Xanthoxin levels and metabolism in the wild-type and wilty mutants of tomato

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Abstract
Using 13C-labelled internal standards and gas chromatography-mass spectrometry/multiple-ion monitoring the levels of xanthoxin (Xan) and 2-trans-xanthoxin (t-Xan) have been determined in stressed and non-stressed leaves of wildtype tomato (Lycopersicon esculentum Mill cv. Ailsa Craig), and the wilty mutants, notabilis (not), flacca (flc) and sitiens (sit). Levels of Xan were very low in all tissues. Ratios of t-Xan: Xan ranged from 10:1 to flc, t-Xan levels increased following stress. The results from feeding experiments using [13C]Xan and t-Xan demonstrated that whilst wild-type and not plants readily converted Xan into abscisic acid (ABA), flc and sit plants converted only a small amount of applied Xan into ABA. In all plants t-Xan was not converted into ABA. These results indicate that the flc and sit mutants are impaired in ABA biosynthesis because they are unable to convert Xan into ABA, whereas the not mutant is blocked at a metabolic step prior to Xan. Another possible ABA precursor, ABA-1′,4′-trans-diol (ABA-t-diol) was found to occur in wild-type and mutant tissue. All four tissues could convert [2H]ABA-t-diol to ABA. Incubation of stressed leaves in the presence of 18O2 provided evidence consistent with Xan and ABA originating via oxidative cleavage of a xanthophyll such as violaxanthin.