Abscisic acid metabolism in a Citrus Sinensis flavedo enzyme system.
This research project had its major objective the unequivocal demonstration that the plant growth regulator, abscisic acid could be produced in vitro from labelled precursors. In addition, this project was intended to demonstrate the metabolic relationship between β, β-xanthophyll turnover and ABA biosynthesis. Finally attempts were made to isolate the enzyme responsible for the cleavage of the immediate xanthophyll precursor to ABA, 9'-cis-neoxanthin. This was achieved using a cell-free system developed from Citrus flavedo which demonstrated formation of xanthoxal and abscisic acid from zeaxanthin, antheraxanthin, violaxanthin and neoxanthin. In addition product abscisic acid was shown to incorporate label from mevalonic acid lactone establishing the terpenoid origin of this plant growth regulator. 9'-cis-neoxanthin cleavage enzyme was present in the Citrus cell-free system, partially purified, and separated from lipoxygenase activity and shown to convert 9'-cis-neoxanthin into xanthoxal and related but unidentified neutral products.