John M. Clarkson

New insights into the mechanisms of fungal pathogenesis in insects

Entomopathogenic fungi are attracting attention as potential biological control agents of insect pests. The mechanisms of pathogenesis have parallels with those of some plant-pathogenic fungi, particularly in the areas of formation of an infection structure, entry into the host and toxin-mediated host death. Understanding these processes will provide a rational basis for strain selection and improvement.

Carbon regulation of the cuticle-degrading enzyme PR1 from Metarhizium anisopliae may involve a trans-acting DNA-binding protein CRR1, a functional equivalent of the Aspergillus nidulans CREA protein

Abstract The pr1 gene of the entomopathogenic fungus Metarhizium anisopliae encodes a serine protease that is highly active towards the insect cuticle and whose synthesis is subject to both carbon and nitrogen repression. The pr1 promoter region was sequenced revealing the presence of putative CREA- and AREA-binding sites. In vitro bandshift experiments demonstrated that an Aspergillus nidulans GST-CREA fusion protein was capable of binding to two of the three putative CREA sites. Using a PCR-based strategy the M. anisopliae crr1 gene was identified; it encodes a putative C2H2-type DNA-binding protein with significant sequence similarity to A. nidulans CREA. Complementation experiments with an A. nidulans strain carrying creA204 demonstrated that CRR1 can partially substitute for CREA function.

Partial characterization of specific inducers of a cuticle-degrading protease from the insect pathogenic fungus Metarhizium anisopliae.

The insect pathogenic fungus Metarhizium anisopliae produces several extracellular cuticle-degrading proteases and evidence is consistent with one of these, PR1, which is a chymoelastase, being a determinant of pathogenicity. We have shown previously that PR1 production is regulated by both carbon catabolite and nitrogen metabolite repression and also by specific induction under derepressed conditions by insect cuticle. In the present work we have established that an enzymically released proteinaceous component(s) of insect cuticle is capable of inducing PR1 (based on appearance of extracellular activity). Cuticle of the desert locust Schistocerca gregaria treated with KOH to remove protein failed to induce PR1 production, whereas cuticle treated with either chloroform or ether to remove lipids still induced PR1. Cuticle digested with either PR1 or the trypsin-like PR2 of M. anisopliae released peptides mainly in the range 150-2000 Da; addition of these peptides generated by PR1 or PR2 at 3 micrograms alanine equivalents ml-1 induced PR1 production to a level similar (75%) to that obtained with untreated insect cuticle. Several amino acids and peptides which are abundant in insect cuticular protein (Ala, Gly, Ala-Ala, Ala-Ala-Ala, Ala-Pro and Pro-Ala) were tested at a range of concentrations and in restricted cultures for their ability to induce PR1. None induced the protease to the levels seen with cuticle or peptides enzymically released from cuticle, although some dimers and notably the monomers Ala and Gly gave 2-2.7-fold enhanced PR1 activity above depressed basal levels (up to 48-57% of that achieved with induced synthesis on cuticle).(ABSTRACT TRUNCATED AT 250 WORDS)

Specific induction of a cuticle-degrading protease of the insect pathogenic fungus Metarhizium anisopliae.

Summary: The insect pathogenic fungus Metarhizium anisopliae produces several extracellular cuticle-degrading proteases and evidence is consistent that one of these, a chymoelastase PR1, is a determinant of pathogenicity. We have shown previously that the wide-domain regulatory circuits of carbon and nitrogen derepression regulate PR1 production. In the present work we have established in addition that PR1 is specifically induced by insect cuticle, but not by other soluble or insoluble proteinaceous substrates. The feeding of elastin or collagen to derepressed established mycelium (starved for carbon and nitrogen) did not enhance PR1 production significantly and the soluble proteins BSA and gelatin rapidly and completely repressed PR1. The carbohydrate polymers cellulose and xylan gave derepressed basal levels of PR1. However, addition of locust cuticle enhanced PR1 production to a level approximately 10-fold that of derepressed mycelium. In order to establish if the enhancing effect of insect cuticle on PR1 production was due to specific induction or merely a reflection of enhanced growth on this insoluble dual carbon and nitrogen source, ergosterol was used as a measure of fungal growth. Expressing enzyme activity per mg dry weight showed that PR1 production in cuticle cultures increased approximately five- and ninefold after 12 and 24 h growth compared with elastin-grown cultures. Thus, the substantial increase in PR1 production on cuticle was shown not to be a function of fungal growth and this confirms that PR1 is induced by a component of insect cuticle; we believe this is the first report of induction by a specific substrate for any microbial protease.

