Fungi reproduce both sexually and asexually, producing a vast array of structures which have evolved over time to suit habitat and in some cases host. These structures are of great economic importance to society. Approximately 48% of the world’s food crop yield is lost due to plant diseases, of which the majority are caused by fungi. For most fungal diseases, the primary sources of inoculum are sexual and/or asexual spores. As well as economic losses, fungi can have positive economic benefits for agriculture, such as biocontrol of plant diseases. Numerous fungi have been successfully developed as biocontrol agents (BCAs) of plant diseases and the majority of these are sold as spore preparations.
The global fungal BCA market is dominated by species of the ubiquitous ascomycete Trichoderma. In general, commercial preparations of Trichoderma spp. for biological control consist of bulk-produced conidia (asexual spores), but good biocontrol activity in the environment relies upon the fungus remaining vegetative, and thus antagonistically active. The ideal Trichoderma BCA produces ample conidia in a cost-effective manner during production and maintains long periods of vigorous vegetative growth during usage. Understanding the factors that control this morphogenic switch from mycelia to conidia is integral to biocontrol research. Over 50 years of studies on conidiation in the genus have established Trichoderma as a model for asexual reproduction in fungi. This review presents what is known about the physiological responses of Trichoderma to the environmental cues that induce conidiation, and provides insights into the molecular basis of these responses, including an examination of the signal transduction pathways which link environmental signals to physiological outputs. Understanding species-specific differences in metabolic adaptations to the environment should assist biocontrol design and implementation. Knowledge of the appropriate conditions for maximal yields of viable spores would likely reduce production costs. Knowledge of survivability and vigour within a complex environment could enable targeting of biocontrol strains to the soil or foliar condition appropriate for their species. It may also be possible to create designer BCAs which incorporate desired traits through protoplast fusion or genetic modification.
Reproduction without sex: conidiation in the filamentous fungus Trichoderma. Microbiology 2010 156: 2887-2900
Trichoderma spp. have served as models for asexual reproduction in filamentous fungi for over 50 years. Physical stimuli, such as light exposure and mechanical injury to the mycelium, trigger conidiation; however, conidiogenesis itself is a holistic response determined by the cell’s metabolic state, as influenced by the environment and endogenous biological rhythms. Key environmental parameters are the carbon and nitrogen status and the C:N ratio, the ambient pH and the level of calcium ions. Recent advances in our understanding of the molecular biology of this fungus have revealed a conserved mechanism of environmental perception through the White Collar orthologues BLR-1 and BLR-2. Also implicated in the molecular regulation are the PacC pathways and the conidial regulator VELVET. Signal transduction cascades which link environmental signals to physiological outputs have also been revealed.