Apical cytoplasmic organization and polar growth
in Rhizoctonia solani are modified by cytochalasins
Stanley N. Grove and James A. Sweigard
Department of Biology, Goshen College, Goshen, IN 46526
Central Research and Development Department, DuPont Experiment Station,
Wilmington, DE 19898
Correspondence should be addressed to: Stanley N. Grove, PhD.
Submitted for publication: October 1996
Keywords: Filamentous fungi, tip growth, polar growth, apical growth,
hyphae, Spitzenkörper, actin, cytochalasin, exocytosis, phase-contrast
microscopy, electron microscopy, endoplasmic reticulum; morphogenesis
Abbreviations and chemicals: Cytochalasin A (CA); Cytochalasin E (CE);
Dimethyl sulfoxide (DMSO); Endoplasmic reticulum (ER); N-ethyl-maleimide
(NEM ); p-chloro-mercuribenzoic acid (PCMB)
Hyphae of Rhizoctonia solani treated with cytochalasin show these
changes 1) immediate reduction in polar growth rate, 2) the typical phase-contrast
dark portion of the Spitzenkörper disappears in <1 min, leaving
a persistent phase light core, 3) the mitochondria lose their typical orientation,
4) continued nonpolar growth forms a bulbous hyphal apex, 5) correlated
light and electron microscopy of the same hyphal tip reveals the persistent
core disengaged from the usual central location and the typical cluster
of apical vesicles, and 6) clusters of smooth ER are found in the apical
cytoplasm. The changes observed support the suggestion that apical growth
and maintenance of a Spitzenkörper are both actin-dependent phenomena.
It is clear that the phase-contrast bright core is a complex feature of
the cytoplasm that interacts with and contains elements of smooth endoplasmic
Growth in the filamentous fungi typically involves a highly polarized system
of exocytosis(1, 2).
Tips of growing septate hyphae contain a Spitzenkörper ("apical
body") composed, in part, of a cluster of cytoplasmic vesicles which
seems to participate in apical growth and fungal morphogenesis (3,
4, 1). Rhizoctonia
solani has a bipartite Spitzenkörper displaying a phase-contrast
light inner zone (core) surrounded by a phase-contrast dark region (5,
6, 7, 8).
Electron micrographs of chemically fixed (5)
or freeze-substituted (9, 2,
10) hyphal tips often show an inner zone or
core devoid of secretory vesicles but containing fibrillar material, microvesicles,
small membranous tubules, and ribosomes. However, the nature and function
of this inner region remains unclear. Mitochondria in the subapical cytoplasm
are also characteristically arranged parallel to the long axis of the growing
hyphae thereby imparting a typical organization not found in hyphae where
apical growth has ceased (11, 5).
Phase contrast microscopy readily reveals these features along with the
linear growth rate of individual hyphae. What is missing, as noted by Howard
(2), is the simultaneous comparison of a light-microscopic
image of a living hyphal apex with an electron-microscopic image of the
same region in the same cell.
Our studies of the effects of cytochalasins on hyphal tip growth have included
direct comparisons of light microscopic and electron microscopic observations
of the same fixed hypha. A previous report shows that the cytochalasins
inhibit sporangiospore germination and hyphal growth in a non-septate fungus
(12). In a preliminary communication (6)
we have shown that the response to cytochalasins in hyphal tips of R.
solani includes inhibition of tip growth and loss of a typical Spitzenkörper,
but persistence of an inner core. Recently this persistence has been independently
confirmed by video-enhanced microscopy (7, 8).
Since the cytochalasins inhibit actin polymerization in vitro, (13,
14) and characteristic microfilamentous structures
can be demonstrated in growing hyphal tips (9,
2, 15, 16,
17, 18, 19,
20, 21) we have
examined the hypothesis that an actin-dependent process is required for
apical growth in R. solani. It seems highly significant that actin
has been demonstrated by ultrastructural immunolocalization in the core-region
of the Spitzenkörper in Magnaporthe grisea (21)
and that an essential myosin I, which probably acts by binding to actin,
is required for polarized growth in Aspergillus nidulans (22).
