Phytochrome behaves as a dimer in vivo

Abstract It is well established that phytochrome exists as a dimer in vitro. A comparison of the relative photoequilibrium concentrations of P_rP_r, P_rP_fr and P_frP_fr, with the relative sizes of the P_fr-pools which undergo dark reversion in the intact plant, leads to the hypothesis that phytochrome also exists as a dimer in vivo, This hypothesis is in accordance with kinetic properties of the phytochrome system under continuous irradiation. Additional support for this view is provided by the observation that P_fr-destruction after a red light flash, which should favour the formation of P_rP_fr dimers, is paralleled by a decay of P_r, even if the presence of P_r cycled through P_fr can be excluded. Preliminary observations could indicate an interaction of the subunits of a phytochrome dimer during the process of phototransformation.