Horseradish peroxidase (HRP) is the enzyme commonly used in the ELISA test for plants due to its high catalytic activity, versatility with substrates, and stability, which enhances the sensitivity and accuracy of the assay in detecting viral pathogens and other targets in plant samples. Horseradish peroxidase is an enzyme that comes from horseradish roots. In an ELISA setup, HRP is conjugated (linked) to a secondary antibody that specifically binds to the target antibody (which in turn binds to the virus or antigen of interest).