Dark Transformations of Phytochrome in vivo. II

Abstract

Previous work (4) showed that the phytochrome in dark-grown seedlings was present entirely as the red-absorbing form, PR. If the seedlings were irradiated with red light and returned to darkness, the PFR formed by the red light, decayed over the course of several hours. After the decay reactions had gone to completion, approximately 20 % of the original amount of phytochrome remained and it was again all PR. It was assumed that the initial irradiation with red light converted essentially all of the phytochrome to PFR and that during the dark period approximately 20 % of the PFR reverted to PR, while 80 % of the PFR was either destroyed or altered so that it was no longer photoreversible. The dark conversion of PFR to PR had been indicated earlier by physiological experiments and had been implicated in the timing mechanism of photoperiodism. More recent work on purified solutions of phytochrome showed that red light does not convert all of the PR to PFR (3, 10). PFR absorbs appreciably in the red region of the spectrum so that red light establishes a photostationary state. This was confirmed in experiments on the denaturation of phytochrome with parachloromercuribenzoate (5) which destroyed the absorbance of PpR but had little effect on the absorbance of PR. A calculation, based on absolute absorption spectra and kinetic data on the rate of conversion of PR and PFR, showed that the photostationary state in red light consisted of 81 % PFR and 19 % PR (3). The fact that about 20 % of the PR was not converted by irradiation with red light alters the previous conclusion that 20 % of the PFR reverted to PR in the dark. Most of the PR which was found in the seedlings several hours after the brief irradiation was due to the PR which was not converted initially. The decay of PFR in the dark-grown seedlings was due mainly to the destruction of PFR; little of the PFR reverted to PR. The time course for the apparent conversion of PFR to PR (4) was that of the destruction of PFR. As PFR disappeared, an irradiation with red light would reestablish the photostationary state and show the