Synthesis of P-protein in mature phloem of Cucurbita maxima

Abstract

Cotyledons of Cucurbita maxima Duch. seedlings were provided with (14)C-labeled amino acids for 12 h. Besides the bulk of labeled amino acids the sieve-tube exudate also carried labeled proteins. 80% of the incorporated radioactivity was found in the P-protein, 20% in a neutral protein, and traces were found in acidic proteins after fractionation on diethyl-aminoethyl cellulose columns. The radioactive elutes were characterized by autoradiographs of both disc- and sodium dodecyl sulfate-gelelectropherograms, and by isoelectric focusing. The P-protein fraction appeared with the void volume from the diethylaminoethyl-cellulose column. Obviously, this is the protein that gels when oxidized and that is reversibly precipitable giving rise to filaments when processed for electron microscopy. Its main component has a molecular weight of 115,000 Dalton. By isoelectric focusing this fraction separated into 3 proteins with isoelectric points of 9.8, 9.4, and 9.2. The isoelectric point 9.2-protein probably is identical with an oligomer of a 30,000 Dalton protein with neutral isoelectric point, which keeps 20% of the incorporated label. Microautoradiographs suggest that the labeled proteins were synthesized in companion cells. The results indicate that P-protein of Cucurbita maxima is synthesized continuously in mature phloem. It can be assumed that P-protein has a relatively high turn-over rate. Therefore it seems unlikely that P-protein is a "structural" protein.

Specific proteins in the sieve-tube exudate of Ricinus commuais L. seedlings: separation, characterization and in-vivo labelling

“…Labelling studies with Vicia faba had already shown that newly synthesized proteins were present in the sieve tubes without concomitant RNA synthesis (Neumann and Wollgiehn 1964). Microautoradiographic studies on the synthesis of Pproteins in Cucurbita maxima demonstrated preferential labelling of proteins in the companion cells (Nuske and Eschrich 1976). If the proteins present within the sieve tubes are synthesized in the companion cells, a mechanism must exist for the transfer of proteins into the sieve tubes.…”

Sieve element occlusion(SEO) genes encode structural phloem proteins involved in wound sealing of the phloem

“…Cucurbits have long served as model plants in phloem research because their large phloem cells are well suited for microscopy and because they bleed profusely after injury, allowing the direct harvest of phloem exudates (51). Electron microscopy revealed the presence of P-protein filaments in C. maxima exudates (52); these filaments were assumed to be composed of PP1, although the ability of the protein to form stable structural components in vivo was debated because of its high turnover rate (53). PP1 is expressed initially in companion cells and then is transported into sieve elements, presumably via pore-plasmodesma units (19).…”

Translocation of Structural P Proteins in the Phloem

“…However, P protein mRNA also can be detected readily in companion cells of mature sieve element-companion cell complexes, especially in the extrafascicular phloem (Bostwick et al, 1992 , 1997). Furthermore, microautoradiography of in vivo-labeled proteins led Nuske and Eschrich (1976) to conclude that P proteins are synthesized continually in the companion cells of mature metaphloem. In bundle phloem, in situ hybridization revealed that PP2 mRNA was easier to detect in early stages of primary growth than later, suggesting that P protein synthesis in the bundle phloem was of limited duration after the onset of translocation .…”