In this report we describe the rapid changes in mode of growth, position
of the Spitzenkörper, and distribution of mitochondria caused by treatment
with cytochalasins. We identify apical growth and maintenance of a Spitzenkörper
as actin-dependent phenomena. Further, we describe the phase-contrast bright
core as a stable feature with fiborous components and closely associated
Organism and Media:
MATERIALS AND METHODS
An isolate of Rhizoctonia solani Kuehn was obtained from Dr. C. E.
Bracker, Department of Botany and Plant Pathology, Purdue University, West
Lafayette, IN. Cultures were grown on yeast-peptone-glucose (YPG) and glucose-asparagine
(GA) media as described previously (12).
Preparation and Observation of Slide Cultures
Slide cultures employed the methods of Howard and Aist (23).
Slides thinly coated with GA agarose medium (1.5% agarose; Type 1, Sigma
Chemical Company, St. Louis, MO) were inoculated in the center with 5 mm
mycelial strips cut from just behind the growing edge of R. solani
YPG cultures. The slide cultures were grown in humidity chambers for 16-28
hours at 25 ° C in the dark.
Preparation of the slides for observation followed the perfusion technique
("An alternative method . . . p. 198) of Howard and Aist (23).
Briefly, channels (1-2 mm) were cut in ca. 15 x 15 mm squares containing
hyphae ca. 10 mm long. A coverslip was gently placed over the remaining
agarose strips and the sides of the coverslip parallel to the channels were
sealed with a 1:1 mixture of paraffin and petroleum jelly. Growing hyphae
displaying typical zonation (11) and located
within 200 µm of a channel were selected for observation.
Liquid media containing the appropriate test chemical was added to the edge
of the cover glass and allowed to perfuse around the hyphal tip. Living
tip cells were observed continually under constant light exposure in a chamber
held at 27.5 ° C. Linear growth rates were determined using an ocular
micrometer. Controls receiving broth with 0.1% DMSO and treated hyphae receiving
broth with cytochalasin (A or E at 1µg/ml) and 0.1% DMSO were observed
alternately. When direct flow of the introduced medium occurred between
the coverslip and the agarose, the observation was terminated because we
have found that very slight environmental changes can cause growth to slow.
Direct comparison of light and electron microscope images
A direct comparison of the light and electron microscopic images
of the same hypha was necessary to identify the ultrastructural equivalents
of components seen in living hyphae, including clues for the composition
of the Spitzenkörper. Since freeze substitution does not lend itself
to the direct microscopic observation of specimens during the fixation process
or to direct before and after comparisons of the same hyphae, we have used
very high quality chemically fixed hyphae to obtain ultrastructural images.
To compare the light and electron microscopic images, a different slide
culture technique was used (Fig. 1). Slide cultures
were grown as above but trimmed and mounted so hyphal tips for examination
were approaching a fresh cut edge of the agar medium beneath a coverglass
attached by drops of wax. This chamber allowed the living tip to be photographed,
perfused with experimental liquid medium, then perfused a second time with
fixative fluid, photographed again, removed from the slide culture and processed
for electron microscopy. A suitable growing tip within 350 µm of the
edge was selected, photographed live, and irrigated with a drop of liquid
medium containing 0.1% DMSO or medium containing CA or CE (1 µg/ml)
and 0.1% DMSO. At the first sign of a reaction to cytochalasin (change in
shape of the Spitzenkörper) the preparation was irrigated again with
several drops of fixative containing 2% formaldehyde (w/v) and 2% glutaraldehyde
(v/v) in 0.1 M sodium cacodylate, pH 7.0. To foster rapid change in the
irrigating solutions a fragment of filter paper, approximately 2 cm2, was
placed at the near edge of the coverglass. This filter paper provided a
sink for excess fluid from under the coverglass when new drops were placed
at the upper right edge of the chamber. After all cytoplasmic motion stopped
in the tip (20-120 sec) a second photograph was recorded, the slide culture
was transferred to a dissecting microscope where the chosen hypha was removed
in a small block of agar, placed in a vial of fresh fixative and prepared
for electron microscopy by staining with OsO4, acetone dehydration and embedding
in Spurr's medium. Serial or partial serial sections were examined after
staining with uranyl acetate and lead citrate (5,
The specimens were observed and photographed on Kodak technical pan film
2415 or Kodak Tri-X with a Zeiss Standard 18 microscope equipped with phase-contrast
optics, a transmitted photo-flash system, and a 35 mm camera. Fine structural
images were observed and recorded in a JEM 100S electron microscope using
DuPont Cronar Ortho Litho film.
