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Trichoderma–Plant–Pathogen Interactions: Advances in Genetics of…
Trichoderma
–Plant–Pathogen Interactions: Advances in Genetics
of Biological Control
Introduction
Trichoderma spp.
teleomorph Hypocrea
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
help to study the molecular mechanisms of interactions of tthe fungi with other biotic and abiotic factors
major limitations of microbe-based fungicides
restricted efficacy
inconsistency under field conditions
slow to act, compared to chemicals
influenced by environmental factors
Interaction With Plants Pathogen
Mycoparasitism is an ancestral trait of trichoderma
Ability to parasitize and kill other fungi has been used as biofungicides
Tyinteraction involve sensing the host/ prey fungus, attraction, attachment, coiling around and lysis by hydrolytic enzyme
Genetic approach has significant progress to understand the mechanism of cell signaling.
G protein couple receptor (Gpr1) is involve in sensing the fungal prey
Binding ligand to the receptor will lead to signalling downstream event via activation of G protein cascade
Tga3 Ga protein-encoding gene is involve in the mycoparasitic ability of T. atroviride
Adenylate cyclase gene tac1 is involve in growth and mycoparasitic abilities of T.virens
MAPK pathway ( comprising MAPKKK, MAPKK & MAPK) act in mycoparasitism and biocontrol
Lessons from Genome Sequencing
T. reesei
is a saprophyte that can be found on decaying wood
It secretes large amounts of cellulases and hemicellulases
Important for industry
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
Genome sequences of
T. reesei, T. atroviridae, T. virens, T. harzanum
and
T. asperellum
are available
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)
Mycoparasitic
Trichoderma
Association with plant roots
Association with living or dead fungal biomass
Has largest sets of proteases
among fungi
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
The expansion of
T. atroviride,
T. virens
and
T. reesei
comprise of genes specific for mycoparasitism such as:
chitinases
some glucanases
those involved in secondary metabolite
biosynthesis
Killing the Host: Production of Hydrolytic Enzymes
and Antibiotics
Trichoderma
deployed chemical arsenals such as hydrolytic enzymes and antibioticsto kill other fungi
Trichoderma spp
. rich in chitinases and glucanases (genes encoding enzymes) also NRPSs (secondary metabolism)
Chitinases and glucanases involved in biocontrol
Deletion of
tvbgn3
(b-1,6-glucanase-encoding) reduced the mycoparasitic and biocontrol potential of T. virens against P. ultimum
Co-overexpression of two
b-glucanases
(Bgn2 and Bgn3) resulted in improved biocontrol of
T. virens
against
R. solani, P. ultimum
and
Rhizopus oryzae
proteases like Prb1/Sp1 are
induced during mycoparasitism also play definitive roles in
biocontrol
Mycoparasitism-relevant signaling pathways of
Trichoderma atroviridae/Trichoderma virens
Trichoderma secretes cell-wall degrading enzymes (CWFDEs)
Result in release of degradation products from the host's cell wall
These act as signals for host recognition in the mycoparasite
Activation of G protein signaling, MAPK and cAMP pathways act as downstream effectors
This happen via phosphorylation, respective targets are regulated resulting in full induction of CDWEs and secondary metabolism
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
Trichoderma–Plant Interactions
opportunistic/facultative symbiosis
Plant provide sucrose/other nutrients
Trichoderma provides:
boosting plant immunity against invading pathogens
improving photosynthetic abilities
evokes a coordinated transcriptomic, proteomic and metabolomic response in the plant
Root Colonization
Trichoderma spp. colonize plant roots externally and internally
Primary step
Trichoderma spp. produce and modulate hormonal signals
promotes root growth
e.g auxins
increasing the available surface area for colonization
promote root growth
gene
accd
encoding ACC deaminase
regulate canola root growth by
T. asperellum
(demonstrated by gene knockout )
facilitate attachment to the root
TasHyd1 protein from
T. asperellum
Qid74 protein of
T. harzianum
facilitate root penetration
secretes expansin-like proteins with cellulose binding modules
secretes endopolygalacturonase
When inside the root :
these fungi can grow inter-cellularly
albeit limited to epidermal layer and outer cortex
Initial suppression of plant defense may facilitate root invasion. Eg:
T. koningii
suppresses the production of phytoalexins during colonization of
Lotus japonicus
roots
Induced Defense
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.
The Endophytic
Trichoderma
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
Attachment to host fungi
Attachment and attack to host are by formation of appressoria/ papillae like structure/ coiling around the host hyphae
Hydrophobins are involve in hydrophobicity and mycoparasitism of T.virens
As adhesion to the host
