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Reactive oxygen species (ROS) are known as toxic metabolic products in plants and other aerobic organisms. An elaborate and highly redundant plant ROS network, composed of antioxidant enzymes, antioxidants and ROS-producing enzymes, is responsible for maintaining ROS levels under tight control. This allows ROS to serve as signaling molecules that coordinate an astonishing range of diverse plant processes. The specificity of the biological response to ROS depends on the chemical identity of ROS, intensity of the signal, sites of production, plant developmental stage, previous stresses encountered and interactions with other signaling molecules such as nitric oxide, lipid messengers and plant hormones. Although many components of the ROS signaling network have recently been identified, the challenge remains to understand how ROS-derived signals are integrated to eventually regulate such biological processes as plant growth, development, stress adaptation and programmed cell death.

SummaryThe oxidative burst is an early response to pathogen attack leading to the production of reactive oxygen species (ROS) including hydrogen peroxide. Two major mechanisms involving either NADPH oxidases or peroxidases that may exist singly or in combination in different plant species have been proposed for the generation of ROS. We identified an Arabidopsis thaliana azide-sensitive but diphenylene iodoniuminsensitive apoplastic oxidative burst that generates H 2 O 2 in response to a Fusarium oxysporum cell-wall preparation. Transgenic Arabidopsis plants expressing an anti-sense cDNA encoding a type III peroxidase, French bean peroxidase type 1 (FBP1) exhibited an impaired oxidative burst and were more susceptible than wild-type plants to both fungal and bacterial pathogens. Transcriptional profiling and RT-PCR analysis showed that the anti-sense (FBP1) transgenic plants had reduced levels of specific peroxidase-encoding mRNAs, including mRNAs corresponding to Arabidopsis genes At3g49120 (AtPCb) and At3g49110 (AtPCa) that encode two class III peroxidases with a high degree of homology to FBP1. These data indicate that peroxidases play a significant role in generating H 2 O 2 during the Arabidopsis defense response and in conferring resistance to a wide range of pathogens.

Immunoglobulin G4 (IgG4)-related disease (IgG4-RD) is an immune-mediated condition that can affect almost any organ and is now being recognized with increasing frequency. IgG4-RD is characterized by a lymphoplasmacytic infiltrate composed of IgG4(+) plasma cells, storiform fibrosis, obliterative phlebitis, and mild to moderate eosinophilia. The diagnosis of IgG4-RD unifies many eponymous fibroinflammatory conditions that had previously been thought to be confined to single organs. IgG4-RD lesions are infiltrated by T helper cells, which likely cause progressive fibrosis and organ damage. IgG4 antibodies are generally regarded as noninflammatory. Although autoreactive IgG4 antibodies are observed in IgG4-RD, there is no evidence that they are directly pathogenic. Rituximab-induced B cell depletion in IgG4-RD leads to rapid clinical and histological improvement accompanied by swift declines in serum IgG4 concentrations. Although IgG autoantibodies against various exocrine gland antigens have been described in IgG4-RD, whether they are members of the IgG4 subclass is unknown. The contribution of autoantibodies to IgG4-RD remains unclear.

A protein phosphatase was cloned that interacts with a serine-threonine receptor-like kinase, RLK5, from Arabidopsis thaliana. The phosphatase, designated KAPP (kinase-associated protein phosphatase), is composed of three domains: an amino-terminal signal anchor, a kinase interaction (KI) domain, and a type 2C protein phosphatase catalytic region. Association of RLK5 with the KI domain is dependent on phosphorylation of RLK5 and can be abolished by dephosphorylation. KAPP may function as a signaling component in a pathway involving RLK5.

