Regulation of sulfate uptake and assimilation in barley (<i>Hordeum vulgare</i>) as affected by rhizospheric and atmospheric sulfur nutrition

To study the regulation of sulfate metabolism in barley (Hordeum vulgare), seedlings were exposed to atmospheric hydrogen sulfide (H2S) in the presence and absence of a sulfate supply. Sulfate deprivation reduced shoot and root biomass production by 60% and 70%, respectively, and it affected the plant’s mineral nutrient composition. It resulted in a 5.7- and 2.9-fold increased shoot and root molybdenum content, respectively, and a decreased content of several other mineral nutrients. Particularly, it decreased shoot and root total sulfur contents by 60% and 70%, respectively. These decreases could be ascribed to decreased sulfate contents. Sulfate deficiency was additionally characterized by significantly lowered cysteine, glutathione and soluble protein levels, enhanced dry matter, nitrate and free amino acid contents, an increased APS reductase (APR) activity and an increased expression and activity of the root sulfate uptake transporters. When sulfate-deprived barley was exposed to 0.6 µl l−1 atmospheric H2S, the decrease in biomass production and the development of other sulfur deficiency symptoms were alleviated. Clearly, barley could use H2S, absorbed by the foliage, as a sulfur source for growth. H2S fumigation of both sulfate-deprived and sulfate-sufficient plants downregulated APR activity as well as the expression and activity of the sulfate uptake transporters. Evidently, barley switched from rhizospheric sulfate to atmospheric H2S as sulfur source. Though this indicates that sulfate utilization in barley is controlled by signals originating in the shoot, the signal transduction pathway involved in the shoot-to-root regulation must be further elucidated

Berichte aus dem Julius-Kühn-Institut 191

25th International Symposium of the Scientific Centre for Fertilizers “Significance of Sulfur inHigh-Input Cropping Systems”, Groningen (Netherlands), September 5-8, 201

Nickel toxicity in Brassica rapa seedlings: Impact on sulfur metabolism and mineral nutrient content

Weltweit haben anthropogene Aktivitäten zu erhöhten Nickel­gehalten im Boden (Ni2+) geführt, was sich negativ auf die Pflanzenproduktivität auswirken kann. Der physiologische Hintergrund der Ni2+ Phytotoxizität ist noch weitgehend unklar. Eine zehntägige Exposition von Brassica rapa Sämlingen mit 1, 2 und 5 μM NiCl2 führte zu stark erhöhten Ni Gehalten im Gewebe, einer verringerten Biomasseproduktion und zu Blattchlorosen bei Konzentrationen von ≥ 2 μM Ni2+. Bei einer Konzentration von 5 μM Ni2+ war kein Pflanzenwachstum mehr zu beobachten. Eine Ni Toxizität trat auf, wenn der Ni Gehalt im Sprosses 1,0 μmol g–1 Trockengewicht und der in der Wurzel 23 μmol g–1 Trockengewicht überschritt. Eine Ni2+ Exposition von 2 μM beein­flusste den Mineralstoffgehalt in Spross und Wurzel nur geringfügig. Daher beeinflusste eine Ni2+ Exposition die Gehalte an Schwefelmetaboliten in der Pflanze kaum. Bei ≥ 1 μM Ni2+ war der Gesamtschwefelgehalt der Wurzel nur geringfügig erniedrigt, was vollständig auf einen verminderten Sulfatgehalt zurückzuführen war. Darüber hinaus war der Gehalt an wasserlöslichen Nicht-Protein-Thiolen sowohl im Spross als auch in der Wurzel nur bei 5 μM Ni2+ erhöht. Aus diesen Ergebnissen geht hervor, dass der Schwefelstoffwechsel wahrscheinlich nicht direkt an den Ni2+ Toleranzmechanismen von B. rapa beteiligt ist.Throughout the world anthropogenic activity has resulted in enhanced soil nickel (Ni2+) levels, which may negatively affect plant productivity. The physiological background of Ni2+ phytotoxicity is still largely unclear. Ten-day exposures of Brassica rapa seedlings to 1, 2 and 5 μM NiCl2 resulted in strongly enhanced tissue Ni levels, a decreased biomass production and leaf chlorosis at ≥ 2 μM Ni2+. At 5 μM Ni2+ plant growth was completely halted. Ni toxicity occurred when the content of the shoot exceeded 1.0 μmol g–1 dry weight and that of the root, 23 μmol g–1 dry weight. Ni2+ exposure at ≤ 2 μM only slightly affected the mineral nutrient content of both shoot and root. Hence, Ni2+ exposure hardly affected the sulfur metabolite content of the plant. At ≥ 1 μM Ni2+ the total sulfur content of the root was only slightly lowered, which could fully be ascribed to a decreased sulfate content. Moreover, the water-soluble non-protein thiol content of both shoot and root was only enhanced at 5 μM Ni2+. From these results it was clear that sulfur metabolism was unlikely to be directly involved in the Ni2+ tolerance mechanisms of B. rapa

Chloride and sulfate salinity differently affect biomass, mineral nutrient composition and expression of sulfate transport and assimilation genes in Brassica rapa

