Ricerc@Sapienza

A screening strategy using micropropagation glass tubes with a gradient of distances between germinating seeds
and a metal-contaminated medium was used for studying alterations in root architecture and morphology of
Arabidopsis thaliana treated with cadmium (Cd) and zinc (Zn) at the concentration of 10–20 ?M and 100–200 ?M,
respectively. Metal concentrations in plant shoots and roots were measured by quadrupole inductively coupled
plasma mass spectrometry. After 21 days from germination, all plants in the tubes were scanned at high resolution
and the root systems analyzed. The localization of indole-3-acetic acid (IAA) in the primary root and
lateral root apices was monitored using DR5:GUS, LAX3:GUS and AUX1:GUS Arabidopsis transgenic lines. Total
phenol content in leaves was measured spectrophotometrically. Shoot and root dry weight and leaf area did not
change in Zn-exposed plants and significantly decreased in Cd-exposed plants, compared to control plants.
Cadmium induced a reduction of root length, of mean number of roots and of total root surface. Both Cd- and Znexposed
plants showed a reduced specific root length. This morphological behavior, together with an observed
increase in root diameter in metal-exposed plants could be interpreted as compensatory growth, and the observed
thicker roots could act as a barrier to protect root from the metals. In comparison with the apical localization
of the IAA signal in the control plants, Zn generally reinforced the intensity of IAA signal, without
affecting its localization. In Cd-exposed plants, IAA localization remained apical but weaker compared to control
plants. Total phenols decreased in plants exposed to Zn and Cd. Therefore, we propose that the remodelling of
the root architecture and the production of some secondary metabolites, such as IAA and phenols could be two
responses of plants subjected to metal stress. This knowledge can open the way to future phytoremediation
strategies of contaminated sites.