The role of calcium in exocytosis and endocytosis in plant cells

Abstract

The role of calcium in the individual cellular events leading to exocytosis is considered. Both vesicle movement processes and vesicle fusion at the cell surface require calcium for completion of specific events in this pathway. Our knowledge of these events is incomplete. In particular the movement of secretory vesicles by the cytoskeleton in response to added calcium is a key event that is beyond our comprehension at present. At the whole cell level, it is shown that external calcium, at the appropriate concentration, is required to elicit secretion at optimal rates. In both plant and animal cells secretion appears to be dependent on, or is triggered by, a rise in the level of internal free calcium ions from about 10‐7 to 10‐6M or even higher. In these eukaryotes internal organelles take up calcium and maintain a low level of calcium in the cell, offsetting the inflow of calcium from the plasma membrane. In some systems the inflow is restricted to a certain part of the plasma membrane, which then acts as a focus for exocytosis and, thereby, establishes a cellular polarity. In plant tissues there appears to be a requirement for some circulation of calcium within the apoplast, to sustain secretion. Recent papers on endocytosis have confirmed its occurrence in plant cells and made significant advances in isolating and characterising the clathrin coats of the coated vesicles involved in the uptake. There is no evidence, at present, for a direct role for calcium in these events. Indirectly, calcium stimulates exocytosis, and hence the delivery of excess membrane to the cell surface, which may be retrieved by an increase in the rate of endocytosis. Quantitative comparisons of the membrane flow occurring in these pathways are not available. Several plant cellular systems have been employed to study secretion and some of these may prove to be superior model systems for the investigation of certain aspects of the control of exocytosis and endocytosis by calcium ions.

Secretion of phenolic bodies following fertilisation inDurvillaea potatorum(Durvillaeales, Phaeophyta)

“…(Hang et aL, 1987). The occurrence of coated pits and vesicles is well documented in higher plant and animal cells (Coleman et al, 1988;Steer, 1988;Robinson & Hillmer, 1990;Mollenhauer et aL, 1991). Coated vesicles in D. potatorum are comparable in size with those in higher plants (72-96 nm) and animals (60-160 nm) (Emons & Traas, 1986).…”

“…In eggs of Laminaria angustata Kjellman, Motomura & Sakai (1984, Figure 22;1988, Figure 2) depicted peripheral osmiophilic vesicles approximately 0-5-1 #m in diameter and rather similar to the phenolic vesicles present in D. potatorum eggs. They reported that the contents of these vesicles appeared to be discharged to the exterior in freshly released eggs, and suggested that they may be physodes (Motomura & Sakai, 1984).…”

Annexins of Plant Cells

“…The evidence that Ca2+ is involved in certain types of secretion in plant cells has been reviewed (Steer, 1988), and the potential role of annexins in exocytosis in plants has been discussed Clark et al, 1995). It is still unclear what the leve1 of free Ca2' is at the site of vesicle fusion with the plasma membrane during secretion in plant cells.…”

Neither compatible nor self‐incompatible pollinations of Brassica napus involve reorganization of the papillar cytoskeleton

“…The plant cytoskeleton plays a critical role in secretory processes such as wall formation (Derksen & Emons, 1990 ;Williamson, 1993) and the release of enzymes (discussed by Steer, 1988) and other extracellular products (Kristen & Lockhausen, 1983). Both actin microfilaments (MFs) and microtubules (MTs) are potentially involved in the movement of secretory vesicles from their point of formation, such as the ER and the Golgi bodies, to their site of exocytosis.…”