Ultrastructure of Conidium Ontogeny in the Deuteromycete Fungus Stachybotrys atra Corda

Ultrastructure of Conidium Ontogeny in the Deuteromycete Fungus Stachybotrys atra CordaAuthor(s): R. CampbellSource: New Phytologist, Vol. 71, No. 6 (Nov., 1972), pp. 1143-1149Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2433583 .

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New Phytol. (I972) 71, II43-I I49.

ULTRASTRUCTURE OF CONIDIUM ONTOGENY IN THE DEUTEROMYCETE FUNGUS

STACHYBOTRYS ATRA CORDA

BY R. CAMPBELL

Department of Botany, The University, Bristol, BS8 i UG

(Received io May I972)

SUMMARY

Various fixation methods were used to study conidium ontogeny in Stachybotrys atra Corda. Phialides arise by subterminal branching of the conidiophore. The first spore originates from the inner layer of the phialide tip, breaking the other wall layers. Thereafter spores are produced from the inside of the phialide neck where new wall layers are formed. Spores are cut off from the phialide by a septum and this splits to release the spore. As spore production ceases the phialide neck becomes plugged, the basal septum of the phialide becomes plugged and the cytoplasm vacuolates. Spores have a rough, pigmented two-layered wall and the usual organelles.

INTRODUCTION

The classification of Deuteromycetes is now based on the details of conidium ontogeny (Hughes, I953; Madelin, I966). Some confusion has developed in the nomenclature of the spore types and the methods of production but this has been largely resolved by recent redefinitions (Ellis, I97I; Kendrick, I97I). Literature on conidium ontogeny has been reviewed by Subramanian (I97Ia) and in the text edited by Kendrick (I97i). However there is still too little detailed information on the structure of conidiogenous cells for some concepts to be placed on a firm factual foundation.

This study describes conidium ontogeny in Stachybotrys atra Corda. The conidia of S. atra are produced from phialides (Ellis, I97i). The phialide opening of Stachybotrys is very small and there is no collarette (Ellis, I97I). The spores are single-celled, have heavily pigmented walls and occur in slime droplets.

Phialides are defined (Kendrick, I97I) as conidiogenous cells which, after the first spore, produce conidia from a fixed conidiogenous locus, each conidium being clad in a new wall to which the wall of the conidiogenous cell does not contribute. There have been several ultrastructural studies of phialides. Delvecchio, Corbaz and Turian (I969)

described how conidium walls of Thielaviopsis were distinct from the wall of the phialide's long collarette, but they did not show the conidiogenous locus so the origin of the walls was not determined. A light microscope study of Thielaviopsis (Cole and Kendrick, I969) has shown that this locus was some way down the phialide, the spores being delimited by the formation of cross-septa. The ultrastructure of Penicillium (Zachariah and Fitz-James, I967; Fletcher, I97I), is very similar to that of Aspergillus (Trinci, Peat and Banbury, I968) and Verticillium (Buckley, Wyllie and Devay, I969). Conidia were produced by wall formation in-the phialide neck where there was often a wall thickening. When the conidium reached its full size a septum formed across the neck; this septum

I I43



I I44 R. CAMPBELL

formed both the bottom of the conidium and the top of the next conidium initial. The presence of a pore in this septum is disputed: Zachariah and Fitz-James (I967) and Fletcher (I97I) claim that there is one in Penicillium; Lowry, Durkee and Sussman (I967) and Oulevey-Matikian and Turian (I968) with Neurospora did not show a pore between the phialide and the conidium, except during formation of the septum. Buckley et al. (I969) do not comment on the matter, though their illustration does not show a pore in Verticillium; Trinci et al. (i968) were unable to determine whether there was a pore or not; Hammill and Wang (1971) state that there definitely is not one in Metarrhizium.

