Negative regulation of defense mechanisms by miRNAs
The Osa-miR156fhl-3p , Osa-miR164a , Osa-miR167d ,Osa-miR169a , Osa-miR319 , Osa-miR396 ,Osa-439a , Osa-miR444 , and Osa-miR1873 have been
identified as negative regulators of rice innate immunity againstM. oryzae. The miR156 belongs to a conserved family of
miRNAs involved in the regulation of plant growth and development and
yield. In rice, Osa-miR156fhl-3p has been shown to negatively
regulate resistance to blast. Overexpression of a target mimic mutant ofOsa-miR156fhl-3p showed stronger resistance to blast through the
enhanced expression of target genes SQUAMOSA promoter-binding
protein-like transcription factor 14 (SPL14 ) and WRKY45transcription factor (Zhang et al. 2020b). Wang et al.(2018) demonstrated the suppression of Osa-miR164a upon M.
oryzae infection, which led to enhanced defense response through the
positive regulation of the expression of target gene OsNAC60 .
Abolition of Osa-miR164a/OsNAC60 regulatory module resulted in
susceptible phenotype. The Osa-miR164a was also shown to control
the fungus Rhizoctonia solani that cuases sheath blight by
activating the salicylic acid (SA) signaling and defense-related gene
expression. The Osa-miR167d modulates plant developmental and
stress responses by targeting auxin responsive factor(ARF ) genes. The Osa-miR167d seemed to play a negative
role in rice immunity against M. oryzae by blocking ARF12genes. Suppression of Osa-miR167d by overexpressing a target
disease mimic mutant of Osa-miR167d led to enhanced resistance to
rice blast (Zhao et al. 2019). Expression of Osa-miR169ahave been shown to condition a strong resistance against M.
oryzae by suppressing the expression of its target gene nuclear
factor Y-A (NF-YA ). These findings indicated a negative role in
mediating immunity. A significant accumulation of Osa-miR169a has
been documented in a susceptible genotype of rice with somewhat
decreased level of resistance. Transgenic plants overexpressingOsa-miR169a have been shown to exhibit higher level of
susceptibility, which was associated with the downregulation of the
expression of target genes and reduced accumulation of intracellular
H2O2 (Li et al. 2017c).
The Osa-miR319 modulates the rice immune response againstM. oryzae in a negative manner. Significantly higher accumulation
of Osa-miR319 transcripts has been observed in the susceptible
genotype through the suppression of the target gene OsTCP21 uponM. oryzae infection. In turn, blocking the conversion of
a-linoleic acid (LnA) to hydroperoxyoctadecadienoic acid (HPODE) has
been shown to inhibit the jasmonic acid (JA) signaling pathway (Zhanget al. 2018b). Characterization of Osa-miR396 revealed its
negative role in defense response against M. oryzae by silencing
multiple OsGRF genes. Overexpression of Osa-miR396 leads
to stronger susceptibility to blast through the downregulation ofOsGRFs genes.Suppression of Osa-miR396 by expressing the
target disease mimic mutant of Osa-miR396 showed enhanced blast
resistance with improved yield (Chandran et al. 2019). TheOsa-miR439a negatively impacted the rice immunity by inhibiting
the defense- related genes and H2O2.
Suppression of the Osa-miR439a using target mimic mutants display
compromised susceptibility to M. oryzae by induction of
H2O2 (Junhua et al.2021).
The Osa-miR444b.2 was identified as a negative regulator of rice
immunity against M. oryzae . Expression of Osa-miR444b.2enhances the susceptibility to M. oryzae with little impact on
its target genes MADS27b and MADS57 . Altered expression of
target mimicry of Osa-miR444b.2 resulted in enhanced resistance
to blast (Xiao et al. 2017). The Osa-miR444a has been shown to
positively regulate immunity against the rice stripe virus (RSV)
by upregulating the OsRDR1 expression, which is facilitated by
the suppression of target MADS -box genes (Wang et
al. 2016). The Osa-miR1873 appeared to regulate resistance to
rice blast in a negative manner. Significant expression ofOsa-miR1873 has been shown to cause susceptibility through the
suppression of a target gene (LOC_Os05g01790 ) with yet to be
identified function. Overexpression of the target gene ofOsa-miR1873 enhances resistance to rice blast. Suppression of theOsa-miR1873 through the expression of the target mimicry mutant
of Osa-miR1873 has been shown to enhance defense response against
blast and fine-tunes rice immunity and plant growth (Zhou et al.2019b).