An in vitro association of soluble phytochrome with a partially purified organelle fraction from barley leaves.

Red light treatment in vitro increases the pelletability of phytochrome in homogenates of etiolated barley (Hordeum vulgare L. cv. 'Julia') leaves. When mixtures of soluble phytochrome (100,000 x g supernatant) and partially-purified organelles (Sephadex G-50 eluate) are irradiated the amount of pelletable phytochrome increases by a factor of two. Pre-irradiation treatments show that phytochrome in both components of the mixture must be in the Pfr form for increased pelletability to be observed. Once associated, photoreversion of Pfr to Pr does not result in decreased pelletability. The results are consistent with a non-artifactual in vitro association of soluble phytochrome to organelle membranes. One possible explanation is that Pfr molecules associate to form dimers.