Cytochalasins A and E were purchased from Aldrich, Milwaukee, WI., N-ethyl-maleimide
(NEM) and p-chloro-mercuribenzoic acid (PCMB) were purchased from Sigma,
St. Louis, MO.
Light Microscopic Observations
Apical organization of growing hyphae includes a dynamic Spitzenkörper
composed of a phase-bright core surrounded by a region that appears as a
dark cloud adjacent to the apical membrane at the tip (Fig.
2). The profile of the bright core is round (0 sec view) except when
the cluster of subtending mitochondria partially obscures its subapical
perimeter (30 and 60 sec views). A region of phase-bright cytoplasm is visible
lateral to the Spitzenkörper and is sometimes penetrated by one or
a few mitochondria. Interspersed in the cluster of mitochondria are pleiomorphic
phase-bright structures which have smooth linear profiles with occasional
enlarged areas. The enlarged or beaded portions seem to migrate along the
strand by a peristalsis-like gliding motion in either direction. These bright
strands appear to make intermittent contact with or contributions to the
subapical perimeter of the core (30 sec view). These bright segments, possibly
endoplasmic reticulum (ER), are scattered in the subapical cytoplasm along
with elongated mitochondria and numerous very dark, round inclusions, thought
to be lipid bodies. Most mitochondria are oriented parallel to the long
axis of the hypha with occasional exceptions. In this study control hyphae
exhibited apical growth rates ranging from 5-15 µm/min.
The exposure of R. solani hyphal tips to cytochalasin A (CA) or cytochalasin
E (CE) yielded a rapid and dramatic effect on hyphal tip organization and
growth (Fig. 3) while control hyphae continued growing
as usual for 5 min (Fig. 4). The effects of CA and
CE were similar except that at equal concentrations the responses to CA
were seen approximately one minute faster than those of CE (Fig.
5). We have noted the following reactions to cytochalasin treatment.
- Reduction in linear growth rate. Three minutes of treatment with CA
or CE rapidly reduced the rate of hyphal tip growth (Fig.
5) from a pretreatment range of 5-15 µm/min down to 25% of the
pretreatment rate for CA and to 35% for CE. Control hyphae maintained the
pretreatment growth rate for at least 5 minutes.
- Disappearance of the phase-contrast dark region of the Spitzenkörper
and persistence of the light core region of the Spitzenkörper. The
first effect (in 20-50 sec after addition) of CA on hyphal tip organization
was a change in shape of the core followed by a dispersion of the dark region
of the Spitzenkörper (compare (Fig. 2, Fig.
3 the 15, 30, and 45 sec views, and Fig. 4 with
Fig. 3 the 60 - 320 sec views). The phase-contrast
light region then retracted (60 sec view) from the hyphal apex and persisted
in the apical cytoplasm for several minutes (90 - 320 sec views). It moved
in the cytoplasm with no apparent pattern (Fig. 6)
and was often in association with mitochondria. We have also noted the persistence
of the inner core of the Spitzenkörper in control hyphae that have
stopped growing due to manipulation during specimen preparation. The phase-contrast
light region in some of these observations persisted for more than twenty
minutes (not shown). Control hyphae (undamaged and not treated with cytochalasins)
maintained a typical Spitzenkörper (Fig. 2
and Fig. 4 ) for at least five minutes (not shown).
