ArticlePDF AvailableLiterature Review
Copyright 1968. All rights reserved
CELL WALL CHEMISTRY, MORPHOGENESIS,
AND TAXONOMY OF FUNGI
1
BY S. BARTNICKI-GARClA
Deparlment of Plan~ Pathology, University of California,
Riverside, California
1504
CONTENTS
GENERAL STRUCTURE ............................................ 88
Protein .......................................... ................. 88
Lipid ............................................................ 88
Polysaccharlde ..................................................... 89
Analytical techniques ............................................... 89
CELL WALL CHEMISTRY AND TAXONOMY ....................... 90
GROUt, L CELL~-LOsE-GLYCOGEI~ ...................................... 92
GRouP II. CELLULOsE-GLucAN ........................................ 93
GROUP III. CELLULOsE-CtIITIN ....................................... 94
GROUP IV. CHITOSAN-CH1TIN ......................................... 94
GROUP V. CHITIN-GL~JCAN ........................................... 95
GROUP VI. MANNAN-GLucAN ......................................... 98
GROUF VII. MANNAN-CH1TIN ......................................... 98
GROUP VIII. POLYOALAETOSAMINE-GALACTAN ........................... 99
CELL WALL STRUCTURE AND MORPHOGENESIS ................. 99
CHEMICAL DIFFERENTIATION OF THE CELL WALL ....................... 99
Ve~etatlve development .............................................. 99
Germination and sporogenesis ........................................ 101
CELL WALL SPLITTING ENZTMES ...................................... 101
Protein disulfide reductase ........................................... 101
Glucanases ........................................................ 102
ULTRA~TRUCTURE OF CELL WALL DEVELOPMENT ....................... 102
Spore germination ................................................. 102
Hyphal morphogenesis .............................................. 103
CELL WALL CONSTRUCTION .......................................... 103
Polymer synthesis .................................................. 103
Polymerization sites ................................................ 104
Assembly patterns .................................................. 104
CONCLUSION ....................................................... 105
The prominent role of the cell wall in the structure and behavior of
fungi needs little elaboration; with few exceptions, the wall, more than any
other cellular part, defines a fungus and distinguishes it from other living
creatures. Only during the brief amoeboid or flagellate stages of a minority
t The survey of the literature pertaining to this review was concluded in January
1968.
87
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88 BARTNICKI-GARCIA
of fungi do we find the fungal protoplast deprived of its characteristic
housing. For most fungi, the wall is a permanent and highly versatile home
--continuously expanded during growth, extensively remodelled during de-
velopment. By manipulating cell wall construction, a fungus may assume a
variety of characteristic morphologies to suit a wide variety of functions:
vegetative growth, substrate colonization, reproduction, dispersal, survival,
host penetration, animal predation, etc. In simplified terms, morphological
development of fungi may be reduced to a question of cell wall morphogen-
esis.
This review deals primarily with recent chemical studies on fungal walls
and how they relate to taxonomy (phylogeny) and to morphogenesis (ontog-
eny). For an introduction to the general features of fungal cell walls, the
reader is referred to Aronson’s chapter (1). The literature dealing specifi-
cally with yeast cell walls was the subject of a recent review by Phaff (2).
GENERAL STRUCTURE
Chemically, the fnngal cell wall is 80 to 90 per cent polysaccharides with
most of the remainder consisting of protein and lipid. Wide departures
from these values are rare; e.g., the cell wall of the yeast Saccharomycopsls
gu~tulata which contains 40 per cent protein (3). Sometimes substantial
amounts of pigments (melanin), polyphosphate, inorganic ions, etc., may
also be present. There is partial evidence for the presence of nucleic acids
in fungal walls (4-7), but to date no decisive proof of their presence has
been provided. Physically, the fungal cell wall is a fabric of interwoven mi-
crofibrils embedded in or cemented by amorphous matrix substances. Chitin
and cellulose are well known as the microfibrillar or skeletal components of
the wall of the vast majorit3~ of fungi; whereas, in most true yeasts, the
skeletal part is seemingly composed of noncellulosic glucans. Proteins and
various polysaeeharides (glucans, mannans, galactans, heteropoly-
saccharides) are probably the cementing substances which bind together
the dit~erent structural components of the wall into macromolecular com-
plexes.
Protein.--Aronson’s (1) cautious reservation on the validity of regard-
ing the protein found in cell wall preparations of filamentous fungi as a
true structural component do6s not seem justified any longer. Although part
of the wall protein may be enzymic, and perhaps another portion could be a
cytoplasmic contaminant, some of the protein is so firmly bound to the rest
of the wall that drastic extractions usually fail to remove it completely [e.g.
(4, 7, 8)]. The latter protein is probably an integral part of the structure
the cell wall. There is also growing evidence for glyeoprotein complexes in
the cell walls of filamentous fungi (9-11), adding to the earlier findings
glycoprotein complexes in yeast walls [see reviews by Nickerson (12, 13)].
L~p~d.--Dyke’s (14) investigations on Nadson~a elongata demonstrated
clearly that the lipid found in the cell wall of this yeast was a bona fide
component and not a cytoplasmic contaminant; contrary to cytoplasmic lip-
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CHEMISTRY OF FUNGAL CELL WALL 89
ids, cell wall lipids lack palmitoleic acid and are mainly composed of satu-
rated fatty acids. The role of lipid in fungal cell walls has not been eluci-
dated. Hurst (15) suggested that lipid contributes to the stiffness of the cell
wall of Saccha,romyces cerevisiae. Another plausible function would be to
confer hydrophobic properties to certain cell structures such as sporangio-
phores and spores. Some of the lipid is firmly bound to the cell wall [e.g.
(4, 7, 8, 16, 17)] and may have a structural role (13).
Polysaccharide.--Polysaccharides of fungal cell walls are built from a
variety of sugars. At least 11 monosaccharides have been reported as occur-
ring in fungal cell walls, but only three, i)-glucose, N-acetylglucosamine,
and D-mannose, are consistently found in most fungi. Their relative propor-
tions, however, vary enormously from traces in certain organisms to princi-
pal components in others. The following monosaccharides are less fre-
quently found and with a more or less characteristic distribution among cer-
tain groups of fungi: D-galactose and D-galactosamine (Ascomycetes), l.-
fueose (Mueorales and Basidiomyeetes), D-glueosamlne (Mueorales),
lose (Basidiomycetes), and D-glucuronic acid (see below). Occasionally,
presence of small quantities of rhamnose (17-19), ribose (4, 7, 17, 20), and
arabinose (21, 22) has been reported. The role of these minor sugars is un-
certain.
As recently as 1965, Aronson (1) commented on the lack of conclusive
evidence for uronic acids in fungal walls; in the intervening time their
presence has been clearly demonstrated (23-25). Gancedo et al. (23)
tected as much as 2 per cent glucuronic acid in Dactylium dendroides and
smaller amounts in Alternaria, Fusarium, Penlcillium, and Aspergillus.
There is also paper chromatographic evidence for small amounts of uronic
acid in Rhieoctonia (40) and Neurospor~ (10). From the cell walls of Pul-
l~daria, pull*d(~,.s, Brown & Lindberg isolated a heteropolymer of mannose,
galaetose, glucose, and glueuronie acid (25). Perhaps the most striking ex-
ample of the presence of uronic acids in fungal walls is that of M~wor rouaqi
in which polymers of ~-glucuronic acid constitute as much as 25 per cent
of the sporangiophore wall (24). There appear to be two kinds of polyuro-
nide in these walls. One is an alkali-soluble heteropolymer ("mucoran")
containing about 50 per cent I~-glucuronic acid plus some fucose, mannose,
and galactose. The other is a highly insoluble, acid-resistant polyuronide,
probably a homopolymer of D-glucuronic acid (24). Skucas (26) found
the papillae of Allomyces sporangia were made of pectic acid material. This
is perhaps the only available example of 9-galactur0nic acid associated with
fungal walls.
