Ionising radiation induces the expression of PARP-1 and PARP-2 genes in Arabidopsis

By screening for Arabidopsis genes activated by ionising radiation (IR)-ind uced DNA damage, we have isolated a cDNA hybridising with a 3.2-kb mRNA tha t accumulates rapidly and strongly in irradiated cell suspensions or whole plants. The cDNA codes for a 110-kDa protein that is highly homologous to t he 116-kDa vertebrate poly(ADP-ribose) polymerase (PARP-1). It is recognise d by a human anti-PARP-1 antibody, binds efficiently to DNA strand interrup tions in vitro, and catalyses DNA damage-dependent (ADP-ribose) polymer syn thesis. We have named this protein AtPARP-1. We have also extended our obse rvations to the Arabidopsis app (AtPARP-2) gene, demonstrating for the firs t time that IR-induced DNA strand interruptions induce rapid and massive ac cumulation of AtPARP-1 and AtPARP-2 transcripts, whereas dehydration and ca dmium preferentially induce the accumulation of AtPARP-2 transcripts. The I R-induced PARP gene expression seen in Arabidopsis is in striking contrast to the post-translational activation of the PARP-1 protein that is associat ed with genotoxic stress in animal cells. AtPARP-1 transcripts accumulate i n all plant organs after exposure to ionising radiation, but this is follow ed by an increase in AtPARP-1 protein levels only in tissues that contain l arge amounts of actively dividing cells. This cell-type specific accumulati on of AtPARP-1 protein in response to DNA damage is compatible with a role for the AtPARP-1 protein in the maintenance of DNA integrity during replica tion, similar to the role of "guardian of the genome" attributed to its ani mal counterpart.