Osa-miR439 Negatively Regulates Rice Immunity Against Magnaporthe oryzae

Osa-miR439 is a rice-specific microRNA family. Here we showed that Osa-miR439 acted as a negative regulator in rice immunity against blast fungus Magnaporthe oryzae. Osa-miR439 differentially responded to M. oryzae between susceptible and resistant rice accessions. The accumulation of Osa-miR439 was constitutively more in the susceptible accession than in the resistant one. Transgenic lines overexpressing Osa-miR439a (OX439a) showed higher susceptibility associating with lower induction of defense-related genes and less hydrogen peroxide (H2O2) accumulation at the infection sites than the control plants. In contrast, transgenic lines expressing a target mimic of Osa-miR439 (MIM439) displayed compromised susceptibility associating with increased H2O2 accumulation. Furthermore, we found that the expression of three predicted target genes was decreased in OX439a but increased in MIM439 in comparison to control plants, and this expression was differential in susceptible and resistant accessions upon M. oryzae infection, indicating that Osa-miR439a may regulate rice blast resistance via these genes. Our results unveiled the role of Osa-miR439a in rice blast resistance and provided the potentiality to improve the blast resistance via miRNA.