SummaryThe heptapeptide Tyr-Gly-Arg-Gly-Asp-Ser-Pro containing the sequence Arg-Gly-Asp (RGD -the essential structure recognised by animal cells in substrate adhesion molecules) was tested on epidermal cells of onion and cultured cells of Arabidopsis upon plasmolysis. Dramatic changes were observed on both types of cells following treatment: on onion cells, Hechtian strands linking the cell wall to the membrane were lost, while Arabidopsis cells changed from concave to convex plasmolysis. A control heptapeptide Tyr-Gly-Asp-Gly-Arg-Ser-Pro had no effect on the shape of plasmolysed cells. Protoplasts isolated from Arabidopsis cells agglutinate in the presence of ProNectinF, a genetically engineered protein of 72 kDa containing 13 RGD sequences: several protoplasts may adhere to a single molecule of ProNectinF. The addition of the RGDheptapeptide disrupted the adhesion between the protoplasts. Purified plasma membrane from Arabidopsis cells exhibits specific binding sites for the iodinated RGDheptapeptide. The binding is saturable, reversible, and two types of high affinity sites (K d1 ≈ 1 nM, and K d2 ≈ 40 nM) can be discerned. Competitive inhibition by several structurally related peptides and proteins noted the specific requirement for the RGD sequence. Thus, the RGD-binding activity of Arabidopsis fulfils the adhesion features of integrins, i.e. peptide specificity, subcellular location, and involvement in plasma membrane-cell wall attachments.

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On addition of inositol trisphosphate, intact vacuoles isolated from Acer pseudoplatanus cell suspension cultures release part of their calcium content. The process was specific, dose-dependent (IC,, = 0.2 PM) and was inhibited by an intracellular calcium antagonist. The calcium efflux elicited by inositol trisphosphate increased with the age of the cell suspension cultures, the maximum effect being obtained when the cultures reached the stationary phase. It is suggested that vacuoles play a role as an endocellular calcium store that is responsive to inositol trisphosphate in plants.