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. Introduction When plants are attacked by pathogens they activate a battery of reactions including both chemical and physical defences (Agrios, 1997). Among the chemical defences the synthesis of antimicrobial compounds such as phytoalexins and defensins have been well documented to play a role in the outcome of an interaction between a host plant and a pathogen (Kuc, 1995; Penninckx et al., 1998). The pathogenesis-related (PR) proteins, comprising enzymes capable of degrading pathogen cell walls, are another example of a successful plant defence mechanism (Van Loon and Van Strien, 1999). They play an important role in basal resistance as well as in systemic acquired resistance (SAR) where they have been shown to be induced not only locally in the attacked plant parts but also systemically in distant areas of the plant (Sticher et al., 1997). This systemic induction is thought to contribute to the heightened defensive capacity of the systemic tissues in SAR. At the cellular level these defence reactions are preceded by numerous changes including the synthesis of salicylic acid, reactive oxygen species (ROS), nitric oxide (NO) and the hypersensitive reaction to name a few (Veronese et al., 2003). All these changes occur in a well-defined sequential order and are mediated by distinct signal transduction pathways picking up the signal upon recognition of the pathogen by the host and leading to the final induction of defensive measures. Classically, two different signalling pathways have been distinguished (Kunkel and Brooks, 2002). One of the pathways is based on SAdependent signalling (Ryals et al., 1996), the other is dependent on a functional jasmonate/ethylene signalling (Thomma et al., 2001). While the former has been shown to be implicated in the defence against biotrophs such as Hyaloperonospora parasitica and Erysiphe orontii or the hemibiotroph Pseudomonas syringae in Arabidopsis, the latter one is implicated when plants are challenged by necrotrophic organisms such as Botrytis cinerea, Alternaria brassicicola or Erwinia carotovora (Rojo et al., 1999; Thomma et al., 2001). This clear distinction observed in Arabidopsis, however, might not be valid for every plant species. Achuo et al. (2004) showed that the SA pathway was effective against Botrytis in tomato, but not in tobacco, while the SA pathway was effective against Oidium in tobacco but not in tomato. Recently, increasing evidence has also been pointing to the involvement of yet another plant hormone, abscisic acid (ABA), in plant–pathogen interactions (Audenaert et al., 2002; Anderson et al., 2004; Thaler and Bostock, 2004; Thaler et al., 2004; Ton and Mauch-Mani, 2004; Ton et al., 2005). When a pathogen tries to invade plant tissues, the first barrier it encounters is the plant cell wall. If ingress can be stopped at this stage, these results in a www.blackwell-synergy.com J. Phytopathology 153, 377–383 (2005) 2005 Blackwell Verlag, Berlin