Variation of photosynthesis, fatty acid composition, ATPase and acid phosphatase activities, and anatomical structure of two tea (Camellia sinensis (L.) O. Kuntze) cultivars in response to fluoride

Transcriptome-based variations effectively untangling the intraspecific relationships and selection signals in Xinyang Maojian tea population

As one of the world's top three popular non-alcoholic beverages, tea is economically and culturally valuable. Xinyang Maojian, this elegant green tea, is one of the top ten famous tea in China and has gained prominence for thousands of years. However, the cultivation history of Xinyang Maojian tea population and selection signals of differentiation from the other major variety Camellia sinensis var. assamica (CSA) remain unclear. We newly generated 94 Camellia sinensis (C. sinensis) transcriptomes including 59 samples in the Xinyang area and 35 samples collected from 13 other major tea planting provinces in China. Comparing the very low resolution of phylogeny inferred from 1785 low-copy nuclear genes with 94 C. sinensis samples, we successfully resolved the phylogeny of C. sinensis samples by 99,115 high-quality SNPs from the coding region. The sources of tea planted in the Xinyang area were extensive and complex. Specifically, Shihe District and Gushi County were the two earliest tea planting areas in Xinyang, reflecting a long history of tea planting. Furthermore, we identified numerous selection sweeps during the differentiation of CSA and CSS and these positive selection genes are involved in many aspects such as regulation of secondary metabolites synthesis, amino acid metabolism, photosynthesis, etc. Numerous specific selective sweeps of modern cultivars were annotated with functions in various different aspects, indicating the CSS and CSA populations possibly underwent independent specific domestication processes. Our study indicated that transcriptome-based SNP-calling is an efficient and cost-effective method in untangling intraspecific phylogenetic relationships. This study provides a significant understanding of the cultivation history of the famous Chinese tea Xinyang Maojian and unravels the genetic basis of physiological and ecological differences between the two major tea subspecies.

Camellia sinensis Chloroplast Fluoride Efflux Gene CsABCB9 Is Involved in the Fluoride Tolerance Mechanism

Soil is a main source of fluoride for plants. The tea plants (Camellia sinensis) accumulate excessive amounts of fluoride in their leaves compared to other plants, but their fluoride tolerance mechanism is poorly understood. A chloroplast fluoride efflux gene (CsABCB9) was newly discovered by using transcriptome analysis, cloned from Camellia sinensis, and its function was demonstrated in the fluoride detoxication mechanism in Escherichia coli/Xenopus laevis oocytes and Arabidopsis thaliana. CsABCB9 is expressed in tea leaves upon F− treatment. The growth of tea, E. coli, and Arabidopsis were inhibited by F− treatment. However, growth of CsABCB9-overexpression in E. coli was shown to increase with lower fluoride content under F− treatment compared to the control. Furthermore, chlorophyll, xanthophyll and soluble sugar contents of CsABCB9-overexpression in Arabidopsis were improved under F− treatment compared to the wild type. CsABCB9 functions in fluoride transport, and the mechanism by which CsABCB9 improves fluoride resistance in tea is mainly chloroplast protection through fluoride efflux.

Two New Members of CsFEXs Couple Proton Gradients to Export Fluoride and Participate in Reducing Fluoride Accumulation in Low-Fluoride Tea Cultivars

The accumulation of fluoride in tea leaves from various cultivars exhibits significant differences. However, the molecular basis and mechanism remain largely unknown. Here, we reported that two genes of CsFEX (fluoride export genes in Camellia sinensis), CsFEX1 and CsFEX2, transport fluoride out of cells, alleviate the cellular fluoride toxin, and rescue the yeast mutant (FEX1ΔFEX2Δ) and Arabidopsis mutant (fex), as their efflux activities are coupled with proton gradients. Further analysis found that CsFEX1 and CsFEX2 localize to the plasma membrane both in yeast and Arabidopsis cells. CsFEX2 is more effective to reduce fluoride toxicity in yeast and Arabidopsis compared with CsFEX1 even at low pH. CsFEX2 induced by fluoride treatment is around tenfold higher in a low-fluoride cultivar (Yunkang 10) than that in a high-fluoride cultivar (Pingyang Tezaocha), suggesting that CsFEX2 possibly plays a critical role in reducing fluoride accumulation in tea leaves.

Location affects fatty acid composition in Camellia sinensis cv Tieguanyin fresh leaves

Tieguanyin tea is a typical representative of oolong tea in China, and is famous for its orchid-like aroma. Fatty acids are one of the important precursors for aroma production. However, fatty acid contents and compositions in Tieguanyin largely remain undefined. In this study, we quantified the fatty acid composition in Tieguanyin and its offspring by gas chromatography-flame ionization detector, and compared the effects of growth sites and harvest time on the leaf fatty acid composition. The results showed that total fatty acid contents in Tieguanyin fresh leaves were higher than its offspring. Growth sites had significant impacts on fatty acid contents. Tieguanyin grown in Anxi County showed higher total fatty acid contents, and higher ratio of total unsaturated fatty acids to total saturated fatty acids. The fresh leaves in the morning showed higher total fatty acid contents compared to samples harvested in the afternoon or evening, suggesting a dynamic fatty acid degradation during day period. During tea processing, unsaturated fatty acids including linolenic acid, linoleic acid and oleic acid (18:1^Δ9c) decreased 13.1%, 13.2% and 84.2%, respectively. The ratio of unsaturated fatty acids to saturated fatty acids still was above 300%. We found that Tieguanyin was a typical 18:3 plant, and the higher ratio of unsaturated fatty acids to saturated fatty acids of Tieguanyin grown in Anxi County may contribute to its characteristics aroma.