Earlier, the mutagenic effects of acridine orange (AO) and ethidium bromide (EB) on prokaryotes and eukaryotes have been studied (Burilkov and Krochik, Laser in the Life Sciences. v.2(4), pp. 253-274, 1988; Dragan, Khrapunov, Cytol. Genet. (USSR) 26:32-35, 1992). We have compared cytogenetic effects of the known sensitizers, AO and EB, and of previously unused ones, such as 6-mercaptopurine (6-MP) and Cloroxine (CX), each used in combination with laser radiation (LR). Maize seedlings A-346 grown on media containing the above sensitizers at a concentration of 0.00001 M each, were exposed to LR for 1 min ( =337.1 nm, I=70 MWt/m m sec). Counts of chromosome aberrations were made in temporary preparations during mitotic anaphase and telophase of maize (Gostimsky, Practical Guide on Cytogenetics, Moscow University Press, Moscow, 1974). The studies have shown that the highest rate of chromosome aberrations occurred when 6-MP was used as a sensitizer. This exceeded the rates of chromosome aberrations resulting from exposure to EB+LR, CX+LR, and AO+LR by factors of 1.5, 4, and 8 respectively.

One possible cause of chromosome aberrations induced by LR and sensitizers may be the formation of one- and two-strand breaks due to laser radiation energy which is transferred from the sensitizer molecule to certain DNA sites. The resultant one- and two-strand breaks may be repaired during mitosis, or they may eventually turn into chromosome aberrations. To test this hypothesis, we studied maize genomic DNA by the gel electrophoresis technique. Electrophoretic patterns and break counts from densitograms (Zhizina et al., Radiobiology (USSR) 23:783-786, 1983) have suggested that the hypothesis is not implausible (Table 1).

The induction of chromosome aberrations in maize plants with the aid of 6-MP and LR has a number of advantages over the conventional techniques:

-it shows relative selectivity: interaction is primarily with DNA;
-it enables the molecular mechanisms of mutation and recombination to be more precisely identified;
-it produces no cytotoxic effects.

P.S. A high level of mutations of the various types inherited in M2, M3 was discovered following the treatment of maize seedlings of the A-346 line. The analysis is in progress.

Table 1. The number of breaks in total DNA of maize resulting from exposure to sensitizers and LR.
 

	C	LR	6MP+LR	EB+LR	AO+LR	CX+LR
L	67	65	45	56	61	60
N	-	0.09	1.50	0.58	0.29	0.34

N-breaks number per molecule; L- average lengths of DNA fragments in the test and control DNA probes; C-control.

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