Volume 12, Part 2, May 1998

What Factors influence the Germination andOutgrowth of fungal Spores?

Fungal spore germination marks the resumptionof vegetative development and the formation of anew individual or colony. Germination, usuallyfollowing a period of dormancy, is said to haveoccurred when a new developing hypha, knownas a germ tube, can be seen emerging from aspore. This represents the start of a new cycle ofdevelopment. However, prior to the outgrowth ofthe germ tube, metabolic activity will alreadyhave begun, leading to the re-integration of bio-chemical processes, followed by morphologicalchanges in the spore, and ultimately the resump-tion of vegetative growth Many spores are sub-ject to variable periods of dormancy. Somerequire that environmental conditions arefavourable (exogenous dormancy) whereas oth-ers, with longer term survival mechanisms (con-sititutive dormancy) may require specificactivation treatments before germination can beinitiated. In some instances a period of physio-logical maturation must occur prior to germina-tion. The period of low metabolic inactivity,necessary for overwintering, must be broken, byspecific activation stimuli, leading to the restora-tion of biochemical processes which were absentor non-functional during the dormant period.

The requirements for activation are often pre-dictable. Dormant spores are relatively dehy-drated and prior to germination most species willrequire the presence of liquid water or conditionsof high relative humidity. As activation occursspores will take in large amounts of free water,rapidly at first, leading to spore swelling. Thisincrease in diameter (sometimes to several timesthe spore diameter during dormancy, e.g.Cunninghamella elegans) and a concomitantincrease in dry weight is a prelude to germ tubeemergence; a first visible sign of impending ger-mination. Cycles of wetting/drying and soaking,such as must occur on leaf surfaces, may alsoserve to wash away, or dilute, chemical inhibitorsto germination. The close proximity of otherspores may be inhibitory to germination, proba-bly owing to levels of inhibitors, but it is also thecase that the germination of some spores is stim-

ulatory to neighbours (e.g Rhizopus stoloniierspores). Newly germinated spores are very vul-nerable to desiccation and it is therefore essen-tial that appropriate moisture levels are presentto support continued outgrowth. In addition,spores absorb water-soluble nutrients (simplesugars and amino acids), which may be used tofuel subsequent metabolic activities and associ-ated processes. In some cases, the presence ofsuitable substrates or exudates to support growth(particularly in the case of pathogens) acts as afurther trigger to the resumption of outgrowth.Most spores also require oxygen for outgrowth tooccur. In other instances environmental changesfunction as activation treatments. These mayrelate directly to the normal ecological habitat.Temperature shocks will often act as triggers tospore activation. Some spores germinate follow-ing exposure to high (HO°C) or low (-50°C) tem-peratures. Conditions which result in theactivation of spores probably cause changes inkey biochemical components of metabolic path-ways, or alterations in membrane permeability,particularly the plasma membrane of the spore.For a time many of these changes can bereversed if external conditions do not remainconducive but eventually spores pass a 'point ofno return'; activation has occurred and germina-tion must ensue. It is clear that there is no uni-versal mechanism for either the maintenance orfor the breakage of dormancy.

In general, a spore is said to have germinatedonce a germ tube has emerged and reached alength equal to the diameter of the swollen (fullyhydrated) spore. Most spores produce a singlegerm tube although spores of some species giverise to more than one. Spore walls are often mul-tilayered, frequently thickened and pigmented.New wall components must be synthesised beforeoutgrowth can begin. Polarity is always estab-lished prior to the germination of spores. Vesiclesaggregate at the site(s) of germ tube emergence inactivated spores,probably carrying cell wall com-ponents and synthetic/lytic enzymes involved incell wall synthesis. It is likely that a combination

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Volume 12, Part 2, May 1998

of swelling due to water uptake, enzyme actionand physical force gives rise to the rupturing ofthe spore wall as germination proceeds. In someinstances germ tubes are formed directly asextensions of the inner spore wall. In other casesand probably of more frequent occurrence, a newwall layer may be laid down and extended outfrom the spore to form the germ tube.

Other changes also take place concurrentlywithin germinating spores. In addition to new wallmaterial, new membrane components are alsosynthesised and there is an increase in theamount of endoplasmic reticulum present.Biochemical processes which do not occur (orocurr at extremely low levels) in dormant sporesare quickly resumed. Mitochondria are seen toincrease in size, number and maturity and rates ofrespiration rise dramatically. As germination isinitiated most of the energy used is derived fromstorage compounds laid down in spores. Lipids,glycogen and trehalose are utilised to fuel initialoutgrowth. Simultaneously there is an increase inthe amount of protein synthesis ocurring and keyenzymes are synthesised rapidly or released fromcompartmentalisation. Essential enzymes areoften present in dormant spores but as inactiveforms or bound to spore components so that they

are not able to interact with substrate. InNeurospora crassa, trehalase which converts tre-halose to glucose is associated with spore wallsand is separated from the substrate until it isreleased by activation processes. All types ofRNA are synthesised early as germination pro-gresses and the spore nucleus increases in size.Perhaps surprisingly, the synthesis of DNA is nota prerequisite of germination (e.g. Neurosporacrassa, Uromycesphaseolus). However, it is essen-tial for the continued outgrowth of the germ tubeand for colony development and normally occursfairly early after extension has begun.

Germ tube emergence usually takes place a fewhours after initiation of the germination process.Initially the young hypha will extend rapidlyacross the new substrate, eventually formingbranches and developing into a colony. Eachhyphal branch will tend to diverge away fromother tips (negative autotropism) and neighbour-ing hyphae so that each can draw on it own areaof substrate and continue to support outgrowth.

Susan IsaacSchool of Biological Sciences,

Life Sciences Building, University of Liverpool,Crown Street, Liverpool, L69 7ZB

Book Reviews

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Fungi and Fish Diseases by L. G. Willoughby(1994). Pp.57, illus. (Softcover). ISBN 0 95211981Pisces Press, Stirling, Scotland. Price £17.50.

To quote from the foreword by R. J. Roberts,this publication brings together a distillation ofthe life's work of Guy Willoughby, 'the doyen ofsaprolegniologists'. It includes much useful prac-tical information, gained the hard way, whenWilloughby worked as a mycologist and fishpathologist. There are eight chapters coveringthe fungi involved in fish diseases, fungal infec-tion of fish, diagnosis of fungal disease, pure cul-ture studies, identification of Saprolegniaceae,numerical assay of fungal spores in water, predis-posing factors to external mycosis and control ofexternal mycosis.

The classification is outdated. There is noreference to heterokont flagella nor to the ideathat the Saprolegiales might not be related tothe Eumycota. There are some curious state-ments e.g. that the Fungi Imperfecti have tradi-tionally been regarded as inhabitants of soilrather than water. What about the ca. 300

species of aquatic hyphomycetes, the aero-aquatic fungi and the species of Fusarium andCylindrocarpon which grow on leaves and twigsin streams? It is stated that the generic nameAspergillus is derived from the distinctivelystalked spores which resemble the aspergillum orholy water brush. Surely it is the whole conidio-phore which bears this resemblance? For thebenefit of those not familiar with the jargon offish pathology it would have been helpful to havehad explanations of such terms as endoph-thalmia, telangiectasis and petechiations. Themeanings of the abbreviations of histologicalstains such as PAS, H + E should have been pro-vided and specification of the salts of the metalsrecommended as micronutrients in culturemedia.

The strength of the book lies in the practicaladvice, for example in rapid preliminary screeningof diseased fish, a method for distinguishingSaprolegnia parasitica by means of the longhooked hairs formed on secondary zoosporecysts, pure culture methods techniques for quan-titative assay of spore concentrations in water,