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The primary inflorescence stem of Arabidopsis thaliana is rich in lignified cell walls, in both vascular bundles and interfascicular fibres. Previous gene expression studies demonstrated a correlation between expression of phenylpropanoid biosynthetic genes and a subset of genes encoding ATP-binding cassette (ABC) transporters, especially in the ABCB/multi-drug resistance/P-glycoprotein (ABCB/MDR/PGP) and ABCG/pleiotropic drug resistance (ABCG/PDR) subfamilies. The objective of this study was to characterize these ABC transporters in terms of their gene expression and their function in development of lignified cells. Based on in silico analyses, four ABC transporters were selected for detailed investigation: ABCB11/MDR8, ABCB14/MDR12, ABCB15/MDR13, and ABCG33/PDR5. Promoter::glucuronidase reporter assays for each gene indicated that promoters of ABCB11, ABCB14, ABCB15, and ABCG33 transporters are active in the vascular tissues of primary stem, and in some cases in interfascicular tissues as well. Homozygous T-DNA insertion mutant lines showed no apparent irregular xylem phenotype or alterations in interfascicular fibre lignification or morphology in comparison with wild type. However, in abcb14-1 mutants, stem vascular morphology was slightly disorganized, with decreased phloem area in the vascular bundle and decreased xylem vessel lumen diameter. In addition, abcb14-1 mutants showed both decreased polar auxin transport through whole stems and altered auxin distribution in the procambium. It is proposed that both ABCB14 and ABCB15 promote auxin transport since inflorescence stems in both mutants showed a reduction in polar auxin transport, which was not observed for any of the ABCG subfamily mutants tested. In the case of ABCB14, the reduction in auxin transport is correlated with a mild disruption of vascular development in the inflorescence stem.

The role of silicon in the enhancement of cucumber defence reactions against powdery mildew fungi was examined. Scanning transmission electron microscopy coupled with energy dispersive X-ray analysis and image analysis were used to map in situ distribution of silicon in cucumber leaves infected with Sphaerotheca fuliginea . Silicon was found in the papillae, in host cell walls, around the haustorial neck and in deposits between the host cell wall and plasma membrane. The deposition of silicon in these sites could be a rapid mechanism for modifying the cell wall during pathogen attack.

Addition of silicate to hydroponic nutrient media has been shown to increase resistance of cucumber to powdery mildew. As a side effect of this treatment, the fruit surface developed an unusual dull appearance, or bloom. The effects of silicate supplementation on cucumber fruit were observed using scanning and transmission electron microscopy, energy-dispersive X-ray analysis, and colourimetric assays. Changes in the fruit trichome morphology occurred: trichomes from + Si fruit had a coarse outer appearance compared to - Si fruit where the trichomes were smooth. X-ray mapping showed high Si content in trichomes but not in the surrounding epidermis or fleshy mesocarp and endocarp tissues. Sections through the epidermal layer reveal that the silica is restricted to the trichomes, primarily in the epicuticular wax.

The response of epidermal cells of cucumber leaf tissue infected by Sphaerotheca fuliginea was examined by light microscopy to understand how silicon in infected host cells affects host defence mechanisms. Leaf pieces from plants treated with nutrient solutions containing 0·05, 0·50 or 2·3 m m of silicon (Si) were harvested at intervals of 24, 48, 72, 96 and 120 h after inoculation with the pathogen and examined after staining with toluidine blue or aniline blue. Si treatments significantly reduced the time to initiation of production and/or accumulation of phenolic materials in infected host epidermal cells, and increased the number of infected cells that produced and/or accumulated phenolics. The number of haustoria produced per colony of S. fuliginea was significantly reduced over time, and conidiophore development was delayed on the leaves of cucumber treated with 2·3 m m Si nutrient solution.

The mobility and deposition of silicon (Si) in cucumber plants grown in hydroponic culture were studied using scanning electron microscopy and energy dispersive X-ray analysis under conditions of powdery mildew fungus infection. Following transfer to Si containing media, plants grown in the absence of Si showed rapid silicification of leafy tissue, primarily in the trichome bases. These plants also exhibited increased disease resistance to Sphaerotheca fuliginea (Schlecht.:Fr.) Poll, and concentration of Si in the leaf epidermis surrounding the invading pathogen. Plants grown in Si-supplemented media and transferred into Si-deficient media contained residual Si in the leaf trichome bases, but failed to display disease resistance or silicification of host tissue surrounding the invading pathogen.

The distribution of silicon in the leaf epidermis of cucumber (Cucumis sativus L.) grown in hydroponic nutrient solutions supplemented with soluble silicates was examined using scanning electron microscopy and energy dispersive x-ray analysis. Silicon was found primarily in cells surrounding the bases of the trichome hairs. During infection by Sphaerotheca fuliginea (Schlecht.:Fr.) Poll., areas of host cell wall adjacent to the germinating hyphae showed altered surface morphology and high concentrations of silicon. Measurements of total hyphal length per colony showed the fungal colonies grown on silicate-treated plants were significantly smaller than fungal colonies grown on control plants. In the early stages of infection, the presence of high silicon surrounding colonies was correlated with lower fungal growth. Key words: silicon, x-ray analysis, hydroponics, Cucumis sativus, Sphaerotheca fuliginea.