CA2+, ANNEXINS, AND GTP MODULATE EXOCYTOSIS FROM MAIZE ROOT CAP PROTOPLASTS

Protoplasts isolated from root cap cells of maize were shown to secret e fucose-rich polysaccharides and were used in a patch-clamp study to monitor changes in whole-cell capacitance. Ca2+ was required for exocy tosis, which was measured as an increase in cell capacitance during in tracellular dialysis with Ca2+ buffers via the patch pipette. Exocytos is was stimulated significantly by small increases above normal restin g [Ca2+]. In the absence of Ca2+, protoplasts decreased in size. In si tu hybridization showed significant expression of the maize annexin p3 5 in root cap cells, differentiating vascular tissue, and elongating c ells, Dialysis of protoplasts with maize annexins stimulated exocytosi s at physiological [Ca2+],and this could be blocked by dialysis with a ntibodies specific to maize annexins. Dialysis with millimolar concent rations of GTP strongly inhibited exocytosis, causing protoplasts to d ecrease in size. GTP gamma S and GDP beta S both caused only a slight inhibition of exocytosis at physiological Ca2+. Protoplasts were shown to internalize plasma membrane actively. The results are discussed in relation to the regulation of exocytosis in what is usually considere d to be a constitutively secreting system; they provide direct evidenc e for a role of annexins in exocytosis in plant cells.