References

8 Nitric Oxide Involvement in Incompatible Plant–Pathogen Interactions 115
through inactivation of NO-generating enzymes (Neill et al. 2003). Various
NO donors have been employed to elevate the level of NO in plant cells.
Although some compounds release NO, others are either iron nitrosyl
substances with a strong NO + character or release equimolar amounts
of NO and superoxide anion (O − 2 )andshouldthereforebeconsideredas
sources of peroxynitrite (ONOO − ).Asaconsequence,dataobtainedusing
these molecules are often difficult to interpret. Recently, it has been
shown that donors releasing NO in different redox forms have different
or even opposite effects on the expression of ferritin (Murgia et al. 2004).
Sodium nitroprusside, a NO donor that releases NO + , induces the accumulation
of ferritin transcripts in A. thaliana cell suspensions. In contrast,
other NO donors such as S-nitroso-N-acetylpenicillamine and S-nitroso-Lglutathione
(GSNO) were not able to induce such an accumulation (Murgia
et al. 2004). The NO scavengers 2-phenyl-4,4,5,5-tetramethylimidazoline-
1-oxyl 3-oxide (PTIO) and carboxyPTIO are commonly used to reduce the
levels of NO (Neill et al. 2003). Mammalian NOS inhibitors are another
effective approach widely used in modulating NO levels in plants (Neill
et al. 2003). Recently, additional genetic-based approaches to manipulate
NO levels in plants in order to clarify the NO involvement in incompatible
plant–pathogen interaction have been proposed (Zeier et al. 2004).
A. thaliana AtNOS1 mutant plants were found to be more susceptible
to virulent Pseudomonas syringae: these plants displayed a much severer
development of disease symptoms and enhanced bacterial growth compared
with wild-type (Zeidler et al. 2004). Furthermore, the expression in
A. thaliana plants as well as in avirulent P. syringae of genes encoding bacterial
flavohemoglobins, which possess a strong NO denitrosylase activity,
revealed that the removal of NO from either the inside or the outside causes
a reduction in hypersensitive cell death (Zeier et al. 2004). Similarly, tobacco
plants that overproduce a nonsymbiontic hemoglobin from alfalfa, which
can act as a NO scavenger, exhibited reduced cell death after inoculation
with avirulent pathogens (Seregelyes et al. 2003) although this protection
was not observed in A. thaliana that express a nonsymbiotic hemoglobin
challenged with an avirulent strain of P. syringae (Perazzolli et al. 2004).
8.5
NO and Cell Death
In animal cells, NO has been shown to cooperate with ROS to induce DNA
fragmentation and cell lysis in murine lymphoma cells, hepatoma cells and
endothelial cells. In this case, cell death is characterized by chromatin condensation,
vacuolization of the cytoplasm and loss of the mitochondrial
membrane electrical potential. Accumulating evidence also indicates a role