Salicylic acid improves salinity tolerance in Arabidopsis by restoring membrane potential and preventing salt-induced K(+) loss via a GORK channel

Despite numerous reports implicating salicylic acid (SA) in plant salinity responses, the specific ionic mechanisms of SA-mediated adaptation to salt stress remain elusive. To address this issue, a non-invasive microelectrode ion flux estimation technique was used to study kinetics of NaCl-induced net ion fluxes in Arabidopsis thaliana in response to various SA concentrations and incubation times. NaCl-induced K+ efflux and H+ influx from the mature root zone were both significantly decreased in roots pretreated with 10–500 μM SA, with strongest effect being observed in the 10–50 μM SA range. Considering temporal dynamics (0–8-h SA pretreatment), the 1-h pretreatment was most effective in enhancing K+ retention in the cytosol. The pharmacological, membrane potential, and shoot K+ and Na+ accumulation data were all consistent with the model in which the SA pretreatment enhanced activity of H+-ATPase, decreased NaCl-induced membrane depolarization, and minimized NaCl-induced K+ leakage from the cell within the first hour of salt stress. In long-term treatments, SA increased shoot K+ and decreased shoot Na+ accumulation. The short-term NaCl-induced K+ efflux was smallest in the gork1-1 mutant, followed by the rbohD mutant, and was highest in the wild type. Most significantly, the SA pretreatment decreased the NaCl-induced K+ efflux from rbohD and the wild type to the level of gork1-1, whereas no effect was observed in gork1-1. These data provide the first direct evidence that the SA pretreatment ameliorates salinity stress by counteracting NaCl-induced membrane depolarization and by decreasing K+ efflux via GORK channels.Maheswari Jayakannan, Jayakumar Bose, Olga Babourina, Zed Rengel, and Sergey Shabal

The NPR1-dependent salicylic acid signalling pathway is pivotal for enhanced salt and oxidative stress tolerance in Arabidopsis

The role of endogenous salicylic acid (SA) signalling cascades in plant responses to salt and oxidative stresses is unclear. Arabidopsis SA signalling mutants, namely npr1-5 (non-expresser of pathogenesis related gene1), which lacks NPR1-dependent SA signalling, and nudt7 (nudix hydrolase7), which has both constitutively expressed NPR1-dependent and NPR1-independent SA signalling pathways, were compared with the wild type (Col-0) during salt or oxidative stresses. Growth and viability staining showed that, compared with wild type, the npr1-5 mutant was sensitive to either salt or oxidative stress, whereas the nudt7 mutant was tolerant. Acute salt stress caused the strongest membrane potential depolarization, highest sodium and proton influx, and potassium loss from npr1-5 roots in comparison with the wild type and nudt7 mutant. Though salt stress-induced hydrogen peroxide production was lowest in the npr1-5 mutant, the reactive oxygen species (ROS) stress (induced by 1mM of hydroxyl-radical-generating copper-ascorbate mix, or either 1 or 10mM hydrogen peroxide) caused a higher potassium loss from the roots of the npr1-5 mutant than the wild type and nudt7 mutant. Long-term salt exposure resulted in the highest sodium and the lowest potassium concentration in the shoots of npr1-5 mutant in comparison with the wild type and nudt7 mutant. The above results demonstrate that NPR1-dependent SA signalling is pivotal to (i) controlling Na(+) entry into the root tissue and its subsequent long-distance transport into the shoot, and (ii) preventing a potassium loss through depolarization-activated outward-rectifying potassium and ROS-activated non-selective cation channels. In conclusion, NPR1-dependent SA signalling is central to the salt and oxidative stress tolerance in Arabidopsis.Maheswari Jayakannan, Jayakumar Bose, Olga Babourina, Sergey Shabala, Amandine Massart, Charlotte Poschenrieder and Zed Renge

Aluminium-induced ion transport in Arabidopsis: the relationship between Al tolerance and root ion flux

Aluminium (Al) rhizotoxicity coincides with low pH; however, it is unclear whether plant tolerance to these two factors is controlled by the same mechanism. To address this question, the Al-resistant alr104 mutant, two Al-sensitive mutants (als3 and als5), and wild-type Arabidopsis thaliana were compared in long-term exposure (solution culture) and in short-term exposure experiments (H+ and K+ fluxes, rhizosphere pH, and plasma membrane potential, Em). Based on biomass accumulation, als5 and alr104 showed tolerance to low pH, whereas alr104 was tolerant to the combined low-pH/Al treatment. The sensitivity of the als5 and als3 mutants to the Al stress was similar. The Al-induced decrease in H+ influx at the distal elongation zone (DEZ) and Al-induced H+ efflux at the mature zone (MZ) were higher in the Al-sensitive mutants (als3 and als5) than in the wild type and the alr104 mutant. Under combined low-pH/Al treatment, alr104 and the wild type had depolarized plasma membranes for the entire 30 min measurement period, whereas in the Al-sensitive mutants (als3 and als5), initial depolarization to around –60 mV became hyperpolarization at –110 mV after 20 min. At the DEZ, the Em changes corresponded to the changes in K+ flux: K+ efflux was higher in alr104 and the wild type than in the als3 and als5 mutants. In conclusion, Al tolerance in the alr104 mutant correlated with Em depolarization, higher K+ efflux, and higher H+ influx, which led to a more alkaline rhizosphere under the combined low-pH/Al stress. Low-pH tolerance (als5) was linked to higher H+ uptake under low-pH stress, which was abolished by Al exposure

