Engineered Maize Hybrids with Diverse Carotenoid Profiles and Potential Applications in Animal Feeding

Multi-gene transformation methods need to be able to introduce multiple transgenes into plants in order to reconstitute a transgenic locus where the introduced genes express in a coordinated manner and do not segregate in subsequent generations. This simultaneous multiple gene transfer enables the study and modulation of the entire metabolic pathways and the elucidation of complex genetic control circuits and regulatory hierarchies. We used combinatorial nuclear transformation to produce multiplex-transgenic maize plants. In proof of principle experiments, we co-expressed five carotenogenic genes in maize endosperm. The resulting combinatorial transgenic maize plant population, equivalent to a "mutant series," allowed us to identify and complement rate-limiting steps in the extended endosperm carotenoid pathway and to recover corn plants with extraordinary levels of β-carotene and other nutritionally important carotenoids. We then introgressed the induced (transgenic) carotenoid pathway in a transgenic line accumulating high levels of nutritionally important carotenoids into a wild-type yellow-endosperm variety with a high β:ε ratio. Novel hybrids accumulated zeaxanthin at unprecedented amounts. We introgressed the same pathway into a different yellow corn line with a low β:ε ratio. The resulting hybrids, in this case, had a very different carotenoid profile. The role of genetic background in determining carotenoid profiles in corn was elucidated, and further rate-limiting steps in the pathway were identified and resolved in hybrids. Astaxanthin accumulation was engineered by overexpression of a β-carotene ketolase in maize endosperm. In early experiments, limited astaxanthin accumulation in transgenic maize plants was attributed to a bottleneck in the conversion of adonixanthin (4-ketozeaxanthin) to astaxanthin. More recent experiments showed that a synthetic β-carotene ketolase with a superior β-carotene/zeaxanthin ketolase activity is critical for the high-yield production of astaxanthin in maize endosperm. Engineered lines were used in animal feeding experiments which demonstrated not only the safety of the engineered lines but also their efficacy in a range of different animal production applications.