MATERIALS AND METHODS

Stachybotrys atra was grown on potato dextrose agar (Oxoid CMI39). Synchronous cultures were grown so that phialides could be observed after producing known numbers of spores (method of Dr G. C. Carroll, University of Oregon; personal communication). In this method, agar plates were inoculated with a spore suspension and covered with a cellophane membrane: after three or four days growth the membrane was stripped off and 7 hours later a uniform growth of conidiophores and phialides was present over the plate and spore production was synchronous while three to four spores formed, after which young conidiophores grew up amongst the synchronous ones. Preliminary light microscope observations were made in various types of slide cultures.

Specimens were prepared for the stereoscan electron microscope by mounting small pieces of agar cut from the colony on metal stubs. The material was freeze-dried and then coated with gold:palladium alloy (60:40; Johnson-Mathey Ltd, London). Speci- mens were examined w'ith a Cambridge 'Stereoscan' Mk IIA.

Material for transmission electron microscopy was prepared by one of the following methods.

(i) Cultures were flooded with I1% (w/v) aqueous potassium permanganate and kept at room temperature for 30 minutes, in part under vacuum.

(2) Cultures were flooded with p-formaldehyde: glutaraldehyde mixture (modified from Karnovsky, i965) with a final concentration of I % (w/v) p-formaldehyde and I .5 00

(v/v) glutaraldehyde in o. i M sodium cacodylate buffer at pH 7.0 and then held at room temperature for 30-60 minutes, in part under vacuum. The material was washed and then post-fixed in 2?% (w/v) osmium tetroxide in the same buffer for 2-4 hours at room temperature. The material from methods (i) and (2) was stained in 2% (w/v) aqueous uranyl acetate for 30 minutes at room temperature then dehydrated in an alcohol: water series, embedded in Epon and sectioned with an LKB ultramicrotome using a diamond knife. Sections were stained in lead citrate and uranyl acetate. f

(3) Specimens were prepared for freeze-etching, using Balzer's apparatus, by covering the cultures with 25% (v/v) dimethylsulphoxide or 3000 (v/v) glycerol for I-7 hours before freezing in freon. Specimens were examined in AEI EM 6B or 6M electron microscopes.

RESULTS

The spores were produced through the tips of the cluster of phialides into the slime drop borne on the top of the conidiophore (Plate i, Nos. i and 2). Roughenings on the conidiophore wall (Plate i, No. 3) were hollow (Plate i ,No. 4) and were formed from the outer layer of the wall (Plate i, No. 5). The conidiophore contained the usual organelles,



Conidial ultrastructure in Stachybotrys I 145

including nuclei, very elongated mitochondria, endoplasmic reticulum (Plate I, No. 6) and ribosomes (Plate I, Nos. 4, 9 and Io). The conidiophore stopped growing when the phialides formed; its tip swelled and a septum delimited it as the first phialide which usually became the central one of the group. New growing points were formed below the septum and the other phialides in the group were produced (Plate I, No. 7). The septa at the junction have a single central pore (Plate I, No. 8) around which several Woronin bodies (Plate I, Nos. 8, 9 and io) were usually found.

The young phialides contained a single nucleus and elaborate endoplasmic reticulum (Plate I, Nos. I and 7). The first spore was produced by the growth of the inner layer of the wall at the phialide neck: the resulting expansion of the spore broke the outer wall layers very near the neck of the phialide and parts of this phialide wall were sometimes seen on the tip of the expanded spore. The spore was coated in slime (Plate 2, No. I I).

