A genetic screen for dominant chloroplast reactive oxygen species signaling mutants reveals life stage-specific singlet oxygen signaling networks.

Introduction: Plants employ intricate molecular mechanisms to respond to abiotic stresses, which often lead to the accumulation of reactive oxygen species (ROS) within organelles such as chloroplasts. Such ROS can produce stress signals that regulate cellular response mechanisms. One ROS, singlet oxygen (O), is predominantly produced in the chloroplast during photosynthesis and can trigger chloroplast degradation, programmed cell death (PCD), and retrograde (organelle-to-nucleus) signaling. However, little is known about the molecular mechanisms involved in these signaling pathways or how many different signaling O pathways may exist.

Methods: The () mutant conditionally accumulates chloroplast O, making a valuable genetic system for studying chloroplast O-initiated signaling. Here, we have used activation tagging in a new forward genetic screen to identify eight dominant activation-tagged () mutations that suppress chloroplast O-initiated PCD.

Results: While O-triggered PCD is blocked in all mutants in the adult stage, such cellular degradation in the seedling stage is blocked in only two mutants. This differential blocking of PCD suggests that life-stage-specific O-response pathways exist. In addition to PCD, mutations generally reduce O-induced retrograde signals. Furthermore, mutants have enhanced tolerance to excess light, a natural mechanism to produce chloroplast O. However, general abiotic stress tolerance was only observed in one mutant (). Together, this suggests that plants can employ general stress tolerance mechanisms to overcome O production but that this screen was mostly specific to O signaling. We also observed that salicylic acid (SA) and jasmonate (JA) stress hormone response marker genes were induced in O-stressed and generally reduced by mutations, suggesting that SA and JA signaling is correlated with active O signaling and PCD.

Discussion: Together, this work highlights the complexity of O signaling by demonstrating that multiple pathways may exist and introduces a suite of new O signaling mutants to investigate the mechanisms controlling chloroplast-initiated degradation, PCD, and retrograde signaling.