On Earth, much life fluoresces, dimly, but to a degree that can be detected given sufficient comparison and filtering and, in fact, this is currently used as a method of detecting from space how much vegetation is present in a region of the Earth's surface. Certain proteins (Fluorescent Proteins) specific to life absorb an Ultraviolet (UV) photon and, in turn, emit longer Wavelength photons to lower the resulting raised State of Excitation. These proteins are considered to have evolved to protect vital life molecules such as DNA from damage due to Photoionization from the UV photons (Fluorescence Protection).
Many exoplanet scenarios would experience UV in similar or greater amounts than Earth receives at its surface, so many possible forms of life would be rewarded for evolving similar mechanisms, which in turn, would offer a possible means of detecting the life from a distance. If likely wavelengths of the emitted light can be identified, an otherwise-unexplained presence of those wavelengths in the glow of a planet would constitute a biosignature. Also, for stars with Stellar Flares that produce additional UV, a simultaneous increase in such wavelengths would constitute such a signature.
Vegetation Red Edge (VRE)