Papaver S-determinants trigger mitochondrially derived ROS production and disrupt energy metabolism in incompatible pollen tubes.

Many plants use self-incompatibility (SI) mechanisms to prevent inbreeding. SI in Papaver rhoeas is triggered by allele-specific interaction between the pollen and pistil S-determinants, activating a Ca2+-dependent signalling network that leads to rapid reactive oxygen species (ROS) production and eventual programmed cell death (PCD) in incompatible pollen. Expression of the Papaver pollen S-determinant (PrpS) in Arabidopsis thaliana recapitulates Papaver SI when challenged with the cognate pistil ligand (PrsS). Using roGFP2-Orp1, a genetically encoded hydrogen peroxide (H2O2) sensor, and measurements of mitochondrial metabolism, reveals a complex SI response. Within minutes, elevated cytosolic Ca2+ (Ca2+cyt) and cytosolic acidification converge to trigger mitochondrial H2O2 production, mitochondrial membrane depolarisation, decreased respiration rate, and ATP depletion. In parallel, oxidative inactivation of GAPDH inhibits glycolysis, resulting in decreased TCA cycle intermediates and providing a feedback loop to enhance mitochondrial disruption. Preceding mitochondrial ROS production, SI rapidly arrests pollen tube growth via inactivation of plasma membrane localised NADPH oxidase (RBOH) mediated superoxide production. This provides insights into how ROS signatures from NADPH oxidase and mitochondria drive distinct processes. We demonstrate that early mitochondrial disruption, likely driven by interconnected Ca2+, pH and redox signalling, is a central feature of this SI response, underpinning rapid disruption of energy metabolism in incompatible pollen tubes prior to PCD.