MicroRNA-mediated regulatory functions under abiotic stresses in legumes - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 233

legumes such as M. truncatula , L. japonicus , soybean,
common bean, peanut, chickpea and cowpea
(Subramanian et al., 2008; Szittya et al., 2008; Arenas-
Huertero et al., 2009; Lelandais-Briere et al., 2009; Lu &
Yang, 2010; Paul et al., 2011). In miRBase ( http://www.
mirbase.org ), an archive of miRNA sequences and anno-
tations, a total of 1256 sequences belonging to 285
miRNA families from legumes have been deposited
(Release 20: June 2013). These miRNAs have the poten-
tial to regulate species-specific biological processes in
legumes (Subramanian et al., 2008; Szittya et al., 2008).
Conserved miRNAs are generally encoded by multi-
genic families (Allen et al., 2004). It is noted that gene
number per miRNA family was generally higher in
soybean compared to M. truncatula . This is probably due
to the large genome of soybean (1115 Mbp) and genome
duplication (Schmutz et al., 2010). However, for some
miRNA families an opposite profile was noted. For
example, miRBase reveals that miR395 and miR399
have more members in M. truncatula than in soybean.
On the other hand, non-conserved miRNAs that
were previously described as crop-specific (soybean:
Subramanian et al., 2008; Wang et al., 2009; Joshi et al.,
2010; or M. truncatula : Szittya et al., 2008; Jagadeeswaran
et al., 2009, Lelandais-Briere et al., 2009) have now been
declared as legume-specific. Due to greater interest in
legume miRNA research, hundreds of such legume-
specific miRNA families have been reported in the last
few years. For example, 229 and 179 miRNA families are
specific to M. truncatula and soybean, respectively.
Recently, De Luis et al. (2012) reported that 35 miRNA
families are specifically present in L. japonicus . This huge
diversity compared to that in rice and wheat is probably
due to more interest in legumes, which resulted in a
great deal of small RNA deep sequencing and analysis.
High-throughput sequencing has now been employed to
systematically identify stress-related miRNAs in legumes
(Li et al., 2010; 2011; Barrera-Figueroa et al., 2011; Wang
et al., 2011; Chen et al., 2012a,b; Zhou et al., 2012a,b).

(Sunkar & Zhu, 2004; Fuji et al., 2005; Aung et al., 2006;
Chiou et al., 2006; Okamura et al., 2008). More than 40
miRNA families have been associated with abiotic stresses
in plants, 13 of which have been found to be responsive to
salinity and drought stresses (Barrera-Figueroa et al., 2012;
Nageshbabu et al., 2013). It is further noted that almost all
stress-related miRNAs are conserved, ultimately suggesting
that miRNA-mediated regulatory roles may be evolution-
arily conserved for corresponding stresses throughout the
plant kingdom. However, a particular miRNA that responds
to a specific abiotic stress in one species may not necessarily
have the same function in another species. For example, at
least 10 miRNAs involved in stress have shown opposite
expression profiles in rice and Arabidopsis under drought
stress (Barrera-Figueroa et al., 2012). Different computa-
tional and experimental approaches were used in the early
studies, which provided low coverage. However, with the
advent of next-generation sequencing (NGS), microarrays
and more advanced computational techniques, it became
much easier and more cost-effective to perform genome-
wide profiling for the identification of stress-responsive
miRNAs. As a result, the discovery of stress-related miR-
NAs has expanded from a few model plants like Arabidopsis
and rice (Zhao et al., 2007; Liu et al., 2008; Zhou et al., 2010)
to other non-model plants (Ding et al., 2009; Jia et al., 2009;
Song et al., 2011; Wang et al., 2011; Barakat et al., 2012;
Eldem et al., 2012; Li et al., 2013; Ozhuner et al., 2013; Shuai
et al., 2013; Yanik et al., 2013), including orphan plants like
legumes (Subramanian et al., 2008; Szittya et al., 2008;
Arenas-Huertero et al., 2009; Jagadeeswaran et al., 2009,
Leladais-Briere et al., 2009; Pant et al., 2009; Wang et al.,
2009; Joshi et al., 2010; Lu & Yang, 2010; Trindade et al.,
2010; Barrera-Figueroa et al., 2012; Chen et al., 2012a,b;
Turner et al., 2012; Dong et al., 2013; Xu et al., 2013).

14.4.1 Drought stress
Drought is one of the main causes of yield reduction in
plants. Due to the limited availability of water for agri-
culture in many areas of the world, it is logical to
investigate the natural mechanism of drought tolerance
as an integral component of understanding the biological
basis of response to drought stress in plants. Legumes are
extensively grown in dry or semi-arid regions of the
world, usually under rain-fed agriculture. There is strong
evidence that miRNAs are involved in drought stress as
well as in diverse physiological processes (Sunkar & Zhu,
2004; Lu et al., 2005). In one of the pioneering studies on
stress-responsive miRNAs, 26 miRNAs from Arabidopsis
were cloned and five were examined for drought

14.4 MicrorNa expression profiling
under abiotic stresses in legumes

Historically, plants have evolved morphological, physio-
logical and molecular adaptations to cope with different
abiotic stresses (Hussain et al., 2011a). Several lines of
evidence have suggested that miRNAs play important
regulatory roles in plants' responses to abiotic stresses