Trichoderma Genes Involved in Interactions with Fungi and Plants - Biotechnology of Fungal Genes - page 156

Invertase and Sucrose Transporters
Even though the fi rst report on internal colonization of plants came
more than a decade ago, the genetic studies on nutritional relationship
were reported very recently. Vargas et al. (2009, 2011) characterized an
invertase and a sucrose transporter in T. virens that play a critical role
in Trichoderma acquiring sucrose from plant cells. It was concluded that
during the symbiosis, the sucrolytic activity in the fungal cells affects
the sink activity of roots, directing carbon partitioning toward roots and
increasing the rate of photosynthesis in leaves. The hydrolysis of sucrose
was also important for regulation of the elicitor protein Sm1. Similar to
plants, T. virens contains a highly specifi c sucrose/H(+) symporter that
is induced in the early stages of root colonization (Vargas et al. 2011).
Using gene deletion, the authors established the involvement of this
sucrose transporter gene in active sucrose transference from the plant to
the fungal cells during the benefi cial associations. It was suggested that
a sucrose-dependent network in the fungal cells regulates the symbiotic
association between plants and Trichoderma .
Hydrophobin and Swollenin
Hydrophobins are proteins containing eight conserved cysteine residues
that occur uniquely in mycelial fungi. Their main function is to confer
hydrophobicity to fungal surfaces in contact with air or during attachment of
hyphae to hydrophobic surfaces of hosts, symbiotic partners or themselves
resulting in morphogenetic signals (Kubicek et al. 2008). A hydrophobin-
like gene ( TasHyd1 ) was isolated during a PCR differential mRNA display
analysis conducted on Trichoderma asperellum mycelia interacting with
plant roots. TasHyd1 deletion mutants had no signifi cant reduction in in
vitro mycoparasitic activity but were altered in their wettability and were
severely impaired in root attachment and colonization (Viterbo and Chet
2006). Swollenin, a cellulolytic protein has a N-terminal carbohydrate-
binding (CBD) domain and a C-terminal expansin-like domain and is
involved in cell-wall expansion. Over-expression of a swollenin in T.
asperellum increased cucumber root-colonization ability of this fungus,
while root colonization rates were reduced in transformants silenced in
swollenin gene expression (Brotman et al. 2008). It was suggested that
the CBD domain might act as a MAMP (microbe-associated molecular
pattern) during plant- Trichoderma interactions.