- Email: [email protected]
‘Clastogenic cross-adaptation’ is dependent on the clastogens used for induction of chromatid aberrations in Vicia faba root-tip meristems
Mutation Research, 144 (1985) 171-175 Elsevier
171
MRLett 0757
'Clastogenic cross-adaptation' is dependent on the clastogens used for induction o f c h r o m a t i d aberrations in Vicia faba root-tip meristems Rigomar Rieger 1, Arnd Michaelis 1 and Shin Takehisa 2 ~Zentralinstitut fiir Genetik und Kulturpflanzenforschung der A d W der DDR, DDR-4325 Gatersleben (German Democratic Republic) and 2Department o f Biology, Keio University, Yokohama 223 (Japan) (Accepted 28 June 1985)
Summary "Clastogenic cross-adaptation" is the triggering, by "conditioning" pretreatment with a low dose of one clastogen, of protective effects against the induction of chromatid aberrations by another ("challenge" treatm.ent) clastogen. All alkylating agents tested in this respect (TEM, NMU, EMS) were found to be able to substitute for each other in arousing such protective effects in Vicia faba root-tip cells. Combinations of M H (pretreatment) with T E M or NMU ('challenge' treatment), and vice versa, did not result in clastogenic cross-adaptation. The data are discussed as to the causes (inducible functions) which may give rise to clastogenic adaptation.
Chronic exposure to low, non-toxic doses of an alkylating agent enables "Escherichia coli to be resistant to subsequent exposures to large acute doses of the same chemical (Samson and Cairns, 1977). The same response was found in mammalian cells in a similar regimen with various alkylating agents (Samson and Schwartz, 1980) and in the response to ionizing radiation (Olivieri et al., 1984). Pretreatments with low, non-toxic doses of certain chemical agents may result in a significant reduction in the yield of chromatid aberrations induced by a subsequent 'challenge' treatment with one and the same chemical. The phenomenon may Address for correspondence: Shin Takehisa, Department of Biology, Keio University, Yokohama 223 (Japan).
be called 'clastogenic adaptation' and is apparently due to inducible functions that exert protective effects and are triggered by the pretreatments. Clastogenic adaptation has been observed in Vicia faba when the same clastogens were used for 'conditioning' pretreatment and 'challenge' treatment (Rieger et al., 1982, 1984a,b). In the present paper we report the results of the experiments in which different clastogens were used for 'conditioning' on the one hand and 'challenging' on the other. The question was whether or not different clastogens can substitute for each other in provoking 'clastogenic adaptation'. In this type of treatment 'clastogenic adaptation' will be called 'clastogenic cross-adaptation'. The data now at hand for a combination of some alkylating agents with maleic hydrazide (MH)
0165-7992/85/$ 03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)
172
allow us to conclude that the occurrence or absence of 'clastogenic cross-adaptation' depends on the specific combination of clastogens ('conditioning' by clastogen A, 'challenging' by clastogen B) being used for the induction of chromatid aberrations. Material and methods
5O MwA %
4O
Primary roots of about 3 cm length of the reconstructed karyotype ACB of Vicia faba (Michaelis and Rieger, 1971; Rieger and Michaelis, 1972) were used for the experiments. The following
30
TEM 20 MwA % 4O
l- i
1
t
~ ~ - "~x~. x x~
10¸
30
[ i !
20
I/////~
// /
I////I
TEMNMU
12
x
' '
\\l NML T T[.M
1/'
//ll/i
15
18
21
h RT
Fig. 2. Reduction of the yield of metaphases with chromatid aberrations (MwA) by 'conditioning' treatment with a low concentration of EMS (1 h 10 -2 M) prior to TEM-'challenging' (0.5 h 10 -4 M TEM). Curve 1:1 h 10 -2 M EMS. Curve 2 : 0 . 5 h 10 -4 M TEM. Curve 3 : 1 h 10 -z M EMS and 2 h later 0.5 h 10 -4 M TEM.
