Amino Acid Transport

Although the chloroplast-localized ascorbate-glutathione cycle is well characterized,
it is now becoming clear that the enzymes of this cycle are also found in mitochondria
and peroxisomes, and they may represent an important antioxidant protection system
against H203 generated in these organelles (Jimhez et al. 1997).
Glutathione transferases may also be involved in conjugating electrophiles generated
from the reaction of hydroxyl radicals with lipids and DNA and, thus, contribute to
oxidative stress resistance (Marrs 1996). Apoplastic GST induction may be mediated by
an oxidative signal in soybean (Flury et al. 1996).
F. P ~ y ~ o c ~ ~
of Heavy
Glutathione serves as a precursor of peptides known as' phytochelatins9 which are composed
of two or more repeating y-glutamylcysteine units, with a terminal glycine residue;
(~-glutamylcysteine)~-gly, where n = 2-11 (Steffens et al, 1986; Reese and ~agner
1987). The enzyme responsible for the synthesis of these peptides is known as phytochelatin
synthase (Rauser 1990, 1995). Phytochelatins (PCs) play an important role in the
detoxification of certain heavy metals (p~ticularly cad~ium) in plants (Rauser 1990,
1995; Howden et al. 1995a,b). Phytochelatin synthase is activated by cadmium (Rauser
1995). In certain plants (notably legumes) that can synthesize homoglutathione, in which
g-alanine is substituted for glycine as the terminal amino acid, homophytochelatins are
synthesized along with PCs in response to Cd (Klapheck et al, 1995).Inmaizeand
certain other species of the Poaceae, a third family of phytochelatins has been found in
which serine is the COOH-te~inal amino acid (Rauser and ~euwly 1995). ~admium
detoxification may require transport of the Cd-phytochelatin complexes into the vacuole;
a transport system has been recently described for these complexes (Salt and Rauser
1995).
In Rubia tinctoru~, phytochelatins (class I11 metallothionein) are induced by many
metal ions, but only a few (Ag, Cd, and Cu) are bound to the PCs that they induce
(Maitani et al. 1996).
v. A
A
During anaerobic stress, pyruvate must be metabolized to either lactate (by lactate dehydrogenase;
LDH) or ethanol (by pyruvate decarboxylase [PDC] and alcohol dehydrogenase
[ADH]) to regenerate the NAD" required to sustain glycolysis and associated ATP
production (Davies 1980). Lactate fermentation may cont~bute to the acidification of the
cytoplasm (Roberts et al, 1984; Ratcliffe 1995), which may progressively inhibit LDH
and activate PDC, promoting alcohol fe~entation (Rivoal and Hanson 1994a). Accumulation
of pyruvate under anaerobic conditions may shift the equilibrium of alanine aminotransferases
toward alanine accumulation; a rapid response of plants to anaerobic stress
(Streeter and Thompson 1972; Stewart and Larher 1980):
Pyruvate + amino acid 3 alanine + 2-keto acid