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).