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by redox enzymes. In this system, manganese oxalate works as a diffusible redox
shuttle, fi rst being oxidized from Mn(II) to Mn(III) by a peroxidase and then being
reduced to Mn(II) by a simultaneous oxidation of the lignin monomers to radicals
that form covalent linkages of the lignin. It suggests involvement of a redox shuttle/
peroxidase system in lignin biosynthesis through activation of polymerization of
monolignols (Önnerud et al.
2002
).
5.18
ROS Signaling System May Activate Transcription
of Defense Genes
ROS signaling system and cognate redox signaling have been shown to be involved
in activation of several defense genes. Redox control of activation of PR genes has
been reported in plants (Fobert and Després
2005
). ROS stress induced the PR pro-
tein PRB1-b, and the antimicrobial protein chitinase class 4 in tobacco (Vranová
et al.
2002
). Thaumatin-like protein (PR-5 protein) is induced by H
2
O
2
(Desikan
et al.
2001
). H
2
O
2
induces PR-1 protein accumulation in tobacco (Neill et al.
2002
).
The ROS stress induced various enzymes involved in phytoalexins synthesis,
including 5-epi-aristoiochene synthase and vestispiradiene synthase in tobacco
(Vranová et al.
2002
). H
2
O
2
are capable of inducing expression of phenylalanine
ammonia-lyase (Neill et al.
2002
). Lignin biosynthesis is also activated by ROS
(Vranová et al.
2002
) and redox signaling system (Önnerud et al.
2002
).
5.19
Pathogens May Cause Disease by Interfering
with ROS Signaling System in Host Plants
ROS signaling system is involved in host plant defense. In the genome of the maize
smut pathogen
Ustilago maydis
, an ortholog of
YAP1
(for
Yeast AP-1-like
) from
Saccharomyces cerevisae
has been identifi ed (Molina and Kahmann
2007
). The
gene is involved in degradation of H
2
O
2
. The Yap1-regulated genes include peroxi-
dase genes and peroxidases are known to degrade H
2
O
2
.
Yap1
gene was found to be
essential for virulence of
U. maydis
and deletion mutants of this gene were attenu-
ated in virulence. These results suggest that the biotrophic pathogen
U. maydis
causes the disease by interfering with the H
2
O
2
signaling pathway (Molina and
Kahmann
2007
).
Another biotrophic fungal pathogen,
Blumeria graminis
f. sp.
hordei
, elicits a
burst of H
2
O
2
in its host barley at sites of germ tube invasion. The fungus produces
catalase during the infection process. The fungal
catB
gene encoding catalase has
been characterized. Enhanced numbers of
catB
transcripts were detected at mature
primary germ tube and appressorium germ tube stages in a susceptible host. Areas
of H
2
O
2
clearing were observed at sites of fungal invasion (Zhang et al.
2004
).
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