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Phenylpropanoids are phenylalanine-derived specialized metabolites and include important structural components of plant cell walls, such as lignin and hydroxycinnamic acids, as well as ultraviolet and visible light-absorbing pigments, such as hydroxycinnamate esters (HCEs) and anthocyanins. Previous work has revealed a remarkable degree of plasticity in HCE biosynthesis, such that most Arabidopsis (Arabidopsis thaliana) mutants with blockages in the pathway simply redirect carbon flux to atypical HCEs. In contrast, the ferulic acid hydroxylase1 (fah1) mutant accumulates greatly reduced levels of HCEs, suggesting that phenylpropanoid biosynthesis may be repressed in response to the loss of FERULATE 5-HYDROXYLASE (F5H) activity. Here, we show that in fah1 mutant plants, the activity of HCE biosynthetic enzymes is not limiting for HCE accumulation, nor is phenylpropanoid flux diverted to the synthesis of cell wall components or flavonol glycosides. We further show that anthocyanin accumulation is also repressed in fah1 mutants and that this repression is specific to tissues in which F5H is normally expressed. Finally, we show that repression of both HCE and anthocyanin biosynthesis in fah1 mutants is dependent on the MED5a/5b subunits of the transcriptional coregulatory complex Mediator, which are similarly required for the repression of lignin biosynthesis and the stunted growth of the phenylpropanoid pathway mutant reduced epidermal fluorescence8. Taken together, these observations show that the synthesis of HCEs and anthocyanins is actively repressed in a MEDIATOR-dependent manner in Arabidopsis fah1 mutants and support an emerging model in which MED5a/5b act as central players in the homeostatic repression of phenylpropanoid metabolism., (© 2015 American Society of Plant Biologists. All Rights Reserved.)

The plant phenylpropanoid pathway produces an array of metabolites that impact human health and the utility of feed and fiber crops. We previously characterized several Arabidopsis thaliana mutants with dominant mutations in REDUCED EPIDERMAL FLUORESCENCE 4 (REF4) that cause dwarfing and decreased accumulation of phenylpropanoids. In contrast, ref4 null plants are of normal stature and have no apparent defect in phenylpropanoid biosynthesis. Here we show that disruption of both REF4 and its paralog, REF4-RELATED 1 (RFR1), results in enhanced expression of multiple phenylpropanoid biosynthetic genes, as well as increased accumulation of numerous downstream products. We also show that the dominant ref4-3 mutant protein interferes with the ability of the PAP1/MYB75 transcription factor to induce the expression of PAL1 and drive anthocyanin accumulation. Consistent with our experimental results, both REF4 and RFR1 have been shown to physically associate with the conserved transcriptional coregulatory complex, Mediator, which transduces information from cis-acting DNA elements to RNA polymerase II at the core promoter. Taken together, our data provide critical genetic support for a functional role of REF4 and RFR1 in the Mediator complex, and for Mediator in the maintenance of phenylpropanoid homeostasis. Finally, we show that wild-type RFR1 substantially mitigates the phenotype of the dominant ref4-3 mutant, suggesting that REF4 and RFR1 may compete with one another for common binding partners or for occupancy in Mediator. Determining the functions of diverse Mediator subunits is essential to understand eukaryotic gene regulation, and to facilitate rational manipulation of plant metabolic pathways to better suit human needs.