MicroRNAs as positive regulators of host immune response
Several miRNAs including Osa-miR159, Osa-miR160, Osa-miR162a ,Osa-miR166 , Osa-miR398b , and Osa-miR7695 have been shown to function as positive regulators of defense response mechanisms in rice (Li et al. 2020). For instance, the Osa-miR159afine-tunes growth and immunity to M. oryzae by inhibiting its target genes OsGAMYBOsGAMYBL , and OsZF . Transgenic lines expressing a short tandem target mimic (STTM) to inhibit the Osa-miR159a showed enhanced susceptibility to this disease. Knock down mutation in the target genes confers resistance toM. oryzae . Altered expression of the Osa-miR159a led to defects in pollens and grain development (Chen et al. 2021). TheOsa-miR160 acts as a positive regulator of rice immunity againstM. oryzae. Transgenic plants overexpressing the Osa-miR160displayed significantly stronger resistance to the rice blast disease caused by M. oryzae . The Osa-miR160a enhances resistance to rice blast disease by suppressing the AUXIN RESPONSE FACTOR 16(ARF16 ), which in turn silence the indole-3-acetic acid (IAA) signaling (Li et al. 2014). It has been shown that theOsa-miR162a acts as a fine-tuner of host innate immunity againstM. oryzae while also involved in the regulation of yield potential. Overexpression of Osa-miR162a showed enhanced resistance to M. oryzae by positively regulating many other defense-related genes and by mediating the accumulation of intracellular hydrogen peroxide (H2O2). Suppression of the target gene Dicer-like 1 (OsDCL1 ) led to enhanced resistance to blast disease in rice (Li et al. 2020).
The miR166 belongs to the highly conserved family of miRNA molecules that targets the class III homeodomain-leucine zipper family (HD ZIP III ) of transcription factors that have been implicated in diverse cellular and physiological processes in plants (Zhu et al. 2011; Li, Lian, Zhao & He 2019c). In rice for example, overexpression of the polycistronic miRNA containing two family members,Osa-miR166k and Osa-miR166h ,has been shown to cause stronger immunity against M. oryzae and Fusarium fujikuroiby targeting the ethylene insensitive 2 (EIN 2 ) gene (Salvador-Guirao, Hsing & San Segundo 2018). The Osa-miR398bpositively regulates host immunity against the rice blast fungus by negatively regulating its target genes superoxide dismutase(SOD ) (Li et al. 2014). Mutation in the SOD genes showed upregulation of cellular H2O2production, which further leads to enhanced resistance to blast (Liet al. 2019d). A new miRNA Osa-miR7695that is likely specific to rice was identified as a positive regulator of rice immune response by mediating the trade-off between defense mechanisms and iron (Fe) homeostasis. Plants overexpressingOsa-miR7695 showed increased resistance to M. oryzae and stronger Fe accumulation at the site of infection. It was proposed that upon challenge with M. oryzae , the expression ofOsa-miR7695 suppresses the target gene Natural Resistance-Associated Macrophage Protein 6 (OsNramp 6 ), which encodes an iron transporter (Campo et al. 2013; Sánchez-Sanuyet al. 2019).