Effects of cadmium and arsenic on root development and auxin distribution in Arabidopsis thaliana Heynh (L.) plantlets

The “heavy” metal cadmium (Cd) and the semimetal arsenic (As) are very toxic elements for all organisms and the environment, where they are released from natural and, mostly, anthropogenic sources (Nagajyoti et al., 2010). Neither Cd nor As are essential elements for plants, but they may easily be absorbed through the root system by using the same transporters of some essential nutrients, and then accumulated in plant tissues, thus entering the food chain (Verbruggen et al., 2009). These pollutants may cause serious damages in sensitive plant species, as Arabidopsis thaliana, in particular to the root, the first organ with which they come in contact. In the most severe cases the root development and function are completely compromised because of alterations to the cells of the apical meristem, and the quiescent centre (QC) contained in it, and of root primary tissues (Brunetti et al., 2011). Damages to the root apparatus have negative consequences on plant growth, development and productivity. The architecture of the root system and the correct histological organization of the roots are finely regulated by the QC. The QC definition and maintenance over time, in primary (PR), lateral (LRs) and adventitious roots (ARs), are controlled by auxin level in its and in the surrounding cells (Della Rovere et al., 2013 and references therein). For these reasons the research was aimed to investigate, in addition to morphological alterations due to the two pollutants, their effects on QC maintenance and functioning, and on auxin level and distribution in the root. Effects of Cd and/or As on auxin transport were also studied in the ARs by expression analyses of the reporter gene uidA driven by the promoters of two IAA polar carriers coding genes, i.e. PIN1 (efflux carrier) and LAX3 (influx carrier). To the aim, A. thaliana plantlets were grown in vitro, in the presence of different concentrations of Cd, as CdSO4, and/or AsV (arsenate), as Na2HAsO4.7H2O. In order to favor LR or AR formation, different light conditions were chosen, i.e. long-day exposition preceded or not by continuous darkness. Primary root and hypocotyl length, LR and AR density were evaluated. In addition to an untransformed line (Columbia ecotype), A. thaliana transgenic GUS lines were used to investigate the QC definition (QC25::GUS line) and free IAA distribution (DR5::GUS line) and transport (PIN1::GUS and LAX3::GUS lines). The results show that Cd and/or AsV presence significantly inhibited the PR and hypocotyl growth. The mean density of LRs and ARs were significantly higher in the plantlets exposed to either Cd alone or both pollutants, while AsV inhibited AR formation. Histochemical GUS analyses indicate that Cd compromised more than AsV the regular QC definition and functionality in the RLs, while, in the ARs, AsV had more drastic effects, putatively related to a reduction on auxin levels during AR development, as shown by the analyses of AsV-treated DR5::GUS plantlets. Both Cd and AsV disturbed the expression patterns of the auxin transporters genes, especially in the early stages of AR development.