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Abstract
Tea (Camellia sinensis L.) leaves synthesize and concentrate a vast array of galloylated catechins (e.g., EGCG and ECG) and non-galloylated catechins (e.g., EGC, catechin, and epicatechin), together constituting 8%–24% of the dry leaf mass. Galloylated catechins account for a major portion of soluble catechins in tea leaves (up to 75%) and make a major contribution to the astringency and bitter taste of the green tea, and their pharmacological activity for human health. However, the catechin galloylation mechanism in tea plants is largely unknown at molecular levels. Previous studies indicated that glucosyltransferases and serine carboxypeptidase-like acyltransferases (SCPL) might be involved in the process. However, details about the roles of SCPLs in the biosynthesis of galloylated catechins remain to be elucidated. Here, we performed the genome-wide identification of SCPL genes in the tea plant genome. Several SCPLs were grouped into clade IA, which encompasses previously characterized SCPL-IA enzymes with an acylation function. Twenty-eight tea genes in this clade were differentially expressed in young leaves and vegetative buds. We characterized three SCPL-IA enzymes (CsSCPL11-IA, CsSCPL13-IA, CsSCPL14-IA) with galloylation activity toward epicatechins using recombinant enzymes. Not only the expression levels of these SCPLIA genes coincide with the accumulation of galloylated catechins in tea plants, but their recombinant enzymes also displayed β-glucogallin:catechin galloyl acyltransferase activity. These findings provide the first insights into the identities of genes encoding glucogallin:catechin galloyl acyltransferases with an active role in the biosynthesis of galloylated catechins in tea plants.
Highlights
Many plant metabolites, specialized compounds, are subject to various modifications, such as glycosylation, malonylation, methylation, acylation, and prenylation (Bowles et al, 2005; Kosma et al, 2012; Bontpart et al, 2015; Ahmad et al, 2017)
About 61% of the CsSCPL-I proteins are predicted to localize to the plasma membrane, whereas 65% of the CsSCPL-II proteins are estimated to localize to the lysosome
BAHD enzymes have been known for a long time to acylate anthocyanins and flavan-3-ols (Zhao, 2015), the molecular identities of the enzymes catalyzing transacylations from 1-O-β-glucose esters or their involvement in the biosynthesis of varied phenolic compounds were not described until the 1990s (Fujiwara et al, 1998; Niemetz and Gross, 2005)
Summary
Specialized compounds, are subject to various modifications, such as glycosylation, malonylation, methylation, acylation, and prenylation (Bowles et al, 2005; Kosma et al, 2012; Bontpart et al, 2015; Ahmad et al, 2017). Plant genomes evolved to owning a large number of gene families encoding modifying enzymes in order to adapt against adverse environments and survive frequent attacks by pathogens and herbivores (Wilson et al, 2016). Among these modifications, acylation is the most common and important of the modifications on plant metabolites, including phenolics, lipid barriers, sugars, and polyamines. Serine carboxypeptidase-like (SCPL) acyltransferases use 1-O-β-glucose esters as acyl donors to facilitate the transacylation reaction to a large variety of phenolics, acids, saponins, and other compounds were only discovered recently (Mugford and Milkowski, 2012)
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