Natural variation in Arabidopsis ISR1 affects iron localization and induced systemic resistance

Beneficial root-associated bacteria can induce systemic resistance (ISR) to foliar pathogens and there is known transcriptional and genetic overlap in the root response to iron deficiency and ISR. A previous study found that there is natural variation in ISR among Arabidopsis accessions. The Ws accession is deficient in ISR, and the responsible recessive genetic locus, named ISR1, was mapped to chromosome 3. To find candidate genes that may underlie ISR deficiency in Ws, we identified genes that are induced in response to the ISR-triggering bacterium Pseudomonas simiae WCS417 and to iron stress and that have non-synonymous mutations in the Ws genome with respect to the ISR-responsive Col-0. We identified a kelch-domain containing protein encoded by At3g07720 that has a genomic rearrangement in Ws. We found that overexpression of Col-0 At3g07720 restores ISR to Ws, indicating that At3g07720 encodes ISR1. Isr1 loss of function mutants do not affect plant growth under iron limiting conditions but have increased levels of apoplastic iron. We found that iron supplementation, P. simiae WCS417, or a loss of isr1 enhance ROS production in a non-additive manner, suggesting they work through the same mechanism to enhance resistance. Our findings show that ISR1 is required for iron localization, immunity, and ISR, and suggest that increased iron uptake induced by ISR-eliciting bacteria may directly contribute to immunity through increased reactive oxygen production.