Unusual Fast Sedimenting Mitochondria Producing Heavy DNA in the Cells of Aging Coleoptiles of Wheat Seedlings

M. D. Kirnos^*, N. I. Alexandrushkina, L. E. Bakeeva, S. B. Kazimirchuk, B. Yu. Shorning, V. A. Alekseeva, L. S. Yaguzhinsky, and B. F. Vanyushin

Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-3181; E-mail: vanyush@genebee.msu.su

^* To whom correspondence should be addressed.

Received April 23, 1998; Revision received September 1, 1998

---

A fraction of unusual fast sedimenting (10 min at 600-1700g) particles with properties of mitochondria has been detected in wheat seedlings. This fraction conventionally called "heavy” mitochondria amounts (by protein) to about 40% of the total subcellular particle fraction sedimented by 10 min centrifugation at 17,000g. The specific feature of these "heavy” mitochondria in aging tissues is an ability to synthesize and even superproduce heavy (rho = 1.718 g/cm³) mitochondrial DNA (H-mtDNA). The share of "heavy” mitochondria sedimented in the interval between 1000 and 1700g and possessing the maximal H-mtDNA synthesis in aging coleoptiles is about 1.5-fold higher than that in young coleoptiles. Although "heavy” mitochondria are present in young plant organs, they seem to be unable to synthesize H-mtDNA; heavy mtDNA 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 mitochondrial fraction. Mitochondria isolated from wheat coleoptiles are practically not stimulated by uncouplers. "Heavy” (600-1700g) and usual (4,300-17,400g) mitochondria are similar in respiration rates, cytochrome compositions, cytochrome c amount (per mg protein) and sensitivities to respiration inhibitors. However, "heavy” mitochondria contain (per mg protein) cytochromes b and aa₃ by 10-20% and Ca²⁺ by 2-3-fold more than normal mitochondria. Ultrastructural analysis showed that the isolated fraction of fast sedimenting mitochondria consists of a suspension of closed membrane vesicles filled with cytoplasm and containing one or a few mitochondria. We observed similar structures in situ in vacuoles of parenchyma cells in the apical part of intact coleoptiles. The process of formation of such structures was detected by serial ultra-thin section analysis. It was shown that tonoplast protrudes into 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 bodies containing mitochondria and, in particular, the superproduction of H-mtDNA in them correlate with processes of aging and cell transition to apoptosis.

---

KEY WORDS: apoptosis, mitochondria, mitochondrial DNA, wheat, plants, DNA synthesis, heavy DNA

---