Structural and biochemical characterization of the C₃–C₄ intermediate Brassica gravinae and relatives, with particular reference to cellular distribution of Rubisco

摘要

On the basis of its CO₂ compensation concentration, Brassica gravinae Ten. has been reported to be a C₃–C₄ intermediate. This study investigated the structural and biochemical features of photosynthetic metabolism in B. gravinae. The cellular distribution of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) was also examined in B. gravinae, B. napus L. (C₃), Raphanus sativus L. (C₃), and Diplotaxis tenuifolia (L.) DC. (C₃–C₄) by immunogold electron microscopy to elucidate Rubisco expression during the evolution from C₃ to C₃–C₄ intermediate plants. The bundle sheath (BS) cells of B. gravinae contained centrifugally located chloroplasts as well as centripetally located chloroplasts and mitochondria. Glycine decarboxylase P-protein was localized in the BS mitochondria. Brassica gravinae had low C₄ enzyme activities and high activities of Rubisco and photorespiratory enzymes, suggesting that it reduces photorespiratory CO₂ loss by the glycine shuttle. In B. gravinae, the labelling density of Rubisco was higher in the mesophyll chloroplasts than in the BS chloroplasts. A similar cellular pattern was found in other Brassicaceae species. These data demonstrate that, during the evolution from C₃ to C₃–C₄ intermediate plants, the intercellular pattern of Rubisco expression did not change greatly, although the amount of chloroplasts in the BS cells increased. It also appears that intracellular variation in Rubisco distribution may occur within the BS cells of B. gravinae.