The FmMYC2-FmLOX6 module promotes pollen germination and tube growth via regulating JA homeostasis in Fraxinus mandshurica.

key messageJasmonic acid (JA) is directly involved in a transcriptional activation loop where the JA signaling transcription factor, FmMYC2 (a bHLH protein), activates the biosynthesis gene FmLOX6 to promote pollen adhesion, germination, and subsequent pollen tube growth. Jasmonic acid (JA) plays a critical role in plant reproduction; yet, its functions during pollen-stigma interaction remain largely unexplored. Transcriptomic analysis of pollinated stigmas in Manchurian ash (Fraxinus mandshurica) revealed that differentially expressed genes (DEGs) were significantly enriched in the JA biosynthesis pathway. Exogenous JA application to stigmas promoted pollen adhesion, germination, and tube growth, whereas inhibition of JA biosynthesis or JA transport in stigmas suppressed these processes. Similarly, in in vitro pollen germination assays, exogenous JA treatment enhanced pollen germination and tube elongation, while inhibition of endogenous JA biosynthesis in pollen reduced both parameters. Combined treatments with JA and transport inhibitors confirmed that functional JA transport is required for pollen germination. Further transcriptomic analysis showed that both FmMYC2 a basic helix-loop-helix (bHLH) transcription factor in the JA signaling network and FmLOX6 a lipoxygenase involved in JA biosynthesis were significantly upregulated during pollen germination. Antisense oligodeoxynucleotide-mediated repression of either FmMYC2 or FmLOX6 inhibited pollen tube growth. Yeast one-hybrid and dual-luciferase reporter assays demonstrated that FmMYC2 directly activates FmLOX6 expression by binding to a G-box motif in its promoter. Our findings reveal a positive feedback loop in which JA signaling via FmMYC2 promotes JA biosynthesis through FmLOX6, thereby enhancing pollen germination and tube growth in F. mandshurica. This study offers new insights into JA-mediated pollination mechanisms in tree species and provides fundamental information for breeding improved varieties.