We have found that a major target for effectors secreted by Pseudomonas syringae is the abscisic acid (ABA) signalling pathway. Microarray data identified a prominent group of effector-induced genes that were associated with ABA biosynthesis and also responses to this plant hormone. Genes upregulated by effector delivery share a 42% overlap with ABA-responsive genes and are also components of networks induced by osmotic stress and drought. Strongly induced were NCED3, encoding a key enzyme of ABA biosynthesis, and the abscisic acid insensitive 1 (ABI1) clade of genes encoding protein phosphatases type 2C (PP2Cs) involved in the regulation of ABA signalling. Modification of PP2C expression resulting in ABA insensitivity or hypersensitivity led to restriction or enhanced multiplication of bacteria, respectively. Levels of ABA increased rapidly during bacterial colonisation. Exogenous ABA application enhanced susceptibility, whereas colonisation was reduced in an ABA biosynthetic mutant. Expression of the bacterial effector AvrPtoB in planta modified host ABA signalling. Our data suggest that a major virulence strategy is effector-mediated manipulation of plant hormone homeostasis, which leads to the suppression of defence responses.

The active oxygen species hydrogen peroxide (H202) was detected cytochemically by its reaction with cerium chloride to produce electron-dense deposits of cerium perhydroxides. In uninoculated lettuce leaves, H202 was typically present within the secondary thickened walls of xylem vessels. Inoculation with wild-type cells of Pseudomonas syringae pv phaseolicola caused a rapid hypersensitive reaction (HR) during which highly localized accumulation of H202 was found in plant cell walls adjacent to attached bacteria. Quantitative analysis indicated a prolonged burst of H202 occurring between 5 to 8 hr after inoculation in cells undergoing the HR during this example of non-host resistance. Cell wall alterations and papilla deposition, which occurred in response to both the wild-type strain and a nonpathogenic hrpD mutant, were not associated with intense staining for H202, unless the responding cell was undergoing the HR. Catalase treatment to decompose H, O, almost entirely eliminated staining, but 3-amino-l,2,4-triazole (catalase inhibitor) did not affect the pattern of distribution of H202 detected. H202 production was reduced more by the inhibition of plant peroxidases (with potassium cyanide and sodium azide) than by inhibition of neutrophil-like NADPH oxidase (with diphenylene iodonium chloride). Results suggest that CeCI, reacts with excess H202 that is not rapidly metabolized during cross-linking reactions occurring in cell walls; such an excess of H202 in the early stages of the plant-bacterium interaction was only produced during the HR. The highly localized accumulation of H,Oz is consistent with its direct role as an antimicrobial agent and as the cause of localized membrane damage at sites of bacterial attachment. INTRODUCTIONThe active resistance of plants to colonization by bacteria and fungi is often expressed by the hypersensitive reaction (HR) of challenged plant cells (Ingram, 1978; Klement, 1982; Mansfield, 1990; Tenhaken et al., 1995). The HR can be recognized as the rapid and localized death of cells in response to an avirulent pathogen; it has been observed during most interactions involving race-specific resistance and also in many examples of non-host resistance (Heath, 1989; Mansfield, 1990; Mansfield et al., 1997). A second form of resistance, more commonly found in non-host reactions, is the highly localized alteration of the cell wall at sites attacked by fungi or bacteria. Modification of the cell wall per se is often associated with the formation of a papilla or apposition at reaction sites (Ride, 1986; Nicholson and Hammerschmidt, 1992; Bestwick et al., 1995). In phytopathogenic bacteria, hypersensitive response and pathogenicity (hrp) genes determine the ability to multiply within susceptible plants and to cause Current address: Division of Biochemical Sciences, Rowett Research Institute, Greenburn Road, Bucksburn, Aberdeen AB21 9SB, UK. To whom correspondence should be addressed. a macroscopic HR in either resistant varieties of their host or in non-host plants (Bonas, 1994). Hrp ...

The plant cell wall is a dynamic and complex structure whose functional integrity is constantly being monitored and maintained during development and interactions with the environment. In response to cell wall damage (CWD), putatively compensatory responses, such as lignin production, are initiated. In this context, lignin deposition could reinforce the cell wall to maintain functional integrity. Lignin is important for the plant's response to environmental stress, for reinforcement during secondary cell wall formation, and for long-distance water transport. Here, we identify two stages and several components of a genetic network that regulate CWD-induced lignin production in Arabidopsis (Arabidopsis thaliana). During the early stage, calcium and diphenyleneiodonium-sensitive reactive oxygen species (ROS) production are required to induce a secondary ROS burst and jasmonic acid (JA) accumulation. During the second stage, ROS derived from the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D and JA-isoleucine generated by JASMONIC ACID RESISTANT1, form a negative feedback loop that can repress each other's production. This feedback loop in turn seems to influence lignin accumulation. Our results characterize a genetic network enabling plants to regulate lignin biosynthesis in response to CWD through dynamic interactions between JA and ROS.