Analytical techniques.--Powder X-ray diffraction continues to be the
most reliable single technique for identifying cell wall polymers such as chi-
tin and cellulose. Its usefulness may be extended to other less frequently
investigated polymers, like noncellulosic glucans and chitosan (27). Infrared
spectroscopy may be used as an auxiliary technique for distinguishing chi-
tinous cell walls from nonchitinous ones (28).
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90 BARTNICKI-GARCIA
CELL WALL CHEMISTRY AND TAXONOMY
Perhaps one of the most mutually satisfying by-products of fungal cell
wall research, to taxonomists and biochemists alike, is the close correlation
that can be established between chemical composition of the cell wall and
major taxonomic groupings elaborated on morphological criteria. This
correlation first became apparent last century when van Wisselingh (29)
showed that cell walls of fungi could be either chitinous or cellulosic; later,
yon Wettstein (30) used this criterion to support the division of aquatic
Phycomycetes into two major taxa. Although some fungi within these
groups were incorrectly assigned owing to dubious cytochemical tests em-
ployed for identifying chitin and cellulose, the more recent X-ray work of
Frey, Aronson, Fuller and co-workers [see review by Aronson (1)] reaf-
firmed the validity of equating the presence of chitin or cellulose with taxo-
nomic position within the Phycomycetes. Confirmation of the simultaneous
presence o~ cellulose and chitin in Rhizidiomyces led Fuller & Barshad
(31) to the conclusion that a third wall category exists which more closely
parallels the currently accepted tripartite division of the aquatic Phycomy-
ceres, made on the basis of flagellation (Oomycetes, Chytridiomycetes, and
Hyphochytridiomycetes ).
With the advent o{ mechanical methods to prepare essentially pure cell
walls, and of paper chromatography to characterize their monomeric com-
ponents, there has been a recent surge of interest in cell wall chemistry of
fungi. Although the data are exceedingly limited in the number of related
organisms examined, diversity of groups selected, and analytical refinement,
it has become increasingly clear that the entire spectrum of fungi may be
subdivided into various categories according to the chemical nature of their
walls, and that these categories closely parallel conventional taxonomic
boundaries. The classification of fungal cell walls proposed in Table I is
based on dual combinations of those polysaccharides which appear to be the
principal components of vegetative walls. These include, in addition to chi-
tin and cellulose, chitosan, mannans, a glycogen-like polymer, and ubiqui-
tous noncellulosic glucans (probably ~1, 3- and ~1, 6-bonded) ~or which the
admittedly ambiguous term "glucan" will be used pending further charac-
terization. Galactose and galactosamine polymers are found in the border-
line group VIII.
For the establishment of the eight wall categories, the presence of small
amounts of other classifying polysaccharides was disregarded (for instance,
traces of chitin are present in the yeasts of category VI). These eight cate-
gories probably represent a minimum of dual combinations. A detailed char-
acterization of glucans (or other components) might, for example, provide
a basis for subdividing the chitin-glucan category, the largest of all. Like-
wise, examination of neglected taxa may reveal new categories. The corre-
lation between wall chemistry and taxonomy may be effectively extended to
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CHEMISTRY OF FUNGAL CELL WALL 91
the genus level, although in such close proximity the differences are apt to
be minor and chiefly, if not entirely, quantitative. Thus, by measuring sugar
ratios in alkali-soluble fractions from the walls of various Basidiomycetes,
O’Brien & Ralph (33) showed clear differences between related genera
(e.g., Fomes versus Fomitopsis; Inonotus versus Pdyporus). Serological
differences which are sometimes used to distinguish species could be reflec-
tions of minor differences in wall composition (34, 35).
Some broad generalizations result from examination of Table I. The
TABLE I
CELL WALL TAXONOMY OF FUNGI
Chemical Category Taxonomic Group
"b Distinctive Features
I. Cellulose-Glycogen
t
tI. Cellulose-Glucan*
III. Celhdose-Chltln
IV. Chltosan-Chltln
V. Chltln-Glucang
VI. Mannan-Glucane
VII. Mannan-Chitln
VIII. Polygalactosamine-
Galactan
Acraslales
Oomycetes
Hyphochy tridlomycetes
Zygomycetes
Chytridlomycetes
Ascomycetes
o
Basidiomycetes
a
Deuteromycetes
°
Saccharomycetaceae
Cryptococcaceae
Sporobolomycetaceae
Rhodotorulaceae
Triehomycetes
pseudoplasmodia
biflagellate zoospores
anteriorly un]flagellate
zoospores
zygospores
posteriorly uniflagellate
zoospores
septate hyphae, ascospores
septate hyphae, basidiospores
septate hyphae
yeast cells, ascospores
yeast cells
yeasts (carotenoid pigment)
ballistospores
yeasts (carotenoid pigment)
heterogenous group,
arthropod parasites
¯After Alexopoulos (32).
b Not all orders or families within each group have been examined for wall com-
position. Further segregation is possible.
~ Except Saccharomycetaceae.
a Except Sporobolomycetaceae.
~Except Cryptococcaceae (and Rhodotorulaceae).
~Incompletely characterized.
¯Incompletely characterized; probably ~1,3- and B1,6-1inked.
vast majority of fungi, including all forms with typical septate mycelium,
have a eh~tin-ghman cell wall (category V). A departure from the myeellal
to the yeast habit is accompanied by a substantial increase in the mannan
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92 BARTNICKI-GARCIA
content of the wall (categories VI and vii), a relationship disclosed some
time ago by Garzuly-Janke (36). Celhflose is characteristic of many but not
all lower fungi. Significantly, the greatest departures in wall composition
from the chltln-glucan axis are {ound in the Acraslales and Trichomycetes,
groups whose inclusion among the authentic flmgi has long been argued.
Cell wall chemistry also serves to distinguish fungi ~rom other living
creatures. The lack of close kinship between Actinomycetes (37-39) and
fungi may be deduced from their entirely different wall composition as well
as other criteria. Likewise, although fungal walls exhibit some similarity to
algal walls (41, 114), distinctive traits can be recognized. For instance,
Parker et el. (42) folmd that two phylogenetically related groups, Vaucher-
iaceous algae and Saprolegniaceous fungi, differ greatly in wall features,
such as content and crystallinity of cellulose, etc.
It seems certain that a greater availability of chemical data on fungal
wails would be of help in dissipating taxonomic ambiguities. Similarly, the
correlation between wall chemistry and taxonomy could serve as a guide to
the biochemist in the exploration of cell wall features throughout the fungi,
and to re-evaluate analytical results inconsistent with taxonomic position.
No drastic differeuces in major strnctnral components are apt to occur
among fungi which are closely related by morphological criteria. For in-
stance, a seemingly simple change like the inversion of the glycosidic link-
age in a glucose polymer, from ~1, 4 to ~1, 4, would cause such vast changes
in the physical properties of the polymer that it is doubtful that the shape
and integrity of the wall could be retained.
The constancy of wall composition is by no means absolute, and a cer-
tain latitude in quantitative, and sometimes qualitative, composition is seen.
