Functional specialization of two jasmonic acid-amido synthetases in rice defense and spikelet development

The phytohormone jasmonic acid (JA) regulates diverse aspects of plant growth, development, and defense, yet how a single hormone coordinates such varied outputs remains poorly understood. Central to jasmonate signaling is the biosynthesis and perception of bioactive JA-amino acid conjugates, such as jasmonoyl-L-isoleucine (JA-Ile), which are synthesized by JASMONATE RESISTANT (JAR) enzymes. Three OsJAR genes are present in the monocot crop rice. Here, we systematically dissect their functions using a complete set of CRISPR-Cas9-derived mutants. OsJAR1 and OsJAR2 redundantly maintain basal JA-Ile levels required for normal vegetative growth, whereas OsJAR3 is functionally attenuated due to low expression and weak enzymatic activity. Notably, OsJAR1 specifically mediates herbivore-induced JA-Ile biosynthesis and confers resistance to the major rice pest, the brown planthopper, under both laboratory and field conditions, whereas OsJAR2 is dispensable for defense. The expression of OsJAR1 is directly controlled by the MYC2-bHLH6 transcriptional cascade, forming a positive feedback loop that sustains JA signaling during herbivory. By contrast, OsJAR2, but not OsJAR1, is expressed during early inflorescence differentiation and contributes to JA-Ile accumulation at this stage, ensuring normal spikelet development. These findings reveal that functional specialization of OsJAR enzymes enables rice to precisely tailor JA responses to distinct developmental and environmental cues.