Molecular characterisation of a Rhodococcus ohp operon

The ohp operon of Rhodococcus strain V49 consists of five genes, ohpR, ohpA, ohpB, ohpC and ohpD which encode putative regulator and transport proteins and confirmed monooxygenase, hydroxymuconic semialdehyde hydrolase and catechol 2,3-dioxygenase enzymes, respectively. These enzymes catalyse the conversion of 3-(2- hydroxyphenyl)propionic acid to the corresponding linear product via a meta-cleavage pathway. Confirmation that the ohp gene cluster formed an operon was provided by gene disruption during which expression of Bacillus levansucrase was confirmed in Rhodococcus. Following biochemical assays of cell-free extracts from recombinant Escherichia coli expressing ohpB (monooxygenase), ohpC (hydroxymuconic-semialdehyde hydrolase) and ohpD (catechol 2,3-dioxygenase), the ortho-hydroxyphenylpropionic acid catabolic pathway in Rhodococcus strain V49 (ATCC 19070) has been predicted.     

Preliminary Evidence for Phytoalexin Production by Eucalyptus urophylla Leaves as a Response to Inoculation with the Incompatible Pathogen Acidovorax avenae pv. avenae

Leaves of eucalyptus ( Eucalyptus urophylla ) were infiltrated with a cell suspension of the incompatible pathogen Acidovorax avenae pv. avenae and showed a typical hypersensitive response within 24h. Necrotic leaf areas were excised, vacuum infiltrated with 40% ethanol and left under continuous agitation at room temperature for 24h. The diffusate was concentrated, partitioned with ethyl-acetate, concentrated to dryness and resuspended in a small volume of methanol. The biological activity of extracts was evaluated by an agar diffusion method against noncompatible bacteria ( A. avenae pv. avenae and Xanthomonas axonopodis pv. manihotis ) and fungi ( Penicillium sp. and Aspergillus sp. ). Inhibition haloes, when present, were always larger in extracts from leaves infiltrated with the incompatible bacterium than the water control. Thin layer chromatography resolution of crude extracts from leaves infiltrated with both incompatible pathogen cell suspension and water, followed by bioautography with Thieleviopsis paradoxa, consistently rendered, in both situations, a large, diffuse inhibition halo near the origin, assumed to be due to preformed antimicrobial substances. However, extracts from leaves infiltrated with the living cells of the incompatible pathogen gave rise to a smaller, second inhibition halo, near the front, that was interpreted as being one or several phytoalexins.     

Use of the rice sucrose synthase-1 promoter to direct phloem-specific expression of {beta}-glucuronidase and snowdrop lectin genes in transgenic tobacco plants

The promoter region from the rice sucrose synthase-1 gene (RSs1)was fused with coding sequences for ß-glucuronidase(GUS) and snowdrop (Galanthus nivalis) lectin (GNA). Tobaccoplants were transformed with these chimaenc genes in order todetermine the expression pattern directed by the RSs1 promoter.Histochemical and immunochemical assays demonstrated that theexpression of both GUS and GNA was restricted to phloem tissue,and was not observed in any other tissues. This phloem-specificexpression pattern was consistent in stem, leaf and root, andin different transgenic plants. Chimaeric genes of RSs 1-GUSand RSs1 GNA were stably inherited in T1 plants. In addition,GNA was detected by immunological assay in the honeydew producedby peach potato aphids (Myzus persicae) feeding on RSs1-GNAtransgenic tobacco plants. This provided direct evidence thatGNA was not only expressed in the phloem tissue, but was alsopresent in the phloem sap of transgenic tobacco plants. TheRSs1 promoter can thus be used to direct expression of an insecticidalprotein, such as GNA, in transgenic plants to control phloemsap-feeding insect pests. Key words: Rice sucrose synthase-1 promoter, phloemspecific, transgenic plants, ß-glucuronidase, Galanthus nivalis agglutinin, gene expression     

Longitudinal effects of herbivory on lotic periphyton assemblages

1. The longitudinal effects of herbivory on stream periphyton assemblages were examined in laboratory stream channels, each of which consisted of an upstream chamber, which either contained snail grazers or not, and downstream chambers, none of which contained grazers. Periphyton assemblages of two ages (0–21 days old and 21–42 days old) were sampled in both upstream and downstream chambers to detect proximate (i.e. localized) and longitudinal (i.e. downstream) effects of herbivory. 2. Both proximate and longitudinal effects were detected, although they differed in their impact on the periphyton assemblage. Periphyton biomass and cell accumulation were lower in grazed than in ungrazed upstream chambers throughout the experimental period. Accumulation rates on initially bare tiles were substantially higher downstream of grazed than of ungrazed chambers, but grazing had no effect on cell densities in established (21–42 day old) assemblages downstream. 3. Longitudinal effects of herbivory were not due to quantitative differences in the flux of propagules or nutrients from grazed and ungrazed chambers. Although not tested in this study, it is hypothesized that differences in the physiological condition of exported propagules may have contributed to differences in downstream colonization rates in grazed and ungrazed streams. 4. The magnitude of longitudinal impacts of herbivory and the importance of different causal mechanisms are predicted to vary depending on the standing crop and productive capacity of the periphyton assemblages as well as the consumptive demand of the herbivore guild.     

Effects of the antifeedant polygodial on plant penetration by aphids, assessed by video and electrical recording

Adult apterousMyzus persicae (Sulz.) discriminated within 2 min between mature Chinese cabbage (Brassica pekinensis L.) leaf halves treated with (±)-polygodial solution and solvent alone, and walked off leaf areas treated with polygodial faster than off the solvent-treated areas. However, when aphids were attached to a fine gold wire and stylet penetration of cabbage leaves was recorded electrically, polygodial treatment did not affect the number or duration of electrically-recorded penetrations, time taken to initiate a first penetration, or total penetration time. As the tethering of aphids causes behavioural restrictions which may negate the response to polygodial, indications of stylet penetration by freely-moving insects were sought. Simultaneous electrical recording and video monitoring showed that stylet penetration duration could be accurately inferred from antennal and body movements, enabling assessment of penetration times without attachment to the wire tether. When freely-movingM. persicae were video recorded during 15 min access to cabbage seedlings, polygodial treatment again had no apparent effect on stylet penetration. However, when aphids were presented with a choice of polygodial- and solvent-treated sides of floating mature cabbage leaf discs, video recordings revealed that the insects spent less time and made fewer penetrations on the polygodial-treated side. In addition to this rapid repellent effect, prolonged exposure to polygodial also produced behavioural changes. After being held for 24 h on polygodial-treated leaves or green paper prior to behavioural examination, aphids penetrated seedlings fewer times but for longer periods. The relevance of the results to virus transmission studies is discussed.