Frequently, these alterations are in response to changes in environmental
conditions, bringing about concomitant changes in cellular shape; hence, the
close interdependence of cell wall chemistry, environment, and morphology
observed in fungi [e.g. (4, 43)].
Salient features and selected examples of each category are briefly dis-
cussed in the following sections.
GROUP I. CELLIILOSE-~LYCOGEIg
By X-ray diffraction a poorly crystalline cellulose was found in the soro-
phores of Dictyostelium and ~4cytostelium (44-46) and the mlcrocyst walls
of Polysphondylh,m (47). Cellulose and a glycogen-like polysaccharide
were found in spore walls of Dictyostelium discoideum prepared by alkali
extraction (48). The extent to which cytoplasmic glycogen may have con-
tributed to this fraction is uncertain. Microcyst wails of Polysphondylium
pallidum prepared by sonic treatment contain approximately equal amounts
of two glueans: cellulose and an alkali-soluble glucan assumed to be glyco-
gen (47). These walls also contain high levels of protein and lipid.
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CHEMISTRY OF FUNGAL CELL WALL 93
GRouP II. CELLULOSE-GLUCAN
Recent papers described the structure of hyphal walls of Oomycetes of
the orders Saprolegniales and Peronosporales with two phytopathogenic gen-
era, Phytophthora and Pythium, receiving the most attention. Except for a
questionable claim (49), there is agreement on the similarity of wall com-
position among these fungi. Traditionally, the Oomycetes have been re-
garded as the cellulosic fungi; surprisingly, cellulose was found not to be
the major component of the hyphal walls of any of the examined genera:
Phytophtho,ra (7, 17, 50), Pythh~m (17, 51), Achlya (42, 50), Saprolegnia
(17, 42), Atkinsiella (50), Brevilegnia, or Dictyuchus (42). Instead, the
principal wall component was an alkali-insoluble and cuprammonium-insolu-
ble gluean(s) containing [31, 3, and [31, 6 linkages (7, 50-52). Cellulose (type
I) was apparently present in a poorly crystalline state (7, 42, 51). Signifi-
cantly, removal of cellulose by cuprammonium extraction did not visibly af-
fect the overall morphology of hyphal walls of Phytophthora (7). Possibly,
the noncellulosic glucan plays a skeletal role in the architecture of these
walls. Although glucose is the principal monomer, small amounts of man-
nose are usually detected in oomycete cell walls (7, 17, 20, 51). The rela-
tively high estimates of mannose for some Saprolegniales may be due to
incomplete removal of cytoplasmic components by the extractive procedure
used in preparing the wall material (42).
The cell wall glucan of Pythium butleri was claimed to be a ~l,2-11nked
polymer analogous to the crown-gall polysaceharide of Agrobacterium tume-
faciens, mainly on the basis of partial similarity in X-ray reflections (49).
However, Novaes-Ledieu et al. (17) and Aronson and collaborators (50,
51) showed convincingly that the hyphal walls of Pythium butleri and P.
debaryanum contain mainly ~1,4-, [31,3-, and ~l,6-1inked glucans as the ~valls
of other Oomycetes do. Although the absence of a small proportion of [31,2
linkages cannot be ruled out, more definite proof is needed to substantiate
their presence in the wall glucans of these fungi.
The absence of any appreciable amount of chitin in these fungi was con-
firmed (7, 51). Only a small amount of hexosamine (usually less than 1
cent) can be detected in oomycete cell walls, but its polymeric state is un-
known. Investigations on yeast cell walls suggested that hexosamine may be
important in the formation of glyeoprotein complexes (53, 54). A distin-
guishing feature of this wall category is the presence of hydroxyproline in
the cell wall protein (7, 17, 20). Failure to detect this and other amino acids
in P. butleri (49) may have resulted from incomplete release and destruc-
tion during alkaline hydrolysis. I-Iydroxyproline is not found in fungi with
chitinous walls (20) ; it is a characteristic amino acid of cellulosic walls
either fungi, algae (55), or higher plants (56). Recent evidence indicates
that hydroxyproline provides an important (glycosidic) link-between poly-
saccharides and protein (57).
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94 BARTNICKI-GARCIA
GROUe IlL CELLULOSE-CHITIN
Only the Hyphochytridiomycetes are presently considered as members of
this category. The presence of both cellulose and chitin was convincingly
demonstrated in Rhi~idiomyces sp. (31, 58), the only representative criti-
cally studied so far. In the absence of quantitative data on wall composition,
the validity of their inclusion in this category rests chiefly on the fact the
Hyphochytridiomycetes constitute an intermediate group of Phycomycetes
distinct from the cellulosic Oomycetes and the chitinous Chytridiomycetes.
The simultaneous presence of cellulose and chitin in the cell walls of a
variety of fungi has long been claimed. Most reports, however, failed to
provide confirmatory X-ray evidence [for examples refer to Frey (59);
Aronson (1)]. At present, Ceratocystis ulmi is the only higher fungus in
which both chitin and cellulose have been demonstrated by X-ray diffraction
(60). In all other mycelial Ascomycetes and Basidiomycetes which have
been critically examined, chitin but not cellulose was consistently detected.
The anomaly posed by the case of C. ulmi may be reconciled in two ways.
Ceratocystis and related genera of the Ophiostomaceae may be a special
group of Ascomycetes legitimately belonging in this category of fungal
walls. Alternatively, in the absence of quantitative information, it is permis-
sible to suggest that the cellulose found on the walls of C. ulmi may repre-
sent a minor portion of the cell wall, the main component being nonce|lu-
losic ghlcan as in the cell walls of other Ascomycetes.
GROUP IV. CHITOSAN-CItITIN
Several representatives of the Zygomycetes have been examined. By
X-ray diffractometry, chitin was detected in the Mucorales [Mucor, Rhizo-
pus (59) Phycomyces (6 1)] an d En tomophthorales [Basidiobolus and En-
tomophthora (59)]. No representatives of the third order (Zoopagales)
have been examined. The highly distinguishing properties of this cell wall
category are derived from limited but consistent evidence regarding several
members of the Mucoraceae: quantitative analyses of Mucor rou.~:ii (4, 9,
24, 62), qualitative study of Zygorhynchus vuilleminii (20), and partial
polymer characterization of Pl*ycomyces blakesleeanus (27). Chitosan, the
group characteristic polymer, is a poorly or nonacetylated polyglucosamine
found by Kreger (27) in the cell walls of the mycelium and sporangio-
phores of Phycomyces. It also occurs in the mycelial, yeast, and sporangio-
phore walls of Mucor (4, 62). This polycation is probably neutralized
the concurrent presence of large amounts of phosphate (inorganic
polyphosphate ?) and polyuronides in the cell walls of Mucor (4, 24).
Fucose, mannose, galactose (4, 20), and substantial quantities of D-glu-
curonic acid (24) are present in this category of cell walls. These sugars
may form part o} heteropolyuronides such as the one isolated from M.
rou~rii cell walls (24). A conspicuous feature of the group is the absence
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CHEMISTRY OF FUNGAL CELL WALL 95
glucose polymers from vegetative walls (Mucor, Zygorhyunchus ). Possi-
bly chitosan fulfills the structural role played by glucan in other fungi. The
absence of glucose would constitute an absolute point of divergency from all
other fungi (save for the disputable Trichomycetes) were it not for the
fact that glucan abruptly appears as the main component of the spore wall
of Mucor rouxii (9). The spore wall also differs from vegetative walls
other chemical features (Table II).
