A nonproteinogenic amino acid, ß-tyrosine, accumulates in young rice leaves via long-distance phloem transport from mature leaves.

Oryza sativa L. ssp. japonica cv. Nipponbare produces a nonproteinogenic amino acid (3R)-ß-tyrosine from l-tyrosine by tyrosine aminomutase (OsTAM1). However, physiological and ecological function(s) of ß-tyrosine have remained obscure. Often an improved understanding of metabolite localization and transport can aid in design of experiments to test physiological functions. In the current study, we investigated the distribution pattern of ß-tyrosine in rice seedlings and found that ß-tyrosine is most abundant in the youngest leaves. Based upon observations of high TAM1 activity in mature leaves, we hypothesized that ß-tyrosine is transported from mature leaves to young leaves. Patterns of predominant mature synthesis and young leaf accumulation were supported by stable isotope studies using labeled ß-tyrosine and the removal of mature leaves. Stem exudate analyses was also consistent with ß-tyrosine transport through phloem. Thus, we identify young leaves as a key target in efforts to understand the biological function(s) of ß-tyrosine in rice.

ß-Tyrosine and its biosynthetic enzyme TAM1 are predominantly distributed in the ancestral subpopulation of japonica rice in Oryza rufipogon.

Intraspecific variation in specialized metabolites plays a crucial role in the adaptive response to diverse environments. Two major subspecies, japonica and indica, are observed in Asian cultivated rice (Oryza sativa L.). Previously, we identified (3R)-ß-tyrosine, a novel nonproteinogenic ß-amino acid in plants, along with the enzyme tyrosine aminomutase (TAM1), which is required for ß-tyrosine biosynthesis, in the japonica cultivar Nipponbare. Notably, TAM1 and ß-tyrosine were preferentially distributed in japonica cultivars compared with indica cultivars. Considering its phytotoxicity and antimicrobial activity, intraspecific variation in ß-tyrosine may contribute to the defensive potential of japonica rice. Investigation of the evolutionary trajectory of TAM1 and ß-tyrosine should enhance our understanding of the evolution of rice defense. However, their distribution patterns in O. rufipogon, the direct ancestor of O. sativa, remain unclear. Therefore, in this study, we extensively examined TAM1 presence/absence and ß-tyrosine content in 110 genetically and geographically diverse O. rufipogon accessions and revealed that they are characteristically observed in the ancestral subpopulation of japonica rice, while being absent or slightly accumulated in other subpopulations. Thus, we conclude that TAM1 and ß-tyrosine in japonica rice are likely derived from its ancestral subpopulation. Furthermore, the high and low TAM1 possession rates and ß-tyrosine content in japonica and indica rice, respectively, could be attributed to distribution patterns of TAM1 and ß-tyrosine in their ancestral subpopulations. This study provides fundamental insights into the evolution of rice defense.