Phagocytosis is coupled to the formation of phagosome-associated podosomes and a transient disruption of podosomes in human macrophages

Phagocytosis consists in ingestion and digestion of large particles, a process strictly dependent on actin re-organization. Using synchronized phagocytosis of IgG-coated latex beads (IgG-LB), zymosan or serum opsonized-zymosan, we report the formation of actin structures on both phagocytic cups and closed phagosomes in human macrophages. Their lifespan, size, protein composition and organization are similar to podosomes. Thus, we called these actin structures phagosome-associated podosomes (PAPs). Concomitantly to the formation of PAPs, a transient disruption of podosomes occurred at the ventral face of macrophages. Similarly to podosomes, which are targeted by vesicles containing proteases, the presence of PAPs correlated with the maturation of phagosomes into phagolysosomes. The ingestion of LB without IgG did not trigger PAPs formation, did not lead to podosome disruption and maturation to phagolysosomes, suggesting that these events are linked together. Although similar to podosomes, we found that PAPs differed by being resistant to the Arp2/3 inhibitor CK666. Thus, we describe a podosome subtype which forms on phagosomes where it probably serves several tasks of this multifunctional structure.     

Shoot Feeding as a Nutrient Acquisition Strategy in Free-Living Psylloids

Shoot feeding by sucking insects is accepted as an adaptation to feeding where plant nutrients are most concentrated and/or of higher quality. Psylloids are an important hemipteran taxon, most of which are free-living and comprise many shoot feeding species, whose nutritional ecology has been largely ignored. I conducted a longitudinal study of Ctenarytaina eucalypti (Maskell) and C. bipartita Burckhardt et al. (Aphalaridae) feeding on eucalypts to document how within-plant (ontogenic) variation in nutritional quality, in particular of free amino acids, determines host suitability and hence the distribution and abundance of nymphs. Nymphs were most abundant within developing apical buds but were not more abundant on branchlets of greater vigour (indicated by rate of extension). Nymphs could be found up to two (C. bipartita) to three (C. eucalypti) alternate leaf pairs distant from apical buds but infrequently and in low numbers; they were never found on older, fully expanded leaves. The position of a leaf on a branchlet (indicative of age) determined its nutritional quality. Younger leaves had higher water contents, lower chlorophyll contents and differed in amino acid (essential and non-essential) composition compared to older leaves. The abundance of C. eucalypti nymphs on expanding leaves and in buds was positively correlated with the concentrations of methionine, valine and threonine in E. globulus leaves at the same or comparable position on a branchlet. The abundance of C. bipartita nymphs was positively correlated with foliar leucine concentrations. Shoot feeding by these two psyllids facilitates access to more concentrated, better quality plant nutrients but may not entirely explain the adaptive significance of their behaviour. The humid microclimate created by the architecture of the hosts’ apical buds protects eggs and nymphs from desiccation and is suggested to have had a significant influence on the evolution of host utilisation strategies of psyllids within this genus.     

Changes in Clonal Poplar Leaf Chemistry Caused by Stem Galls Alter Herbivory and Leaf Litter Decomposition

Gall-inducing insects are highly specialized herbivores that modify the phenotype of their host plants. Beyond the direct manipulation of plant morphology and physiology in the immediate environment of the gall, there is also evidence of plant-mediated effects of gall-inducing insects on other species of the assemblages and ecosystem processes associated with the host plant. We analysed the impact of gall infestation by the aphid Pemphigus spirothecae on chemical leaf traits of clonal Lombardy poplars (Populus nigra var. italica) and the subsequent effects on intensity of herbivory and decomposition of leaves across five sites. We measured the herbivory of two feeding guilds: leaf-chewing insects that feed on the blade (e.g. caterpillars and sawfly larvae) and skeletonising insects that feed on the mesophyll of the leaves (e.g. larvae of beetles). Galled leaves had higher phenol (35%) and lower nitrogen and cholorophyll contents (35% respectively 37%) than non-galled leaves, and these differences were stronger in August than in June. Total herbivory intensity was 27% higher on galled than on non-galled leaves; damage by leaf chewers was on average 61% higher on gall infested leaves, whereas damage by skeletonising insects was on average 39% higher on non-galled leaves. After nine months the decomposition rate of galled leaf litter was 15% lower than that of non-galled leaf litter presumably because of the lower nitrogen content of the galled leaf litter. This indicated after-life effects of gall infestation on the decomposers. We found no evidence for galling x environment interactions.     

Effects of ambient temperature on avian incubation behavior

Ambient temperature is commonly thought to influence avian incubation behavior. However, results of empirical studies examining correlationsbetween ambient temperature and bout duration are equivocal.We propose that these equivocal results can be partly explainedby developing a conceptual understanding of how we should expecttemperature to influence incubation. We demonstrate why linearcorrelation analyses across a wide range of temperatures canbe inappropriate based on development of an incubation model for small birds that incorporates how ambient temperature influencesboth embryonic development and adult metabolism. We found supportfor predictions of the model using incubation data from orange-crownedwarblers (Vermivora celata) in Arizona. Both off- and on-boutduration were positively correlated with ambient temperaturebetween 9° and 26°C, but unrelated to ambient temperature<9° and 26-40°C. Bout durations declined as ambienttemperature approached or exceeded 40°C. Incubating orange-crowned warblers appeared to avoid bouts off the nest <7 min andbouts on the nest <20 min. Time of day, duration of theprevious bout, and variation among nests all explained variationin both on- and off-bout duration. Although we found supportfor the general shape of the incubation model, temperature still explained only a small portion of the overall variation in on-and off-bout duration. Results of previous studies were generallyconsistent with the model for off-bout duration; most studiesin colder environments reported positive correlations withtemperature, and the one negative correlation reported was from a hot environment. However, the relationships between on-boutduration and temperature reported in previous studies wereless consistent with our model and our data. Although somediscrepancies could be explained by considering our model,some studies reported negative correlations in cold environments.The effect of ambient temperature on duration of on-bouts probablydiffers among species based on the amount of fat reserves females typically carry during incubation and the extent of male incubationfeeding. Additional studies of the effects of temperature onavian incubation will help improve the general model and ultimatelyaid our understanding of energetic and ecological constraintson avian incubation.     