TABLE II
CHEMICAL DIFFERENTIATION OF THE CELL WALl, IN Tt~E LIFE CYCLE
OF MUCOR ROUXII (4, 9, 24, 62)
i
Wall Component Yeasts Hyphae Sporangiophores Spores
Chitin 8.4 9.4 18.0 2.1
b
Chitosan 27.9 32.7 20.6 9.5
b
Mannose 8.9 1.6 0.9 4.8
Fucose 3.2 3.8 2.1 0.0
Galactose 1.1 1.6 0.8 0.0
Glucuronic Acid 12.2 11.8 25.0 1.9
Glucose 0.0 0.0 0.1 42.6
Protein - 10.3 6.3 9.2 16.1
Lipid 5.7 7.8 4.8 9.8
Phosphate 22.1 23.3 0.8 2.6
Melanin 0.0 0.0 0.0 10.3
~ Values are per cent dry wt of the cell ~vall.
b Not confirmed by X ray. Value of spore chitin represents
amine; chitosan is nonacetylated glucosamine. N-acetylated glucoso
Apart from morphogenetic considerations (9), one could also conjecture
that the shift in wall chemistry, from a chitosan-chitin type in vegetative
cells to a glucan-polyglucosamine (chitin?) type in spores, represents
transition from group IV to group V; hence, we may be witnessing an on-
togenetic recapitulation of a phylogenetic relationship.
GROUP V. CHITIN-GLucAN
This is by far the most numerous category, harboring all mycelial forms
of the Ascomycetes, Basidiomycetes, and Deuteromycetes. Also included is
a segment of lower fungi, the Chytridiomycetes. This inclusion is tentative
pending characterization of the wall glucan(s). Presently, the very limited
information available on the glucose polymers of .4llornyces walls (6, 63)
does not rule out the possibility that they may be similar, at least in part, to
the wall glucans o~ higher fungi. Confirmation of a close similarity in glucan
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96 BARTNICKI-GARCIA
structure would strengthen those phylogenetic schemes which propose that
higher fungi evolved from ancestral Chytridiomyeetes [see Klein & Cron-
quist (148) for a recent review].
The chitin content oscillates widely from 3 to 5 per cent in Schizophfl-
lure commune (64) to about 60 per cent in Allomyces macrogynus (6)
Sclerotium rolfsii (65). The balance consists largely of noneellulosic glu-
cans which have been only superficially characterized, if at all. At least two
different kinds of glucans may be present; these are usually separated by
their solubility in alkali. Although seldom attempted, the noneellulosie glu-
cans may be identified by X-ray diffraction [Kreger (27)]. Thus, the alka-
li-soluble fraction of cell walls of Penicillium, Endomyces, Agaricus (27),
and Schizophyllum (64) exhibited sharp reflections corresponding to those
of a glucan first discovered in Schizosaccharomyces (27). The nature
this soluble glucan, abbreviated S-gluean (64) is not well known; seemingly
it contains mainly ~1,6 linkages [Wessels (64)]. The alkali-insoluble frac-
tion may be characterized as "yeast" glucan upon conversion to its hydro-
glucan by acid treatment, as has been done for the cell walls of various En-
domycetales (27), Schi~ophyllum co~nmune (64), and Verticillium albo-
atrum (66). It is not known if both glueans occur throughout the entire
spectrum of fungi with ehitin-gluean cell walls. There is already one excep-
tion, Agaricus, which apparently lacks "yeast" glucan (27).
The linkage most commonly reported in these noncellulosic glucans is ~1,3.
In addition to the ~1,6 mentioned above, a-l,3 and a-l,4 have also been de-
tected. Repeated claims for the presence of cellulose (~1,4 glucan) in walls
of higher fungi have long been made. Some recent ones include Fusariu~u
(67), AspertTillus (68), Schi,~ophyllum (69), Venturia (70), and Ceratocystis
(60). Except ~or the last one (see Group III) no conclusive X-ray evidence
was submitted to substantiate these claims. In fact, X-ray analyses of the
cell walls of Fusarium (59), Aspergillus (8), Schi~ophyllum (64), and
variety of other representatives of this wall category, have failed to reveal
the presence of cellulose.
The major classes of {ungi included in this category differ from one an-
other with regard to the presence of minor sugars. Barring exceptions
which could have been the result of incomplete hydrolysis or incorrect iden-
tification, the following rules seem applicable. Ascomycetes have galactose
and galactosamine in their walls, while lacking xylose and fucose. The re-
verse is true in the ]3asidiomycetes. Mannose occurs in both classes (20,
33). In the Chytrldlomycetes (Atlomyces) none of these sugars was de-
tected (6, 63).
Quantitative analyses of wall composition have been published for As-
pergillus (5, 8, 21, 65), Neurospora (10, 43, 71), Pithomyces (72, 73), Pen-
icillium (18, 74), Sclerotium (65, 75), Rhizoctonia (40, 76), Fusarium
(77), Venluria (70), Schi~ophyllum (64), H istoplasma (78), Trlchophyton
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CHEMISTRY OF FUNGAL CELL WALL 97
(79). dspergillus and Neurospora will be discussed in some detail.
There is general agreement on the presence of N-aeetylglucosamine, glu-
cose, mannose, and galactose in Aspergillus (5, 8, 20, 21, 65) but galactos-
amine has not been always reported even though Aspergfllus spp. were
found to have the highest content of myeelial galaetosaminoglyean from
among a variety of fungi examined (80). The insoluble glucan of Aspergillus
was proven long ago not to be cellulose (81). Enzymic digestion of cell walls
of various species of dspergillus indicated a preponderance of ~l,3-1inked
glucose residues (5, 65, 77, 83). Johnston (21, 82) separated three glucan
fractions from the mycelial walls of A. niger. Two glucans were alkali-solu-
ble; one of them was further soluble in hot water and was identified as
nigeran (alternating al,3 and al,4 bonds). The main component of this
fraction was a glucan composed chiefly of el,3 bonds and a small propor-
tion of ~1,4. The alkali-insoluble gluean was not characterized, but it prob-
ably contains the ~1,3 linkages found by others in Asperyillus cell walls.
According to Ruiz-Herrera (8), mannose and galactose are associated
with the alkali-insoluble glucan. He found no evidence for nigeran in an
dspergillus sp. Nigeran was found to be only a minor component of the
walls of a strain of d. niger examined by Johnston (21) and, even though
its presence in the wall has been confirmed (84), its role in wall structure
would seem doubtful; nigeran is appareutly a dispensable component of
d. niger (85).
About two thirds of the hyphal wall of Neurospora crassa has been ac-
connted for as chitin and glucan (10, 43, 71, 86). The protein content was
calculated from total N values to be as high as 14 per cent; this figure is
probably an overestimate, since hexosamine-N is usually incompletely ac-
counted for. The presence of ~1,3 and ~1,6 linkages in Neurospora cell wall
glucan(s) has been reported (10, 71). Small amounts of galactosamine
detected in walls of N. crctssa (10, 87) and N. sitophila (20). Mahadevan
’.Tatum (10) extracted the walls of a variety of strains and mutants of
crassa into four fractions. Galactosamine was found in the hydrolyzate of
an alkali-soluble fraction which also contained glucuronic acid, glucose, and
amino acids. These monomers seemingly form part of a g]ycoprotein complex
important to hyphal morphology (see below) ; the glycoprotein appears under
the electron microscope as coarse microfibrils located in the outer portions
of the wall (86). By comparing the digestive action of ~1,3 glncanase and
chitinase, tested alone and in combination, against the hyphal walls of N.
crassa, it was concluded that chitin is probably surrounded or protected by
glucan molecules (71, 86). Lyric studies of this sort may be helpful in de-
termining structural differences among members of the chitin-glucan group.
