_E_)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBDP). HMBDP can be further reduced to produce IPP and, to a lesser extent, DMAPP. Both HMBDP and DMAPP can serve as precursors of cytokinin synthesis, though it is worth noting that the level of HMBDP in plastids is approximately five times higher than that of DMAPP [102]. MVA can be converted into an intermediate IPP (in cytosol), which is an intermediate in both carotenoid and steroid pathways. Furthermore, in plastids in which carotenoid synthesis occurs, IPP is generated through the MEP pathway using pyruvate and glyceraldehyde-3-phosphate. The enzyme responsible for catalyzing the initial step in the non-MEP biosynthetic pathway is called DOXP synthase (DXS).