Leucoanthocyanidin Dioxygenase in Arabidopsis thaliana: characterization of mutant alleles and regulation by MYB-BHLH-TTG1 transcription factor complexes

In Arabidopsis thaliana, most mutants impaired in flavonoid accumulation were identified through screens for altered seed pigmentation. Mutations in more than 20 loci have been described that can result in a transparent testa (tt) or tannin deficient seed (tds) phenotype. For some of these mutants it is still unclear if they represent additional loci or if they are allelic to known mutations. In this study, we found that tt17 is allelic to tt11 and tds4 and identified a point mutation in tt17 that affects the gene encoding Leucoanthocyanidin Dioxygenase (LDOX). The mutation results in replacement of a cysteine close to the active site of the enzyme by the hydrophobic amino acid tyrosine. Effects of this mutation on protein structure and activity are discussed in the context of LDOX sequences from various genotypes. Regulation of the LDOX promoter was analyzed and found to be directly controlled by different MYB-BHLH-TTG1 transcription factor complexes containing the BHLH factors EGL3 and TT8. Experiments with single and double loss-of-function mutants identified EGL3 and TT8 as necessary regulators of anthocyanin accumulation in developing A. thaliana seedlings.

Analysis of PRODUCTION OF FLAVONOL GLYCOSIDES-dependent flavonol glycoside accumulation in Arabidopsis thaliana plants reveals MYB11-, MYB12- and MYB111-independent flavonol glycoside accumulation

The flavonol branch of flavonoid biosynthesis is under transcriptional control of the R2R3-MYBs production of flavonol glycoside1 (PFG1/MYB12, PFG2/MYB11 and PFG3/MYB111) in Arabidopsis thaliana. Here, we investigated the influence of specific PFG transcription factors on flavonol distribution in various organs. A combination of genetic and metabolite analysis was used to identify transcription factor gene-metabolite correlations of the flavonol metabolic pathway. Flavonol glycoside accumulation patterns have been analysed in wild-type and multiple R2R3-MYB PFG mutants in an organ- and development-dependent manner using high-performance thin-layer chromatography, supplemented with liquid chromatography-mass spectroscopy metabolite profiling. Our results clearly demonstrate a differential influence of MYB11, MYB12 and MYB111 on the spatial accumulation of specific flavonol derivatives in leaves, stems, inflorescences, siliques and roots. In addition, MYB11-, MYB12- and MYB111-independent flavonol glycoside accumulation was observed in pollen grains and siliques/seeds. The highly complex tissue- and developmental-specific regulation of flavonol biosynthesis in A. thaliana is orchestrated by at least four PFG transcription factors, differentially influencing the spatial accumulation of specific flavonol derivatives. We present evidence that a separate flavonol control mechanism might be at play in pollen.