Volume 11, Part 2, May 1997

HOW DO FUNGI DEGRADE AND OBTAINNUTRIENTS FROM CELLULOSE?

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A great deal of research activity has been direct-ed towards increasing our understanding of themechanism(s) by which fungi affect cellulose inthe natural environment. In very general terms,cellulose degradation occurs as the result of theproduction of appropriate extracellular enzymes(cellulases) and a large number of fungal speciesdo produce such enzymes. However, informationrelating to the activity of fungal cellulases hasbeen pieced together from many experimentsusing purified enzymes and culture filtrates fromfungi normally exhibiting degradative activity. Itis quite clear that the degradative process iscomplex and occurs as the result of several bio-chemical steps. The impetus for such researchhas arisen partly from an academic viewpointbut also in relation to the commercial exploita-tion of fungal cellulases. In addition, it providesgreater understanding of the means by whichdecomposition of plant materials occurs and alsoof the mechanisms by which pathogenic fungigain entry into and exploit healthy plants.

Fungi require sources of organic carbon forgrowth and development. Most organic carbonin the environment occurs as carbohydratesbound into the major components of plant tis-sues. However, plant materials are highly vari-able in composition and those carbohydrates arebound in different forms and in different relativeamounts. Much plant carbohydrate is present incomplex forms. In general terms, plant tissuesare made up from celluloses (polymer of D-glu-cos e), hemicelluloses (xylans and mannans),lignins (polymers of phenylpropane), other poly-saccharides and glycoproteins. These polymersare laid down in close associations and the spe-cific mix of neighbouring components may influ-ence the accessibility of cellulose for microbialenzyme degradation. Plant cell walls are multi-layered, each linked together but with slightlydifferent constituent components. However, alarge number of fungi are efficient cellulosedegraders (e.g. Trichoderma reesei, T. viride) and,providing that sufficient water is available, thepenetrative nature of hyphal growth ensures that

extracellular enzymes quickly reach into planttissues.

Cellulose is a major component of plant mate-rials and is composed of microfibrils (chains ofmolecules complexed together). It is a polymerof D-linked glucose and is hydrolysed by theaction of cellulases. In this process, cellulose fib-rils are degraded by a series of enzyme reactions.In the first step the enzyme endo ~(1-4)-glu­

canase splits the cross links (glucosyl bonds)between the component glucan chains whichresults in the formation of unbroken, single ~(1­

4)-glucan chains. A second enzyme ~(1-4)-D­

glucan cellobiohydrolase degrades those ~(1-4)­

glucan chains (cellulose chains) to give cellobiose(a disaccharide). Subsequently, the cellobiose isconverted to glucose by the activity of anotherenzyme ~(1-4)-glucosidase. In this way glucose isliberated from cellulose microfibrils and can beabsorbed into fungal hyphae. It is clear that thefirst enzyme (endo ~(1-4)-D-glucanase) createssites at which the second enzyme (~(1-4)-D-glu­

can cellobiohydrolase) can act. The full enzymecomplex is required for efficient degradation andnutrient release since the enzymes act synergis-tically i.e, their action in combination is greaterthan the sum of the individual activities.

It is possible that some fungi produce severalforms of these enzymes and this apparent multi-plicity of form has complicated the elucidation ofthe degradation system. However, it is likelythat at least some confusion may have arisenfrom the presence of artifacts of purification andvery low levels of impurities in assay systems.Further investigations at molecular level are like-ly to improve our understanding in this area.Cellulase enzymes are produced by fungi inresponse to the presence of cellobiose (disaccha-ride) which initially acts as an inducer, althoughhigher levels of cellobiose cause repression of cel-lulase enzyme activity. It is also important tonote that in addition to the complex of cellulaseenzymes mentioned here other oxidativeenzymes may also have a role to play in cellulosedegradation e.g. cellobiose oxidase.

Some plant materials are woody and thick-ened, containing large amounts of lignin laiddown in cell walls. This provides mechanicalstrength and resistance to plant tissues since thelignin component is not easily degraded bymicrobial enzymes (much plant cellulose isencrusted with lignin and is therefore unavailableto many fungal species). These thickened tis-sues are recalcitrant to degradation and aremuch less easily broken down by fungi.Relatively few fungi are capable of lignin degra-dation but some species are efficient degraders(e.g. white rot fungi such as Phanerochaetechrysosporium, Rigidoporus ulmarius, Trametesspp) and these species can delignify plant cellu-lose. Fungi that cause wood rot without destroy-ing lignin are known as brown rots e.g. Serpulalacrymans. In the natural environment plantmaterials are usually colonised by a number ofdifferent fungal species and also by bacteria, and

Volume 11,Part 2, May 1997

the suite of microbial enzymes that are releasedby that population often results in efficientdecomposition of the plant debris.

It is clear that cellulases are also importantenzymes for plant pathogens, aiding entry intoand ramification through living plant tissues ,providing the proliferating mycelium with nutri-ents. However, some virulent pathogens effi-ciently enter plant tissues without causing agreat deal of disruption to the structural integri-ty and although these enzymes have a role toplay they are by no means the only determiningfactor in pathogenicity.

Susan IsaacSchool of Biological Sciences,

Life Sciences Building,University of Liverpool, Crown Street,

Liverpool , L69 7ZB

BOOK REVIEWSPatterns in Fungal development edited bySiu-Wai Chiu & D. Moore (1996). Pp . xii + 226.Hardcover. ISBN 0 521 56047 O. CambridgeUniversity Press, Cambridge, UK. Price £35($US 54.95).

This is an important and stimulating little book.Its eight chapters are based on a symposiumheld at IMC5 in Vancouver in 1994. They pro-vide an important but somewhat uneven contri-bution to facts and ideas on pattern formation infungi - a topic which mycologists have been slowto investigate experimentally. In this latterrespect, the first (Moore), sixth (Bourne, Chiu &Moore) and seventh (Frazer) chapters provideboth a useful summary of the facts available ondevelopment of the basidiocarp, particularly thatof Coprinus, and also a number of provocativeideas and hypotheses which need to be thoughtabout further and tested by experiment. Indeed,if the ideas developed in these chapters result inmore such work then the book will have morethan justified its production. Captivating ques -tions such as:

how are particular morphogenetic processesinitiated and sustained?

what is the nature of the postulated morpho-genetic fields and do they really have a

material basis?do fungal morphogenetic hormones exist, what

is their nature and how do they operate?how is coordination achieved in developing tis-

sues and within the developing basidiocarp?

are discussed within the very real limits of exist-ing knowledge. It is difficult to read about themwithout wanting to take these and related mat-ters further, both mentally and in the laboratory.Nor is the field mycologist neglected, forWatling's speculative chapter, embodying andsynthesizing a huge observational corpus as itdoes , suggests ways in which the form of basidio-carps in the wild are related to the conditions inwhich they grow and their functional require-ment to promote effective spore dispersal. Itbrings a dynamic view to the form, adaptationand function of the basidiocarp in nature.

The remaining chapters are less provocativesave perhaps that on hyphal tip extension andsubsequent development (Johnson, Calleja &Yoo). This is a salutary reminder that anyhypotheses to account for wall extension andgrowth need to encompass the phenomenon infungal unicells as well as hyphal forms. Thegenetics of morphogenesis in Neurospora(Vierula) adds little to what is already knownwhile those on pattern formation in the myceli-

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