References

114 M.D. Stefano, A. Ferrarini, M. Delledonne
ascorbate and phenolic compounds (Romero-Puertas et al. 2004). Recently,
the synthesis of NO via nonenzymatic reduction of apoplastic nitrite in
seeds has been demonstrated (Bethke et al. 2004), although it is unlikely
that this route of production of NO is significant in response to pathogen
attack as the apoplastic pH is likely to be too high (Bolwell et al. 2002). The
enzymatic routes of NO synthesis include reduction of nitrite by nitrate
reductase (NR), conversion of nitrite by xanthine oxidoreductase (Rockel
et al. 2002) and conversion of arginine to citrulline by NOS (Guo et al.
2003). As NR is involved in many physiological responses (Neill et al.
2003), no significant differences in NO accumulation have been observed
in response to infection with avirulent pathogens between wild-type Arabidopsis
plants and a mutant impaired in NR activity (Zhang et al. 2003).
Recently, AtNOS1 from Arabidopsis thaliana was identified on the basis
of its sequence similarity to a protein implicated in NO synthesis in the
snail Helix pomatia (Guo et al. 2003). AtNOS1 displays flavin-, heme- and
tetrahydrobiopterin-independent NOS activity and appears to be constitutively
expressed (Zeidler et al. 2004).
In plants, NO production in response to infection has been shown under
conditions in which the generation of ROS is also activated (Delledonne et
al. 1998; Clarke et al. 2000; Foissner et al. 2000; Allan and Fluhr 1997). This
event is concomitant with the avirulent gene-dependent oxidative burst that
occurs immediately prior to the onset of hypersensitive cell death (Romero-
Puertas et al. 2004). In A. thaliana and soybean, NO production rapidly
increases following challenge with avirulent bacteria and is maintained over
a 6-h period (Clarke et al. 2000; Delledonne et al. 1998; Zhang et al. 2003).
Moreover,directcontactofavirulentcrownrustfunguswithoatplants
induces the production of NO in an early stage in the defense response (Tada
et al. 2004). In addition, epidermal tobacco cells treated with the fungal
elicitor cryptogein (Foissner et al. 2000; Lamotte et al. 2004) and potato
tubers treated with an elicitor from Phytophthora infestans (Yamamoto et
al. 2004) accumulate NO within minutes. Lipopolysaccharides extracted
from Gram-negative plant and animal pathogens have also been found to
elicit a strong and rapid burst of NO in A. thaliana plants (Zeidler et al.
2004).
8.4
Experimental Approaches for Manipulation
of Endogenous NO Levels
Most of the current information about the function of NO in plants relies
on data obtained by pharmacological studies. These approaches include
application of NO donors or NO scavengers, and loss of NO synthesis