After this first spore, further ones were formed by the production of wall material by a region on the inside of the wall of the phialide neck (Plate 2, No. 12). The young spore rapidly increased in size and its wall became two layered, the outer one being very electron dense, but even when nearly mature the continuity of the wall with the inside of the phialide neck was still apparent (Plate 2, No. I3). The spore stopped enlarging at the time that a septum formed across its base, cutting it off from the phialide. An invagination of the plasmalemma started at the phialide neck and proceeded centripetally (Plate 2, No. I4) until the closure was complete (Plate 2, No. I5). A study of serial sections of synchronously grown material of known age and of stereoscan pictures (Plate 4, No. .26) suggested that this septum did not have a pore, except during its formation. The spore broke away from -the phialide at this stage as the septum split (Plate 2, No. I6) into two layers. One layer formed the base of the spore, the other was left covering the opening at the tip of the phialide (Plate 3, No. I7). The next spore was produced by a new wall layer, formed in the phialide neck, that was continuous with this latter layer of the septum. Thus the more spores a phialide produced the greater was the thickening at the neck and this often had a layered structure, each layer being the point of origin of one spore (Plate 3, No. i8).

As the phialides aged they began to vacuolate (Plate 3, Nos. I7 and I9). The vacuoles often contained membrane complexes (Plate 3, Nos. I-9 and 2I) which were sometimes associated with the nuclear membrane (Plate 3, Nos., 20 and 2i). Two nuclei were often seen in these old phialides (Plate 3, No. 22). There were organelles of unknown function, usually near the neck, which consisted of bundles of regularly orientated fibres which were membrane bound (Plate 3, No. 22; Plate 4, No. 23) and which were usually associated with the endoplasmic reticulum (Plate 4, No. 24): when seen in tangential section the delimiting membrane was not always obvious (Plate 4, No. 24). The wall at the neck invariably had very heavy thickening on the inside and this sometimes occluded the pore, or the pore was plugged by electron dense material (Plate 3, Nos. I9 and 22).

The septal pores at the junction of the phialides and the conidiophore were also plugged (Plate 4, No. 25), presumably with material derived from the Woronin bodies which were noted earlier (p. I I45).

The spore had a two-layered wall. The outer layer was electron dense, presumably with melanin, and had a rough surface (Plate I, No. I ; Plate 4, Nos. 26 and 27). The spore bases were truncated where the septum had cut the spore off from the phialide (Plate I,

No. I; Plate 4, No. 26); no pores were seen in these bases. The usual organelles were present and many of the spores also contained lipid droplets (Plate I, No. I; Plate 4, No. 27). The outer surface of the plasmalemma was grooved (Plate 4, No. 28) and there



I I46 R. CAMPBELL

were corresponding ridges in the inner surface. Similar grooves and ridges have now been seen in several fungi (Moor and Miihlethaler, I963; Sassen, Remsen and Hess, I967; Campbell, I969) but their function, if any, is not known.

DISCUSSION

The conidium ontogeny- described agreed with the electron microscope studies of other phialidic fungi mentioned earlier (Trinci et al., I968; Buckley et al., I969; Fletcher, I97I) and was also similar to the production of microconidia in Neurospora (Lowry et al., I967; Oulevey-Matikian and Turian, I968). The light microscope studies of Boerema (I965) on Phoma and of Cole and Kendrick (I969) on Phialophora and Penicillium are also essentially in agreement, concerning the relationship between phialide and conidium walls, with the present study. The characteristic features of the phialidic method of spore production are firstly the formation of the conidium wall in a fixed conidiogenous region inside the phialide neck, causing a thickening as successive spores are produced, and secondly the cutting off of the spore with a septum. It is suggested above that the fully formed septum (the double septum of Kendrick (I97I)), does not have a pore. The length of the collarette varies greatly between genera depending on the point at which the phialide wall breaks to release the first conidium (Kendrick in discussion of Subramanian, I97Ib). The collarette is so short in Stachybotrys that Ellis (I97I) was unable to see it with the light microscope. This is presumably because the outer layers of the phialide wall break very soon after the first spore starts expanding. It is generally agreed that this first spore from the phialide may be different in wall origin from the others, for it includes the wall of the phialide tip (Kendrick, I971). Subramanian (I97ib) suggested on the basis of unpublished light microscope work of Seshadri, that the first conidium of S. chartarum (Ehrenb.) Hughes has a wall formed, at least in part, by the blowing out of the tip of the phialide. S. atra resembles S. chartarum in this respect since the first conidium is an extension of the inner wall of the phialide.

