that the multiplicity of responses is consistent with the various aspects of ethylene in development and re-
sponses to other environmental stimuli (e.g., wounding).
- Oxygen Radicals
A third enzyme worth noting is superoxide dismutase (SOD). The action of SOD in a low-O 2 environ-
ment may seem counterintuitive; however, the generation of superoxide radicals, leading to lipid peroxi-
dation, has been implicated in postanoxic tissue damage [179]. In anoxia-intolerant Iris germanicarhi-
zomes, lipid peroxidation was widespread compared with anoxia-tolerant Iris pseudoacorusrhizomes.
Monk et al. [180] demonstrated that SOD activity rises after extended anoxia in Iris pseudoacorusrhi-
zomes but not in Iris germanicaorGlyceria maxima. This suggests that one of the strategies of tolerance
to anoxia may be to synthesize proteins that anticipate the return of an aerobic environment [175].
- Signal Transduction
This signal transduction pathway or network is incompletely understood. Drew [169] indicated that some
evidence for an O 2 sensor exists, but not conclusively. In contrast, changes in Ca^2 fluxes definitely ap-
pear to play a role, with changes in cytoplasmic pH or decreased energy metabolism possibly acting to in-
duce these fluxes [169].
Regulation of aerenchyma formation, via ethylene, is apparently complex and an example of pro-
grammed cell death. Drew et al. [181] propose a model in which elevated levels of ethylene eventually
activate phospholipase C, which then catalyzes a lipid-derived signal cascade culminating in the protein
kinase–mediated phosphorylation of some target proteins involved in PCD. It is quite likely that this sig-
nal transduction pathway interacts with others to ensure its specificity to the roots.
D. Air Pollution
Air pollution is still emerging as an area of research for alterations of protein synthesis. Ozone (O 3 ) and
sulfur dioxide (SO 2 ) are usually considered to be the primary culprits in damage due to air pollution. How-
ever, these molecules eventually generate toxic oxygen species via the reaction H 2 O 2 O- 2 ⇒O 2 OH..
Changes in both protein [182] and mRNA [183,184] are known to occur, generally resulting in the
action of antioxidants or detoxifying enzymes to minimize the damage to cellular membranes or macro-
molecules. Of the enzymes, superoxide dismutase (SOD) is the best characterized, catalyzing the reaction
2O- 2 2H⇒H 2 O 2 O 2. Bowler et al. [14] summarized information indicating that SOD protection in
response to O 3 stress is often contradictory and extremely dependent on the conditions and plant species
under consideration. Indeed, Badiani et al. [185] reported that with Phaseolus vulgarisL., fluctuations in
the level of antioxidants and detoxifying enzymes occur during the day. Previously, SOD in P. vulgaris
was reported to be unaffected by ozone treatment [186]. In contrast, there appears to be much better evi-
dence for a role in SO 2 protection.
Of the antioxidant molecules, glutathione (GSH) has received the most attention. Glutathione is a
polypeptide of the sequence -glutamyl-cysteinyl-glycine and acts to maintain the redox state of cysteine
groups in proteins via its cysteinyl side chain. Reduction of protein disulfide bonds results in the forma-
tion of oxidized glutathione (GSSG). Glutathione, its synthetic enzymes, and glutathione reductase are
known to be induced by O 3 [187]. As with SOD, however, the actual value of enhanced glutathione lev-
els is equivocal, again being species and condition dependent.
E. Metal Ion Stress
Heavy metal stress is frequently encountered by plants in areas of industrial pollution, as a result of min-
ing activity, or, in the case of Al, acidic soils in the tropics and subtropics. In animals, the primary means
of heavy metal sequestration is by the metallothionein family of proteins [188]. These proteins are Cys
rich, relying on those residues to chelate metal ions. Although evidence of such proteins exist in plants,
the primary defense against heavy metal toxicity relies on polypeptide equivalents, the phyochleatins.
These polypeptides are inducible by heavy metals and are found throughout the plant kingdom [189].
Phytochelatins are related to glutathione, having the primary structure ( -Glu-Cys)n-Gly or ( -Glu-
Cys)n--Ala, where n2 to 11 [188]. There is evidence for a metal-inducible phytochelatin synthase,
catalyzing the transfer of -glutamylcysteine to glutathione, thus generating the ( -Glu-Cys)nportion of
the molecule [189].
INDUCTION OF PROTEINS IN RESPONSE TO STRESSES 673