Plant Molecular Biology 38: 111–125, 1998.
© 1998 Kluwer Academic Publishers. Printed in The Netherlands.
Exocytosis in plants
Gerhard Thiel and Nick Battey
A. van Haller Institute for Plant Sciences, University of Göttingen, Göttingen 37073, Germany;
Laboratories, University of Reading, Whiteknights, Reading RG6 6AS, UK (
author for correspondence; e-mail
Key words: exocytosis, endocytosis, vesicle fusion, patch clamp, membrane capacitance, plasma membrane
Exocytosis is the ﬁnal event in the secretory pathway and requires the fusion of the secretory vesicle membrane
with the plasma membrane. It results in the release to the outside of vesicle cargo from the cell interior and also the
delivery of vesicle membrane and proteins to the plasma membrane. An electrophysiological assay that measures
changes in membrane capacitance has recently been used to monitor exocytosis in plants. This complements
information derived from earlier light and electron microscope studies, and allows both transient and irreversible
fusion of single exocytotic vesicles to be followed with high resolution in protoplasts. It also provides a tool
to investigate bulk exocytotic activity in single protoplasts under the inﬂuence of cytoplasmic modulators. This
research highlights the role of intracellular Ca
, GTP and pressure in the control of exocytosis in plants.
In parallel to these functional studies, plant proteins with the potential to regulate exocytosis are being iden-
tiﬁed by molecular analysis. In this review we describe these electrophysiological and molecular advances, and
emphasise the need for parallel biochemical work to provide a complete picture of the mechanisms controlling
vesicle fusion at the plasma membrane of plant cells.
On reaching the plasma membrane secretory vesicles
undergo a complex series of steps that leads to mem-
brane fusion and content release (Figure 1). In this
review we shall ﬁrst give an overview of this exo-
cytotic process, based mainly on information from
animal cells. Then we shall discuss recent advances
in understanding of plant exocytosis, which derive
largely from the application of the patch clamp method
to measure membrane capacitance.
Overview of the exocytotic process: background
mainly from animal cells
Vesicle docking brings the vesicle membrane in close
proximity with the plasma membrane (Figure 1),
and is probably mediated in many cell types by the
interaction of NSF (N-ethylmaleimide-sensitive fac-
tor), SNAPs (soluble NSF attachment proteins) and
SNAREs (SNAP receptors) . Rabs (small GTP-
binding proteins of the Rab family) regulate this
interaction ([6, 79], Robinson et al., this volume).
This recognition event may form the molecular basis
for the targeting of secretory vesicles to the plasma
membrane in plant cells, and be involved in the po-
larised secretion found in the hypersecretory cells of
the root cap, Fucus rhizoid, pollen tube, and other
non-isodiametrically expanding cells of the plant body
. In animal neurons, vesicles remain docked in
readiness for fusion upon the arrival of the Ca
transient associated with membrane depolarisation. In
chromafﬁn cells this docked state can last for only
100 s, before the vesicle returns to the cytoplasm, in-
dicating that docking can be reversible . In plants
the residence time is not known, but it is likely that in,
for example, the pollen tube tip it is very short, given
the extremely dynamic nature of the secretory zone
(see ). However, in the Fucus rhizoid something
more akin to triggered fusion may take place [114, 94].