ISSN 10214437, Russian Journal of Plant Physiology, 2010, Vol. 57, No. 2, pp. 290–296. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © N.L. Klyachko, 2010, published in Fiziologiya Rastenii, 2010, Vol. 57, No.2, pp. 304–311.
The pathways of signal transduction in plant cells
are the target for active investigations, and now we
know much about molecular components of these
pathways and their interactions both within separate
signaling chains and between various pathways com
bined in a complex network.
We know much less about spatial organization of
these signal transduction pathways, in particular about
early stages of signal transduction from membrane
receptors to cell acceptor compartments.
Until recently, it was commonly accepted that sig
nal transduction from the plasma membrane (PM) to
the nucleus occurs by free diffusion of the chain com
ponents in the cytosol, whereas receptor internaliza
tion (i.e., its traslocation from the membrane inside
the cell) is a way for a decrease in its concentration in
the PM, i.e., the way for signal downregulation
(which occurs really in many cases during receptor
transport to lysosomes). However, in the last decade,
some findings start to accumulate that receptor could
continue or even start sending the signal from endo
somes as well and that the endosome could be the site
for localization of other components of the signaling
chain, which allows them more efficiently interact
with the receptor (see reviews [1–3] and many others).
Even a special expression appeared, e.g., receptor
mediated endocytosis. When signaling from endo
somes in animals is a reliably proved fact, for plants,
for which the initial arguments for endocytosis occur
rence per se appeared only 20 years ago, this problem
was outside researcher attention until recently.
INTERNALIZATION OF PLASMA MEMBRANE
RECEPTORS BY ENDOCYTOSIS
Internalization of various biologically active com
ponents of PM, proteins in particular, was repeatedly
demonstrated for both animals and plants . Among
such plant proteins (arabidopsis proteins) are PIN and
AUX transporters of auxins [5, 6] and borate trans
porters AtBOR1 , leucinerich repeat serine/threo
nine protein kinase AtSERK1 , a homolog of the
receptorlike kinase CRINKLY4 (ACR4; ), or
human transferrin receptor (hTfR) in transformed arabi
dopsis protoplasts . Different proteins resided in the
PM could be internalized by different pathways: for
example, endocytosis of different auxin transporters
manifests different sensitivity to specific inhibitors
[11, 12]. Processes of internalization into intracellular
endocytotic compartment are described in detail in
some excellent reviews ([13–19] and others).
The occurrence of receptormediated endocytosis
in plants was relatively well studied only in two sys
tems. The both cases in point are internalization of
leucinerich repeat receptor Ser/Thr kinases (LRR
Ser/Thr kinases) in arabidopsis. This class of kinases is
most numerous receptors residing in the PM (more
than 200 in arabidopsis).
Receptor internalization might be induced by its
interaction with the ligand or be constitutive. The
example of the first process is internalization of the
flagellin receptor FLS2 (f
lagellinsensitive 2); after
interaction with flg22 (a conserved peptide of
22 amino acid residues, the fragment of bacterial
flagellin), this transmembrane LRR kinase triggers a
cascade of signal transduction resulting in improved
plant resistance to the pathogen. Robatzek et al. 
demonstrated internalization of this receptor after
interaction with flg22. After long interaction with this
Plant Signaling Endosomes and Endosome Trafficking
N. L. Klyachko
Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya ul. 35, Moscow, 127276 Russia;
fax: 7 (495) 9778018; email: firstname.lastname@example.org
Received March 12, 2009
—A discovery of a possibility for signal transduction from endosomes differing quantitatively and
qualitatively from signaling from the plasma membrane became a reliably proved fact for animal and yeast
receptors but was unaddressed by plant researches for a long time. In this lecture, I describe briefly recent
progress in this research area and also the involvement of the actin cytoskeleton in endosome transport from
the plasma membrane to acceptor compartments.
Key words: plant endosomes receptors signaling actin cytoskeleton
BR—brassinosteroids; PM—plasma membrane.