The multifaceted role of ABA in disease resistance.

Long known only for its role in abiotic stress tolerance, recent evidence shows that abscisic acid (ABA) also has a prominent role in biotic stress. Although it acts as a negative regulator of disease resistance, ABA can also promote plant defense and is involved in a complicated network of synergistic and antagonistic interactions. Its role in disease resistance depends on the type of pathogen, its specific way of entering the host and, hence, the timing of the defense response and the type of affected plant tissue. Here, we discuss the controversial evidence pointing to either a repression or a promotion of resistance by ABA. Furthermore, we propose a model in which both possibilities are integrated.