Effect of atmospheric CO2 on plant defense against leaf and root pathogens of Arabidopsis

Climate change and the associated increase in atmospheric CO2 levels may affect the severity of plant diseases and threaten future crop yields. Here, we compared responses of the model plant Arabidopsis thaliana to leaf and root pathogens with hemi-biotrophic or necrotrophic infection strategies under pre-industrial, current, and future atmospheric CO2 conditions. Defenses against biotrophs are generally regulated by salicylic acid (SA) signaling, whereas jasmonic acid (JA) signaling controls defenses against necrotrophs. Under the CO2 conditions tested, basal expression of the JA-responsive marker gene PDF1.2 increased at increasing CO2 concentrations. The SA-responsive marker genes ICS1 and FRK1 showed an opposite behavior, being lower expressed under high CO2 and higher expressed under low CO2, respectively. Accordingly, plants showed enhanced resistance to the necrotrophic leaf pathogen Botrytis cinerea under high CO2, while resistance to the hemi-biotrophic leaf pathogen Pseudomonas syringae pv. tomato was reduced. The opposite was true for plants grown under low CO2. Disease severity caused by the soil-borne pathogens Fusarium oxysporum f.sp. raphani and Rhizoctonia solani was similar under all CO2 conditions tested. Collectively, our results stress the notion that atmospheric CO2 impacts the balance between SA- and JA-dependent defenses and concomitant resistance against foliar (hemi)biotrophic and necrotrophic pathogens. The direction of the CO2-mediated effects on SA- andJA-mediated defenses varies between reportedstudies, suggesting that the defense output is influenced by environmental context. These findings highlight that a wider dynamic range of climate change parameters should be studied simultaneously to harness plant traits for the development of future climate-resilient crops.