M.Sc Thesis

M.Sc StudentTrushina Naomi
SubjectRole of the PacC pH Sensing Pathway of Trichoderma virens
in Biocontrol
DepartmentDepartment of Biology
Supervisor PROF. Benjamin Horwitz
Full Thesis textFull thesis text - English Version


Trichoderma virens is a soil fungus which is found in the rhizosphere of plants, and antagonizes other fungi, including many that are pathogenic. T. virens is therefore being investigated as a biocontrol agent. Little is known about the mechanisms of interaction between T. virens, plant pathogenic fungi and susceptible plants. Understanding of the signaling steps and the control of gene expression which underlie these interactions is essential for improving biocontrol strains, and therefore has great agricultural importance. 

Human, plant, and insect pathogenic fungi produce extracellular enzymes that degrade the integuments of their hosts, whether skin, lipid-based or chitin-based cuticle, respectively. Explanation of the mechanisms that control the production and activity of these enzymes is central to understanding pathogen development in the host. Some of these mechanisms may be pH-dependent. Fungi are able to thrive over a wide range of pH and likely have a genetic regulatory system that tailors gene expression to the pHs at which gene products can function effectively. The transcription factor PacC is a central player in the detection of ambient pH.

To find out whether PacC is important for mycoparasitism I deleted its ortholog in T. virens. After isolating mutants, growth on different media and sensitivity to different stresses were characterized. The mutant was unable to grow at pH 9, as compared to WT which grew, although slower than at acidic pH. These data imply that T. virens PacC might be involved in alkaline pH sensing.  Mycoparasitic ability of WT and the mutant was compared. This was done in two different ways; both assays showed the same result: the ability of WT to overgrow and inhibit growth of Rhizoctonia solani was better than of the PacC mutant. These results suggest that the transcription factor PacC is important for antagonism of host fungi by Trichoderma virens.

One explanation is that the local pH changes during mycoparasitic interaction, and that sensing this pH change by T. virens has a role in programming the transcriptional pattern responsible for antagonism in the fungal-fungal interaction. Indeed, I have shown that the local pH increases at the interaction front between the mycoparasite and host. It is also possible that the external pH change acts as a stress, and that its detection, through PacC, allows maintenance of pH homeostasis. Testing these hypotheses, combined with studies on the mechanism of action of PacC in fungi, will be promising routes to follow in future work.