No abstract

(+)-Abscisin II[(+)-dormin] has been identified in the ether soluble acid fraction of twenty plant species, including mono-and dicotyledons and a fern. (+)-Abscisin II was identified by its general physical properties, biological activity, chromatographic behaviour and optical rotatory dispersion. Synthetic abscisin II runs on paper chromatograms at the same RF as inhibitor-β from potato tuber and lemon fruit in the solvent mixtures used by previous workers, furthermore the growth-inhibitory activity of these extracts was attributable to the (+)-abscisin II contents. The concentrations of (+)-abscisin II in extracts of a number of plants were calculated from the specific rotation at the first, positive extremum (289 mμ) of the optical rotatory dispersion curve after extraction and chromatographic procedures.A new method of measurement was evolved, analogous to inverse isotope dilution, where a known quantity of racemic abscisin II was added to a tissue homogenate and then the total abscisin II in a highly purified extract was measured spectrophotometrically and the natural (+)-abscisin II measured spectropolarimetrically. The (+)-abscisin II content in the original sample was then calculated from the ratio of these two measurements and the known weight of racemic material added. Concentrations of (+)-abscisin II within tissues obtained by this technique were about three times higher than by the standard extraction procedure.Identification of several previously described growth-inhibitory compounds as (+)-abscisin II has extended the known physiological effects of the compound; its roles as a dormancy and an abscission factor and as a germination inhibitor are discussed.

The pathway of biosynthesis of abscisic acid (ABA) can be considered to comprise three stages: (i) early reactions in which small phosphorylated intermediates are assembled as precursors of (ii) intermediate reactions which begin with the formation of the uncyclized C40 carotenoid phytoene and end with the cleavage of 9'-cis-neoxanthin (iii) to form xanthoxal, the C15 skeleton of ABA. The final phase comprising C15 intermediates is not yet completely defined, but the evidence suggests that xanthoxal is first oxidized to xanthoxic acid by a molybdenum-containing aldehyde oxidase and this is defective in the aba3 mutant of Arabidopsis and present in a 1-fold acetone precipitate of bean leaf proteins. This oxidation precludes the involvement of AB-aldehyde as an intermediate. The oxidation of the 4'-hydroxyl group to the ketone and the isomerization of the 1',2'-epoxy group to the 1'-hydroxy-2'-ene may be brought about by one enzyme which is defective in the aba2 mutant and is present in the 3-fold acetone fraction of bean leaves. Isopentenyl diphosphate (IPP) is now known to be derived by the pyruvate-triose (Methyl Erythritol Phosphate, MEP) pathway in chloroplasts. (14C)IPP is incorporated into ABA by washed, intact chloroplasts of spinach leaves, but (14C)mevalonate is not, consequently, all three phases of biosynthesis of ABA occur within chloroplasts. The incorporation of labelled mevalonate into ABA by avocado fruit and orange peel is interpreted as uptake of IPP made in the cytoplasm, where it is the normal precursor of sterols, and incorporated into carotenoids after uptake by a carrier in the chloroplast envelope. An alternative bypass pathway becomes more important in aldehyde oxidase mutants, which may explain why so many wilty mutants have been found with this defect. The C-1 alcohol group is oxidized, possibly by a mono-oxygenase, to give the C-1 carboxyl of ABA. The 2-cis double bond of ABA is essential for its biological activity but it is not known how the relevant trans bond in neoxanthin is isomerized.

No abstract

No abstract