Tradeoffs between foliar silicon and carbon‐based defences: evidence from vegetation communities of contrasting soil types

Silica is ubiquitous in plants and can constitute up to 10% of plant dry mass, varying with phylogeny and soil silicon availability. Plant silicon is an important alleviator of abiotic (salinity, heavy metal, drought) and biotic (herbivore and fungal pathogen) stress. As well as playing an important role in reducing the impact of abiotic stresses, silicon may be an alternative to carbon‐based and other chemical defences. Knowledge of silicon function is predominantly derived from agricultural species and model systems. We investigated the abundance and role of plant silicon at a community level by comparing leaf silicon concentration with defence chemicals, carbon compound concentrations and invertebrate assemblages in vegetation communities from two different soil types with contrasting levels of plant available silicon. We found that the concentrations of silicon in the leaves did not reflect the silicon availability in the soil at a community level. The leaf silica concentration range in the vegetation communities was comparable to other diverse communities reported in the literature, suggesting that the species rather than the environment determine leaf silica concentration. Across sites, leaf silica concentration was significantly negatively correlated with concentrations of carbon, total phenols and weakly with tannins but not with other measured defence compounds. Leaf silica concentration was also negatively correlated with Coleoptera abundance, but not the abundance of any other invertebrate groups measured. Our results suggest that tradeoffs exist between phenolic‐ and tannin‐based defences and provide evidence that leaf silicification may be a more effective defence against some chewing herbivore groups than others.     

Plant-soil feedbacks do not explain invasion success of Acacia species in introduced range populations in Australia

Legumes, especially acacias, are considered amongst the most successful invaders globally. However there is still very little known about the role of soil microbial communities in their invasion success in novel ranges. We examined the role of the soil microbial community in the invasion success of four Acacia species (A. cyclops, A. longifolia, A. melanoxylon and A. saligna) and a close relative Paraserianthes lophantha, introduced into novel regions within Australia using a “black-box” approach. Seed and soil material were collected from multiple populations within each species’ native and introduced range within Australia and used in a plant-soil feedback experiment to assess the effect of the soil microbial community on plant growth and nodulation. We found no effect, either positive or negative, of soil origin on species’ performance, however there was a significant interaction between species and seed origin. Seed origin had a significant effect on the biomass of two species, A. cyclops and A. saligna. A. cyclops plants from the native range performed better across all soils than plants from the introduced range. The opposite trend was observed for A. saligna, with plants from the introduced range performing better overall than plants from the native range. Seed or soil origin did not have a significant effect on the presence and number of nodules suggesting that rhizobia do not constrain the invasion success of these legumes. Our results suggest that plant-soil feedbacks are unlikely to have played a significant role in the invasion success of these five species introduced into novel regions within Australia. This may be due to the widespread occurrence of acacias and their associated soil microbial communities throughout the Australian continent.     

An Anti-Human ICAM-1 Antibody Inhibits Rhinovirus-Induced Exacerbations of Lung Inflammation

Human rhinoviruses (HRV) cause the majority of common colds and acute exacerbations of asthma and chronic obstructive pulmonary disease (COPD). Effective therapies are urgently needed, but no licensed treatments or vaccines currently exist. Of the 100 identified serotypes, ∼90% bind domain 1 of human intercellular adhesion molecule-1 (ICAM-1) as their cellular receptor, making this an attractive target for development of therapies; however, ICAM-1 domain 1 is also required for host defence and regulation of cell trafficking, principally via its major ligand LFA-1. Using a mouse anti-human ICAM-1 antibody (14C11) that specifically binds domain 1 of human ICAM-1, we show that 14C11 administered topically or systemically prevented entry of two major groups of rhinoviruses, HRV16 and HRV14, and reduced cellular inflammation, pro-inflammatory cytokine induction and virus load in vivo. 14C11 also reduced cellular inflammation and Th2 cytokine/chemokine production in a model of major group HRV-induced asthma exacerbation. Interestingly, 14C11 did not prevent cell adhesion via human ICAM-1/LFA-1 interactions in vitro, suggesting the epitope targeted by 14C11 was specific for viral entry. Thus a human ICAM-1 domain-1-specific antibody can prevent major group HRV entry and induction of airway inflammation in vivo.     

