The final steps in the biosynthesis of the plant hormones abscisic acid (ABA) and indole-3-acetic acid (IAA) have been shown to be catalyzed by aldehyde oxidases (AO). We have cloned three putative functional AO genes (TAO1, TAO2 and TAO3) and two putative AO pseudogenes (TAO4 and TAO5) in tomato. The TAO1 cDNA described here includes the correct amino terminus of the encoded TAO1 protein and is different at the 5'-end from the TAO1 sequence in GenBank (accession number U82558). Northern analysis shows that TAO1 is expressed mainly in vegetative tissues and TAO2 is expressed in both vegetative and reproductive tissues. TAO3 expression was not detectable by Northern hybridization. These results suggest that each AO may play different roles in the regulation of tomato growth and development.

The molybdenum cofactor (MoCo)-containing enzymes aldehyde oxidase (AO; EC 1.2.3.1) and xanthine dehydrogenase (XDH; EC 1.2.1.37) require for activity a sulfuration step that inserts a terminal sulfur ligand into the MoCo. The tomato flacca mutation was originally isolated as a wilty phenotype due to a lack of abscisic acid (ABA) that is related to simultaneous loss of AO and XDH activities. An expressed sequence tag candidate from tomato was selected on the basis of homology to sulfurases from animals, fungi and the recently isolated Arabidopsis genes LOS5/ABA3. The tomato homologue maps as a single gene to the bottom of chromosome 7, consistent with the genetic location of the flacca mutation. The structure of FLACCA shows a multidomain protein with an N-terminal NifS-like sulfurase domain; a mammal-specific intermediate section; and a C-terminus containing conserved motifs. Prominent among these are molybdopterin oxidoreductases and thioredoxin redox-active centre/iron-sulfur-binding region signatures which may be relevant to the specific sulfuration of MoCo. Indeed, the molecular analysis of flacca identifies the mutation in a highly conserved motif located in the C-terminus. Activity gel assays show that FLACCA is expressed throughout the plant. Transient and stable complementation of flacca and the Arabidopsis aba3 mutants with Aspergillus nidulans hxB and FLACCA yielded full, partial and tissue-specific types of Mo-hydroxylase activities. Restoration of activity in the root alone is sufficient to augment plant ABA content and rectify the wild-type phenotype. Thus the pleiotropic flacca phenotype is due to the loss of activity of enzymes requiring a sulfurated MoCo.