GmMMP9 confers resistance to soybean mosaic virus via interactions with SMV-6K1 and GmPsaN to induce a reactive oxygen species response

Soybean mosaic virus (SMV) is a widespread disease that significantly affects the yield and quality of soybean (Glycine max), but our understanding of the molecular mechanisms of resistance to SMV remains limited. In this study, we characterized 27 soybean matrix metalloproteinase (MMP) family members and we report the function and regulatory mechanisms of GmMMP9 in resistance. Soybean plants overexpressing MMP9 exhibited enhanced resistance to SMV, while CRISPR/Cas9-mediated MMP9 homozygous mutants were more susceptible. Staining with DAB and Trypan Blue revealed that MMP9 conferred SMV resistance through production of reactive oxygen species (ROS) and programmed cell death triggered by the hypersensitive response. Furthermore, we determined that MMP9 was involved in endoplasmic reticulum stress induced by SMV via its interaction with the SMV protein 6K1 (SMV-6K1). This interaction was abolished only when all four 6K1-interacting residues in MMP9 were mutated, suggesting that it inhibited SMV-6K1 function via a structural envelopment mechanism. RNA-sequencing analysis of MMP9-overexpressing, knockout, and wild type plants infected with SMV revealed the presence of several differentially expressed genes that were involved in photosynthesis-related processes. In addition, we found that a PSI reaction center subunit N, PsaN, interacted with MMP9. Silencing PsaN in MMP9-overexpressing lines, knockout lines, and wild type plants led to reductions in chlorophyll content, superoxide dismutase activity, H2O2 levels, and the transcriptional expression of ROS-signaling genes, demonstrating that the MMP9-PsaN interaction affected PSI activity and thereby triggered a ROS response. Our findings show that MMP9 is a novel regulator of soybean resistance to SMV, and suggest its potential use for soybean molecular breeding.