Superproduction of "Heavy" DNA in Non-dividing Cells of Aging Coleoptile of Wheat Seedlings during Apoptosis

M. D. Kirnos,^1,2 N. I. Alexandrushkina,¹ B. Yu. Shorning,¹ S. N. Bubenshchikova,¹ and B. F. Vanyushin¹

¹Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-3181; E-mail: vanyushin@moo.genebee.msu.su

²To whom correspondence should be addressed.

Submitted June 27, 1997; revision submitted August 26, 1997.

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"Heavy" (rho = 1.718 g/cm³) DNA (H-DNA) forms and accumulates in aging cells of the wheat seedling coleoptile on a background of apoptosis. The amount of H-DNA in coleoptiles of wheat seedlings of 190 h age and older is comparable with that of nuclear DNA (nDNA, rho = 1.700 g/cm³), and it reaches up to 50% of total coleoptile DNA. H-DNA was not observed in coleoptiles of young seedlings of 144 h age and younger; it was not detected in CsCl density gradients by UV-light adsorption measurements but it was found there earlier [1] only by determination of radioactivity. H-DNA, in contrast to nuclear DNA, does not contain 5-methylcytosine and it is formed in the cell as a result of an unusual active DNA synthesis. Superproduction of H-DNA was not suppressed by known inhibitors of DNA synthesis such as cycloheximide and ethidium bromide but is significantly stimulated by these compounds. A signal for formation and accumulation of H-DNA appears during the fifth cycle of synchronous DNA replication in non-dividing cells of leaf and coleoptile (seedling age is 135-145 h), and it seems do not migrate along the plant. H-DNA superproduction is a specific programmed and synchronously appearing event in the aging cells of the whole plant organ. The appearance and accumulation of H-DNA in plants may to some extent serve as an indicator of aging.

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KEY WORDS: aging, apoptosis, cycloheximide, DNA fractionation, DNA fragmentation, DNA synthesis, ethidium bromide, inhibitors, ontogenesis, plants, wheat.

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