Phytochrome and phosphotungstate-chromate-positive vesicles from Cucurbita pepo L.

The phosphotungstic acid-chromic acid (PTA-CrO3) stain, putatively specific for the plasma membrane of plants, has been used in an attempt to monitor the distribution of this membrane in a 20,000 x g particulate fraction from Cucurbita hypocotyl hooks. On discontinuous sucrose gradients, the relative distributions of the phytochrome and PTA-CrO3-positive vesicles present in this fraction appear to be correlated. When intact tissue is stained, however, other components, in addition to the plasma membrane, react positively to the stain. These components include prolamellar-body membranes, lipid droplets, and ribosomes. This lack of specificity calls into question the reliability of the technique for the unequivocal identification and accurate quantitation of plasma-membrane fragments in isolated particulate fractions. The present data do not, therefore, provide unambiguous evidence that phytochrome is associated with plasma membrane in tissue homogenates from Cucurbita.