According to Kendrick's (I97I) definition, the conidium wall in second and subsequent conidia is formed at a fixed conidiogenous locus and the conidiogenous cell wall (phialide wall) does not contribute to the conidium wall. This definition required that the thickening at the neck of the conidiogenous cell, described here and by other authors, should not be considered part of the wall of that cell: this is shown to be true in Stachybotrys in that the formation of the thickening was entirely the result of spore formation and occurred at a different time from the formation of the phialide wall. Phialoconidia are also said to be enteroblastic (the outer layers of the conidiogenous cell wall are not involved in the formation of the conidium wall (Kendrick, 197I)): conidium ontogeny in Stachybotrys agreed with this.

Subramanian (I97Ib) suggested that there were three types of phialides which varied in the position of the conidiogenous locus and in the degree of continuity between the walls of successive conidia: this latter is reflected in the difference between those phialides with loose chains or slime drops and those, such as Penicillium with a true chain. Subra- manian stressed that his groups are at present based on a limited knowledge of a few species. However, I suspect that fungi with differing phialide structure may be interpreted as a continuum rather than as a series of groups. Stachybotrys atra, as here described, would, however, fit into Subramanian's group i characterized by Phialophora lagerbergii (Melin and Nannf.) Conant and including Neurospora, Verticillium and Stachy- botrys chartarum.



Conidial ultrastructure in Stachybotrys I I47

This study shows that ontogeny in Stachybotrys supports the definitions and concepts of phialides and phialoconidium production as proposed in the text edited by Kendrick ('97').

The complex membranes in the old phialides were thought to be a stage in the degeneration of the cytoplasm: the acceptable fixation of other parts of the same phialide group and of neighbouring conidiophores in the same sections suggested that the membranes were not fixation artifacts.

The site of nuclear division has not been conclusively determined. Active phialides always had one nucleus; nuclear division must, therefore, immediately precede migration of the daughter nucleus into the new spore. Nuclear membranes containing arrays of microtubules, which could be interpreted as a stage in nuclear division, have been seen in phialides, but only very rarely. The two nuclei in old phialides could be the result of nuclear division continuing after spore formation had stopped.

What caused the phialide to stop producing spores? The blocking of the phialide neck by the thickening caused by spore formation or by the electron-dense plug coincided, as nearly as could be determined, with blocking of the septal pore between the phialide and the conidiophore. Vacuolation, which started while spores were still being produced, became conspicuous at this time, and the membrane complexes were formed. Which of these events was the cause and which the effect of the cessation of spore production is not clear, but the blocking of the phialide neck is probably the main cause.

The spores were borne in a very obvious slime droplet; the slime was seen attached to the phialide apices in permanganate fixation (Plate i, No. i) and in stereoscan pre- parations (Plate i, No. 2). Since no source of this slime was seen, such as vesicles passing through the plasmalemma to the outside, it is assumed that it polymerizes in situ from soluble components.

ACKNOWLEDGMENTS

This work was supported by a Science Research Council grant (B/SR/9o7I). I thank the Long Ashton Research Station and the Department of Biochemistry of the University of Bristol for the use of stereoscan and freeze-etching facilities respectively. Professor L. E. Hawker, Dr M. F. Madelin and Dr A. Beckett gave helpful advice during the preparation of this typescript.

REFERENCES

BUCKLEY, P. M., WYLLIE, T. D. & DEVAY, J. E. (I969). Fine structure of conidia and conidium formation in Verticillium albo-atrum and V. nigrescens. Mycologia, 6I, 240.

BOEREMA, G. H. (I965). Spore development in the form-genus Phoma. Persoonia, 3, 413. CAMPBELL, R. (I969). Further electron microscope studies of the conidium of Alternaria brassicicola. Arch.

