Molecular and Biochemical Genetics of 2-Oxoglutarate-Dependent Dioxygenases Required for Flavonoid Biosynthesis in Arabidopsis thaliana

Three 2-oxoglutarate-dependent dioxygenases required for flavonoid biosynthesis were characterized in Arabidopsis thaliana. Genes encoding flavanone 3-hydroxylase (F3H), flavonol synthase (FLS), and leucoanthocyanidin dioxygenase (LDOX) were cloned and sequenced. The predicted proteins encoded by each of these Arabidopsis genes shared high homology with all F3H, FLS, or LDOX sequences available in Genbank. Low-stringency DNA blot analysis indicated that F3H and LDOX are encoded by a single gene in Arabidopsis, while FLS may be encoded by two or three genes.

RNA blot analysis was performed to determine the expression patterns of these three genes relative to previously-cloned genes encoding flavonoid biosynthetic enzymes. Light-induction experiments and analysis of regulatory mutants showed that the CHS, CHI, F3H, and FLS1 are coordinately regulated in Arabidopsis seedlings, encode enzymes acting near the beginning of the pathway, and are therefore referred to as "early" genes. The same experiments showed that DFR and LDOX are regulated distinctly from "early" genes, share similar expression patterns in response to light, and are not expressed in the ttg mutant. DFR and LDOX are therefore referred to as "late" genes due to the timing of expression in response to light and the fact that they encode enzymes acting late in flavonoid biosynthesis.

To determine whether any of the previously-identified transparent testa mutants were defective in F3H, FLS, or LDOX, the chromosomal locations of these genes in the Arabidopsis genome were determined. The positions of these genes suggested that no previously-identified tt mutant was defective in the cloned FLS or LDOX structural genes, while tt6 was potentially the F3H locus. The coding region of F3H was amplified by PCR from tt6 genomic DNA and sequenced, and several point mutations were found in the coding region of this allele, three of which are predicted to result in amino acid substitutions.

Polyclonal antibodies were also developed using four different purified, recombinant flavonoid enzymes as antigens. These antibodies were used to determine the pattern of accumulation of flavonoid enzymes in developing seedlings. Immunoblot analysis was also performed to determine whether mutations in genes encoding specific flavonoid enzymes or an enzyme in pathways that compete for or provide substrate for flavonoid biosynthesis (mutants defective in tryptophan or ferulic acid biosynthesis) affect the levels of flavonoid enzymes. These analyses showed that mutant seedlings which lacked specific flavonoid or tryptophan biosynthetic enzymes accumulated higher steady-state levels of other enzymes in the pathway. These results suggest that the accumulation of specific flavonoid intermediates or indole can lead directly or indirectly to higher levels of flavonoid enzymes.