- Mitochondrial reorientation. Mitochondria in control hyphae, usually
oriented parallel to the long axis of the hypha, did not penetrate the extreme
apical dome (the Spitzenkörper region) but remained just behind the
Spitzenkörper (Fig. 2). After CA treatment,
usually within a minute after disappearance of the phase contrast dark region
of the Spitzenkörper, mitochondria were seen in the extreme apical
dome and exhibited an altered orientation in the hypha (Fig.
3 the 60 - 320 sec views). This phenomenon started in the apical cytoplasm
and gradually spread to the subapical cytoplasm.
- Hyphal tip swelling. Changes in the Spitzenkörper were accompanied
by changes in the profiles of CA-treated hyphae which soon developed swollen
tips (Fig. 3 the 60 - 320 sec views, Fig.
6 the 60 - 240 second views). This nonpolar growth continued during
the observation period. Control hyphae (Fig. 4)
maintained normal apical shape and polar growth for at least 5 minutes (not
Since our working hypothesis was that cytochalasins act primarily as anti-microfilament
agents, we examined other compounds that might mimic other potential effects
of the cytochalasins. NEM and PCMB are general sulfhydryl reagents and were
used as controls for the sulfhydryl reactivity that has been demonstrated
for CA (24). At 100 times the concentration
of CA, NEM and PCMB had less effect on linear growth rate than CA (Fig.
7). Moreover, this slowing of growth was not accompanied by the hyphal
tip swelling that is typical of cytochalasin treatment (Fig.
3 & Fig. 6).
Electron Microscopic Observations
In near-median sections of control hyphae the region corresponding to the
Spitzenkörper core of a fixed hypha viewed with phase contrast optics
(Fig. 8a, 8b), a small core area was devoid of
vesicles (Fig. 8c, 8d) but containing granular
or filamentous matrix and small elements of tubular smooth membrane (
Fig. 9a - 9c).
The characteristic tight cluster of large and small vesicles
surrounded this core and some of the vesicles seemed to be embedded in the
outer part of the core matrix. The tubular smooth membrane and small vesicular
profiles (Fig. 9a - 9c) seemed much more frequent
in the lateral and subapical portions of the Spitzenkörper region.
They were present here in frequencies similar to those found in the subapical
cytoplasm. Microfilaments are visible in the core and extend out between
the vesicles (Fig. 9a &9c). Mitochondria
were absent in the apical dome, but ribosomes and some vesicles were seen
in apical cytoplasm surrounding the Spitzenkörper
(Fig. 8c, 8d).
In specimens treated with cytochalasin E for one min or less and then fixed
in situ, the persistent Spitzenkörper core seen in a fixed hypha
using phase contrast optics (Fig. 10a & 10b)
corresponded to a region in electron micrographs which was granular, free
of microfibrils, and was usually off center in a more rounded or blunt apical
dome (Fig. 10c). The apical cytoplasm in these
hyphae contained a few scattered apical vesicles. Mitochondria and other
cytoplasmic components where dispersed in the area usually occupied by an
organized cluster of vesicles (Fig. 10c). The location
corresponding to the core (see Fig. 10c enlargement)
lacked apical vesicles, mitochondria, and other organelles except for a
few ribosomes. Many of the mitochondrial profiles were in cross section
and some were located at the apical, lateral and subapical peripheries of
the core. The cytoplasm surrounding the core also contained microvesicles,
small tubular membrane profiles, and some ribosomes. In addition, the apical
cytoplasm contained prominant clusters of tubular ER
(Fig. 10c) not seen in controls
(Fig. 8c, 8d). The
subapical cytoplasm within 1.5 hyphal diameters of the apex in the treated
tips also contained similar accumulations of tubular ER
(Fig. 10c) which were not present in controls
(Fig. 8c, 8d). The ER in the subapical cytoplasm appeared as long, rough cisternae
with limited smooth regions and very few of the unusual tubular clusters
Responses to cytochalasins
Our primary purpose in using CA and CE was to investigate the role of microfilaments
in hyphal tip growth. We previously presented evidence supporting CA as
a microfilament inhibitor in fungi (12). Here
we have provided additional indirect support for this suggestion since two
general sulfhydral reagents fail to mimic the effect of CA, thereby making
unlikely a potential mechanism of action for CA (24)
in our study. Though we are unable to prove that cytochalasins are acting
by inhibiting actin polymerization, accepting this hypothesis yields the
most straightforward interpretation of our observations and is consistent
with the observations of others.
