Papaver S-determinants trigger an integrated network of mitochondrially derived ROS and disruption of energy metabolism in incompatible pollen tubes

Abstract

Many plants use self-incompatibility (SI) mechanisms to prevent inbreeding by inhibiting pollen tube growth in "self" pistils. In Papaver rhoeas, SI 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 tubes. 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, combined with measurements of mitochondrial metabolism we have revealed a complex SI response. Within minutes, elevated cytosolic Ca2+ ([Ca2+]cyt) and cytosolic acidification converge to trigger mitochondrial H2O2 production (with consequent cytosolic and plastid H2O2 increase), membrane depolarisation, decreased respiration rate, and ATP depletion. In parallel, oxidative inactivation of cytosolic GAPDH inhibits glycolytic flux, resulting in decreased TCA cycle intermediates and providing a feedback loop to enhance mitochondrial disruption. Preceding production of mitochondrially-sourced ROS, we show that SI rapidly arrests pollen tube growth via inactivation of plasma membrane localised NADPH oxidase (RBOH) mediated superoxide production. Our findings provide new insights into how different ROS signatures from NADPH oxidase and mitochondria drive distinct processes. We demonstrate that mitochondrial disruption is an early and central event in the SI-response, likely driven by positive feedback loops involving altered Ca2+ signalling, cytosolic pH and ROS. Our findings provide novel insights into how SI rapidly disrupts energy metabolism in pollen tubes through interconnected signalling networks centred on mitochondria prior to PCD.