further showed that a gene product they termed HOS1 acts as a negative regulator of pathway IV at an
early stage and appears to interact positively with both the ABA-dependent and ABA-independent signal
transduction pathways. The nature of HOS1 remains elusive but is indicative of the extent of cross talk
between these signal transduction pathways.
C. Oxygen Deprivation
- Physiology
The phrase oxygen deprivationis a general term for an area of study that has had a considerable variation
in terminology. Anaerobicmeans O 2 -free,anoxiarefers to O 2 levels so low that ATP production by ox-
idative phosphorylation is essentially nil, and hypoxiadefines O 2 levels that limit ATP production by mi-
tochondria [168]. The actual levels of O 2 that correspond to these states are highly dependent on the tis-
sue utilized and the physiological process under investigation. In general, O 2 levels from 2 to 10%
(compared with the normal atmospheric concentration of 21%) result in a hypoxic state. For additional
information, see Ref. 169. Low-O 2 environments are associated with excess water in the soil and rela-
tively low diffusibility of O 2 in water compared with air. Proper soil aeration is prevented, which leads
subsequently to consumption of available O 2 by aerobic organisms.
The effects of O 2 deprivation on protein synthesis are similar to those encountered during heat shock.
In maize, there is a decrease in normal aerobic protein synthesis, associated with a loss of polysomes
[170–172]. This is followed by the concomitant synthesis of approximately 20 proteins [170] under tran-
scriptional and posttranscriptional regulation [172,173]. There may be one protein commonly expressed
under both oxygen deprivation and heat shock [171].
- Anaerobic Polypeptides
The preferentially synthesized proteins may be divided into two temporally regulated groups. Members
of the first group are translated primarily during the first 5 hr of anoxia and are referred to as transition
polypeptides (TPs) [170]. These proteins are stable, lasting long after their synthesis declines [171]. The
second group, the anaerobic polypeptides (ANPs), begins to appear after approximately 90 min of anoxia,
with synthesis continuing for several days, until cell death [170]. It has been noted that some plants dif-
fer in their tolerance to anoxia, e.g., maize (tolerant) and soybean (less tolerant). Both Sachs [171] and
Hwang and Van Toai [174] speculated that a possible reason for the difference in anoxia tolerance is the
number or types of ANPs synthesized.
Rather than acting to ameliorate protein denaturation, as in heat shock, most of these proteins are ap-
parently involved in maintaining cellular ATP levels. In particular, several of the ANPs have been iden-
tified as glycolytic or fermentative enzymes. They include sucrose synthase, phosphoglucoisomerase, al-
dolase, alcohol dehydrogenase (ADH), and pyruvate decarboxylase (PDC) (summarized by Drew
[169,175]).
Of these enzymes, ADH is the best characterized. Andrews et al. [176] examined Adhgene expres-
sion and enzyme activity in several tissues of maize under different O 2 concentrations. They showed that
Adhgene expression is maximal with anoxia or extreme hypoxia (i.e., 0 to 4% O 2 ) in both root tips and
axes. However, Adhtranscripts did not always parallel ADH activity. The authors concluded that hypoxia
is apparently crucial to increased ADH induction and activity. They suggested that a delay between Adh
induction and enhanced activity provides a mechanism for survival during the anoxic state that would fol-
low hypoxia. Indeed, Drew [175] indicated that in maize, tolerance to anoxia can be improved by expo-
sure to hypoxic conditions. The regulatory pathway by which Adhis induced is not completely under-
stood; however, common sequence elements have been found between Adh1 and aldolase gene promoter
regions [173].
- Ethylene Synthesis
Another important enzyme that increases during O 2 deprivation is 1-aminocarboxylate-1-cyclopropane
synthase (ACC synthase). This enzyme catalyzes the rate-limiting step in the synthesis of ethylene, which
increases dramatically in response to hypoxia [177]. One of ethylene’s actions is to stimulate the forma-
tion of aerenchyma in the stem (and possibly roots), thus providing more O 2 to deprived tissues. Zarem-
binski and Theologis [178] reported that in rice, several ACC synthase genes are induced by anoxia, and
that differential expression occurs under different hormonal and environmental signals. They suggested
672 ARTLIP AND WISNIEWSKI