Functions of peptide pheromones in Hypocrea jecorina

Species of fungal genus Trichoderma are saprophytes, and their respective teleomorphs lignicolous and necrotrophic ascomycetes of the genus Hypocrea. Hypocrea jecorina (the teleomorph of Trichoderma reesei), is probably the most prominent cellulose degrader within the genus Trichoderma. Apart from studies on the enzymology of cellulose degradation, there has also been a continuing interest in understanding how the synthesis of these enzymes is regulated. It is generally believed that oligosaccharides, which are released from cellulose, serve as the actual compounds triggering the high level of induction of cellulase expression. However, only little is known about how these environmental stimuli launch the signal transduction cascade finally leading to enhanced transcription of the cellulase genes and which components are involved in this cascade. Recently, the involvement of the light regulatory protein Envoy in cellulase regulation was reported, thus establishing a link between cellulose signalling and light response. Also the finding that the G-alpha proteins GNA1 and GNA3 influence cellulase gene transcription in dependence of light points into this direction. Studies on genes putatively involved in cellulase signalling further showed that H. jecorina possesses an unusual a-factor pheromone, which is involved in light- dependent cellulase gene transcription. Since signal transduction via heterotrimeric G-proteins is involved in pheromone signalling and also because of the links between cellulose- and pheromone signalling as well as light response as suggested by our own studies we intend to elucidate the interconnection of these pathways and to amend our first sketch of this signal transduction network in H. jecorina. Therefore we want to investigate the effect of the two peptide pheromone precursor genes min1 and ppg1 on cellulase gene expression by gene deletion, overexpression of these genes in light and darkness. Also the influence of the light regulatory gene env1 on this signaling cascade as well as the impact to the regulatory pathway including both light response and pheromone expression/response on the known cellulase regulators ACE1, ACE2 and XYR1 shall be studied.

Trichoderma reesei (the anamorph of Hypocrea jecorina), is the most prominent cellulose degrader within the genus Trichoderma. In recent years, this characteristic of Trichoderma gained increased attention because of the importance of cellulases for production of second generation biofuels made from agricultural waste instead of food crops. We therefore aimed to understand the physiology of this fungus by investigating the important process of sexual development and the necessary transmission of pheromone signals inducing it. Additionally, we wanted to elucidate a possible interconnection of the regulation of sexual development with cellulase gene expression. In the course of this project we could show that indeed T. reesei is capable of sexual development although this fungus was reported to be asexual for decades. This is especially important for the widespread industrial application of T. reesei, because using this tool tremendously facilitates strain improvement and combination of different properties of production strains. Nevertheless, sexual development in T. reesei also opens up a whole new topic for research with this fungus and provides valuable tools for investigation of its physiology. In the following we further described a novel class of peptide pheromone precursors (h-type in addition to a-type and alpha-type), with T. reesei HPP1 as first member. HPP1 assumes the role of an a-type peptide pheromone in T. reesei and potential homologues with similar structure were found in several more heterothallic ascomycetes, which opens a new chapter in research with peptide pheromones in fungi. Sexual development is dependent on the light status in fungi and this is also true for T. reesei. Our analysis of the role of photoreceptors BLR1, BLR2 and ENV1 revealed that these factors indeed influence fruiting body formation and pheromone gene transcription of T. reesei, but are not essential for sexual development. With respect to an interconnection between sexual development and cellulase gene expression, we could also confirm our hypothesis. Since the pheromone precursor gene hpp1 is differentially expressed between a cellulase producing and a non producing strain, we assumed that pheromone precursor genes might be involved in regulation of cellulase gene expression. We found that the two pheromone precursor genes hpp1 (h-type) and ppg1 (alpha- type) not only influence cellulase gene expression but also pectinase and xylanase gene expression. Consequently, our studies showed a previously unknown crosstalk between sexual development and regulation of carbon source utilization in fungi.