Genome-wide transcriptional analysis of salinity stressed japonica and indica rice genotypes during panicle initiation stage

Rice yield is most sensitive to salinity stress imposed during the panicle initiation (PI) stage. In this study, we have focused on physiological and transcriptional responses of four rice genotypes exposed to salinity stress during PI. The genotypes selected included a pair of indicas (IR63731 and IR29) and a pair of japonica (Agami and M103) rice subspecies with contrasting salt tolerance. Physiological characterization showed that tolerant genotypes maintained a much lower shoot Na(+) concentration relative to sensitive genotypes under salinity stress. Global gene expression analysis revealed a strikingly large number of genes which are induced by salinity stress in sensitive genotypes, IR29 and M103 relative to tolerant lines. We found 19 probe sets to be commonly induced in all four genotypes. We found several salinity modulated, ion homeostasis related genes from our analysis. We also studied the expression of SKC1, a cation transporter reported by others as a major source of variation in salt tolerance in rice. The transcript abundance of SKC1 did not change in response to salinity stress at PI stage in the shoot tissue of all four genotypes. However, we found the transcript abundance of SKC1 to be significantly higher in tolerant japonica Agami relative to sensitive japonica M103 under control and stressed conditions during PI stage. ELECTRONIC SUPPLEMENTARY MATERIAL: Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s11103-006-9112-0 and is accessible for authorized users

Aluminium-sensitive Arabidopsis thaliana mutants als3 and als5 differ in response to acidic stress

Plants sensitive to Al³⁺ may also be sensitive to acidity. We exposed two Arabidopsis Al³⁺ -sensitive mutants (als3 and als5) and wild type (WT) to different pH (5.5 or 4.2) with or without 0.5 mM homo PIPES buffer. Homo PIPES kept pH at starting 4.2 or 5.5 for 24 hours. Acidity decreased als3 and WT shoot biomass, whereas als5 showed acid resistance. Acidity reduced accumulation of Ca²⁺ and Mg²⁺ but not K⁺ in all genotypes. Under acidic stress als5 had higher shoot Ca²⁺ and Mg²⁺ concentrations than WT, which might be responsible for als5 acid resistance.Jayakumar Bose, Olga Babourina and Zed Renge

Low-pH and aluminum resistance in Arabidopsis correlates with high cytosolic magnesium content and increased magnesium uptake by plant roots

Low-pH stress and Al3+ toxicity affect root growth in acid soils. It was hypothesized that the capacity of genotypes to maintain Mg2+ uptake in acidic environments may contribute to low-pH and Al resistance, but explicit evidence is lacking. In this work, an Al-resistant alr104 mutant and two Al-sensitive mutants (als5 and als3) of Arabidopsis thaliana were compared with the wild type (Col-0) for Mg2+ uptake and intracellular Mg2+ concentration under low-pH and combined low-pH/Al stresses. Magnesium accumulation in roots was measured in long-term (7 d) experiments. The Mg2+ fluxes were measured using ion-sensitive microelectrodes at the distal elongation and the mature root zones in short-term (0–60 min) experiments. Intracellular Mg2+ concentrations were measured in intact root cells at the distal elongation zone using magnesium-specific fluorescent dye and fluorescent lifetime imaging (FLIM) analysis. Under low-pH stress, Arabidopsis mutants als5 and alr104 maintained a higher Mg concentration in roots, and had greater Mg2+ influx than the wild type and the als3 mutant. Under combined low-pH/Al treatment, Al-resistant genotypes (wild type and alr104) maintained a higher Mg2+ accumulation, and had a higher Mg2+ influx and higher intracellular Mg2+ concentration than Al-sensitive genotypes (als3 and als5). Overall, these results show that increased Mg2+ uptake correlates with an enhanced capacity of Arabidopsis genotypes to cope with low-pH and combined low-pH/Al stresses.Jayakumar Bose, Olga Babourina, Sergey Shabala, and Zed Renge

Nitrate supply affects ammonium transport in canola roots

Plants may suffer from ammonium (NH4 +) toxicity when NH4 + is the sole nitrogen source. Nitrate (NO3 –) is known to alleviate NH4 + toxicity, but the mechanisms are unknown. This study has evaluated possible mechanisms of NO3 – alleviation of NH4 + toxicity in canola (Brassica napus L.). Dynamics of net fluxes of NH4 +, H+, K+ and Ca2+ were assessed, using a non-invasive microelectrode (MIFE) technique, in plants having different NO3 – supplies, after single or several subsequent increases in external NH4Cl concentration. After an increase in external NH4Cl without NO3 –, NH4 + net fluxes demonstrated three distinct stages: release (t1), return to uptake (t2), and a decrease in uptake rate (t3). The presence of NO3 – in the bathing medium prevented the t1 release and also resulted in slower activation of the t3 stage. Net fluxes of Ca2+ were in the opposite direction to NH4 + net fluxes, regardless of NO3 – supply. In contrast, H+ and K+ net fluxes and change in external pH were not correlated with NH4 + net fluxes. It is concluded that (i) NO3 – primarily affects the NH4 + lowaffinity influx system; and (ii) NH4 + transport is inversely linked to Ca2+ net flux