Localized attack by a necrotizing pathogen induces systemic acquired resistance (SAR) to subsequent attack by a broad range of normally virulent pathogens. Salicylic acid accumulation is required for activation of local defenses, such as pathogenesis-related protein accumulation, at the initial site of attack, and for subsequent expression of SAR upon secondary, distant challenge. Although salicylic acid moves through the plant, it is apparently not an essential mobile signal. We screened Agrobacterium tumefaciens transfer DNA (tDNA) tagged lines of Arabidopsis thaliana for mutants specifically compromized in SAR. Here we show that Defective in induced resistance 1-1 (dir1-1) exhibits wild-type local resistance to avirulent and virulent Pseudomonas syringae, but that pathogenesis-related gene expression is abolished in uninoculated distant leaves and dir1-1 fails to develop SAR to virulent Pseudomonas or Peronospora parasitica. Petiole exudate experiments indicate that dir1-1 is defective in the production or transmission from the inoculated leaf of an essential mobile signal. DIR1 encodes a putative apoplastic lipid transfer protein and we propose that DIR1 interacts with a lipid-derived molecule to promote long distance signalling.

Age-related resistance (ARR) has been observed in a number of plant species; however, little is known about the biochemical or molecular mechanisms involved in this response. Arabidopsis becomes more resistant, or less susceptible, to virulent Pseudomonas syringae (pv tomato or maculicola ) as plants mature (in planta bacterial growth reduction of 10-to 100-fold). An ARR-like response also was observed in response to certain environmental conditions that accelerate Arabidopsis development. ARR occurs in the Arabidopsis mutants pad3-1 , eds7-1 , npr1-1 , and etr1-4 , suggesting that ARR is a distinct defense response, unlike the induced systemic resistance or systemic acquired resistance responses. However, three salicylic acid (SA) accumulation-deficient plant lines, NahG , sid1 , and sid2 , did not exhibit ARR. A heat-stable antibacterial activity was detected in intercellular washing fluids in response to Pst inoculation in wild-type ARR-competent plants but not in NahG . These data suggest that the ability to accumulate SA is necessary for the ARR response and that SA may act as a signal for the production of the ARR-associated antimicrobial compound(s) and/or it may possess direct antibacterial activity against P. syringae . INTRODUCTIONThe relationship between plant age and disease resistance has been investigated in many plant-pathogen systems (Bateman and Lumsden, 1965; Griffey and Leach, 1965;Hunter et al., 1977;Lazarovits et al., 1981;Ward et al., 1981;Miller, 1983; Chase and Jones, 1986;Reuveni et al., 1986;Pretorius et al., 1988;Koch and Mew, 1991; Chang et al., 1992;Heath, 1993;Rupe and Gbur, 1995). Some plants become more susceptible to certain pathogens as they develop (Miller, 1983); however, susceptibility decreases with increasing leaf age in the rice/ Xanthomonas campestris pv oryzae (Koch and Mew, 1991) and rice/ Pyricularia oryzae (Roumen et al., 1992) interactions. In contrast, older plants (both young and mature leaves) display increased resistance in the wheat/ Puccinia recondita f.sp. tritici (Pretorius et al., 1988) and tobacco/ Peronospora tabacina (Reuveni et al., 1986) interactions. When older leaves/plants display increased resistance or reduced susceptibility to pathogens, this form of resistance often is referred to as age-related resistance (ARR).The actual mechanisms responsible for the different forms of ARR have been studied in a preliminary manner in only a few cases. In cowpea/rust and cereal/rust interactions, an ARR response is thought to be controlled by single resistance genes expressed in adult plants (Roelfs, 1984;Heath, 1993). Ward et al. (1981) and Lazarovits et al. (1981) observed a positive correlation between increasing plant age, glyceollin production, and resistance to Phytophthora megasperma var sojae in soybean. A similar correlation was observed for the accumulation of a cotton phytoalexin in response to Verticillium albo-atrum infection (Bell, 1969), constitutive accumulation of terpenoids in older cotton plants (Hunter et al., 1977), or capsidiol accumulati...

Salicylic acid (SA)-mediated innate immune responses are activated in plants perceiving volatile monoterpenes. Here, we show that monoterpene-associated responses are propagated in feed-forward loops involving the systemic acquired resistance (SAR) signaling components pipecolic acid, glycerol-3-phosphate, and LEGUME LECTIN-LIKE PROTEIN1 (LLP1). In this cascade, LLP1 forms a key regulatory unit in both within-plant and between-plant propagation of immunity. The data integrate molecular components of SAR into systemic signaling networks that are separate from conventional, SA-associated innate immune mechanisms. These networks are central to plant-to-plant propagation of immunity, potentially raising SAR to the population level. In this process, monoterpenes act as microbe-inducible plant volatiles, which as part of plant-derived volatile blends have the potential to promote the generation of a wave of innate immune signaling within canopies or plant stands. Hence, plant-to-plant propagation of SAR holds significant potential to fortify future durable crop protection strategies following a single volatile trigger.

The Arabidopsis proline-rich extensin-like receptor kinase (PERK) family consists of 15 predicted receptor kinases. A comprehensive expression analysis was undertaken to identify overlapping and unique expression patterns within this family relative to their phylogeny. Three different approaches were used to study AtPERK gene family expression, and included analyses of the EST, MPSS and NASCArrays databases as well as experimental RNA blot analyses. Some of the AtPERK members were identified as tissue-specific genes while others were more broadly expressed. While in some cases there was a good association between these different expression patterns and the position of the AtPERK members in the kinase phylogeny, in other cases divergence of expression patterns was seen. The PERK expression data identified by the bioinformatics and experimental approaches were found generally to show similar trends and supported the use of data from large-scale expression studies for obtaining preliminary expression data. Thus, the bioinformatics survey for ESTs and microarrays is a powerful comprehensive approach for obtaining a genome-wide view of genes in a multigene family.