abscisate biosynthesis pathway ABA biosynthesis UPa:UPA00090 GO:0009688 Biosynthesis of abscisic acid (ABA), a sesquiterpenoid (15-carbon) plant hormon. Abscisic acid is partially produced via the mevalonic pathway in chloroplasts and other plastids. Because it is synthesized partially in the chloroplasts, it makes sense that biosynthesis primarily occurs in the leaves. The production of ABA is accentuated by stresses such as water loss and freezing temperatures. plant hormone biosynthesis sesquiterpene biosynthesis