Rhizobacteria-mediated induced systemic resistance (ISR) in Arabidopsis requires sensitivity to jasmonate and ethylene but is not accompanied by an increase in their production

Abstract

Plants develop an enhanced defensive capacity against a broad spectrum of plant pathogens after colonization of the roots by selected strains of nonpathogenic biocontrol bacteria. In Arabidopsis thaliana, this induced systemic resistance (ISR) functions independently of salicylic acid but requires an intact response to the plant hormones jasmonic acid (JA) and ethylene. To further investigate the roles of JA and ethylene in the ISR signalling pathway, the levels of these signalling molecules were determined in A. thaliana upon induction of ISR by Pseudomonas¯uorescens WCS417r and subsequent challenge inoculation with Pseudomonas syringae pv. tomato DC3000. Upon treatment of the roots with ISR-inducing WCS417r bacteria, neither the JA content, nor the level of ethylene evolution was altered in systemically resistant leaves. In®ltration of leaves with WCS417r triggered the JA-and ethylene-dependent ISR pathway, but did not cause local changes in the production of either of these signalling molecules. These results indicate that rhizobacteria-mediated ISR is not based on the induction of changes in the biosynthesis of either JA or ethylene. However, in ISR-expressing plants the capacity to convert 1-aminocyclopropane-1-carboxylate (ACC) to ethylene was signi®cantly enhanced, providing a greater potential to produce ethylene upon pathogen attack.*

Priming in plant–pathogen interactions

“…Analysis of local and systemic levels of jasmonic acid and ethylene revealed that ISR is not associated with changes in the production of these signal molecules [48]. This suggests that the induced resistance is based on an enhanced sensitivity to these plant hormones rather than on an increase in their production.…”

“…Although ethylene plays an important role in the ISR signalling pathway of Arabidopsis, ethylene production is not increased in ISRexpressing tissue [52]. However, after treatment with a saturating 1 mM dose of the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC), ISR-expressing plants emit significantly more ethylene than ACC-treated control plants [48,52]. Evidently, the capacity to convert ACC to ethylene is increased in ISR-expressing plants.…”

“…Because ACC levels increase rapidly in infected tissues as a result of pathogen-induced ACC synthase activity, the enhanced ACC-converting capacity observed in ISR-expressing plants might prime the plant for a faster or greater production of ethylene upon pathogen attack. Interestingly, exogenous application of ACC has been shown to induce resistance against Pst DC3000 in Arabidopsis [10,47,48]. Therefore, a faster or greater production of ethylene in the initial phase of infection might contribute to enhanced resistance against this pathogen.…”

Impact of root exudates and plant defense signaling on bacterial communities in the rhizosphere. A review

“…Whereas SAR is associated with local and systemic accumulation of SA and the expression of pathogenesis-related (PR) genes (Métraux et al 1990;Uknes et al 1992;Sticher et al 1997;Mauch-Mani and Métraux 1998;Durrant and Dong 2004), ISR requires responsiveness of the plant to JA and ET (Pieterse et al 1996(Pieterse et al , 1998. However, ISR is not associated with an increased synthesis of these hormones, nor with an increased expression of known defense-related genes (Van Wees et al 1999;Pieterse et al 2000;Verhagen et al 2004).…”

Colonization of Arabidopsis roots by Pseudomonas fluorescens primes the plant to produce higher levels of ethylene upon pathogen infection

“…This suggests that the resistance attained was not associated with major increases in the levels of either JA or ethylene. Indeed, analysis of JA levels in, and ethylene production by, leaves treated with WCS417r and untreated plant parts expressing ISR, revealed no changes in the production of these signal molecules [20]. Therefore, it is assumed that the JA and ethylene dependency of ISR is based on enhanced sensitivity to these hormones, rather than on an increase in their production.…”