Perspectives of plant-associated microbes in heavy metal phytoremediation

"Phytoremediation" know-how to do-how is rapidly expanding and is being commercialized by harnessing the phyto-microbial diversity. This technology employs biodiversity to remove/contain pollutants from the air, soil and water. In recent years, there has been a considerable knowledge explosion in understanding plant-microbes-heavy metals interactions. Novel applications of plant-associated microbes have opened up promising areas of research in the field of phytoremediation technology. Various metabolites (e.g., 1-aminocyclopropane-1-carboxylic acid deaminase, indole-3-acetic acid, siderophores, organic acids, etc.) produced by plant-associated microbes (e.g., plant growth promoting bacteria, mycorrhizae) have been proposed to be involved in many biogeochemical processes operating in the rhizosphere. The salient functions include nutrient acquisition, cell elongation, metal detoxification and alleviation of biotic/abiotic stress in plants. Rhizosphere microbes accelerate metal mobility, or immobilization. Plants and associated microbes release inorganic and organic compounds possessing acidifying, chelating and/or reductive power. These functions are implicated to play an essential role in plant metal uptake. Overall the plant-associated beneficial microbes enhance the efficiency of phytoremediation process directly by altering the metal accumulation in plant tissues and indirectly by promoting the shoot and root biomass production. The present work aims to provide a comprehensive review of some of the promising processes mediated by plant-associated microbes and to illustrate how such processes influence heavy metal uptake through various biogeochemical processes including translocation, transformation, chelation, immobilization, solubilization, precipitation, volatilization and complexation of heavy metals ultimately facilitating phytoremediation.     

An overview of Cistus ectomycorrhizal fungi

The genus Cistus comprises a group of about 20 shrub species found in wide areas throughout the whole Mediterranean region to the Caucasus. Being one of the main constituents of the Mediterranean-type maquis, this plant genus is peculiar in that it has developed a range of specific adaptations to resist summer drought and frequent disturbance events, such as fire and grazing. In addition, it can form both ectomycorrhizas and arbuscular mycorrhizas. In this paper, we review the information available on the ectomycorrhizal fungi of Cistus across its entire geographic range, as gathered and critically sifted from both published literature sources and personal observations. Although the resulting data matrix was based primarily on accounts of sporocarp inventories in the field, existing knowledge on the features of Cistus natural and synthesized ectomycorrhizas was also included and discussed. In total, more than 200 fungal species belonging to 40 genera have been reported so far to be associated with Cistus. An analysis of the pattern of ectomycorrhizal diversity and host specificity revealed that members of the Cortinariaceae and Russulaceae make the most of both Cistus-aspecific and Cistus-specific mycobionts. Further studies are needed to expand our preliminary knowledge of the mycorrhizal ecology and biology of Cistus and its fungal associates, focusing on topics such as mycobiont diversity, host specificity, fungal succession, mycorrhizal influence on stress tolerance, and impact of disturbances, while comparing the findings with those from other ecosystems.     

Soil modification by different tree species influences the extent of seedling ectomycorrhizal infection

Established vegetation can facilitate the ectomycorrhizal infection of seedlings, but it is not known whether this interaction is limited by the phylogenetic relatedness of trees and seedlings. We use a series of bioassay experiments to test whether soil modification by different ectomycorrhizal tree species causes different levels of seedling infection, whether the extent of seedling infection is a function of the relatedness of tree and seedling, and whether the effect of trees on seedlings is mediated by biotic or abiotic soil factors. We found that soils from under different tree species do vary in their mycorrhizal infectiveness. However, this variation is not related to the genetic relatedness of trees and seedlings but instead, appears to be an attribute of the overstory species, irrespective of seedling species, mediated through a suite of humus- and base-cation-related abiotic effects on soils. Modification of abiotic soil properties by overstory trees should be considered as an important factor in the effect of different overstory trees on the extent of seedling mycorrhizal infection.     

Molecular diversity of arbuscular mycorrhizal fungi in onion roots from organic and conventional farming systems in the Netherlands

Diversity and colonization levels of naturally occurring arbuscular mycorrhizal fungi (AMF) in onion roots were studied to compare organic and conventional farming systems in the Netherlands. In 2004, 20 onion fields were sampled in a balanced survey between farming systems and between two regions, namely, Zeeland and Flevoland. In 2005, nine conventional and ten organic fields were additionally surveyed in Flevoland. AMF phylotypes were identified by rDNA sequencing. All plants were colonized, with 60% for arbuscular colonization and 84% for hyphal colonization as grand means. In Zeeland, onion roots from organic fields had higher fractional colonization levels than those from conventional fields. Onion yields in conventional farming were positively correlated with colonization level. Overall, 14 AMF phylotypes were identified. The number of phylotypes per field ranged from one to six. Two phylotypes associated with the Glomus mosseae–coronatum and the G. caledonium–geosporum species complexes were the most abundant, whereas other phylotypes were infrequently found. Organic and conventional farming systems had similar number of phylotypes per field and Shannon diversity indices. A few organic and conventional fields had larger number of phylotypes, including phylotypes associated with the genera Glomus-B, Archaeospora, and Paraglomus. This suggests that farming systems as such did not influence AMF diversity, but rather specific environmental conditions or agricultural practices. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Authors Guillermo A. Galván and István Parádi contributed equally to this research and share first co-authorship.     