Background and aimsIt remains uncertain whether a higher toxicity of either NaCl or Na2SO4 in plants is due to an altered toxicity of sodium or a different toxicity of the anions. The aim of this study was to determine the contributions of sodium and the two anions to the different toxicities of chloride and sulfate salinity. The effects of the different salts on physiological parameters, mineral nutrient composition and expression of genes of sulfate transport and assimilation were studied. Methods Seedlings of Brassica rapa L. have been exposed to NaCl, Na2SO4, KCl and K2SO4 to assess the potential synergistic effect of the anions with the toxic cation sodium, as well as their separate toxicities if accompanied by the non-toxic cation potassium. Biomass production, stomatal resistance and Fv/fm were measured to determine differences in ionic and osmotic stress caused by the salts. Anion content (HPLC), mineral nutrient composition (ICP-AES) and gene expression of sulfate transporters and sulfur assimilatory enzymes (real-time qPCR) were analyzed. ResultsNa2SO4 impeded growth to a higher extent than NaCl and was the only salt to decrease Fv/fm. K2SO4 reduced plant growth more than NaCl. Analysis of mineral nutrient contents of plant tissue revealed that differences in sodium accumulation could not explain the increased toxicity of sulfate over chloride salts. Shoot contents of calcium, manganese and phosphorus were decreased more strongly by exposure to Na2SO4 than by NaCl. The expression levels of genes encoding proteins for sulfate transport and assimilation were differently affected by the different salts. While gene expression of primary sulfate uptake at roots was down-regulated upon exposure to sulfate salts, presumably to prevent an excessive uptake, genes encoding for the vacuolar sulfate transporter Sultr4;1 were upregulated. Gene expression of ATP sulfurylase was hardly affected by salinity in shoot and roots, the transcript level of 5′-adenylylsulfate reductase (APR) was decreased upon exposure to sulfate salts in roots. Sulfite reductase was decreased in the shoot by all salts similarly and remained unaffected in roots. Conclusions The higher toxicity of Na2SO4 over NaCl in B. rapa seemed to be due to an increased toxicity of sulfate over chloride, as indicated by the higher toxicity of K2SO4 over KCl. Thus, toxicity of sodium was not promoted by sulfate. The observed stronger negative effect on the tissue contents of calcium, manganese and phosphorus could contribute to the increased toxicity of sulfate over chloride. The upregulation of Sultr4;1 and 4;2 under sulfate salinity might lead to a detrimental efflux of stored sulfate from the vacuole into the cytosol and the chloroplasts. It remains unclear why expression of Sultr4;1 and 4;2 was upregulated. A possible explanation is a control of the gene expression of these transporters by the sulfate gradient across the tonoplast

Interactions of sulfate with other nutrients as revealed by H2S fumigation of Chinese cabbage

Sulfur deficiency in plants has severe impacts on both growth and nutrient composition. Fumigation with sub-lethal concentrations of H2S facilitates the supply of reduced sulfur via the leaves while sulfate is depleted from the roots. This restores growth while sulfate levels in the plant tissue remain low. In the present study this system was used to reveal interactions of sulfur with other nutrients in the plant and to ascertain whether these changes are due to the absence or presence of sulfate or rather to changes in growth and organic sulfur. There was a complex reaction of the mineral composition to sulfur deficiency, however, the changes in content of many nutrients were prevented by H2S fumigation. Under sulfur deficiency these nutrients accumulated on a fresh weight basis but were diluted on a dry weight basis, presumably due to a higher dry matter content. The pattern differed, however, between leaves and roots which led to changes in shoot to root partitioning. Only the potassium, molybdenum and zinc contents were strongly linked to the sulfate supply. Potassium was the only nutrient amongst those measured which showed a positive correlation with sulfur content in shoots, highlighting a role as a counter cation for sulfate during xylem loading and vacuolar storage in leaves. This was supported by an accumulation of potassium in roots of the sulfur-deprived plants. Molybdenum and zinc increased substantially under sulfur deficiency, which was only partly prevented by H2S fumigation. While the causes of increased molybdenum under sulfur deficiency have been previously studied, the relation between sulfate and zinc uptake needs further clarification

Atmospheric H₂S exposure does not affect stomatal aperture in maize

Main conclusion: Stomatal aperture in maize is not affected by exposure to a subtoxic concentration of atmospheric H2S. At least in maize, H2S, thus, is not a gaseous signal molecule that controls stomatal aperture. Abstract: Sulfur is an indispensable element for the physiological functioning of plants with hydrogen sulfide (H2S) potentially acting as gasotransmitter in the regulation of stomatal aperture. It is often assumed that H2S is metabolized into cysteine to stimulate stomatal closure. To study the significance of H2S for the regulation of stomatal closure, maize was exposed to a subtoxic atmospheric H2S level in the presence or absence of a sulfate supply to the root. Similar to other plants, maize could use H2S as a sulfur source for growth. Whereas sulfate-deprived plants had a lower biomass than sulfate-sufficient plants, exposure to H2S alleviated this growth reduction. Shoot sulfate, glutathione, and cysteine levels were significantly higher in H2S-fumigated plants compared to non-fumigated plants. Nevertheless, this was not associated with changes in the leaf area, stomatal density, stomatal resistance, and transpiration rate of plants, meaning that H2S exposure did not affect the transpiration rate per stoma. Hence, it did not affect stomatal aperture, indicating that, at least in maize, H2S is not a gaseous signal molecule controlling this aperture