Long-Distance Electrical and Calcium Signals Evoked by Hydrogen Peroxide in Physcomitrella

Electrical and calcium signals in plants are some of the basic carriers of information that are transmitted over a long distance. Together with reactive oxygen species (ROS) waves, electrical and calcium signals can participate in cell-to-cell signaling, conveying information about different stimuli, e.g. abiotic stress, pathogen infection or mechanical injury. There is no information on the ability of ROS to evoke systemic electrical or calcium signals in the model moss Physcomitrella nor on the relationships between these responses. Here, we show that the external application of hydrogen peroxide (H2O2) evokes electrical signals in the form of long-distance changes in the membrane potential, which transmit through the plant instantly after stimulation. The responses were calcium-dependent since their generation was inhibited by lanthanum, a calcium channel inhibitor (2 mM), and EDTA, a calcium chelator (0.5 mM). The electrical signals were partially dependent on glutamate receptor (GLR) ion channels since knocking-out the GLR genes only slightly reduced the amplitude of the responses. The basal part of the gametophyte, which is rich in protonema cells, was the most sensitive to H2O2. The measurements carried out on the protonema expressing fluorescent calcium biosensor GCaMP3 proved that calcium signals propagated slowly (>5 µm/s) and showed a decrement. We also demonstrate upregulation of a stress-related gene that appears in a distant section of the moss 8 min after the H2O2 treatment. The results help understand the importance of both types of signals in the transmission of information about the appearance of ROS in the plant cell apoplast.