Summary mi RNA s contribute to plant resistance against pathogens. Previously, we found that the function of miR398b in immunity in rice differs from that in Arabidopsis. However, the underlying mechanisms are unclear. In this study, we characterized the mutants of miR398b target genes and demonstrated that multiple superoxide dismutase genes contribute to miR398b‐regulated rice immunity against the blast fungus Magnaporthe oryzae . Out of the four target genes of miR398b, mutations in Cu/Zn‐Superoxidase Dismutase1 ( CSD 1 ), CSD 2 and Os11g09780 ( Superoxide DismutaseX , SODX ) led to enhanced resistance to M. oryzae and increased hydrogen peroxide (H 2 O 2 ) accumulation. By contrast, mutations in Copper Chaperone for Superoxide Dismutase ( CCSD ) resulted in enhanced susceptibility. Biochemical studies revealed that csd1 , csd2 and sodx displayed altered expression of CSD s and other superoxide dismutase ( SOD ) family members, leading to increased total SOD enzyme activity that positively contributed to higher H 2 O 2 production. By contrast, the ccsd mutant showed CSD protein deletion, resulting in decreased CSD and total SOD enzyme activity. Our results demonstrate the roles of different SOD s in miR398b‐regulated resistance to rice blast disease, and uncover an integrative regulatory network in which miR398b boosts total SOD activity to upregulate H 2 O 2 concentration and thereby improve disease resistance.

Fitness cost is a common phenomenon in rice blast disease-resistance breeding. MiR396 is a highly conserved microRNA (miRNA) family targeting Growth Regulating Factor (OsGRF) genes. Mutation at the target site of miR396 in certain OsGRF gene or blocking miR396 expression leads to increased grain yield. Here we demonstrated that fitness cost can be trade-off in miR396-OsGRFs module via balancing growth and immunity against the blast fungus. The accumulation of miR396 isoforms was significantly increased in a susceptible accession, but fluctuated in a resistant accession upon infection of Magnaporthe oryzae. The transgenic lines over-expressing different miR396 isoforms were highly susceptible to M. oryzae. In contrast, overexpressing target mimicry of miR396 to block its function led to enhanced resistance to M. oryzae in addition to improved yield traits. Moreover, transgenic plants overexpressing OsGRF6, OsGRF7, OsGRF8, and OsGRF9 exhibited enhanced resistance to M. oryzae, but showed different alteration of growth. While overexpression of OsGRF7 led to defects in growth, overexpression of OsGRF6, OsGRF8, and OsGRF9 resulted in better or no significant change of yield traits. Collectively, our results indicate that miR396 negatively regulates rice blast disease- resistance via suppressing multiple OsGRFs, which in turn differentially control growth and yield. Therefore, miR396-OsGRFs could be a potential module to demolish fitness cost in rice blast disease-resistance breeding.

MicroRNAs (miRNAs) are known to fine‐tune growth, development, and stress‐induced responses. Osa‐miR1873 is a rice‐specific miRNA targeting LOC_Os05g01790. Here, we show that Osa‐miR1873 fine‐tunes rice immunity against Magnaporthe oryzae and yield traits via LOC_Os05g01790. Osa‐miR1873 was significantly upregulated in a susceptible accession but downregulated in a resistance accession at 24 h post‐inoculation (hpi) of M. oryzae. Overexpressing Osa‐miR1873 enhanced susceptibility to M. oryzae and compromised induction of defense responses. In contrast, blocking Osa‐miR1873 through target mimicry compromised susceptibility to M. oryzae and enhanced induction of defense responses. Altered expression of Osa‐miR1873 also resulted in some defects in yield traits, including grain numbers and seed setting rate. Moreover, overexpression of the target gene LOC_Os05g01790 increased rice blast disease resistance but severely penalized growth and yield. Taken together, we demonstrate that Osa‐miR1873 fine‐tunes the rice immunity‐growth trade‐off via LOC_Os05g01790, and blocking Osa‐miR1873 could improve blast disease resistance without significant yield penalty. Thus, the Osa‐miR1873‐LOC_Os05g01790 regulatory module is valuable in balancing yield traits and blast resistance.