Saprotrophic and Mycoparasitic Components of Aggressiveness of Trichoderma harzianum Groups toward the Commercial Mushroom Agaricus bisporus

ABSTRACT We examined the mycoparasitic and saprotrophic behavior of isolates representing groups of Trichoderma harzianum to establish a mechanism for the aggressiveness towards Agaricus bisporus in infested commercial compost. Mycoparasitic structures were infrequently observed in interaction zones on various media, including compost, with cryoscanning electron microscopy. T. harzianum grows prolifically in compost in the absence or presence of A. bisporus, and the aggressive European (Th2) and North American (Th4) isolates produced significantly higher biomasses (6.8- and 7.5-fold, respectively) in compost than did nonaggressive, group 1 isolates. All groups secreted depolymerases that could attack the cell walls of A. bisporus and of wheat straw, and some were linked to aggressiveness. Growth on mushroom cell walls in vitro resulted in rapid production of chymoelastase and trypsin-like proteases by only the Th2 and Th4 isolates. These isolates also produced a dominant protease isoform (pI 6.22) and additional chitinase isoforms. On wheat straw, Th4 produced distinct isoforms of cellulase and laminarinase, but there was no consistent association between levels or isoforms of depolymerases and aggressiveness. Th3s distinctive profiles confirmed its reclassification as Trichoderma atroviride. Proteases and glycanases were detected for the first time in sterilized compost colonized by T. harzianum. Xylanase dominated, and some isoforms were unique to compost, as were some laminarinases. We hypothesize that aggressiveness results from competition, antagonism, or parasitism but only as a component of, or following, extensive saprotrophic growth involving degradation of wheat straw cell walls.

Characterization of SNP1, a Cell Wall-Degrading Trypsin, Produced During Infection by Stagonospora nodorum

Stagonospora (= Septoria) nodorum when grown in liquid culture with wheat cell walls as the sole carbon and nitrogen source secretes numerous extracellular depolymerases, including a rapidly produced, alkaline, trypsin-like protease (SNP1). The enzyme was purified 417-fold by cation exchange chromatography and has a molecular mass of 25 kDa on sodium dodecyl sulfate gels, pI 8.7, and pH optimum of 8.5. It cleaved peptide bonds on the carboxyl side of lysine or arginine, was strongly inhibited by the trypsin inhibitors aprotinin and leupeptin and weakly by phenylmethylsulfonyl fluoride, and its activity was stimulated by calcium. SNP1 has the characteristic, conserved, fungal, trypsin N terminus. Polymerase chain reaction (PCR) primers based on this sequence and the conserved trypsin active site were used to amplify a DNA fragment that facilitated isolation of the corresponding genomic clone from a lambda library of S. nodorum. The full-length sequence confirmed its identity as a trypsin-like protease containing the N-terminal sequence of the previously purified enzyme. Infected leaf tissue contained a protease, not present in controls, that coeluted with the fungal trypsin from cation exchange, and had properties (pI and inhibitor characteristics) similar to those of the fungal trypsin. SNP1 expression in planta was detected by Northern (RNA) blotting, reverse transcription PCR, and green fluorescent protein confocal microscopy. SNP1 released hydroxyproline from wheat cell walls. The release of hydroxyproline, together with its early expression in planta, suggests that SNP1 participates in the degradation of host cell walls during infection.