1 a~ ~ ] / / ~ 9
12
15
TEM 18
T 9
I !N~ 12
lf>
~, , N T
Fig. 1. Reduction in the yield of metaphases with chromatid aberrations (cross-adaptation) in the treatments in which T E M was used for 'conditioning' and NMU for 'challenge' treatment (left side), and vice versa (right side). Each data point represents the mean of the percentage of metaphases with chromatid aberrations (MwA o70). The vertical lines represent the standard error of the mean. RT, recovery time, Left: 0.5 h 10 -5 M TEM; 1 h 10 -3 M NMU; 0.5 h 10 -5 M T E M and 2 h later 1 h 10 -3 M N M U (broken line). Right: I h 10 -4 M NMU; 0.5 h 10 -4 M T E M ; 1 h 10 -4 M N M U and 2 h later 0.5 h 10 -4 M T E M (broken line).
agents were applied for induction of chromatid aberration~ at 24°C: N-methyl-N-nitrosourea (NMU); triethylenemelamine (triethylene-iminotriazine, TEM); maleic hydrazide (1,2-dihydropyridazine-3,6-dione, MH); ethyl methanesulfonate (EMS). For treatment concentration, time intervals between 'conditioning' and 'challenge' treatments, and other conditions see the legends of Figs. 1-4. After various recovery times in running tapwater (24°C) following treatment, the roots were treated with 0.05°70 colchicine for 2 h, fixed in absolute ethanol and glacial acetic acid (3"1), and Feulgen-stained.
173
l / i " . . iM~
50-
MwA %
~III/II
•
/
\
i
\ \
I
L ~ \
40-
• NM[
l
MH
30 ¸
~I
NMU
o f TEM prior to NMU 'challenging' resulted in a significant reduction in the percentage of metaphases with chromatid aberrations over the whole sequence of recovery times (9-18 h) tested (Fig. 1). Analogous results were obtained when the 'conditioning' was NMU and the 'challenge' treatment was TEM. Pretreatment with the monofunctional alkylating agent EMS and 'challenging' by TEM (Fig. 2) also resulted in 'clastogenic crossadaptation'; the yield of metaphases with chromatid aberrations after 'conditioning' with EMS was significantly lower than that of 'challenge' treatment with TEM alone. Different results were obtained when MH was
T
MwA
]
9
12
{
15
18
6
T
"~
]
1'5
7
ii
%
40-
I/
l
I\\
18 h R~
Fig. 3. No reduction in the yield of metaphases with chromatid aberrations (absence of 'cross-adaptation') when NMU was used for 'conditioning' and MH for 'challenge' treatment, and vice versa. Left: 1 h 10 4 M NMU; 0.5 h 4 x 10 -4 M MH; 1 h l0 -4 M NMU and 2 h later 0.5 h 4 x 10 -4 M MH (broken line). Right: 0 . 5 h 5 x 10 - S M M H ; 1 h 10 - 3 M N M U ; 0 . 5 h 5 x 10 5 M M H a n d 2 h l a t e r l h . 1 0 -3MNMU(brokenline).
For each recovery time, 200 metaphases of first cell-cycle after treatment were scored from 4 roottips, and the percentage of metaphases with isochromatid breaks, chromatid translocations, duplication deletions, and intercalary deletions was calculated. At least two repetitions of each experiment were made and the data were pooled. Results
Figs. 1 and 2 show the results obtained in the experiments in which 'conditioning' and 'challenge' treatments were done with different alkylating agents. A 'conditioning' treatment by a small dose
30
20.
10
~ T E M 15
18
21
24
! lI ~ t ~, ; ~ tI
15
18
"21
T PMH
24 h RT
Fig. 4. No reduction in the yield of meataphases with chromatid aberrations (absence of 'cross-adaptation') when TEM was used for 'conditioning' and MH for 'challenge' treatment, and vice versa. Left: 0 . 5 h 10 - S M T E M ; 0 . 5 h 5 x 10 - 4 M MH; 0.5 h 10 - 5 M TEM and 2 h later 0.5 h 5 × 10 -4 M MH (broken line). Right: 0.5 h 5 x 10 -5 M M H ; 0 . 5 h 10 - 4 M T E M ; 0 . 5 h 5 × 10 -5 M MH and 2 h later 0.5 h 1 0 - 4 M TEM (broken line).