Lake Erie Microcystis: Relationship between microcystin production, dynamics of genotypes and environmental parameters in a large lake

Cyanobacteria of genus Microcystis sp. have been commonly found in Lake Erie waters during recent summer seasons. In an effort to elucidate relationships between microcystin production, genotypic composition of Microcystis community and environmental parameters in a large lake ecosystem, we collected DNA samples and environmental data during a three-year (2003–2005) survey within Lake Erie and used the data to perform a series of correlation analyses. Cyanobacteria and Microcystis genotypes were quantified using quantitative real-time PCR (qPCR). Our data show that Microcystis in Lake Erie forms up to 42% of all cyanobacteria, and that Microcystis exists as a mixed population of potentially toxic and (primarily) non-toxic genotypes. In the entire lake, the total abundance of Microcystis as well as the abundance of microcystin-producing Microcystis is strongly correlated with the abundance of cyanobacteria suggesting that Microcystis is a significant component of the cyanobacterial community in Lake Erie during summer seasons. The proportion of total Microcystis of all cyanobacteria was strongly linked to the microcystin concentrations, while the percentage of microcystin-producing genotypes within Microcystis population showed no correlation with microcystin concentrations. Correlation analysis indicated that increasing total phosphorus concentrations correlate strongly with increasing microcystin concentrations as well as with the total abundance of Microcystis and microcystin-producing Microcystis.     

Different growth response of five co-existing stoloniferous plant species to inoculation with native arbuscular mycorrhizal fungi

Five species of stoloniferous plants originating from the same field site (Galeobdolon montanum, Glechoma hederacea, Potentilla anserina, Ranunculus repens and Trifolium repens) were studied with respect to their interaction with arbuscular mycorrhizal (AM) fungi. More specifically, the question was addressed whether mycorrhizal growth response of host plant species could be related to their vegetative mobility. The roots of all the species examined were colonised with AM fungi in the field, with the percentage of colonisation varying among species from approximately 40% to 90%. In a subsequent pot experiment, plants of all the species were either left non-inoculated or were inoculated with a mixture of three native AM fungi isolated from the site of plant origin (Glomus mosseae, G. intraradices and G. microaggregatum). AM fungi increased phosphorus uptake in all the plant species; however, plant growth response to inoculation varied widely from negative to positive. In addition to the biomass response, AM inoculation led to a change in clonal growth traits such as stolon number and length or ramet number in some species. Possible causes of the observed differences in mycorrhizal growth response of various stoloniferous plants are discussed.     

具反馈控制的时滞阶段结构种群模型的稳定性

研究具反馈控制的时滞阶段结构种群模型,证明了正平衡点的局部渐近稳定性, 并给出了正平衡点全局渐近稳定的充分条件．     

Root exudate of pmi tomato mutant M161 reduces AM fungal proliferation in vitro

Soluble factors released from roots of the pre-mycorrhizal infection (pmi) myc(-) tomato mutant M161 were analyzed and compared with normal wild-type released factors. Aseptic whole exudates from the M161 mutant retarded the proliferation of Glomus intraradices in vitro. When the whole exudate was further fractionated on a C18 SEPAK cartridge, the 50/70% methanol fraction showed an activity against hyphal tip growth of Gigaspora gigantea and Gl. intraradices. Preliminary characterization of the exudate suggests that the inhibitory moieties are heat labile, bind to PVPP (polyvinyl polypyrrolidone), and are not volatile. This is the first reported instance of the inhibition by a myc(-) plant being ascribed to inhibitory component(s) released in root exudate.     

Platelet serotonin concentration in alcoholic subjects

Serotonin (5-hydroxytryptamine, 5-HT) is assumed to play a role in the pathophysiology of different psychiatric disorders including alcoholism. Since platelets and central serotonergic synaptosomes share similar pharmacodynamics of 5-HT, this study determined platelet 5-HT concentration in 148 male and 42 female drug-free subjects with alcohol dependency, according to the DSM-IV criteria, and in sex-and age-matched controls. Male and female alcoholics had significantly lower platelet 5-HT concentration than 110 male and 123 female healthy controls. Sex differences, i.e. higher platelet 5-HT concentration in men than in women, were found both in healthy and alcoholic subjects. Platelet 5-HT concentration differed significantly in male and female alcoholic subjects with or without different psychiatric comorbidities. Platelet 5-HT concentration was higher in male alcoholics with comorbid posttraumatic stress disorder (PTSD) than in male alcoholics with comorbid anxious-depressive disorder, or depression, or male alcoholics without any psychiatric comorbidities. Comorbid depression in female alcoholics slightly elevated platelet 5-HT levels but these values were still reduced compared to values in healthy women. Smoking status did not affect platelet 5-HT concentration either in healthy or in alcoholic subjects. The data from our study show sex differences, and reduced platelet 5-HT values, regardless of the nicotine dependence, in the large groups of male and female alcoholic subjects. Among male alcoholics the presence of comorbid PTSD partly normalized the decreased platelet 5-HT values. The results of the present study support the hypothesis that alterations in 5-HT system might be related to alcoholism.