A V232M missense mutation in rice OsLSZ1, a lycopene β-cyclase, inhibits β-carotene biosynthesis and impairs chloroplast development during seedling stage at low temperatures

Carotenoids are essential for chloroplast development, with lycopene β-cyclase as the rate-limiting enzyme in the biosynthesis of β-carotene and α-carotene. This study investigates the carotenoid biosynthetic alterations in a low-temperature-sensitive zebra-leaf rice mutant, oslsz1, which exhibits yellow or yellow-green striped leaves before the five-leaf stage at lower temperatures (22 °C and 28 °C) but develops normally at higher temperature (34 °C). Metabolomics revealed that at 22 °C, levels of β-carotene and xanthophylls decreased, while levels of (E/Z)-phytoene, lycopene, α-carotene, and γ-carotene increased in oslsz1 as compared with the wild type. Transmission electron microscopy demonstrated defective chloroplast development in oslsz1 at 22 °C. Through map-based cloning, the causative gene was identified as OsLCYb, which encodes lycopene β-cyclase with a V232M missense mutation. mRNA quantification showed that OsLSZ1/OsLCYb is ubiquitously expressed, especially in young-seedling leaves, with suppression at low temperatures. Heterologous expression of oslsz1/oslcyb in Escherichia coli confirmed a near-complete loss of lycopene conversion to β-carotene at 22 °C. Further domain dissection indicated that the N-terminal 181 amino acids of OsLSZ1/OsLCYb are critical for low-temperature sensitivity, while its C-terminal extension enhances this response. In summary, these findings underscore the crucial role of OsLSZ1/OsLCYb in carotenoid biosynthesis and chloroplast development in rice at low temperatures.