Cadmium detoxification in roots of Pisum sativum seedlings: relationship between toxicity levels, thiol pool alterations and growth

The evaluation of thiol metabolism in plant adaptation to relevant levels of cadmium stress is important for understanding the real importance of phytochelatins and related thiols in stress coping. The present work was designed to study the process of stress adaptation in roots of Pisum sativum L. plants during an exposure to different cadmium concentrations, ranging from more realistic exposures to those usually employed in PC studies. The balance between individual PCs and their homologous hPCs in constitutive thiol pools and root growth was also accessed. Roots of intact plants were submitted to 1, 3, 30, 60 or 120 micromolar Cd and harvested after 1, 3, 6 and 9 days after exposure. Growth parameters and root tissue cadmium accumulation were analysed. High-performance liquid chromatography (HPLC) with fluorescence detection was used due to its high sensitivity. Root growth was only affected in concentrations higher than 30 micromolar Cd, but the presence of low cadmium concentrations induced significant alterations in constitutive thiols and triggered the synthesis of PCs and hPCs, bearing two to four olygomeric repeats. Increasing Cd stress levels were generally associated with higher polythiol production; however, with the time-course of the experiments, higher degrees of toxicity were associated with a reduction in thiol production. This behaviour was attributed to the Cys and GSH depletion, which limited polythiol synthesis, as well as root growth. In tolerable concentrations, the rate of root length recovery matched the increase in PC and hPC synthesis. In higher concentrations (60 and 120 micromolar), the reduction in non-protein polythiol synthesis was associated with higher Cd toxicity, leading to a severe growth reduction. The synthesis of hPCs seemed to have a reduced importance in tolerance; however, their production was stimulated when the GSH deficit was higher. Our results suggest that the reductions in PC levels, observed in higher degrees of stress, were not related to the activation of other tolerance mechanisms but were instead associated with the high metabolic cost of this thiol-based tolerance mechanism.