Sieve-tube exudate from Ricinus communis L. seedlings contains ubiquitin and chaperones

Abstract

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.

The movement protein NSm of tomato spotted wilt tospovirus (TSWV): RNA binding, interaction with the TSWV N protein, and identification of interacting plant proteins

“…Besides a direct association between a viral movement protein and cytoskeleton, which is shown for the TMV P30 protein (52,64), an indirect contact via a Hsp-70 protein, either as an intrinsic viral component or recruited by viral structures, is also conceivable. The fact that Hsp-70 has been detected in phloem exudates (65) indicates that it might have some intrinsic properties that facilitate its intercellular transport through plasmodesmata. This leads to speculation that the protein itself may be involved in transport processes.…”

Salicylic Acid, an Ambimobile Molecule Exhibiting a High Ability to Accumulate in the Phloem

“…The Ricinus system is a biological model widely employed to study the phloem uptake of nutrients Orlich and Komor, 1992;Zhong et al, 1998) to identify endogenous molecules moving in sieve tubes (Schobert et al, 1995;Antognoni et al, 1998) and to evaluate phloem systemicity of xenobiotics (Bromilow et al, 1987;Delétage-Grandon et al, 2001). Because the castor bean is a symplastic-apoplastic loader (Orlich and Komor, 1992), exogenous SA molecules found in the phloem sap may be taken up from the phloem apoplast or may come, via the symplastic route, from other cells and, especially, from the cotyledon epidermis.…”

The Phloem Sieve Element: A River Runs through It.

“…By contrast, proteolysis in differentiating sieve elements is amazingly selective and is almost "surgical" in its precision. Although ubiquitin has been detected in STEPs from R. communis (Schobert et al, 1995), and polyubiquitin was encoded by clones obtained from a cDNA library developed from the phloem of pine (Alosi-Carter et al, 1995), the role of ubiquitin in sieve element differentiation remains to be established. Although protein synthesis and trafficking from companion cells are important, it is also important to recognize that sieve element differentiation takes place when the cell is nucleated and while it still has ribosomes.…”