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

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 <500:1. In the wild-type and 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.Abbreviations ABA abscisic acid - ABA-t-diol abscisic acid-1,4-trans-diol - DDC sodium diethyldithiocarbamate - FW fresh weight - GC-MS gas chromatography-mass spectrometry - i.d. internal diameter - MIM multiple-ion monitoring - PA phaseic acid - Xan xanthoxin - flc flacca - not notabilis - sit sitiens