Sesquiterpene-like inhibitors of a 9-cis-epoxycarotenoid dioxygenase regulating abscisic acid biosynthesis in higher plants.

Abscisic acid (ABA) is a carotenoid-derived plant hormone known to regulate critical functions in growth, development and responses to environmental stress. The key enzyme which carries out the first committed step in ABA biosynthesis is the carotenoid cleavage 9-cis-epoxycarotenoid dioxygenase (NCED). We have developed a series of sulfur and nitrogen-containing compounds as potential ABA biosynthesis inhibitors of the NCED, based on modification of the sesquiterpenoid segment of the 9-cis-xanthophyll substrates and product. In in vitro assays, three sesquiterpene-like carotenoid cleavage dioxygenase (SLCCD) inhibitor compounds 13, 17 and 18 were found to act as inhibitors of Arabidopsis thaliana NCED 3 (AtNCED3) with K(i)'s of 93, 57 and 87 microM, respectively. Computational docking to a model of AtNCED3 supports a mechanism of inhibition through coordination of the heteroatom with the non-heme iron in the enzyme active site. In pilot studies, pretreatment of osmotically stressed Arabidopsis plants with compound 13 resulted lower levels of ABA and catabolite accumulation compared to levels in mannitol-stressed plant controls. This same inhibitor moderated known ABA-induced gene regulation effects and was only weakly active in inhibition of seed germination. Interestingly, all three inhibitors led to moderation of the stress-induced transcription of AtNCED3 itself, which could further contribute to lowering ABA biosynthesis in planta. Overall, these sesquiterpenoid-like inhibitors present new tools for controlling and investigating ABA biosynthesis and regulation.