Abstract
Caffeine is a metabolite of great economic importance, especially in coffee, where it influences the sensorial and physiological impacts of the beverage. Caffeine metabolism in the Coffea species begins with the degradation of purine nucleotides through three specific N-methyltransferases: XMT, MXMT and DXMT. A comparative analysis was performed to clarify the molecular reasons behind differences in caffeine accumulation in two Coffea species, namely Coffea arabica and Coffea canephora var. robusta. Three different genes encoding N-methyltransferase were amplified in the doubled haploid Coffea canephora: CcXMT1, CcMXMT1 and CcDXMT. Six genes were amplified in the haploid Coffea arabica: CaXMT1, CaXMT2, CaMXMT1, CaMXMT2, CaDXMT1, and CaDXMT2. A complete phylogenic analysis was performed to identify specific key amino acids defining enzymatic function for each protein identified. Furthermore, a quantitative gene-expression analysis was conducted on leaves and on maturing coffee beans, simultaneously analyzing caffeine content. In the different varieties analyzed, caffeine accumulation is higher in leaves than in the coffee bean maturation period, higher in Robusta than in Arabica. In Robusta, CcXMT1 and CcDXMT gene expressions are predominant and transcriptional activity is higher in leaves than in maturing beans, and is highly correlated to caffeine accumulation. In Arabica, the CaXMT1 expression level is high in leaves and CaDXMT2 as well to a lesser extent, while global transcriptional activity is weak during bean maturation, suggesting that the transcriptional control of caffeine-related genes differs within different organs and between Arabica and Robusta. These findings indicate that caffeine accumulation in Coffea species has been modulated by a combination of differential transcriptional regulation and genome evolution.Electronic supplementary materialThe online version of this article (doi:10.1007/s00425-014-2170-7) contains supplementary material, which is available to authorized users.
Highlights
Coffee is an important crop with more than seven million tons of green beans produced every year
To identify the different genes involved in the final steps of the caffeine biosynthesis pathway in two Coffea species characterized by different levels of caffeine content, we decided to make use of the fact that previous sequences shared high similarity (Ogawa et al 2001; Uefuji et al 2003; Mizuno et al 2003b; McCarthy and McCarthy 2007) and to design ‘‘universal’’ caffeine gene primers
To eliminate the natural polymorphism/allelism existing in Coffea species that could interefere with sequence annotation, we selected two specific Coffeas: the doubled-haploid Coffea canephora Robusta DH-200-94, diploıd but strictly homozygous, and the haploid Coffea arabica ET39-DH3 which has only one copy of each subgenome
Summary
Coffee is an important crop with more than seven million tons of green beans produced every year. In terms of cup quality, Coffea arabica (Arabica) is appreciated by consumers due to better flavor and high acidity compared with Coffea canephora (Robusta), famous for its bitterness and intense dark flavor. With complex variables, depends significantly on the fluctuating biochemical composition of the bean during fruit maturation (Simkin et al 2006; Lepelley et al 2007; Privat et al 2008; Salmona et al 2008) as well as control on the genetic level (Montagnon et al 1998; Leroy et al 2006). The principal molecules accumulated in coffee beans are caffeine, chlorogenic acids, lipids, sucrose, fat, and proteins. These different aroma precursors are transformed during roasting through Maillard reactions. Chlorogenic acids and caffeine are responsible for coffee bitterness, while free carbohydrates like sucrose generate flavor compounds by interacting with amino acids. Among the compounds present in coffee beans, caffeine is among the best known and most commonly studied because of its physiological effects on humans and its role in coffee plant resistance
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