Leaf variegation in the rice zebra2 mutant is caused by photoperiodic accumulation of tetra-Cis-lycopene and singlet oxygen

In field conditions, the zebra2 (z2) mutant in rice (Oryza sativa) produces leaves with transverse pale-green/yellow stripes. It was recently reported that ZEBRA2 encodes carotenoid isomerase (CRTISO) and that low levels of lutein, an essential carotenoid for non-photochemical quenching, cause leaf variegation in z2 mutants. However, we found that the z2 mutant phenotype was completely suppressed by growth under continuous light (CL; permissive) conditions, with concentrations of chlorophyll, carotenoids and chloroplast proteins at normal levels in z2 mutants under CL. In addition, three types of reactive oxygen species (ROS; superoxide O₂ ⁻], hydrogen peroxide H₂O₂], and singlet oxygen ¹O₂]) accumulated to high levels in z2 mutants grown under short-day conditions (SD; alternate 10-h light/14-h dark; restrictive), but do not accumulate under CL conditions. However, the levels of lutein and zeaxanthin in z2 leaves were much lower than normal in both permissive CL and restrictive SD growth conditions, indicating that deficiency of these two carotenoids is not responsible for the leaf variegation phenotype. We found that the CRTISO substrate tetra-Cis-lycopene accumulated during the dark periods under SD, but not under CL conditions. Its accumulation was also positively correlated with ¹O₂ levels generated during the light period, which consequently altered the expression of ¹O₂-responsive and cell death-related genes in the variegated z2 leaves. Taking these results together, we propose that the z2 leaf variegation can be largely attributed to photoperiodic accumulation of tetra-cis-lycopene and generation of excessive ¹O₂ under natural day-night conditions.