Trichoderma in the light of day--physiology and development

Abstract

In recent years, considerable progress has been made in the elucidation of photoresponses and the mechanisms responsible for their induction in species of the genus Trichoderma. Although an influence of light on these fungi had already been reported five decades ago, their response is not limited to photoconidiation. While early studies on the molecular level concentrated on signaling via the secondary messenger cAMP, a more comprehensive scheme is available today. The photoreceptor-orthologs BLR1 and BLR2 are known to mediate almost all known light responses in these fungi and another light-regulatory protein, ENVOY, is suggested to establish the connection between light response and nutrient signaling. As a central regulatory mechanism, this light signaling machinery impacts diverse downstream pathways including vegetative growth, reproduction, carbon and sulfur metabolism, response to oxidative stress and biosynthesis of peptaibols. These responses involve several signaling cascades, for example the heterotrimeric G-protein and MAP-kinase cascades, resulting in an integrated response to environmental conditions.

Fig. 1

Effect of blue light on Trichoderma. (A) The pictures show the effect of a short (5 min) pulse of blue light (BL) on T. atroviride. The upper photographs show a colony of Trichoderma growing in the dark (D). The lower photograph shows a colony of Trichoderma 36 h after exposure to light, with the characteristic ring of green conidia at the what was the colony perimeter at the time of exposure. Photographs at the right correspond to scanning electron micrographs of the indicated area (fine lines), hyphae (top), and hyphae and conidiophores (bottom). (B) The pictures show the dramatic effect of light on colony growth in the Denv1 mutant of T. reesei (left), as compared to the wild type strain (right). T. reesei was grown under continuous exposure to blue light (BL) for 72 h.

Fig. 2

Schematic representation of the eight potential photoreceptors found in Trichoderma species and their domain structure. Conserved domains in all putative photoreceptors were determined using Pfam (http://pfam.sanger.ac.uk/search) and SMART (http://smart.embl-heidelberg.de/). Nomenclature: HDAC histone deacetylase interaction domain; LOV, Light, Oxygen, and Voltage; PAS, domain first found in the Per – period circadian protein, Arnt – Ah receptor nuclear translocator protein, and Sim – single-minded protein; GATA, GATA-type Zinc finger; PHY, characteristic phytochrome domain; GAF, cGMP phosphodiesterase/Adenylate cyclase/FhlA; HIS kinase, Histidine kinase domain.

Fig. 3

Diagram of the signaling network involved in regulation of light responses in Trichoderma species. The diagram has been divided in two sections for simplicity. (A) The photoreceptor complex BLR1/BLR2 controls expression of blue light up-regulated (blu) and down-regulated (bld) genes. The BLR proteins control transcription of the env1 gene. ENV1 in turn establishes a feedback regulatory loop with the BLR proteins, and is involved in photoadaptation. Blu genes impact stress tolerance and DNA repair. Both blu and bld genes regulate growth. The photoreceptor BLR1/BLR2 is also crucial for light dependent carbon source utilization. In addition, light causes oxidative stress and influences peptaibol biosynthesis, which is also regulated by the G-protein alpha subunit GNA3. (B) The BLR1/BLR2 complex regulates photoconidiation, which is also influenced by a second light input involving the cAMP-pathway, constituted by adenylate cyclase (ACY), cAMP, and protein kinase A (PKA). Light enhanced cellulase gene expression involves the function of the photoreceptors BLR1/BLR2 and ENV1, the sulfur signaling pathway, the heterotrimeric G-protein alpha subunits GNA1 and GNA3 and the cAMP-pathway. Arrows indicate positive regulation and lines with a bar at the end indicate negative regulation. Solid lines are used when there is direct experimental evidence and dotted lines when the role is still hypothetical. Overlapping lines appear in gray for clarity.