plants capable of accumulating uncommonly high Zn levels. In 1935, Byers documented the accumulation of Remediation of sites contaminated with toxic metals is particularly selenium in Astragalus spp. One decade later, Minguzzi challenging. Unlike organic compounds, metals cannot be degraded, and the cleanup usually requires their removal. However, this energy- and Vergnano (1948) identified plants capable of hyperintensive approach can be prohibitively expensive. In addition, the accumulating up to 1% Ni in shoots. Following the idenmetal removing process often employs stringent physicochemical tification of these and other hyperaccumulator species, agents which can dramatically inhibit soil fertility with subsequent a great deal of research has been conducted to elucidate negative impacts on the ecosystem. Phytoremediation has been pro- the physiology and biochemistry of metal hyperaccumuposed as a cost-effective, environmental-friendly alternative technol- lation in plants. Significant results have been obtained, ogy. A great deal of research indicates that plants have the genetic and the understanding of metal accumulating mechapotential to remove many toxic metals from the soil. Despite this nisms substantially advanced. However, a better undpotential, phytoremediation is yet to become a commercially available erstanding of the biological processes is needed if phytechnology. Progress in the field is hindered by a lack of understanding toextraction is to become a reliable, commercially of complex interactions in the rhizosphere and plant-based mechanisms which allow metal translocation and accumulation in plants. In available technology. this paper, four research areas relevant to metal phytoextraction from The success of phytoextraction, as an environmental contaminated soil are reviewed. The review concludes with an assess- cleanup technology, depends on several factors includment of the current status of technology deployment and suggestions ing the extent of soil contamination, metal availability for future phytoremediation research. for uptake into roots (bioavailability), and plant ability to intercept, absorb, and accumulate metals in shoots (Ernst, 1996). Ultimately, the potential for phytoextracP hytoremediation, the use of plants for environmen- tion depends on the interaction between soil, metal, and tal restoration, is an emerging cleanup technology. plant. The complexity of this interaction, controlled by To exploit plant potential to remediate soil and water climatic conditions, argues against generic and in favor contaminated with a variety of compounds, several tech- of a site specific phytoremediating approach. This undernological subsets have been proposed. Phytoextraction lines the importance of understanding the mechanisms is the use of higher plants to remove inorganic contami- and processes that govern metal uptake and accumulanants, primarily metals, from polluted soil. In this ap- tion in plants. In this review, four research areas, releproach, plants capable of accumulating high levels of vant to soil and plant interaction as it relates to metal metals are grown in contaminated soil. At maturity, phytoextraction, have been identified. The significance metal-enriched aboveground biomass is harvested and of these areas is briefly discussed below.

https://clu-in.org/download/remed/lasat_article.pdf

Phytoextraction of toxic metals: a review of biological mechanisms.

Beneficial plant–microbe interactions in the rhizosphere are primary determinants of plant health and soil fertility. Arbuscular mycorrhizas are the most important microbial symbioses for the majority of plants and, under conditions of P-limitation, influence plant community development, nutrient uptake, water relations and above-ground productivity. They also act as bioprotectants against pathogens and toxic stresses. This review discusses the mechanism by which these benefits are conferred through abiotic and biotic interactions in the rhizosphere. Attention is paid to the conservation of biodiversity in arbuscular mycorrhizal fungi (AMF). Examples are provided in which the ecology of AMF has been taken into account and has had an impact in landscape regeneration, horticulture, alleviation of desertification and in the bioremediation of contaminated soils. It is vital that soil scientists and agriculturalists pay due attention to the management of AMF in any schemes to increase, restore or maintain soil fertility.

The contribution of arbuscular mycorrhizal fungi in sustainable maintenance of plant health and soil fertility

http://clemkuek.com/papers/heavymetalreview.pdf

Role of plants, mycorrhizae and phytochelators in heavy metal contaminated land remediation.