Enzymes of the eukaryotic protein kinase superfamily catalyze the reversible transfer of the y-phosphate from ATP t o amino acid side chains of proteins. Protein kinase function can be counteracted by the action of phosphoprotein phosphatases. Phosphorylation status of a protein can have profound effects on its activity and interaction with other proteins. An estimated 1 t o 3% of functional eukaryotic genes encode protein kinases, suggesting that they are involved in many aspects of cellular regulation and metabolism. In plants, protein phosphorylation has been implicated in responses t o many signals, including light, pathogen invasion, hormones, temperature stress, and nutrient deprivation. Activities of severa1 plant metabolic and regulatory enzymes are also controlled by reversible phosphorylation. As might be expected from this diversity of function, there is a large array of different protein kinases. Purification of protein kinases and their subsequent cloning, facilitated by the PCR and advances in homology-based cloning techniques, as well as functional analyses, including complementation of conditional yeast mutants and positional cloning of mutant plant genes, has already led t o identification of more than 70 plant protein kinase genes. However, the precise functional roles of specific protein kinases and phosphatases during plant growth and development have been elucidated for only a few. This update will focus on the eukaryotic protein kinases that have been classified by sequence similarity into related families (Hanks and Hunter, 1995). The predominant features of the five major plant protein kinase families will be discussed, and the similarities and differences between the plant protein kinases and the other eukaryotic protein kinases will be highlighted. This analysis has led to some interesting observations regarding signal transduction in higher plants. THE EUKARYOTIC PROTEIN KINASE SUPERFAMILYEnzymes belonging to the eukaryotic protein kinase superfamily are related by homologous protein kinase catalytic domains. Typically, eukaryotic protein kinases have been subdivided into those that phosphorylate Ser and/or

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We have established an Arabidopsis protoplast model system to study plant cell death signaling. The fungal toxin fumonisin B1 (FB1) induces apoptosis-like programmed cell death (PCD) in wild-type protoplasts. FB1, however, only marginally affects the viability of protoplasts isolated from transgenic NahG plants, in which salicylic acid (SA) is metabolically degraded; from pad4-1 mutant plants, in which an SA amplification mechanism is thought to be impaired; or from jar1-1 or etr1-1 mutant plants, which are insensitive to jasmonate (JA) or ethylene (ET), respectively. FB1 susceptibility of wild-type protoplasts decreases in the dark, as does the cellular content of phenylalanine ammonia-lyase, a light-inducible enzyme involved in SA biosynthesis. Interestingly, however, FB1-induced PCD does not require the SA signal transmitter NPR1, given that npr1-1 protoplasts display wild-type FB1 susceptibility. Arabidopsis cpr1-1 , cpr6-1 , and acd2-2 protoplasts, in which the SA signaling pathway is constitutively activated, exhibit increased susceptibility to FB1. The cpr6-1 and acd2-2 mutants also constitutively express the JA and ET signaling pathways, but only the acd2-2 protoplasts undergo PCD in the absence of FB1. These results demonstrate that FB1 killing of Arabidopsis is light dependent and requires SA-, JA-, and ET-mediated signaling pathways as well as one or more unidentified factors activated by FB1 and the acd2-2 mutation. INTRODUCTIONPlant cell death is often the consequence of plant-pathogen interactions in both compatible and incompatible relationships (Greenberg, 1997). A notable example is localized cell collapse, called the hypersensitive response (HR), which is induced rapidly in a resistant plant at the infection site of an avirulent pathogen (Staskawicz et al., 1995; Bent, 1996; Dangl et al., 1996; Hammond-Kosack and Jones, 1996). Hypersensitive cell death, which is distinct from necrosis caused by metabolic toxins or severe trauma, is genetically programmed (programmed cell death [PCD]) and requires active host cell metabolism Morel and Dangl, 1997;Pennell and Lamb, 1997; Gilchrist, 1998; Gray and Johal, 1998; Heath, 1998;Richberg et al., 1998). In fact, plant cells undergoing the HR display several molecular and morphological markers characteristic of animal apoptosis (a specialized form of PCD), including systematic DNA degradation and formation of apoptotic-like bodies, which suggests that the terminal steps in PCD are well conserved in animals and plants (Levine et al., 1996;Ryerson and Heath, 1996; Wang et al., 1996). It remains to be determined, however, whether the signal transduction mechanisms leading to the onset of PCD are also equally conserved between the two kingdoms.Salicylic acid (SA) is the best-characterized signaling molecule in plant defense responses (Ryals et al., 1996; Delaney, 1997; Durner et al., 1997). Application of exogenous SA or SA analogs activates the expression of a variety of pathogenesis-related ( PR ) genes and enhances resistance to a variety of pathogens (W...