Nitric oxide (NO) has recently emerged as an important cellular mediator in plant defense responses. However, elucidation of the biochemical mechanisms by which NO participates in this signaling pathway is still in its infancy. We previously demonstrated that cryptogein, an elicitor of tobacco defense responses, triggers a NO burst within minutes in epidermal sections from tobacco leaves (Nicotiana tabacum cv Xanthi). Here, we investigate the signaling events that mediate NO production, and analyze NO signaling activities in the cryptogein transduction pathway. Using flow cytometry and spectrofluorometry, we observed that cryptogein-induced NO production in tobacco cell suspensions is sensitive to nitric oxide synthase inhibitors and may be catalyzed by variant P, a recently identified pathogen-inducible plant nitric oxide synthase. NO Moreover, NO appears to be involved in the pathway(s) leading to the accumulation of transcripts encoding the heat shock protein TLHS-1, the ethyleneforming enzyme cEFE-26, and cell death. In contrast, NO does not act upstream of the elicitor-induced activation of mitogenactivated protein kinase, the opening of anion channels, nor expression of GST, LOX-1, PAL, and PR-3 genes. Collectively, our data indicate that NO is intimately involved in the signal transduction processes leading to cryptogein-induced defense responses.Plants are frequently challenged by potential pathogens and have therefore evolved inducible defense mechanisms to survive in their environment. The activation of plant defense responses is initiated through the recognition of microorganism-derived molecules called elicitors, which trigger rapid defense responses via complex signal transduction pathways (Scheel, 1998). Plant defense responses classically include the production of active oxygen species (AOS), reinforcement of cell walls, and enhanced expression of a large number of defense-related genes, including those encoding cell wall proteins, enzymes involved in the phenylpropanoid biosynthetic pathway, and pathogenesis-related (PR) proteins (Fritig et al., 1998). Plants resisting pathogen attack sometimes develop a hypersensitive response (HR) in which rapid, localized cell death is associated with the restriction of invaders to the infection site (Lam et al., 2001). The HR shares some features of the apoptotic cell death process in animals, and it may likewise represent a type of programmed cell death (PCD; Lam et al., 2001). Moreover, plants usually develop a long-lasting resistance to diverse pathogens via systemic acquired resistance (SAR) or the less well understood induced systemic resistance (ISR; Pieterse et al., 1998).Cryptogein is a basic 10-kD proteinaceous elicitor secreted by the hemibiotrophic oomycete Phytophthora cryptogea (Ricci, 1997). The application of nanomolar concentrations of cryptogein to tobacco plants triggers expression of defense-related genes, induces an HR-like response, and inaugurates the protection of plants against invasion by a broad spectrum of microorganisms including...

Caspases play important roles in apoptotic cell death and in some other functions, such as cytokine maturation, inflammation, or differentiation. We show here that the 5-flanking region of the human CASP-2 gene contains three functional response elements for sterol regulatory element binding proteins (SREBPs), proteins that mediate the transcriptional activation of genes involved in cholesterol, triacylglycerol, and fatty acid synthesis. Exposure of several human cell lines to statins, lipid-lowering drugs that drive SREBP proteolytic activation, induced the CASP-2 gene to an extent similar to that for known targets of SREBP proteins. Adenoviral vector-mediated transfer of active SREBP-2 also induced expression of the CASP-2 gene and the caspase-2 protein and increased the cholesterol and triacylglycerol cellular content. These rises in lipids were strongly impaired following small interfering RNA-mediated silencing of the CASP-2 gene. Taken together, our results identify the human CASP-2 gene as a member of the SREBP-responsive gene battery that senses lipid levels in cells and raise the possibility that caspase-2 participates in the control of cholesterol and triacylglycerol levels.

Cryptogein is a 10-kD protein secreted by the oomycete Phytophthora cryptogea that induces a hypersensitive response on tobacco (Nicotiana tabacum var. Xanthi) plants and a systemic acquired resistance against various pathogens. The mode of action of this elicitor has been studied using tobacco cell suspensions. Our previous data indicated that within minutes, cryptogein signaling involves various events including changes in ion fluxes, protein phosphorylation, sugar metabolism, and, eventually, cell death. These results suggested that transport of sugars could be affected and, thus, involved in the complex relationships between plant and microorganisms via elicitors. This led us to investigate the effects of cryptogein on glucose (Glc) uptake and mitochondrial activity in tobacco cells. Cryptogein induces an immediate inhibition of Glc uptake, which is not attributable to plasma membrane (PM) depolarization. Conversely, cryptogein-induced valine uptake is because of PM depolarization. Inhibition of the PM Glc transporter(s) was shown to be mediated by a calcium-dependent phosphorylation process, and is independent of active oxygen species production. This inhibition was associated with a strong decrease in O 2 uptake rate by cells and a large mitochondrial membrane depolarization. Thus, inhibition of Glc uptake accompanied by inhibition of phosphorylative oxidation may participate in hypersensitive cell death. These results are discussed in the context of competition between plants and microorganisms for apoplastic sugars.Many incompatible plant-microorganism interactions are mediated by elicitors of defense responses. Studies on the mode of action of elicitors have revealed that they first activate plasma membrane (PM) proteins involved in recognition (Boller, 1995) and signal transduction. The latter phenomenon involves Ca 2ϩ channels (Zimmermann et al., 1997; Lecourieux et al., 2002), anionic and K ϩ channels (Nü rnberger et al., 1994;Wendehenne et al., 2002), NADPH oxidase (Keller et al., 1998), phospholipases (van der Luit et al., 2000, and probably other proteins that have not yet been identified. When activated, these proteins trigger within a few minutes a complex network of second messengers (free cytosolic calcium increase, cytosolic pH decrease, active oxygen species [AOS], PM depolarization, and changes in metabolism; Batz el al., 1998; Lebrun-Garcia et al., 1999), which in turn triggers defense reactions, as well as the systemic acquired resistance (SAR).Little information is available concerning the exchange of organic solutes at the PM during plant/ microorganism interactions. Both the plant and the phytopathogen compete for the solutes contained in the apoplast that separates them. This competition is particularly important in the case of sugars, which provide both a source of energy and carbohydrate skeletons. Thus, the relative capacity for plants and microorganisms to control the uptake of sugars and other nutrients from the apoplast may be a determinant in the final outcome of the inter...