Unusual fast sedimenting mitochondria producing heavy DNA in the cells of aging coleoptiles of wheat seedlings

A fraction of unusual fast sedimenting (10 min at 600-1700g) particles with properties of mitochondria has been detected in wheat seedlings. This frac tion conventionally called "heavy" mitochondria amounts (by protein) to abo ut 40% of the total subcellular particle fraction sedimented by 10 min cent rifugation at 17,000g. The specific feature of these "heavy" mitochondria i n aging tissues is an ability to synthesize and even superproduce heavy (rh o = 1.718 g/cm(3)) mitochondrial DNA (H-mtDNA). The share of "heavy" mitoch ondria sedimented in the interval between 1000 and 1700g and possessing the maximal H-mtDNA synthesis in aging coleoptiles is about 1.5-fold higher th an that in young coleoptiles. Although "heavy" mitochondria are present in young plant organs, they seem to be unable to synthesize H-mtDNA; heavy mtD NA forms only in mitochondria of aging or old cells. Thus, aging in plants is accompanied by a change in population of mitochondria and appearance of the ability for selective H-mtDNA superproduction in a certain mitochondria l fraction. Mitochondria isolated from wheat coleoptiles are practically no t stimulated by uncouplers. "Heavy" (600-1700g) and usual (4,300-17,400g) m itochondria are similar in respiration rates, cytochrome compositions, cyto chrome c amount (per mg protein) and sensitivities to respiration inhibitor s. However, "heavy" mitochondria contain (per mg protein) cytochromes b and aa, by 10-20% and Ca2+ by 2-3-fold more than normal mitochondria. Ultrastr uctural analysis showed that the isolated fraction of fast sedimenting mito chondria consists of a suspension of closed membrane vesicles filled with c ytoplasm and containing one or a few mitochondria. We observed similar stru ctures in situ in vacuoles of parenchyma cells in the apical part of intact coleoptiles. The process of formation of such structures was detected by s erial ultra-thin section analysis. It was shown that tonoplast protrudes in to vacuoles, the separate mitochondria translocate into these protrusions, and then these structures separate. As a result, the suspended cytoplasmic bodies containing mitochondria appear in vacuoles. Appearance of these bodi es containing mitochondria and, in particular, the superproduction of H-mtD NA in them correlate with processes of aging and cell transition to apoptos is.