MicroRNA-mediated regulatory functions under abiotic stresses in legumes - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 237

Several miRNAs have been reported to be involved
in nutrient signalling (Fuji et al., 2005; Aung et al.,
2006; Bari et al., 2006; Chiou et al., 2006). Therefore,
miRNAs were considered as a vital element in gene
regulatory networks under nutrient deprivation condi-
tions, particularly P-starvation stress (Schachtman &
Shin, 2007; Chen et al., 2008; Guo et al., 2008; Valdes-
Lopez & Hernandez, 2008; Yuan & Liu, 2008;
Hackenberg et al., 2013a). In Arabidopsis , P homeo-
stasis is post-transcriptionally regulated by miR399,
which forms an essential component of the phosphate
starvation response 1 (PHR1) signalling pathway (Fuji
et al., 2005; Aung et al., 2006; Bari et al., 2006; Chiou
et al., 2006). miRNA399 was shown to be positively
regulated by AtPHR1 (Aung et al., 2006; Bari et al., 2006)
but negatively regulated by IPS1/At4 by means of a
target mimicry mechanism (Franco-Zorilla et al., 2007).
A similar case was also observed in barley (Hackenberg
et al., 2013b). miR399 targets UBC24 , which encodes a
ubiquitin-conjugating E2 enzyme, also known as
PHOSPHATE 2 (PHO2) (Fuji et al., 2005; Aung et al.,
2006; Bari et al., 2006). The PHO2 gene negatively reg-
ulates phosphate uptake and root-to-shoot allocation.
miR399 is also involved in P-deficiency signalling in
rapeseed and pumpkin phloem sap and common bean
roots (Pant et al., 2008; Valdes-Lopez et al., 2008).
Overexpression of miR399 in mutant plants rescued
the phenotype, resulting in high phosphate (Pi) levels
in shoots and significantly reducing levels of the target
transcript (Chiou et al., 2006). Similarly, Pant et al.
(2008) demonstrated that miR399 accumulated at a
high level in phloem sap of rapeseed and pumpkin
under P-deficiency, indicating a role in long-distance
communication. This hypothesis was verified by
reciprocal grafting experiments between transgenic
Arabidopsis expressing miR399 and wild-type plants.
This experiment revealed that miR399 can move from
shoots to roots but not in the opposite direction.
Furthermore, a reduced level of PHO2 mRNA in the
rootstock was observed. This confirmed that translo-
cated miR399 is also functional (Lin et al., 2008; Pant
et al., 2008; Buhtz et al., 2010).
Recently, Xu et al. (2013) identified an additional
miRNA (miR2111) that was induced under P-deficiency
in soybean. This miRNA was almost undetectable under
normal conditions (full nutrition) but became highly
abundant under limited P. In addition, miR2111 also
accumulated in phloem sap like miR399 during the

P-deficient stress, indicating that miR2111 might also be
involved in long-distance communication (Pant et al.,
2009). A Kelch repeat-containing F-box protein was
identified in Arabidopsis and soybean as one of its targets,
suggesting that miR2111 has a possible role in the con-
trol of protein abundance under P-limitation (Hsieh et al.,
2009). Moreover, several reports have revealed that
miR156, miR778 and miR827 were upregulated whereas
miR169, miR395 and miR398 were downregulated in
Arabidopsis under P deficiency (Hsieh et al., 2009; Pant
et al., 2009). Similarly, many miRNAs (miR157, miR160,
miR165, miR166, miR169, miR393, pvu-miR2118, gma-
miR1524, gma-miR1526 and gma-miR1532) showed
differential expression upon P deficiency in common
beans (Valdes-Lopez et al., 2010). Interestingly, 167 miR-
NAs belonging to 35 miRNA families showed differential
expression in white lupin in response to P deficiency
(Zhu et al., 2010). Furthermore, Zeng et al. (2010) dem-
onstrated that 27 miRNA families representing at least 57
miRNA members showed significant changes in expres-
sion level under P starvation in soybean.
In regard to other nutrients, miR397, miR398, miR408
and miR857 responded to copper (Cu alteration) and
have been proposed to be involved in Cu homeostasis
through the regulation of Cu:Zn superoxide dismutase
(CSD1 and CSD2), plastocyanin and various laccases
(Abdel-Ghany & Pilon, 2008; Yamasaki et al., 2008; Buhtz
et al., 2008; 2010; Dugas & Bartel, 2008). Similarly,
miR395, which is involved in regulation of two ATP sul-
phurylases (APS1 and APS4) and a low-affinity sulphate
transporter (SULTR2;1), has a relevant role in sulphur
homeostasis during S limitation (Allen et al., 2004; Jones-
Rhoades et al., 2006; Buhtz et al., 2008, 2010; Kawashima
et al., 2009). miR167 is associated with lateral root growth
under nitrogen (N) deficiency, miR169 and miR398a
were repressed upon N limitation (Pant et al., 2009; Zhao
et al., 2010; Wang et al., 2013) and miR393 was induced
by high nitrate and controls root architecture (Gifford et
al., 2008; Vidal et al., 2010). It is further demonstrated
that changes in one nutrient significantly affect the avail-
ability of other nutrients, which may result in unexpected
interactions among miRNAs and consequently gene reg-
ulation (Haydon & Cobbett, 2007). A striking example is
that under iron (Fe) deficiency, expression of miR399
and miR2111 was decreased in phloem and almost unde-
tectable in roots and leaves (Buhtz et al., 2008, 2010),
which indicates that changes in Fe availability signifi-
cantly alter P homeostasis.