Biomedical Engineering Reference
In-Depth Information
also had reduced production of the antifungal secondary metabolite
6-pentyl-alpha-pyrone, but enhanced growth inhibition of
R. solani
in
confrontation assay. Contrary to Tga1, the deletion of another G
α
protein
Tga3 resulted in reduced intracellular cAMP level in
T. atroviride
(Zeilinger
et al. 2005). Interestingly, here too, the mutants had reduced growth
rate and continuously produced conidia on solid medium in a light-
independent manner. The mutants had reduced conidial germination
and were avirulent against the host fungi. It is interesting to note that
the effect of deletion of Tga1 or Tga3 is nearly identical although the
former mutation resulted in elevated cAMP levels and the latter one in
reduced intracellular cAMP levels. Recently, Casas-Flores
et al. (2006)
studied the role of a protein kinase. A regulatory subunit in blue-light
induced conidiation of
T. atroviride
. Expression of an antisense copy
of this gene resulted in a non-sporulating phenotype, whereas over-
expression resulted in light-independent conidiation. Contrary to all the
fungi studied, including the closely related
T
.
atroviride
, the deletion of
the three G
α
proteins individually had no effect on growth, sporulation or
spore germination frequency in
T. virens
(Mukherjee et al. 2004, Pardovitz-
Kedmi et al. 2006). Deletion of TgaA or TgaB individually had no effect on
hyphal coiling of
R. solani
, but ΔTgaA mutants were less effective in the
parasitism of sclerotia of
S. rolfsii
. Deletion of the MAPK TmkA in
T. virens
resulted in light-independent conidiation and attenuation of sclerotial
parasitism of
S. rolfsii
and
R. solani
, while the hyphal parasitism was
unaltered (Mukherjee et al. 2003b). The TmkA mutants also had reduced
ability to induce resistance in cucumber seedlings, even though there
was no effect on root colonization (Viterbo et al. 2005). The mutants also
had reduced biocontrol of
S. rolfsii
in greenhouse test. In another strain
of
T.
virens
, deletion of the MAP kinase gene
tvk1
resulted in enhanced
antagonism and biocontrol (Mendoza-Mendoza et al. 2003). Zeilinger
(2004) and Reithner et al (2007) also demonstrated that deletion of the
MAPK Tmk1 results in light-independent conidiation in
T. atroviride
.
Using SSH, a set of genes have been identifi ed to be down-regulated by
MAPK TmkA (Mukherjee et al. 2006b). One of them,
mrsp1
, encodes for a
cysteine-rich secreted protein MRSP1 (MAPK repressed secreted protein 1)
the expression of which is very tightly regulated by TmkA. Recently, using
the RNAi mediated gene silencing, a
T. harzianum
stress-response MAPK
ThHOG1 has been identifi ed to be involved in osmotic and oxidative
stress tolerance (Delgado-Jarana et al. 2006). Deletion of
tac1
adenylate
cyclase gene in
T.
virens
greatly impairs its growth, conidial germination,
mycoparasitism and secondary metabolite production thereby implying
a direct correlation of cAMP signaling in these processes (Mukherjee et
al. 2007). Like the
tmkA
, deletion of another conserved MAP kinase gene
tmkB
resulted in overlapping phenotypes like constitutive conidiation