174
used for 'conditioning' and alkylating agents for 'challenging', and vice versa (Figs. 3 and 4). As evident from Fig. 3, treatment with NMU prior to M H 'challenging' did not reduce the yield of metaphases with chromatid aberrations induced by MH. The opposite treatment ('conditioning' by MH, 'challenging' by NMU) gave similar results, i.e., the percentage of metaphases with aberrations obtained after the 'conditioning' and 'challenge' treatment was even higher than that obtained after 'challenge' treatment with either M H or NMU alone. Similar results were obtained when M H was used for 'conditioning' and TEM for 'challenging', and vice versa (Fig. 4). Discussion
'Conditioning' pretreatments of Viciafaba roottip meristems with small, non-toxic doses of certain clastogens may exert protection against a subsequently administered 'challenge' (application o f a high dose) treatment with the same agents. The protective effect of pretreatment was found to be dependent on the clastogen concentration used for pretreatment and on the time span between 'conditioning' and 'challenging'. Inhibition of protein synthesis prior to 'conditioning' treatment made the protective effect of 'conditioning' disappear (Rieger et al., 1982, 1984a,b). It was first reported in Escherichia coli that 'conditioning' pretreatment renders the cells resistant to the killing and mutation induced by a subsequent 'challenge' treatment (Samson and Cairns, 1977). Resistance was due to induction of an adaptive DNA-repair pathway and was called the adaptive response. A response analogous to the adaptive response was found in C H O cells and human skin fibroblast cells for the damage induced by alkylating agents and measured by cell survival and sister-chromatid exchange (Samson and Schwartz, 1980). It also occurred for ionizing radiation-induced chromatid aberrations in human lymphocytes (Olivieri et al., 1984). In the experiments the cells were given 150 rad of X-rays after exposure to chronic radiation from an incorporated radioisotope. A response analogous to the
adaptive response occurred not only in mammalian cells but also in plants. The response in Vicia faba reported in the present paper is one example. Analogous observations have been made in barley (Nicoloff et al., 1985) and Tradescantia (Veleminsky et al., 1983). The data presently at hand suggest that in Vicia faba 'conditioning' pretreatments with low clastogen doses trigger inducible functions which eventually make the root-tip cells less responsive to 'challenge' treatment with the same clastogens than before. Whether or not these functions represent inducible repair processes is a matter of speculation. Appropriate biochemical studies will be able to solve this problem and are being done. The experimental results reported in this communication allow some additional conclusions with respect to 'clastogenic adaptation'. The phenomenon is without doubt dependent on the clastogens being used for 'conditioning' and 'challenging'. All the alkylating agents tested were found to be able to substitute for each other in provoking 'clastogenic adaptation' or 'clastogenic cross-adaptation', i.e., the cellular functions induced by 'conditioning' with different alkylating agents were in all cases actively protecting cells themselves against the alkylating agents used for 'challenging'. The situation was quite different when MH and alkylating agents were being used for 'conditioning' and 'challenge' treatment, respectively. Absence of 'clastogenic crossadaptation' under these circumstances indicates that the inducible function triggered by MH, on the one hand, and alkylating agents, on the other hand, are different, and cannot exert a protective influence on the damage induced by the other agent in the respective combination (MH ~ NMU and vice versa, M H ~ TEM and vice versa). Whatever the nature of the inducible functions may be, the absence of 'clastogenic crossadaptation' in certain combinations of clastogens indicates that 'conditioning', depending on the agent being used, may induce a number of different (possibly lesion-specific) functions that can or cannot exert protection against specific chromosome-damaging agents.
175 'Clastogenic
cross-adaptation'
has
also
been
r e p o r t e d f o r V 7 9 C h i n e s e h a m s t e r cells p r e t r e a t e d with
N-methyl-N' -nitro-N-nitrosoguanidine
(MNNG) agents.
and No
'challenged'
such
effect
was
with
methylating
observed
when
' c h a l l e n g i n g ' w i t h e t h y l a t i n g a g e n t s ( K a i n a , 1983).
