1.4.41
GENOMIC DNA HYPOMETHYLATION IS ASSOCIATED WITH INDUCED RESISTANCE INPLANTS


GENOMIC DNA HYPOMETHYLATION IS ASSOCIATED WITH INDUCED RESISTANCE INPLANTS

T KRASKA, U STEINER, H.-W. DEHNE

Institut fuer Pflanzenkrankheiten, Universitaet Bonn, Nussallee 9, D-53115 Bonn, Germany

Background

The mechanisms of induced resistance in plants are not understand completely, but it requires the activation of plant defence genes. It could be shown that the transcription rates of PR-proteins increase. DNA hypomethylation is reported to be associated with (re-)activation of genes in plants. A reduced level of 5-methylcytosin (5mC) is in general accompanied by a higher transcription rate of the gene showing a hypomethylation. In this study we tested the hypothesis, if also in the activation process of defence genes in induced resistant plants a genomic DNA hypomethylation could be observed.

Materials and methods

DNA was isolated according to ROGERS & BENDICH (1985), DNA purification and analysis of DNA methylation are described in KRASKA (1996).

Results and Conclusions

In comparative study, including various known resistance inducers, it was shown that all treatments reduced the infection density of powdery mildew on cucumber leaves. The degree of resistance varied between 20% and 65% as compared to the untreated control (=100%). The DNA methylation of cucumber leaves at the time point of inoculation with the pathogen was reduced after treatment with resistance inducers. The reduction varied between 70% and 83% as compared to the control (=100%) and was significant for all treatments. Two fungicides, which controlled powdery mildew very effectively in these experiments, do not affected DNA methylation. From these experiments it could be concluded that resistance inducers have an effect on DNA methylation. Vice versa, we tested known DNA methylation inhibitors in the same pathosystem. Methylation was reduced between 77% and 82% and was nearly in the same range as it was measured for the resistance inducers. Beside this expected effect on methylation the treated planst showed a higher resistance against the pathogen. Although it was less (68% - 72%) than the reduction observed for the resistance inducers, it was significant. At last we tested the effect of the methylation inhibtors on enzyme activity. 5-Azacytidine (a well known inhibitor) increased the activity of peroxidase and -1,3-glucanase in cucumber. The activity of peroxidase was approximately 6 fold higher and -1,3-glucanase 3 fold higher as in controls 5 days after treatment. It can be concluded that resistance inducers have an effect on DNA methylation and that vice versa methylation inhibitors showed some of the effects known from plants exhibiting an induced resistance. It has now to verified, if DNA methylation is directly involved in the activation of defence genes in induced resitant plants or if it is only a secondary effect of the treatments.

Referneces

1. Rogers SO, AJ Bendich, 1985. Plant Mol. Biol. 5, 69-75

2. Kraska T, 1996. Thesis, University of Hanover