Phaseic acid biosynthesis is a pathway that begins in the chloroplast and ends in the cytosol by which violaxanthin becomes phaseic acid (PA) and its derivative dihydrophaseic acid (DPA), synthesizing abscisic acid in the process. PA and DPA are inactive forms of abscisic acid whose synthesis provides a mechanism for controlling abscisic acid concentration. First, neoxanthin synthase catalyzes the opening of the violaxanthin epoxide ring to form neoxanthin. Second, a yet unidentified neoxanthin isomerase is theorized to isomerize neoxanthin to 9'-cis-neoxanthin. Third, 9-cis-epoxycarotenoid dioxygenase (NCED) uses oxygen to cleave 9'-cis-neoxanthin to form xanthoxin and C25-allenic-apo-aldehyde. This enzyme requires Fe2+ as a cofactor. Next, a xanthoxin transporter is theorized to export xanthoxin from the chloroplast into the cytosol to continue abscisic acid biosynthesis, but it has yet to be discovered. Fourth, xanthoxin dehydrogenase, located in the cytosol, catalyzes the conversion of xanthoxin and NAD to abscisic aldehyde, NADH, and a proton with the help of a molybdenum cofactor (MoCo). Fifth, abscisic-aldehyde oxidase converts abscisic aldehyde, water, and oxygen into hydrogen peroxide, hydrogen ion, and abscisic acid. Sixth, abscisic acid 8'-hydroxylase / abscisic acid 9'-hydroxylase uses NADPH, oxygen, and a proton to convert abscisic acid into 8'-hydroxyabscisate and water. Seventh, 8'-hydroxyabscisate spontaneously becomes phaseic acid. Eighth, the predicted enzyme phaseic acid reductase (coloured orange in the image) is theorized to catalyze the conversion of phaseic acid into dihydroxyphaseic acid.

Phaseic Acid Biosynthesis References