טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentZaid Rinat
SubjectThe Impact of the Secondary Metabolite Gliotoxin, Secreted
by Trichoderma, on the Fungal-Host Interaction
DepartmentDepartment of Biology
Supervisor Dr. Benjamin Horwitz
Full Thesis textFull thesis text - English Version


Abstract

Trichoderma spp. interactions with plant crops have long been established to be beneficial, and many members of this genus are employed as bio-fertilizers and biocontrol agents in agricultural fields worldwide. Although the fungal-root interaction is generally beneficial, the Trichoderma genomes show intrinsic pathogenic potential. This raises questions about the limits of symbiosis, priming of plant immunity, and disease in the rhizosphere.

The Trichoderma-plant interactions are studied as a complex molecular crosstalk between Trichoderma and a plant host. The fungal bioactive compound repertoire impacts the balance between plant immunity and growth. Thus, the fungal partner modulates the sensitivity to jasmonate, salicylate, gibberellin and ethylene, but relatively little is known about the details of signaling and response. It is clear that the outcome of the interaction depends on the fungal and plant species. Less obvious, even within the same species pair the interactions are strain-specific, suggesting that genetic compatibility is important.

Most of what is known in the literature about this fungal-host molecular dialogue is the role of fungal proteinaceous effectors and elicitors. Although many symbiotic fungi, Trichoderma included, produce a plethora of well-studied secondary metabolites, their functional effects on plants, and on the fungal-plant interaction, is often overlooked. Characterizing the functional effects of fungal secondary metabolites might lead to a paradigm shift in the understanding of plant-fungal interactions.

This work describes how the fungal secondary metabolite gliotoxin, that is considered a virulence factor in plant and animal pathogens, has a role as a plant defense elicitor in interaction with tomato seedlings. Comparison of a wild type gliotoxin producer with a mutant lacking the metabolite showed that gliotoxin is responsible, as expected, for a large portion of the detrimental effect on plant growth. Perhaps surprisingly, gliotoxin production evoked strong transcriptomic impacts on the plant host as well. The gliotoxin producer induced the expression of a large set of genes related to defense, development and hormone response. A T. virens strain that does not produce gliotoxin also primes the plant immune system for defense against subsequent pathogen attack, but the transcriptomic signature is different, and overall attenuated as compared to the response to the gliotoxin producer. The transcriptomic signatures of tomato seedlings in interaction with the three strains studied here help understand how Trichoderma evokes plant defense, including oxidative-burst response, synthesis of anti-microbial compounds, the accumulation of pathogenesis-related proteins like PR-1, and modulating phytohormonal axes, thus facilitating plant disease resistance and immunity against pathogens, and robustly affecting the plant’s metabolic decision-making.

Successfully linking these results to fungal molecular processes may provide tools for improving the performance of Trichoderma based agricultural products to its best potential.