Support for a significant role for actin in hyphal tip growth is increasing.
Highly polar F-actin networks have been demonstrated in a variety of fungi
(reviewed by Heath in 19) and ultrastructural
observations reveal a microfilament network in the hyphal apex (see review
by Heath in 19, 10).
The microfilaments were found to be most abundant in the core region (2,
25) and were identified in the core as actin
via ultrastructural immunolocalization (21).
Howard (2) suggested that this microfilamentous
network serves as a mechanism to control the localized secretion of growth
materials in the tip. Actin microfilaments might be involved in movement
of vesicles from subapical regions to the hyphal tip. They have also been
found in association with mitochondria and other organelles in a variety
of organisms (26, 27,
28, 29) and may
help determine the distribution and orientation of these organelles. If
CA and CE are inhibiting actin polymerization then our observations support
all of these roles for microfilaments. Cytochalasin treatment leads to rapid
depolarization of growth, an observation consistent with a role for microfilaments
in supporting localized secretion. Cytochalasin treatment also leads to
dispersal of the cluster of vesicles and to a switch from polar to nonpolar
growth. This observation is consistent with a role for microfilaments in
the movement of vesicles to the hyphal apex and in organizing the vesicle
cluster. Finally, cytochalasin treatment leads to a reorientation of mitochondria
suggesting that together with microtubules (23,
30), microfilaments help determine the position
of these organelles.
Bartnicki-Gracia, et al (4) proposed
a mathematical model to explain hyphal tip growth. The model hypothesizes
that a vesicle supply center (VSC) controls hyphal morphogenesis. The model
predicts that the position of VSC determines the polarity of growth and
that as the VSC moves away from the apex, growth becomes nonpolar. Cytochalasin
treatment of growing hyphal tips leads to retraction of the phase contrast
light portion of the Spitzenkörper from the hyphal tip and a change
from polar to nonpolar growth. This observation suggests that the phase
contrast light core of the Spitzenkörper is the natural equivalent
of the proposed VSC.
The persistent inner core of the Spitzenkörper:
Our novel observation that the core of the Spitzenkörper is a stable
component of the cytoplasm also suggests that the core is the equivalent
of the VSC. Earlier the Spitzenkörper was found to accompany tip growth
in septate fungi (31, 32,
5) but its nature and content have not been
adequately defined. Apical vesicles are clearly present and are assumed
to have a secretory function. But controversy exists over whether the cluster
of vesicles constitutes the whole of the structure or if other components
contribute (32, 5,
2). Evidence for cytoskeletal material within
the apical cytoplasm has been found using fluorochromes to label specific
materials (16, 18,
15, 17) and in
freeze-substituted specimens observed with electron microscopy (33,
9, 2, 25,
10). This study indicates that the phase contrast
dark portion of the visible Spitzenkörper may include only those vesicles
tightly clustered around the periphery of a more complex phase contrast
bright core. This central core includes microfilaments (10,
25, 21) and smooth
membrane profiles (10, 25).
This core is retained and can be identified even after the usual structure
and position are disrupted by anti-microfilament agents as shown in the
Little attention has been given to the potential contribution of endoplasmic
reticulum to tip growth although small tubules or cisternae were seen in
and around the Spitzenkörper (11, 25).
In Sclerotium rolfsii after 30 min of treatment with 0.1 µg/ml
of the demethylase inhibitor, cyproconazole, the Spitzenkörper was
still present but a proliferation of smooth ER had occurred (10).