Native and exotic invasive plants have fundamentally similar carbon capture strategies

1.  Leaf trait relationships of native and exotic invasive species from a range of habitats were compared to assess consistency across habitats and the role of disturbance.
2.  One hundred and twenty-two native and exotic species were sampled in five habitats in eastern Australia. Specific leaf area, foliar nitrogen ( N _mass), assimilation rate ( A _mass) and dark respiration ( R _mass) were measured for each species. Plants were classified into four types: native undisturbed, native disturbed, exotic invasive undisturbed and exotic invasive disturbed.
3.  All traits were positively correlated and slopes were homogeneous within habitats. Significant differences between plant types in slope elevation were found in only two of 18 cases. There were significant shifts in group means along a common slope between plant types within habitats. These shifts were associated with disturbed vs. undisturbed areas, with plant types from disturbed areas having higher trait values.
4.   Synthesis . Exotic invasive and native species do not have fundamentally different carbon capture strategies. The carbon capture strategy of a species is strongly associated with disturbance, with species from disturbed sites having traits that confer capacity for fast growth. Thus, differences between exotic invasives and natives may reflect differences in the environmental conditions of the sites where they occur rather than differences between exotic invasives and natives per se .     

Landscape-scale detection and mapping of invasive African Olive (Olea europaea L. ssp. cuspidata Wall ex G. Don Ciferri) in SW Sydney,Australia using satellite remote sensing

Question: Is satellite imagery an effective tool for mapping and examining the distribution of the invasive species Olea europaea L. ssp. cuspidata at a regional landscape scale? Location: Southwest Sydney, Australia. Methods: Remote sensing software was used to classify pixels of Olea europaea L. ssp. cuspidata (African Olive) and major vegetation types from satellite imagery, using a “supervised classification” technique across a 721 km² study area in the Cumberland Plain region of western Sydney. A map of African Olive distribution was produced from the image analysis and checked for accuracy at 337 random locations using ground observation and comparison with existing vegetation maps. The African Olive distribution data were then used in a GIS analysis with additional spatial datasets to investigate the relationship between the distribution of African Olive and environmental factors, and to quantify the conservation threat to endangered native vegetation. Results: A total area of 1907 ha of dense African Olive infestation was identified, with an omission error of 7.5% and a commission error of 5.4%. African Olive was found to occur on the steepest slopes (mean slope 14.3°) of the vegetation classes examined, with aspect analysis identifying a high prevalence on south- and southwest-facing slopes. The analysis also quantified the level of African Olive infestation in endangered ecological communities, with Western Sydney Dry Rainforest (25% affected) and Moist Shale Woodland (28% affected) identified as most vulnerable to African Olive invasion. Conclusion: The distribution of African Olive can be efficiently mapped at a landscape scale. This technique, used in association with additional spatial datasets, identified African Olive as a significant environmental weed in SW Sydney, occupying a greater area than previously recognised and threatening several endangered native vegetation communities.     

Th1 and Th2 CD4+ T cells provide help for B cell clonal expansion and antibody synthesis in a similar manner in vivo

The relative ability of Th1 and Th2 T cells to help B cells remains controversial as do the mechanisms by which both T cell subsets provide help in vivo. Whether this help affects the clonal expansion and/or differentiation of B cells has been difficult to assess due to the low frequency of Ag-specific T and B lymphocytes. We have employed a novel technique to directly monitor the clonal expansion of Ag-specific T and B lymphocytes in vivo. OVA-specific TCR transgenic T lymphocytes were polarized toward a Th1 or Th2 phenotype in vitro. These cells were then transferred into syngeneic recipients, along with B cell receptor transgenic hen egg lysozyme-specific B lymphocytes. Our results indicate that Th1 and Th2 cells support B cell responses to a similar extent in vivo and that they achieve this in the same manner by migrating into B cell follicles to promote CD154-dependent B cell clonal expansion and Ab production.     

