Background: Brassinosteroids (BRs) are a type of sterol plant hormone that play an important role in various biochemical and physiological reactions such as promoting cell growth, increasing biomass, and improving stress resistance. Results: To investigate the regulatory and molecular mechanism of BRs on the growth and development of tea plants (Camellia sinensis L.), changes in cell structure and gene expression levels of tea leaves treated with exogenous BRs were analyzed by electron microscopy and high-throughput Illumina RNA-Seq technology. The results showed that the number of starch granules in the chloroplasts and lipid globules increased and thylakoids expanded after BR treatment compared with the control. Transcriptome analysis showed that in the four BR treatments (CAA: BR treatment for 3 h, CAB: BR treatment for 9 h, CAC: BR treatment for 24 h, and CAD: BR treatment for 48 h), 3861 (1867 upregulated and 1994 downregulated), 5030 (2461 upregulated and 2569 downregulated), 1626 (815 upregulated and 811 downregulated), and 2050 (1004 upregulated and 1046 downregulated) differentially expressed genes were detected, respectively, compared with CAK (BR treatment for 0 h). Using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases, metabolic pathway enrichment analysis showed that the differentially expressed genes of CAA vs. CAK, CAB vs. CAK, CAC vs. CAK, and CAD vs. CAK significantly enriched the functional categories of signal transduction, cell cycle regulation, and starch, sucrose, and flavonoid biosynthesis and metabolism pathways. We also found that after spraying BR, the key genes for caffeine synthesis were downregulated. The results of qRT-PCR coincided with the findings of transcriptomic analysis. Conclusions: The present study improved our understanding of the effects of BRs on the growth and development of tea leaves and laid the foundation for the in-depth analysis of signal transduction pathways of BRs in tea leaves.

Agrobacterium-mediated transformation is a widely used method for plant genetic modification. However, its efficiency in tea plants is notably low, and the underlying molecular mechanisms remain unclear, hindering advancements in the molecular breeding and biology of tea plants. In this study, tobacco was utilized as a model to investigate the effects of various concentrations of epigallocatechin-3-gallate (EGCG) on Agrobacterium transformation efficiency. The results demonstrated that at an EGCG concentration of 0.4 mg/mL, Agrobacterium nearly lost its ability to transform tobacco. Additionally, malondialdehyde content in Agrobacterium was measured before and after EGCG treatment. The findings indicated that EGCG treatment led to an increase in malondialdehyde content. Transcriptome sequencing analysis revealed that differentially expressed genes (DEGs) involved in Agrobacterium flagellar synthesis and secretion systems were down-regulated under EGCG stress. Furthermore, flgE, virB4, and virB6 were identified as hub genes through weighted gene co-expression network analysis (WGCNA). These results elucidate the dynamic mechanisms by which EGCG affects Agrobacterium at both the physicochemical and molecular levels, providing a theoretical basis for optimizing genetic transformation in tea plants.