Oxygen radicals and cytoplasm zoning in growing lily pollen tubes

Key message Differential modulation of ROS content of the microenvironment (O over bar /MnTMPP/OH center dot) affects growth speed and morphology in lily pollen tubes. Oxygen radicals influence ionic zoning: membrane potential and pH gradients. Recently, redox-regulation of tip growth has been extensively studied, but differential sensitivity of growing cells to particular ROS and their subcellular localization is still unclear. Here, we used specific dyes to provide mapping of H2O2 and O-2(center dot) over bar in short and long pollen tubes. We found apical accumulation of H2O2 and H2O2-producing organelles in the shank that were not colocalized with O-2(center dot) over bar -producing mitochondria. Differential modulation of ROS content of the germination medium affected both growth speed and pollen tube morphology. Oxygen radicals affected ionic zoning: membrane potential and pH gradients. OH center dot caused depolarization all along the tube while O-2(center dot) over bar provoked hyperpolarization and cytoplasm alkalinization. O-2(center dot) over bar accelerated growth and reduced tube diameter, indicating that this ROS can be considered as pollen tube growth stimulator. Serious structural disturbances were observed upon exposure to OH center dot and ROS quencher MnTMPP: pollen tube growth slowed down and ballooned tips formed in both cases, but OH center dot affected membrane transport and organelle distribution as well. OH center dot, thus, can be considered as a negative regulator of pollen tube growth. Pollen tubes, in turn, are able to reduce OH center dot concentration, which was assessed by electron paramagnetic resonance spectroscopy (EPR).