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.
Biosynthesis of Ribulose-1,5-Bisphosphate Carboxylase in Spinach Leaf Protoplasts