Gene duplication in the diversification of secondary metabolism: Tandem 2-oxoglutarate-dependent dioxygenases control glucosinolate biosynthesis in arabidopsis

Secondary metabolites are a diverse set of plant compounds believed to have numerous functions in plant-environment interactions. The large chemical d iversity of secondary metabolites undoubtedly arises from an equally divers e set of enzymes responsible for their biosynthesis. However, little is kno wn about the evolution of enzymes involved in secondary metabolism. We are studying the biosynthesis of glucosinolates, a large group of secondary met abolites, in Arabidopsis to investigate the evolution of enzymes involved i n secondary metabolism. Arabidopsis contains natural variations in the pres ence of methylsulfinylalkyl, alkenyl, and hydroxyalkyl glucosinolates. In t his article, we report the identification of genes encoding two 2-oxoglutar ate-dependent dioxygenases that are responsible for this variation. These g enes, AOP2 and AOP3, which map to the same position on chromosome IV, resul t from an apparent gene duplication and control the conversion of methylsul finylalkyl glucosinolate to either the alkenyl or the hydroxyalkyl form. By heterologous expression in Escherichia and the correlation of gene express ion patterns to the glucosinolate phenotype, we show that AOP2 catalyzes th e conversion of methylsulfinylalkyl glucosinolates to alkenyl glucosinolate s. Conversely, AOP3 directs the formation of hydroxyalkyl glucosinolates fr om methylsulfinylalkyl glucosinolates. No ecotype coexpressed both genes. F urthermore, the absence of functional AOP2 and AOP3 leads to the accumulati on of the precursor methylsulfinylalkyl glucosinolates. A third member of t his gene family, AOP1, is present in at least two forms and found in all ec otypes examined. However, its catalytic role is still uncertain.