|Ph.D Student||Lamdan Netta-Li|
|Subject||Secretome of the Biocontrol Fungus Trichoderma|
Virens Co-Cultured with Maize Roots: Role
in Induced Systemic Resistance
|Department||Department of Biology||Supervisor||Professor Benjamin Horwitz|
|Full Thesis text|
Trichoderma species are biocontrol agents used in agriculture to antagonize pathogens of crop plants. In addition to direct mycoparasitism of soil-borne fungal pathogens, Trichoderma interacts with plant roots. This interaction induces systemic resistance (ISR), reducing diseases of above-ground parts of the plant. In the complex molecular dialog between fungus and plant leading to ISR, proteins secreted by Trichoderma can provide signals. Only a few such proteins have been characterized previously. In order to study the role of secreted proteins in ISR, we first looked for Trichoderma species with strong ISR ability. We chose two species known to have good ISR ability, T. atroviride and T. virens and compared their ISR ability. For evaluation of the ISR ability we developed a hydroponic culture system, combined with a bioassay to evaluate the level of ISR in maize leaves infected by the pathogen Cochliobolus heterostrophus. Maize plants interacting with T. virens expressed higher levels of ISR in our system. We then studied the ISR activity of the proteins secreted by T. virens, using the same system, and found that a secreted protein fraction from co-culture of maize roots and T. virens (Tv) has higher ISR activity than from T.virens grown alone (Tv). The secretome was compared between the two conditions. A total of 280 fungal proteins was identified, 66 showing significant differences in abundance between the two conditions, 32 were higher in Tv while 34 were higher in Tv. Out of the 34 found in higher abundance in Tv and negatively regulated by the maize roots was a large group of 13 SSCPs (small, secreted, cysteine rich proteins), known to be important in the molecular dialog between plants and fungi. Using a genetic approach of gene knock-out we studied the role of 4 SSCPs in ISR. We found out that all 4 knock-out lines showed better ISR activity than WT without affecting the ability of the fungus to colonize maize roots. Furthermore, the secreted protein fraction obtained from each of the mutant lines showed improved ISR activity compared to WT. These SSCPs, apparently, act as negative effectors reducing the defense levels in the plant and may be important for the fine tuning of ISR by Trichoderma. The down-regulation of SSCPs in interaction with plant roots implies a revision of the current model for the Trichoderma-plant symbiosis and its induction of resistance to pathogens.