Production of the Non-Protein Amino Acid β-Tyrosine in Rice

Plants under attack by herbivores produce the hormone jasmonic acid, which induces defense responses including the expected changes in transcription. To go beyond the expected, Yan et al. (2015) looked for metabolites induced by jasmonate treatment of leaves of the rice (Oryza sativa) cultivar Nipponbare. Specifically, they used liquid chromatography to screen for non-protein amino acids and used gas chromatography-mass spectrometry to identify β-tyrosine. In this isomer of α-tyrosine, the amino group is on the β-carbon, rather than the α-carbon (see figure). The authors found β-tyrosine in leaves, seeds, and roots of rice. The tyrosine aminomutase (TAM) enzyme produces (R)-β-tyrosine (top) from (S)-α-tyrosine (bottom) by exchanging the hydrogen and the amino group on the α- and β-carbon atoms. Examination of recombinant inbred lines from Nipponbare and IR64, a rice cultivar without α-tyrosine, allowed them to map the TAM1 locus. They also confirmed the activity of the encoded protein by transient expression in Nicotiana benthamiana and by examination of point mutations of the endogenous rice locus.

The tyrosine aminomutase enzyme produces (R)-β-tyrosine (top) from (S)-α-tyrosine (bottom). (Reprinted from Yan et al. [2015], Figure 1B.)

This raises the next question: What does β-tyrosine do? Non-protein amino acids such as homoserine and ornithine function in primary metabolism. However, some rice cultivars have TAM1, particularly many of the temperate japonica types, but many do not, indicating that β-tyrosine does not have essential functions. In other systems, non-protein amino acids function in defense, such as against insects (reviewed in Huang et al., 2011). Also, β-phenylalanine is a precursor of taxol in yew trees (Taxus sp), and β-tyrosine is a precursor of various antibiotics in different species of bacteria. However, the authors found that β-tyrosine taken up into tobacco leaves had no effect on feeding aphids and had no effect on growth of lepidopterans (black cutworms [Agrotis ipsilon] and sugarcane borers [Diatraea saccharalis]). β-Tyrosine at physiologically relevant concentrations did significantly affect growth of the bacterial pathogen Pseudomonas syringae in liquid growth media, indicating a potential antibacterial effect.

Different plant metabolites also function in allelopathy, in which plants secrete compounds that kill or impair nearby plants—a phenomenon familiar to gardeners who have tried to grow anything underneath a black walnut tree (Juglans nigra). Many rice cultivars show allelopathy, for example, based on secreted diterpenoids (Xu et al., 2012). Hydroponically grown Nipponbare plants secrete β-tyrosine into the medium. Also, relevant concentrations of β-tyrosine inhibited root growth of Arabidopsis thaliana and other dicots but had less of an effect on monocots; addition of α-tyrosine to the medium reversed this inhibition. Exploiting allelopathy has great potential as a mechanism to suppress weeds in agriculture. Therefore, TAM1 may have immediate relevance in rice breeding efforts. Certainly, research to figure out the effects of β-tyrosine on pathogens, other plants, and the effect of dietary intake of β-tyrosine on human health may also have long-term relevance for efforts to improve rice and other crops.