Endonuclease activities in the coleoptile and the first leaf of developing etiolated wheat seedlings

DNase activity in coleoptiles and the first leaf apices of winter wheat (Triticum aestivum L., cv. Mironovskaya 808) etiolated seedlings was found to increase significantly during seedling growth, peaking on the eighth day of plant development. The maximum of DNase activity was coincident with apoptotic internucleosomal DNA fragmentation in these organs. Wheat endonucleases are capable of hydrolyzing both singleand double-stranded DNA of various origins. The leaf and coleoptiles were found to exhibit nuclease activities that hydrolyzed the lambda phage DNA with N⁶-methyladenine and 5-methylcytosine more actively compared to the hydrolysis of similar unmethylated DNAs. Thus, the endonucleases of wheat seedlings are sensitive to the methylation status of their substrate DNAs. The leaves and coleoptiles exhibited both Ca²⁺/Mg²⁺- and Zn²⁺-dependent nuclease activities that underwent differential changes during development and senescence of seedling organs. EDTA at a concentration of 50 mM fully inhibited the total DNase activity. Electrophoretic heterogeneity was observed for DNase activities operating simultaneously in the coleoptile and the first leaf at different stages of seedling development. Proteins exhibiting DNase activity (16–80 kD mol wt) were revealed in the first leaf and the coleoptile; these proteins were mostly nucleases with the pH optimum around 7.0. Some endonucleases (mol wts of 36, 39, and 28 kD) were present in both organs of the seedling. Some other DNases (mol wts of 16, 56, and about 80 kD) were found in the coleoptile; these DNases hydrolyzed DNA in the nucleus at terminal stages of apoptosis. Different suites of DNase activities were revealed in the nucleus and the cytoplasm, the nuclear DNase activities being more diverse than the cytoplasmic ones. Thus, the cellular (organspecific) and subcellular heterogeneity in composition and activities of DNases has been revealed in wheat plants. These DNases undergo specific changes during seedling development, serving at various stages of programmed cell death in seedling tissues.