Identification of the ubiquinone-binding domain in the disulfide catalyst disulfide bond protein B.

Disulfide bond (Dsb) formation is catalyzed in the periplasm of prokaryotes by the Dsb proteins. DsbB, a key enzyme in this process, generates disulfides de novo by using the oxidizing power of quinones. To explore the mechanism of this newly described enzymatic activity, we decided to study the ubiquinone-protein interaction and identify the ubiquinone-binding domain in DsbB by cross-linking to photoactivatable quinone analogues. When purified Escherichia coli DsbB was incubated with an azidoubiquinone derivative, 3-azido-2-methyl-5-[(3)H]methoxy-6-decyl-1,4-benzoquinone ([(3)H]azido-Q), and illuminated with long wavelength UV light, the decrease in enzymatic activity correlated with the amount of 3-azido-2-methyl-5-methoxy-6-decyl-1,4-benzoquinone (azido-Q) incorporated into the protein. One azido-Q-linked peptide with a retention time of 33.5 min was obtained by high performance liquid chromatography of the V8 digest of [(3)H]azido-Q-labeled DsbB. This peptide has a partial NH(2)-terminal amino acid sequence of NH(2)-HTMLQLY corresponding to residues 91-97. This sequence occurs in the second periplasmic domain of the inner membrane protein DsbB in a loop connecting transmembrane helices 3 and 4. We propose that the quinone-binding site is within or very near to this sequence.     

Cross-pathogenicity of Rhizoctonia solani strains on pasture legumes in pasture-crop rotations

In Australia, in the past, pasture legumes were rotated mainly with cereals, but increasingly these rotations now involve pasture legumes with a wider range of crops, including legumes. This increasing frequency of the leguminous host in the rotation system may be associated with increased root rots in legumes in the current pasture-crop rotations. The primary aim of this study was to see whether the pathogenicity on pasture legumes of strains of Rhizoctonia solani sourced from lupins and cereals (common crops in rotation with pastures) is associated with increased incidence of root rots in pasture legumes in the disease conducive sandy soils of the Mediterranean regions of southern Australia. The second aim was to determine sources of resistance among newly introduced pasture legumes to R. solani strains originating from rotational crops as this would reduce the impact of disease in the pasture phase. Fifteen pasture legume genotypes were assessed for their resistance/susceptibility to five different zymogram groups (ZG) of the root rot pathogen R. solani under glasshouse conditions. Of the R. solani groups tested, ZG1–5 and ZG1–4 (both known to be pathogenic on cereals and legumes) overall, caused the most severe root disease across the genotypes tested, significantly more than ZG6 (known to be pathogenic on legumes), in turn significantly >ZG4 (known to be pathogenic on legumes) which in turn was >ZG11 (known to be pathogenic on legumes including tropical species). Overall, Ornithopus sativus Brot. cvs Cadiz and Margurita, Trifolium michelianum Savi. cvs Paradana and Frontier and T. purpureum Loisel. cv. Electro showed a significant level of resistance to root rot caused by R. solani ZG11 (root disease scores ≤1.2 on a 1–3 scale where 3 = maximum disease severity) while O. sativus cvs Cadiz and Erica showed a significant level of resistance to root rot caused by R. solani ZG4 (scores ≤1.2). O. compressus L. cvs Charano and Frontier, O. sativus cv. Erica, and T. purpureum cv. Electro showed some useful resistance to root rot caused by R. solani ZG6 (scores ≤1.8). This is the first time that cvs Cadiz, Electro, Frontier, Margurita and Paradana have been recognised for their levels of resistance to root rot caused by R. solani ZG11; and similarly for cvs Cadiz and Erica against ZG4; and for cvs Charano, Erica, and Electro against ZG6. These genotypes with resistance may also serve as useful sources of resistance in pasture legume breeding programs and also could potentially be exploited directly into areas where other rotation crops are affected by these R. solani strains. None of the tested genotypes showed useful resistance to R. solani ZG1–4 (scores ≥2.0) or ZG1–5 (scores ≥2.5). This study demonstrates the relative potential of the various R. solani ZG strains, and particularly ZG1–4, ZG1–5, ZG4 and ZG6 to attack legume pastures and pose a significant threat to non-pasture crop species susceptible to these strains grown in rotation with these pasture legumes. Significantly, the cross-pathogenicity of these strains could result in the continuous build-up of inoculum of these strains that may seriously affect the productivity eventually of legumes in all rotations. In particular, when choosing pasture legumes as rotation crops, caution needs to be exercised so that the cultivars deployed are those with the best resistance to the R. solani ZGs most likely to be prevalent at the location.