INTRODUCTIONCytoplasmic free Ca 2 ϩ ([Ca 2 ϩ ] cyt ) serves as a second messenger in plant processes as diverse as root nodule formation, phytochrome phototransduction, stomatal closure, geotropism, circadian rhythm, pollen tube growth, and stress adaptation (Rudd and Franklin-Tong, 1999). Stimulusspecific and spatially and temporally defined Ca 2 ϩ signatures of characteristic magnitude, frequency, and duration are assumed to maintain signal specificity of transduction cascades (Thuleau et al., 1998;Trewavas, 1999). Subsequently, the binding of [Ca 2 ϩ ] cyt to calmodulin, Ca 2 ϩ -dependent protein kinases, Ca 2 ϩ -dependent protein phosphatases, Ca 2 ϩ -gated ion channels, or Ca 2 ϩ -activated phospholipases facilitates downstream signal transduction directed toward activation of a signal-specific cellular response (Blumwald et al., 1998).Expression of the Aequorea aequorea apoaequorin gene in the cytoplasm of plant cells provides a means for accurate, noninvasive quantification of changes in [Ca 2 ϩ ] cyt (Knight et al., 1991). When reconstituted with coelenterazine, holoaequorin acts as a bioluminescent indicator of [Ca 2 ϩ ] cyt . Since the pioneering work of Knight et al. (1991), aequorin technology has been widely applied in plants to report changes in [Ca 2 ϩ ] cyt in response to abiotic stimuli, such as touch, wind, cold, heat, and drought (Knight et al., 1991(Knight et al., , 1992(Knight et al., , 1997Haley et al., 1995;Gong et al., 1998;Plieth et al., 1999); blue light ; circadian rhythm (Johnson et al., 1995); ozone (Clayton et al., 1999); anoxia (Sedbrook et al., 1996); oxidative stress (Price et al., 1994); and hypoosmotic shock (Chandra and Low, 1997;Takahashi et al., 1997;Cessna et al., 1998).Numerous recent studies have provided evidence that Ca 2 ϩ plays a pivotal role in activating the plant's surveillance system against attempted microbial invasion (Yang et al., 1997;Scheel, 1998). Activation of plant defense is believed to be receptor mediated through recognition of pathogenderived elicitors (Yang et al., 1997;Scheel, 1998;Nürnberger, 1999). In contrast to elicitors from phytopathogenic bacteria, elicitors of fungal or oomycete origin appear to be recognized by high-affinity receptors residing in the plasma membrane of plant cells. Although several such plasma membrane binding sites have been characterized kinetically and structurally, our knowledge of the molecular mode of fungal pathogen perception in plants remains fragmentary: only one elicitor receptor has been isolated thus far, a soybean 70-kD plasma membrane protein that binds Phytophthora sojae-derived ␤ -glucans (Umemoto et al., 1997).Receptor-ligand interaction initiates an intracellular signal transduction cascade that mediates activation of the defense against the pathogen. Cellular components shown to be modulated by elicitor treatment include GTP binding proteins (Bischoff et al., 1999); plasma membrane ion channels 1 Current address: LION Bioscience AG, Im Neuenheimer Feld 515-517, D-69120 Heidelberg, Germany. 2 To whom corre...

Transient influx of Ca(2+) constitutes an early element of signaling cascades triggering pathogen defense responses in plant cells. Treatment with the Phytophthora sojae-derived oligopeptide elicitor, Pep-13, of parsley cells stably expressing apoaequorin revealed a rapid increase in cytoplasmic free calcium ([Ca(2+)](cyt)), which peaked at approximately 1 microM and subsequently declined to sustained values of 300 nM. Activation of this biphasic [Ca(2+)](cyt) signature was achieved by elicitor concentrations sufficient to stimulate Ca(2+) influx across the plasma membrane, oxidative burst, and phytoalexin production. Sustained concentrations of [Ca(2+)](cyt) but not the rapidly induced [Ca(2+)](cyt) transient peak are required for activation of defense-associated responses. Modulation by pharmacological effectors of Ca(2+) influx across the plasma membrane or of Ca(2+) release from internal stores suggests that the elicitor-induced sustained increase of [Ca(2+)](cyt) predominantly results from the influx of extracellular Ca(2+). Identical structural features of Pep-13 were found to be essential for receptor binding, increases in [Ca(2+)](cyt), and activation of defense-associated responses. Thus, a receptor-mediated increase in [Ca(2+)](cyt) is causally involved in signaling the activation of pathogen defense in parsley.

Advanced glycation end products (AGEs) are formed in vivo by a non-enzymatic reaction of proteins with carbohydrates and accumulate in many tissues during ageing. They are discussed as being responsible for many age- and diabetes-related diseases. On the other hand, AGEs are formed by the heating of food and are taken up by the nutrition. The contribution of endogenously formed versus exogenous intake of AGEs to age-related diseases is still under discussion.

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Life span in individual humans is very heterogeneous.Thus, the ageing rate, measured as the decline of functional capacity and stress resistance, is different in every individual. There have been attempts made to analyse this individual age, the so-called biological age, in comparison to chronological age. Biomarkers of ageing should help to characterise this biological age and, as age is a major risk factor in many degenerative diseases,could be subsequently used to identify individuals at high risk of developing age-associated diseases or disabilities. Markers based on oxidative stress, protein glycation,inflammation, cellular senescence and hormonal deregulation are discussed.