MicroRNA-mediated regulatory functions under abiotic stresses in legumes - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 235

help in understanding the regulatory roles of miRNAs in
drought tolerance of legumes.

known miRNAs belonging to six miRNA families with
128 novel miRNAs belonging to 64 miRNA families.
Further analysis of the 128 novel miRNAs revealed that
66 miRNAs belonging to 27 miRNA families have iden-
tifiable loci in plant species, and that 12 miRNAs were
conserved in legumes, of which 10 were found only in
soybean. Furthermore, a total of 770 putative targets
were identified for 224 miRNAs. The majority of miR-
NAs each have multiple targets varying from 2 to 10.
Computational prediction of functions of potential tar-
gets revealed that many of them have been associated
with stress responses in plants (Sunkar & Zhu, 2004; Liu
et al., 2008; Jia et al., 2009). Interestingly, this study shed
light on the potential role of miRNAs in soybean and
revealed a dynamic regulation pattern of miRNAs dur-
ing salt stress in functional nodules of soybean. However,
further characterization of highly expressed individual
miRNAs will help to elucidate the molecular mechanism
of salt tolerance of functional soybean nodules.

14.4.2 Salt stress
Salt stress is another important constraint for plant
growth. In particular, legume production has been
greatly affected by salinity (Bayuelo-Jimenez et al., 2002;
Wang et al., 2003; C. Chen et al., 2007). MicroRNAs have
a role in gene expression under different abiotic stresses
and several studies have measured the differential
expression of miRNAs under salt stress in different plants
(Sunkar & Zhu, 2004; Liu et al., 2008; Lu et al., 2008;
Arenas-Huertero et al., 2009). To date, limited
information is available on the role of miRNAs under salt
stress in legumes. However, potential candidates have
been identified in soybean and other legumes using in
silico techniques (Zhang et al., 2008; Chen et al., 2009).
Recently, a study followed the expression levels of 12
miRNAs in P. vulgaris under salinity and drought stresses
(Nageshbabu & Jyothi, 2013). Eleven of the 12 miRNAs
had been demonstrated to be involved in either salinity
or drought stress responses in previous studies in
Arabidopsis and rice. The authors did not find any
dramatic change in the expression pattern of miRNAs
under salt/drought stress. This points towards the
existence of fine-tuning mechanisms in plants that may
be important for the regulation of gene expression
without negatively impacting plant growth and
development under stress. Similarly, using a compara-
tive genomics technique, Paul et al. (2011) identified 18
conserved miRNAs belonging to 16 distinct miRNA fam-
ilies in cowpea ( Vigna unguiculata ) under salt stress.
Fifteen potential targets were predicted by BLASTn
searches using the miRNA sequences. Interestingly, all
of the predicted targets were identified as transcription
factors. Expression patterns of these miRNAs under salt
stress were studied using qRT-PCR and results con-
firmed the presence and upregulation of seven miRNAs
in roots under salt stress. The results showed the
induction of miR498 under salt stress in cowpea. In
Arabidopsis , miR498 was upregulated only during cold
and drought stresses (Liu et al., 2008). These results sug-
gest that plants employ different miRNA-mediated
regulatory strategies under different stresses.
Recently, Dong et al. (2013) studied the effect of salt
on soybean nodules using deep sequencing of non-
stressed nodules (NSN) and salt-stressed nodules (SSN).
Computational analysis of sequence reads identified 110

14.4.3 extreme temperature stress
Altered expression of miRNAs under extreme tempera-
tures (cold and heat) has been reported in different
model plant species. Heat stress disturbs cellular homeo-
stasis and can lead to leaf etiolation, severe retardation
in growth and development, and even death. Different
high-throughput approaches like bioinformatics, micro-
arrays, transcriptomics and direct cloning have been
used to identify cold- and heat-responsive miRNAs in
plants (Zhou et al., 2008; Jian et al., 2010; Xin
et al., 2010; Yu et al., 2011). In Arabidopsis , the expres-
sion levels of miR393 and miR319c were upregulated by
cold (Sunkar & Zhu, 2004). Similarly, several miRNAs
like miR165/166, miR169, miR172, miR393, miR396,
miR397 and miR408 were significantly upregulated,
while other miRNAs like miR156/157, miR159/319,
miR164, miR394 and miR398 showed either transient
or mild regulation under cold stress (Zhou et al., 2008).
MicroRNA contents of Arabidopsis and Populus were also
analysed using high-throughput microarray techniques
(Liu et al., 2008; Lu et al., 2008). Deep sequencing led to
the identification of 28 cold-responsive miRNAs in
Brachypodium (Zhang et al., 2009).
Wang and Long (2010) analysed four miRNAs
(miR319, miR393, miR397 and miR402) from
Arabidopsis for cold tolerance with the assumption that
they might also be involved in cold tolerance in sweet
pea. The results showed the presence of these miRNAs