Direct biocontrol mode of action in Trichoderma-plant pathogen interaction
includes mycoparasitism and antibiosis [2, 70, 79, 80]. Other Trichoderma mechanisms to exert biocontrol against plant pathogenic microorganisms involve also
competition by nutrients and space [28, 81]. Mycoparasitism involves host recognition, attachment to and coiling around the host hyphae. This mechanism requires
tropic growth of the biocontrol agent toward the targeted fungi, lectin-mediated
coiling of Trichoderma hyphae to the pathogen, and finally the attack [79]. Furthermore, during mycoparasitism, secretion of antibiotic metabolites also takes place,
resulting in disarming the pathogen and killing it [77, 78, 82]. For example, the
anthraquinone pachybasin, identified in T. harzianum, increases the number of coils
of the biocontrol agent against R. solani [83]. During mycoparasitism production
and release of atpenins, potent and specific inhibitors of mitochondria metabolism in
the parasite have also been reported [84].
Mycoparasitism has been also studied at the genetic level. The transcriptome
response of the biocontrol agent T. atroviride with the plant pathogens B. cinerea and
R. solani revealed that some genes of T. atroviride were upregulated in the early
stage of the physical contact between microorganisms. The upregulated genes
involved those for nitrogen metabolism, stress response, signal transduction, and
lipid catabolism [85]. Undoubtedly, mycoparasitism is a complex process that is
fine-tuned by Trichoderma to coordinate the gene expression and production of
effective secondary metabolites against its prey.
3.1
The Role of Released Fungal Siderophores in the Rhizosphere
Trichoderma produce several compounds that chelate iron and form Fe (III)
complexes. Then, the microorganisms can reutilize sequestered iron in
a physiological mechanism where the charged siderophore is taken up by ferricchelate transporters [51, 86]. Coprogen, coprogen B, ferricrocin, and fusarin are
well-known fungal siderophores (Fig. 6). A recent study has shown that
T. atroviride, T. asperellum, T. gamsii, T. hamatum, T. harzianum, T. virens, T. polysporum, and T. reesei produce coprogen, fusigen, fusarin A, and ferricrocin [87].
Fungal siderophores are catalyzed by NRPS, generally from L-ornithine-derived N5acyl-N5-hydroxy-L-ornithine with different possible acyl groups whereby the NRPS
covalently links these units via ester or peptide bonds to linear or cyclic oligomers
that later can be modified to give different siderophores [1, 87, 88]. The production
of siderophores can stop the growth of plant pathogen microorganisms by depriving
them of iron. These compounds can solubilize unavailable iron for plants [51]. More
recently, it was reported that HA also is a metabolite that binds iron with good
affinity [89].
3.2
Antibiotic Production by Trichoderma
Trichoderma species are known due to their ability to produce metabolites with
antibiotic activity (Fig. 7). Of these, alkyl pyrones, isonitriles, polyketides,
12 Interactions of Trichoderma with Plants, Insects, and Plant Pathogen. . .
275
includes mycoparasitism and antibiosis [2, 70, 79, 80]. Other Trichoderma mechanisms to exert biocontrol against plant pathogenic microorganisms involve also
competition by nutrients and space [28, 81]. Mycoparasitism involves host recognition, attachment to and coiling around the host hyphae. This mechanism requires
tropic growth of the biocontrol agent toward the targeted fungi, lectin-mediated
coiling of Trichoderma hyphae to the pathogen, and finally the attack [79]. Furthermore, during mycoparasitism, secretion of antibiotic metabolites also takes place,
resulting in disarming the pathogen and killing it [77, 78, 82]. For example, the
anthraquinone pachybasin, identified in T. harzianum, increases the number of coils
of the biocontrol agent against R. solani [83]. During mycoparasitism production
and release of atpenins, potent and specific inhibitors of mitochondria metabolism in
the parasite have also been reported [84].
Mycoparasitism has been also studied at the genetic level. The transcriptome
response of the biocontrol agent T. atroviride with the plant pathogens B. cinerea and
R. solani revealed that some genes of T. atroviride were upregulated in the early
stage of the physical contact between microorganisms. The upregulated genes
involved those for nitrogen metabolism, stress response, signal transduction, and
lipid catabolism [85]. Undoubtedly, mycoparasitism is a complex process that is
fine-tuned by Trichoderma to coordinate the gene expression and production of
effective secondary metabolites against its prey.
3.1
The Role of Released Fungal Siderophores in the Rhizosphere
Trichoderma produce several compounds that chelate iron and form Fe (III)
complexes. Then, the microorganisms can reutilize sequestered iron in
a physiological mechanism where the charged siderophore is taken up by ferricchelate transporters [51, 86]. Coprogen, coprogen B, ferricrocin, and fusarin are
well-known fungal siderophores (Fig. 6). A recent study has shown that
T. atroviride, T. asperellum, T. gamsii, T. hamatum, T. harzianum, T. virens, T. polysporum, and T. reesei produce coprogen, fusigen, fusarin A, and ferricrocin [87].
Fungal siderophores are catalyzed by NRPS, generally from L-ornithine-derived N5acyl-N5-hydroxy-L-ornithine with different possible acyl groups whereby the NRPS
covalently links these units via ester or peptide bonds to linear or cyclic oligomers
that later can be modified to give different siderophores [1, 87, 88]. The production
of siderophores can stop the growth of plant pathogen microorganisms by depriving
them of iron. These compounds can solubilize unavailable iron for plants [51]. More
recently, it was reported that HA also is a metabolite that binds iron with good
affinity [89].
3.2
Antibiotic Production by Trichoderma
Trichoderma species are known due to their ability to produce metabolites with
antibiotic activity (Fig. 7). Of these, alkyl pyrones, isonitriles, polyketides,
12 Interactions of Trichoderma with Plants, Insects, and Plant Pathogen. . .
275