Endogenous biosynthetic precursors of (+)-abscisic acid. VI. Carotenoids and ABA are formed by the ‘non-mevalonate’ triose-pyruvate pathway in chloroplasts

Abstract

A cell-free system from avocado fruit which routinely incorporated [14C]mevalonate into ABA (1000 dpm per 5 mL of preparation), and into carotenoids, has now been shown to incorporate [14C]pyruvate even more successfully (1620 dpm). Intact chloroplasts from spinach leaf protoplasts incorporated 2990 dpm of [14C]pyruvate (from 2 x 106 dpm) into ABA compared with 990 dpm from [3-R-5-14C]mevalonate (also from 2 x 106 dpm). The intact chloroplasts also produced [14C]ABA (1575 dpm) when supplied with [14C]isopentenyl diphosphate. This result establishes that the whole pathway of biosynthesis of ABA can occur within chloroplasts. Little [14C]acetate or [14C]alanine was incorporated into ABA by avocado fruit mesocarp. Most of the ABA in leaf tissue now appears to be formed by the triose-pyruvate pathway in chloroplasts and incorporation of [14C]mevalonate occurs after activation in the cytoplasm and importation of a later intermediate into the plastids.