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regulation (Sunkar & Zhu, 2004). The result showed
that three miRNAs (miR393, miR397b and miR402)
were upregulated, one (miR389a) was downregulated
and one (miR319c) was non-regulated by drought.
Notably, all these miRNAs were also regulated by cold,
NaCl and/or abscisic acid (ABA) treatments. Also in
Arabidopsis
, miR169 was found to be downregulated
by drought stress in an ABA-dependent pathway.
Downregulation of miR169 leads to higher expression of
its target gene,
NFYA5
, which in turn enhances the
drought tolerance of the plant (Li
et al.,
2008). Several
miRNAs involved in drought tolerance were discovered
by global miRNA expression profiling using high-
throughput techniques in legumes.
The expression pattern of miRNAs varies among dif-
ferent species under drought stress. For example,
miR169 is downregulated in
M. truncatula
(Trindade
et
al.,
2010; Wang
et al.,
2011) but upregulated in rice and
common bean under drought (Arenas-Huertero
et al.,
2009; Zhao
et al.,
2009). Similarly, miR1510 is upregu-
lated in
Glycine max
and downregulated in
M. truncatula
,
while miR396 is down-regulated in
M. truncatula
and
Vigna unguiculata
, and upregulated in
G. max
(Mantri
et al.,
2013). Some miRNAs can be differentially expressed
in the same plant species under drought conditions, for
example, miR398a/b in
M. truncatula
. In one study
(Trindade
et al.,
2010) it was upregulated, but in another
study (Wang
et al.,
2011) it was downregulated by
drought. This difference may be attributed to experi-
mental conditions, particularly the extent and duration
of drought stress.
Wang
et al.
(2011) performed small RNA expression
profiling of two libraries of
M. truncatula
under drought
stress. Twenty-two miRNAs of four families and 10 miR-
NAs of six families were found to be upregulated and
downregulated, respectively, under drought stress and
this was also verified by high-throughput sequencing
and quantitative real-time polymerase chain reaction
(qRT-PCR). They also identified 29 new members of
known miRNA families, of which eight miRNAs were
found to be responsive to drought stress. Recently, a
study investigated the effect of drought and salt stress
on expression of conserved miRNAs in
Phaseolus vulgaris
(Nageshbabu
et al.,
2013). Results showed that miRNAs
were less sensitive to drought than salinity. However,
both stresses altered the expression pattern in a dose-
dependent manner. Significant changes in expression
level under light drought stress were shown by miR156
and miR162, suggesting that these miRNAs may have
important functions under drought stress. Furthermore,
Arenas-Huertero
et al.
(2009) identified a number of
novel legume-specific miRNAs in
P. vulgaris
under
drought and ABA treatment. Targets of novel miRNAs
were found to be involved in diverse cellular processes
unique to legumes.
A complex study comprising drought and rust suscep-
tible and resistant genotypes of soybean identified 256
miRNAs from different small RNA libraries using deep
sequencing technology (Kulcheski
et al.,
2011). Of these
identified miRNAs, 71 miRNAs belong to conserved
miRNA soybean families, 15 miRNAs belonging to six
miRNA families were found to be conserved in other
plant species, and 29 miRNAs belonging to 24 novel
families were reported for the first time. The authors
also identified 121 isoforms (miRNA variants) derived
from 22 conserved miRNA families and four novel
miRNA families. Interestingly, of the 11 novel miRNAs
analysed, all were expressed differently from each other
during drought stress. However, the majority were
upregulated in the susceptible genotype but downregu-
lated in the tolerant genotype under drought. This
distinct miRNA behaviour under drought stress in two
contrasting genotypes may potentially be involved in
regulating genes associated with tolerance/intolerance.
However, this needs further investigations. Similarly,
Barrera-Figueroa
et al.
(2011) used deep sequencing of
small RNA libraries from two cowpea genotypes
(drought tolerant and susceptible) and identified 157
miRNAs belonging to 89 families by mapping sequenced
small RNA reads to genomic sequences of cowpea.
Forty-four drought-associated miRNAs belonging to 28
families were identified by comparing the expression
levels of experimental samples with control samples.
Furthermore, 30 out of 44 drought-associated miRNAs
were upregulated and the rest were downregulated. The
44 drought-associated miRNAs included ones from
miRNA families previously known to be associated with
drought in other plant species, indicating that miRNAs
are involved in conserved drought response pathways.
In addition, target genes for 32 miRNAs were predicted,
encoding diverse functions. Most of these targets are
transcription factors. Some miRNA genes, despite high
expression in libraries, were not identified mainly due
to non-availability of complete genome sequences of
cowpea. In conclusion, detailed investigation of the tar-
gets of drought-associated miRNAs identified so far will
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