ACCUMULATION OF TRACE ELEMENTS AND SULFUR USE EFFICIENCY IN MODEL PLANTS

Sulfur is an essential element for all living organisms. It is found in a broad variety of compounds [two amino acids (cysteine and methionine), glutathione (GSH), phytochelatins (PCs), vitamins, iron-sulfur clusters, cofactors and other molecules]. For plants, the main source of sulfur is sulfate ion that is taken up, by roots, from the soil solution. Once inside the cells, sulfate is reduced and assimilated into cysteine from which, GSH is synthetized enzymatically. This tripeptide (i.e., GSH) is involved in the maintaining of the redox homeostasis of the cells and in the detoxification of toxins. In plants not exposed to cadmium (Cd, a toxic not essential heavy metal), GSH represents the main thiol in the cells. However, under Cd exposure, plants, starting from GSH, immediately synthetize PCs, which, in turn, become the most abundant class of thiols. These Cys-rich peptides are able to chelate Cd, reducing the levels of free Cd ions in the cell and the damage induced by the metal. The large amount of PCs represents a sulfate additional sink that increases the request for Cys and GSH and, consequently, the total amount of sulfur necessary for both mitigation of stressing conditions and plant growth. In this thesis, two experimental works are presented. The aim concerned the improvement of the knowledge on the molecular and physiological relationships existing among Cd accumulation, Cd tolerance, sulfur metabolism and sulfur use efficiency in two different model plants: barley and Arabidopsis. In the first work, six barley cultivars widely differing for Cd tolerance, partitioning, and translocation were analyzed in relation to their thiol metabolism. The data analysis indicated that Cd tolerance was not clearly related to the total amount of Cd absorbed by plants, but it is closely dependent on the capacity of the cultivars to chelate and immobilize the metal at root level. Such behaviors suggested the existence of root mechanisms preserving shoots from Cd-induced oxidative damages, as indicated by the analysis of thiobarbituric acid-reactive substances (diagnostic indicators of oxidative stress), whose levels increased in the shoots and they were negatively related to Cd root retention and tolerance. Cd exposure differentially affected GSH and PC levels in the tissues of each barley cultivar. The capacity to produce PCs appeared as a specific characteristic of each barley cultivar, since it did not depend on Cd concentration in the roots and resulted negatively related to the concentration of the metal in the shoots, indicating the existence of a cultivar-specific interference of Cd on GSH biosynthesis, as confirmed by the presence of close positive linear relationships between the effect of Cd on GSH levels and PC accumulation in both roots and shoots. The six barley cultivars also differed for their capacity to load Cd ions into the xylem, which was negatively related to PC content in the roots. All these data indicated that the different capacity of each cultivar to maintain GSH homeostasis under Cd stress may strongly affect PC accumulation and, thus, Cd tolerance and translocation. Concerning the second work, plants of Arabidopsis thaliana were grown in complete hydroponic solutions containing different sulfate concentrations and exposed or not to different levels of Cd, for short or long period. Concerning shoot, long-term Cd exposure induced an increment of the external critical sulfate concentration ([SO42-]crit, i.e. the sulfate concentration in the growing medium that produced the 95% of the maximum amount of fresh weight). Moreover, in this experimental condition, shoot tolerance to relatively low Cd concentration increased as sulfate availability in the growing medium did, whilst at root level the strong inhibition induced by Cd was independent from external sulfate concentration. Conversely, under short-term Cd exposure, [SO42-]crit did not change statistically in both shoot and roots and the inhibitory effect exerted by the metal on shoot and root growth was independent from external sulfate availability. Interestingly, the presence of Cd for both short and long period induced an increment of the relative expression levels of genes codifying for high-affinity sulfate transporters enhancing, consequently, the sulfate uptake. On the other hand, increases of the sulfate availability in the growing solution reduced the amount of sulfate taken up by roots. However, only under short-term Cd exposure the increments of sulfate uptake were coupled with increases of non-protein thiol levels indicating that long-term Cd exposure decreases the capacity of the Arabidopsis roots to efficiently use the available sulfate ions in the growing medium to promote the growth. Such a behavior is likely due to the effect exerted by Cd accumulation which, reducing the development of root apparatus, makes the adaptive response of the high-affinity sulfate transporters “per se” not enough to optimize the growth at sulfate external concentrations lower than [SO42-]crit. Finally, reassuming, the main results show that the capacity of plant tissues to maintain GSH homeostasis under Cd stress may strongly affect PC accumulation and, thus, Cd tolerance and translocation. Moreover, such a capacity seems to be related to the total amount of sulfur available for plant nutrition in the growing medium, since adequate levels of sulfate modulate thiol metabolism and partitioning, reducing the negative effects produced by Cd at shoot level. This confirms that sulfur plays a pivotal role in the mechanisms involved in Cd detoxification suggesting that the manipulation of both sulfate transport and thiol metabolism may represent a useful strategy for the selection of low Cd-accumulating cultivars or more Cd-tolerant plants when grown in Cd-contaminated soils.