Oxygen radicals and cytoplasm zoning in growing lily pollen tubes
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- 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--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* caused depolarization all along the tube while O*2- provoked hyperpolarization and cytoplasm alkalinization. O*2- accelerated growth and reduced tube diameter, indicating that this ROS can be considered as pollen tube growth stimulator along with H2O2. Serious structural disturbances were observed upon exposure to OH* and H2O2 and O*2- quencher MnTMPP: pollen tube growth slowed down and ballooned tips formed in both cases, but in the presence of OH* membrane transport and organelle distribution was affected as well. OH*, thus, can be considered as a negative influence on pollen tubes which, presumably, have mechanisms for leveling it. The assumption was confirmed by EPR spectroscopy: pollen tubes actively reduce OH* content in the incubation medium.