XVI International Botanical Congess
The transport cells of phloem in dicotyledons developmentally accumulate a suite of proteins known as P-proteins. Of these, the two most abundant, PP1 and PP2, have been studied anatomically and biochemically. Recently, proteins identified as serine proteinase inhibitors have been isolated from pumpkin fruit exudates. Antibodies against the pumpkin fruit trypsin inhibitor (PFTI) were used to immunolocalize PFTI specifically to the phloem. PFTI occurs in the internal and external phloem as well as the extrafascicular phloem of pumpkin hypocotyls. It localizes to both sieve elements and companion cells. Developmentally, PFTI does not occur at early stages of hypocotyl development but is evident at later stages. This protein appears after accumulation of the other major P-proteins PP1 and PP2.