Xanthoxin Metabolism in Cell-free Preparations from Wild Type and Wilty Mutants of Tomato

Abstract

Extracts prepared from the turgid and water-stressed leaves of wild-type tomato (Lycopersicon esculentum Mill cv Ailsa Craig) and the wilty mutants sitiens, notabilis, and lacca were tested for their ability to metabolize xanthoxin to ABA. Extracts from wild type and notabilis converted xanthoxin at similar rates, while extracts from sitiens and flacca showed little or no activity. We also observed no activity when extracts of sitiens and flacca were mixed. Similar results were obtained when ABA aldehyde was used as a substrate, in that extracts from wild type and notabilis were equally active, but extracts fromflacca and sitiens showed little activity. None of the tomato extracts showed significant activity with xanthoxin acid, xanthoxin alcohol, or ABA-i',4-'Trans-diol as substrates. Extracts from bean leaves (Phaseolus vulgaris L. cv Blue Lake) were similar to the wild-type tomato extracts in their ability to convert the various substrates to ABA, although excised bean leaves did convert ABA-I ',4'-trans-diol and xanthoxin alcohol to ABA when these substances were taken up through the petiole. These results are consistent with a role for xanthoxin as a normal intermediate on the ABA biosyn-thetic pathway, and they suggest that ABA aldehyde is the final ABA precursor. Although the details of the ABA biosynthetic pathway have not yet been delineated, several lines of evidence suggest that the pathway includes the oxidative cleavage of a xanthophyll to a C-15 compound that is metabolized to ABA. Mutants lacking xanthophylls, or in which xanthophyll biosynthesis is blocked by inhibitors, show a greatly reduced ability to accumulate ABA (11-13). ABA produced by water-stressed plants in the presence