Transcriptome analysis reveals fundamental differences in plant response to acute and chronic exposure to ionizing radiation.

We analyzed the influence of acute and chronic ionizing radiation (IR) on plant genome stability and global genome expression. Plants from the "chronic" group were grown for 21 days on (137)Cs-artificially contaminated soil, and received a cumulative dose of 1Gy. The "acute" plant group was exposed to an equal dose of radiation delivered as a single pulse. Analysis of homologous recombination (HR) events revealed a significantly higher increase in HR frequency (HRF) in the "chronic" group as compared to "acute" group. To understand the observed difference we performed global genome expression analysis. RNA profiling at 2h and 24h after acute irradiation showed two-third of up- and down-regulated genes to be similarly regulated at both time points. In contrast, less than 10% of the genes up- or down-regulated at 2h or 24h post-acute irradiation were similarly changed after chronic exposure. Promoter analysis revealed substantial differences in the specific regulatory elements found in acute and chronic transcriptomes. Further comparison of the data with existing profiles for several stresses, including UVC and heavy metals, showed substantial transcriptome similarities with the acute but not the chronic transcriptome. Plants exposed to chronic but not acute radiation showed early flowering; transcriptome analysis also revealed induction of flowering genes in "chronic" group.