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. Th3's 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.

Proteomic response of Trichoderma aggressivum f. europaeum to Agaricus bisporus tissue and mushroom compost

“…It has been previously shown that A. bisporus metabolites increase the rate of extension of T. aggressivum hyphae (Muthumeenakshi et al 1998) and that and saprotrophic growth in mushroom compost is key to its survival strategy (Williams et al 2003). The re-arrangement of the actin cytoskeleton may be an aspect of both of these behaviours.…”

“…This may be partly due to the fact that the fungus was grown under non-inducing conditions. Previous studies have shown the secretion of fungal cell wall associated depolymerising enzymes by T. aggressivum in the presence of A. bisporus (Williams et al 2003;Guthrie & Castle 2006) and the production of these enzymes is considered to be indicative of mycoparasitic activity. However, in both studies the depolymerising enzymes were secreted into the extracellular space of T. aggressivum and so it may be that these proteins are insensitive to change or are present in relatively low abundance in the intracellular protein fraction.…”

“…The mechanisms of T. aggressivum colonisation of mushroom compost are not fully understood but it is thought to involve both mycoparasitic and saprotrophic components (Williams et al 2003). Trichoderma aggressivum can colonise mushroom compost in the presence or absence of A. bisporus, but it sporulates more heavily when A. bisporus is present (Largeteau & Savoie 2010).…”

“…The tolerance of T. aggressivum to the presence of compost microbiota and A. bisporus and its ability to acquire nutrition from the mushroom compost and mushroom spawn have been proposed as the attributes that make it so harmful to mushroom agriculture (Savoie et al 2001a;Williams et al 2003;Fletcher & Gaze 2008). Trichoderma aggressivum is known to produce metabolites which are toxic to A. bisporus (Mumpuni et al 1998;Krupke et al 2003;Guthrie & Castle 2006), but other Trichoderma species, which do not cause severe green mould disease, are known to produce metabolites which may be as toxic (Mumpuni et al 1998).…”

“…Previous studies (Savoie et al 2001b;Williams et al 2003;Guthrie & Castle 2006) have described the importance of proteins secreted by T. aggressivum (prev. Trichoderma harzianum) in its interaction with A. bisporus, however there is little information published on the intracellular protein fraction of T. aggressivum, which is the focus of this study.…”