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rate (Turner et al. 1981). Fast coleoptile elon-
gation could facilitate contact with air in water-
logged or flooded soils and maintain adequate
aeration of the growing embryo. This accelerated
elongation was independent of ethylene synthe-
sis (Pearce et al. 1992), but rather dependent on
the extent of alcohol fermentation and ethanol
synthesis (Setter and Ella 1994), which empha-
sizes the importance of anaerobic metabolism
during germination and early seedling growth.
However, this rapid growth as a mechanism of
avoiding oxygen stress in anaerobic soils con-
trasts with tolerance of transient flooding at later
vegetative stages of growth, when submergence
injury is worsened by rapid growth and use of
carbohydrate reserves (Das et al. 2005; Bailey-
Serres et al. 2010). Recent studies showed that
it is possible to combine tolerance of anaerobic
conditions during germination and early growth
with that during the vegetative stage conferred
by the
SUB1
gene, with the latter causing inacti-
vation of shoot elongation (Mackill et al. 2010).
The regulatory mechanisms that mediate these
opposing responses are yet to be determined, but
seem to be stage-specific, and probably mediated
by sugar and light signaling, as the seedlings shift
from being dependent on storage reserves to car-
bohydrates supplied through photosynthesis.
Another important and well-characterized
survival mechanism of anaerobic conditions is
the primary shift from aerobic metabolism to
anaerobic fermentation to generate the energy
needed to sustain growth of the germinating
embryo (Guglielminetti et al. 1995). Unlike other
cereals, rice seeds sustain their ability to break
down starch into readily fermentable carbohy-
drates when germinating under hypoxic or even
anoxic conditions (Atwell and Greenway 1987).
Rice seeds are known to have all the enzymes
needed for degradation of starch and its use for
the growing embryo, but the activities of these
enzymes are affected by oxygen availability. The
importance of
under anoxia (Perata et al. 1993; Guglielminetti
et al. 1995; Perata et al. 1997; Hwang et al. 1999).
Recently, Ismail and colleagues (2009) reported
genetic variation in the ability of rice to break
down starch reserves, with genotypes tolerant of
anoxia showing better ability to mobilize and use
stored reserves. The process was also associated
with higher expression of
RAMY3D
, a member of
the amylase gene family known to be expressed
under anaerobic conditions.
Germinating rice seeds can therefore degrade
starch to soluble sugars under anaerobic condi-
tions. The next bottleneck is the use of these
sugars as substrates to generate ATP, as oxy-
gen is the terminal electron accepter in aerobic
respiration. This is achieved through switching
to anaerobic metabolism when oxygen is lacking
(Avadhani et al. 1978; Jackson et al. 1982; Gibbs
et al. 2000). The key enzymes for anaerobic fer-
mentation are pyruvate decarboxylase (PDC),
which catalyzes the decarboxylation of pyru-
vate to yield carbon dioxide and acetaldehyde,
and alcohol dehydrogenase (ADH) that reduces
acetaldehyde to ethanol, with the reduced form
of nicotinamide adenine dinucleotide (NADH)
being oxidized in the process to maintain gly-
colysis. We observed genetic variation in the
extent of anaerobic respiration in rice cultivars
contrasting in tolerance of anaerobic conditions
during germination and early seedling growth
(Ismail et al. 2009). Activities of PDC and ADH
increased shortly after imbibition in both toler-
ant and sensitive genotypes, but were substan-
tially higher in the tolerant genotypes. However,
expression of
PDC1
and
PDC2
and
ADH1
and
ADH2
was simultaneously high in both toler-
ant and sensitive genotypes. The data support
the notion that anaerobic respiration is important
for the survival of rice seeds germinating under
anaerobic conditions, and that tolerant lines are
capable of greater induction of this pathway than
sensitive lines; however, this induction seems to
be regulated post-transcriptionally. Further stud-
ies are ongoing to identify and validate the traits
that are associated with the tolerant phenotype
-amylases in starch degradation
when O
2
is limiting was reported, in which both
mRNA and protein accumulation were observed
α
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