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.

Osa-miR439是水稻特有的microRNA家族。在这里我们显示Osa-miR439在水稻对稻瘟病菌Magnaporthe oryzae的免疫中起负调节作用。Osa-miR439对敏感和抗性水稻品种对米曲霉的反应不同。Osa-miR439的积累在易感种质上比在抗性种质上的积累更多。过表达Osa-miR439a（OX439a）的转基因品系显示较高的敏感性，与防御相关基因的诱导程度较低和过氧化氢（H ₂ O ₂）含量较低相关）在感染部位的积累要比对照植物多。相反，表达Osa-miR439（MIM439）靶标模拟物的转基因品系显示出易感性与H ₂ O ₂积累增加有关。此外，我们发现与对照植物相比，OX439a中三个预测靶基因的表达降低，而MIM439中三个预测靶基因的表达增加，并且此表达在米曲霉感染后的易感和抗性材料中有所差异，表明Osa-miR439a可能调控水稻通过这些基因产生抗药性。我们的结果揭示了Osa-miR439a在水稻稻瘟病抗性中的作用，并提供了通过miRNA改善稻瘟病抗性的潜力。