Identification of a cytochrome P450, CYP6CY6v2, as a cytokinin side-chain hydroxylase from the aphid Acyrthosiphon pisum

Previous surveys of endogenous cytokinins (a class of plant hormones) across diverse insect taxa have revealed a strong correlation between the presence of trans-zeatin, or its hydroxylated precursors, and the ability of insects to induce gall formation. In this study, we identified a cytokinin hydroxylase from the pea aphid, Acyrthosiphon pisum, a model insect that possesses hydroxylated cytokinins. The enzyme, named CYP6CY6v2, belongs to a cytochrome P450 family specific to insects and is phylogenetically distinct from CYP735A, the cytokinin hydroxylase found in plants. CYP735A exclusively uses dimethylallylated nucleotide-type cytokinins with phosphate groups as substrates. By contrast, CYP6CY6v2 hydroxylates nucleotide-type cytokinins, a nucleoside-type cytokinin lacking phosphate groups, and a free-base form lacking both phosphate and ribose moieties. The identification of CYP6CY6v2 provides a valuable insight into the evolutionary acquisition of hydroxylated cytokinin biosynthesis in insects.