Transcriptional Regulation of the Phenylpropanoid Pathway in Malus x domestica

Abstract

Compounds within the phenylpropanoid pathway are used by plants for defence against both biotic and abiotic stressors. Additionally these compounds make up a large proportion of the secondary metabolites consumed by people and have attributed health properties. Apples contain a large variety of these compounds within both the flesh and skin and their composition can vary widely amongst the variety of cultivars available. Differences in the regulation of enzymes controlling biosynthetic steps that produce specific phenylpropanoid compounds will be partially responsible for determining the phenolic composition. The initial control point for regulating the production of these enzymes is the production of their gene transcripts. Currently only two compound families, the anthocyanins and pro-anthocyanidins, have published information as to how these compounds are transcriptionally regulated within apple fruit. The proteins responsible for this regulation, transcription factors (TF), have been shown consistently throughout the plant kingdom to rely upon one major family called MYB TFs. In order to understand transcriptional regulation of the entire phenylpropanoid pathway in apples, MYBs were selected and screened against the promoters of biosynthetic genes in the pathway. Using transient expression of candidates in Nicotiana benthamiana and Nicotiana tabacum confirmed their ability to regulate this pathway. These candidate regulators altered the three main pathways beginning at phenylalanine.The specificity of MdMYB10 and MdMYB110a for anthocyanin production was confirmed. In addition the remaining two pathways, flavonol and chlorogenic acid production, appear to be regulated by MdMYB22 and MdMYB19 respectively. This extends our current knowledge of phenylpropanoid regulation in apples, identifies candidate flavonol regulators in apples, and a candidate regulator for chlorogenic acid in plants.