Previously it was shown that the clastogenic effi-The frequency of O 6 -MeG immediately after expociency of high doses of alkylating agents in plant sure to a conditioning and a subsequent challenge root meristems can be reduced significantly by con-treatment was reduced by 43% as compared to ditioning pretreatment with either a low dose of the treatment with only the high dose. At a recovery time same agents, a sublethal heat shock, or heavy metal of 18 hours the corresponding frequency of adducts salts. The molecular mechanisms responsible for was reduced by 68.3% (related to the initial level) these protective effects are still unclear.
after treatment with the high dose alone, and by Here we report on the quantification of O 6 -meth-81.3% under adaptive conditions. Sublethal heat ylguanine (O 6 -MeG) by immuno-slot-blot analysis in shock or heavy metal salt used as conditioning pre-DNA of root tip meristems of field bean (Vicia faba) treatments also resulted in a decrease of adducts seedlings under conditions of clastogenic adapta-immediately after treatment with the challenge dose. tion. When root tips were pretreated with a low, From these data and from prevention of the effects conditioning dose of N-methyl-N-nitrosourea (MNU, by pretreatment with cycloheximide or O 6 -benzyl-10 04 M, 1 hour) 2 hours before exposure to a high guanine we conclude that under conditions of clastodose of the same clastogen (10 03 M, 1 hour), the genic adaptation O 6 -MeG is more efficiently refrequency of chromatid aberrations was reduced by moved from the DNA, presumably by induction of more than 50% at a recovery time of 18 hours, as an alkyl acceptor protein such as O 6 -methylguaninecompared to treatment with the high dose alone. The DNA methyltransferase (MGMT). This could explain same was observed when conditioning pretreatment the observed protective effects (clastogenic adaptawas by a sublethal heat shock (10 minutes, 40ะC) tion).