References Kaina, B. (1983) Cross-resistance studies with V79 Chinese hamster cells adapted to the mutagenic or clastogenic effect of N-methyl-N'-nitro-N-nitrosoguanidine, Mutation Res., 111, 341-352. Michaelis, A., and R. Rieger (1971) New karyotypes of Vicia faba, Chromosoma, 35, 1-8. Nicoloff, H., K. Gecheff, R. Rieger and A. Michaelis (1985) 'Clastogenic adaptation' in barley: Differential response of presoaked and dry seeds, Mutation Res., in press. Olivieri, G., J. Bodycote and S. Wolff (1984) Adaptive response of human lymphocytes to low concentrations of radioactive thymidine, Science, 223, 594-597. Rieger, R., and A. Michaelis (1972) Effects of chromosome repatterning in Vicia faba L., I. Aberration distribution, aberration spectrum and karyotype sensitivity after treat-
ment with ethanol of differently reconstructed chromosome complements, Biol. ZhI., 91, 151-169. Rieger, R., A. Michaelis and H. Nicoloff (1982) Inducible repair processes in plant root tip meristems? 'Belowadditivity effects' of unequally fractionated clastogen concentrations, Biol. Zbl., 101, 125-138. Rieger, R., A. Michaelis and H. Nicoloff (1984a) 'Clastogenic adaptation' of the Viciafaba root-tip meristem as affected by various treatment parameters, Mutation Res., 140, 99-102. Rieger, R., A. Michaelis and H. Nicoloff (1984b) Pretreatment of Vicia faba root tip meristems with low clastogen doses protects against aberration induction by subsequent treatments: Induction of repair processes? in: G.P. Chapman and S.A. Tarawali (Eds.), Systems for cytogenetic analysis in Vicia faba L., Proc. 2nd Vicia faba Cytogenetic Review Meeting (Wye/England), Nijhoff and Junk, Amsterdam, pp. 40-51. Samson, L., and J. Cairns (1977) A new pathway for DNA repair in Escherichia coli, Nature (London), 267, 281-283. Samson, L., and J.L. Schwartz (1980) Evidence for an adaptive DNA repair pathway in CHO and human skin fibroblast cell lines, Nature (London), 287, 861-863. Veleminsky, J., T. Gichner and T.J. Satava (1983) Reduction in the frequency of N-methyl-N-nitrosourea-induced somatic mutations in Tradescantia by pretreatment with low doses of alkylating agents, Mutation Res., 122, 229-234.
171
MRLett 0757
'Clastogenic cross-adaptation' is dependent on the clastogens used for induction o f c h r o m a t i d aberrations in Vicia faba root-tip meristems Rigomar Rieger 1, Arnd Michaelis 1 and Shin Takehisa 2 ~Zentralinstitut fiir Genetik und Kulturpflanzenforschung der A d W der DDR, DDR-4325 Gatersleben (German Democratic Republic) and 2Department o f Biology, Keio University, Yokohama 223 (Japan) (Accepted 28 June 1985)
Summary "Clastogenic cross-adaptation" is the triggering, by "conditioning" pretreatment with a low dose of one clastogen, of protective effects against the induction of chromatid aberrations by another ("challenge" treatm.ent) clastogen. All alkylating agents tested in this respect (TEM, NMU, EMS) were found to be able to substitute for each other in arousing such protective effects in Vicia faba root-tip cells. Combinations of M H (pretreatment) with T E M or NMU ('challenge' treatment), and vice versa, did not result in clastogenic cross-adaptation. The data are discussed as to the causes (inducible functions) which may give rise to clastogenic adaptation.
Chronic exposure to low, non-toxic doses of an alkylating agent enables "Escherichia coli to be resistant to subsequent exposures to large acute doses of the same chemical (Samson and Cairns, 1977). The same response was found in mammalian cells in a similar regimen with various alkylating agents (Samson and Schwartz, 1980) and in the response to ionizing radiation (Olivieri et al., 1984). Pretreatments with low, non-toxic doses of certain chemical agents may result in a significant reduction in the yield of chromatid aberrations induced by a subsequent 'challenge' treatment with one and the same chemical. The phenomenon may Address for correspondence: Shin Takehisa, Department of Biology, Keio University, Yokohama 223 (Japan).
