MicroRNA-mediated regulatory functions under abiotic stresses in legumes - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 236

in sweet pea. RT-PCR analysis showed that the
expression levels of these miRNAs increased after
cold treatment in sweet pea. On the other hand, Yu
et al. (2011) identified 35 conserved miRNA families
in Brassica rapa using a deep sequencing approach.
Among them, five miRNA families were responsive to
heat tolerance. Two miRNAs (miR156h and miR156g)
of the miR156 family were induced by heat stress and
their putative target, SPL2 , was downregulated follow-
ing the stress. These results show that plant miRNAs
can be regulated by extreme temperature stress
and may function in critical defence systems for stress
fitness.

tolerance genes confirmed the above results. Taken
together, the data suggest that miR395 is involved in
detoxification of Cd in B. napus . Previous studies showed
that miR319 is also regulated under various metal
stresses. MiR319 was upregulated in M. truncatula
exposed to 80 μM Cd, 20 μM Hg and 200 μM Mn,
whereas it was downregulated in B. napus after
treatment with 80 μM Cd and 10 μM Hg (Zhou et al.,
2008, 2012a; Valdes-Lopez et al., 2010). Similarly,
miR171 and miR392 also showed induction/suppres-
sion patterns in B. napus, and M. truncatula upon
treatment with different stresses like Cd, Hg and Al (Xie
et al., 2007; Zhou et al., 2008, 2012a,2012b; Ding et al.,
2011; Lima et al., 2011).
In conclusion, it is clear from the above reports that
miRNAs constitute an integral part of regulatory circuits
conferring metal stress tolerance in plants.

14.4.4 heavy metal stress
To date, there is a lack of detailed information on the
role of miRNAs under heavy metal stress in legumes.
However, a few reports have demonstrated the involve-
ment of miRNAs in response to different heavy metal
stresses - Cd (cadmium), Hg (mercury), Al (aluminium),
As (arsenic) - in legumes like M. truncatula , Brassica
napus , P. vulgaris and Glycine max .
Several metal stress-responsive miRNAs were iden-
tified from a small RNA library of B. napus seedlings
exposed to 80 μM Cd (Zhou et al., 2012a). Comparative
analysis of four libraries (treated and control roots and
shoots) revealed that Cd stress differentially regulated
the expression of some miRNAs, suggesting that these
miRNAs are directly or indirectly involved in different
physiological processes leading to plant tolerance to
Cd stress. Furthermore, the authors reported some
non-conserved miRNAs were also regulated by Cd
exposure. Interestingly, no species-specific miRNA
was detected, probably due to the limited number of
EST sequences of B. napus that are used for the
identification of species-specific miRNAs. While a
number of targets were detected for most of the con-
served miRNAs, no target was found for non-conserved
miRNAs possibly due to the low expression of these
targets or miRNAs.
Recently Zhang et al. (2013) demonstrated that trans-
genic B. napus overexpressing miR395 showed improved
plant growth under Cd stress (80-120 μM). The mani-
fested transgenic plants exhibited enhanced chlorophyll
accumulation and attenuation of the oxidative damage
to root cells. Furthermore, the transgenic plants showed
low translocation of Cd from roots to shoots compared
to wild-type plants. The expression of several Cd

14.4.5 Nutrient deficiency stress
Phosphorus (P) is one of the essential macronutri-
ents important for plant growth and development
(Hackenberg et al., 2013a). It is involved in many
cellular processes and constitutes an integral
building block of many macromolecules such as
nucleic acids and biomembrane phospholipids (Vance
et al., 2003; Valdes-Lopez et al., 2008; Yuan & Liu,
2008). Low P availability severely affects plant
growth, and 30-80% of organic and insoluble forms
of P are not accessible to plants (Abel et al., 2002; Sha
et al., 2012). To react to nutrient deprivation, plants
have evolved a broad spectrum of morphological,
physiological and metabolic adaptations that include:
decreased growth rate; establishment of mycorrhizal
symbiosis for environmental P fixation and utiliza-
tion; modification of root system architecture (RSA)
for increased surface area; enhanced expression of
phosphorus transporter genes; and adaptation of
some metabolic pathways for bypassing P-requiring
steps (Shulaev et al., 2008; Yuan & Liu, 2008; Kehr,
2013). Over the past few years, collective studies
have revealed the roles of several metabolic genes,
transcription factors, ribo-regulators, plant hormones
and ubiquitin-related proteins (Rubio et al., 2001,
2009; Hammond et al., 2003; Wu et al., 2003; Franco-
Zorilla et al., 2004; Mission et al., 2005; Z.H. Chen
et al., 2007; Devaiah et al., 2007; Morcuende et al.,
2007; Mueller et al., 2007; Nilsson et al., 2007; Yuan &
Liu, 2008).