Down-regulation of cinnamoyl-CoA reductase in tomato (Lycopersicon esculentum Mill.) induces dramatic changes in soluble phenolic pools

Abstract

Health-beneficial properties of many plant secondary metabolites have driven much interest into the control of their biosynthesis in crop species. Phenolic compounds, including flavonoids, hydroxycinnamates and tannins, make up an important group of such phytonutrients. They are formed via the phenylpropanoid pathway and share common precursors with lignin, an insoluble cell wall-associated polymer. In this study, we aimed at reducing lignin biosynthesis to enhance availability of these precursors and thereby stimulate the production of soluble, potentially health-promoting, phenolic compounds in tomato (Lycopersicon esculentum Mill.). We first identified and characterized two tomato genes encoding cinnamoyl-CoA reductase (CCR), a key enzyme in the formation of lignin monomers. Transgenic plants exhibiting a reduced lignin content were subsequently obtained through an RNAi strategy targeting one of these genes. As anticipated, the total level of soluble phenolics was higher in stems and leaves of the transformants as compared to control plants. This was correlated with an increased antioxidant capacity of the corresponding plant extracts. Analysis of the soluble phenolic fraction by HPLC-MS revealed that vegetative organs of CCR down-regulated plants contained higher amounts of chlorogenic acid and rutin, and accumulated new metabolites undetectable in the wild type, such as N-caffeoyl putrescine and kaempferol rutinoside. In fruits, CCR down-regulation triggered the moderate accumulation of two new compounds in the flesh, but the total phenolic content was not affected. Although the prospects of exploiting such a strategy for crop improvement are limited, our results provide further insight into the control of the phenylpropanoid pathway in Solanaceae.