Suppression of Granal Development and Accumulation of Rubisco in Different Bundle Sheath Chloroplasts of the C 4Succulent Plant Portulaca grandiflora

Abstract

Chlorenchyma of Portulaca grandiflora, an NADP-malic enzyme type succulent C 4plant, consists of several types of green cells. Typical bundle sheath and mesophyll cells were located around the peripheral vascular bundles in the cylindrical leaf. Water storage cells with small and few chloroplasts were located in the inner part of the leaf. Another vascular bundle was located in the centre of the leaf, which was surrounded by water storage cells. Typical bundle sheath cells were not observed around the central vascular bundle. Granal development was suppressed in the chloroplasts of typical bundle sheath cells. Mesophyll chloroplasts and the chloroplasts in most of the water storage cells possessed well-developed grana. However, granal development was suppressed in the chloroplasts of the innermost water storage cells adjacent to the central vascular bundle. Immunogold localization revealed that ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) accumulated in bundle sheath chloroplasts and in the chloroplasts of the water storage cells adjacent to the central vascular bundle. Labelling of Rubisco was markedly reduced in mesophyll chloroplasts and in the chloroplasts of remaining water storage cells. Gradient in labelling of Rubisco and granal development was not observed in the remaining water storage cells. Therefore, the accumulation of Rubisco and the suppression of granal development were restricted to the chloroplasts in the cells adjacent to the vascular bundles.