Role of Sulfate Transporters in Chromium Tolerance in Scenedesmus acutus M. (Sphaeropleales)

Sulfur (S) is essential for the synthesis of important defense compounds and in the scavenging potential of oxidative stress, conferring increased capacity to cope with biotic and abiotic stresses. Chromate can induce a sort of S-starvation by competing for uptake with SO42− and causing a depletion of cellular reduced compounds, thus emphasizing the role of S-transporters in heavy-metal tolerance. In this work we analyzed the sulfate transporter system in the freshwater green algae Scenedesmus acutus, that proved to possess both H+/SO42− (SULTRs) and Na+/SO42− (SLTs) plasma membrane sulfate transporters and a chloroplast-envelope localized ABC-type holocomplex. We discuss the sulfate uptake system of S. acutus in comparison with other taxa, enlightening differences among the clade Sphaeropleales and Volvocales/Chlamydomonadales. To define the role of S transporters in chromium tolerance, we analyzed the expression of SULTRs and SULPs components of the chloroplast ABC transporter in two strains of S. acutus with different Cr(VI) sensitivity. Their differential expression in response to Cr(VI) exposure and S availability seems directly linked to Cr(VI) tolerance, confirming the role of sulfate uptake/assimilation pathways in the metal stress response. The SULTRs up-regulation, observed in both strains after S-starvation, may directly contribute to enhancing Cr-tolerance by limiting Cr(VI) uptake and increasing sulfur availability for the synthesis of sulfur-containing defense molecules.

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.

Modular Chitosan-Based Adsorbents for Tunable Uptake of Sulfate from Water

The context of this study responds to the need for sorbent technology development to address the controlled removal of inorganic sulfate (SO42−) from saline water and the promising potential of chitosan as a carrier system for organosulfates in pharmaceutical and nutraceutical applications. This study aims to address the controlled removal of sulfate using chitosan as a sustainable biopolymer platform, where a modular synthetic approach was used for chitosan bead preparation that displays tunable sulfate uptake. The beads were prepared via phase-inversion synthesis, followed by cross-linking with glutaraldehyde, and impregnation of Ca2+ ions. The sulfate adsorption properties of the beads were studied at pH 5 and variable sulfate levels (50–1000 ppm), where beads with low cross-linking showed moderate sulfate uptake (35 mg/g), while cross-linked beads imbibed with Ca2+ had greater sulfate adsorption (140 mg/g). Bead stability, adsorption properties, and the point-of-zero charge (PZC) from 6.5 to 6.8 were found to depend on the cross-linking ratio and the presence of Ca2+. The beads were regenerated over multiple adsorption-desorption cycles to demonstrate the favorable uptake properties and bead stability. This study contributes to the development of chitosan-based adsorbent technology via a modular materials design strategy for the controlled removal of sulfate. The results of this study are relevant to diverse pharmaceutical and nutraceutical applications that range from the controlled removal of dextran sulfate from water to the controlled release of chondroitin sulfate.

SLIM1 Transcription Factor Promotes Sulfate Uptake and Distribution to Shoot, Along with Phytochelatin Accumulation, Under Cadmium Stress in Arabidopsis thaliana

Sulfur (S) assimilation, which is initiated by sulfate uptake, generates cysteine, the substrate for glutathione (GSH) and phytochelatin (PC) synthesis. GSH and PC contribute to cadmium (Cd) detoxification by capturing it for sequestration. Although Cd exposure is known to induce the expression of S-assimilating enzyme genes, including sulfate transporters (SULTRs), mechanisms of their transcriptional regulation are not well understood. Transcription factor SLIM1 controls transcriptional changes during S deficiency (−S) in Arabidopsis thaliana. We examined the potential involvement of SLIM1 in inducing the S assimilation pathway and PC accumulation. Cd treatment reduced the shoot fresh weight in the sulfur limitation1 (slim1) mutant but not in the parental line (1;2PGN). Cd-induced increases of sulfate uptake and SULTR1;2 expressions were diminished in the slim1 mutant, suggesting that SLIM1 is involved in inducing sulfate uptake during Cd exposure. The GSH and PC levels were lower in slim1 than in the parental line, indicating that SLIM1 was required for increasing PC during Cd treatment. Hence, SLIM1 indirectly contributes to Cd tolerance of plants by inducing −S responses in the cell caused by depleting the GSH pool, which is consumed by enhanced PC synthesis and sequestration to the vacuole.

Contribution of Root Hair Development to Sulfate Uptake in Arabidopsis

Root hairs often contribute to nutrient uptake from environments, but the contribution varies among nutrients. In Arabidopsis, two high-affinity sulfate transporters, SULTR1;1 and SULTR1;2, are responsible for sulfate uptake by roots. Their increased expression under sulfur deficiency (−S) stimulates sulfate uptake. Inspired by the higher and lower expression, respectively, of SULTR1;1 in mutants with more (werwolf [wer]) and fewer (caprice [cpc]) root hairs, we examined the contribution of root hairs to sulfate uptake. Sulfate uptake rates were similar among plant lines under both sulfur sufficiency (+S) and −S. Under −S, the expression of SULTR1;1 and SULTR1;2 was negatively correlated with the number of root hairs. These results suggest that both −S-induced SULTR expression and sulfate uptake rates were independent of the number of root hairs. In addition, we observed (1) a negative correlation between primary root lengths and number of root hairs and (2) a greater number of root hairs under −S than under +S. These observations suggested that under both +S and −S, sulfate uptake was influenced by the root biomass rather than the number of root hairs.

Sulfate transport systems in plants: functional diversity and molecular mechanisms underlying regulatory coordination

Sulfate transporters are integral membrane proteins controlling the flux of sulfate (SO42–) entering the cells and subcellular compartments across the membrane lipid bilayers. Sulfate uptake is a dynamic biological process that occurs in multiple cell layers and organs in plants. In vascular plants, sulfate ions are taken up from the soil environment to the outermost cell layers of roots and horizontally transferred to the vascular tissues for further distribution to distant organs. The amount of sulfate ions being metabolized in the cytosol and chloroplast/plastid or temporarily stored in the vacuole depends on expression levels and functionalities of sulfate transporters bound specifically to the plasma membrane, chloroplast/plastid envelopes, and tonoplast membrane. The entire system for sulfate homeostasis, therefore, requires different types of sulfate transporters to be expressed and coordinately regulated in specific organs, cell types, and subcellular compartments. Transcriptional and post-transcriptional regulatory mechanisms control the expression levels and functions of sulfate transporters to optimize sulfate uptake and internal distribution in response to sulfate availability and demands for synthesis of organic sulfur metabolites. This review article provides an overview of sulfate transport systems and discusses their regulatory aspects investigated in the model plant species Arabidopsis thaliana.