Arabidopsis root K+-efflux conductance activated by hydroxyl radicals: single-channel properties, genetic basis and involvement in stress-induced cell death

Abstract

SummaryReactive oxygen species (ROS) are central to plant stress response, signalling, development and a multitude of other processes. In this study, the plasma-membrane hydroxyl radical (HR)-activated K + channel responsible for K + efflux from root cells during stress accompanied by ROS generation is characterised. The channel showed 16-pS unitary conductance and was sensitive to Ca 2+ , tetraethylammonium, Ba 2+ , Cs + and free-radical scavengers. The channel was not found in the gork1-1 mutant, which lacks a major plasma-membrane outwardly rectifying K + channel. In intact Arabidopsis roots, both HRs and stress induced a dramatic K + efflux that was much smaller in gork1-1 plants. Tests with electron paramagnetic resonance spectroscopy showed that NaCl can stimulate HR generation in roots and this might lead to K + -channel activation. In animals, activation of K + -efflux channels by HRs can trigger programmed cell death (PCD). PCD symptoms in Arabidopsis roots developed much more slowly in gork1-1 and wild-type plants treated with K + -channel blockers or HR scavengers. Therefore, similar to animal counterparts, plant HR-activated K + channels are also involved in PCD. Overall, this study provides new insight into the regulation of plant cation transport by ROS and demonstrates possible physiological properties of plant HR-activated K + channels.

Cell-Type-Specific H+-ATPase Activity in Root Tissues Enables K+ Retention and Mediates Acclimation of Barley (Hordeum vulgare) to Salinity Stress

“…Plant GLRs were confirmed recently to be NSCC (Price et al, 2012;Forde, 2014). Therefore, activated by Glu, GLR may mediate stress-induced Ca 2+ uptake, with a consequent rise in cytosolic Ca 2+ , that in turn can activate the plasma membrane NADPH oxidase (Lecourieux et al, 2002), leading to elevated H 2 O 2 levels and a consequent formation of OH $ (as discussed above), leading to a massive K + efflux via OH $ -activated outward-rectified (GORK) K + channels (Demidchik et al, 2010). Thus, the strong increase in Glu levels found in the root apex (Table I) may be an additional factor exacerbating a stress-induced decrease in the cytosolic K + pool in this zone, leading to the activation of caspase-like enzymes and, finally, programmed cell death (Fig.…”

“…By interacting with transition metals such as either copper or iron, increased accumulation of hydrogen peroxide (H 2 O 2 ) may result in the formation of highly reactive hydroxyl radicals (OH $ ) in both the apoplast (Demidchik, 2015) and cytosol (Rodrigo-Moreno et al, 2013). Both H 2 O 2 and OH $ may cause a major perturbation in intracellular ionic homeostasis by activating a range of cation-permeable ion channels (Demidchik and Maathuis, 2007;Demidchik et al, 2010;Shabala and Pottosin, 2014). Thus, tissuespecific salinity stress sensitivity between the root apex and mature zones may be causally related to the patterns of OH $ production and/or sensitivity of membrane transporters to OH $ .…”

“…On the other hand, the highly reactive OH $ , formed upon H 2 O 2 reduction by Fe 2+ or Cu + , causes membrane depolarization and the activation of GORK and a variety of nonselective conductances, culminating in a massive K + efflux (Demidchik et al, 2010;Shabala and Pottosin, 2014). The OH $ -induced K + current was 3-fold higher in the protoplasts from the elongation zone (Fig.…”

“…High cytosolic Na + concentrations are considered to be toxic for cell metabolism and, thus, are reduced by various means (Tester and Davenport, 2003;Ji et al, 2013;Flowers et al, 2015). At the same time, superior K + retention and a cell's ability to maintain cytosolic K + homeostasis correlate with salinity tolerance in a broad range of plant species (Anschütz et al, 2014;Shabala and Pottosin, 2014) and are essential for preventing salinityinduced programmed cell death (Shabala, 2009;Demidchik et al, 2010). High cytosolic K + levels also are essential to maintain high vacuolar H + -PPase activity, thus enabling the operation of tonoplast NHX proteins that mediate vacuolar Na + sequestration (Shabala, 2013).…”

Transition metals: A double edge sward in ROS generation and signaling

“…7 To deal with Fe-or Cu-induced toxicity, plants need to enhance antioxidant defenses; this phenomenon has been reported for a wide range of species such as radicals-activated K + channels are involved in programmed cell death in plant cells. 33 Recently, Lahoavist et al 36 showed that the hydroxyl-radical activated Ca 2+ and K + conductance in Arabidopsis is mediated by annexin1 at the mature and elongation zone levels. In all these studies, the hydroxyl radical generation occurred at the external side of the plasma membrane, and under non-physiological (1mM of the hydroxyl-radical generation mixture Cu/Asc) conditions.…”