Reactive oxygen intermediates (ROI) are closely related to defence reactions of plants against pathogens. A prominent role in the production of ROI has been attributed to the plant respiratory burst oxidase homologues (RBOH) of the human phagocyte GP91(phox). A barley RBOH, which encodes a putative superoxide (O2·−) producing NADPH oxidase, is described here. Histochemical analysis of the barley-Blumeria graminis f. sp. hordei (Bgh) interaction showed that O2·− is produced locally at the site of penetration. In contrast, hydrogen peroxide (H2O2) is produced in non-penetrated cell wall appositions. A barley RBOHA cDNA was isolated and a minor induction of expression of RBOHA was observed during the interactions of barley with Bgh. Transient RNA interference-mediated gene silencing of HvRBOHA during the penetration process of Bgh led to an increase of basal penetration resistance. The results support a potential role of HvRBOHA in cellular accessibility to Blumeria graminis.

Cadmium is a major environmental pollutant that enters human food via accumulation in crop plants. Responses of plants to cadmium exposure—which directly influence accumulation rates—are not well understood. In general, little is known about stress-elicited changes in plants at the proteome level. Alterations in the root proteome of hydroponically grown Arabidopsis thaliana plants treated with 10 μM Cd2+ for 24 h are reported here. These conditions trigger the synthesis of phytochelatins (PCs), glutathione-derived metal-binding peptides, shown here as PC2 accumulation. Two-dimensional gel electrophoresis using different pH gradients in the first dimension detected on average ∼1100 spots per gel type. Forty-one spots indicated significant changes in protein abundance upon Cd2+ treatment. Seventeen proteins found in 25 spots were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Selected results were independently confirmed by western analysis and selective enrichment of a protein family (glutathione S-transferases) through affinity chromatography. Most of the identified proteins belong to four different classes: metabolic enzymes such as ATP sulphurylase, glycine hydroxymethyltransferase, and trehalose-6-phosphate phosphatase; glutathione S-transferases; latex allergen-like proteins; and unknown proteins. These results represent a basis for reverse genetics studies to better understand plant responses to toxic metal exposure and to the generation of internal sinks for reduced sulphur.

Extracellular invertases are suggested to play a crucial role in the arbuscular mycorrhiza (AM) symbiosis to fulfil the increased sink function of the mycorrhizal root and the supply of the obligate biotrophic AM fungus with hexoses. In tomato (Lycopersicon esculentum), LIN6 represents an apoplastic invertase which is described as a key enzyme in establishing and maintaining sink metabolism. In this study, transcript levels of LIN6 were analysed in tomato roots colonized with the AM fungus Glomus intraradices. Using real-time RT–PCR, a nearly 3-fold increase in LIN6 mRNA levels was detected at late stages of mycorrhization (11 weeks after inoculation). A 1.8-fold induction could already be achieved at earlier stages (5 weeks after inoculation) using higher inoculum concentrations, whereas wounding of non-mycorrhizal roots resulted in up to 12-fold enhanced LIN6 transcripts. As revealed by in situ hybridization, the expression of LIN6 upon mycorrhization was specifically restricted to colonized cells and to the central cylinder. Such a strongly localized pattern due to mycorrhizal cells and to the central core could also be shown for promoter activity using transgenic Nicotiana tabacum plants expressing the gene coding for β-glucuronidase under the control of the LIN6 promoter. The moderate induction of LIN6 expression in mycorrhizal tomato roots compared with stress-stimulated induction suggested a fine-tuning in the activation of sink metabolism in the mutualistic interaction, avoiding stress-induced defence reactions.

During the development of the haustorium, searching hyphae of the parasite and the host parenchyma cells are connected by plasmodesmata. Using transgenic tobacco plants expressing a GFP-labelled movement protein of the tobacco mosaic virus, it was demonstrated that the interspecific plasmodesmata are open. The transfer of substances in the phloem from host to the parasite is not selective. After simultaneous application of 3H-sucrose and 14C-labelled phloem-mobile amino acids, phytohormones, and xenobiotica to the host, corresponding percentages of the translocated compounds are found in the parasite. An open continuity between the host phloem and the Cuscuta phloem via the haustorium was demonstrated in CLSM pictures after application of the phloem-mobile fluorescent probes, carboxyfluorescein (CF) and hydroxypyrene trisulphonic acid (HPTS), to the host. Using a Cuscuta bridge 14C-sucrose and the virus PVYN were transferred from one host plant to the another. The results of translocation experiments with labelled compounds, phloem-mobile dyes and the virus should be considered as unequivocal evidence for a symplastic transfer of phloem solutes between Cuscuta species and their compatible hosts.