Installing the neurospora carotenoid pathway in plants enables cytosolic formation of provitamin A and its sequestration in lipid droplets

Abstract

Vitamin A deficiency remains a severe global health issue, which creates a need to biofortify crops with provitamin A carotenoids (PACs). Expanding plant cell capacity for synthesis and storing of PACs outside the plastids is a promising biofortification strategy that has been little explored. Here, we engineered PAC formation and sequestration in the cytosol of Nicotiana benthamiana leaves, Arabidopsis seeds, and citrus callus cells, using a fungal (Neurospora crassa) carotenoid pathway that consists of only three enzymes converting C₅ isopentenyl building blocks formed from mevalonic acid into PACs, including β-carotene. This strategy led to the accumulation of significant amounts of phytoene and γ- and β-carotene, in addition to fungal, health-promoting carotenes with 13 conjugated double bonds, such as the PAC torulene, in the cytosol. Increasing the isopentenyl diphosphate pool by adding a truncated Arabidopsis hydroxymethylglutaryl-coenzyme A reductase substantially increased cytosolic carotene production. Engineered carotenes accumulate in cytosolic lipid droplets (CLDs), which represent a novel sequestering sink for storing these pigments in plant cytosol. Importantly, β-carotene accumulated in the cytosol of citrus callus cells was more light stable compared to compared with plastidial β-carotene. Moreover, engineering cytosolic carotene formation increased the number of large-sized CLDs and the levels of β-apocarotenoids, including retinal, the aldehyde corresponding to vitamin A. Collectively, our study opens up the possibility of exploiting the high-flux mevalonic acid pathway for PAC biosynthesis and enhancing carotenoid sink capacity in green and non-green plant tissues, especially in lipid-storing seeds, and thus paves the way for further optimization of carotenoid biofortification in crops.

Keywords: Neurospora crassa; carotenoid biosynthesis; carotenoid sequestration; carotenoid stability; cytosolic lipid droplets; metabolic engineering; provitamin A; synthetic metabolism.