Isoprenoid Biosynthesis and Stability in Developing Green and Achlorophyllous Leaves of Rye (Secale cereale L.)

Formation o f major prenylquinones and carotenoids was investigated by comparing the incorporation o f [14C]mevalonate into segments o f different age from green and etiolated leaves o f 22 °C-grown rye seedlings (Secale cereale L.) and from 32 °C-grown rye leaves which contained bleached and proplastid-like ribosome-deficient plastids, due to a heat-sensitivity o f 70S ribosome formation. The contents o f plastidic isoprenoids were much lower (between 2 -30% ) in the achlorophyllous than in green leaves. In green leaves [l4C]mevalonate incor­ poration into non-polar lipids and into plastoquinone was partially inhibited in the presence o f gabaculin, an inhibitor o f chlorophyll synthesis. However, except for ß-carotene, [,4C]mevalonate incorporation into isoprenoids continuously increased with age also in achlorophyllous etiolated or 32 °C-grown, as in green, leaves and was, except for ß-carotene and plastoquinone, higher in etiolated than in green leaves. In bleached 32 °C-grown leaves [l4C]mevalonate incorporation into all plastidic isoprenoids was strikingly (up to 45-fold) higher than in green control leaves. While degradation o f ß-carotene was greatly enhanced in bleached 32 °C-grown leaves, relative to green control leaves, and could thus compensate for a higher apparent synthesis, chase experiments did not reveal any marked differences o f the turnover o f other isoprenoids. The half times o f plastoquinone, phylloquinone and lutein were in the order o f 2 3 days. Within a 24 h chase period a-tocopherol degradation did not become apparent. Uptake o f [14C]mevalonate and [l4C]isopentenyl pyrophosphate by isolated bleached plastids from 32 °C-grown leaves was much more rapid than by chloroplasts and resulted in higher precursor accumulation within the organelle. While mevalonate incorporation into iso­ prenoid lipids was not detected, isopentenyl pyrophosphate was incorporated into isoprenoid lipids, including plastoquinone. Rates o f incorporation by isolated chloroplasts or bleached plastids were o f similar order. The results illustrate that divergent types o f plastid differentia­ tion are associated with fundamental developmental changes o f the metabolic flow o f iso­ prenoid precursors between different products and compartments and, in particular, with changes o f import into the plastid compartment.