The present study finds a rapid proliferation of tubular ER in response
to applied inhibitor and suggests that much shorter times (<1 min) are
needed to detect direct responses of growing hyphal tips to environmental
stresses. It is interesting that during growth of control hyphae intermittent
interactions are observed via phase-contrast microscopy between ER components
and the core of the Spitzenkörper (personal observations not shown).
It is significant that similar interactions were recently video recorded
by Rosa Maria Lopez-Franko (34 and personal
communication). Could these interactions represent the contribution of membrane
and luminal material from the ER directly to the core? If so, does the rapid
proliferation of tubular ER in treated tips indicate a buildup of material
due to blocked transfer caused by the disintegration of core structure and
function? Does this reveal an alternate to transport by apical vesicles,
possibly a system for transferring a subset of products from ER directly
to the apex?
To better illuminate the nature of the Spitzenkörper or proposed VSC
(4) a method is needed to allow direct observation
by video-enhanced microscopy of responses to applied stresses followed shortly
by rapid preservation and study of the same hypha by electron microscopy.
Until then the video-enhanced observations can be profitably coupled with
standard chemical fixation of high quality to extend our understanding of
the growing hyphal apex. In the study of such delicate and complex phenomena
the value of correlations made on the same specimen with different appropriate
techniques should not be underestimated.
This research was supported in part by grants from the Faculty Research
Fund at Goshen College, The William and Flora Hewlett Foundation via Research
Corporation, and the NIH (AI 14678).
- Grove, S.N. (1978) The cytology of hyphal
tip growth. In: The Filamentous Fungi (Smith JE, Berry DR Eds), Vol. III
pp 28-50 Arnold, London.
- Howard, R.J. (1981) Ultrastructural analysis
of hyphal tip cell growth in fungi: Spitzenkörper, cytoskeleton and
endomembranes after freeze-substitution. Journal of Cell Science 48, 89-103.
- Bartnicki-Garcia, S. (1973) Fundamental
aspects of hyphal morphogenesis. Symp Soc Gen Microbiol. 23. 245-267.
- Bartnicki-Garcia, S., Hergert, F., &
Gierz, G. (1989) Computer simulation of fungal morphogenesis and the mathematical
basis for hyphal (tip) growth. Protoplasma 153, 6-57.
- Grove, S.N. & Bracker, C.E. (1970).
Protoplasmic organization of hyphal tips among fungi: vesicles and Spitzenkörper.
Journal of Bacteriology 104, 989-1009.  MEDLINE
- Grove, S.N. & Sweigard, J.A. (1981)
The Spitzenkörper core persists after tip growth is arrested by cytochalasins.
Mycological Society of America Newsletter 32, 33.
- Bracker, C.E., & Lopez-Franco, R. (1990)
The hyphal tip: a cell designed for growing, and growing, and... Abstracts
4th Intl Mycological Congress Regensburg, Germany 1B-69/4.
- Lopez-Franco, R., Howard, R.J., &
Bracker, C.E. (1990) Video-microscopy of growing hyphal tips. Abstracts,
4th International Mycological Congress at Regensburg, Germany 1B-83/2.
- Hoch, H.C. & Howard, R.J. (1980) Ultrastructure
of freeze-substituted hyphae of the Basidiomycete Laetisaria arvalis.
Protoplasma 103, 281-297.
- Roberson, R.W. & Fuller, M.S. (1990)
Effects of the demethylase inhibitor, cyproconazole, on hyphal tip cells
of Sclerotium rolfsii. II. An electron microscope study. Experimental
Mycology 14, 124-135.
- Grove, S.N., Bracker, C.E. & Morre',
D.J. (1970) An ultrastructural basis for hyphal tip growth in Pythium
ultimum. American Journal of Botany 57:245-266.
- Grove, S.N. & Sweigard, J.A. (1980)
Cytochalasin A inhibits spore germination and hyphal tip growth in Gilbertella
persicaria. Experimental Mycology 4, 239-250.
- Howard, T.H. & Lin, S. (1979) Specific
interaction of cytochalasins with muscle and platelet actin filaments in
vitro. Journal of Supramolecular Structure 11, 283-293.
