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

A cell-free system from avocado fruit which routinely incorporated [C-14]mevalonate into ABA (1000 dpm per 5 mL of preparation), and into carotenoids, has now been shown to incorporate [C-14]pyruvate even more successfully (1620 dpm). Intact chloroplasts from spinach leaf protoplasts incorporated 2990 dpm of [C-14]pyruvate (from 2 x 10(6) dpm) into ABA compared with 990 dpm from [3-R-5-C-14]mevalonate (also from 2 x 10(6) dpm). The intact chloroplasts also produced [C-14]ABA (1575 dpm) when supplied with [C-14]isopentenyl diphosphate. This result establishes that the whole pathway of biosynthesis of ABA can occur within chloroplasts. Little [C-14]acetate or [C-14]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 [C-14]mevalonate occurs after activation in the cytoplasm and importation of a later intermediate into the plastids.