Specific and constitutive expression of oxalate oxidase during the ageing of leaf sheaths of ryegrass stubble

Effective pathogenesis by the fungus Sclerotinia sclerotiorum requires the secretion of oxalic acid. Studies were conducted to determine whether oxalate aids pathogen compatibility by modulating the oxidative burst of the host plant. Inoculation of tobacco leaves with an oxalate-deficient nonpathogenic mutant of S. sclerotiorum induced measurable oxidant biosynthesis, but inoculation with an oxalate-secreting strain did not. Oxalate inhibited production of H(2)O(2) in tobacco and soybean cultured cell lines with a median inhibitory concentration of approximately 4 to 5 mM, a concentration less than that measured in preparations of the virulent fungus. Several observations also indicate that the inhibitory effects of oxalate are largely independent of both its acidity and its affinity for Ca(2)+. These and other data demonstrate that oxalate may inhibit a signaling step positioned upstream of oxidase assembly/activation but downstream of Ca(2)+ fluxes into the plant cell cytosol.

Radish seeds have previously been shown to contain two homologous, 5-kD cysteine-rich proteins designated Raphanus sativus-antifungal protein 1 (Rs-AFP1) and Rs-AFP2, both of which exhibit potent antifungal activity in vitro. We now demonstrate that these proteins are located in the cell wall and occur predominantly in the outer cell layers lining different seed organs. Moreover, Rs-AFPs are preferentially released during seed germination after disruption of the seed coat. The amount of released proteins is sufficient to create a microenvironment around the seed in which fungal growth is suppressed. Both the cDNAs and the intron-containing genomic regions encoding the Rs-AFP preproteins were cloned. Transcripts (0.55 kb) hybridizing with an Rs-AFP1 cDNA-derived probe were present in near-mature and mature seeds. Such transcripts as well as the corresponding proteins were barely detectable in healthy uninfected leaves but accumulated systemically at high levels after localized fungal infection. The induced leaf proteins (designated Rs-AFP3 and Rs-AFP4) were purified and shown to be homologous to seed Rs-AFPs and to exert similar antifungal activity in vitro. A chimeric Rs-AFP2 gene under the control of the constitutive cauliflower mosaic virus 35S promoter conferred enhanced resistance to the foliar pathogen Alternaria longipes in transgenic tobacco. The term "plant defensins" is proposed to denote these defense-related proteins.

Programmed cell death (PCD) is a physiological process occurring during development and in pathological conditions of animals and plants. The cell death program can be subdivided into three functionally different phases: a stimulus-dependent induction phase, an effector phase during which the wide range of death-stimuli are translated to a central coordinator, and a degradation phase during which the alterations commonly considered to define PCD (apoptotic morphology of the nucleus and chromatin fragmentation) become apparent. Recent studies suggest that mitochondrial permeability transition is the central coordinator of PCD and deciding whether or not a cell will die. There is increasing evidence that reactive oxygen intermediates (ROI) serve as direct and indirect mediators of PCD in mammalian and plant cells. Overexpression of genes encoding pro- and antioxidant enzymes in transgenic animals and plants has been informative regarding the function of ROI. Recent data imply a dual role of ROI in the apoptotic process: first, as a facultative signal during the induction phase, and, second, as a common consequence of mitochondrial permeability transition leading to the final destruction of the cell. The present review discusses and compares new insights into the function of ROI during PCD in mammalian cells and in human and plant diseases.