Biophoton distress flares signal the onset of the hypersensitive reaction.

Detection of biophoton emission, a natural bioluminescence, has emerged as a non-destructive method to mark the onset of the hypersensitive resistance reaction in Arabidopsis, bean and tomato. Rapid biophoton emission in Arabidopsis requires an intact R-gene signalling network and increased levels of cytosolic calcium and nitric oxide. The burst of biophotons precedes macroscopic symptoms by several hours and its timing is characteristic for specific gene-for-gene interactions. The ability to monitor biophoton emission from whole plants in real time should allow detailed dissection of plant defence responses.