Protein filaments-structural components of the phloem exudate : I. Observations with Cucurbita and Nicotiana

Fine structure and chemical composition of the phloem exudate of Cucurbita maxima and Nicotaiana glauca x suaveolens are investigated. Filamentous structures, several microns in length, are identified as structural components of the exudate by means of negative staining and electron microscopy. Two types of filaments are described: one form measures nearly 40 Å in diameter and shows a beaded appearance with regular spacings of about 50 Å; it is termed "elementary filament". The second form has a diameter of about 90 Å and presumably consists of two helically arranged 40 Å subunits.The proteinaceous nature of the filaments is indicated by chemical analysis. The main macromolecular component of the exudate is demonstrated to be protein. Only traces of nucleic acids are detectable, lipids and polysaccharides cannot be found. The identity of the protein filaments with the filamentous structures ("slime", "P-protein"), as revealed in thin sections of mature sieve tubes, is discussed.