Abstract
The FLAGELLIN-SENSING2 (FLS2) receptor kinase recognizes bacterial flagellin and initiates a battery of downstream defense responses to reduce bacterial invasion through stomata in the epidermis and bacterial multiplication in the apoplast of infected plants. Recent studies have shown that during Pseudomonas syringae pv tomato (Pst) DC3000 infection of Arabidopsis (Arabidopsis thaliana), FLS2-mediated immunity is actively suppressed by effector proteins (such as AvrPto and AvrPtoB) secreted through the bacterial type III secretion system (T3SS). We provide evidence here that T3SS effector-based suppression does not appear to be sufficient to overcome FLS2-based immunity during Pst DC3000 infection, but that the phytotoxin coronatine (COR) produced by Pst DC3000 also plays a critical role. COR-deficient mutants of Pst DC3000 are severely reduced in virulence when inoculated onto the leaf surface of wild-type Columbia-0 plants, but this defect was rescued almost fully in fls2 mutant plants. Although bacteria are thought to carry multiple microbe-associated molecular patterns, stomata of fls2 plants are completely unresponsive to COR-deficient mutant Pst DC3000 bacteria. The responses of fls2 plants were similar to those of the Arabidopsis G-protein alpha subunit1-3 mutant, which is defective in abscisic acid-regulated stomatal closure, but were distinct from those of the Arabidopsis non-expressor of PR genes1 mutant, which is defective in salicylic acid-dependent stomatal closure and apoplast defense. Epistasis analyses show that salicylic acid signaling acts upstream of abscisic acid signaling in bacterium-triggered stomatal closure. Taken together, these results suggest a particularly important role of FLS2-mediated resistance to COR-deficient mutant Pst DC3000 bacteria, and nonredundant roles of COR and T3SS effector proteins in the suppression of FLS2-mediated resistance in the Arabidopsis-Pst DC3000 interaction.
Highlights
The FLAGELLIN-SENSING2 (FLS2) receptor kinase recognizes bacterial flagellin and initiates a battery of downstream defense responses to reduce bacterial invasion through stomata in the epidermis and bacterial multiplication in the apoplast of infected plants
Stomatal guard cells respond to purified microbe-associated molecular patterns (MAMPs), such as chitosan, a polymer of b-1,4-glucosamine residues derived from fungal chitin (Lee et al, 1999; Amborabe et al, 2008), flg22, a 22-amino acid peptide derived from bacterial flagellin (Melotto et al, 2006; Cho et al, 2008; Desikan et al, 2008; Zhang et al, 2008), and bacterial lipopolysaccharides (LPSs; Melotto et al, 2006; Cho et al, 2008)
MAMPs are recognized by plant pattern-recognition receptors, such as Arabidopsis proteins FLAGELLIN-SENSING2 (FLS2) that recognizes bacterial flagellin (GomezGomez and Boller, 2000), EF-TU RECEPTOR (EFR) that recognizes bacterial elongation factor TU (Zipfel et al, 2006), and CHITIN ELICITOR RECEPTOR KINASE1 (CERK1) that perceives an unknown MAMP from Pst DC3000 (Gimenez-Ibanez et al, 2009a, 2009b)
Summary
The FLAGELLIN-SENSING2 (FLS2) receptor kinase recognizes bacterial flagellin and initiates a battery of downstream defense responses to reduce bacterial invasion through stomata in the epidermis and bacterial multiplication in the apoplast of infected plants. Fls2 Plants Are Defective in Stomatal Closure in Response to Live COR-Deficient Mutant Bacteria
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