Pathogen stress increases somatic recombination frequency in <i>Arabidopsis</i>

Evolution is based on genetic variability and subsequent phenotypic selection. Mechanisms that modulate the rate of mutation according to environmental cues, and thus control the balance between genetic stability and flexibility, might provide a distinct evolutionary advantage ^1-4. Stress-induced mutations stimulated by unfavorable environments, and possible mechanisms for their induction, have been described for several organisms ^2-4, but research in this area has mainly focused on microorganisms. We have analyzed the influence of adverse environmental conditions on the genetic stability of the higher plant <i>Arabidopsis</i> thaliana. Here we show that a biotic stress factor—attack by the oomycete pathogen <i>Peronospora parasitica</i>—can stimulate somatic recombination in <i>Arabidopsis</i>. The same effect was observed when plant pathogen-defense mechanisms were activated by the chemicals 2,6-dichloroisonicotinic acid (INA) or benzothiadiazole (BTH), or by a mutation (<i>cim3</i>). Together with previous studies of recombination induced by abiotic factors, these findings suggest that increased somatic recombination is a general stress response in plants. The increased genetic flexibility might facilitate evolutionary adaptation of plant populations to stressful environments.