Ca²⁺, Annexins, and GTP Modulate Exocytosis from Maize Root Cap Protoplasts

Abstract

Protoplasts isolated from root cap cells of maize were shown to secrete fucose-rich polysaccharides and were used in a patch-clamp study to monitor changes in whole-cell capacitance. Ca2+ was required for exocytosis, which was measured as an increase in cell capacitance during intracellular dialysis with Ca2+ buffers via the patch pipette. Exocytosis was stimulated significantly by small increases above normal resting [Ca2+]. In the absence of Ca2+, protoplasts decreased in size. In situ hybridization showed significant expression of the maize annexin p35 in root cap cells, differ-entiating vascular tissue, and elongating cells. Dialysis of protoplasts with maize annexins stimulated exocytosis at physiological [Ca2+], and this could be blocked by dialysis with antibodies specific to maize annexins. Dialysis with milli-molar concentrations of GTP strongly inhibited exocytosis, causing protoplasts to decrease in size. GTPgammaS and GDPbetaS 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 considered to be a constitutively secreting system; they provide direct evidence for a role of annexins in exocytosis in plant cells.

Transcription Profile Analyses Identify Genes and Pathways Central to Root Cap Functions in Maize

“…Results from the qRT-PCR were consistent with that obtained from the GeneChip experiments (Table 2). In addition, we showed that a number of genes previously reported by others as being highly expressed in the maize RC, were likewise upregulated in the RC microarray data (Supplement 1), including OS001597_r (ZmRCP1) (Matsuyama et al, 1999a), OS003906.1 (ZmRCP2) (Matsuyama et al, 1999a), OS007905 (ANNEXIN,) (Carroll et al, 1998), OS011991_i (GDP-D-Mannose-4,6-Dehydrogenase) (Ponce et al, 2000), and OS014195_i (ZmGRP4) (Matsuyama et al, 1999b), thereby providing further support for the reliability of the microarray data.…”

The cytoskeleton in plant cell growth: lessons from root hairs

“…The calcium influx takes place only at the tips of growing root hairs (Herrman & Felle, 1995; Felle & Hepler, 1997). The high [Ca 2+ ] c is probably required for exocytosis (Carroll et al. , 1998).…”

TheAspergillus nigerannexin,anxC3.1is constitutively expressed and is not essential for protein secretion

“…In mammals, annexins have been implicated in a number of membrane events including regulated and constitutive exocytosis, endocytosis and membrane organisation [7][8][9]. In plants, a role for annexins in membrane trafficking and/or secretion, calcium channel formation and calcium hemostasis has been suggested [10][11][12][13].…”