Mitochondria and Chloroplast Peripheral Reticulum in the C4 Plants Amaranthus edulis and Atriplex spongiosa

Cytochemical techniques were used to show that all the many large mitochondrion-like bodies present in bundle sheath cells of the aspartate-donor C4 plants, Amaranthus edulis and Atriplex spongiosa, possessed mitochondrial characteristics. The techniques involved the location of cytochrome c oxidase activity with 3,3'-diaminobenzidine and examination of configurational changes in mitochondria after incubation in the inhibitors of oxidative phosphorylation, rotenone and oligomycin, and of substrate phosphorylation, iodoacetate, or the uncouplers of oxidative phosphorylation 2,4-dinitrophenol and carbonyl cyanide p-trifluoromethoxyphenylhydrazone. The peripheral reticulum, which is a prominent feature of C4 mesophyll and bundle sheath chloroplasts and whose internal structure closely resembles that of the bundle sheath mitochondria, was also examined cytochemically. Frequent protrusions or buds from the peripheral reticulum in the bundle sheath chloroplasts strongly suggested that it could give rise to all or some of the mitochondria. Cytochemical investigation with diaminobenzidine, however, demonstrated that the peripheral reticulum does not possess cytochrome c oxidase activity and that the mitochondrial population is homogeneous. Studies of developing leaves of Amaranthus edulis showed that the mitochondria were a prominent feature in the bundle sheath cells before the appearance of the peripheral reticulum. The close proximity of the peripheral reticulum to mitochondria or to peroxisomes and sometimes also to the cell wall plasmodesmata indicates that it may play a role in metabolite transport.