Mikrobiol., 69, 6o. COLE, G. T. & KENDRICK, W. B. (I969). Conidium ontogeny in hyphomycetes. The phialides of Phialophora,

Penicillium, and Ceratocystis. Can. J3. Bot., 47, 779. DELVECCHIO, V. G., CORBAZ, R. & TURIAN, G. (I969). An ultrastructural study of the hyphae, endoconidia

and chlamydospores of Thielaviopsis basicola. J3. gen. Microbiol., 58, 23. ELLIS, M. B. (I97i). Dematiaceous Hyphomycetes. Commonwealth Mycological Institute, Kew, England. FLETCHER, J. (I 97I). Conidium ontogeny in Penicillium. J. gen. Microbiol., 67, 207. HAMMILL, T. M. and WANG, C. J. K. (I97I). Fine structure of phialoconidiation in Metarrhizium anisopliae

(Hyphomycetes). Am. J3. Bot., 58, 474. HUGHES, S. J. (1953). Conidiophores, conidia and classification. Can. J3. Bot., 31, 577. KARNOVSKY, M. J. (I965). A formaldehyde-glutaraldehyde fixative of high osmolarity for use in electron

microscopy. J. Cell Biol., 27, 137. KENDRICK, B. (Ed. and part author) (197I). Taxonomy of Fungi Imperfecti. University of Toronto Press,

Toronto. LOWRY, R. J,, DURKEE, T. L. & SUSSMAN, A. S. (I967). Ultrastructural studies of microconidium formation

in Neurospora crassa. 5'. Bact., 94, 1757.



1148 R. CAMPBELL

EXPLANATION OF PLATES Key to lettering: c, conidiophore; er, endoplasmic reticulum; 1, lipid; m, mitochondrion; n, nucleus; p, phialide; r, ribosomes; s, spore; sl, slime; v, vacuole; w, Woronin body. All magnifications are approximate.

PLATE I

The spore drop and the conidiophore. No. I. Phialide group with spores in the slime drop. Spores of various ages are shown and when in median section they have truncated bases. A new spore is being produced by the phialide on the left. KMnO4; x 4200.

No. 2. A complete slime droplet with embedded spores; the conidiophore and phialides are not covered by slime. Stereoscan; x 2300.

No. 3. A broken conidiophore showing the wall roughenings are bubbles. Stereoscan; X 7000. No. 4. A cross section of a conidiophore with wall roughenings. Glutaraldehyde-formaldehyde; x I3,300. No. 5. Cross section of the wall roughenings which are hollow bubbles of the outer wall layer. KMnO4; x I7,000. No. 6. Longitudinal section of the conidiophore with typical organelles. KMnO4; x 5,600.

No. 7. A young conidiophore tip which has formed the central, terminal, phialide: the second phialide is formed by branching below septum of the first. KMnO4; x 7900. No. 8. Septum at the phialide base in median section with the single central pore. KMnO4; X 19,500.

No. 9. A group of phialide bases with Woronin bodies. Glutaraldehyde-formaldehyde; X 20,500.

No. io. Detail of No. 9 with the unit membrane around the Woronin bodies. Glutaraldehyde- formaldehyde; x 70,000.

PLATE 2

Spore production: the phialide is towards the bottom of each photograph and the spore is at the top. No. i I. The first spore produced by a phialide. The spore wall is continuous with the inner layer of the phialide wall, the outer layer of which is broken. The new spore is surrounded by slime. KMnO4; X 22,000.

No. 12. The start of the second spore produced by a phialide. Wall production takes place by the formation of a new layer within the phialide wall (boundary at arrows). Glutaraldehyde- formaldehyde. x 38,300. No. I3. The second spore now nearly mature: the continuity of the spore wall with the new inner layer of the phialide wall is clearly seen. Notice the broken edges of the phialide wall forming the very short collarette. KMnO4; x 41,400.

No. I4. The septum between the spore and the phialide beginning to form by invagination around the phialide neck. KMnO4; x 26,700.

