Two important flavonoids, kaempferol and quercetin possess remarkably potent biological impacts on human health. However, their structural complexity and low abundance in nature make both bulk chemical synthesis and extraction from native plants difficult. Therefore microbial production via heterologous expression of plant enzymes can be a safe and sustainable route for their production. Despite several attempts reported in microbial hosts, the production levels of kaempferol and quercetin still stay far behind compared to many other microbial-produced flavonoids.

In this study, Saccharomyces cerevisiae was engineered for high production of kaempferol and quercetin in minimal media from glucose. First, the kaempferol biosynthetic pathway was reconstructed via screening various F3H and FLS enzymes. In addition, we demonstrated that amplification of the rate-limiting enzyme AtFLS could reduce the dihydrokaempferol accumulation and improve kaempferol production. Increasing the availability of precursor malonyl-CoA further improved the production of kaempferol and quercetin. Furthermore, the highest amount of 956 mg L^− 1 of kaempferol and 930 mg L^− 1 of quercetin in yeast was reached in fed-batch fermentations.

De novo biosynthesis of kaempferol and quercetin in yeast was improved through increasing the upstream naringenin biosynthesis and debugging the flux-limiting enzymes together with fed-batch fermentations, up to gram per liter level. Our work provides a promising platform for sustainable and scalable production of kaempferol, quercetin and compounds derived thereof.