Abscisic acid and callose: Team players in defence against pathogens?

Abscisic acid (ABA) plays an important role as a plant hormone and as such is involved in many different steps of plant development. It has also been shown to modulate plant responses to abiotic stress situations and in recent years, it has become evident that it is partaking in processes of plant defence against pathogens. Although ABA's role in influencing the outcome of plant-pathogen interactions is controversial, with most research pointing into the direction of increased susceptibility, recent results have shown that ABA can also be involved in rendering plants more resistant to pathogen attack. In these cases, ABA interacts with callose deposition allowing an early and efficient build up of papillae at the sites of infection. The present review tries to shed some light on a possible interplay between ABA and callose in the protection of plants against invading pathogens.