Raising the cytosolic Ca2+ concentration increases the membrane capacitance of maize coleoptile protoplasts: Evidence for Ca2+-stimulated exocytosis

Abstract

Abstract. Enhanced elongation of coleoptile cells hasbeen proposed to be related to a rise in secretory activity. Therefore, to obtain a direct measurement of exocytotic events in maize (Zea mays L.) coleoptile protoplasts we used the patch-clamp method to record changes in membrane capacitance (Cm) as a parameter proportional to fluctuations of the membrane surface area. The secretory activity of protoplasts was correlated with the cytosolic free Ca 2 + concentration ([Ca 2 +]cyt): dialyzing protoplasts with 1 laM [Ca2+]cy t caused a steady rise in Cm of 3.3_ pF-s 1. In contrast, dialysis with a solution containing <20 nM Ca 2+ produced a small and persistent decrease in Cm. This demonstrates that secretory activity in coleoptile cells can be controlled by factors which modulate [Ca2+]cy t.

Unitary exocytotic and endocytotic events in Zea mays L. coleoptile protoplasts

“…For comparison the C m on-and off-steps from Figure 2 were pooled and converted into corresponding vesicle diameters. For this conversion a specific capacitance of 8 mF m -2 was considered (Thiel et al, 1994). Again, the global distribution of the electrically derived vesicle diameters was in good agreement with the diameter of morphologically detectable vesicle structures in grass coleoptiles.…”

“…A C m step of 100 aF would be expected when a spherical vesicle with a diameter of ജ 63 nm interacts with the membrane. Calculation of the latter value is based on the fact that C m is proportional to the membrane surface area and that the plasma membrane of coleoptile cells has a specific capacitance of 8 mF m -2 (Thiel et al, 1994).…”

“…Considering a specific membrane capacitance of 8 mF m -2 for the plasma membrane of coleoptile protoplasts (Thiel et al, 1994) Step-size distribution of exocytotic (on-step, n ϭ 94) and endocytotic (off-steps, n ϭ 107) events obtained from cell-attached recordings in 20 coleoptile protoplasts. Scale for vesicle diameter calculated assuming spherical vesicle shape and specific membrane capacitance of 8 mF m -2 .…”

“…Protoplasts from the apex of etiolated grown coleoptiles of maize (cv, mutin; KWS Saatzucht, Einbeck, Germany) were prepared as described previously (Thiel et al, 1994). Secretory activity of these protoplasts bathed in 250 mM KCl, 10 mM CaCl 2 , 20 mM HEPES/ KOH, pH 7, was assayed with the patch clamp technique in the cell-attached mode as described by Kreft and Zorec (1997).…”

Localization of calcium ions in wounded characean internodal cells

“…Therefore, a small influx, if any, is probably sufficient for the deposition of amorphous wound walls via signal amplification. In plant cells, cytosolic Ca# + concentrations as low as 1 µ stimulate exocytosis (Zorec & Tester, 1992 ;Thiel et al, 1994). Detection of small fluxes requires the use of methods such as Ca# + sensitive microelectrodes (Felle, 1991) or Ca# + indicating dyes, which are difficult to apply to the cortex of characean internodal cells (Plieth & Hansen, 1996 and pers.…”