ISSN 1021-4437, Russian Journal of Plant Physiology, 2009, Vol. 56, No. 5, pp. 627–634. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © N.A. Burmistrova, M.S. Krasavina, E.N. Akanov, 2009, published in Fiziologiya Rastenii, 2009, Vol. 56, No. 5, pp. 695–703.
Sucrose is the main transport form of carbon ﬁxed
during photosynthesis. Sucrose partition in the plant is
determined by several processes: its loading into the
complex sieve element/companion cell in the phloem
of ﬁne veins of the source leaf, its transport within
phloem sieve elements (mass ﬂow), and its exit from
the transport route in sinks (sieve element unloading).
Further sucrose partition between the parenchyma cells
is called as post-phloem transport. Transport processes
occurring in sinks on the route from the sieve element
to the site of sucrose utilization in corresponding cells
were called as phloem unloading.
In young seedlings, the main sink is the tip of
actively growing root. In this region, all cells are con-
nected by a great number of plasmodesmata, which
allows symplast exit of assimilates from the phloem
and their further partition between root parenchymal
cells [1–3]. Biochemical data conﬁrmed predominantly
symplastic pathway of sucrose transport in the root tip:
transported sucrose was not detected in the apoplast,
exogenous sugars did not suppress unloading, the ine-
quality of distribution of
C-hexose components of
asymmetrically labeled sucrose applied to the source
was retained . The pattern of distribution in the root
tip of ﬂuorescent dyes not penetrating across the
plasma membrane also demonstrated clearly the sym-
plastic way of sucrose transport in the root tip. Being
introduced into the cells of the source organ or distant
root region, these compounds accumulated in all the
cells of the root tip, to be exact, in the basal zone of the
It is still unclear what regulates compound distribu-
tion in the growing root. Some reliable facts indicate
that there are factors affecting plasmodesmal conduc-
tivity. A dependence of plasmodesmal conductivity on
plant developmental stage, activity of cell metabolism,
and external factors (temperature, medium acidity, the
presence of symbionts and pathogens) was demon-
strated [2, 3, 7]. In recent time, the data appeared that
some endogenous molecules (transcription factors,
RNAs, and proteins, including phloem proteins) can
regulate plasmodesmal conductivity [8, 9].
In some experiments, it was shown that plasmodes-
mal conductivity in the sink tissues was much higher
than in other tissues not displaying sink activity. Thus,
mature mesophyll cells are connected by plasmodes-
mata transmitting compounds with the size of 0.8–1.0 kD.
At the same time, young cells, including the cells in the
root tip, can transport compounds of much higher size,
for example such proteins as green ﬂuorescent protein
from jellyﬁsh (GFP, 27 kD) and fused dextrans (
and references therein).
There are some indications that plasmodesmata
with such a high conductivity are poorly regulated .
Salicylic Acid Can Regulate Phloem Unloading in the Root Tip
N. A. Burmistrova
, M. S. Krasavina
, and E. N. Akanov
Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya ul. 35, Moscow, 127276 Russia;
fax: 7 (495) 977-8018; e-mail: email@example.com
Pryanishnikov Research Institute of Agrochemistry, Russian Academy of Agricultural Sciences, Moscow
Received October 13, 2008
—In three-day-old maize (
L.) seedlings, we removed the endosperm, coleoptile with leaf-
lets, and adventitious roots. Primary roots were exposed to 0–10
M salicylic acid (SA) for 1–5 h; scutellum,
M 2-desoxy-D-glucose (2dG). 2dG-sucrose synthesized from 2dG was transported from scutella to the
roots along the phloem. Its accumulation in 5-mm-long root tips was the measure of phloem unloading. At the
concentrations higher than 10
M, SA suppressed unloading. Simultaneously, the uptake of
xazolidinedione (DMO) by root segments was inhibited, indicating cytoplasm acidiﬁcation. 10
M SA also
inhibited root respiration and growth. The lower SA concentrations (10
M) activated unloading under
conditions of weak sucrose phloem transport to the root. They did not affect DMO uptake, respiration, and growth.
M SA stimulated unloading during 1- or 2-h exposure but did not affect it at longer treatments. A depen-
dence of SA action on its concentration and exposure duration implies its involvement in the control of phloem
unloading in the root tip.
Key words: Zea mays - root - phloem unloading - salicylic acid
: 2dG—2-deoxy-D-glucose; DMO—
oxazolidinedione; SA—salicylic acid.