Inverse dark reversion of phytochrome: An explanation

Pea epicotyl tissue freeze dried with phytochrome (P) in the red absorbing (Pr) form, on exposure to red light does not form the far-red absorbing form of P (Pfr), but forms the intermediate P698 which reverts to Pr in darkness. Similar tissue containing the pigment as Pfr undergoes a photoreversible reaction on exposure to alternate red and far-red light. This represents the photoreversibility between Pfr and the intermediate P650. The difference spectrum of this reaction is similar to that of phytochrome photoconversion in dry cucumber seeds. P650 is shown to revert slowly to Pfr in darkness and it is proposed that this reaction accounts for the observation of apparent inverse dark reversion in cucumber seeds. Partial rehydration of freeze-dried tissue containing Pr, by means of 80% glycerol: 20% water (v/v), partially restores photoreversibility between Pr and Pfr. In such samples Pfr formation from intermediates, however, is slow and continues for several min in the dark after exposure to red light. This reaction can simulate the apparent inverse dark reversion observed in many seeds during early stages of imbibition. In this case Pfr appears from an intermediate produced by exposure to red light, which has not had time to form Pfr during the normal assay period. These two processes of Pfr production from intermediates in darkness, present under conditionsof partial or extreme dehydration, can explain previous experimental observations interpreted as inverse dark reversion.It is therefore proposed that the process, formerly described as 'inverse dark reversion', is not a transformation of Pr to Pfr and that there is not a separate form of phytochrome responsible for this reaction.