be called 'clastogenic adaptation' and is apparently due to inducible functions that exert protective effects and are triggered by the pretreatments. Clastogenic adaptation has been observed in Vicia faba when the same clastogens were used for 'conditioning' pretreatment and 'challenge' treatment (Rieger et al., 1982, 1984a,b). In the present paper we report the results of the experiments in which different clastogens were used for 'conditioning' on the one hand and 'challenging' on the other. The question was whether or not different clastogens can substitute for each other in provoking 'clastogenic adaptation'. In this type of treatment 'clastogenic adaptation' will be called 'clastogenic cross-adaptation'. The data now at hand for a combination of some alkylating agents with maleic hydrazide (MH)
0165-7992/85/$ 03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)
172
allow us to conclude that the occurrence or absence of 'clastogenic cross-adaptation' depends on the specific combination of clastogens ('conditioning' by clastogen A, 'challenging' by clastogen B) being used for the induction of chromatid aberrations. Material and methods
5O MwA %
4O
Primary roots of about 3 cm length of the reconstructed karyotype ACB of Vicia faba (Michaelis and Rieger, 1971; Rieger and Michaelis, 1972) were used for the experiments. The following
30
TEM 20 MwA % 4O
l- i
1
t
~ ~ - "~x~. x x~
10¸
30
[ i !
20
I/////~
// /
I////I
TEMNMU
12
x
' '
\\l NML T T[.M
1/'
//ll/i
15
18
21
h RT
Fig. 2. Reduction of the yield of metaphases with chromatid aberrations (MwA) by 'conditioning' treatment with a low concentration of EMS (1 h 10 -2 M) prior to TEM-'challenging' (0.5 h 10 -4 M TEM). Curve 1:1 h 10 -2 M EMS. Curve 2 : 0 . 5 h 10 -4 M TEM. Curve 3 : 1 h 10 -z M EMS and 2 h later 0.5 h 10 -4 M TEM.
1 a~ ~ ] / / ~ 9
12
15
TEM 18
T 9
I !N~ 12
lf>
~, , N T
Fig. 1. Reduction in the yield of metaphases with chromatid aberrations (cross-adaptation) in the treatments in which T E M was used for 'conditioning' and NMU for 'challenge' treatment (left side), and vice versa (right side). Each data point represents the mean of the percentage of metaphases with chromatid aberrations (MwA o70). The vertical lines represent the standard error of the mean. RT, recovery time, Left: 0.5 h 10 -5 M TEM; 1 h 10 -3 M NMU; 0.5 h 10 -5 M T E M and 2 h later 1 h 10 -3 M N M U (broken line). Right: I h 10 -4 M NMU; 0.5 h 10 -4 M T E M ; 1 h 10 -4 M N M U and 2 h later 0.5 h 10 -4 M T E M (broken line).
agents were applied for induction of chromatid aberration~ at 24°C: N-methyl-N-nitrosourea (NMU); triethylenemelamine (triethylene-iminotriazine, TEM); maleic hydrazide (1,2-dihydropyridazine-3,6-dione, MH); ethyl methanesulfonate (EMS). For treatment concentration, time intervals between 'conditioning' and 'challenge' treatments, and other conditions see the legends of Figs. 1-4. After various recovery times in running tapwater (24°C) following treatment, the roots were treated with 0.05°70 colchicine for 2 h, fixed in absolute ethanol and glacial acetic acid (3"1), and Feulgen-stained.
173
l / i " . . iM~
50-
MwA %
~III/II
•
/
\
i
\ \
I
L ~ \
40-
• NM[
l
MH
30 ¸
~I
NMU
o f TEM prior to NMU 'challenging' resulted in a significant reduction in the percentage of metaphases with chromatid aberrations over the whole sequence of recovery times (9-18 h) tested (Fig. 1). Analogous results were obtained when the 'conditioning' was NMU and the 'challenge' treatment was TEM. Pretreatment with the monofunctional alkylating agent EMS and 'challenging' by TEM (Fig. 2) also resulted in 'clastogenic crossadaptation'; the yield of metaphases with chromatid aberrations after 'conditioning' with EMS was significantly lower than that of 'challenge' treatment with TEM alone. Different results were obtained when MH was
T
MwA
]
9
12
{
15
18
6
T
"~
]
1'5
7
ii
%
40-
I/
l
I\\
18 h R~
Fig. 3. No reduction in the yield of metaphases with chromatid aberrations (absence of 'cross-adaptation') when NMU was used for 'conditioning' and MH for 'challenge' treatment, and vice versa. Left: 1 h 10 4 M NMU; 0.5 h 4 x 10 -4 M MH; 1 h l0 -4 M NMU and 2 h later 0.5 h 4 x 10 -4 M MH (broken line). Right: 0 . 5 h 5 x 10 - S M M H ; 1 h 10 - 3 M N M U ; 0 . 5 h 5 x 10 5 M M H a n d 2 h l a t e r l h . 1 0 -3MNMU(brokenline).
