A ROS-Assisted Calcium Wave Dependent on the AtRBOHD NADPH Oxidase and TPC1 Cation Channel Propagates the Systemic Response to Salt Stress

Abstract

ORCID IDs: 0000-0001-9597-6839 (S.G.); 0000-0003-3080-2613 (R.J.M.).Plants exhibit rapid, systemic signaling systems that allow them to coordinate physiological and developmental responses throughout the plant body, even to highly localized and quickly changing environmental stresses. The propagation of these signals is thought to include processes ranging from electrical and hydraulic networks to waves of reactive oxygen species (ROS) and cytoplasmic Ca 2+ traveling throughout the plant. For the Ca 2+ wave system, the involvement of the vacuolar ion channel TWO PORE CHANNEL1 (TPC1) has been reported. However, the precise role of this channel and the mechanism of cell-to-cell propagation of the wave have remained largely undefined. Here, we use the fire-diffuse-fire model to analyze the behavior of a Ca 2+ wave originating from Ca 2+ release involving the TPC1 channel in Arabidopsis (Arabidopsis thaliana). We conclude that a Ca 2+ diffusion-dominated calcium-induced calcium-release mechanism is insufficient to explain the observed wave transmission speeds. The addition of a ROS-triggered element, however, is able to quantitatively reproduce the observed transmission characteristics. The treatment of roots with the ROS scavenger ascorbate and the NADPH oxidase inhibitor diphenyliodonium and analysis of Ca 2+ wave propagation in the Arabidopsis respiratory burst oxidase homolog D (AtrbohD) knockout background all led to reductions in Ca 2+ wave transmission speeds consistent with this model. Furthermore, imaging of extracellular ROS production revealed a systemic spread of ROS release that is dependent on both AtRBOHD and TPC1. These results suggest that, in the root, plant systemic signaling is supported by a ROS-assisted calcium-induced calcium-release mechanism intimately involving ROS production by AtRBOHD and Ca 2+ release dependent on the vacuolar channel TPC1.

Interplay of Plasma Membrane and Vacuolar Ion Channels, Together with BAK1, Elicits Rapid Cytosolic Calcium Elevations in Arabidopsis during Aphid Feeding

“…Since the feeding site [Ca 2+ ] cyt elevations are attenuated, but not abolished in the tpc1-2 mutant, it appears that release of vacuolar Ca 2+ by TPC1 is downstream of and dependent on extracellular Ca 2+ release by the GLRs. This finding agrees with work showing that TPC1 activity is positively regulated by [Ca 2+ ] cyt (Hedrich and Neher, 1987;Ward and Schroeder, 1994;Allen and Sanders, 1996;Guo et al, 2016;Kintzer and Stroud, 2016) and plays a role in systemically propagating Ca 2+ -induced Ca 2+ release (Dubiella et al, 2013;Evans et al, 2016;Gilroy et al, 2016;Choi et al, 2016). Consequently, TPC1 appears to be activated by GLR-mediated Ca 2+ influx and involved in the cell-to-cell spread of Ca 2+ during biotic interactions.…”

“…TPC1 is a tonoplast-localized Ca 2+ -permeable channel whose activity is regulated by voltage and Ca 2+ (Hedrich and Neher, 1987;Ward and Schroeder, 1994;Peiter et al, 2005;Gradogna et al, 2009;Guo et al, 2016Guo et al, , 2017Kintzer and Stroud, 2016). TPC1 also has an established role in systemic Ca 2+ signaling in response to salt stress (Choi et al, 2014;Evans et al, 2016) and wounding (Kiep et al, 2015) via its positive regulation by Ca 2+ in a process termed Ca 2+ -induced Ca 2+ release. However, the mechanism by which Ca 2+ -induced Ca 2+ release is triggered in plants remains unknown.…”

The Ever-Closer Union of Signals: Propagating Waves of Calcium and ROS Are Inextricably Linked

“…TPC1 encodes the Slow Vacuolar cation channel, which is activated by [Ca 21 ] cyt elevation, and hence may mediate [Ca 21 ] cytinduced Ca 21 release (CICR) from the vacuole, the plant cell's largest Ca 21 store (Ward and Schroeder, 1994). However, the ability of this channel to release Ca 21 is still a matter of debate (Hedrich and Marten, 2011;Peiter, 2011) Evans et al (2016) raise the interesting notion that RbohD-generated ROS, besides propelling the Ca 21 wave from cell to cell, may also determine the waveform and prevent the flooding of the cytosol with vacuolar Ca 21 by desensitizing TPC1, which operates only in reducing conditions (Hedrich and Marten, 2011;Peiter, 2011).…”

“…The article of Evans et al (2016) advances our understanding of plant-wide communication and hence of whole-plant responses to the environment. As with any breakthrough, their studies trigger a host of new questions that now need to be addressed.…”

“…The work of Evans et al (2016) has inextricably linked two mechanisms of rapid systemic signaling in plants, the importance of which for plant defense and acclimation is becoming increasingly apparent. Now we need to unravel the probably complex apparatus that governs the initiation, propagation, and impact of this union of signals.…”