| Summary: | To study the regulation of sulfate metabolism in barley (<i>Hordeum vulgare</i>), seedlings were exposed to atmospheric hydrogen sulfide (H<sub>2</sub>S) 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<sup>−1</sup> atmospheric H<sub>2</sub>S, the decrease in biomass production and the development of other sulfur deficiency symptoms were alleviated. Clearly, barley could use H<sub>2</sub>S, absorbed by the foliage, as a sulfur source for growth. H<sub>2</sub>S 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 H<sub>2</sub>S 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.
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