Plant activators are compounds, such as analogs of the defense hormone salicylic acid (SA), that protect plants from pathogens by activating the plant immune system. Although some plant activators have been widely used in agriculture, the molecular mechanisms of immune induction are largely unknown. Using a newly established high-throughput screening procedure that screens for compounds that specifically potentiate pathogen-activated cell death in Arabidopsis thaliana cultured suspension cells, we identified five compounds that prime the immune response. These compounds enhanced disease resistance against pathogenic Pseudomonas bacteria in Arabidopsis plants. Pretreatments increased the accumulation of endogenous SA, but reduced its metabolite, SA-O-β-d-glucoside. Inducing compounds inhibited two SA glucosyltransferases (SAGTs) in vitro. Double knockout plants that lack both SAGTs consistently exhibited enhanced disease resistance. Our results demonstrate that manipulation of the active free SA pool via SA-inactivating enzymes can be a useful strategy for fortifying plant disease resistance and may identify useful crop protectants.

The voltage-dependent anion channels (VDACs), VDAC1, VDAC2, and VDAC3, are pore-forming proteins that control metabolite flux between mitochondria and cytoplasm. VDAC1 and VDAC2 have voltage-dependent gating activity, whereas VDAC3 is thought to have weak activity. The aim of this study was to analyze the channel properties of all three human VDAC isoforms and to clarify the channel function of VDAC3. Bacterially expressed recombinant human VDAC proteins were reconstituted into artificial planar lipid bilayers and their gating activities were evaluated. VDAC1 and VDAC2 had typical voltage-dependent gating activity, whereas the gating of VDAC3 was weak, as reported. However, gating of VDAC3 was evoked by dithiothreitol (DTT) and S-nitrosoglutathione (GSNO), which are thought to suppress disulfide-bond formation. Several cysteine mutants of VDAC3 also exhibited typical voltage-gating. Our results indicate that channel gating was induced by reduction of a disulfide-bond linking the N-terminal region to the bottom of the pore. Thus, channel gating of VDAC3 might be controlled by redox sensing under physiological conditions.

Melatonin exists in a considerable variety of plant species. However, the physiological roles of melatonin in plants are not well understood. In this study, the distribution and accumulation of melatonin during leaf and fruit development were analyzed in Micro-Tom, a model cultivar of tomato (Solanum lycopersicum L.). Melatonin was extracted using an acetone-methanol method and measured by enzyme-linked immunosorbent assay. Melatonin was detected in leaves, stems, roots, flowers, fruits, seedlings and seeds in the range of 1.5-66.6 ng/g fresh weight, with seeds containing the highest concentration of melatonin. In fruits and leaves, melatonin concentrations varied depending on the developmental stage, suggesting that melatonin controls some of the processes involved in plant maturation.

Melatonin exists in numerous living organisms including vertebrates, insects, fungi, bacteria, and plants. Extensive studies have been conducted on the physiological roles of melatonin in various plant species. In plants, melatonin seems to act in antioxidant protection, as a growth promoter, and in photoperiodism. However, the mechanisms by which melatonin carries out these roles remain unclear. We manipulated the endogenous melatonin content in tomato plants by modifying the metabolic enzyme indoleamine 2,3-dioxygenase (IDO). The OsIDO gene was isolated from rice (Oryza sativa) and characterized using 3-D homology modeling and reverse genetic approaches. The amino acid sequence of OsIDO showed high homology to the Ustilago maydis IDO. The 3-D model structure of OsIDO is composed of a small and a large domain. Transgenic tomato plants constitutively expressing the OsIDO gene exhibited a decrease in their melatonin content. Moreover, the number of lateral leaflets decreased in transgenic plants. Protein extracts taken from these plants showed activity degradation, demonstrating the function of OsIDO. These results suggest the involvement of IDO in plant melatonin metabolism and a possible role in plant leaf development.