Topic 23.15

ABA May Play a Role in Plant Pathogen Responses

ABA has long been recognized as a mediator of response to abiotic stress, but it is now receiving more attention for its role in pathogen response (Ton et al., 2009). Several lines of evidence support this connection. Physiological studies have shown that increased ABA levels, due to stress or external application, are correlated with increased susceptibility to infection by some pathogens. However, the role of ABA is complex in that it also increases resistance to other pathogens , and mutations affecting ABA response may have opposite effects on susceptibility to different pathogens . It has been suggested that these apparent contradictions reflect differences in the mode of entry for different pathogens, which affects the timing of ABA participation in the defense response.

Attempts to define the molecular basis of ABA effects on pathogen response have used global comparisons of gene expression (transcriptomes); these studies show that nearly half the genes regulated by ABA are also regulated by certain bacterial virulence factors (Type III). This may partly reflect the importance of common secondary messengers acting in both abiotic and biotic stress. For example, Ca²⁺ and ROS regulate stomatal aperture, whether in the context of gas exchange or as a potential entry point for fungal or bacterial pathogens.

In general, ABA promotes early defense responses such as stomatal closure, may either increase or decrease callose deposition in phloem sieve elements (see textbook Chapter 10) depending on whether the pathogen is fungal or bacterial, and is more likely to inhibit late defense responses by suppressing salicylic acid-dependent responses and modulating jasmonic acid- and ethylene-dependent defenses. Further details about plant defense responses can be found in textbook Chapter 13.