The Tyrosine Aminomutase TAM1 Is Required for β-Tyrosine Biosynthesis in Rice

Non-protein amino acids, often isomers of the standard 20 protein amino acids, have defense-related functions in many plant species. A targeted search for jasmonate-induced metabolites in cultivated rice (Oryza sativa) identified (R)-beta-tyrosine, an isomer of the common amino acid (S)-alpha-tyrosine in the seeds, leaves, roots, and root exudates of the Nipponbare cultivar. Assays with 119 diverse cultivars showed a distinct presence/absence polymorphism, with b-tyrosine being most prevalent in temperate japonica cultivars. Genetic mapping identified a candidate gene on chromosome 12, which was confirmed to encode a tyrosine aminomutase (TAM1) by transient expression in Nicotiana benthamiana and in vitro enzyme assays. A point mutation in TAM1 eliminated beta-tyrosine production in Nipponbare. Rice cultivars that do not produce b-tyrosine have a chromosome 12 deletion that encompasses TAM1. Although beta-tyrosine accumulation was induced by the plant defense signaling molecule jasmonic acid, bioassays with hemipteran and lepidopteran herbivores showed no negative effects at physiologically relevant beta-tyrosine concentrations. In contrast, root growth of Arabidopsis thaliana and other tested dicot plants was inhibited by concentrations as low as 1 mM. As beta-tyrosine is exuded into hydroponic medium at higher concentrations, it may contribute to the allelopathic potential of rice.