Biology Reference
In-Depth Information
growth, respiration, stomatal movements and adaptation to biotic and abi-
otic stresses. Critically, plant NO has been implicated in playing a role in
mediating defences against bacterial pathogens: NO collaborates with reac-
tive oxygen species (ROS) to play crucial roles in the execution of the plant
hypersensitive response (HR), a localized programmed of cell death
(
Delledonne, Zeier, Marocco, & Lamb, 2001; Mur, Carver, & Prats,
2006
). NO accumulation in host tissue when challenged by a pathogen is
well documented using cytochemical methods (
Mur, Mandon, Cristescu,
Harren, & Prats, 2011; Prats, Carver, & Mur, 2008
).
Although HR-induced cell death is critical in establishing disease resis-
tance, the effect of the plant defence response in necrotrophic interactions is
not clearly understood. Necrotrophic fungi are not deterred by the hyper-
sensitive reaction of the host (
Govrin & Levine, 2000
). In fact, the HR has
been reported to stimulate and facilitate the colonization of necrotrophs
(
Malolepsza & Rozalska, 2005
). Thus, certain plant pathogens are able to
evade the radical burst from the host and/or exploit the activated host
defence for pathogenic development and colonization. A study of the bac-
terium
Erwinia chrysanthemi
(
Boccara et al., 2005
) has shed some light on the
possibility of the participation of plant pathogens in regulating NO levels
during infection. Mutation of the FHb gene,
hmpX
, resulted in a strong burst
of NO that accumulated in the infected leaf, which led to a HR-like
response in a compatible plant-pathogen interaction. Introduction of
hmpX
into an incompatible strain,
Pseudomonas syringae
, suppressed the HR elicited
in
Arabidopsis
. Thus the regulation of NO levels is a prerequisite for a suc-
cessful infection and the mode of regulation was demonstrated to be via the
NO-detoxifying protein FHb. This suggests that the roles of
NO-detoxifying globins in plant pathogens are likely to be more complex
and subtle than simply eliminating all NO from their environments.
Generalized conclusions from
Tables 9.4
and Supplementary Table S2 at
discern because the fungi represented have such diverse habitats and hosts.
However, it appears to be the case that the true biotrophs lack genes for FHbs
(common in other fungal groups) or indeed any globin. Examples in
Supplementary Table S2 at
http://www.elsevierdirect.com/companions/
The former is a biotrophic fungus causing barley powdery mildew. Infection
responds to internal and external stimuli. Spore germination follows separa-
tion from the spore chain in the mother colony, but recognition of host
features is necessary for elongation and differentiation of the appressorial
