tripeptide loses its Gly, and the two other remaining amino acids are bound to the PC. Formation of longer
PC chains increases the possibility for metal ions to be sequestrated promptly and protect HM-sensitive
enzymes. The enzyme phytochelatin synthase ( -glutamylcysteinyltransferase, EC 2.3.2.15) was found
to ensure PC synthesis [78]. Consequently, as PCs become longer, the glutathione concentration drops
drastically (Figure 2). Prolonged PC synthesis requires de novo synthesis of the glutathione-synthesizing
enzymes [80]. When PC concentrations are sufficient to chelate the metals, the enzyme synthesis is turned
off [81]. The appearance of PC only 10 to 15 min after the entry of HM in cells is well correlated with the
fact that the enzymes involved in PC synthesis are constitutive in nature [54]. Chen et al. [74] demon-
strated that PC synthase requires free metal ions such as Cd^2 or Agfor activity.
When HM pollution occurs, plants synthesize metal-glutathione complexes [53,54], which are ex-
ported to the vacuoles [82] or transfer the metal (Cd^2 , Pb^2 , Cu, and Hg^2 ) to PCs that have a higher
affinity for metals than glutathione itself [83–85]. PC-metal complexes accumulate in vacuoles, where in-
organic sulfide and sulfite are incorporated, making the complexes more stable [86–88]. Increasing sul-
fide incorporation into PC leads to a substantial increase in Cd^2 /PC stoichiometry [88,89]. Mehra et al.
[90] suggested that the sulfate reduction pathway, leading to the production of sulfide, may allow the for-
mation of PC-Cd-S complexes in Candida glabratacells before internalization in the vacuole. Others
have suggested that the formation of PC-Cd-S complexes in fission yeast may take place in vacuoles [91].
An ATP-dependent Cd-PC active transport has been described in the tonoplast of oat root cells [92]. Once
in the vacuoles, the metals are liberated and the PC moiety is degraded [93]. The metals are complexed
with vacuolar organic acids while the individual amino acids can enter the cytosol again [54].
Although PC production by algae is known from laboratory experiments [56,94,95], until recently
there was no evidence for metal toxicity in phytoplankton communities in situ. In 1997, Ahner et al. [96]
indicated that PCs were synthesized in situ, suggesting that they can be used to detect trace metal pollu-
tion because the PC concentration increases well before other physiological parameters such as growth
rate or chlorophyll content are affected.
All the studies, except Ref. 97, report that PC synthase is activated by metals [78,80]. The efficiency
of various metals for PC synthase activation has been tested with various plant material [53,54,74,98]. Al-
though the capacity of different metals to induce PC synthesis varies from one study to another, all des-
ignate Cd^2 as the strongest inducer. Despite the fact that many metals are able to trigger PC synthesis,
756 BERTRAND ET AL.Figure 2 Time course of PC induction and glutathione (GSH) consumption after administration of 200 M
Cd(NO 3 ) 2 to a cell suspension culture of Rauvolfia serpentina. Quantities of glutathione (GSH,—), total PC
(-----), and individual PCs with n(number of -glutamylcysteine units per molecule) 2 (······), 3 (— —),
4 (—·—), or 5 (–··–) are expressed as mol of -glutamylcysteine per g of cell dry weight. (From Ref. 79.) With
permission, from the Annual Review of Plant Physiology and Plant Molecular Biology, Volume 41, © 1990,
by Annual Reviews.