Theanine (Thea) is a unique metabolite in tea plants, but its physiological functions remain elusive. A low soil pH increases cadmium (Cd) availability, affecting the quality of tea plant products. In this study, we found that Thea reversed the Cd-induced reduction in free amino acid (FAA) and caffeine (CAF) in the young tea leaves, as well as the down-regulation in the expression of nitrate transporters CsNRT1.2 and CsNRT2.5, and genes responsible for the nitrogen (N) assimilation. We demonstrated that Thea could alleviate Cd-induced oxidative stresses and enhance photosynthesis. Moreover, an ATP-binding cassette (ABC) transporter, CsABCG11.2, could uptake distinct Cd substrates and the five major amino acids in tea plants. Heterologous expression of CsABCG11.2 in yeast indicated a competitive absorption between Cd and Thea in a concentration-dependent pattern. CsABCG11.2-overexpressing Arabidopsis plants exhibited increased sensitivity to Cd due to enhanced Cd concentration, accumulation in the shoots, and reduction in the primary root length. Exogenous application of Thea at environmentally regular levels attenuated the adverse effects of Cd-induced growth inhibition and chlorosis in CsABCG11.2-overexpressing Arabidopsis plants. Knockdown of CsABCG11.2 tea plants significantly lowered Cd levels in young shoots. Our results suggest that Thea plays beneficial roles in alleviating Cd stress directly or indirectly by modulating CsABCG11.2-mediated Cd uptake and translocation within plants.

Nitrogen (N) is a macroelement with an indispensable role in the growth and development of plants, and tea plant (Camellia sinensis) is an evergreen perennial woody species with young shoots for harvest. During senescence or upon N stress, autophagy has been shown to be induced in leaves, involving a variety of autophagy-related genes (ATGs), which have not been characterized in tea plant yet. In this study, a genome-wide survey in tea plant genome identified a total of 80 Camellia Sinensis autophagy-related genes, CsATGs. The expression of CsATG8s in the tea plant showed an obvious increase from S1 (stage 1) to S4 (stage 4), especially for CsATG8e. The expression levels of AtATGs (Arabidopsis thaliana) and genes involved in N transport and assimilation were greatly improved in CsATG8e-overexpressed Arabidopsis. Compared with wild type, the overexpression plants showed earlier bolting, an increase in amino N content, as well as a decrease in biomass and the levels of N, phosphorus and potassium. However, the N level was found significantly higher in APER (aerial part excluding rosette) in the overexpression plants relative to wild type. All these results demonstrated a convincing function of CsATG8e in N remobilization and plant development, indicating CsATG8e as a potential gene for modifying plant nutrient utilization.

The vigor of tea plants (Camellia sinensis) and tea quality are strongly influenced by the abundance and forms of nitrogen, principally NO₃⁻, NH₄⁺, and amino acids. Mechanisms to access different nitrogen sources and the regulatory cues remain largely elusive in tea plants. A transcriptome analysis was performed to categorize differentially expressed genes (DEGs) in roots and young leaves during the early response to four nitrogen treatments. Relative to the continuously nitrogen-replete control, the three nitrogen-deprived and resupplied treatments shared 237 DEGs in the shoots and 21 DEGs in the root. Gene-ontology characterization revealed that transcripts encoding genes predicted to participate in nitrogen uptake, assimilation, and translocation were among the most differentially expressed after exposure to the different nitrogen regimes. Because of its high transcript level regardless of nitrogen condition, a putative amino acid transporter, TEA020444/CsCAT9.1, was further characterized in Arabidopsis and found to mediate the acquisition of a broad spectrum of amino acids, suggesting a role in amino acid uptake, transport, and deposition in sinks as an internal reservoir. Our results enhance our understanding of nitrogen-regulated transcript level patterns in tea plants and pinpoint candidate genes that function in nitrogen transport and metabolism, allowing tea plants to adjust to variable nitrogen environments.