Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 209

The occurrence of simultaneous biotic and abiotic stressors presents an added
degree of complexity, as the responses to these are largely controlled by different
hormone signalling pathways that may interact with or inhibit one another. The
exposure of plants to pathogens often increases the effects of abiotic stress, while
long-term abiotic stress may weaken plant defences and cause enhanced pathogen
susceptibility (Atkinson and Urwin 2012 ). On the other hand, one stressor may
cause an acclimation response, possibly be resulting in cross-tolerance to a second
stressor. Studies on the joint effect of an abiotic stress and pathogens have revealed
both positive and negative interactions depending on the timing, nature and
severity of each stress.
It was demonstrated that pre-exposure to a mild dose of Cd increased plant
resistance to viral or fungus infection (Ghoshroy et al. 1998 ; Mittra et al. 2004 ),
suggesting that heavy metals may induce defence pathways and increase resistance
to biotic stress. The cross-talk between pathogens and heavy metal stress is also
supported by the finding that various heavy metal ions induce ethylene and JA
accumulation, and that JA and heavy metals have a similar effect on gene
expression (Maksymiec 2007 ). Externally applied methyl JA had a protective
effect against excess Cd and Cu in Arabidopsis (Maksymiec and Krupa 2002 ).
Plants exposed to ozone exhibited enhanced resistance to virulent Pseudomonas
syringae strains. The results indicate that there is overlapping between the
development of ozone- and pathogen-induced resistance and that both are SA-
dependent (Sharma et al. 1996 ). Like ozone, UV radiation was shown to induce the
accumulation of SA, while also stimulating PR-protein synthesis and inducing
virus resistance in tobacco plants. The results suggest that UV light, ozone
fumigation and tobacco mosaic virus activate a common signal transduction
pathway that leads to SA and PR-protein accumulation and increased disease
resistance (Yalpani et al. 1994 ). In barley, increasing levels of salt-induced
osmotic stress were directly correlated with resistance to powdery mildew (Wiese
et al. 2004 ), while drought stress can enhance resistance to the fungus Botrytis
cinerea in tomato (Achuo et al. 2006 ). Infection with viruses can actually provide
protection against drought stress (Xu et al. 2008 ). Futhermore, many types of
bacteria and arbuscular mycorrhizal fungi are known to enhance stress tolerance in
a range of crop species by producing antioxidants, suppressing ethylene produc-
tion, stabilizing the soil structure, increasing osmolyte production, and improving
ABA regulation (Atkinson and Urwin 2012 ).
Drought stress resulted in a two-fold increase in endogenous ABA as well as a
50 % reduction in B. cinerea infection and a significant suppression of Oidium
neolycopersici on tomato cv. Money maker. Although salt stress did not affect B.
cinerea infection, it significantly reduced infection by O. neolycopersici, but with
no obvious increase in endogenous ABA. Compared with the wild type, the ABA-
deficient sitiens mutant was more resistant to O. neolycopersici and B. cinerea.
Exogenous ABA resulted in increased susceptibility of the sitiens mutant to both
pathogens, but did not increase the basal susceptibility of wild-type tomato plants
(Achuo et al. 2006 ). It can be concluded that drought and salt stress stimulate