Tea (Camellia sinensis [L.] O. Kuntze) is an important global economic crop and is considered to enhance health. However, the functions of many genes in tea plants are unknown. Virus-induced gene silencing (VIGS) mediated by tobacco rattle virus (TRV) is an effective tool for the analysis of gene functions, although this method has rarely been reported in tea plants. In this study, we established an effective VIGS-mediated gene knockout technology to understand the functional identification of large-scale genomic sequences in tea plants. The results showed that the VIGS system was verified by detecting the virus and using a real-time quantitative reverse transcription PCR (qRT-PCR) analysis. The reporter gene CsPOR1 (protochlorophyllide oxidoreductase) was silenced using the vacuum infiltration method, and typical photobleaching and albino symptoms were observed in newly sprouted leaves at the whole plant level of tea after infection for 12 d and 25 d. After optimization, the VIGS system was successfully used to silence the tea plant CsTCS1 (caffeine synthase) gene. The results showed that the relative caffeine content was reduced 6.26-fold compared with the control, and the level of expression of CsPOR1 decreased by approximately 3.12-fold in plants in which CsPOR1 was silenced. These results demonstrate that VIGS can be quickly and efficiently used to analyze the function of genes in tea plants. The successful establishment of VIGS could eliminate the need for tissue culture by providing an effective method to study gene function in tea plants and accelerate the process of functional genome research in tea.

Brassinosteroids (BRs) are important hormones that play crucial roles in plant growth, reproduction, and responses to abiotic and biotic stresses. CYP85A1 is a castasterone (CS) synthase that catalyzes C-6 oxidation of 6-deoxocastasterone (6-deoxoCS) to CS, after which CS is converted into brassinolide (BL) in a reaction catalyzed by CYP85A2. Here, we report the functional characteristics of rice (Oryza sativa L.) OsCYP85A1. Constitutive expression of OsCYP85A1 driven by the cauliflower mosaic virus 35S promoter increased endogenous BR levels and significantly promoted growth and biomass production in three groups of transgenic Populus tomentosa lines. The plant height and stem diameter of the transgenic poplar plants were increased by 17.6% and 33.6%, respectively, in comparison with control plants. Simultaneously, we showed that expression of OsCYP85A1 enhanced xylem formation in transgenic poplar without affecting cell wall thickness or the composition of cellulose. Our findings suggest that OsCYP85A1 represents a potential target candidate gene for engineering fast-growing trees with improved wood production.