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This work investigated if the assessment of tolerance to trace metals can depend on plant density in the experimental design. A non-metallicolous and a metallicolous populations of Silene paradoxa were hydroponically cultivated at increasing density and in both the absence (−Cu conditions) and excess of copper (+Cu conditions). In −Cu conditions, the metallicolous population showed a lower susceptibility to plant density in comparison to the non-metallicolous one, explained by a higher capacity of the metallicolous population to exploit resources. In +Cu conditions, an alleviating effect of increasing density was found in roots. Such effect was present to a greater extent in the non-metallicolous population, thus making the populations equally copper-tolerant at the highest density used. In shoots, an additive effect of increasing plant density to copper toxicity was reported. Its higher intensity in the metallicolous population reverted the copper tolerance rela- tionship at the highest plant densities used. In both populations, a density-induced decrease in root copper accumulation was observed, thus concurring to the reported mitigation in +Cu conditions. Our work revealed the importance of density studies on the optimization of eco-toxicological bio-assays and of metal tolerance assessment and it can be considered the first example of an alleviating effect of increasing plant number on copper stress in a metallophyte.

Metal contaminated soils are increasing worldwide. Metal-tolerant plants growing on metalliferous soils are fascinating genetic and microbial resources. Seeds can vertically transmit endophytic microorganisms that can assist next generations to cope with environmental stresses, through yet poorly understood mechanisms. The aims of this study were to identify the core seed endophyte microbiome of the pioneer metallophyte Crotalaria pumila throughout three generations, and to better understand the plant colonisation of the seed endophyte Methylobacterium sp. Cp3. Strain Cp3 was detected in C. pumila seeds across three successive generations and showed the most dominant community member. When inoculated in the soil at the time of flowering, strain Cp3 migrated from soil to seeds. Using confocal microscopy, Cp3-mCherry was demonstrated to colonise the root cortex cells and xylem vessels of the stem under metal stress. Moreover, strain Cp3 showed genetic and in planta potential to promote seed germination and seedling development. We revealed, for the first time, that the seed microbiome of a pioneer plant growing in its natural environment, and the colonisation behaviour of an important plant growth promoting systemic seed endophyte. Future characterization of seed microbiota will lead to a better understanding of their functional contribution and the potential use for seed-fortification applications. This research was supported by a BOF-BILA grant from Hasselt University, the UHasselt Methusalem project 08M03VGRJ

Campo rupestre is a kind of Brazilian rocky outcrop with high biodiversity and many endemic and threatened species. It occurs mainly in the Espinhaco Range in a contact region between Cerrado and/or Caatinga and Atlantic Forest. The Espinhaco Range is recognized as a region with the highest floristic diversity in South America and with many endemic species, most of which are associated with rocky outcrop environments. These, among other peculiarities, recently granted the Espinhaco Range the status of Biosphere Reserve. The relationship between soil and vegetation was studied in campo rupestre areas with quartzite and itabirite rocks. Three habitats in both lithologies were defined by geomorphology as: 1. Slopes with grasslands; 2. Plateaus with grasslands and 3. Valleys with woody savannas. In each lithology, 30 plots (10 × 10 m), 10 in each habitat, were defined. The species and their respective coverage were recorded and soil was sampled to perform chemical and physical analyses. The analyzed soils were similar in being sandy, acidic and with low fertility. Nevertheless, they exhibited differences in chemical and physical properties. Altogether there were 272 species, belonging to 70 families. The canonical correspondence analysis of soil variables and species coverage showed a clear segregation of lithological sites due mainly to the exchangeable content of Ca, Cu, Mg, Mn and S; soil particle size – central tendency and sorting; and the percentage of silt, fine soil and bare rocks. A strong correlation between plant species coverage and soil properties was also found.

