Theanine, a tea-plant-specific non-proteinogenic amino acid, is involved in the regulation of lateral root development in response to nitrogen status

Abstract

Glutamine synthetase type I (GSI)-like proteins are proposed to mediate nitrogen signaling and developmental fate by synthesizing yet unidentified metabolites. Theanine, the most abundant non-proteinogenic amino acid in tea plants, is the first identified metabolite synthesized by a GSI-like protein (CsTSI) in a living system. However, the roles of theanine in nitrogen signaling and development are little understood. In this study we found that nitrogen deficiency significantly reduced theanine accumulation and increased lateral root development in tea plant seedlings. Exogenous theanine feeding significantly repressed lateral root development of seedlings of tea plants and the model plant Arabidopsis. The transcriptomic analysis revealed that the differentially expressed genes in the roots under theanine feeding were enriched in the apoplastic pathway and H₂O₂ metabolism. Consistently, theanine feeding reduced H₂O₂ levels in the roots. Importantly, when co-treated with H₂O₂, theanine abolished the promoting effect of H₂O₂ on lateral root development in both tea plant and Arabidopsis seedlings. The results of histochemical assays confirmed that theanine inhibited reactive oxygen species accumulation in the roots. Further transcriptomic analyses suggested the expression of genes encoding enzymes involved in H₂O₂ generation and scavenging was down- and upregulated by theanine, respectively. Moreover, the expression of genes involved in auxin metabolism and signaling, cell division, and cell expansion was also regulated by theanine. Collectively, these results suggested that CsTSI-synthesized theanine is likely involved in the regulation of lateral root development, via modulating H₂O₂ accumulation, in response to nitrogen levels in tea plants. This study also implied that the module consisting of GSI-like protein and theanine-like metabolite is probably conserved in regulating development in response to nitrogen status in plant species.

Acknowledgements

This work was supported by the National Key R&D Program of China (2021YFD1601101, 2018YFD1000601) and grants from the National Natural Science Foundation of China (32072624) and the Base of Introducing Talents for Tea Plant Biology and Quality Chemistry (D20026). We would like to thank the Tea Plant Cultivar and Germplasm Resource Garden in Guohe Town, Anhui Agricultural University, for providing tea plant material. We also thank Professor William J. Lucas of the University of California, Davis, for his suggestions on this study.

Author contributions

Z.Z., X.W., G.X., and J.Z. conceived this project and designed the experiments. T.C., S.L., Z.C., T.Y., and S.Z. carried out the experiments and analyzed the data. T.C. and S.L. drafted the manuscript. Z.Z. finalized the manuscript. All authors read and approved the manuscript.

Data availability

All relevant data in this study are provided in the article and its supplementary files.

Conflict of interest

The authors declare that they have no conflict of interest.

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