Iron (Fe) is an essential micronutrient whose availability is limiting in many soils. During Fe deficiency, plants alter the expression of many genes to increase Fe uptake, distribution, and utilization. In a genetic screen for suppressors of Fe sensitivity in the E3 ligase mutant bts-3, we isolated an allele of the bHLH transcription factor (TF) ILR3, ilr3-4 We identified a striking leaf bleaching phenotype in ilr3 mutants that was suppressed by limiting light intensity, indicating that ILR3 is required for phototolerance during Fe deficiency. Among its paralogs that are thought to be partially redundant, only ILR3 was required for phototolerance as well as repression of genes under Fe deficiency. A mutation in the gene-encoding PYE, a known transcriptional repressor under Fe deficiency, also caused leaf bleaching. We identified singlet oxygen as the accumulating reactive oxygen species (ROS) in ilr3-4 and pye, suggesting photosensitivity is due to a PSII defect resulting in ROS production. During Fe deficiency, ilr3-4 and pye chloroplasts retain normal ultrastructure and, unlike wild type (WT), contain stacked grana similar to Fe-sufficient plants. Additionally, we found that the D1 subunit of PSII is destabilized in WT during Fe deficiency but not in ilr3-4 and pye, suggesting that PSII repair is accelerated during Fe deficiency in an ILR3- and PYE-dependent manner. Collectively, our results indicate that ILR3 and PYE confer photoprotection during Fe deficiency to prevent the accumulation of singlet oxygen, potentially by promoting reduction of grana stacking to limit excitation and facilitate repair of the photosynthetic machinery.
Keywords: Arabidopsis; ROS; chloroplasts; grana; iron deficiency.