Interaction between atmospheric and pedospheric sulfur nutrition

 

In addition to wet deposited atmospheric sulfur, which may be taken up as sulfate by the root, plants are also able to utilize foliarly taken up sulfur gases as sulfur source for growth (Figure 1). The absorbed SO₂ and H₂S may directly enter the sulfur assimilatory pathway and be metabolized into organic sulfur compounds and contribute to plant sulfur nutrition. For instance, it has been established, that a continuous exposure of curly kale (Brassica olereacea) to  ≥ 0.06 µ l l^-1 H₂S appeared to be sufficient to cover the sulfur requirement for growth, in the absence of sulfate in the root environment. However, the ability of Chinese cabbage (Brassica pekinensis) to utilize SO₂ as sulfur source strongly depends on the sulfur status and/or developmental stage of the plant and prolonged sulfate-deprived plants benefited only little from SO₂ exposure. It was unclear to what extent the latter effects can be explained by an interfering toxicity of SO₂ in the absence of sulfate supply.

At the whole plant level, the uptake of sulfate by the root and its transport and assimilation in the shoot is coordinated by and balanced with the actual sulfur requirement for growth. Exposure of plants to SO₂ and H₂S may affect the uptake of sulfate by the root, and its transfer to and its reduction in the shoot. For instance, upon H₂S exposure of B. pekinensis or B. oleracea at  ≥ 0.1  µl l^-1, this species switched from utilizing sulfate taken up by the root to sulfide taken up by the shoot as the sulfur source for structural growth, which resulted in a partial decrease in the uptake of sulfate by the root and its reduction, the latter illustrated by a decrease in activity and expression of the APS reductase activity in the shoot (Figure 2).

Sulfate deprivation of the root generally induces multiple responses enabling an enhanced sulfate uptake efficiency on a whole plant basis. For instance, sulfate deprivation generally results in a rapidly induced mass expression of the sulfate transporters mRNAs accompanied with an enhanced sulfate uptake capacity by the roots, whereas more prolonged sulfate deprivation results in an altered shoot to root biomass partitioning in favor of that of the root. Despite the fact that plants are able to transfer from sulfate taken up by the root to absorbed SO₂ and H₂S as sole sulfur source for growth, the enhanced sulfate uptake capacity, a mass expression of the various sulfate transporters in the root and the altered shoot to root partitioning in favor of that of the root upon sulfate deprivation were not rapidly alleviated upon exposure . Apparently in the absence of sulfate in the root environment there was a poor shoot to root signaling for the regulation of sulfate uptake and shoot to root partitioning.

Regulation of the uptake and distribution of sulfate

 

The uptake and distribution of sulfate in Brassica oleracea, a species characterized by its high sulfate content in root and shoot, are highly coordinated and adjusted to the sulfur requirement for growth even at external sulfate concentrations close to the K_m value of the high affinity sulfate transporters. Plants were able to grow normally and maintain a high sulfur content even if grown at 5 or 10 μ M sulfate in the root environment. Abundance of mRNAs for the high affinity sulfate transporters, BolSultr1;1 and BolSultr1;2, in the roots were enhanced at  £ 25 μ M sulfate, and this was accompanied with an up to three-fold increase in the sulfate uptake capacity (Fig. 3), whereas sulfate, organic sulfur and thiol contents were only slightly affected. Upon sulfate deprivation, there was a mass induction of the sulfate transporters, BolSultr1;1, BolSultr1;2, BolSultr1;3, BolSultr2;1 and BolSultr4;1, whilst the sulfate uptake capacity was only increased up to four-fold. P lant growth and shoot to root biomass partitioning were affected only upon sulfate deprivation and not at low external sulfate concentrations. It is likely that the internal sulfate concentration itself acts as a determining factor in the regulation of the activity and expression of the sulfate transporters and shoot to root partitioning in B. oleracea.

Glutathione, the “S” factor in plant function, plant health and food quality

 

Glutathione is a primary plant metabolite and plays a central role in plant sulfur nutrition, its adaptation to climate change and environmental stress, resistance to pathogen and wounding, detoxification of herbicides and heavy metals, and - as an antioxidant and presumed anti-carcinogen - in determining the nutritional quality of plant-derived foodstuff. Its presumed involvement in plant functioning as reductant, redox regulator via sulfide/disulfide exchange, signal compound and anti-oxidant requires a complex but precise regulation of the intracellular glutathione levels. However, there is still insufficient information on in situ turnover and the subcellular localization of glutathione in plants under various environmental conditions. The in situ glutathione level in plants appears to be very dynamic and strongly affected by physiological and environmental factors. These gaps in our understanding of glutathione metabolism in plants preclude a more efficient management of agro- and natural ecosystems for nutrition, stress resistance, pathogen resistance and risk assessment (“Sulfur-Induced Resistance”), and adaptation to global change as well as streamlining food production and processing to optimize nutritional quality. The impact of the environment, and biotic and abiotic stress on the regulation of glutathione synthesis and cellular and subcellular distribution during growth and development is currently investigated. 

 

 

Whole tree regulation of sulfur and nitrogen uptake and metabolism in Eucalyptus as affected by climate change factors

 

Climate change will affect ecosystems in a number of sometimes-contradictory ways, such as promoting plant growth by elevated CO₂, or limiting plant growth by increasing environmental stress due to e.g. increasingly dry conditions. Given the outstanding importance of the genus Eucalyptus, understanding the responses of trees is crucial to understand climate change effects on forest ecosystems. One significant challenge in responding to climate change will be the adaptation and management of tree nutrition to the changed demand; a task that cannot rely on presently used empirical knowledge, but will instead require a functional and mechanistic understanding of plant nutrition. Novel molecular and experimental techniques enabled functional insights into the uptake and assimilation of nitrogen and sulfur, two major plant nutrients with closely linked functions. Due to the long life spans, long transport and signaling distances, and the access to large storage tissues in trees, their regulation and metabolism of nutrition is significantly different from that of herbaceous plants. Research is aimed at studying the whole tree regulation of the uptake, distribution and assimilation of S and N related to growth and development of Eucalyptus as affected by the global change factors elevated CO₂ and drought. The regulation of transporter genes and key assimilation genes, the nutrient fluxes, and major N and S metabolites will be determined in relation to the growth rates of the trees to give a whole-tree picture of the regulation. In summary, t he following research questions are currently addressed: i) how is regulation of N and S metabolism tied to the demand on a whole plant level, ii) is there shoot to root signaling of N and S status, iii) what are the effect of global change and stress factors on the regulation of N and S metabolism and iv) are there differences between Eucaluptus species.