Ketocarotenoids accumulation in the leaves of engineered Brassica napus restricts photosynthetic efficiency and plant growth

Astaxanthin is a high-valued ketocarotenoid that is rarely synthesized in higher plants due to their lack of a carotenoid ketolase. Overexpression of a heterologous 8-carotene ketolase gene coupled with other pathway genes in higher plants has triggered the production of various ketocarotenoids in the transformants. Here we reported that constitutive and simultaneous overexpression of four genes involved in astaxanthin biosynthesis in Brassica napus resulted in the accumulation of the nonnative ketocarotenoids such as astaxanthin and ketolutein, and the decreased production of chlorophylls, lutein and violaxanthin in leaves. As a result, transgenic plants showed lower non-photochemical quenching (NPQ) under high light, leading to moderate photoinhibition of photosystem II (PSII). Such PSII photoinhibition depressed linear electron transport and thus restricted the rate of CO2 assimilation. Consequently, transgenic plants grew slower and showed less biomass than wild type. These results indicated that endogenous synthesis of ketocarotenoids negatively affected photosynthesis and impaired plant growth in B. napus. Therefore, to prevent the side effect on normal growth, seed specific expression of the transgenes may be necessary for enhanced production of astaxanthin in seeds.

Automated summary

Overexpression of four genes involved in astaxanthin biosynthesis in Brassica napus resulted in the accumulation of nonnative ketocarotenoids, decreased levels of chlorophylls and other carotenoids, and ultimately led to slower growth and decreased biomass production in the transgenic plants. The reduced photosynthetic efficiency caused by the altered carotenoid levels negatively impacted plant growth in B. napus.