Biochemical and historical studies on the contents of sieve tubes with special reference to the slime substance

Abstract

Correlated studies have been made on the sieve tube contents of Heracleum, and in some cases Tetragonia and Lycopersicum using electron microscopy, and analytical techniques on extracted materials. The latter have provided information on the physicochemical properties of P-protein, and on the organic and inorganic ions of the phloemThe studies on the ultrastructure of sieve elements of Totragonia, Hcracleum and Lycopersicum showednearly all the sieve plate pores plugged with a fibrillarAmaterial. Mitochondria were less scarcely seen in the sieve element of Heracleum. The sieve tube slime of Tetragonia and Lvconersicum petiolar phloem was subjected to proteolytic enzymes both before and after embedding and sectioning; electron microscopical examination confirmed that the material is protein in nature and provided evidence that the protein is acidic rather than basic. Histochemical experiments showed that neither ATP-ase nor alkaline phosphatase was associated with P-protein in the sieve element of Tetragonia; indications of some acid phosphatase activity on F-protein are probably spurious. These enzymes appeared to be mainly confined to the plasmalemma region. The absence of enzymes associated with slime favours the view that it is a protein of structural type. A filamentous protein was isolated from the phloem tissue of Heracleum. Two methods (sequential fractionation and gradient elution chromatography) were successfully employed to isolate this fibrillar material. Biochemical studies suggest that this protein has the requirements to be classed as a glycoprotein. The relative abundance, solubility properties andappearance under negative staining in the electron microscope suggested that the material was P-protein. Isoelectric focusing in a sucrose gradient and in polyacrylamide gel indicated that the isoelectric point was about 4.9. Potassium was present in high concentra tions in the phloem extracts of Hcraoleum. Organic and inorganic acids appeared to be insufficient to supply the anionic equivalence to K' in the sieve element. It is suggested very tentatively that perhaps the anionic balance is made up by the P-protein.Of the six sugars identified sucrose seemed to be tho main carbohydrate component in the phloem extract of Heraoleum. Thirteen amino acids were also identified in it. Among the other constituents six phenolic and tannin components were detected. The findings of this study lend suoport to a active transport mechanism dependent, like electroosmosis, on a structural rather than an enzymaiic interpretation of the P-protein.