that attract the fungus toward the root [26]. Sucrose derived from plants seems to
be a key metabolite in the Trichoderma-root association [27, 28]. It is known
that ThPG1 from T. harzianum T34 is a plant cell wall-degrading enzyme required
for fungal root colonization [29]. In the fungus-root association process,
these fungi penetrate the epidermis and the first cortical cell layers in the root [30,
31 ]. Commonly, Trichoderma growth is limited to the apoplast among root cells
[32]. In the early stage of root colonization, the plant limits the endophytic
colonization of Trichoderma through the cell wall reinforcement and accumulation
of both antimicrobial compounds and reactive oxygen species [33].
On the other hand, it was observed that when T. virens colonizes maize roots
through the root apoplast, the fungus releases several proteins that are likely
involved in the suppression of the plant immunity, which facilitate the fungal root
colonization. Fungal proteins secreted in the apoplast corresponded with cell wall
hydrolysis, scavenging of reactive oxygen species and secondary metabolism [32].
Trichoderma-secreted enzymes that must facilitate the root colonization involve
glycoside hydrolases, glycosyltransferases, polysaccharide lyases, carbohydrate
esterases, and carbohydrate-binding proteins. Among these proteins glycoside
hydrolases are the most abundant enzymes in T. atroviride IMI 206040, T. virens
Gv29-8, T. reesei QM6a, T. reesei Rut C-30, T. guizhouense NJAU 4742, T.
harzianum T6776, T. parareesei CBS 125925, and T. gamsii T6085 [31].
Plant hormones have been reported play a key role in regulating Trichoderma root
colonization. For example, T. harzianum T-78 increased the root colonization of the
A. thaliana sid2 mutant that accumulates lower amount of SA compared with its
background, the wild-type Columbia-0, suggesting that SA is a key regulator of
Trichoderma root colonization [34]. Root colonization of A. thaliana by T.
asperelloides T203 resulted in the substantial alteration of the plant transcriptome
with marked changes in the expression of defense response-related genes [35]. When
these fungi colonize roots, different plant processes at chemical, biochemical, and
molecular levels are activated and can cause plant growth promotion, increased
nutrient uptake, and inductions of local and systemic defense [7, 23, 36]. Concerning
nutrient uptake, it is known that T. asperellum T42 improves tobacco plants with
nitrogen utilization efficiency, which directly improved plant growth [36].
Trichoderma species also induce a beneficial impact on plants that have been
grown in soils polluted with toxic elements such as sodium (Na
+
) or arsenic (As).
In the experimental case of A. thaliana seedlings grown in detrimental concentrations of salt (100 mM NaCl), T. virens and T. atroviride improved plant growth
ä
Fig. 2 (continued) 4-days. (a) Control plants, (b) plants inoculated with T. virens Gv29.8. Notice
the abundance of lateral root and shoot size in inoculated plants. Expression partner of CycB1::
GUS, a transgenic marker of the phase G2/M in the division cell cycle in the root tip of the primary
root of (c) a control plant and (d) a plant inoculated with T. virens as indicated above. (e and f) show
the expression of the CycB1::GUS marker in lateral roots of control and T. virens-inoculated plants,
respectively. Notice that CycB1::GUS is upregulated in the presence of the fungus, which suggests a
modulation of the division cell cycle
268
H. A. Contreras-Cornejo et al.
be a key metabolite in the Trichoderma-root association [27, 28]. It is known
that ThPG1 from T. harzianum T34 is a plant cell wall-degrading enzyme required
for fungal root colonization [29]. In the fungus-root association process,
these fungi penetrate the epidermis and the first cortical cell layers in the root [30,
31 ]. Commonly, Trichoderma growth is limited to the apoplast among root cells
[32]. In the early stage of root colonization, the plant limits the endophytic
colonization of Trichoderma through the cell wall reinforcement and accumulation
of both antimicrobial compounds and reactive oxygen species [33].
On the other hand, it was observed that when T. virens colonizes maize roots
through the root apoplast, the fungus releases several proteins that are likely
involved in the suppression of the plant immunity, which facilitate the fungal root
colonization. Fungal proteins secreted in the apoplast corresponded with cell wall
hydrolysis, scavenging of reactive oxygen species and secondary metabolism [32].
Trichoderma-secreted enzymes that must facilitate the root colonization involve
glycoside hydrolases, glycosyltransferases, polysaccharide lyases, carbohydrate
esterases, and carbohydrate-binding proteins. Among these proteins glycoside
hydrolases are the most abundant enzymes in T. atroviride IMI 206040, T. virens
Gv29-8, T. reesei QM6a, T. reesei Rut C-30, T. guizhouense NJAU 4742, T.
harzianum T6776, T. parareesei CBS 125925, and T. gamsii T6085 [31].
Plant hormones have been reported play a key role in regulating Trichoderma root
colonization. For example, T. harzianum T-78 increased the root colonization of the
A. thaliana sid2 mutant that accumulates lower amount of SA compared with its
background, the wild-type Columbia-0, suggesting that SA is a key regulator of
Trichoderma root colonization [34]. Root colonization of A. thaliana by T.
asperelloides T203 resulted in the substantial alteration of the plant transcriptome
with marked changes in the expression of defense response-related genes [35]. When
these fungi colonize roots, different plant processes at chemical, biochemical, and
molecular levels are activated and can cause plant growth promotion, increased
nutrient uptake, and inductions of local and systemic defense [7, 23, 36]. Concerning
nutrient uptake, it is known that T. asperellum T42 improves tobacco plants with
nitrogen utilization efficiency, which directly improved plant growth [36].
Trichoderma species also induce a beneficial impact on plants that have been
grown in soils polluted with toxic elements such as sodium (Na
+
) or arsenic (As).
In the experimental case of A. thaliana seedlings grown in detrimental concentrations of salt (100 mM NaCl), T. virens and T. atroviride improved plant growth
ä
Fig. 2 (continued) 4-days. (a) Control plants, (b) plants inoculated with T. virens Gv29.8. Notice
the abundance of lateral root and shoot size in inoculated plants. Expression partner of CycB1::
GUS, a transgenic marker of the phase G2/M in the division cell cycle in the root tip of the primary
root of (c) a control plant and (d) a plant inoculated with T. virens as indicated above. (e and f) show
the expression of the CycB1::GUS marker in lateral roots of control and T. virens-inoculated plants,
respectively. Notice that CycB1::GUS is upregulated in the presence of the fungus, which suggests a
modulation of the division cell cycle
268
H. A. Contreras-Cornejo et al.