The maize cytochrome P450 CYP79A61 produces phenylacetaldoxime and indole-3-acetaldoxime in heterologous systems and might contribute to plant defense and auxin formation

Plants produce a group of aldoxime metabolites that are well known as volatiles and as intermediates in cyanogenic glycoside and glucosinolate biosynthesis in particular plant families. Recently it has been demonstrated that aldoximes can also accumulate as part of direct plant defense in poplar. Cytochrome P450 enzymes of the CYP79 family were shown to be responsible for the formation of aldoximes from their amino acid precursors. Here we describe the identification and characterization of maize CYP79A61 which was heterologously expressed in yeast and Nicotiana benthamiana and shown to catalyze the formation of (E/Z)-phenylacetaldoxime and (E/Z)-indole-3-acetaldoxime from L-phenylalanine and L-tryptophan, respectively. Simulated herbivory on maize leaves resulted in an increased CYP79A61 transcript accumulation and in elevated levels of L-phenylalanine and (E/Z)-phenylacetaldoxime. Although L-tryptophan levels were also increased after the treatment, (E/Z)-indole-3-acetaldoxime could not be detected in the damaged leaves. However, simulated herbivory caused a significant increase in auxin concentration. Our data suggest that CYP79A61 might contribute to the formation of (E/Z)-phenylacetaldoxime in maize. Since aldoximes have been described as toxic compounds for insect herbivores and pathogens, the increased accumulation of (E/Z)-phenylacetaldoxime after simulated herbivory indicates that this compound plays a role in plant defense. In addition, it is conceivable that (E/Z)-indole-3-acetaldoxime produced by recombinant CYP79A61 could be further converted into the plant hormone indole-3-acetic acid after herbivore feeding in maize.