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...

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