Salicylic Acid Biosynthesis and Role in Modulating Terpenoid and Flavonoid Metabolism in Plant Responses to Abiotic Stress - Salicylic Acid: Plant Growth and Development - page 149

examples of SA and ROS interplay during abiotic stress situations. The rapid and
transient production of ROS during these stress conditions is analogous to the
''oxidative burst'' commonly observed during plant-pathogen interactions but also
to that reported in mammalian macrophage and neutrophils during pathogen inva-
sion (Langebartels et al. 2002 ). During many biotic and abiotic stress situations SA-
induced ROS formation is crucial for the development of an appropriate response.
Arabidopsis eto1 and eto3 mutants, ethylene overproducers, constitutively accu-
mulate high SA levels and exhibit a rapid increase in free SA prior to lesion for-
mation in response to ozone fumigation (Rao et al. 2002 ). In contrast, in rice, which
contains much higher basal levels of free SA (at least 10 times more than infected
tobacco or Arabidopsis plants), SA does not appear to be an effective signal mole-
cule during disease responses (Yang et al. 2004 ). SA-deficient transgenic rice has
higher ROS levels and reduced antioxidant capacity, as well as spontaneous lesion
formation in an age- and light-dependent manner. Symptom development was in fact
suppressed by exogenous application of the SA analog, benzothiadiazol (Yang et al.
2004 ). Although unexpected, these results may be explained by the fact that in plants
SA may directly function as an antioxidant as reported for animals (Castagne et al.
1999 ). SA may scavenge hydroxyl radicals and thus protect plants against catalase
inactivation by H 2 O 2 (Durner and Klessig 1996 ). Hence, in rice for instance, SA may
play a critical role in modulating the cell redox balance, hence protecting the plant
against oxidative damage (Yang et al. 2004 ).
Despite of the evident involvement of SA in plant responses to drought and salt
stress, the drought or salt stress signal transduction mechanisms downstream SA
remain obscure. The extensive search for a SA receptor has yielded several SA-
binding proteins. One of these proteins is CAT, which is specifically inactivated
upon SA binding. Moreover, SA also inhibits cAPX activity directly, thus indi-
cating that SA may inhibit ROS scavenging systems to facilitate an oxidative burst
(reviewed by Vlot et al. 2009 ; Fig. 2 ). In biotic interactions the task of SA sensing
lies largely upon NONEXPRESSION OF PR GENES1 (NPR1) (Pieterse and Van
Loon 2004 ) (Fig. 2 ). Whether NPR is involved in SA-dependent abiotic stress
responses is still unclear, but npr1 mutants show delayed senescence phenotype
(Morris et al. 2000 ). The involvement of several Mitogen-Activated Protein
Kinases (MAPKs or MPKs) such as the tobacco SA-INDUCED PROTEIN
KINASE (SIPK) and WOUND-INDUCED PROTEIN KINASE (WIPK), and their
respective Arabidopsis orthologues AtMPK3 and 6, has been shown in plant
responses to both drought and osmotic stress (a component of salt stress)
(reviewed by Vlot et al. 2009 ) and, in agreement, MAPK cascades operating under
drought and salt stress have been partially described (MEKK1-MKK2-MPK4 and
MKK1-MPK4, respectively) (see Zhang and Klessig 2001 for review). Still, both
the stress sensor upstream the MAPK cascades and the effectors downstream them
remain unknown (Fig. 2 ).