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.