Studies on the phytochrome regulation of membrane permeability.

The plant photoreceptor, phytochrome was extracted from dark-grown cereal shoots, and partially purified. Its interactions with artificial membrane-vesicles (liposomes) were studied. Phytochrome was found not to bind to liposomes under a variety of conditions. It increased the permeability of liposomes to glucose only when applied in an impure form, or when incubated under continuous light. Red and far-red light treatments were not antagonistic, suggesting that the effect is not related to phytochrome-modulated photomorphogenesis. Using liposomes filled with the calcium-sensitive dye, murexide, an apparent increase in permeability to Ca++ was noted, on the addition of impure phytochrome. This was considered to be due to a contaminant in the phytochrome preparation, which caused fusion of the vesicles, thus affecting the turbidity of the liposome suspension. Pure phytochrome had no effect on the permeability of liposomes to Ca++. Substances which are known to enhance membrane permeability (termed membranophoretic agents) were found unable to simulate the red light-promotion of unrolling of barley leaf segments. This result failed to support the theory that Pfr brings about this response by increasing membrane permeability. Other reasons for the lack of response are discussed, however. It is concluded that phytochrome action may require a more complex membrane structure than the one provided by the artificial vesicles used in this study.