Induction of Systemic Resistance toBotrytis cinereain Tomato byPseudomonas aeruginosa7NSK2: Role of Salicylic Acid, Pyochelin, and Pyocyanin

Abstract

The rhizobacterium Pseudomonas aeruginosa 7NSK2 produces secondary metabolites such as pyochelin (Pch), its precursor salicylic acid (SA), and the phenazine compound pyocyanin. Both 7NSK2 and mutant KMPCH (Pch-negative, SA-positive) induced resistance to Botrytis cinerea in wild-type but not in transgenic NahG tomato. SA-negative mutants of both strains lost the capacity to induce resistance. On tomato roots, KMPCH produced SA and induced phenylalanine ammonia lyase activity, while this was not the case for 7NSK2. In 7NSK2, SA is probably very efficiently converted to Pch. However, Pch alone appeared not to be sufficient to induce resistance. In mammalian cells, Fe-Pch and pyocyanin can act synergistically to generate highly reactive hydroxyl radicals that cause cell damage. Reactive oxygen species are known to play an important role in plant defense. To study the role of pyocyanin in induced resistance, a pyocyanin-negative mutant of 7NSK2, PHZ1, was generated. PHZ1 is mutated in the phzM gene encoding an O-methyltransferase. PHZ1 was unable to induce resistance to B. cinerea, whereas complementation for pyocyanin production or co-inoculation with mutant 7NSK2-562 (Pch-negative, SA-negative, pyocyaninpositive) restored induced resistance. These results suggest that pyocyanin and Pch, rather than SA, are the determinants for induced resistance in wild-type P. aeruginosa 7NSK2.Additional keywords: phenazine-1-carboxylate, siderophores.Induced resistance is a state of enhanced defensive capacity developed by a plant when appropriately stimulated (van Loon et al. 1998). Induced resistance is generally systemic and can be triggered by pathogens, certain chemicals, and nonpathogenic rhizosphere bacteria. The mechanisms involved in rhizobacteria-mediated induced systemic resistance (ISR) appear to vary among bacterial strains and pathosystems. Bacterial determinants of ISR which have been identified are lipopolysaccharides and siderophores (van Loon et al. 1998). Siderophores are high-affinity iron(III)-chelating compounds that are produced by most microorganisms under iron-limiting conditions. The catechol siderophore biosynthesis genes of Serratia marcescens are involved in ISR to Colletotrichum orbiculare on cucumber (Press et al. 2001). The purified pyoverdine-type siderophore of Pseudomonas putida WCS374 induced resistance to Fusarium wilt in radish (Leeman et al. 1996), while a pyoverdine-negative mutant of P. fluorescens CHA0 was less effective in inducing resistance to Tobacco necrosis virus on tobacco than the wild-type strain (Maurhofer et al. 1994). Another iron-chelating molecule that is well-studied with respect to induced plant defense is salicylic acid (SA). Although the siderophore capacity of SA is rather poor (Chipperfield and Ratledge 2000), it appears to be an important molecule in induced resistance by the rhizobacterium P. aeruginosa 7NSK2. This bacterium produces three siderophores under iron-limiting conditions (pyoverdine, pyochelin (Pch), and SA) and can induce resistance to plant diseases ca...

Isochorismate Synthase (PchA), the First and Rate-limiting Enzyme in Salicylate Biosynthesis of Pseudomonas aeruginosa

“…Similar observations have recently been reported for another strain of P. aeruginosa (37). It is not known, however, whether there are regulatory links between pyochelin and cysteine synthesis in P. aeruginosa.…”

Plant responses to plant growth-promoting rhizobacteria

“…For tomato it was established that it is not the SA which is produced by this strain that triggers ISR, but synthesis of the SA-containing siderophore pyochelin and the antibiotic pyocyanin. In combination, pyochelin and pyocyanin induce the formation of oxygen free radicals in the roots, which triggers SA production in the plant and subsequent activation of an SA-dependent enhanced resistance (Audenaert et al 2002). Bacillus spp.…”

“…Siderophores and antibiotics are produced by the bacteria to compete for iron and to inhibit other strains in the rhizosphere, respectively, and thus have dual functions in microbial antagonism on the one hand, and elicitation of ISR on the other. The siderophore pyochelin and the antibiotic pyocyanin are both required for the induction of systemic resistance in tomato by P. aeruginosa 7NSK2 (Audenaert et al 2002). QS N-acylhomoserine lactones were recently shown to act as inducers of systemic resistance in tomato against Alternaria alternata (Shuhegger et al 2006).…”

Microbial Siderophores Exert a Subtle Role in Arabidopsis during Infection by Manipulating the Immune Response and the Iron Status

“…Some siderophores secreted by soil-borne Pseudomonas species (pyoverdin and pyocyanin) can promote systemic plant protection against soil and foliar pathogens, a phenomenon known as induced systemic resistance (Audenaert et al, 2002;Haas and Défago, 2005). Induced systemic resistance is known to be dependent on the ET and JA pathways and independent of the SA pathway (Pieterse et al, 1998).…”