Arabidopsis root formation is altered by cadmium and arsenic

The semimetal arsenic (As) and the heavy metal cadmium (Cd) are highly toxic for plants and animals, evoking enormous concern due to their widespread and persistent presence in polluted ecosystems. Both elements are not essential for plants but easily absorbed by their roots using the same membrane transporters of essential nutrients. The exposure to Cd or As causes inhibition of plant growth, especially in sensitive plants as Arabidopsis thaliana, the model species used in this research. It was reported that Cd and As mainly localize in root meristems. The correct organization and functionality of primary (PR), lateral (LR) and adventitious roots (AR) depends on the integrity of their apical meristem, on the correct activity and maintenance over time of a small group of cells which rarely divide, i.e. the quiescent centre (QC) cells. The QC inhibits the differentiation of the surrounding stem cells, allowing the apical root growth and the correct root differentiation. In A. thaliana LR and AR originate from pericycle founder cells in the PR and hypocotyl, respectively, their QC is established in a precise stage of primordium development. It was demonstrated that the positioning and maintenance of the QC in these roots is strictly related to a correct transport and biosynthesis of indole-3-acetic acid (IAA), the main plant auxin. To the aim to investigate the effect of Cd and As on auxin-mediated LR and AR development and QC maintenance, the expression of the IAA-sensitive DR5: GUS, of QC25: GUS (QC-marker), of the auxin biosynthetic gene YUCCA6, of the IAA carriers GUS-lines PIN1: GUS and LAX3: GUS and IAA levels in seedlings exposed to Na2HAsO47H2O and/or CdSO4 were evaluated. Results indicate that Cd and As alter auxin biosynthesis and transport during root formation, with consequent negative effects on their growth.