For instance, the combination ~1,3 glucanase + chitinase dissolved only 40
per cent of the wall of Neurospora crassa; whereas, the same treatment
completely disintegrated the walls of Fusarium solanl [Potgleter & Alex-
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98 BARTNICKI-GARCIA
ander (71)]. The exact structural reason for this discrepancy is not known;
Manocha & Colvin (11) suggested that wall protein may have been respon-
sible for the retention of morphology of N. crassa after enzymic attack.
GROUP VI. IVIANNAN-GLUCAN
In this category are included the yeast forms of the Ascomycetes and
Deuteromycctes. A description of the corresponding literature on yeast
wall structure will not be attempted, the reader is advised to consult Phaff
(2) and Nickerson (12, 13). Typical members of this category (5:accharo-
myces, Candida, Hanseniaspora, Kloeckera, etc.) have mannan (s) and glu-
can(s) as their principal wall components. Chitin is present only in very
small amounts. Mannan and part of the glucan are alkali-soluble. Glycopro-
tein complexes of these polysaccharides have been isolated (12, 13). The
alkali-insoluble glucan, the so-called "yeast" glucan, is usually considered
the skeletal component of the wall, but its native fibrillar state is uncertain
(88). Also, its fine chemical structure has yet to be clearly elucidated; the
most recent report regards it as a branched polymer with ~l,5-1inked main
chains and ~l,3-1inked branches (89). A predominant mannan-glucan com-
position cannot be established for all organisms belonging to the Hemiasco-
mycetes. However, the discrepancies correlate to a large extent with the de-
gree of morphological dissimilarity between the divergent organism and the
typical budding yeasts. Thus, compared to budding yeasts, the mycellal Endo-
mycetales (Endomycopsis, Endomyces, Eremascus) have a reduced "yeast"
glucan content, a high chitin, and a very low mannan content (27) ; there-
fore, they are much closer to and probably belong with the typical mycelial
fungi of group V. Yeasts with unusual characters also depart in chemical
composition, e.g., the "triangular" yeast Trigonopsis variabilis (90) con-
tains little or no alkali-insoluble "yeast" ghmau. Several species of Schizo-
saccharomyces, the fission yeasts, contain "yeast" glucan but seem to lack
chitin and manan (27). Instead, they have in their walls an alkali-soluble
glucan (S-glucan, see group V) which is present in the Endomycetales and
in typical higher filamentous fungi (27). The virtual absence of mannan
Schizosaccharomyces has been confirmed (20). In other yeasts, e.g., Nadso-
nia elongata, conflicting claims of presence or absence of mannose polymers
have been made (14, 91 ).
GROUP VII. MANNAN-CHITIN
The pink yeasts of the genera Rhodotorula and Sporobolomyces fall in
this category. Their cell walls differ from those of yeasts of group VI in
that they contain only a small quantity of glucose polymers (20), seemingly
lack "yeast" glucan, and their chitin content is high (27). Their mannan
may also be different since it does not form an insoluble copper complex,
which probably explains previous negative findings (27, 36) [see also com-
ments on mannan detection (13) ]. Actually, mannose occurred abundantly
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CHEMISTRY OF FUNGAL CELL WALL 99
wall hydrolyzates of Sporobolomyces and Rhodotorula [Crook & Johnston
(20)]. Furthermore, these yeasts are the only ones having galactose, fu-
cose, and -l,-aminobutyric acid in their walls (20). The presence of fueose
significant since it is also characteristic of other Basidiomycetes wherein
these yeasts are classified. Interestingly, Tsuchiya et al. (35) reclassified
Rhodotorula with Sporobolomyces in the same subfamily (Sporobolomy-
cetoideae) on the basis of serological (wall antigens ?) similaritie~ (34,
as well as other known common properties. This relationship might also
have been deduced from examination of their unique wall composition men-
tioned above.
G~ouP vuI. POLYGALACTOSAMINE-GALACTAN
The properties of this category are typified by those of Amoebidium pa-
rasiticum (92), the only Trichomycete so far examined. Since this is a bet-
erogenous group, the cell wall composition may not be uniform throughout
the Trichomycetes. In marked contrast to the walls of other fungi, the thal-
lus walls of A. parasltlcum are entirely soluble in alkali. The insoluble
polymers of glucose and (acetyl) glucosamlne, characteristic of other fungi,
are absent in A. parasiHcum. Instead, Trotter & Whistler (92) found chro-
matographic evidence for polymers of galactosamine, the predominant com-
ponent, and galactose. Smaller amounts of xylose were also present; glucose
was absent.
CELL WALL STRUCTURE AND MORPHOGENESIS
Nickerson’s (93) pioneering approach to the elucidation of biochemical
bases of morphogenesis in fungi via a better understanding of cell wall
properties and behavior has continued to receive increasing attention. The
evidence, though circumstantial, does indicate that morphological diferen-
tiation is correlated with changes directly affecting cell wall metabolisml
thus strengthening the main premise of equating morphological differentia-
tion with cell wall differentiation (94). Elucidating the nature of the wall
bricks and cementing material is of course part of the story only. The cell
wall-building enzymes and their chemical as well as physical relationship to
the interior of the cell, constitute at the moment a formidable and barely
understood complex. The following selected examples illustrate some of the
chemical, enzymological, or cytological tactics that have been used in a stra-
tegic attack on the mystery of cell wall morphogenesis.
CHEMICAL DIFFERENTIATION OF THE CELL WALL
Vegetative development.--]Veurospora crassa may be induced, phenotypi-
tally or genotypically, to change its normal long, straight, sparsely branched
hyphae into short, undulating, highly branched ones (the so-called colonial
growth). Tatum and co-workers (10, 43, 86) have shown that this morpho-
logical change is accompanied by a marked increase in the ratio of
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100 BARTNICKI-GARCIA
hexosamine/glueose in the wall polymers. A glycoprotein seemingly made of
glucose, glucuronic acid, and galactosamine was the wall component most
critically affected during colonial growth of a variety of strains of N.
cr~ssc~ [Mahadevan & Tatum (10)]. Closely related is the observation that
an "osmotic" mutant of N. crassa,, with its less rigid and irregularly shaped
walls, contained in its walls much more galactosamine and less glueosamine
than the wild type (95). This suggests that a proper balance of hexosamine
polymers may be essential for maintaining the rigidity and regularity of hy-
phal walls of this fungus. Interestingly, selective removal of various wall
components from isolated normal hyphae of N. crassa (86) did not destroy
the integrity of the wall--an indication that the critical time for attaining
proper balance of wall polymers is during cell wall assembly. Colonial
growth has been traced to single-gene mutations affecting, in one case, the
structure of glucose-6-phosphate dehydrogenase with a resulting in vivo in-
crease in the level of glueose-6-phosphate; in another colonial mutant there
was a severe deficiency in phosphoglucomutase leading to the accumulation
of glucose-l-phosphate [Brody & Tatum (96, 97)]. Conceivably, these su-
perficially simple metabolic changes bring about, in an as yet unknown man-
ner, an alteration of the ratios and(or) fine structure of wall polymers, ultl-
mately resulting in "colonial" morphology.
