A feeling for the micro-organism: structure on a small scale. Biofilms on plant roots

Biofilms are structured communities of bacterial cells enclosed in a self-produced polymeric matrix and adherent to an inert or living surface; they have clinical, industrial and environmental impacts. Biofilms that are established by bacteria on plants are found on the surfaces of roots, leaves, seeds and internal vascular tissues where the microbes live in commensal, mutualistic or parasitic/pathogenic associations with their host. The study of the structure of plant-associated biofilms has been considerably helped by the development of techniques using fluorescent markers coupled with confocal scanning laser microscopy as well as scanning electron microscopy. We review several of these techniques as well as some of the research that has dealt with plant-associated biofilms. Our investigations focus on biofilm formation in the early stages of the Rhizobium –legume symbiosis, in which Gram-negative rhizobia provide fixed nitrogen to a host legume, and in return, the legume provides carbon-containing molecules. Because root colonization is an important early step in the establishment of the nitrogen-fixing symbiosis, we looked at Sinorhizobium meliloti attachment and biofilm establishment on the roots of its legume hosts, Medicago sativa L. and Melilotus alba Desr. We also examined biofilm formation by Rhizobium leguminosarum bv. viciae on the roots of Arabidopsis thaliana (L.) Heynh., a non-legume and non-host. Our ultimate goal is to characterize the rhizobial genes involved in aggregation and attachment to roots because several of these appear to be shared in biofilm formation and rhizobial entry of legume root cells.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 150 , 79–88.     

A Rhetorical and Sociolinguistic Model for the Analysis of Narrative

In this paper, the rhetorical theory of Kenneth Burke and the sociolinguistic method of William Labov, are synthesized to derive a formal and functional analytical framework for the study of narrative. The resulting framework provides for the identification of narrative style and performance in any sub-genre of narrative.     

Trophallaxis by temporal subcastes in the fire ant, Solenopsis invicta, in response to honey

ABSTRACT. Trophallactic behaviour of temporal subcastes in Solenopsis invicta Buren colonies was studied in two separate experiments. First, radiolabeled honey was presented to small colonies containing larvae, queens and marked workers; radioactivity in the head, thorax and abdomen of each ant was measured after 0.25, 1, 6, 12, 24, 48 and 72 h. The quantity of honey per subcaste was dependent on the total amount of honey collected. As more honey entered the colony, the quantity in foragers, queens and reserves increased more rapidly than that in nurses or larvae. The internal distribution of radioactivity in the head, thorax or abdomen indicated that differences between subcaste members in either the rate of food exchange or digestion existed. In a second experiment behavioural observations were made on marked workers in colonies before and after the presentation of honey. Reserves spent significantly more time engaged in trophallaxis than either foragers or nurses, both as active donors and active receivers. Based on number of encounters, nurses were more likely to receive honey while reserves and foragers were more likely to donate honey. The duration of encounters involving nurses tended to be shorter than those involving reserves or foragers.     

Distribution and feeding behaviour of field populations of the water cricket Velio, caprai (Hemiptera)

1. The distribution of individuals, prey capture success and prey choice were studied in a water cricket (Velia caprai) population in southern Ireland. Groups of different sizes were observed in the field during the summer and all potential prey animals entering the groups were counted. Surface drift samples were collected to assess the proportion of profitable prey items. 2. Two types of Velia groups were observed: mixed groups (adults and juvenile instars 4 and 5) and juvenile groups (instars 1–3), A greater proportion of the available prey was captured by the mixed groups. 3. Individuals in large groups had a lower capture rate than individuals in small groups. 4. No differences in capture rates could be found between males, females and juveniles, but differences in prey choice were apparent. Females captured more large prey than expected. 5. Water cricket distribution in the stream did not follow the ‘ideal free’ model; too few individuals were found at the most profitable sites. Food might not be the limiting factor during summer, and other factors such as intraspecific interference and predator avoidance could be more important.     

Plant sugars are crucial players in the oxidative challenge during abiotic stress: extending the traditional concept

Plants suffering from abiotic stress are commonly facing an enhanced accumulation of reactive oxygen species (ROS) with damaging as well as signalling effects at organellar and cellular levels. The outcome of an environmental challenge highly depends on the delicate balance between ROS production and scavenging by both enzymatic and metabolic antioxidants. However, this traditional classification is in need of renewal and reform, as it is becoming increasingly clear that soluble sugars such as disaccharides, raffinose family oligosaccharides and fructans – next to their associated metabolic enzymes – are strongly related to stress‐induced ROS accumulation in plants. Therefore, this review aims at extending the current concept of antioxidants functioning during abiotic stress, with special focus on the emanate role of sugars as true ROS scavengers. Examples are given based on their cellular location, as different organelles seem to exploit distinct mechanisms. Moreover, the vacuole comes into the picture as important player in the ROS signalling network of plants. Elucidating the interplay between the mechanisms controlling ROS signalling during abiotic stress will facilitate the development of strategies to enhance crop tolerance to stressful environmental conditions.     

