Parsley cells recognize the fungal plant pathogen Phytophthora sojae through a plasma membrane receptor. A pathogen-derived oligopeptide elicitor binds to this receptor and thereby stimulates a multicomponent defense response through sequential activation of ion channels and an oxidative burst. An elicitor-responsive mitogen-activated protein (MAP) kinase was identified that acts downstream of the ion channels but independently or upstream of the oxidative burst. Upon receptor-mediated activation, the MAP kinase is translocated to the nucleus where it might interact with transcription factors that induce expression of defense genes.

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The plant hormone abscisic acid (ABA) participates in the control of several important physiological processes in plants such as stomata regulation, seed dormancy and stress tolerance. A new strategy was developed to study these phenomena by blocking abscisic acid with intracellularly expressed specific single-chain variable fragment (scFv) antibodies. Here evidence is presented that the expression of single-chain Fv antibodies against abscisic acid in the endoplasmic reticulum of transgenic tobacco cells leads to a wilty phenotype. Stomatal conductance is increased at high CO2 concentrations dependent on the level of antibody expression in leaves. Symptoms of abscisic acid deficiency were generated in the transformants although they have even higher levels of abscisic acid than wild-type plants.

ions exert their toxic effects on cellular metabolism in the ER rather than in the cytosol.Ions of heavy metals such as iron, copper, zinc, cobalt, or nickel are essential micronutrients, required for function of a large number of proteins. At supraoptimal concentrations, however, these metal ions can be detrimental. Furthermore, living organisms can be exposed to the highly toxic ions of cadmium, lead, mercury, and other metals that are generally considered non-essential. Consequently, a complex network of transport, chelation, and sequestration processes has evolved that functions to maintain the concentrations of essential metal ions in different cellular compartments within the narrow physiological range and to minimize the damage caused by the entry of non-essential metal ions into the cytosol (1, 2). The exquisitely tight control of free ion concentrations has been demonstrated for copper (3). Most recent evidence suggests a similar degree of control also for zinc (4). The intracellular mechanisms of storage and cellular distribution, however, are largely unknown.Proteins belonging to the cation diffusion facilitator family (CDF) 1 (5, 6) could potentially play a major role in metal homeostasis. They have been shown in bacteria and budding yeast to confer tolerance of Zn 2ϩ , Co 2ϩ , or Cd 2ϩ ions (7-10). ZRC1 and COT1 in Saccharomyces cerevisiae localize to the vacuolar membrane and are hypothesized to contribute to the storage of Zn 2ϩ and Co 2ϩ ions, respectively (11, 12). Mammalian members of the CDF family appear to be involved mainly in the removal of Zn 2ϩ ions from the cytosol either through the plasma membrane (Zn-T1) (13) or into endosomal vesicles (Zn-T2) (14).We are interested in the physiological role of CDFs and other putative metal transporters for cellular metal homeostasis, tolerance, and accumulation in organisms that express phytochelatin synthases, using fission yeast as the most suitable model system. The synthesis of phytochelatins (PCs), small metal-binding peptides derived from glutathione (15, 16), represents one of the main metal chelation and detoxification mechanisms in plants, fungi, marine diatoms, and also certain animals (17-19). Here we report on the functional characterization of a Schizosaccharomyces pombe CDF (named Zhf, for zinc homeostasis factor), whose function affects tolerance to a range of metal ions in drastically different fashion; disruption of the gene renders S. pombe cells Zn 2ϩ -and Co 2ϩ -hypersensitive yet significantly enhances tolerance toward Cd 2ϩ and Ni 2ϩ . Electron microscopic protein and zinc localization indicate Zhf-dependent zinc accumulation in the ER. Our findings represent novel evidence for the role of the respective compartment in metal homeostasis and identify a major pathway of zinc storage in the ER. Furthermore, the data provide new insights into the still poorly understood cellular mechanisms of cadmium toxicity. EXPERIMENTAL PROCEDURESS. pombe Strains and Media-The S. pombe strains employed in this study were derived from FY261 (h ϩ ...

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Abstract. We report on the stress-independent, tissue-specific expression of the heat-stress protein HSP17 in developing seeds of different plant species and on its intracellular localization. Though HSP17 expression during seed development seems to be a general phenomenon, the isoform patterns, the relative amounts in embryonic tissues and the intracellular localization show species-specific variations. In contrast to the results on the stressinduced protein forming large cytoplasmic aggregates (heat stress granules) the developmentally expressed HSP17 is mainly found in nuclei. But in addition, a considerable part is also detected in protein bodies of mature seeds of Lycopersicon esculentum and Vicia faba, but not of Zea mays. The mechanism of this transition into the vacuolar compartment remains to be investigated.

SummaryThe conformational dynamism and aggregate state of small heat shock proteins (sHSPs) may be crucial for their functions in thermoprotection of plant cells from the detrimental effects of heat stress. Ectopic expression of single chain fragment variable (scFv) antibodies against cytosolic sHSPs was used as new tool to generate sHSP loss-of-function mutants by antibody-mediated prevention of the sHSP assembly in vivo. Anti-sHSP scFv antibodies transiently expressed in heat-stressed tobacco protoplasts were not only able to recognize the endogenous sHSPs but also prevented their assembly into heat stress granula (HSGs). Constitutive expression of the same scFv antibodies in transgenic plants did not alter their phenotype at normal growth temperatures, but their leaves turned yellow and died after prolonged stress at sublethal temperatures. Structural analysis revealed a regular cytosolic distribution of stress-induced sHSPs in mesophyll cells of stress-treated transgenic plants, whereas extensive formation of HSGs was observed in control cells. After prolonged stress at sublethal temperatures, mesophyll cells of transgenic plants suffered destruction of all cellular membranes and finally underwent cell death. In contrast, mesophyll cells of the stressed controls showed HSG disintegration accompanied by appearance of polysomes, dictyosomes and rough endoplasmic reticulum indicating normalization of cell functions. Apparently, the ability of sHSPs to assemble into HSGs as well as the HSG disintegration is a prerequisite for survival of plant cells under continuous stress conditions at sublethal temperatures.