Ferruginous "campos rupestres" are a particular type of vegetation growing on iron-rich primary soils. We investigated the influence of soil properties on plant species abundance at two sites of ferruginous "campos rupestres" and one site of quartzitic "campo rupestre", all of them in "Quadrilátero Ferrífero", in Minas Gerais State, southeastern Brazil. In each site, 30 quadrats were sampled to assess plant species composition and abundance, and soil samples were taken to perform chemical and physical analyses. The analyzed soils are strongly acidic and presented low fertility and high levels of metallic cations; a principal component analysis of soil data showed a clear segregation among sites due mainly to fertility and heavy metals content, especially Cu, Zn, and Pb. The canonical correspondence analysis indicated a strong correlation between plant species abundance and soil properties, also segregating the sites. Os campos ferruginosos são um tipo particular de vegetação que cresce sobre solos primários ricos em ferro. Estudou-se a influência do solo sobre a abundância em espécies vegetais em duas áreas de campos ferruginosos e uma área de campo rupestre quartzítico, todos no Quadrilátero Ferrífero, em Minas Gerais, sudeste do Brasil. Em cada área foram amostradas 30 parcelas para avaliar a composição florística e a abundância, e analisar o solo. Os solos das áreas estudadas são fortemente ácidos, com baixa fertilidade e altos níveis de cátions metálicos, e mostraram clara separação na análise de componentes principais, devido principalmente à fertilidade e ao teor de metais pesados, especialmente Cu, Zn e Pb. A análise de correspondência canônica indicou uma forte correlação entre a abundância e as propriedades do solo, também separando as áreas.

It is hypothesized that metal hyperaccumulator plants have specific rhizosphere conditions, potentially modifying the bioavailability of soil metals. This article aims to further the knowledge about the rhizosphere of the hyperaccumulator Thlaspi caerulescens, focusing on its microflora isolated from metalliferous soils collected in situ where the plants grow naturally. We characterized the cultivable microbial communities isolated from the rhizosphere of one population of this Ni hyperaccumulator species grown on a serpentine soil. The rhizosphere soil harbored a wide variety of microorganisms, predominantly bacteria, confirming the stimulatory effect of the T. caerulescens rhizosphere on microbial growth and proliferation. We tested the hypothesis that the rhizosphere of T. caerulescens influences (1) the metabolic diversity of the bacterial community and (2) the bacterial resistance to metals. The principal component analysis of the Biolog plate's data confirmed a structural effect of the rhizosphere of T. caerulescens on bacterial communities. The percentage of Ni-resistant bacteria was higher in the rhizosphere than in the bulk soil, suggesting a direct effect of the rhizosphere on Ni tolerance, reflecting a greater bacterial tolerance to Ni in the rhizosphere.

The highly specialized flora of localities affected by former metal ore mining and metallurgy is endangered by succession or intentional afforestation all over Europe. Its last remnants therefore deserve our attention. We examined whether Scots pine encroaching on a heavy-metal grassland (Olkusz Zn-Pb mining area, S Poland) is outcompeting specialized herbaceous species, as has been observed elsewhere. Plant species composition and richness sampled at 124 plots were analysed in relation to pine stand parameters (canopy cover, stand age, stand basal area), abiotic environmental factors (e.g. soil properties) and spatial variables (e.g. plot coordinates). Plots were divided into three shading categories and compared in terms of vegetation and habitat parameters. Scots pine outcompeted several light-demanding species, leading to a decrease of total species richness and cover. Characteristic species of this grassland (Biscutella laevigata, Silene vulgaris) and some metal-tolerant plants were clearly insensitive to shading. For these early successional species, more important was the availability of microsites with shallow skeletal soil or bare subsoil. Tree stand parameters differently affected grassland vegetation: canopy cover caused primarily a compositional shift in the community, while stand age was the principal agent of decline in species richness. Scots pine increased the soil concentrations of available Ca and Mg, and negatively affected soil development (organic matter and mineral particle accumulation), which might be beneficial to some shade-tolerant grassland species. Maintaining the studied grassland’s present species richness and composition would require cutting woody plants less frequently than recommended for dry grasslands of non-metalliferous sites, and disturbing the soil surface.