The cell wall mannan content of various dimorphic fungi has been ob-
served to vary according to morphology (mycelial versus yeastlike), al-
beit in conflicting directions. For instance, yeast walls of Mucor ro~:ff have
5 to 6 times as much mannose as the hyphal walls of aerobically grown my-
celium. Since the architectural role of this carbohydrate is not known, phy-
logenetic reasoning was invoked to support a casual relationship between
the increase in wall mannan and yeast morphogenesis (4, 98). In partial
agreement, slightly more m~nnose was detected in yeast walls than in fila-
mentous walls of P~ll~l~rf~ p~ll~lans [Brown & Nickerson (19)]. On the
other hand, Domer et al. (78) found that the yeast form of Histoplasm(~
ca.psulatum had nearly one fifth the mannose content of the mycelial form.
Likewise, 2.5 times less mannan was detected (Cu complex) in ellipsoidal
cell walls compared to the walls of highly elongated cells of
sch~eyyii [Sundhagul & Hedrick (99)]. By growing Sacchc~rornyces cerevi-
siae nnder NH~
+ limitation (100), cell shape may be varied from spheroid
to cylindroid, yet the mannan content of the cell wall is not necessarily al-
tered [McMurrough & Rose (101)]. From the foregoing, it is obvious that
measurements of overall mannose content do not support a universal role
of mannan in dimorphism. Yet, the possibility of mannose polymers playing
a causal role in yeast morphogenesis cannot be ruled out entirely. The con-
flicting mannose content/morphology relationships could arise from the fact
that the measurements did not discriminate between different types of man-
naris, i.e., glucomannans (16), phosphomannans (53, 102), heteropolyuro-
nides (24, 25), each one possibly having a different architectural influence
on the cell wall.
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CHEMISTRY OF FUNGAL CELL WALL 101
Germination and sporoyenesis.--Chemical comparisons of vegetative cell
walls versus spore walls may be useful in understanding the biochemical
basis of sporulation and spore germination, but few studies have been made
in this regard. Partial characterizations of hyphal walls and spore walls of
Aspergillus ory~ae (5), Aspergillus phoenicis (65), Pithomyces chartarum
(72, 73, 103), Fusarium culmorum (67), hyphal walls of Allomyces macro-
gynus (6), and sporangial walls of A. neo-moniliformis (63), revealed
differences that ~vere largely quantitative. Among the qualitative changes
recorded were the deposition of melanin (5, 63, 65) and the absence of
phospho-glycoprotein (73). It is quite possible that minor structural
changes of morphogenetic significance may not be detectable by gross chem-
ical analysis. The conclusion for the time being would be that asexual spo-
rogenesis and spore germination in the above fungi (group V) are accom-
plished with retention of the basic chitin-glucan structure of the cell wall.
In Mucor rouxil the situation is entirely different. A comparison of cell
wall composition in four stages of development (Table II) revealed only
quantitative differences between hyphae, yeast cells, and sporangiophores
(4, 24, 62), but the spore wall manifested a drastically different qualitative
composition (9). A glucan appears de novo during sporogenesis as the prin-
cipal polymer of M. rouxii spore wails ; various hyphal wall components are
either diminished or absent. Upon germination, spore wall composition re-
verses to the chitosan-chitin type. These shifts in chemical composition indi-
cate a recurring reorganization of cell wall metabolism in each turn of the
asexual life cycle of Mucor. Accompanying the chemical shift, there is a
structural discontinuity between the spore walls and vegetative walls (104).
A phylogenetic interpretation for the shift was suggested above (group
IV). It has also been proposed that the cell wall shift may be a critical part
of a morphogenetic mechanism by which the ~ungus divert~ its resources
from the synthesis of vegetative walls to spore walls (9).
CELL WALL SPLITTING ENZYMES
Endogenous enzymes capable of splitting cell wall polymers or their
complexes seemingly play an indispensable role in a multitude of morphoge-
netic processes. Furthermore, it is conceivable that some of these degrada-
rive enzymes occur at the sites of cell wall formation and operate in har-
mony with cell wall synthetases.
Protein disulfide reductase.--Nickerson & Falcone (93) interpreted the
dimorphism of Candida albicans as a case of different budding ability. The
ellipsoidal, actively budding cell of C. albicans is believed to have an unim-
paired capacity to plasticize localized areas of the cell wall where bud ex-
trusion takes place. The softening of the wall was attributed to the specific
action of a protein disulfide reductase capable of splitting S-S cross-links in
the glycoproteins of the wall fabric. Accordingly, the levels of protein disul-
fide reductase found in a filamentous mutant of C. albicans, displaying little
or no budding capacity, were much lower than those in the normal budding.
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102 BARTNICKI-GARCIA
form. Although the cytology and rapidity of budding are in controversy
[compare (93, 105, 106, 109)], the correlation between protein disulfide re-
ductase and budding is supported by the recent observation of Brown &
Hough (107) who found that elongated cells of Saccharomyces cerevisiae
contained considerably lower levels of the enzyme than did typical ellip-
soidal cells. Moor’s (108) freeze-etching electron microscopy of S. cerevisiae
suggests a centrifugal progression of elements of the endoplasmie retieulum
toward the budding sites; it was postulated that these vesleles, rather than
mitoehondria, were the carriers of the protein disulfide reduetase.
Glucanases.--Enzymes active against cell wall glucans have been re-
ported to participate in morphogenetic phenomena; for instance, in the fu-
sion of conjugating cells of Hansenula wingei (110, 111), in the hormone-
induced development of antheridial branchings in Achlya (112), and in the
formation of pilei from dikaryons of Schizophyllum commune (64, 113). In
the latter, Wessels observed that pileus morphogenesis was correlated with
substantial increases in the levels of a specific, repressible glueanase which
hydrolyzes the alkali-insoluble gluean of myeelial walls. The degraded my-
eelial polysaccharide was subsequently utilized for pileus development. Inhi-
bition of pileus morphogenesis by exogenous glucose was interpreted as
being a result of eatabolite repression of the glueanase in question (113).
ULTRASTRUCTURE OF CELL WALL DEVELOPMENT
The following are only two aspects of the rapidly expanding field of fine
structure of developing cell walls of fungi. For a discussion of other se-
lected areas, consult Hawker (115) and Bracker (116).
Spore 9ermination.--Although relatively few fungi have been examined,
it appears that there are three basically different mechanisms of vegetative
wall formation during spore germination, each type being characteristic of
certain groups of fungi.
Type I. The vegetative wall is derived directly as an extension of the
spore wall, or one of its innermost layers. This mode of genesis is perhaps
the most common and appears throughout the higher fungi; e.g., the conidia
of Botrytis (117), Aspergillus (118, 119), uredospores of Melampsora
(120), conidia and ascospores of Neurospora (121, 122). There is probably
no fundamental change in the composition of the cell wall du~-ing this type
of germination [e.g., in Aspergillus (5)].
Type II. De novo formation of a cell wall on a naked protoplast. This
occurs during the encystment of the zoospores of aquatic Phycomycetes
(123-125).
Type III. De novo formation of a vegetative wall under the spore wall.
