Biology Reference
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conditions, including upon re-oxygenation, the
terminal methionine of RAP2.12 is removed by
a methionine amino peptidase (MetAP). This
exposes the second amino acid, a cysteine (C 2 )
within the consensus sequence MCGGAII, to
oxidation, followed by conjugation of an argi-
nine residue to the amino-terminus in a reac-
tion catalyzed by arginine transferase (ATE),
triggering ubiquitination by the ligase PROTE-
OLYSIS 6 (PRT6), and targeting RAP2.12 for
proteasome degradation (Licausi et al. 2011).
It was further shown that another member of
the ERF subfamily VII, HYPOXIA RESPON-
SIVE 2 (HRE2), is stabilized under aerated con-
ditions in the prt6 mutant, resulting in constitu-
tive accumulation of hypoxia-responsive genes
(Gibbs et al. 2011). However, as has been shown
by Gibbs and colleagues (2011) using an in vitro
assay that SUB1A-1 protein is not degraded by
this pathway even after the N-terminal conserved
N-end rule target motif (MCGG E VI) was modi-
fied (MCGG A VI) to better match the consensus
sequence. Resistance of SUB1A-1 to degrada-
tion prior to severe oxygen deficiency and upon
re-oxygenation might indeed be very important
for survival of submergence as well as protection
against cell damage by ROS as outlined above.
Interestingly, it has been shown that SUB1A
enhanced the survival of rice seedlings after
drought stress in a pot experiment and that genes
related to drought tolerance (DREB1A , DREB1E ,
AP59 ) are induced in M202-Sub1 (Fukao et al.
2011). It therefore seems that tolerance of sub-
mergence and drought stress is conferred by sim-
ilar and partially overlapping pathways. This is
currently being validated under different envi-
ronmental conditions. It has been shown that
SUB1A is specifically expressed in the grow-
ing parts of leaves (leaf base and leaf collar)
and in the shoot apex of young rice seedlings
(Singh et al. 2010; unpublished data). These tis-
sues are the actively growing parts of rice plants
and are therefore critical for growth and regen-
eration after exposure to stress. Detailed studies
are now under way to specify a putative role of
SUB1A in maintaining and protecting meristem-
atic cells under stress.
Tolerance of Flooding during
Germination
Seeds of most cereal crops (maize, wheat, bar-
ley, oats, sorghum) are extremely sensitive to
low-oxygen stress during germination, and they
normally fail to germinate in flooded or even
saturated soils (Perata et al. 1997; Vartapetian
and Jackson 1997). Unlike other cereals, rice is
capable of germination under flooded conditions
(Taylor 1942; Yamauchi et al. 1993; Ella and Set-
ter 1999; Angaji et al. 2010), but only through
elongation of the coleoptiles with failure to form
roots and leaves (Biswas and Yamauchi 1997).
However, substantial genetic variation in abil-
ity to germinate and establish in flooded soil
was found in rice after screening a large number
of germplasm accessions and breeding lines of
different origins (Yamauchi et al. 1993; Biswas
and Yamauchi 1997; Jiang et al. 2004; Angaji
et al. 2010). A few rice genotypes with greater
tolerance of flooding during germination and
early seedling growth were identified and sub-
sequently characterized to assess the physiolog-
ical and molecular bases of tolerance (Ismail
et al. 2009). Germinating embryos in flooded
soils can suffer from hypoxia or even anoxia
in severe cases, and this hinders further growth
as oxygen is necessary for functioning of the
enzymes involved in the breakdown and mobi-
lization of stored carbohydrates and the oxida-
tive pathways to generate energy for the grow-
ing embryos (Drew 1990; Greenway and Setter
1996).
Germinating rice seeds tolerate oxygen defi-
ciency in flooded soils through various growth
and metabolic adjustments. One of the most
spectacular adaptive growth features is the exces-
sive growth of the coleoptile, which can elon-
gate even faster at low O 2 concentrations than
in air (Alpi and Beevers 1983), and with consid-
erable variation among genotypes in elongation
 
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