Provided is a component having plant defense activity. A PR1 gene expression activator containing a culture of a unicellular alga containing a green photosynthetic pigment as an active ingredient; A plant defense activator containing a culture of a unicellular alga containing a green photosynthetic pigment as an active ingredient; A plant disease preventive or ameliorating agent containing a culture of a unicellular alga containing a green photosynthetic pigment as an active ingredient; and a method of producing a plant defense activating component containing: cultivating a unicellular alga containing a green photosynthetic pigment under an aerobic condition to produce a culture containing an extracellular polysaccharide.