This kind, first observed by Hawker & Abbott in Rh&opus (126), is also
found in other Mueorales : Cunninghamella (127), Gilbertella (128), Mucor
(104). This unique mode of germination may be exclusive of fungi with
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CHEMISTRY OF FUNGAL CELL WALL 103
vegetative walls of the chitosan-chitin type (category IV) and may be
direct consequence of the changeover in cell wall composition that occurs
during germination. Because of the distinctly different appearance of spore
wall versus vegetative ~vall in the Mucorales, the formation of a new cell
wall during germination could be ascertained with confidence. In other fungi,
the demarcation between the older and newer parts of the wall was not as
clear, and claims for the formation of an entirely new wall during germina-
tion may have been premature [e.g., in the conidia of Fusarium culmorum
(129)
Hyphal morphogenesis.--Fungal hyphae grow by the commonly known
but poorly understood process of apical growth. Essentially, apical growth
is a mechanism by which the fungus restricts cell wall synthesis to the hy-
phal apex. Some recent studies, seeking intracellular structures associated
with and possibly responsible for apical growth, have shown an increased
degree of vesicular differentiation in the apical cytoplasm. Bracker and col-
laborators (116) found an accumulation of vesicles, similar to those dis-
charged by dictyosomes, in the hyphal apices of Pythium ultimum. Mar-
chant et al. (130) proposed that vesicles from the endoplasmic reticulum
were responsible for "primary" cell wall synthesis. They considered that lo-
masomes, structures previously suspected of having a role in cell wall syn-
thesis (131, 132), were responsible for "secondary" wall synthesis. In ger-
minating spores of Mucor rouxii, a seemingly unique organelle was found
in association with the apical wall of germ tubes (104). It was postulated
that this extracytoplasmic apical corpuscle could be responsible for both
germ tube emergence and its continued apical growth.
CELL WALL CONSTRUCTION
Polymer synthesis.--Our knowledge of cell wall biosynthesis in fungi is
restricted to data concerning a few individual polysaccharidcs. Chitin is
synthesized from the same precursor by all fungi so far examined. The ini-
tial observation of uridine-diphospho-N-acetyl-glucosamine serving as the
glucosyl donor for chitin synthesis in Neurospora crassa (133) has been
extended to Venturia inaequalis (134), ,411omyces macrogynus (134, 135),
Blastocladiella emersonii (136), Mucor rouxii (137), and Schizophyllum
commune (138). Uridine-diphospho-glucose (UDPG) was reported to
the natural precursor for the synthesis of noncellulosic glucan(s) of Philo-
phthora cinnamo~ni; both ~1, 3 and ~1, 6 bonds were formed (139). UDPG
is also the glycosyl donor for the synthesis of a glycogen-like polymer of
Dictyostelium di.~coideum spore coats (48). The mode of cellulose biosyn-
thesis in fungi is not clear. Although cellulose is present in both Phyto-
phthora and Dictyostel~um, there was little or no cellulose synthesized in
the cell-free preparations containing UDPG as precursor (48, 139). Guano-
sine-diphospho-glucose, the other known cellulose precursor (140) was not
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104 BARTNICKI-GARCIA
effective either (139). Guanosine-diphospho-mannose serves as glyeosyl
donor for the synthesis of mannan in gaccharomyces carlsbergensgs (141)
and phosphomannan in Hansbnula holstll (142). Cell-free extracts of
rou,ii catalyzed the synthesis of wall polyuronides from uridine-diphos-
pho-glucuronic acid (143).
Polymerization s#es.--Solutions to some of the most important problems
of fungal morphogenesis probably depend on the availability of answers
to .the following questions: Where ere cell wall structural polymers
synthesized? Are they polymerized in some intracellular site (plasma-
lemma, dictyosomes, lomasomes, etc.) and somehow transported in an or-
derly way to their final destination in the cell wall ? Alternatively, is it more
reasonable to expect cell wall synthesis (polymerization) to take place
situ (94)? Data obtained from cell-free studies are too fragmentary and
contradictory to permit definite conclusions about the subcellular site of cell
wall polysaccharide synthesis. The only generalization that can be safely
made is that the polymerizing enzymes are not soluble but particulate. The
actual cellular organelles responsible for cell ~vall polysaccharide synthesis
have not been conclusively characterized. Significantly, in those instances
wherein the cell wall fraction was assayed, most of the enzymic activity
was detected in this fraction (48, 139). These results tend to support the
concept of in silu synthesis, at least for some cell wall polymers; however,
additional evidence is needed to refute the possibility that the observed
association between synthesizing enzyme and cell wall resulted from in-
complete removal of cytoplasm.
Assembly patterns.--The techniques of autoradiography and fluorescent-
antibody labeling have been employed to elucidate the patterns of cell wall
assembly in budding yeasts, but they have led to conflicting conclusions. Au-
toradiographic studies of incorporation of glucose-all into the cell wall of
Pichia farinosa and Saccharomyces cerevisiae led Johnson & Gibson (144,
145) to conclude that wall extension was primarily by tip growth (distal
end of the bud). However, Chung et al. (146) observed that the buds of
cerevislae labeled with fluorescent antibody incorporated new wall material
mainly into an annular region at the base of the bud. These two conflicting
observations may be reconciled by the possibi!ity that each technique fol-
lowed the formation of entirely different wall components, each with a spe-
cial distribution and function in the cell.
Autoradiographic evidence was collected to support the hypothesis (98)
that the dimorphism of M. rouxli results from the operation of two differ-
ent mechanisms of cell wall assembly (localized versus disperse). Accord-
ingly, hyphae showed the expected strongly localized apical pattern of cell
wall synthesis, whereas budding yeastlike cells of the same fungus exhibited
a more diffuse pattern, with a tendency for increased wall synthesis toward
the base of the bud. The increased synthesis at the base might be related to
bud abscission rather than bud growth (147).
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CHEMISTRY OF FUNGAL CELL WALL 105
CONCLUSION
In the previous pages I have attempted to summarize and emphasize the
role of the cell wall in two major aspects of the biology of fungi, systemat-
ics and morphogenesis. Despite our present level of ignorance, some close
correlations can be established between cell wall properties and the proper-
ties o~ the whole fungus. This constitutes a reassuring indication o{ the po-
tential value that cell wall studies may have in the elucidation of biochemi-
cal bases of ontogenetic and phylogenetic development of the fungi.
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106 BARTNICKI-GARCIA
LITERATURE CITED
1. Aronson, J. M., in The Fungi, 49-76.
(Ainsworth, G. C., Sussman, A.
S., Eds., Academic Press, New
York, 748 pp., 1965)
2. Phaff, H. J., Ann. Rev. Microblol.,
17, 15-30 (1963)
3. Shifrine, M., Phaff, H. J., Antonle
~an Leeuwenhoek, 24, 274-80
(1958)
4. 13artnlcki-Garcla, S., Nickerson, W.
I., Biochlm. Biophys. Acta, 58,
102-19 (1962)
5. Horikoshl, K., Iida, S., Biochlm. Bio-
phys. /lcta, 83s 197-203 (1964)
6. Aronson, J. M., Maehlis, L., Am. a
r. Botany, 46, 292-300 (1959)
7. 13artnieki-Garcia, $., I. Gen. Micro-
biol., 42, 57-69 (1966)
8. Ruiz-Herrera, J., ~lrch. Biochem.
Biophys., 122, 118-25 (1967)
9. Bartnieki-Garcia, S., Reyes, E., Arch.
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... The beginning of the interactions in the antagonism process involving fungi is thought to be the cell wall, which serves as a barrier to safeguard the acts carried out by microorganisms. Chitin, 1,6-glucans, and other polysaccharides make up the majority of the cell wall of fungi (Bartinicki-Garcia, 1968). Chitinases break down the cell walls of fungi to produce oligomers that activate additional hydrolytic enzyme genes, intensifying the host attack (Viterbo et al., 2002). ...
