Binding of phytochrome to plasma membranes in vivo

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Abstract
A microsomal fraction was prepared by differential centrifugation from the homogenate of dark grown shoots of oats (Avena sativa L. cv. Sol II). Plasma membranes were prepared from the microsomal fraction by means of an aqueous polymer two phase partition method. The content of phytochrome in the microsomal fraction and the plasma membrane fraction, respectively, were studied after different irradiation treatments of the intact shoots. Red irradiation increased the content of phytochrome in both the microsomal and plasma membrane fraction, especially in the presence of Mg2+. The increase induced by red light was fully reversible by far‐red light for the plasma membrane fraction both in the presence and absence of Mg2+, in contrast to the microsomal fraction where Mg2+ had to be omitted. KI treatment of the membranes destroyed the binding of phytochrome whereas agents such as KCI, EDTA, CaCl2 and Triton X‐100, did not have this effect, indicating that the phytochrome attachment to the membrane is hydrophobic. This in vivo binding resembles to a large extent the one obtained in vitro by Sundqvist and Widell (Physiol. Plant. 59: 35–41, 1983) even though some differences between the phytochrome species and the membrane side exposed probably occur; so that the present interaction between phytochrome and the plasma membrane does not necessarily reflect the interaction that leads to physiological responses, and there could be more than one type of interaction.