Biophoton imaging: a nondestructive method for assaying R gene responses.

Plant disease resistance (R) proteins of the nucleotide binding-leucine rich repeat class are responsible for pathogen recognition and activation of defense signaling networks leading to the hypersensitive response (HR). Genetically, R-protein signaling appears to be integrated through a limited set of common downstream components. However, the timing of development of visible HR is unique to individual R proteins. By utilizing the phenomena of ultraweak photon emission from leaves undergoing an incompatible interacttion, a powerful nondestructive and facile assay is described to compare timing of defense responses elicited by different R proteins. We demonstrate that ultraweak photon emission, or "biophoton generation," is demonstrated to be associated with hypersensitive cell death. Biophoton emission requires an intact R signaling network and increases in cytosolic calcium and nitric oxide, but elevated reactive oxygen species are not necessary. Importantly, the assay is robust and applicable to a range of incompatible interactions in various plant species. The ability to assay R responses nondestructively in real time and a chosen genetic background makes this technique amenable to subtle genetic dissection of plant defense responses.