Abstract
Polyphenol oxidase (PPO) plays a role in stress response, secondary metabolism, and other physiological processes during plant growth and development, and is also a critical enzyme in black tea production. However, the regulatory mechanisms of PPO genes and their activity in tea plants are still unclear. In this study, we measured PPO activity in two different tea cultivars, Taoyuandaye (TYDY) and Bixiangzao (BXZ), which are commonly used to produce black tea and green tea, respectively. The expression pattern of CsPPO1 was assessed and validated via transcriptomics and quantitative polymerase chain reaction in both tea varieties. In addition, we isolated and identified an R2R3-MYB transcription factor CsMYB59 that may regulate CsPPO1 expression. CsMYB59 was found to be a nuclear protein, and its expression in tea leaves was positively correlated with CsPPO1 expression and PPO activity. Transcriptional activity analysis showed that CsMYB59 was a transcriptional activator, and the dual-luciferase assay indicated that CsMYB59 could activate the expression of CsPPO1 in tobacco leaves. In summary, our study demonstrates that CsMYB59 represents a transcriptional activator in tea plants and may mediate the regulation of PPO activity by activating CsPPO1 expression. These findings provide novel insights into the regulatory mechanism of PPO gene in Camellia sinensis, which might help to breed tea cultivars with high PPO activity.
Highlights
Polyphenol oxidases (PPOs) are a group of terminal copper-containing oxidases commonly found in various tissues and organs of plants, and play an important role in the enzymatic browning reaction
The CsPPO1 gene expression was significantly higher in TYDY than in BXZ based on the RNA sequencing (RNA-seq) data, and this result was consistent with the trend of PPO activity; quantitative real time polymerase chain reaction (qRT-PCR) analysis verified this result
We found that CsMYB59 showed transcriptional activity in yeast cells, and the results were verified via transcriptional activity analysis in tobacco leaves which confirmed that CsMYB59 may function as a transcriptional activator
Summary
Polyphenol oxidases (PPOs) are a group of terminal copper-containing oxidases commonly found in various tissues and organs of plants, and play an important role in the enzymatic browning reaction. Tyrosinase has monophenolase activity and catalyzes the formation of o-diphenol from monophenols, which is oxidized to produce the corresponding quinone. Catechol-oxidases show diphenolase activity and catechol oxidase activity, which catalyze the oxidation of diphenolic compounds to generate quinone (Yu et al, 2015; Liu et al, 2019). When the cells are subjected to mechanical damage, the contact of PPO enzymes with the substrates causes rapid oxidation of polyphenols, which is responsible for the negative effects on the appearance, nutritional quality, and commercial value of agricultural products (Hojnik Podrepšek et al, 2020). PPO is one of the most essential enzymes in black tea processing
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