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