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

Formation of major prenylquinones and carotenoids was investigated by comparing the incorporation of [¹⁴C]mevalonate into segments of different age from green and etiolated leaves of 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 of 70S ribosome formation. The contents of plastidic isoprenoids were much lower (between 2 - 30%) in the achlorophyllous than in green leaves. In green leaves [¹⁴C]mevalonate incorporation into non-polar lipids and into plastoquinone was partially inhibited in the presence of gabaculin, an inhibitor of chlorophyll synthesis. However, except for β-carotene, [¹⁴C]mevalonate incorporation into isoprenoids continuously increased with age also in achlorophyllous etiolated or 32 °C-grown, as in green, leaves and was, except for P-carotene and plastoquinone, higher in etiolated than in green leaves. In bleached °32 C-grown leaves [¹⁴C]mevalonate incorporation into all plastidic isoprenoids was strikingly (up to 45-fold) higher than in green control leaves. While degradation of P-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 of the turnover of other isoprenoids. The half times of plastoquinone. phylloquinone and lutein were in the order of 2-3 days. Within a 24 h chase period a-tocopherol degradation did not become apparent. Uptake of [¹⁴C]mevalonate and [¹⁴C]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 isoprenoid lipids was not detected, isopentenyl pyrophosphate was incorporated into isoprenoid lipids, including plastoquinone. Rates of incorporation by isolated chloroplasts or bleached plastids were of similar order. The results illustrate that divergent types of plastid differentiation are associated with fundamental developmental changes of the metabolic flow of isoprenoid precursors between different products and compartments and, in particular, with changes of import into the plastid compartment.