Cloning and sequence analysis of an intron-containing domain from a peptide synthetase-encoding gene of the entomopathogenic fungus Metarhizium anisopliae

A gene encoding a putative peptide synthetase has been cloned and partially sequenced from the filamentous fungus, Metarhizium anisopliae. The deduced amino acid sequence of one entire domain and the following spacer is typical of fungal peptide synthetases, showing good conservation of the six expected core sequences. There are two introns within this region, the first interrupting core 5 (RLDLTDIE) of the domain and the second in a conserved area of the spacer region.

A Selective Medium for Quantitative Reisolation of Trichoderma harzianum from Agaricus bisporus Compost

ABSTRACT We adapted a selective medium, previously developed for reisolation of Trichoderma spp. from soil, for quantitative determination of growth of T. harzianum from commercial Agaricus bisporus composts. This medium enables comparisons of aggressive (sensu inhibition of A. bisporus yield) with nonaggressive T. harzianum groups. The resulting medium contains the antimicrobials chloramphenicol, streptomycin, quintozene, and propamocarb and was highly selective, allowing the recovery of T. harzianum, as viable conidia and hyphal fragments, in compact colonies with the absence of visible microbial contaminants.

Isolation of a nitrogen response regulator gene (nrr1) from Metarhizium anisopliae

Attempts to improve the effectiveness of entomopathogenic fungi as biological control agents require a clear understanding of the pathogenicity determinants at both the biochemical and molecular level. Proteases play a key role in entomopathogenicity, allowing the fungus to penetrate the insect cuticle and rapidly invade the host. The most extensively studied of these protease activities, PR1A and PR2, are both subject to nitrogen derepression. The Metarhizium anisopliae nrr1 (nitrogen response regulator 1) gene was identified using a PCR-based strategy; it encodes a putative DNA-binding protein with a single zinc finger motif defined by the C-X2-C-X17-C-X2-C sequence. M. anisopliae NRR1 shows a significant sequence similarity to Neurospora crassa NIT2. Sequence analysis identified the presence of two introns, suggesting a greater degree of similarity to N. crassa nit2 than to the areA-like genes that have been identified. However, functional equivalence of nrr1 to areA was demonstrated, by co-transformation and complementation of an A. nidulans areA loss-of-function mutant (areA18 argB2 pabaA1 inoB2) with the M. anisopliae nrr1 gene. The areA-/nrr1+ Aspergillus transformants were able to grow on media with nitrate and glutamate as the sole nitrogen source, whereas the areA- strain is unable to grow under these conditions. The possible relevance of nitrogen regulation to pathogenicity is discussed.

Trehalose-hydrolysing enzymes of Metarhizium anisopliae and their role in pathogenesis of the tobacco hornworm, Manduca sexta

Trehalose is the main haemolymph sugar in most insects including the tobacco hornworm, Manduca sexta, and is potentially a prime target for an invading pathogenic fungus. There was considerably more trehalose-hydrolysing activity in the haemolymph of caterpillars infected with Metarhizium anisopliae than in controls. This appeared to be due primarily to additional isoforms; one of which could also hydrolyse maltose and was designated an alpha-glucosidase. A comparable isoform was identified in in vitro culture of the fungus, supporting a fungal origin for the in vivo enzyme. The in vitro fungal enzyme, alpha-glucosidase-1 (alpha-gluc-1), was purified to homogeneity and partially characterised. A study with the trehalase inhibitor trehazolin and C14 trehalose suggested that extracellular hydrolysis is important for fungal mobilisation of trehalose. Haemolymph glucose increases significantly during mycosis of tobacco hornworm larvae by M. anisopliae, consistent with the hydrolysis of trehalose by extracellular fungal enzymes. The implications for the host insect are discussed.