Enhanced drought tolerance of foxtail millet seedlings by sulfur dioxide fumigation.

Recently, sulfur dioxide (SO2) has been considered to be a beneficial bio-regulator in animals. However, the positive roles of SO2 in plant adaptation to drought stress are still unclear. In this study, we investigated the physiological and molecular changes that are induced by SO2 fumigation to improve the drought tolerance of foxtail millet seedlings. The relative water content in the leaves of drought-stressed seedlings was significantly improved by pre-exposure to 30 mg/m3 SO2. These responses might be related to decreased stomatal apertures and a reduced leaf transpiration rate, which were induced by SO2 under drought conditions. In addition, the SO2 pretreatment markedly enhanced proline accumulation in the leaves of drought-stressed seedlings, which was supported by increased Δ1-pyrroline-5-carboxylate synthetase (P5CS) activity, decreased proline dehydrogenase (ProDH) activity, and the corresponding transcripts. Moreover, the SO2 application upregulated the enzyme activity of catalase (CAT) and peroxidase (POD) in the leaves of drought-stressed plants, as well as their transcripts, which contributed to the scavenging of hydrogen peroxide (H2O2) and alleviated drought-induced oxidative damage, as indicated by the decreased malondialdehyde (MDA) level in SO2-pretreated plants. Together, these results indicate that the application of SO2 might enhance drought tolerance by reducing stomatal apertures, increasing proline accumulation, and promoting antioxidant defence in foxtail millet seedlings. This study presents new insight into the beneficial roles of SO2 in plant responses to drought stress.

Sulfur dioxide derivatives alleviate cadmium toxicity by enhancing antioxidant defence and reducing Cd2+ uptake and translocation in foxtail millet seedlings.

Sulfur dioxide (SO2) was recently proposed as a novel bio-regulator in mammals. However, the possible advantageous effects of SO2 in plant adaptation to heavy metal-contaminated environments are largely unknown. In the present study, using Na2SO3/NaHSO3 derivatives as SO2 donors, we investigated the possible roles and regulation mechanisms of SO2 in alleviating Cd2+ toxicity in foxtail millet seedlings. Exogenous SO2 derivatives (0.5 mM) application significantly reduced the seedling growth inhibition caused by Cd2+ stress. Cd2+-induced oxidative damage was also alleviated by SO2 derivatives, which was supported by the decreased malondialdehyde (MDA) level in the leaves of seedlings pretreated with SO2 derivatives. These responses were related to the enhanced activities of representative antioxidant enzymes, including catalase and superoxide dismutase, as well as the up-regulation of ascorbate-glutathione cycle, which contributed to the scavenging of Cd2+-elicited O2•－ and H2O2 within the leaves of foxtail millet seedlings. Also, SO2 derivative application promoted sulfur assimilation and increased the content of glutathione and phytochelatins, which may help to enhance Cd2+ detoxification capacity in foxtail millet seedlings. Moreover, application of SO2 derivatives caused down-regulation of the transcript expression levels of several genes involved in Cd2+ uptake and translocation, such as NRAMP1, NRAMP6, IRT1, IRT2, HMA2, and HMA4, thus resulting in reduced Cd2+ accumulation in the shoots and roots of Cd2+-stressed seedlings. Collectively, these results suggest that exogenous SO2 derivative application can alleviate oxidative damage and restrict Cd2+ buildup, thereby reducing Cd2+-induced growth inhibition in foxtail millet seedlings upon Cd2+ exposure. This novel finding indicates that the usage of SO2 derivatives may be an effective approach for enhancing Cd2+ tolerance in foxtail millet and other crops.

Reactive oxygen species and Ca2+ are involved in cadmium-induced cell killing in yeast cells.

Cadmium (Cd) is one of the most toxic heavy metals. It is of great environmental concern and its toxicity has been investigated in a variety of cells. In this study, we elucidated the toxic effects of Cd in cells of yeast (Saccharomyces cerevisiae). Our results showed that Cd2+ (0.05-5.0 mmol·L-1) significantly inhibited yeast cell growth, and the inhibitory effect was positively correlated with Cd2+ concentrations. Cd2+ caused loss of yeast cell viability in a concentration- and duration-dependent manner. Intracellular reactive oxygen species (ROS) and Ca2+ levels increased in yeast cells after exposure to 5.0 mmol·L-1 Cd for 6 h. Cd2+-caused cell viability loss was blocked by antioxidants (0.5 mmol·L-1 ascorbic acid or 500 U·mL-1 catalase) or Ca2+ antagonists (0.5 mmol·L-1 ethylene glycol tetraacetic acid or 0.5 mmol·L-1 LaCl3). Moreover, a collapse of mitochondrial membrane potential (ΔΨm) was observed in Cd2+-treated yeast cells. These results indicate that Cd-induced yeast cell killing was associated with the elevation of intracellular ROS and Ca2+ levels and also the loss of ΔΨm.

Populus euphratica XTH overexpression enhances salinity tolerance by the development of leaf succulence in transgenic tobacco plants.

