Cadmium increases the optimal external concentration for sulfate in Arabidopsis thaliana

The main sulfur (S) source for plants is the sulfate ion (SO42-), which is taken up by roots through specific plasma membrane high-affinity SO42- transporters. Once inside the plant, SO42- is allocated to different sinks where it is first assimilated into cysteine (Cys), a key intermediate from which most of S-containing compounds are synthetized. Considering the central role of Cys, it appears evident that both SO42- uptake and assimilation have to be finely modulated to meet the metabolic demand for S arising from Cys consuming activities, which largely contribute to define the total S requirement of plants. Such a demand may consistently vary under the different environmental conditions that plants may experience during their growth. Abiotic stresses may increase the metabolic demand for some Cys derived compounds, causing an increase in the activity of the SO4 2- assimilatory pathway. An example of this has been largely described in plants exposed to cadmium (Cd) in which the activation of glutathione- (GSH) consuming adaptive responses may increase the demand for both SO42- and S metabolites. Indeed, GSH is the key intermediate for the synthesis of phytochelatins (PCs), a class of Cys-rich metal-binding peptides involved in Cd detoxification. The large amount of PCs produced by plants represents an additional sink for S, which generates a typical demand-driven transcriptional regulation of genes involved in SO42- uptake, SO42- assimilation and GSH biosynthesis. To better understand the relationship existing between Cd accumulation and S metabolism, we analyzed the growth of Arabidopsis thaliana under a wide range of SO42- external concentrations (from 1 to 150 μM), in the presence or absence of 0.1 μM Cd2+. Plant growth was dependent on SO42- external concentration in both the experimental conditions. The growth isotherms were saturating curves properly described by the following single exponential rise to maximum function: = !"# ∙ (1 − !! !"! !! ) Such a function allowed us to estimate the theoretical critical concentration for SO42-, defined as the external concentration of the anion, which produces 95% of the maximum fresh weight. Data analysis indicated that the presence of Cd significantly changed the demand for S to maximize shoot growth, since the critical value for SO42- external concentration was higher in Cd treated (40 μM) than in control plants (29 μM). Moreover, the negative effect exerted by Cd on shoot growth decreased as SO42- concentration in the external medium increased. No effects of Cd on shoot growth were observed for SO42- external concentrations higher than the critical one. Differently, the detrimental effect exerted by Cd on root growth was independent of SO42- external concentration. Finally, Cd exposure negatively affected thiol metabolism, since shoot GSH concentration was lower in Cd-treated than in control plants, in all the experimental conditions. However, the effect of Cd on GSH levels decreased as SO42- external concentration increased, up to reach the critical value. Thus, at least our experimental conditions, the minimum shoot GSH concentration that have to be maintained to contrast Cd toxicity is around 375 nmol g-1 FW.