Functional Conservation of Plant Secondary Metabolic Enzymes Revealed by Complementation of Arabidopsis Flavonoid Mutants with Maize Genes

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Abstract
Mutations in the transparent testa (tt) loci abolish pigment production in Arabidopsis seed coats. The TT4, TT5, and TT3 loci encode chalcone synthase, chalcone isomerase, and dihydroflavonol 4-reductase, respectively, which are essential for anthocyanin accumulation and may form a macromolecular complex. Here, we show that the products of the maize (Zea mays) C2, CHI1, and A1 genes complement Arabidopsis tt4, tt5, and tt3 mutants, restoring the ability of these mutants to accumulate pigments in seed coats and seedlings. Overexpression of the maize genes in wild-type Arabidopsis seedlings does not result in increased anthocyanin accumulation, suggesting that the steps catalyzed by these enzymes are not rate limiting in the conditions assayed. The expression of the maize A1 gene in the flavonoid 3' hydroxylase Arabidopsis tt7 mutant resulted in an increased accumulation of pelargonidin. We conclude that enzymes involved in secondary metabolism can be functionally exchangeable between plants separated by large evolutionary distances. This is in sharp contrast to the notion that the more relaxed selective constrains to which secondary metabolic pathways are subjected is responsible for the rapid divergence of the corresponding enzymes.