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The  phyB-401  mutant is 10 3  fold more sensitive to red light than its wild-type analogue and shows loss of photoreversibility of hypocotyl growth inhibition. The  phyB-401  photoreceptor displays normal spectral properties and shows almost no dark reversion when expressed in yeast cells. To gain insight into the molecular mechanism underlying this complex phenotype, we generated transgenic lines expressing the mutant and wild-type phyB in  phyB-9  background. Analysis of these transgenic lines demonstrated that the mutant photoreceptor displays a reduced rate of dark-reversion but normal P fr  to P r  photoconversion  in vivo  and shows an altered pattern of association/dissociation with nuclear bodies compared to wild-type phyB. In addition we show (i) an enhanced responsiveness to far-red light for hypocotyl growth inhibition and  CAB2  expression and (ii) that far-red light mediated photoreversibility of red light induced responses, including inhibition of hypocotyl growth, formation of nuclear bodies and induction of  CAB2  expression is reduced in these transgenic lines. We hypothesize that the incomplete photoreversibility of signalling is due to the fact that far-red light induced photoconversion of the chromophore is at least partially uncoupled from the P fr  to P r  conformation change of the protein. It follows that the  phyB-401  photoreceptor retains a P fr -like structure (P r   * ) for a few hours after the far-red light treatment. The greatly reduced rate of dark reversion and the formation of a biologically active P r   *  conformer satisfactorily explain the complex phenotype of the  phyB-401  mutant and suggest that amino acid residues surrounding the position 564 G play an important role in fine-tuning phyB signalling.

Altered Dark- and Photoconversion of Phytochrome B Mediate Extreme Light Sensitivity and Loss of Photoreversibility of the phyB-401 Mutant