Continuous Turnover of Carotenes and Chlorophyll a in Mature Leaves of Arabidopsis Revealed by CO2 Pulse-Chase Labeling

Carotenoid turnover was investigated in mature leaves of Arabidopsis (Arabidopsis thaliana) by 14CO2 pulse-chase labeling under control-light (CL; 130 mmol photons m s) and high-light (HL; 1,000 mmol photons m s) conditions. Following a 30-min 14CO2 administration, photosynthetically fixed C was quickly incorporated in b-carotene (b-C) and chlorophyll a (Chl a) in all samples during a chase of up to 10 h. In contrast, C was not detected in Chl b and xanthophylls, even when steady-state amounts of the xanthophyll-cycle pigments and lutein increased markedly, presumably by de novo synthesis, in CL-grown plants under HL. Different light conditions during the chase did not affect the C fractions incorporated in b-C and Chl a, whereas long-term HL acclimation significantly enhanced C labeling of Chl a but not b-C. Consequently, the maximal C signal ratio between b-C and Chl a was much lower in HL-grown plants (1:10) than in CL-grown plants (1:4). In lut5 mutants, containing a-carotene (a-C) together with reduced amounts of b-C, remarkably high C labeling was found for a-C while the labeling efficiency of Chl a was similar to that of wild-type plants. The maximum C ratios between carotenes and Chl a were 1:2 for a-C:Chl a and 1:5 for b-C:Chl a in CL-grown lut5 plants, suggesting high turnover of a-C. The data demonstrate continuous synthesis and degradation of carotenes and Chl a in photosynthesizing leaves and indicate distinct acclimatory responses of their turnover to changing irradiance. In addition, the results are discussed in the context of photosystem II repair cycle and D1 protein turnover.