only some of them (i.e., Cd^2 , Hg^2 , Pb^2 , Cu, Ag) are able to form complexes with PC [98–101]. An
alternative role for PC in metal tolerance could be a shuttle activity for the transfer of metals from the cy-
toplasm to the vacuole [102] and vice versa because metals used for growth can be stored in vacuoles
[103,104]. Interestingly, Mn^2 and Zn^2 concentrations are higher in the vacuoles of the green alga
Chlorellathan in the cytoplasm [35]. In higher plants, Zn accumulates in vacuoles [105], usually chelated
by organic acids [106,107] or precipitated as Zn-phytate [108]. In fact, PC and PC synthase could be in-
volved in homeostasis [93].
- Proline
Cu, Cd, and Na induce proline accumulation in some freshwater algae [13,109] and higher plants
[110–112]. In Chlorella, proline accumulation reduces the internalization of Cu. Although the exact
mechanism is still unknown, it is hypothesized that proline decreases Cu absorption [13], probably
through inhibition of Kleakage as demonstrated with the cyanobacterium Anacystis nidulans[109]. It
is interesting to note that proline may stabilize membranes [113].
In higher plants, proline can also be involved in the chelation of excess cytoplasmic metal ions that
have a preference for nitrogen or oxygen coordination [114]. These metals, e.g., Ni or Zn, are usually poor
inducers of PC [53]. Although there is no conclusive evidence for a direct contribution of proline in cel-
lular HM detoxification, it is interesting to note that constitutive proline levels are higher in metal-toler-
ant ecotypes of Silene vulgaris[115] and Armeria maritima[116]. Proline production could not be a di-
rect effect of HM stress but rather a consequence of the water stress induced by metals.
VI. CAN METAL RESISTANCE OF PLANTS BE IMPROVED?
From the preceding sections, it is apparent that much work remains to be done to unveil fully the mecha-
nisms involved in plant metal resistance. However, using the data already available, some remediation of
inorganic contaminants has been successfully achieved thanks to genetic assays.
The insertion of the animal MT (class I) gene into the genome of either higher plants [117] or
cyanobacteria [118] confers a stimulation of HM tolerance in the transformants.
We reported earlier that an Al resistance is linked to the secretion of citric acid [46]. One way to in-
crease the resistance is to make the plant produce more citric acid. This was achieved for two Al-sensi-
tive species (tobacco and papaya) after insertion in their genome of the bacterial gene coding for the cit-
ric acid synthase. Consequently, roots of the transgenic plants secreted 5–6 times more citric acid and
were in turn 10 times more metal tolerant than the wild type [119,120] as they absorbed less Al.
Bacteria can reduce a number of HM ions and oxyanions to less toxic oxidation states [121]. For in-
stance, Hg resistance in gram-negative bacteria is located on an operon that encodes different kinds of pro-
teins: (1) transport proteins that bind and transfer Hg into the cell; (2) an organomercury lyase that catalyzes
the protonolysis of CMHg bonds, releasing Hg; and (3) a mercuric ion reductase that reduces Hgto Hg^0
that is in turn volatilized from the cell [122,123]. The gene encoding the mercuric ion reductase was slightly
modified and cloned successfully in Arabidopsis, which became resistant to mercury [124].
Only these examples let us hope that other current trials in genetic engineering [125] will provide
transformants with improved resistance to toxic metals.
ACKNOWLEDGMENTS
The authors thank Dr. R. K. Mehra (University of California, San Diego) for his constructive help and H.
Küpper (University of South Bohemia, Czech Republic) for revealing his latest results. B. Schoefs and
M. Bertrand thank the Ministry of Education, Youth and Sports of the Czech Republic (grant VS-96085)
for their financial support.
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