The OsmiR396–OsGRF8–OsF3H‐flavonoid pathway mediates resistance to the brown planthopper in rice (Oryza sativa)

Abstract

Summary Multi‐functional micro RNA s (mi RNA s) are emerging as key modulators of plant–pathogen interactions. Although the involvement of some mi RNA s in plant–insect interactions has been revealed, the underlying mechanisms are still elusive. The brown planthopper ( BPH ) is the most notorious rice ( Oryza sativa )‐specific insect that causes severe yield losses each year and requires urgent biological control. To reveal the mi RNA s involved in rice– BPH interactions, we performed mi RNA sequencing and identified BPH ‐responsive OsmiR396. Sequestering OsmiR396 by overexpressing target mimicry ( MIM 396) in three genetic backgrounds indicated that OsmiR396 negatively regulated BPH resistance. Overexpression of one BPH ‐responsive target gene of OsmiR396, growth regulating factor 8 ( Os GRF 8 ), showed resistance to BPH . Furthermore, the flavonoid contents increased in both the OsmiR396‐sequestered and the Os GRF 8 overexpressing plants. By analysing 39 natural rice varieties, the elevated flavonoid contents were found to correlate with enhanced BPH resistance. Artificial applications of flavonoids to wild type ( WT ) plants also increased resistance to BPH . A BPH ‐responsive flavanone 3‐hydroxylase ( OsF3H ) gene in the flavonoid biosynthetic pathway was proved to be directly regulated by Os GRF 8. A genetic functional analysis of OsF3H revealed its positive role in mediating both the flavonoid contents and BPH resistance. And analysis of the genetic correlation between OsmiR396 and OsF3H showed that down‐regulation of OsF3H complemented the BPH resistance characteristic and simultaneously decreased the flavonoid contents of the MIM 396 plants. Thus, we revealed a new BPH resistance mechanism mediated by the OsmiR396–Os GRF 8–OsF3H–flavonoid pathway. Our study suggests potential applications of mi RNA s in BPH resistance breeding.

Current understanding of the molecular players involved in resistance to rice planthoppers

“…Plants may use flavonoids to respond to insects. Flavonoid contents were positively correlated with BPH resistance . Another multi‐functional miRNA, OsmiR396, targets growth regulating factor 8 ( OsGRF8 ) and directly activates the BPH‐responsive flavanone 3‐hydroxylase ( OsF3H ) gene in the flavonoid biosynthetic pathway .…”

“…The OsmiR396–OsGRF8–OsF3H–flavonoid pathway has been identified to mediate resistance to BPH. OsmiR396 cleaves OsGRF8 transcripts and represses their expression; the downstream flavonoid biogenesis is thereafter suppressed, leading to rice vulnerability to BPH attack …”

“…The metabolic cost of producing secondary metabolites may result in low rice yield. Therefore, a major challenge for the practical utilization of vital genes in the biosynthetic pathways of these secondary metabolites is to anticipate that the allocation of defensive metabolites for resistance will have no negative impact on desirable traits such as seed production and quality, as is the case for OsmiR396 . The ideal would be to find a molecular player that mediates anti‐planthopper innate immunity in rice but also enables the rice plants to maintain normal growth and grain yield, as is the case for Bph6 , Bph9 and Bph14 …”

“…48 Another multi-functional miRNA, OsmiR396, targets growth regulating factor 8 (OsGRF8) and directly activates the BPH-responsive flavanone 3-hydroxylase (OsF3H) gene in the flavonoid biosynthetic pathway. 48 OsmiR396-sequestered transgenic rice plants with increased flavonoid contents, enlarged grains and improved seed quality exhibit enhanced resistance to BPH. Moreover, over-expression of OsGRF8 increased flavonoid content and showed resistance to BPH.…”

“…Therefore, a major challenge for the practical utilization of vital genes in the biosynthetic pathways of these secondary metabolites is to anticipate that the allocation of defensive metabolites for resistance will have no negative impact on desirable traits such as seed production and quality, as is the case for OsmiR396. 48 The ideal would be to find a molecular player that mediates anti-planthopper innate immunity in rice but also enables the rice plants to maintain normal growth and grain yield, as is the case for Bph6, 31 Bph9 32 and Bph14. 36 Over recent decades, to promote sustainable agriculture, biotechnological approaches to controlling the planthopper outbreaks have moved from using chemical pesticides to exploring the natural defenses of the host.…”