Aldehyde oxidase and xanthine dehydrogenase in a flacca tomato mutant withdeficient abscisic acid and wilty phenotype

The flacca tomato (Lycopersicon esculentum) mutant displays a wilty phenoty pe as a result of abscisic acid (ABA) deficiency. The Mo cofactor (MoCo)-co ntaining aldehyde oxidases (AO; EC 1.2.3.1) are thought to play a role in t he final oxidation step required for ABA biosynthesis. AO and related MoCo- containing enzymes xanthine dehydrogenase (XDH; EC 1.2.1.37) and nitrate re ductase (EC 1.6.6.1) were examined in extracts of the flacca tomato genotyp e and of wild-type (WT) roots and shoots. The levels of MoCo were found to be similar in both genotypes. No significant XDH or AO (MoCo-containing hyd roxylases) activities were detected in flacca leaves; however, the mutant e xhibited considerable MoCo-containing hydroxylase activity in the roots, wh ich contained notable amounts of ABA. Native western blots probed with an a ntibody to MoCo-containing hydroxylases revealed substantial, albeit reduce d, levels of cross-reactive protein in the flacca mutant shoots and roots. The ABA xylem-loading rate was significantly lower than that in the WT, ind icating that the flacca is also defective in ABA transport to the shoot. Si gnificantly, in vitro sulfurylation with Na2S reactivated preexisting XDH a nd AO proteins in extracts from flacca, particularly from the shoots, and s uperinduced the basal-level activity in the WT extracts. The results indica te that in flacca, MoCo-sulfurylase activity is impaired in a tissue-depend ent manner.