Browsing Pathways

Abscisic acid biosynthesis is a pathway that begins in the chloroplast and ends in the cytosol by which violaxanthin becomes abscisic acid, a plant hormone that plays a role in many plant developmental processes, including bud dormancy . 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.

Abscisic acid glucose ester metabolism is a pathway that begins in the chloroplast and enters the cytosol and endoplasmic reticulum body by which violaxanthin becomes abscisic acid glucose ester, synthesizing abscisic acid in the process. Abscisic acid glucose ester synthesis and reformation back to abscisic acid provides a mechanism for precisely controlling abscisic acid concentration (quickly removing and adding abscisic acid when required). 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 glucosyltransferase uses UDP to convert abscisic acid into abscisic acid glucose ester. Abscisic acid glucose ester can then be converted back to abscisic acid via abscisic acid glucose ester beta-glucosidase located in the endoplasmic reticulum body (coloured dark green in the image). Consequently, it is theorized that ABA-GE transporters are required for this enzyme to access its substrates from the cytosol.