HIGHER ACTIVITY OF AN ALDEHYDE OXIDASE IN THE AUXIN-OVERPRODUCING SUPERROOT1 MUTANT OF ARABIDOPSIS-THALIANA

Aldehyde oxidase (AO; EC 1.2.3.1) activity was measured in seedlings o f wild type or an auxin-overproducing mutant, superroot1 (sur1), of Ar abidopsis thaliana. Activity staining for AO after native polyacrylami de gel electrophoresis separation of seedling extracts revealed that t here were three major bands with AO activity (AO1-3) in wild-type and mutant seedlings. One of them (AO1) had a higher substrate preference for indole-3-aldehyde. This AO activity was significantly higher in su r1 mutant seedlings than in the wild type. The difference in activity was most apparent 7 d after germination, the same time required for th e appearance of the remarkable sur1 phenotype, which includes epinasti c cotyledons, elongated hypocotyls, and enhanced root development. Hig her activity was observed in the root and hypocotyl region of the muta nt seedlings. We also assayed the indole-3-acetaldehyde oxidase activi ty in extracts by high-performance liquid chromatography detection of indole-3-acetic acid (IAA). The activity was about 5 times higher in t he extract of the sur1 seedlings, indicating that AO1 also has a subst rate preference for abscisic aldehyde. Treatment of the wild-type seed lings with picloram or IAA caused no significant increase in AO1 activ ity. This result suggested that the higher activity of AO1 in sur1 mut ant seedlings was not induced by IAA accumulation and, thus, strongly supports the possible role of AO1 in IAA biosynthesis in Arabidopsis s eedlings.