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Unusual closed membrane vesicles containing one or more mitochondria were isolated from homogenates of aging wheat coleoptiles. Very similar (or the same) bodies were shown to exist in situ in vacuoles of undividing cells in the apical part of intact senescent coleoptiles. Vesicles isolated from coleoptile homogenate free of nuclei by 10 min centrifugation at 1700U Ug and traditional mitochondria (sedimented at between 4300U Ug and 17 400U Ug) are similar in respiration rate, composition and content of cytochromes and sensitivity to respiration inhibitors. However, vesicles contain about 2-fold more Ca 2+ ions than free mitochondria do. The specific feature of vesicles containing mitochondria in aging coleoptiles is an intensive synthesis of heavy (b b = 1.718 g/cm 3 ) mitochondrial DNA (H-mtDNA). Thus, aging in plants is accompanied by an increased selective H-mtDNA production and change in subcellular organization of mitochondria.z 1999 Federation of European Biochemical Societies.

After incubation of etiolated wheat seedlings in the presence of [8‐14C]adenine the radioactive N 6‐methyladenine (m6A) has been detected in the newly synthesized mitochondrial DNA (mtDNA) (ML = 100·m6A/(m6A+A)=0.4–0.6). This DNA is a low molecular mass (7.7 S) fraction of the mtDNA population. The detection of N 6‐methyladenine in the mtDNA of wheat seedlings indicates the presence of adenine DNA methylase in mitochondria of higher plants. The presence of m6A in plant mtDNAs makes them distinct from animal mtDNAs in which, as we know, this additional base has not been found.