THE ROLE OF PHYTOCHELATIN OVERPRODUCTION
and
OF THE VACUOLAR TRANSPORTER ABCC3 IN Cd TOLERANCE OF ARABIDOPSIS
AND TOBACCO PLANTS
Patrizia Brunetti
Phytochelatins (PCs) are metal binding peptides, found in plants, fungi, nematodes and all groups of
algae including cyanobacteria. In plants PCs are enzymatically synthesized from gluthatione (GSH)
by PC synthase (PCS1) and form specific complexes with Cd. PC-Cd complexes are sequestered
into the vacuole by specific membrane transport proteins whereby the metals do not harm the cell.
We previously demonstrated that in tobacco seedlings AtPCS1 overexpression increases Cd
tolerance and accumulation in the presence of exogenous GSH (Pomponi et al., 2006). We also
compared the effects of PC overproduction on Cd tolerance of Arabidopsis and tobacco seedlings
in relation to endogenous GSH content, (Brunetti et al., 2011).
We are currently evaluating the role of the ABC vacuolar protein AtABCC3 in the transport of PCCd complexes into the vacuole and on Cd tolerance (Brunetti et al., manuscript in preparation). We
showed that Arabidopsis abcc3 seedlings, defective in AtABCC3, have an increased sensitivity to
different Cd concentrations, and that seedlings overexpressing AtABCC3, AtABCC3ox, have an
increased Cd tolerance. In contrast overexpression of AtABCC3 in cad1-3 mutant seedlings,
defective in PC production, had no effect on Cd tolerance, suggesting that AtABCC3 acts via PCs.
Accordingly in abcc3 mutant protoplasts Cd is mostly localized in the cytosol, whereas in
protoplasts from AtABCC3ox plants there is an increase in vacuolar Cd compared to wild type
protoplasts. We also analysed Cd sensitivity of abcc3 mutant seedlings in comparison to that of the
double mutant atabcc1 atabcc2 defective in the transporters AtABCC1 AtABCC2, also involved in
Cd tolerance. We showed that abcc3 single mutant and atabcc1 atabcc2 double mutant seedlings
showed slight differences in Cd sensitivity. Moreover, the level of AtABCC3 transcript in wild type
seedlings increases after Cd exposure, and further increased in atabcc1 atabcc2
mutant
background.
Our results indicate that AtABCC3 is an independent transporter of PCs-Cd complexes, and that its
activity is regulated by Cd and is concerted with the activity of AtABCC1/AtABCC2.