Tea (Camellia sinensis) is one of the most important cash crops in the world. Theanine, as an important amino acid component in tea, is a key quality index for excellent tea quality and high economic value. People increase theanine accumulation in tea mainly through the application of nitrogen fertilizer, shading and pruning. However, these methods are not effective. In this study, we treated tea buds with a 100 μM solution of GA₃ containing 1‰ tween-20, investigated the effects of GA₃ on theanine accumulation, bud yield, chlorophyll fluorescence parameters and expression level of theanine biosynthesis pathway genes in tea plant by qPCR, LC-MS/MS etc. Results showed that change trends of theanine and GA₃ was extremely positively correlated with each other. Exogenous GA₃ upregulated the expression level of theanine biosynthesis pathway genes, caused an increase of theanine content (mg·g^-1) by 27% in tea leaves compared with Mock, and accelerated the germination of buds and elongation of shoots, which lead to a significant increase of tea yield by 56% (w/w). Moreover, the decrease of chlorophyll contents, photochemical quenching coefficient (qP) and relative electron transport rate (rETR) under GA₃ treatment suggested that GA₃ reduced photosynthesis in the tender tea leaves, indicating that the decline of carbon assimilation in tea plants was conducive to the nitrogen metabolism, and it was beneficial to the accumulation of theanine. This study provided a new technical and theoretical support for the precise control of tea quality components and phenophase.

Theanine is a crucial indicator of tea quality, its significance is closely tied to the economic value of tea. There were many reports on the regulation mechanism of theanine synthesis and accumulation, but the mechanism of gibberellin regulates theanine synthesis in tea plants is poorly understood. Previous studies have shown that the content of theanine experiences significant changed in the growth stages of tea shoots, displayed a strong correlation with gibberellin. This study confirmed that gibberellin significantly promoted the expression of the major gene of theanine synthesis, known as CsTSI. Additionally, the study identified CsWRKY71, a transcription factor mediated the regulation of gibberellin on theanine synthesis in tea plants. CsWRKY71 was localized in the nucleus and had a typical WRKY domain. It was a member of class IIC sub- and its expression was significantly suppressed following exogenous GA₃ treatment. Further assay such as EMSA, dual luciferase and asODN interfering demonstrated that CsWRKY71 significantly interacted with the promoter of CsTSI, which inhibited theanine synthesis by binding to the cis-acting element (C/T)TGAC(T/C) of CsTSI promoter. Overall, the addition of exogenous gibberellin alleviated the inhibition of CsTSI by down-regulating the expression of CsWRKY71, ultimately facilitated the rapid biosynthesis of theanine. This study elucidated the molecular mechanism of CsWRKY71 mediated gibberellin regulation of theanine synthesis in tea plant. The findings not only enhance our understanding of the regulatory processes involved in theanine synthesis in tea plants, but also provide important references for maintaining the characteristics of high theanine in tea plant.

Nitrogen is an important nutrient for the tea plant (Camellia sinensis), which profoundly affects the quality and value of tea. In this study, the variations of biochemical activities, antioxidant systems, and tea quality of two tea varieties under four levels of nitrogen fertilizers were analyzed to explore the responses of tea plants to nitrogen stress. The primary maximum photochemical efficiency of PSII(F_v/F_m), the photochemical quenching coefficient (qP) and the relative electron transport rate (rETR) decreased under nitrogen deficiency (ND) and high nitrogen treatments (HN) in tea plant. Meanwhile, the levels of reactive oxygen species (ROS) increased significantly under ND/HN treatments, and the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) increased under HN/ND treatments. The relative expressions of antioxidant biosynthesis enzyme genes (CsSOD, CsPOD, CsCAT and CsAPX) were up-regulated under ND/HN treatments. Furthermore, the change trend of total free amino acid content under ND/HN treatments showed that nitrogen stress was not conducive to the accumulation of free amino acid content in tea, which may be related to the increase in ROS. This study presents a potential approach to improve tea quality by balancing ROS and antioxidant systems.

The substitution of chemical nitrogen (N) with organic fertilizers in tea plantations has been widely recognized as a strategy to maintain tea yield and improve soil quality, ensuring the sustainability of tea production systems. However, the effects of long-term organic-fertilizer substitution on tea yield and quality, soil properties, and bacterial communities have yet to be fully investigated, and the underlying mechanisms affecting tea yield and quality remain unclear. We conducted a six-year-long field experiment in a tea plantation to investigate the relationships among soil properties, bacterial communities, and the yield and quality of tea. Four treatments were compared: no fertilizer (NF), conventional fertilization (CF), 50% chemical N fertilizer substituted with a microbial organic fertilizer (MF), and 50% chemical N fertilizer substituted with a special organic fertilizer for tea (OF). The results showed that the substitution of organic fertilizers increased the spring tea yield by 6.4%~8.5% and the amino acid content of tea by up to 7.8%, while reducing tea polyphenol levels by 1.2–4.4% compared to CF. The soil quality improved significantly, with total phosphorus rising by 20.0% (MF) and 22.9% (OF), and soil organic matter increasing notably in the MF treatment group. The soil quality index (SQI) improved by 38.6% in the OF treatment group compared to the CF treatment group. Organic treatments reshaped bacterial communities, with the OF boosting Acidobacteriota (36.4%) and Planctomycetota (444.4%), and the MF enriching Actinobacteria and Gemmatimonadetes. Bacterial diversity (Shannon and Chao1 indices) correlated positively with the soil organic matter, total nitrogen, and pH. Changes in microbial communities were driven by pH, soil organic matter, and nitrogen levels. The partial least squares path model analysis confirmed that fertilization indirectly influenced tea yield (67% variance explained) and quality (79% variance explained) via soil properties and bacterial communities. These findings highlight the potential of organic-fertilizer substitution to promote sustainable tea production.