The mature, functional sieve-tube system in higher plants is dependent upon protein import from the companion cells to maintain a functional long-distance transport system. Soluble proteins present within the sieve-tube lumen were investigated by analysis of sievetube exudates which revealed the presence of distinct sets of polypeptides in seven monocotyledonous and dicotyledonous plant species. Antibodies directed against sieve-tube exudate proteins from Ricinus communis L. demonstrated the presence of shared antigens in the phloem sap collected from Triticum aestivum L., Oryza sativa L., Yucca ®lamentosa L., Cucurbita maxima Duch., Robinia pseudoacacia L. and Tilia platyphyllos L. Speci®c antibodies were employed to identify major polypeptides. Molecular chaperones related to Rubiscosubunit-binding protein and cyclophilin, as well as ubiquitin and the redox proteins, thioredoxin h and glutaredoxin, were detected in the sieve-tube exudate of all species examined. Actin and pro®lin, a modulator of actin polymerization, were also present in all analyzed phloem exudates. However, some proteins were highly species-speci®c, e.g. cystatin, a protease-inhibitor was present in R. communis but was not detected in exudates from other species, and orthologs of the well-known squash phloem lectin, phloem protein 2, were only identi®ed in the sieve-tube exudate of R. communis and R. pseudoacacia. These ®ndings are discussed in terms of the likely roles played by phloem proteins in the maintenance and function of the enucleate sieve-tube system of higher plants.
Abstract. The cut hypocotyl of Ricinus communis L.seedlings exudes phloem sap which contains a characteristic set of proteins (Sakuth et al. 1993, Planta 191, 207-213). These sieve-tube exudate proteins were probed with antibodies to highly conserved proteins, namely ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), Rubisco-subunit-binding protein, heat-shock protein (HSP 70), chaperonin GroEL and ubiquitin. Homologous proteins in the sieve-tube exudate were identified with antisera to HSP 70, Rubisco-subunit-binding protein and ubiquitin. Ribulose-l,5-bisphosphate carboxylase-oxygenase, which was present in the tissue, was not detected. Of all the cross-reactive proteins detected, ubiquitin was special because the ubiquitin-to-protein ratio in the sieve-tube exudate was higher than in both the surrounding hypocotyl and in the cotyledonary tissues. Therefore, ubiquitin features properties which favour its transfer into the sieve tubes and which might rely on efficient transport through plasmodesmata. It is assumed that chaperones and ubiquitin are needed for the maintenance of sieve-tube function, e.g. to ensure correct folding of proteins. Their possible involvement in protein translocation through plasmodesmata from companion cells to sieve tubes is discussed.
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