Greenland Ice Sheet surface melt amplified by snowline migration and bare ice exposure.

Greenland Ice Sheet mass loss has recently increased because of enhanced surface melt and runoff. Since melt is critically modulated by surface albedo, understanding the processes and feedbacks that alter albedo is a prerequisite for accurately forecasting mass loss. Using satellite imagery, we demonstrate the importance of Greenland's seasonally fluctuating snowline, which reduces ice sheet albedo and enhances melt by exposing dark bare ice. From 2001 to 2017, this process drove 53% of net shortwave radiation variability in the ablation zone and amplified ice sheet melt five times more than hydrological and biological processes that darken bare ice itself. In a warmer climate, snowline fluctuations will exert an even greater control on melt due to flatter ice sheet topography at higher elevations. Current climate models, however, inaccurately predict snowline elevations during high melt years, portending an unforeseen uncertainty in forecasts of Greenland's runoff contribution to global sea level rise.

Regulation of the plant-type 5'-adenylyl sulfate reductase by oxidative stress.

5'-Adenylyl sulfate (APS) reductase (EC 1.8.4.9) catalyzes a key reaction in the plant sulfate assimilation pathway leading to the synthesis of cysteine and the antioxidant glutathione. In Arabidopsis thaliana APS reductase is encoded by a family of three genes. In vitro biochemical studies revealed that the enzyme product derived from one of them (APR1) is activated by oxidation, probably through the formation of a disulfide bond. The APR1 enzyme is 45-fold more active when expressed in a trxB strain of Escherichia coli than in a trxB(+) wild type. The enzyme is inactivated in vitro by treatment with disulfide reductants and is reactivated with thiol oxidants. Redox titrations show that the regulation site has a midpoint potential of -330 mV at pH 8.5 and involves a two-electron redox reaction. Exposure of a variety of plants to ozone induces a rapid increase in APS reductase activity that correlates with the oxidation of the glutathione pool and is followed by an increase in free cysteine and total glutathione. During the response to ozone, the level of immunodetectable APS reductase enzyme does not increase. Treatment of A. thaliana seedlings with oxidized glutathione or paraquat induces APS reductase activity even when transcription or translation is blocked with inhibitors. The results suggest that a posttranslational mechanism controls APS reductase. A model is proposed whereby redox regulation of APS reductase provides a rapidly responding, self-regulating mechanism to control the glutathione synthesis necessary to combat oxidative stress.

Overproduction of Ascorbate Peroxidase in the Tobacco Chloroplast Does Not Provide Protection against Ozone.

Transgenic tobacco (Nicotiana tabacum cv Bel W3) plants were used to test the hypothesis that protection from O3 injury could be conferred by overproduction of ascorbate peroxidase (APX) in the chloroplast. The 10-fold increase in soluble APX activity in the chloroplast was expected to alleviate an implied increase in oxidative potential and prevent damage caused by O3. Three different O3 exposure experiments (one acute and two chronic) with two replicates each were conducted. APX activity in nontransgenic plants increased in response to chronic O3 exposure. However, most responses to O3 were similar between transgenic and nontransgenic plants. These included reductions in net photosynthesis and stomatal conductance, increases in ethylene emission and visible injury, and a decline in the level of the small subunit of ribulose-1,5-biphosphate carboxylase/oxygenase mRNA transcripts observed in response to the air pollutant in the acute and/or chronic experiments. No O3-induced effect on ribulose-1,5-biphosphate carboxylase/oxygenase quantity was observed in the chronic experiments. O3 did not induce acceleration of senescence, as expected from studies with most other species; rather, the tobacco plants rapidly developed necrotic lesions. Thus, overproduction of APX in the chloroplast did not protect this cultivar of tobacco from O3.

Overexpression of Copper/Zinc Superoxide Dismutase in the Cytosol of Transgenic Tobacco Confers Partial Resistance to Ozone-Induced Foliar Necrosis.

Ozone damage to plants has been attributed to the action of oxygen free-radicals and other ozone degradation products against which cellular antioxidant systems have been considered to be a front-line defense. The activity of superoxide dismutase (SOD), one such antioxidant, has been shown to increase in ozonated plants. Past work with pea (Pisum sativum L.) in our laboratory showed that the cytosolic Cu/Zn-SOD isoform and its transcript were most responsive to ozone, compared to chloroplastic Cu/Zn-SOD. In the current work we tested the hypothesis that plants that constitutively overexpress cytosolic SOD are more tolerant of ozone. Pea cytosolic Cu/Zn-SOD was overproduced in the cytosol of two cultivars of transformed tobacco (Nicotiana tabacum), Bel W3 and Wisconsin 38. Young and recently expanded leaves of transgenic plants of both cultivars showed less foliar necrosis than nontransformed controls when exposed to acute doses of ozone. We suggest that this may demonstrate the importance of Cu/Zn-SOD in the cytosol as a protector of the integrity of the plasma membrane and possibly other cellular constituents.

The Antiozonant Ethylenediurea Does Not Act via Superoxide Dismutase Induction in Bean.

It has been proposed that the mode of action of ethylenediurea, a very effective antiozonant, is via an increase in the antioxidant enzyme superoxide dismutase (EH Lee, JH Bennett [1982] Plant Physiol 69: 1444-1449). Data presented here refute that hypothesis. No ethylenediurea-associated increases in Cu/Zn-superoxide dismutase or Mn-superoxide dismutase activity, nor in steady-state Cu/Zn-superoxide dismutase protein levels, were found in soluble extracts of bean (Phaseolus vulgaris L. cv Bush Blue Lake 290) leaves. However, the cytosolic Cu/Zn-superoxide dismutase increased as a result of ozone fumigation and subsequent injury. Also noted was a developmentally related difference between chloroplastic and cytosolic Cu/Zn-superoxide dismutase, the latter declining during maturation of the leaf.

Overproduction of petunia chloroplastic copper/zinc superoxide dismutase does not confer ozone tolerance in transgenic tobacco.

Transgenic tobacco (Nicotiana tabacum cultivar W38) plants that overproduce petunia chloroplastic Cu/Zn superoxide dismutase were exposed to ozone dosages that injure control tobacco plants. Based on foliar injury ratings, there was no consistent protection provided to the transgenic plants. These data indicate that an increase in the chloroplastic Cu/Zn superoxide dismutase alone is not sufficient to reduce ozone toxicity.