For each recovery time, 200 metaphases of first cell-cycle after treatment were scored from 4 roottips, and the percentage of metaphases with isochromatid breaks, chromatid translocations, duplication deletions, and intercalary deletions was calculated. At least two repetitions of each experiment were made and the data were pooled. Results
Figs. 1 and 2 show the results obtained in the experiments in which 'conditioning' and 'challenge' treatments were done with different alkylating agents. A 'conditioning' treatment by a small dose
30
20.
10
~ T E M 15
18
21
24
! lI ~ t ~, ; ~ tI
15
18
"21
T PMH
24 h RT
Fig. 4. No reduction in the yield of meataphases with chromatid aberrations (absence of 'cross-adaptation') when TEM was used for 'conditioning' and MH for 'challenge' treatment, and vice versa. Left: 0 . 5 h 10 - S M T E M ; 0 . 5 h 5 x 10 - 4 M MH; 0.5 h 10 - 5 M TEM and 2 h later 0.5 h 5 × 10 -4 M MH (broken line). Right: 0.5 h 5 x 10 -5 M M H ; 0 . 5 h 10 - 4 M T E M ; 0 . 5 h 5 × 10 -5 M MH and 2 h later 0.5 h 1 0 - 4 M TEM (broken line).
174
used for 'conditioning' and alkylating agents for 'challenging', and vice versa (Figs. 3 and 4). As evident from Fig. 3, treatment with NMU prior to M H 'challenging' did not reduce the yield of metaphases with chromatid aberrations induced by MH. The opposite treatment ('conditioning' by MH, 'challenging' by NMU) gave similar results, i.e., the percentage of metaphases with aberrations obtained after the 'conditioning' and 'challenge' treatment was even higher than that obtained after 'challenge' treatment with either M H or NMU alone. Similar results were obtained when M H was used for 'conditioning' and TEM for 'challenging', and vice versa (Fig. 4). Discussion
'Conditioning' pretreatments of Viciafaba roottip meristems with small, non-toxic doses of certain clastogens may exert protection against a subsequently administered 'challenge' (application o f a high dose) treatment with the same agents. The protective effect of pretreatment was found to be dependent on the clastogen concentration used for pretreatment and on the time span between 'conditioning' and 'challenging'. Inhibition of protein synthesis prior to 'conditioning' treatment made the protective effect of 'conditioning' disappear (Rieger et al., 1982, 1984a,b). It was first reported in Escherichia coli that 'conditioning' pretreatment renders the cells resistant to the killing and mutation induced by a subsequent 'challenge' treatment (Samson and Cairns, 1977). Resistance was due to induction of an adaptive DNA-repair pathway and was called the adaptive response. A response analogous to the adaptive response was found in C H O cells and human skin fibroblast cells for the damage induced by alkylating agents and measured by cell survival and sister-chromatid exchange (Samson and Schwartz, 1980). It also occurred for ionizing radiation-induced chromatid aberrations in human lymphocytes (Olivieri et al., 1984). In the experiments the cells were given 150 rad of X-rays after exposure to chronic radiation from an incorporated radioisotope. A response analogous to the
adaptive response occurred not only in mammalian cells but also in plants. The response in Vicia faba reported in the present paper is one example. Analogous observations have been made in barley (Nicoloff et al., 1985) and Tradescantia (Veleminsky et al., 1983). The data presently at hand suggest that in Vicia faba 'conditioning' pretreatments with low clastogen doses trigger inducible functions which eventually make the root-tip cells less responsive to 'challenge' treatment with the same clastogens than before. Whether or not these functions represent inducible repair processes is a matter of speculation. Appropriate biochemical studies will be able to solve this problem and are being done. The experimental results reported in this communication allow some additional