Populus euphratica is a salt-tolerant tree species that develops leaf succulence after a prolonged period of salinity stress. In the present study, a putative xyloglucan endotransglucosylase/hydrolase gene (PeXTH) from P. euphratica was isolated and transferred to tobacco plants. PeXTH localized exclusively to the endoplasmic reticulum and cell wall. Plants overexpressing PeXTH were more salt tolerant than wild-type tobacco with respect to root and leaf growth, and survival. The increased capacity for salt tolerance was due mainly to the anatomical and physiological alterations caused by PeXTH overexpression. Compared with the wild type, PeXTH-transgenic plants contained 36% higher water content per unit area and 39% higher ratio of fresh weight to dry weight, a hallmark of leaf succulence. However, the increased water storage in the leaves in PeXTH-transgenic plants was not accompanied by greater leaf thickness but was due to highly packed palisade parenchyma cells and fewer intercellular air spaces between mesophyll cells. In addition to the salt dilution effect in response to NaCl, these anatomical changes increased leaf water-retaining capacity, which lowered the increase of salt concentration in the succulent tissues and mesophyll cells. Moreover, the increased number of mesophyll cells reduced the intercellular air space, which improved carbon economy and resulted in a 47-78% greater net photosynthesis under control and salt treatments (100-150 mM NaCl). Taken together, the results indicate that PeXTH overexpression enhanced salt tolerance by the development of succulent leaves in tobacco plants without swelling.

Overexpression of copper/zinc superoxide dismutase from mangrove Kandelia candel in tobacco enhances salinity tolerance by the reduction of reactive oxygen species in chloroplast.

Na(+) uptake and transport in Kandelia candel and antioxidative defense were investigated under rising NaCl stress from 100 to 300 mM. Salinized K. candel roots had a net Na(+) efflux with a declined flux rate during an extended NaCl exposure. Na(+) buildup in leaves enhanced H2O2 levels, superoxide dismutase (SOD) activity, and increased transcription of CSD gene encoding a Cu/Zn SOD. Sequence and subcellular localization analyses have revealed that KcCSD is a typical Cu/Zn SOD in chloroplast. The transgenic tobacco experimental system was used as a functional genetics model to test the effect of KcCSD on salinity tolerance. KcCSD-transgenic lines were more Na(+) tolerant than wild-type (WT) tobacco in terms of lipid peroxidation, root growth, and survival rate. In the latter, 100 mM NaCl led to a remarkable reduction in chlorophyll content and a/b ratio, decreased maximal chlorophyll a fluorescence, and photochemical efficiency of photosystem II. NaCl stress in WT resulted from H2O2 burst in chloroplast. Na(+) injury to chloroplast was less pronounced in KcCSD-transgenic plants due to upregulated antioxidant defense. KcCSD-transgenic tobacco enhanced SOD activity by an increment in SOD isoenzymes under 100 mM NaCl stress from 24 h to 7 day. Catalase activity rose in KcCSD overexpressing tobacco plants. KcCSD-transgenic plants better scavenged NaCl-elicited reactive oxygen species (ROS) compared to WT ones. In conclusion, K. candel effectively excluded Na(+) in roots during a short exposure; and increased CSD expression to reduce ROS in chloroplast in a long-term and high saline environment.

Exogenous hydrogen peroxide, nitric oxide and calcium mediate root ion fluxes in two non-secretor mangrove species subjected to NaCl stress.

Using 3-month-old seedlings of Bruguiera gymnorrhiza (L.) Savigny and Kandelia candel (L.) Druce, we compared species differences in ionic homeostasis control between the two non-secretor mangrove species. A high salinity (400 mM NaCl, 4 weeks) resulted in a decline of the K(+)/Na(+) ratio in root and leaf tissues, and the reduction was more pronounced in K. candel (41-66%) as compared with B. gymnorrhiza (5-36%). Salt-altered flux profiles of Na(+), K(+), H(+) and Ca(2+) in roots and effects of exogenous hydrogen peroxide (H(2)O(2)), nitric oxide (NO) and Ca(2+) on root ion fluxes were examined in seedlings that were hydroponically treated short term with 100 mM NaCl (ST, 24 h) and long term with 200 mM NaCl (LT, 7 days). Short term and LT salinity resulted in Na(+) efflux and a correspondingly increased H(+) influx in roots of both species, although a more pronounced effect was observed in B. gymnorrhiza. The salt-enhanced exchange of Na(+) with H(+) was obviously inhibited by amiloride (a Na(+)/H(+) antiporter inhibitor) or sodium orthovanadate (a plasma membrane H(+)-ATPase inhibitor), indicating that the Na(+) efflux resulted from active Na(+) exclusion across the plasma membrane. Short term and LT salinity accelerated K(+) efflux in the two species, but K. candel exhibited a higher flux rate. The salt-induced K(+) efflux was markedly restricted by the K(+) channel blocker, tetraethylammonium chloride, indicating that the K(+) efflux is mediated by depolarization-activated channels, e.g., KORCs (outward rectifying K(+) channels) and NSCCs (non-selective cation channels). Exogenous H(2)O(2) application (10 mM) markedly increased the apparent Na(+) efflux and limited K(+) efflux in ST-treated roots, although H(2)O(2) caused a higher Na(+) efflux in B. gymnorrhiza roots. CaCl(2) (10 mM) reduced the efflux of K(+) in salinized roots of the two mangroves, but its enhancement of Na(+) efflux was found only in B. gymnorrhiza. Under ST treatment, sodium nitroprusside (SNP) (100 ∝M, an NO donor) increased Na(+) efflux at the root apex of the two species; however, its inhibition of K(+) loss was seen only in K. candel. Of note, NaCl caused an obvious influx of Ca(2+) in B. gymnorrhiza roots, which was enhanced by H(2)O(2) (10 mM). Therefore, the salt-induced Ca(2+) benefits B. gymnorrhiza in maintaining K(+)/Na(+) homeostasis under high external salinity.