High concentrations of heavy metals in soil have an adverse effect on micro-organisms and microbial processes. Among soil microorganisms, mycorrhizal fungi are the only ones providing a direct link between soil and roots, and can therefore be of great importance in heavy metal availability and toxicity to plants. This review discusses various aspects of the interactions between heavy metals and mycorrhizal fungi, including the effects of heavy metals on the occurrence of mycorrhizal fungi, heavy metal tolerance in these micro-organisms, and their effect on metal uptake and transfer to plants. Mechanisms involved in metal tolerance, uptake and accumulation by mycorrhizal hyphae and by endo- or ectomycorrhizae are covered. The possible use of mycorrhizal fungi as bioremediation agents in polluted soils or as bioindicators of pollution is also discussed.

Effect of heavy metal pollution on mycorrhizal colonization and function: physiological, ecological and applied aspects

Effects of biofertilizer containing N-fixer, P and K solubilizers and AM fungi on maize growth: a greenhouse trial

Effects of mycorrhizae and other soil microbes on revegetation of heavy metal contaminated mine spoil.

Effects of mycorrhizae and fertilizer amendments on zinc tolerance of plants.

Soils from a heavy metal contaminated southeast Kansas mining site were characterized for microbial resistance to zinc, cadmium, and lead. Plant availability indices (DTPA-extractable) for zinc were very high in the waste material (610 Μg g−1), while levels of lead and cadmium were more moderate, 45 and 4.3 Μg g−1, respectively. Soil contamination decreased with distance from the mine tailings or ‘chat’. In the rhizosphere, bacterial resistance to zinc and cadmium increased as the DTPA-extractable plant availability indices increased. Mycorrhizal root colonization was not affected by the heavy metal concentration in the soil. The established vegetation in the contaminated area included warm- and cool-season grasses as well as forbs and sedges.

Biological characterization of a Southeast Kansas mining site

Characterization of a heavy metal contaminated site

Huanglongbing (HLB), also known as citrus greening, is an insidious disease in citrus and has become a threat to the sustainability of the citrus industry worldwide. In the U.S., Candidatus Liberibacter asiaticus (CLas) is the pathogen that is associated with HLB, an unculturable, phloem-limited bacteria, vectored by the Asian Citrus Psyllid (ACP, Diaphorina citri). There is no known cure nor treatment to effectively control HLB, and current control methods are primarily based on the use of insecticides and antibiotics, where effectiveness is limited and may have negative impacts on beneficial and non-target organisms. Thus, there is an urgent need for the development of effective and sustainable treatment options to reduce or eliminate CLas from infected trees. In the present study, we screened citrus-derived endophytes, their cell-free culture supernatants (CFCS), and crude plant extracts for antimicrobial activity against two culturable surrogates of CLas, Sinorhizobium meliloti and Liberibacter crescens. Candidates considered high-potential antimicrobial agents were assessed directly against CLas in vitro, using a propidium monoazide–based assay. As compared to the negative controls, statistically significant reductions of viable CLas cells were observed for each of the five bacterial CFCS. Subsequent 16S rRNA gene sequencing revealed that each of the five bacterial isolates were most closely related to Bacillus amyloliquefaciens, a species dominating the market of biological control products. As such, the aboveground endosphere of asymptomatic survivor citrus trees, grown in an organic orchard, were found to host bacterial endophytes capable of effectively disrupting CLas cell membranes. These results concur with the theory that native members of the citrus microbiome play a role in the development of HLB. Here, we identify five strains of Bacillus amyloliquefaciens demonstrating notable potential to be used as sources of novel antimicrobials for the sustainable management of HLB.

K. G. Shetty

Papers

Effects of mycorrhizae and other soil microbes on revegetation of heavy metal contaminated mine spoil.

Effects of mycorrhizae and fertilizer amendments on zinc tolerance of plants.

Biological characterization of a Southeast Kansas mining site

Characterization of a heavy metal contaminated site

Endophytes and Plant Extracts as Potential Antimicrobial Agents against Candidatus Liberibacter Asiaticus, Causal Agent of Huanglongbing