384
ISSN 1021-4437, Russian Journal of Plant Physiology, 2018, Vol. 65, No. 3, pp. 384–393. © Pleiades Publishing, Ltd., 2018.
Cytochalasin B Treatment of Apple (Malus pumila Mill.) Pollen Tubes
Alters the Cytoplasmic Calcium Gradient and Causes Major Changes
in the Cell Wall Components
1
K. F. Fang
a, b
, Q. Zhang
b, c
, R. Yang
c
, Q. Q. Cao
b, c
, and L. Qin
b, c,
*
a
College of Landscape Architecture, Beijing University of Agriculture, Beijing, 102206 China
b
Beijing Collaborative Innovation Center for Eco-environmental Improvement with Forestry and Fruit Trees,
Beijing, 102206 China
c
Key Laboratory for Agricultural Application and New Technique, College of Plant Science and Technology,
Beijing University of Agriculture, Beijing, 102206 China
*e-mail: qinlingbac@126.com
Received May 22, 2017
Abstract⎯It is well established that the actin cytoskeleton is absolutely essential to pollen germination and
tube growth. In this study we investigated the effects of cytochalasin B (CB), which affects actin polymeriza-
tion by binding to the barbed end of actin filaments, on apple (Malus pumila Mill.) pollen tube growth.
Results showed that CB altered the morphology of pollen tubes, which had a larger diameter than control
tubes beside inhibiting pollen germination and tube growth. Meantime CB also caused an abnormal distri-
bution of actin filaments in the shank of the treated pollen tubes. Fluo-3/AM labeling indicated that the gra-
dient of cytosolic calcium ([Ca
2+
]c) in the pollen tube tip was abolished by exposure to CB, which induced a
much stronger signal in the cytoplasm. Cellulose and callose distribution in the tube apex changed due to the
CB treatment. Immunolabeling with different pectin and arabinogalactan protein (AGP) antibodies illus-
trated that CB induced an accumulation of pectins and AGPs in the tube cytoplasm and apex wall. The above
results were further supported by Fourier-transform infrared (FTIR) analysis. The results suggest the disrup-
tion of actin can result in abnormal growth by disturbing the [Ca
2+
]c gradient and the distribution of cell wall
components at the pollen tube apex.
Keywords: Malus pumila, actin, [Ca
2+
]c, pollen tube, wall components
DOI: 10.1134/S1021443718030111
INTRODUCTION
Pollen tubes are tip-growing cells, each of which
carries a pair of sperm cells to the ovule, where double
fertilization process takes place, and hence are crucial
for seed and fruit formation [1]. Lopez et al. [2]
reported that the gradual replacement of large F-
actin aggregates by a filamentous network in the veg-
etative cytoplasm activated and initated pollen ger-
mination. After germination, actin filaments are bun-
dled into long, thick cables, which are arranged in a
longitudinal orientation throughout the pollen tube.
These cables provide the main track for the transport
of organelles and Golgi-derived secretory vesicles
containing wall precursors to the tube wall [2]. Previ-
ous studies have demonstrated that the actin cyto-
skeleton influences cell polarity, growth, and cyto-
plasmic streaming [3–5].
Besides F-actin, calcium plays an essential role in
pollen tube growth. A number of studies have shown
that there is a “tip-focused” gradient of free
Ca
2+
([Ca
2+
]) in pollen tubes and that Ca
2+
plays an
essential role as a second messenger to control many
various processes [6]. Fu [6] summarized that the
[Ca
2+
] gradient was derived largely from an extracellu-
lar Ca
2+
influx at the pollen tube tip and partly by the
release of Ca
2+
from internal stores. Adding Ca
2+
channel blocker or Ca
2+
to the buffer would dissipate
the cytosolic calcium ([Ca
2+
]c) gradient and thus
inhibit pollen tube growth [6]. Malhó and Trewavas [7]
reported that artificially re-directing the [Ca
2+
]c gra-
dient to a new site altered the direction of pollen tube
growth. Therefore, the [Ca
2+
]c gradient acts an active
part in regulating pollen tube tip growth [8].
The fungal metabolites, cytochalasins, inf luence
actin polymerization by binding to the barbed end of
1
The article is published in the original.
Abbreviations: AGP⎯arabinogalactan protein; [Ca
2+
]c—cyto-
solic calcium; CB⎯cytochalasin B; FTIR⎯Fourier Transform
Infrared; LSCM⎯laser-scanning confocal microscope.
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