Inductie of systemic resistence in plants by Pseudomonas aeruginosa: role of phenazines and salicyclic acid.

Since many years the research group at the University of Gent (Monica Hofte) and the VUB (Pierre Cornelis and Nico Koedam) are studying antagonistic fluorescent pseudomonads. Especially antagonistic Pseudomonas aeruginosa strains are investigated such as the Belgian isolate 7NSK2, the Indian isolate PNA1, the isolate from Sri Lanka SA44 and the clinical isolate PA01 Root colorazation with Pseudomonas aeruginosa results in a direct antagonism against soil-borne plant pathogens such as Pythium sp., but can also protect plants against leaf pathogens by inducing systemic resistance. Induced systemic resistance (ISR) refers to an increased broad-spectrum and plant-mediated resistance that is induced throughout the whole plant after a stimulus applied to only a part of the plant. A pathogen infection is the most documented stimulus of ISR and the events between the pathogen and the enhanced resistance state are characterized down to the molecular level. The enhanced resistant state is induced via a salicylic add dependent signal transduction pathway and is assodated with an accumulation of distinct pathogenesis-related (PR) proteins. This whole phenomenon is designated systemic acquired resistance (SAR) (Ryals et al., 1996). An alternative stimulus that can lead to ISR is root colonization with selected strains of plant growth-promoting rhizobacteria. Although this stimulus results in a systemic resistance that is similar to SAR, both phenomena are not necessarily mediated by the same mechanisms. Some rootkolonizing bacteria induce resistance via the salicylic add dependent SAR pathway, while other bacteria require jasmonate and ethylene perception by the plant for the development of ISR (van Loon et al., 1998). Under iron-limiting conditions Pseudomonas aeruginosa produces three iron-chelating compounds or siderophores: pyoverdine, a yellow-green fluorescent compound; pyocheline, a thiazoline derivative; and salicylic add. In addition, salicylic add is a precursor in the biosynthesis of pyocheline. Buysens et al. (1996) have shown that both pyoverdine and pyocheline play a role in the direct antagonism of P. aeruginosa 7NSK2 against Pythium splendens. Via its siderophore salicylic add, P. aeruginosa 7NSK2 can induce resistance against Botrytis dnerea on bean (De Meyer & Hofte, 1997) and tomato (Audenaert et al., unpublished) and against tobacco mosaic virus (TMV) in tobacco. It was also shown that P. aeruginosa 7NSK2 is no longer able to induce resistance against TMV intransgenic NahG tobacco plants in which salicylic add is converted to catechol (De Meyer & Hofte, 1998). These results show that ISR by P. aeruginosa 7NSK2, just like SAR by TMV, depends on in planta accumulation of salicylic add. In contrast to SAR by TMV, however, ISR by 7NSK2 is not associated with the expression of PRla, a gene that codes for a PR protein assodated with SAR. Besides siderophores, Pseudomonas aeruginosa strains also produce phenazines. Phenazines are heterocyclic compounds, produced as secondary metabolites (Turner & Messenger, 1986). One single strain can produce different types of phenazine, as was shown e.g. in P. aeruginosa strain PNA1, a strain isolated in previous research (VUB). PNA1 produces both phenazine-1-carboxylate (PCA) and oxychlororaphin (OCP). The function of phenazines for the bacteria has as yet not been established, but these molecules have a number of interesting properties. PCA and OCP both have antibacterial and antifungal activity (Thomashow &Weller, 1988; Anjaiah.