Stoneflies as ecological engineers – hungry predators reduce fine sediments in stream beds

1. We conducted experiments in a Colorado Rocky Mountain stream to measure the effect of foraging by predatory stoneflies ( Megarcys signata ) on fine sediment deposition and removal.
2. Cages containing one of four treatments were placed in the stream substratum and removed 3 days after fine sediment load to the stream was experimentally increased. Cages containing a stonefly but no prey accumulated less sediment than cages initially with no benthic invertebrates. Although cages with Megarcys plus prey also tended to have less sediment than controls, sediments were not reduced in cages with prey only.
3. Distance from sediment source, depth and current velocity at cages, final prey density and numbers of prey per predator gut at the end of the experiment had no effects on sediment accumulation in cages.
4. To determine the mechanisms underlying this effect, behavioural tests were conducted in a stream-side artificial stream system. To simulate the different hunger levels of stoneflies in cages, Megarcys were fed or starved for 3 days before behavioural trials that were repeated during high (night) and low (day) stonefly feeding periods. During night trials, foraging movements by starved Megarcys were more prolonged and active than those of fed stoneflies, regardless of the presence of prey. However, neither hunger level nor prey presence affected stonefly foraging behaviour during day trials.
5. Results of field experiments suggest that the presence of stoneflies enhances sediment removal from interstitial spaces. Behavioural observations indicate that nocturnal searching for prey by hungry Megarcys dislodges fine sediments from interstitial spaces.
6. Future studies should explore beyond the direct impacts of predators on stream invertebrate communities, and investigate the potential impact of predators on benthic microhabitat quality.     

Competition with Egyptian Geese Alopochen aegyptiaca overrides environmental factors in determining productivity of Black Sparrowhawks Accipiter melanoleucus

We studied a population of Black Sparrowhawks Accipiter melanoleucus that has recently colonized the Cape Peninsula, South Africa. Black Sparrowhawks are winter breeders: over most of their range winters are dry, but the Cape Peninsula has a Mediterranean climate, with most rain falling in winter. The birds bred at an average density of 38 pairs/100 km². Monitoring of 70 breeding attempts by 26 pairs showed that the birds favoured large stands of trees for breeding. Birds breeding in smaller stands were more randomly spaced, dispersion being determined by habitat availability. Parameters related to climate, habitat and aspects of nest-site selection failed to predict the birds' productivity despite the fact that they had recently colonized a novel climatic window. Rather, productivity was best explained by the probability of nest usurpation by highly aggressive Egyptian Geese Alopochen aegyptiaca . Pairs encountering Goose usurpation problems raised an average of 0.76 chicks per breeding attempt, compared with 1.58 chicks for pairs without Geese. The only defence against nest usurpation by Geese appears to be the construction of multiple nests, an option most available to and most exploited by birds breeding in large stands of trees.     

Ecological Studies of Seaweeds in McMurdo Sound, Antarctica

Three species of benthic marine macroalgae comprise the chiefcomponents of the seaweed flora of McMurdo Sound, Antarctica.Quantitative studies at Cape Evans demonstrate a depth-relateddistribution pattern, with Iridaea cordata (Turner) Bory inshallow water, Phyllophora antarctica A. and E. S. Gepp abundantand fertile at intermediate depths, and Leptophytum coulmanicum(Foslie) Adey dominant below 20 m. The vertical distributionof species is correlated with irradiance levels. At sites withthinner annual sea ice and less snow accumulation (e.g., CapeEvans, Cape Royds, and Granite Harbor), vertical distributionsare shifted downward relative to those at sites that remaincovered most of the year with thick or snow-covered fast ice(e.g., Cape Armitage and New Harbor). Disturbance caused byice scour and anchor ice probably determines the upper limitof algal distribution; herbivory is apparently absent. There is a disproportionate representation of cystocarpic femalegametophytes in populations of I. cordata and P. antarctica.Perennation via persistent basal crusts and apogamic recyclingof gametophytes are suggested as factors structuring phase distribution.Although the overall size of I. cordata blades is greater (andcystocarpic plants more abundant) at Cape Royds than at CapeEvans, cystocarpic blades are similar in minimum size at bothsites, indicating a threshhold size or age for the onset ofreproduction in this species.     

Purinoceptors in blood feeding behaviour in the stable fly, Stomoxys calcitrans

Abstract Both sexes of the stable fly, Stomoxys calcitrans L. (Diptera: Muscidae), have receptors on their mouthparts that mediate blood feeding. The potency ranking of the adenine nucleotides (ATP > ADP > AMP > adenosine) in eliciting feeding and suppressing the NaCl-sensitive cell may indicate the involvement of a P₂-type receptor. This is supported by the lack of effect on feeding by methyl xanthines. Feeding-related behavioural and electrophysiological results demonstrate that the potency of CH₃-S-ATP is not greater than that of ATP. These ATP-mediated responses are antagonized by ANAPP₃. Results support the conclusion that the putative 'ATP receptor' involved in stable fly phagostimulation resembles the P_2x purinoceptor of vertebrates.