Abstract
Proanthocyanidins (PAs) are the major component of phenolics in apple, but mechanisms involved in PA biosynthesis remain unclear. Here, the relationship between the PA biosynthesis and the expression of genes encoding leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR) was investigated in fruit skin of one apple cultivar and three crabapples. Transcript levels of LAR1 and ANR2 genes were significantly correlated with the contents of catechin and epicatechin, respectively, which suggests their active roles in PA synthesis. Surprisingly, transcript levels for both LAR1 and LAR2 genes were almost undetectable in two crabapples that accumulated both flavan-3-ols and PAs. This contradicts the previous finding that LAR1 gene is a strong candidate regulating the accumulation of metabolites such as epicatechin and PAs in apple. Ectopic expression of apple MdLAR1 gene in tobacco suppresses expression of the late genes in anthocyanin biosynthetic pathway, resulting in loss of anthocyanin in flowers. Interestingly, a decrease in PA biosynthesis was also observed in flowers of transgenic tobacco plants overexpressing the MdLAR1 gene, which could be attributed to decreased expression of both the NtANR1 and NtANR2 genes. Our study not only confirms the in vivo function of apple LAR1 gene, but it is also helpful for understanding the mechanism of PA biosynthesis.
Highlights
Apple (Malus × domestica Borkh.), a member of the Rosaceae family, is one of the most widely cultivated fruit crops in the world
Proanthocyanidins (PAs), called condensed tannins, are phenolic polymers formed by condensation of flavan-3ol monomeric units such as catechin and epicatechin, which are synthesized via a branch of anthocyanin biosynthesis pathway under the catalyzation of two enzymes, leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR)
Our study indicates that the apple LAR1 and ANR2 genes probably play an important role in the biosynthesis of catechin and epicatechin, respectively, and ectopic expression of apple LAR1 genes in tobacco causes a significant decrease in both anthocyanin and PA accumulation in flowers
Summary
Apple (Malus × domestica Borkh.), a member of the Rosaceae family, is one of the most widely cultivated fruit crops in the world. Proanthocyanidins (PAs), called condensed tannins, are phenolic polymers formed by condensation of flavan-3ol monomeric units such as catechin and epicatechin, which are synthesized via a branch of anthocyanin biosynthesis pathway under the catalyzation of two enzymes, leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR). LAR catalyzes the conversion of leucoanthocyanidin (flavan3,4-diol) to catechin, while ANR catalyzes the synthesis of epicatechin from anthocyanidin (Tanner et al, 2003; Xie et al, 2003, 2004). A more recent study indicates that ectopic expression of the tea CsLAR gene in tobacco results in the accumulation of higher level of epicatechin than that of catechin, suggesting that LAR may be involved in the biosynthesis of epicatechin (Pang et al, 2013). Many questions regarding the transport of PAs from cytosol to vacuoles and the polymerization of flavan-3-ol monomers are still open (Zhao et al, 2010)
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