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The fermentation of endophytic Nigrospora chinensis GGY-3 resulted in the isolation of tropolone stipitaldehyde (1), which exhibited broad-spectrum inhibition activity against fungi and bacteria, especially against Phytophthora capsici, with an EC50 value of 0.83 μg/mL and Xanthomonas oryzae pv. oryzicola, with a minimum inhibitory concentration value of 4.0 μg/mL. The in vitro and in vivo assays demonstrated that 1 had a significant protective effect on P. capsici. Furthermore, 1 inhibited the spore germination of P. capsici and damaged the plasma membrane structure. As observed by SEM and TEM, after exposure to 1, mycelia exhibited swelling, shrunken, branch-increasing phenomena, cell wall and membrane damage, and disordered content. Transcriptome analysis revealed that 1 might affect starch and sucrose metabolism and fatty acid biosynthesis by suppressing the expression of genes relevant to cell wall synthetases and cell membrane-associated genes. These findings indicate that 1 may be a potential agrochemical fungicide for controlling phytophthora blight.
Chapter
Mycelium composites are a class biopolymeric composites, consisting of cost-effective and environmentally sustainable materials. Globally, this class of composites is currently experiencing burgeoning research interest. With increasing pressure on cheaper materials with sustainable and ‘green’ credentials, mycelium composites hold some promise in this space, particularly in the construction industry, where the cost-performance indicator is a critical consideration. This material type uses the biological phenomenon of fungal growth to transition agri-waste materials to low-cost and low energy-embodied construction materials. Mycelium composites are inherently lossy in constitution and hence, have natural thermal and acoustic insulating properties. They have also shown impressive fire-resistant properties. These lossy properties, however, do not attribute good mechanical properties to mycelium composites, which are further compounded by its low hydrophobicity. However, some recent developments in the processing of the mycelium composites using 3D printing technologies by chemical manipulation of its constituents and self-healing mycelium structures, point this class of composites towards more flexural, robust, and strength-based semi-structural applications.
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Quantitative analyses of the cell wall of the penicillin-producing mold, Penicillium notatum, are described. The cell walls consist mainly of a glucose polysaccharide, a glucosamine polysaccharide which is probably chitin, and some protein. In addition, smaller quantities of galactose, mannose, and galactosamine have been found. There is no evidence for any mucopeptide-like component in the cell wall. (See footnote 1 in text.)
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Cell-free extracts and culture fluids ofSchizophyllum commune were assayed for enzymatic activity effecting the degradation of an alkali-insoluble cell-wall component of this mushroom, a glucan containingβ-(1→3) linkages (R-glucan). The activity of R-glucanase as determinedin vitro with isolated R-glucan as a substrate was found to increase from the onset of pileus formation, a process accompanied by R-glucan degradation in the mycelium. This R-glucanase activity is influenced by the presence of glucose in the culture medium, probably through a mechanism by which glucose represses synthesis of the enzyme. A morphological mutant (cup mutant) producing no pilei and exhibiting a lower degradation of R-glucanin vivo, produced levels of R-glucanase comparable to those of the wild-type stock and gave even higher levels in young cultures. The difference between the wild-type stock and the cup mutant with respect to degradation of R-glucan during development is most probably to be sought in the structure of the cell wall, the R-glucan in isolated cell walls of the cup mutant being less susceptible to enzymatic attack. High resistance to R-glucanase activity was also encountered in certain cell-wall preparations of the wild-type stock e.g. in those prepared from developing pilei. This suggests that cell-wall glucan degradation during pileus formation is controlled by both the level of R-glucanase, as influenced by glucose in the medium, and differences in protection of R-glucan in the cell wall against enzymatic attack.
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Sonic oscillation was used for the purpose of obtaining clean, chemically intact cell walls. The rate of disruption was determined for cells ofHanseniaspora uvarum andSaccharomyces cerevisiae. The carbohydrate fractions of cell walls ofHanseniaspora uvarum, H. valbyensis, Kloeckera apiculata, Saccharomycodes ludwigii andSaccharmyces cerevisiae were shown to be similar. Chromatography of cell wall hydrolysates of all these species demonstrated that glucose and mannose were the only sugars present (in about equal amounts) besides traces of glucosamine. The cell walls ofH. uvarum contained 78.1 per cent carbohydrates, 7 per cent protein and approximately 0.05 per cent of chitin. Fractionation of the polysaccharides lead to a recovery of 83.3 per cent of the carbohydrates present (30.4 per cent glucan and 34.9 per cent mannan). Saccharomyces cerevisiae cell walls were found to have a carbohydrate content of 82.8 per cent, 6.5 per cent protein and a trace of chitin (0.04 per cent). Nadsonia elongata contained a relatively large amount of chitin (ca. 5 per cent) and lacked mannan in its cell walls. It was concluded thatHanseniaspora andSaccharomycodes are closely related to theSaccharomyceteae but they have little in common with species ofNadsonia.
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Chemically intact cell walls of filamentous and yeast-like forms of Mucor rouxii were isolated. Comparative studies were made on their composition and structure to explore possible morphogenetic implications. Both types of cell walls exhibited a complex chemical composition consisting of polysaccharides (glucosamine, galactose, mannose and fucose), phosphate, proteins (at least 13 common amino acids), lipids (readily extracted and bound), purines and pyrimidines (RNA type), Mg2+ and Ca2+. Chitosan was the most abundant component of both types of cell walls. Chitin was present in smaller quantities. No qualitative differences were found between the two types of cell walls. Major quantitative differences were found in protein, purine-pyrimidine, and especially mannose content, all of which were higher in the yeast walls. Electron microscopy of ultrathin section of whole cells showed pronounced differences in thickness and fine structure of the walls. Whereas yeast walls were seemingly composed of two layers, no distinct layering was apparent in filamentous walls, which were only one tenth as thick as yeast walls.
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The difference between the budding process of Paracoccidioides brasiliensis and Blastomyces dermatitidis is reported herein. A characteristic feature in P. brasiliensis is that the optical density of the cell wall increases at the site where budding begins and at the neck of the dividing cell, whereas B. dermatitidis does not undergo this alteration. The neck which is formed between the mother and daughter cell at the site of division is much wider in B. dermatitidis than in P. brasiliensis. The bud scar in P. brasiliensis appears as a truncated cone, the top of which is covered only by the inner layer of the cell wall; in comparison, in B. dermatitidis the bud scar exhibits a flattened surface covered by the cell wall. Both fungi show an increase in the number of mitochondria and infoldings of the cytoplasmic membrane at the site of separation, which indicates that at this site there is an increase of metabolic activity.
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The structure and composition of the cell walls of hyphae of Neurospora crassa were investigated by electron microscopy, chemical analysis, and X-ray diffraction both before and after progressive enzymatic degradation by snail gut enzymes, chitinase, and trypsin. The wall consists of two phases: randomly disposed skeletal microfibrils of chitin only and an amorphous matrix which contains both beta-glucans and protein. The protein contains a high percentage of the amides of aspartic and glutamic acid but no hydroxy-proline or cysteine. A portion of this protein is a component of or is associated with a system of pores which is embedded in the matrix of the wall. These pores, 40 to 70 A in outside diameter, sometimes branch and seem to provide a three-dimensional network from one side of the wall to the other. They may be a general system of transport across the walls.