Trichoderma–Plant–Pathogen Interactions: Advances in Genetics of Biological Control
INTRODUCTION
INTERACTION WITH PLANT PATHOGENS
-teleomorph Hypocrea
-most successful biofungicides used in agriculture (>60 % of the registered biofungicides world-wide is Trichoderma-based)
Mycoparasitism
Biofungicides
Bio-nematicides
Limitation using microbe-based fungicides -slow - depend on environmental factors
‘‘genetic intervention’’ - to design strains that more effective than the native ones
- by gaining information on molecular mechanisms of interactions of organisms with other biotic and abiotic factors.
Mycoparasitic interaction
Attachment
Coiling around
Attraction
Lysis
Sensing of host/prey fungus
Trichoderma - Plant Interaction
Mechanisms of
cell signaling in Trichoderma (genetic approaches)
- Driven by the ability of Trichoderma to derive sucrose or other nutrients from plant
- Boost plant immunity against invade pathogens
- Improve Photosynthetic ability
- Presence of Trichoderma: evoked transcriptomic, proteomic & metabolomic response
Seven transmembrane G protein coupled receptor Gpr1
Function : sensing the fungal prey
Root Colonization
Primary Steps
silencing of the gpr1 gene in T. atroviride caused the mycoparasite unable to respond to the presence of the host fungus
Attachment
- secrete cysteine-rich hydrophobin-like proteins (facilitate anchoring/attachment) - TasHyd1 from T. asperellum - Qid74 of T. harzianum
Penetration
- secrete expansin-like protein will cellulose binding molecule & endopolygalactunose (facilitate root penetration)
Internal colonization of plant roots
- albeit to epidermal and outer cortex
- Trichoderma produces and modulate hormonal signal - auxins promotes root growth - increase surface area: facilitate colonization
Role of aacd
- encode ACC deminase (gene knockout by T. asperellu of canola root growth)
Induced Defense
The Endophytic Trichoderma
- Trichoderma spp.* not restricted to outer root tissues -also can live in the plant as ‘‘true’’ endophytes
Binding of a ligand to GPR1 receptor
leads to downstream signaling events via activation of G-protein cascades
Endophytic Trichoderma (new species)
T. stromaticum, T. amazonicum, T. evansii, T. martiale, T. taxi and T. theobromicola
-isolated from soil/rhizosphere -Phylogenetic analysis undergo recent evolutionary origin
Tga3 Ga protein-encoding gene
- Induce transcriptomic changes in plants
- Some protect plants from diseases and abiotic stresses
- Some colonize the surface of glandular trichomes, form appressoria-like structures (uses a ‘‘non-root’’ mode of entry into the plant).
Deletion of the Tga3 Ga protein-encoding gene affected the mycoparasitic abilities of T. atroviride in a similar way to loss of Gpr1
Adenylate cyclase gene tac1
Deletion of the adenylate cyclase gene tac1 severely impaired growth and mycoparasitic abilities of T. virens
- Salicylate (SA) & jasmonate/ethylene (JA/ET) signaling - acquire to tolerance pathogen invasion
- JA/ET-mediated induced systemic resistance (ISR) - resemble response triggered by pgp rhizobacteria
MAPK cascades comprising MAPKKK, MAPKK and MAPK
- Higher inoculum doses Trichoderma (trigger SA-mediated systemic acquired resistance (SAR) response - similar to that invoked by necrotrophic pathogens)
- Mitogen-activated protein kinase (MAPK) from cucumber & MAPK from T. virens in molecular cross-talk between plant and Trichoderma - trigger downstream defense response
- Xynalase & peptaibols (alamethinine and trichovirin II) - elicit immune response in plant
- PKS/NRPS hybrid enzyme defense respons e in maize plant
Elicitor (Sm1/Ep11)
CONCLUSION
- Genetic basic of interaction of Trichoderma with other organisms must be followed by understanding the mechanisms
Lack of whole genome: Expected to change with availability of five Trichoderma genomes
- know the function of each gene by high throughput gene knockouts as accomplishes in N. crassa in an exemplary community effort
Future studies
Transcriptome analyses under conditions of mycoparasitism and plant root colonization
Identify novel candidate genes involved in the interaction of Trichoderma spp. with plant & plant pathogens
deletion of this gene impairs elicition in maize
Sm1 monomeric form in glycodylated state essential for elicitation properties
Sm1 reactive as inducer of ISR in monomeric, non-glycosylated form susceptible to oxidative-driven
small, cysteine-rich hydrophobin-like protein of the cerato-platanin (CP) family
- Rapid ion fluxes & oxidative burst
- Deposition of cellulose
- Synthesis of polyphenols
Lessons from Genome Sequencing
Available genome sequences
T. virens
T. harzianum
T. atroviride
T. asperellum
T. reesei
Found on decaying wood
Can secrete large amounts of cellulases and hemicellulases (industrial importance)
36.1 Mb, 11,863 gene models
37.4 Mb , 12,586 gene models
38.8 Mb, 12,427 gene models
40.98 Mb, 14,095 gene models)
T. atroviride, T. virens, T. harzianum and T. asperellum, T. reesei has the smallest genome (34.1 Mb, 9,129 gene models) probably resulting from a loss of mycoparasitism specific genes
Mycoparasitic Trichoderma species frequently live in association with plant roots and living or dead fungal biomass
T. atroviride and T. asperellum are phylogenetically ancestral species and both are powerful antagonists of other fungi (necrotrophic mycoparasites)
T. virens and T. harzianum are aggressive parasites of phytopathogenic fungi, and are particularly effective in the stimulation of plant defense responses
Comparative genome analysis between T. atroviride, T. virens and T. reesei revealed an expansion of several gene families in the mycoparasites relative to T. reesei or other ascomycetes
Expansions comprise genes specific for mycoparasitism such as chitinases and some glucanases and those involved in secondary metabolite biosynthesis
Recent secretome analysis further revealed that Trichoderma may have one of the largest sets of proteases among fungi
Subtilisin-like proteases of the S8 family, dipeptidyl and tripeptidyl peptidases are expanded in the mycoparasites
Further support the adaptation of the mycoparasitic
Trichoderma species to their antagonistic lifestyle
Attachment to and attack of host fungi
Accompanied by the formation of appressoria or papillae-like structures and/or coiling around host hyphae
Indirect support for the involvement of hydrophobins (coats hydrophobic (water-repellent) coating on the surface of an object
T.virens mutants in the transcriptional regulator of secondary metabolism and morphogenesis Vel 1, which have decreased hydrophobin expression were defective in both hydrophobicity and mycoparasitism
Killing the Host: Production of hydrolytic enzymes and antibiotics
Hydrolytic enzymes and antibiotics are among the most important members of the chemical arsenals deployed by Trichodema to kill other fungi
Trichoderma spp rich in genes encoding enzymes like chitinases and glucanases and those for secondary metabolism like NRPSs
Involvement of chitinases in biocontrol through the effects of deletion of chit421ech42 were not very drastic due to large reservoir of genes with a compensatory effect.
Glucanases is another group of cell wall-lytic enzymes with roles in mycoparasitism or biocontrol
Deletion of tvbgn3 reduced the mycoparasitic and biocontrol potential of T.virens against P.ultimum
Co-overexpression of two Beta glucanases (Bgn2 and Bgn3) resulted in improved biocontrol of T.virens against R.solani, P.ultimum and Rhizopus oryzae