Gamma irradiation with different dose rates induces different DNA damage responses in Petunia x hybrida cells

In plants, there is evidence that depending on the dose rate exposure to gamma (g) rays can cause different biological effects. Dynamics of DNA damage accumulation and molecular mechanisms that regulate recovery from radiation injury as a function of dose rate are poorly explored. To highlight dose-rate dependent differences in DNA damage, single cell gel electrophoresis was carried out on regenerating Petunia x hybrida leaf discs exposed to LDR (total dose 50 Gy, delivered at 0.33 Gy min-1) and HDR (total doses 50 and 100 Gy, delivered at 5.15 Gy min-1) g-ray in the 0-24 h time period after treatments. Significant fluctuations of double strand breaks and different repair capacities were observed between treatments in the 0-4 h time period following irradiation. Dose-rate dependent changes in the expression of the PhMT2 and PhAPX genes encoding a type 2 metallothionein and the cytosolic isoform of ascorbate peroxidase, respectively, were detected by Quantitative RealTime-Polymerase Chain Reaction. The PhMT2 and PhAPX genes were significantly up-regulated (3.0- and 0.7-fold) in response to HDR. Results are discussed in view of the potential practical applications of LDR-based treatments in mutation breeding.