A new type of mutation in the plant photoreceptor phytochrome B causes loss of photoreversibility and an extremely enhanced light sensitivity.

Photoreversibility, reversion of the inductive effect of a brief red light pulse by a subsequent far-red light pulse, is a property of photoresponses regulated by the plant photoreceptor phytochrome B (phyB). We screened for mutants with impaired photoreversibility to gain better insight into the phyB-specific signalling cascade. The phenotype of the mutant described is caused by a single amino acid exchange in a phyB subdomain that is highly conserved in all phytochromes but whose functional significance was unknown. The mutated phyB showed a slower dark reversion but no major alterations in its spectral properties. In addition to its loss of photoreversibility, the mutant also exhibited a hypersensitivity towards continuous red-light irradiation and an altered phenotype of adult plants under short-day conditions.