Leaf inner structure and immunogold localization of some key enzymes involved in carbon metabolism in CAM plants

There are relatively few reports on the leaf structure and in situ immunolocalization of carbon metabolism enzymes in crassulacean acid metabolism (CAM) plants, compared with reports on C₄ plants. The leaf inner structure and the subcellular location of some key CAM enzymes for a phosphoenolpyruvate carboxykinase (PCK) CAM species, Ananas comosus, and three malic enzyme (ME) CAM species, Mesembryanthemum crystallinum, Kalanchoe daigremontiana, and K. pinnata, was investigated by immunogold labelling and electron microscopy in this study. The leaves of these species had few intercellular air spaces in the mesophyll. A large vacuole occupied the mesophyll cells, and many vesicles of various sizes occurred in the cytosol. Immunocytochemical study revealed that labelling was present for phosphoenolpyruvate carboxylase in the cytosol and for ribulose-1,5-bisphosphate carboxylase/oxygenase in the chloroplasts of the mesophyll cells in all species. No specific labelling for pyruvate orthophosphate dikinase (PPDK) was observed in the PCK-CAM species. In the ME-CAM species, the patterns of labelling for PPDK differed. In M. crystallinum labelling for PPDK was present only in the chloroplasts, whereas in the two Kalanchoe species it occurred in the cytosol as well as in the chloroplasts. These results suggest that the subcellular localization of PPDK varies with ME-CAM species, in contrast to the conventional belief that it is localized in the chloroplasts.