Reactive oxygen species in pollination drops of coniferous plants

In flowering plants, reactive oxygen species (ROS) are actively involved in the regulation of sexual reproduction. Here we present the first evidence on the participation of ROS in sexual interactions in coniferous plants. In most gymnosperms, pollen hydrates and germinates in a pollination drop—a special type of ovular secretion. We studied the composition of this liquid for four conifer species from three families for the presence of ROS using the most sensitive method for detecting low concentrations of ROS—EPR (electron paramagnetic resonance) spectroscopy. ROS were present in pollination drops of all the plants studied, however, the composition of ROS was different: in Taxus, Pinus and Picea the main form was O^⋅₂⁻, in Thuja it was H₂O₂, reaching values comparable to stigma exudates of flowering plants. We tested the potential involvement of superoxide in the regulation of spruce pollen germination in vitro and found a strong stimulating effect on both total and bipolar germination efficiency. Thus, we found that conifers are characterized by the presence of ROS in the liquid intended for pollen germination, as well as in flowering plants. Hydrogen peroxide and superoxide radical, apparently represent a variety of ROS-based mechanisms for the regulation of pollen germination in vivo.