Signal transduction pathways influencing development of Trichoderma reesei

Abstract

In their natural habitat, fungi constantly face the challenge to outcompete other organisms in complex ecosystems. Therefore they developed powerful enzyme systems for degradation of substrate, which provide for fast growth and efficient colonization of their environment. However, fungi also evolved the ability to kill, or at least inhibit the growth of their competitors by producing a versatile array of secondary metabolites. Application of these different survival utilities has to be tightly controlled in order to balance assignment of resources for feeding to succeed by superior growth or fighting to decrease the chances of survival for competitors. In Trichoderma reesei (teleomorph Hypocrea jecorina), light is a crucial environmental factor for initiation of sexual development and considerably influences expression of glycoside hydrolases. In both processes, the light regulatory protein ENV1 plays a key role. Transcriptome analysis revealed that vel1 (encoding the VELVET orthologue) transcription is regulated by the carbon source in the medium. Moreover, ENV1 negatively regulates vel1 in light. VELVET proteins are global regulators of development and secondary metabolism in fungi and their function is influenced by light. In the course of this PhD thesis, we intended to expand the current knowledge on regulators of development and metabolism in T. reesei. Our data indicated that VEL1 indeed plays a pivotal role in regulation and coordination of development, secondary metabolism and light response. VEL1 was shown to be essential for conidiation irrespective of light condition and carbon source. Deletion of vel1 resulted in female sterility in light and complete abolishment of mating in darkness. Additionally, we could show that VEL1 regulates communication between confronting mating partners by adjusting the expression level of pheromone system and production of secondary metabolites. Considering that both ENV1 and VEL1 are light regulatory proteins, and based on our initial data on a regulatory effect of ENV1 on expression of vel1, we further studied a possible interrelation between these proteins in regulation of development in T. reesei. Our data showed that ENV1 regulates expression of vel1 both under sexual and asexual growth with the most significant regulation upon encounter of a mating partner. VEL1 also impacted expression of env1 in a mating type dependent manner. Although our initial results showed that -env1 and -vel1 did not mate in light, we could cross these strains with the application of an altered light regime. The fruiting bodies then showed highly defected ascosporogenesis but we could still obtain -env1 -vel1 double mutants. Phenotypic analysis of -env1 -vel1 mutants showed that they are incapable of conidiation which was similar to a -vel1 mutant. Moreover, these double mutants showed a decreased growth compared to wild type which was observed also in single mutants. However, the growth defect was alleviated compared to single mutants of -env1. Growth of double mutants on 3% (w/v) malt extract agar resulted in a banding phenotype which was not observed in a -vel1 single mutant or darkness. Analysis of sexual development of -env1-vel1 mutants showed similar results to a deletion of vel1 in light. The double mutants were able to mate with the wild type which resulted in formation of fruiting bodies with pale color and severe defect in ascosporogenesis. Moreover, these double mutants were female sterile both in light and darkness. Interestingly, in darkness we could cross -env1 -vel1 MAT 1-1 with a wild type strain or a -env1 strain which indicated that mating defect due to deletion of vel1 in darkness is alleviated when env1 is also deleted in a mating type dependent manner. Transcriptional analysis of the pheromone system also showed altered expression of pheromone precursor genes in the double mutants. Taken together, ENV1 and VEL1 integrate in a complex regulatory mechanism with both complementary and antagonistic effects influencing growth and development of T. reesei.