A phloem-specific, lectin-like protein is located in pine sieve-element plastids by immunocytochemistry.

Two co-purifying phloem polypeptides of 24 and 25 kilodaltons (kDa) were isolated from homogenates of Pinus sabiniana Dougl. phloem by differential centrifugation, selective solubilization and electrophoresis, and rabbit antibodies raised against them. The antisera were found to be specific for doublet bands between 23 and 25 kDa in Western blots of whole phloem extracts of Pinus species; no xylem polypeptides were labelled, nor did labelling occur in blots of phloem extracts from other genera in the Pinaceae. Solubilized phloem polypeptides bind strongly to chitin (oligomeric N-acetylglucosamine) columns and are sensitive to thiol reagents, both characteristics which relate them to phloemspecific lectins isolated from angiosperm species (C. Allen, 1979, Biochem. J. 183, 133-137; A.K. Gietl et al., 1979, Planta 144, 367-371). Fluorescence microscopy and immuno-gold electron microscopic cytochemistry demonstrated antigenic sites specifically associated with protein crystals peculiar to the sieve-element plastids of the Pinaceae.