Research area at MPIZ Koeln

General research interest:

Functional analysis of transcription factors mediating the formation of flavonols in Arabidopsis thaliana

Plants are exposed to sun light which contains significant amounts of potentially harmful high-energy photons. A potential protective mechanism against this deleterious radiation is the accumulation of pigments, and it has long been proposed that flavonoids are such pigments.

Flavonoids, and more specifically flavonol glycosides, absorb UV-B light. They accumulate after an inductive light treatment in their glycosylated form in the vacuoles of epidermal cells (Weisshaar and Jenkins, 1998). Four 'structural' genes are required for the formation of the C-15 basic flavonol aglycon from central biosynthesis intermediates. 4-coumaroyl-CoA is the main substrate of the first enzyme specific for flavonoid biosynthesis, chalcone synthase (CHS). Chalcone-flavanone isomerase (CFI) and flavanone 3-hydroxylase (F3H) are required for synthesis of 3-hydroxy-flavanone, which serves as a substrate for either dihydroflavonol reductase (DFR) or flavonol synthase (FLS). DFR reduces the heterocyclic C-ring of dihydroflavonols and leads to the formation of pigments absorbing in the visible range of the spectrum, such as anthocyanins, phlobaphenes, or condensed tannins. In contrast, FLS oxidises the C-ring and results in the formation of colourless flavonols which predominantly absorb UV-light. UV and blue light stimulate the transcription of the CHS, CFI, F3H, and FLS genes, but they are also activated at certain developmental stages and in specific tissues. The various signals are integrated and transmitted to the nucleus, and the set of genes required for accumulation of the respective flavonol(s) must become active in a coordinated way. Our studies concentrated on the coordinated activation of a number of genes, but include also experiments to analyse the biochemisty and physiology of flavonoid biosynthesis.

  • Related links:
    the Natural Flavonoids page (maintained by Stefan Martens, Dept. of Gert Forkmann)

Regulation of gene activation in plants

Initial experiments to study light-dependent CHS gene activation were performed with parsley (Petroselinum crispum). A light-regulatory unit 1 (LRU1PcCHS) was defined which is sufficient to mediate light-induced CHS gene expression and consists of at least two distinct cis-acting elements, ACEPcCHS and MREPcCHS (ACE: ACGT-containing element; MRE: MYB recognition element). A number of factors of the bZIP and MYB type were isolated which, respectively, recognise these elements (Feldbrügge et al., 1997; Kircher et al., 1998). From the biochemical data collected with parsley plants and cultured parsley cells it became clear that genetic tools will be needed to further unravel the mechanisms controling flavonol accumulation in plants. Therefore, work with Arabidopsis thaliana was initiated.

Flavonol biosynthesis & flavonoid accumulation in response to UV light

In parallel to using genetic and functional genomics approaches to find the transcription factor(s) co-regulating flavonol biosynthesis genes, we also analysed transparent testa (tt) mutants of A. thaliana. While A. thaliana wildtype plants produce dark brown seeds, tt mutant seeds show a yellow to pale brown color. This is due to the absence of condensed tannins (flavonoid-derived brown pigments) from the seed coat. The TT loci include structural genes required for flavonoid biosynthesis. However, for some of the so far unkown TT genes (TT1, TT2, TT8) there are indications for a regulatory function.

Current projects & some abstracts:

  • Abstract of the 1999 MPIZ report
  • Establishment of high-efficiency SNP-based mapping tools (a GABI project)