Larger seeds in tropical floras: consistent patterns independent of growth form and dispersal mode

Many species of tropical moist forests have large seeds compared to those found in temperate floras. This could be attributed to a prevalence of woody growth forms, or adaptation to dispersal by vertebrates, or the dense shade of a closed canopy, rather than to an intrinsic tropical v. temperate difference. We compared tropical v. temperate seed mass data at two geographic scales; firstly within Australia between tropical, subtropical and temperate open woodlands, then between six tropical and temperate datasets from five continents. Within Australia we found that seed mass increased with decreasing latitude in ten out of eleven growth form-dispersal mode combinations: only wind-dispersed graminoids showed no significant effect. While the pattern of generally larger seeds in the tropical flora was associated with tropical families tending to have larger seeds than temperate families, we also found that tropical species had larger seeds than temperate cofamilials. As a preliminary test of the generality of these findings we included a further four tropical and temperate data sets in a second analysis. Average tropical seed masses were larger than temperate seed masses in all testable growth form-dispersal mode combinations, and statistically larger in five out of seven cases: unassisted, vertebrate- and wind-dispersed woody species, and unassisted and vertebrate-dispersed forbs. No difference was found for wind-dispersed forbs or for graminoids lacking dispersal structures. That an effect was found in these combinations rules out explanations based solely on characteristics of tropical v. temperate dispersers, although this may be a contributing factor. Instead we suggest that some aspect of tropical systems, for example higher metabolic costs of seedling production, has selected for a general increase in seed mass among tropical taxa.     

Seed size and shape are not related to persistence in soil in Australia in the same way as in Britain

1. Previous studies have shown that among British species, seeds that persist in the soil tend to be small and compact compared with non-persistent seeds. We tested whether or not this pattern is repeated among 101 Australian species, from a range of habitats.
2. Seed mass was plotted against variance of seed dimensions, across all species. Species with persistent seeds were found across the whole range of seed mass (0·217–648·9 mg) and variance (0·0000–0·2497), providing no evidence for a critical mass or variance which separated persistent from transient seeds.
3. We tested whether or not divergence within individual clades between persistent and transient seeds was associated with increased seed mass or seed dimension variance, using phylogenetically independent contrasts (PICs). There was no consistent tendency found.
4. Thus for Australian species, persistent seeds were not smaller or more compact than transient seeds when compared across all species or when compared using PICs. Presumably the natural history of burial and disturbance operates differently in British and Australian habitats.     

Have your cake and eat it too: greater dispersal ability and faster germination towards range edges of an invasive plant species in eastern Australia

The process of range expansion often selects for traits that maximize invasion success at range edges. For example, during range expansion, individuals with greater dispersal and colonization ability will be selected for towards range edges. For wind dispersed plants, however, there exists a fundamental trade-off between dispersal and colonization ability (germination success and growth) that is mediated by seed size; smaller seeds often have greater dispersal ability but poorer colonization ability. We investigated the nature of the dispersal/colonization trade-off by comparing dispersal ability (wing loading ratio: seed mass/wing area), germination success and growth related traits across multiple populations of a coastal exotic invasive plant species (Gladiolus gueinzii Kunze) along its entire introduced distribution in eastern Australia. We found that G. gueinzii had significantly greater dispersal ability towards its range edges which was mediated by a decrease in seed mass. However, this was not associated with a decrease in probability of germination or growth after 3 months. In fact, seeds from range edge populations had significantly faster germination times. Our results suggest that a shift towards greater dispersal ability does not have an associated negative effect on the colonization ability of G. gueinzii and may be a key factor in promoting further range expansion of this exotic invasive species.