Abstract
The genome of Arabidopsis (Arabidopsis thaliana) contains five sequences with high similarity to FLAVONOL SYNTHASE1 (AtFLS1), a previously characterized flavonol synthase gene that plays a central role in flavonoid metabolism. This apparent redundancy suggests the possibility that Arabidopsis uses multiple isoforms of FLS with different substrate specificities to mediate the production of the flavonols, quercetin and kaempferol, in a tissue-specific and inducible manner. However, biochemical and genetic analysis of the six AtFLS sequences indicates that, although several of the members are expressed, only AtFLS1 encodes a catalytically competent protein. AtFLS1 also appears to be the only member of this group that influences flavonoid levels and the root gravitropic response in seedlings under nonstressed conditions. This study showed that the other expressed AtFLS sequences have tissue- and cell type-specific promoter activities that overlap with those of AtFLS1 and encode proteins that interact with other flavonoid enzymes in yeast two-hybrid assays. Thus, it is possible that these "pseudogenes" have alternative, noncatalytic functions that have not yet been uncovered.
Highlights
The genome of Arabidopsis (Arabidopsis thaliana) contains five sequences with high similarity to FLAVONOL SYNTHASE1 (AtFLS1), a previously characterized flavonol synthase gene that plays a central role in flavonoid metabolism
We describe an effort to test this hypothesis by examining the expression patterns and biochemical characteristics of the six Arabidopsis FLS isoforms as well as the impact of knockout mutations on phenotypes associated with flavonoid metabolism
While AtFLS1 expression was visible in leaf tissues for all transgenic lines that we investigated, this was not the case for AtFLS3 and -5, in which expression was limited to a few lines each, consistent with the overall lower gene expression levels for these isoforms as assessed by reverse transcription (RT)-PCR
Summary
The genome of Arabidopsis (Arabidopsis thaliana) contains five sequences with high similarity to FLAVONOL SYNTHASE1 (AtFLS1), a previously characterized flavonol synthase gene that plays a central role in flavonoid metabolism This apparent redundancy suggests the possibility that Arabidopsis uses multiple isoforms of FLS with different substrate specificities to mediate the production of the flavonols, quercetin and kaempferol, in a tissue-specific and inducible manner. The exception is FLS, for which we have identified six homologs in the Arabidopsis genome This raises the possibility that gene duplication has led to a group of differentially regulated genes encoding isoforms with varying substrate specificities, facilitating the synthesis of different flavonols to meet the dynamic physiological needs of the plant. The results of these experiments provide new insights into the mechanisms controlling flavonol accumulation in vivo
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