conclusions with respect to 'clastogenic adaptation'. The phenomenon is without doubt dependent on the clastogens being used for 'conditioning' and 'challenging'. All the alkylating agents tested were found to be able to substitute for each other in provoking 'clastogenic adaptation' or 'clastogenic cross-adaptation', i.e., the cellular functions induced by 'conditioning' with different alkylating agents were in all cases actively protecting cells themselves against the alkylating agents used for 'challenging'. The situation was quite different when MH and alkylating agents were being used for 'conditioning' and 'challenge' treatment, respectively. Absence of 'clastogenic crossadaptation' under these circumstances indicates that the inducible function triggered by MH, on the one hand, and alkylating agents, on the other hand, are different, and cannot exert a protective influence on the damage induced by the other agent in the respective combination (MH ~ NMU and vice versa, M H ~ TEM and vice versa). Whatever the nature of the inducible functions may be, the absence of 'clastogenic crossadaptation' in certain combinations of clastogens indicates that 'conditioning', depending on the agent being used, may induce a number of different (possibly lesion-specific) functions that can or cannot exert protection against specific chromosome-damaging agents.
175 'Clastogenic
cross-adaptation'
has
also
been
r e p o r t e d f o r V 7 9 C h i n e s e h a m s t e r cells p r e t r e a t e d with
N-methyl-N' -nitro-N-nitrosoguanidine
(MNNG) agents.
and No
'challenged'
such
effect
was
with
methylating
observed
when
' c h a l l e n g i n g ' w i t h e t h y l a t i n g a g e n t s ( K a i n a , 1983).
References Kaina, B. (1983) Cross-resistance studies with V79 Chinese hamster cells adapted to the mutagenic or clastogenic effect of N-methyl-N'-nitro-N-nitrosoguanidine, Mutation Res., 111, 341-352. Michaelis, A., and R. Rieger (1971) New karyotypes of Vicia faba, Chromosoma, 35, 1-8. Nicoloff, H., K. Gecheff, R. Rieger and A. Michaelis (1985) 'Clastogenic adaptation' in barley: Differential response of presoaked and dry seeds, Mutation Res., in press. Olivieri, G., J. Bodycote and S. Wolff (1984) Adaptive response of human lymphocytes to low concentrations of radioactive thymidine, Science, 223, 594-597. Rieger, R., and A. Michaelis (1972) Effects of chromosome repatterning in Vicia faba L., I. Aberration distribution, aberration spectrum and karyotype sensitivity after treat-
ment with ethanol of differently reconstructed chromosome complements, Biol. ZhI., 91, 151-169. Rieger, R., A. Michaelis and H. Nicoloff (1982) Inducible repair processes in plant root tip meristems? 'Belowadditivity effects' of unequally fractionated clastogen concentrations, Biol. Zbl., 101, 125-138. Rieger, R., A. Michaelis and H. Nicoloff (1984a) 'Clastogenic adaptation' of the Viciafaba root-tip meristem as affected by various treatment parameters, Mutation Res., 140, 99-102. Rieger, R., A. Michaelis and H. Nicoloff (1984b) Pretreatment of Vicia faba root tip meristems with low clastogen doses protects against aberration induction by subsequent treatments: Induction of repair processes? in: G.P. Chapman and S.A. Tarawali (Eds.), Systems for cytogenetic analysis in Vicia faba L., Proc. 2nd Vicia faba Cytogenetic Review Meeting (Wye/England), Nijhoff and Junk, Amsterdam, pp. 40-51. Samson, L., and J. Cairns (1977) A new pathway for DNA repair in Escherichia coli, Nature (London), 267, 281-283. Samson, L., and J.L. Schwartz (1980) Evidence for an adaptive DNA repair pathway in CHO and human skin fibroblast cell lines, Nature (London), 287, 861-863. Veleminsky, J., T. Gichner and T.J. Satava (1983) Reduction in the frequency of N-methyl-N-nitrosourea-induced somatic mutations in Tradescantia by pretreatment with low doses of alkylating agents, Mutation Res., 122, 229-234.