Synergistic metabolism in hybrid corn indicates bottlenecks in the carotenoid pathway and leads to the accumulation of extraordinary levels of the nutritionally important carotenoid zeaxanthin

Lutein and zeaxanthin cannot be synthesized de novo in humans, and although lutein is abundant in fruit and vegetables, good dietary sources of zeaxanthin are scarce. Certain corn varieties provide adequate amounts because the ratio of endosperm β : ε lycopene cyclase activity favours the β-carotene/zeaxanthin branch of the carotenoid pathway. We previously described a transgenic corn line expressing the early enzymes in the pathway (including lycopene β-cyclase) and therefore accumulating extraordinary levels of β-carotene. Here, we demonstrate that introgressing the transgenic mini-pathway into wild-type yellow endosperm varieties gives rise to hybrids in which the β : ε ratio is altered additively. Where the β : ε ratio in the genetic background is high, introgression of the mini-pathway allows zeaxanthin production at an unprecedented 56 μg/g dry weight. This result shows that metabolic synergy between endogenous and heterologous pathways can be used to enhance the levels of nutritionally important metabolites.