RGD-MEDIATED MEMBRANE-MATRIX ADHESION TRIGGERS AGAROSE-INDUCED EMBRYOID FORMATION IN SUNFLOWER PROTOPLASTS

Agarose embedding of sunflower (Helianthus annuus L.) hypocotyl protoplasts induces an asymmetric division pattern and subsequent polarized development leading to embryoid formation. We cultured protoplasts in media with different mannitol concentrations. Induction of plasmolysis of agarose-embedded protoplasts by increasing the mannitol concentration lowered the proportion of embryoids formed. This indicates that adhesion sites between the plasma membrane and the agarose matrix are involved in embryoid formation. The involvement of such adhesion sites was tested by incubating embedded protoplasts with RGD peptide. 1 μM RGD heptapeptide reduced embryoid formation by 50% as compared to the control DGR peptide. We also showed that RGD heptapeptide acts on the cytoskeleton by disrupting cortical microtubules. The results are discussed in terms of a model in which the anchorage of the protoplast membrane to the agarose matrix is mediated by RGD-binding proteins connected with microtubules, determining asymmetric division of the cell and polarized development.