No. I5. A completed septum with no pore. Notice the clear outline of the spore base and the slight protrusions (at arrows and also present in No. 14) which are the start of the next spore producing layer. KMnO4; x 34,300.

No. I6. The release of the second spore from a phialide. The double septum is splitting down the centre (arrow) as the new spore is forming beneath in the phialide neck. KMnO4; x 24,400.

MADELIN, M. F. (I966). The genesis of spores of Higher Fungi. In: The Fungus Spore (Colston Papers, I8, I5). Butterworth, London.

MOOR, H. & MOHLETHALER, K. (I963). Fine structure in frozen-etched yeast cells. 7. Cell Biol., 17, 609. OULEVEY-MATIKIAN, N. & TURIAN, G. (I968). Controle metabolique et aspects ultrastructuraux de la

conidiation (macro-microconidies) de Neurospora crassa. Arch. Mikrobiol., 6o, 35. SASSEN, M. M. A., REMSEN, C. C. & HESS, W. M. (I967). Fine structure of Penicillium megasporum conidio-

spores. Protoplasma, 64, 75. SUBRAMANIAN, C. V. (097Ia). Conidium ontogeny in fungi imperfecti with special reference to cell wall

relationships. 3. Indian Bot. Soc. Golden Jubilee Volume 5oA, 5 I. SUBRAMANIAN, C. V. (I97Ib). The phialide. In: Taxonomy of Fungi Imperfecti (Ed. by B. Kendrick), p. 92.

University of Toronto Press, Toronto. TRINCI, A. P. J_., PEAT, A. & BANBURY, G. H. (I968). Fine structure of phialide and conidiospore develop-

ment in Aspergillus, giganteus Wehmer. Ann. Bot., N.S., 32, 241. ZACHARTAH, K. & FITZ-JAMES, P. C. (I967). The structure of phialides of Penicillium claviforme. Can. 37.

MWicrobiol., I3; 249.



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Conidial ultrastructure in Stachybotrys 1149

PLATE 3 Older phialides. No. I7. A phialide whose spore has just broken away leaving the tip of the phialide closed by half of the septum. The phialide is just starting to vacuolate. KMnO4; x I8,400. No. i8. A phialide that has produced many (probably six or seven) spores: the laminated thickening at the neck is very clear, with the present spore wall still in continuity with the inner layer. This is not quite a median section: some wall material is seen apparently in the channel between phialide and spore but this is not part of a septum. KMnO4; x 22,500.

No. i 9. An old vacuolated phialide with membrane complexes and a plugged tip. Glutaralde- hyde-formaldehyde; x 12,400.

No. 20. Multiple membranes associated with a nucleus in an old phialide. Glutaraldehyde- formaldehyde; x 23,000. No. 2I. Tubular and whorled membranes associated with both the nucleus and vacuole in an old phialide. Glutaraldehyde-formaldehyde; x 26,ioo. No. 22. An old phialide with two nuclei and a plugged tip. Note the position of the two unknown fibrous organelles (arrows). Glutaraldehyde-formaldehyde; x I7,300.

PLATE 4

Older phialides and spores. No. 23. A membrane bound fibrous organelle. Glutaraldehyde-formaldehyde; x 82,500. No. 24. A fibrous organelle in tangential section. Notice the association with the endoplasmic reticulum. Glutaraldehyde-formaldehyde; x 84,400. No. 25. The junction of old phialides with the conidiophore. The septa are plugged. KMnO4; X 30,000.

No. 26. Spores with their rough surfaces and the truncated bases. The bases of all the spores are pointing towards the left. Stereoscan; X 23,900. No. 27. A spore, with its rough wall, containing a nucleus and prominent lipid droplets. Freeze-etch; x i8,200.

No. 28. The spore with the prominent grooves in plasmalemma (looking from outside the spore towards the inside). The spore wall is two layered. Freeze-etch; x I8,400.



Documents

Ultrastructure of Conidium Ontogeny in the Deuteromycete Fungus Stachybotrys atra Corda