Molybdate toxicity in Chinese cabbage is not the direct consequence of changes in sulphur metabolism

Regulation of Sulfate Uptake and Assimilation in Barley (Hordeum vulgare) 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.