Translocation of structural P proteins in the phloem

Phloem-specific proteins (P proteins) are particularly useful markers to in vestigate long-distance trafficking of macromolecules in plants. In this st udy, genus-specific molecular probes were used in combination with intergen eric grafts to reveal the presence of a pool of translocatable P protein su bunits. Immunoblot analyses demonstrated that Cucurbits spp P proteins PP1 and PP2 are translocated from Cucurbita maxims stocks and accumulate in Cuc umis sativus scions. Cucurbita maxims or Cucurbita ficifolia PP1 and PP2 mR NAs were not detected in Cucumis sativus scions by either RNA gel blot anal ysis or reverse transcription-polymerase chain reaction, indicating that th e proteins, rather than transcripts, are translocated. Tissue prints of the Cucumis sativus scion, using antibodies raised against Cucurbita maxims PP 1 or PP2, detected both proteins in the fascicular phloem of the stem at po ints distal to the graft union and in the petiole of a developing leaf, sug gesting that the proteins move within the assimilate stream toward sink tis sues. Cucurbita maxims PP1 was immunolocalized by light microscopy in sieve elements of the extrafascicular phloem of Cucumis sativus scions, whereas Cucurbita maxims PP2 was detected in both sieve elements and companion cell s.