- Cooper, J.A. (1987) Effect of cytochalasin
and phalloidin on actin. Journal of Cell Biol. 105, 1473-1478. 
- Hoch, H.C. & Staples, R.C. (1985)
The microtubule cytoskeleton in hyphae of Uromyces phaseoli germlings:
its relationship to the region of nucleation and to the F-actin cytoskeleton.
Protoplasma 124, 112-122.
- Anderson, J.M. & Soll, D.R. (1986)
Differences in actin localization during bud and hypha formation in the
yeast Candida albicans. Journal of General Microbiology 132, 2035-2047.
- Tucker, B.E., Hoch, H.C. & Staples,
R.C. (1986) The involvement of F-actin in Uromyces cell differentiation:
the effects of cytochalasin E and phalloidin. Protoplasma 135, 88-101.
- Heath, I.B. (1987) Preservation of a labile
cortical array of actin filaments in growing hyphal tips of the fungus Saprolegnia
ferax. European Journal of Cell Biology 44,10-16.
- Heath, I.B. (1990) The roles of actin
in tip growth of fungi. International Review of Cytology 123, 95-127.
- Jackson, S.L. & Heath, I.B. (1990)
Evidence that actin reinforces the extendable hyphal apex of the oomycete
Saprolegnia ferax. Protoplasma 157, 144-153.
- Bourett, T.M. & Howard, R.J. (1991)
Ultrastructural immunolocalization of actin in a fungus. Protoplasma 163,
- McGoldrick, C. A., Gruver,C. & May,
G.S. (1995) myoA of Aspergillus nidulans encodes an essential myosin
I required for secretion and polarized growth. J. Cell Biol. 128, 577-587.
- Howard, R.J. & Aist, J.R. (1977) Effects
of MBC on hyphal tip organization, growth, and mitosis of Fusarium acuminatum,
and their antagonism by D2O. Protoplasma 92,195-210.
- Lagunoff, D. (1976) The reaction of cytochalasin
A with sulfhydryl groups. Biochemical and Biophysical Research Communication
- Roberson, R.W. & Fuller, M.S. (1988)
Ultrastructural aspects of the hyphal tip of Sclerotium rolfsii preserved
by freeze substitution. Protoplasma 146, 143-149.
- Palevitz, B.A. & Hepler, P.K. (1975)
Identification of actin in situ at the ectoplasm-endoplasm interface of
Nitella. Microfilament-chloroplast Association. Journal of Cell Biology
- Bradley, T.J., & Satir, P. (1979)
Evidence of microfilament-associated mitochondrial movement. Journal of
Supramolar Structure 12, 165-175.
- Hirokawa, N. (1982) Cross-linker system
between neurofilaments, microtubules, and membranous organelles in frog
axons revealed by quick-freeze, deep-etching method. Journal of Cell Biology
94,129-142.  MEDLINE
- Hoch, H.C. & Staples, R.C. (1983)
Ultrastructural organization of the non-differentiated uredospore germling
of Uromyces phaseoli variety typica.. Mycologia 75, 795-824.
- Herr, F.B. & Heath, M.C. (1982) The
effects of antimicrotubule agents on organelle positioning in the cow pea
rust fungus, Uromyces phaseoli var vignae. Experimental Mycology
- McClure, W.K., Park, D., & Robinson,
P.M. (1968) Apical organization in the somatic hyphae of fungi. Journal
of General Microbiology 50, 177-182.  MEDLINE
- Girbardt, M. (1969) Die Ultrastruktur
der Apikalregion von Pilzhyphen. Protoplasma 67, 413-441.
- Howard, R.J. & Aist, J.R. (1979) Hyphal
ultrastructure of the fungus Fusarium: improved preservation by freeze-substitution.
Journal of Ultrastruct Research 66, 224-234.
- Lopez-Franco, R., S. Bartnicki-Garcia
& C. E. Bracker (1994) Pulsed growth of fungal hyphal tips. Proc Natl
Acad Sci U S A 9127:12228-12232  MEDLINE
©1996 Epress Inc.