L-theanine, a unique non-protein amino acid predominantly found in tea plants (Camellia sinensis), plays a pivotal role in plant responses to abiotic stress and significantly influences tea quality. In this review, the metabolism and transport mechanisms of L-theanine are comprehensively discussed, highlighting its spatial distribution in tea plants, where it is most abundant in young leaves and less so in roots, stems, and older leaves. The biosynthesis of L-theanine occurs through the enzymatic conversion of glutamate and ethylamine, catalyzed by theanine synthase, primarily in the roots, from where it is transported to aerial parts of the plant for further catabolism. Environmental factors such as temperature, light, drought, elevated CO₂, nutrient unavailability, and heavy metals significantly affect theanine biosynthesis and hydrolysis, with plant hormones and transcription factors playing crucial regulatory roles. Furthermore, it has been demonstrated that applying L-theanine exogenously improves other crops’ resistance to a range of abiotic stresses, suggesting its potential utility in improving crop resilience amid climate change. This review aims to elucidate the physiological mechanisms and biological functions of L-theanine metabolism under stress conditions, providing a theoretical foundation for enhancing tea quality and stress resistance in tea cultivation.

Tambocerus elongatus (Shen) (Hemiptera: Cicadellidae) is a devastating insect pest species of Camellia sinensis, significantly affecting the yield and quality of tea. Due to growing concerns over the irrational use of insecticides and associated food safety, it is crucial to better understand the innate resistance mechanism of tea trees to T. elongatus. This study aims to explore the responses of tea trees to different levels of T. elongatus infestation. We first focused on the primary metabolism and found that the amino acid levels decreased significantly with increasing T. elongatus infestation, while sugar accumulation showed an opposite trend. Moreover, secondary metabolite analysis showed a significant increase in flavonoid compounds and lignin content after T. elongatus infestation. Metabolomics analysis of the flavonoid compounds revealed a decrease in the proanthocyanidin level and an increase in anthocyanidin glycosides (anthocyanins and their derivatives) after T. elongatus infestation. T. elongatus infestation also caused a decrease in the abundance of non-ester catechins and an increase in the abundance of ester catechins. Furthermore, the gene expression analysis revealed that transcripts of genes involved in flavonoid biosynthesis, such as CsCHI, CsF3H, CsF3′H, CsFNS, CsFLS, and CsUFGT, were down-regulated, while genes involved in the lignin pathway were up-regulated by insect infestation, suggesting that lignin probably plays a pivotal role in tea plant response to T. elongatus infestation. Analysis of the expression of related genes indicates that the jasmonate (JA) pathway primarily responds to leafhopper damage. These findings suggest that the lignin pathway and JA play a preferential role in tea plant response to T. elongatus. Furthermore, the production of saccharides and the accumulation of anthocyanin glycosides in the flavonoid metabolic pathway are critical during this stress response. Further exploration of the roles of anthocyanin glycosides and lignin in tea tree resistance could provide a theoretical basis for understanding the defense mechanism of tea trees against T. elongatus damage.

Low temperatures severely restrict tea plant (Camellia sinensis) growth and yield stability, yet how brassinosteroid (BR) signaling modulates cold acclimation at a systems level remains insufficiently defined. Here, we integrated transcriptomic and UHPLC–MS metabolomic profiling of tea leaves under Control, 24-epibrassinolide (EBR), Cold, and Cold + EBR treatments to delineate BR-potentiated cold responses. Principal component analyses revealed clear treatment-specific separation and tight clustering of biological replicates at both omics levels. Quantitatively, cold stress induced extensive reprogramming (4075 differentially expressed genes (DEGs) and 298 differentially accumulated metabolites (DAMs)), whereas EBR alone exerted relatively modest effects (231 DEGs and 50 DAMs). Notably, EBR under cold conditions further reshaped cold-responsive networks (371 BR-modulated DEGs and 17 BR-modulated DAMs), consistent with a potentiating role for BR signaling. Functional enrichment analyses highlighted phenylpropanoid metabolism and hormone signal transduction as core responsive modules, with coordinated activation of key gateway genes (PAL, C4H, and 4CL) and concurrent engagement of lignin-, flavonoid-, and catechin-associated branches under Cold + EBR. Metabolomic analyses identified flavonoids as the dominant responsive metabolite class (49.31%), particularly anthocyanins and flavonol glycosides. Integrative TF–metabolite–gene correlation networks prioritized WRKY transcription factors (TEA001162, TEA027058) and a UDP-glycosyltransferase gene (TEA025792) as candidate hub genes linking hormone signaling to phenylpropanoid outputs. Collectively, this work provides a systems-level framework of co-regulated transcript–metabolite modules and candidate molecular targets, offering a foundation for functional validation and practical improvement of cold resilience in tea production.