through the activation of the auxin signaling and biochemical changes that included
the accumulation of ABA, the antioxidant compound ascorbic acid, and the
osmolyte L-proline [20]. In the case of Eucalyptus globulus grown in the presence
of As, an element that decreases also plant growth, T. harzianum promoted E.
globulus growth and induced the accumulation of chlorophyll, and in this scenario,
T. harzianum increased As accumulation in E. globulus roots, thus showing As
bioremediation potential [37].
2.2
Fungal Metabolites Involved in Plant Growth
Microbial synthesis of the phytohormone auxin has been known for a long time. This
property is best documented for fungi that interact with plants because fungal auxin
can interfere with many plant developmental processes [38]. Based on the occurrence of IAA intermediates described in plants, different pathways that share Ltryptophan (L-Trp) as a common precursor have been reported in microorganisms
[39]. Production of IAA through the indole-3-pyruvic acid (IPA) pathway was
identified in the fungus Colletotrichum acutum [40]. HPLC analysis and chromogenic stains after a fluorescence TLC separation unambiguously identified IAA,
indole-3-ethanol (IEt), indole-3-acetaldehyde (IAAld), and IPA from cultures
supplemented with L-Trp. Interestingly, increasing L-Trp concentrations drastically
increased the levels of IEt but not IAA [40].
The ability of Trichoderma to produce indole auxins in pure culture has been
demonstrated when L-Trp was provided in the medium [41]. Contreras-Cornejo and
coworkers [11] reported that culture filtrates of T. virens Gv29-8 contained IAA
and its concentrations increased from 13.48 Æ 0.97 to 233.64 Æ 3.06 μg l
À1 when
100 mg l
À1 of L-Trp were added to the culture medium. Furthermore, T. virens also
produced the indole-derived compounds IAAld, IEt, and indole-3-carboxaldehyde
(ICAld), likely involved in the biosynthetic and catalytic pathways of IAA. Other
fungi such as Amanita muscaria, Paxillus involutus, Suillus luteus, Suillus bovinus,
and Rhizopogon luteolus isolated from Pinus sylvestris also produce auxins [42].
Pharmacological analysis revealed that plants have different sensitivity to indolederived substances during Trichoderma-plant interactions [11, 23]. For example,
IAAld induced lateral root and root hair formation in Arabidopsis thaliana, but
ICAld induced adventitious root formation [11, 23]. T. asperellum promotes maize
seedlings growth and produces IAA in concentrations of 72.52 Æ 15.14 μg/g of dry
weight. In addition, T. asperellum increased the IAA content in the shoot and root of
maize plants. Most likely, T. asperellum promoted the maize growth by activating
the plasma membrane H
+
-ATPase [43]. Figure 3 shows chemical structures of
indolic compounds identified in T. virens and T. atroviride and the proposed biosynthetic pathway for IAA.
T. virens and T. atroviride also produce ethylene (ET) a gaseous hydrocarbonated
compound derived from L-methionine [44]. In plants, ET induces root hair
formation and controls root branching in a cross-talk mechanism with IAA [44].
More recently, it was identified that T. virens and T. atroviride also produce cis,
12 Interactions of Trichoderma with Plants, Insects, and Plant Pathogen. . .
269
the accumulation of ABA, the antioxidant compound ascorbic acid, and the
osmolyte L-proline [20]. In the case of Eucalyptus globulus grown in the presence
of As, an element that decreases also plant growth, T. harzianum promoted E.
globulus growth and induced the accumulation of chlorophyll, and in this scenario,
T. harzianum increased As accumulation in E. globulus roots, thus showing As
bioremediation potential [37].
2.2
Fungal Metabolites Involved in Plant Growth
Microbial synthesis of the phytohormone auxin has been known for a long time. This
property is best documented for fungi that interact with plants because fungal auxin
can interfere with many plant developmental processes [38]. Based on the occurrence of IAA intermediates described in plants, different pathways that share Ltryptophan (L-Trp) as a common precursor have been reported in microorganisms
[39]. Production of IAA through the indole-3-pyruvic acid (IPA) pathway was
identified in the fungus Colletotrichum acutum [40]. HPLC analysis and chromogenic stains after a fluorescence TLC separation unambiguously identified IAA,
indole-3-ethanol (IEt), indole-3-acetaldehyde (IAAld), and IPA from cultures
supplemented with L-Trp. Interestingly, increasing L-Trp concentrations drastically
increased the levels of IEt but not IAA [40].
The ability of Trichoderma to produce indole auxins in pure culture has been
demonstrated when L-Trp was provided in the medium [41]. Contreras-Cornejo and
coworkers [11] reported that culture filtrates of T. virens Gv29-8 contained IAA
and its concentrations increased from 13.48 Æ 0.97 to 233.64 Æ 3.06 μg l
À1 when
100 mg l
À1 of L-Trp were added to the culture medium. Furthermore, T. virens also
produced the indole-derived compounds IAAld, IEt, and indole-3-carboxaldehyde
(ICAld), likely involved in the biosynthetic and catalytic pathways of IAA. Other
fungi such as Amanita muscaria, Paxillus involutus, Suillus luteus, Suillus bovinus,
and Rhizopogon luteolus isolated from Pinus sylvestris also produce auxins [42].
Pharmacological analysis revealed that plants have different sensitivity to indolederived substances during Trichoderma-plant interactions [11, 23]. For example,
IAAld induced lateral root and root hair formation in Arabidopsis thaliana, but
ICAld induced adventitious root formation [11, 23]. T. asperellum promotes maize
seedlings growth and produces IAA in concentrations of 72.52 Æ 15.14 μg/g of dry
weight. In addition, T. asperellum increased the IAA content in the shoot and root of
maize plants. Most likely, T. asperellum promoted the maize growth by activating
the plasma membrane H
+
-ATPase [43]. Figure 3 shows chemical structures of
indolic compounds identified in T. virens and T. atroviride and the proposed biosynthetic pathway for IAA.
T. virens and T. atroviride also produce ethylene (ET) a gaseous hydrocarbonated
compound derived from L-methionine [44]. In plants, ET induces root hair
formation and controls root branching in a cross-talk mechanism with IAA [44].
More recently, it was identified that T. virens and T. atroviride also produce cis,
12 Interactions of Trichoderma with Plants, Insects, and Plant Pathogen. . .
269