Turnip Z-ISO gene promotes zeaxanthin accumulation by modulating carotenoid metabolic flux and is involoved in negative responses to low-temperature stress.

Turnip, Brassica rapa ssp. rapa, is a biennial herbaceous root-type subspecies that is widely cultivated in East Asia. Carotenoids, important plastidial isoprenoid pigments and antioxidants, have important coloring functions in plants and contribute to health maintenance in humans. Here, the last key gene isolated in the carotenoid metabolic pathway, 15-cis-ζ-carotene isomerase (Z-ISO), which encodes 356 amino acids, has identified. It was expressed significantly more in the swollen root of yellow-turnip W25 than in white-turnip W21, and the BrrZ-ISO protein was located in the nucleus and cell membrane. Gene expression and carotenoid-targeted metabolomics analyses of Arabidopsis thaliana transgenic lines showed that overexpression of BrrZ-ISO enhanced the overall activity of the carotenoid biosynthesis pathway, altered the metabolic flux distribution at the lycopene branch point, and prioritized the promotion of β-chain metabolism to facilitate the accumulation of downstream components, such as zeaxanthin. Under low-temperature stress, BrrZ-ISO overexpression in Arabidopsis lines increased the malondialdehyde (MDA) content, delayed the early accumulation of proline, and reduced the total protein levels, thereby diminishing cold resistance. In addition, low-temperature stress induced the downregulation of carotenoid structural genes, thereby inhibiting carotenoids accumulation in the Arabidopsis BrrZ-ISO overexpressed lines. Our research findings not only increase our understanding of the BrrZ-ISO gene's role in turnip carotenoids accumulation and also establish a theoretical foundation for the study of carotenoids in non-photosynthetic tissues.