Modeling the Action Potential in Characeae Nitellopsis obtusa: Effect of Saline Stress

Abstract

Action potentials (AP) of characean cells were the first electrical transients identified in plants. APs provide information about plethora of environmental cues. Salinity stress is critical for plants and impacts on excitability. The AP of brackish Characeae Nitellopsis obtusa , obtained in artificial pond water (APW) and under osmotic stress of 90 or 180 mM sorbitol APW or saline stress of 50 or 100 mM NaCl APW, were simulated by the Thiel-Beilby model (Beilby and Al Khazaaly, 2016 ). The model is based on a paradigm from animal systems, featuring the second messenger inositol 1,4,5-triphosphate (IP 3 ) mediating the opening of Ca 2+ channels on internal stores. In plants the IP 3 receptors have not been identified, so other second messengers might translate the threshold plasma membrane depolarization to Ca 2+ release. The increased Ca 2+ concentration in the cytoplasm activates Cl − channels, which lead to the depolarizing phase of the AP. The repolarization to normal resting potential difference (PD) results from the Ca 2+ being re-sequestered by the Ca 2+ pumps, the closure of the Cl − channels, efflux of K + through the depolarization-activated outward rectifier channels and the continuing activity of the proton pump. The Nitellopsis AP form is longer in APW compared to that of Chara , with more gradual repolarization. The tonoplast component of the AP is larger than that in Chara australis . The plasma membrane AP is prolonged by the exposure to saline to a “rectangular” shape, similar to that in Chara . However, the changes are more gradual, allowing more insight into the mechanism of the process. It is possible that the cells recover the original AP form after prolonged exposure to brackish conditions. Some cells experience tonoplast APs only. As in Chara , the proton pump is transiently inhibited by the high cytoplasmic Ca 2+ and gradually declines in saline media. However, if the cells are very hyperpolarized at the start of the experiment, the pump inhibition both by the AP and by the saline medium is mitigated. The model parameters and their changes with salinity are comparable to those in Chara .

Nutrient cycling is an important mechanism for homeostasis in plant cells

“…The resting membrane voltage V is determined by the activity of all ion transporters in the membrane and is in the range of -200 mV < V < -100 mV (Scherzer et al, 2019; Iosip et al, 2020). Considering that K + and anions like Cl - are the main players in the execution of plant APs (Beilby and Coster, 1979; Beilby and Walker, 1996; Beilby and Al Khazaaly, 2016; Beilby and Al Khazaaly, 2017; Cuin et al, 2018; Kisnieriene et al, 2019) such a condition could be achieved by a transporter network of case 17 or 12. While case 17 can flexibly realize all three possibilities (i) V < E K , (ii) V = E K , or (iii) E K < V , case 12 is limited to V = E K .…”

Impact of Mammalian Two-Pore Channel Inhibitors on Long-Distance Electrical Signals in the Characean Macroalga Nitellopsis obtusa and the Early Terrestrial Liverwort Marchantia polymorpha

“…Prolongation of repolarization can be attributed to inhibition of Ca 2+ -ATPase activity. Ca 2+ -ATPase governs Ca 2+ resequestration to inner sources after excitation, and thus reduces the activity of Ca 2+ -dependent Cl − channels [ 45 ]. A blocking effect of verapamil on K + channels cannot be excluded either.…”

“…All AP parameters of N. obtusa were distributed unimodally because the electrode tip was always located in the vacuole (determinable by the RP, peak value, and shape of AP) [ 45 ]. Similar to verapamil, NED-19 depolarized the membrane excitation threshold E th , leading to decreased AP amplitude A Eth .…”

Using Plant Cells of Nitellopsis obtusa for Biophysical Education

“…Characeaen cells have the structure of a typical plant cell (Fig 1): they possess a cell wall, an excitable plasmalemma, cytoplasm, a layer of chloroplasts, a vacuolar membrane (tonoplast, which is also excitable) (8,9), multiple nuclei, and a vacuole that comprises up to 95% of the cell volume (7). During laboratory practice, students should become familiar with the morphology of characeaen cells and learn how their cellular structure has evolved to support specific functions.…”

“…Spontaneous APs are usually related to salt stress (7,9). Application of a 3 M KCl solution elicited an extreme change in the extracellular environment, immediately changing the transmembrane Nernst potential for K þ ions, depolarizing the plasmalemma, and bringing it to a level more positive than its normal excitation threshold.…”