Phytopathogens can manipulate plant hormone signaling to access nutrients and counteract defense responses. Pseudomonas syringae produces coronatine, a toxin that mimics the plant hormone jasmonic acid isoleucine and promotes opening of stomata for bacterial entry, bacterial growth in the apoplast, systemic susceptibility and disease symptoms. We examined the mechanisms underlying coronatine-mediated virulence and show that coronatine activates three homologous NAC transcription factor (TF) genes, ANAC019, ANAC055 and ANAC072, through direct activity of the TF, MYC2. Genetic characterization of NAC TF mutants demonstrates that these TFs mediate coronatine-induced stomatal reopening and bacterial propagation in both local and systemic tissues by inhibiting the accumulation of the key plant immune signal salicylic acid (SA). These NAC TFs exert this inhibitory effect by repressing ICS1 and activating BSMT1, genes involved in SA biosynthesis and metabolism, respectively. Thus, a signaling cascade by which coronatine confers its multiple virulence activities has been elucidated.

SummaryCoronatine is an important virulence factor produced by several pathovars of the bacterial pathogen Pseudomonas syringae. The structure of coronatine is similar to that of a class of plant hormones called jasmonates (JAs). An important step in JA signaling is the SCF COI1 E3 ubiquitin ligase-dependent degradation of JAZ repressor proteins. We have recently shown that jasmonoyl isoleucine (JA-Ile) promotes physical interaction between Arabidopsis JAZ1 and COI1 (the F-box component of SCF COI1) proteins, and that the JAIle-dependent COI1-JAZ1 interaction could be reconstituted in yeast cells (i.e. in the absence of other plant proteins). Here we show that coronatine, but not its two biosynthetic precursors, also promotes interaction between Arabidopsis COI1 and multiple JAZ proteins. The C-terminal Jas motif, but not the N-terminal (NT) domain or central ZIM domain of JAZ proteins, is critical for JA-Ile/coronatine-dependent interaction with COI1. Two positively charged amino acid residues in the Jas domain were identified as essential for coronatinedependent COI1-JAZ interactions. Mutations of these two residues did not affect the ability of JAZ1 and JAZ9 to interact with the transcription factor AtMYC2. Importantly, transgenic Arabidopsis plants expressing JAZ1 carrying these two mutations exhibited JA-insensitive phenotypes, including male sterility and enhanced resistance to P. syringae infection. These results not only suggest that coronatine and JA-Ile target the physical interaction between COI1 and the Jas domain of JAZ repressors, but also illustrate the critical role of positively charged amino acids in the Jas domain in mediating the JA-Ile/coronatine-dependent JAZ interaction with COI1.

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.

Cell walls are crucial for development, signal transduction, and disease resistance in plants. Cell walls are made of cellulose, hemicelluloses, and pectins. Xyloglucan (XG), the principal load-bearing hemicellulose of dicotyledonous plants, has a terminal fucosyl residue. A 60-kilodalton fucosyltransferase (FTase) that adds this residue was purified from pea epicotyls. Peptide sequence information from the pea FTase allowed the cloning of a homologous gene, AtFT1, from Arabidopsis. Antibodies raised against recombinant AtFTase immunoprecipitate FTase enzyme activity from solubilized Arabidopsis membrane proteins, and AtFT1 expressed in mammalian COS cells results in the presence of XG FTase activity in these cells.

Stomata are microscopic pores formed by pairs of guard cells in the epidermis of terrestrial plants; they are essential for gas exchange with the environment and controlling water loss. Accordingly, plants regulate stomatal aperture in response to environmental conditions, such as relative humidity, CO2 concentration, and light intensity. Stomatal openings are also a major route of pathogen entry into the plant and plants have evolved mechanisms to regulate stomatal aperture as an immune response against bacterial invasion. In this review, we highlight studies that begin to elucidate signaling events involved in bacterium-triggered stomatal closure and discuss how pathogens may have exploited environmental conditions or, in some cases, have evolved virulence factors to actively counter stomatal closure to facilitate invasion.