Trichoderma–Plant–Pathogen Interactions: Advances in Genetics
of Biological Control

Introduction

Interaction With Plants Pathogen

Lessons from Genome Sequencing

Killing the Host: Production of Hydrolytic Enzymes
and Antibiotics

T. reesei is a saprophyte that can be found on decaying wood

It secretes large amounts of cellulases and hemicellulases

Important for industry

Genome sequences of T. reesei, T. atroviridae, T. virens, T. harzanum and T. asperellum are available

Trichoderma spp.

teleomorph Hypocrea

Mycoparasitic Trichoderma

successful biofungicides

60 % of the registered biofungicides world-wide

250 products are available for field applications (India)

has extensive data on their molecular genetics and physiology

major limitations of microbe-based fungicides

restricted efficacy

inconsistency under field conditions

slow to act, compared to chemicals

influenced by environmental factors

This species has the smallest genome (34.1 Mb, 9,129 gene models) as compared to the other mycoparasitic species

Due to the loss of mycoparasitism specific genes

T. harzanum (40.98 Mb
and 14,095 gene models)

T. asperellum ( 37.4 Mb and, 12,586 gene models)

T. virens (38.8 Mb and, 12,427 gene models)

T. atroviridae (36.1 Mb and 11,863 gene
models)

help to study the molecular mechanisms of interactions of tthe fungi with other biotic and abiotic factors

Mycoparasitism is an ancestral trait of trichoderma

Trichoderma–Plant Interactions

Genetic approach has significant progress to understand the mechanism of cell signaling.

Ability to parasitize and kill other fungi has been used as biofungicides

opportunistic/facultative symbiosis

Association with plant roots

G protein couple receptor (Gpr1) is involve in sensing the fungal prey

Association with living or dead fungal biomass

Tyinteraction involve sensing the host/ prey fungus, attraction, attachment, coiling around and lysis by hydrolytic enzyme

Plant provide sucrose/other nutrients

T. atroviride and T. asperellum are phylogenetically ancestral species and powerful antagonists of other fungi (necrotrophic mycoparasites).

T. virens and T. harzianum are aggressive
parasites of phytopathogenic fungi,

These species are effective for the stimulation of plant defense response

Trichoderma provides:

boosting plant immunity against invading pathogens

improving photosynthetic abilities

evokes a coordinated transcriptomic, proteomic and metabolomic response in the plant

The expansion of T. atroviride,
T. virens and T. reesei comprise of genes specific for mycoparasitism such as:

Root Colonization

chitinases

some glucanases

those involved in secondary metabolite
biosynthesis

Induced Defense

The Endophytic Trichoderma

Has largest sets of proteases
among fungi

Binding ligand to the receptor will lead to signalling downstream event via activation of G protein cascade

Trichoderma spp. colonize plant roots externally and internally

Primary step

Trichoderma spp. produce and modulate hormonal signals

Capture

Tga3 Ga protein-encoding gene is involve in the mycoparasitic ability of T. atroviride

Trichoderma deployed chemical arsenals such as hydrolytic enzymes and antibioticsto kill other fungi

  1. promotes root growth

e.g auxins

increasing the available surface area for colonization

Adenylate cyclase gene tac1 is involve in growth and mycoparasitic abilities of T.virens

Trichoderma spp. rich in chitinases and glucanases (genes encoding enzymes) also NRPSs (secondary metabolism)

gene accd

encoding ACC deaminase

MAPK pathway ( comprising MAPKKK, MAPKK & MAPK) act in mycoparasitism and biocontrol

regulate canola root growth by T. asperellum (demonstrated by gene knockout )

Chitinases and glucanases involved in biocontrol

promote root growth

Deletion of tvbgn3 (b-1,6-glucanase-encoding) reduced the mycoparasitic and biocontrol potential of T. virens against P. ultimum

Attachment to host fungi

Co-overexpression of two b-glucanases (Bgn2 and Bgn3) resulted in improved biocontrol of T. virens against R. solani, P. ultimum and Rhizopus oryzae

  1. facilitate attachment to the root

Attachment and attack to host are by formation of appressoria/ papillae like structure/ coiling around the host hyphae

TasHyd1 protein from T. asperellum

Qid74 protein of T. harzianum

  1. facilitate root penetration

secretes expansin-like proteins with cellulose binding modules

proteases like Prb1/Sp1 are
induced during mycoparasitism also play definitive roles in
biocontrol

secretes endopolygalacturonase

Hydrophobins are involve in hydrophobicity and mycoparasitism of T.virens

Mycoparasitism-relevant signaling pathways of Trichoderma atroviridae/Trichoderma virens

When inside the root :

  1. Trichoderma secretes cell-wall degrading enzymes (CWFDEs)

these fungi can grow inter-cellularly

albeit limited to epidermal layer and outer cortex

  1. Result in release of degradation products from the host's cell wall
  1. These act as signals for host recognition in the mycoparasite
  1. Activation of G protein signaling, MAPK and cAMP pathways act as downstream effectors

As adhesion to the host

Initial suppression of plant defense may facilitate root invasion. Eg:

T. koningii

  1. This happen via phosphorylation, respective targets are regulated resulting in full induction of CDWEs and secondary metabolism

suppresses the production of phytoalexins during colonization of Lotus japonicus roots

Examples

T. atroviride produce the volatile metabolite 6-pentyl-2H-pyran-2-one (6-PP) which plays an important role in Trichoderma–plant and Trichoderma–fungal interactions

The T. pseudokoningii peptaibol trichokonin VI induce programmed cell death in Fusarium oxysporum

click to edit

  • rapid ion fluxes, oxidative burst and deposition of callose and synthesis of polyphenols (make plants respond immediately to Trichoderma invasion.
  • subsequent event: salycilate (SA) and jasmonate/ethylene (JA/ET)- signaling : results- plant acquire varying degrees of tolerance to pathogen invasion.
  • JA/ET-mediated induces systemic resistance (ISR) and resembles the response triggered by plant growth-promoting rhizobacteria (PGPR).
  • higher inoculum doses Trichoderma can trigger SA mediated systemic cquired resistance (SAR) response - similare to nvoked by necrotrophic pathogens.
  • hint from mitogen-activated protein kinase (MAPK) from cucumber and MAPK from T. virens - triggering the downstream defense responses.
  • xylanase and peptaibols (alamethicin and trichovirin II) produced by Trichoderma elicit an immune response in plant.
  • PKS/NRPS hybrid enzyme identified; involve is defense response in maize.
  • SM1/EpII: the best characterized elicitor produced- secreted abundantly, small cystein-rich hydrophobin-like protein of cerato-platanin (CP) family.
  • monomeric form in the non-glycosylated state: susceptible to oxidative-driven dimerization in plants rendering Sm1 inactive as inducer of ISR.
  • 3-D structure of the Ceratocystis platani cerato-platanin has been resolved. The carbohydrate residue (an oligomer of N-acetyl glucosamine) that binds to it has been identified.
  • CP protein family is highly conserved, its structure and carbohydrate-binding properties may suggest a mechanism for the elicitation properties of Sm1.
  • some can live in plant as "true" endophytes (not restricted to outer root tissue
  • being discovered as new species because they are different from the species that were being isolated from soil/rhizopsphere.
  • eg: T. stromaticum, T. amazonicum, T. evansii, T. martiale, T. taxi and T. theobromicola
  • phylogenetic analysis - these species are of recent evolutionary origin
  • some preferentially colonize the surface of glandular trichomes and form appressoria-like structures
  • reported to induce transcriptomic changes in plants; some protect plants from diseases and abiotic stresses
  • it uses non-root mode of entry into the plant