Abstract
The chlorotic tea variety Huangjinya, a natural mutant, contains enhanced levels of free amino acids in its leaves, which improves the drinking quality of its brewed tea. Consequently, this chlorotic mutant has a higher economic value than the non-chlorotic varieties. However, the molecular mechanisms behind the increased levels of free amino acids in this mutant are mostly unknown, as are the possible effects of this mutation on the overall metabolome and biosynthetic pathways in tea leaves. To gain further insight into the effects of chlorosis on the global metabolome and biosynthetic pathways in this mutant, Huangjinya plants were grown under normal and reduced sunlight, resulting in chlorotic and non-chlorotic leaves, respectively; their leaves were analyzed using transcriptomics as well as targeted and untargeted metabolomics. Approximately 5,000 genes (8.5% of the total analyzed) and ca. 300 metabolites (14.5% of the total detected) were significantly differentially regulated, thus indicating the occurrence of marked effects of light on the biosynthetic pathways in this mutant plant. Considering primary metabolism, including that of sugars, amino acids, and organic acids, significant changes were observed in the expression of genes involved in both nitrogen (N) and carbon metabolism. The suite of changes not only generated an increase in amino acids, including glutamic acid, glutamine, and theanine, but it also elevated the levels of free ammonium, citrate, and α-ketoglutarate, and lowered the levels of mono- and di-saccharides and of caffeine as compared with the non-chlorotic leaves. Taken together, our results suggest that the increased levels of amino acids in the chlorotic vs. non-chlorotic leaves are likely due to increased protein catabolism and/or decreased glycolysis and diminished biosynthesis of nitrogen-containing compounds other than amino acids, including chlorophyll, purines, nucleotides, and alkaloids.
Highlights
The flavor and quality of tea are attributed to the presence of polyphenols, alkaloids, and amino acids
The leaves were chlorotic in the tea plants grown in full sunlight, while the leaves of plants shaded to 60% of the full sunlight intensity turned green (Figure 1A and Figure S2)
The chloroplasts of the chlorotic leaves were arrested at the proplastid stage, and they did not develop a clear sheet membrane, nor did they have grana structures but instead they had ubiquitous osmiophilic granules (Figure 1C)
Summary
The flavor and quality of tea are attributed to the presence of polyphenols, alkaloids, and amino acids. Each free amino acid (or amide) has its own taste–as one of or a combination of sweet, salty, sour, bitter, and umami–which is why amides are recognized as the principal contributor to the mellow taste of brewed green tea (Zhang and Ruan, 2016). The concentrations of total free amino acids (including amides) and polyphenols in tea leaves can range from 1 to 5% and 20 to 40% (Zhang and Ruan, 2016), respectively. In premium green teas, a lower ratio of polyphenols to amino acids is required to balance the astringent and the mellow tastes. Too high a light intensity or temperature (or both) will not favor the accumulation of amino acids in tea plants (Zhang et al, 2014)
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