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Subjects:
Ecology
Heavy metals are among the primary environmental factors affecting plants and microorganisms. The use of beneficial microorganisms is considered one of the most promising methods of increasing stress tolerance since plant-associated microbes reduce metal accumulation, so the research focuses on plant–microorganism interactions and their practical application in phytoremediation.
- heavy metals
- plants
- microorganisms
- environmental problems
1. Introduction
Unfavorable environmental factors have a negative impact on living organisms. One of the serious consequences of unfavorable environmental factors is the reduction of biodiversity and changes in the relationships between plants and microorganisms. Currently, global warming is of particular concern, which, according to most environmental scientists, may exacerbate other environmental problems [1,2].
According to several authors, anthropogenic climate change led to an increase in flooded areas of northern Eurasia and other regions in 2010–2013 [3].
A number of scientists believe that flooding can increase average concentrations of heavy metals in the soil [5]. Floods have been shown to have different effects on heavy metal content in the soil; however, regular floods can contaminate the entire ecosystem, making it less suitable for agricultural activities [6]. Thus, the problems of flooding and soil contamination with heavy metals are becoming increasingly urgent.
2. Effects of Heavy Metals on Plants and Microorganisms
The prolonged influence of heavy metals in the environment poses a threat to various populations of plants and microorganisms affecting biocenoses and putting the integrity of ecosystems at risk. High concentrations of heavy metals are typical not only for the soils of areas of ore metal deposits but also for other ecosystems. For instance, heavy metals, along with deicing agents, are priority pollutants of the soil in urban ecosystems and have an adverse effect on plants [7,8].
Contamination by Cu, Pb, Zn, and Cd is common in urban soils and urban road dust [9,10]. Heavy metals reduce ornamental qualities and significantly limit the diversity of plant species and varieties used in urban areas. This is a serious ecological problem, so issues of increasing plant tolerance to the urban environment are an important field of biological sciences [11,12].
Plants that are not resistant to heavy metals show altered metabolism, reduced growth, biomass production, and yields [13]. High concentrations of heavy metals in the soil have a strong negative impact on plant biodiversity. The number of cereals, legumes, and compositae is decreasing, while other species (including ruderals) are affected to a lesser extent [14].
When studying the effects of various mining land restoration measures on plant diversity, eight families and 10 species of surviving plants were observed, with most of them being herbs [15,16]. Heavy metal pollution can also affect microbial communities causing changes in their structure and biodiversity [17].
Differences in the abundance of fungi, as well as in the composition of communities and the relationship of soil bacterial and fungal communities between the soils of urban and suburban parks were noted. Soil-available Zn played an important role in the formation of bacterial and fungal community structures in the park soils of Shanghai [18].
The metal resistance depends on the type of microorganism and the specific metal. Bacterial communities show different reactions to soil contamination depending on the metal. A negative dose-dependent response was found for Cu but not for Cd on the diversity of bacteria in the environment [19].
Consortia may show greater tolerance to heavy metals than pure cultures [20].
3. The Influence of Heavy Metals on Plant–Microorganism Interactions
There are various works describing the interaction between plants and microorganisms under the high content of heavy metals in the soil.
Plants release a variety of compounds that attract and stimulate soil microbial communities. C. ambrosioides feeds a diverse bacterial community in its rhizosphere soils [21]. Pseudomonas and Arthrobacter predominated among the metal-resistant bacteria [21]. Plant root exudates are useful sources of nutrients and energy for soil microorganisms [22].
Some microorganisms stimulate plant growth in soils through the production of important substances for the growth of plants, including the solubilization/transformation of minerals (phosphorus, potassium nitrogen, and iron), synthesis of phytohormones, siderophores and specific enzymes, as well as indirectly by controlling plant pathogens or inducing systemic plant resistance to them [22,23].
Plant growth-promoting microorganisms can modify the bioavailability of metals in the soil through different mechanisms, such as acidification, precipitation, chelation, complexation, and redox reactions [22].
Studies are being conducted to show the effects of microorganisms on plant growth. Copper-resistant rhizobacteria isolated from Elsholtzia splendens exhibit growth-enhancing features [24].
Bacterial isolates from Bacillus cereus, B. atrophaeus, B. pumilus, B. amyloliquefaciens, B. tropicus, B. subtilis, B. halotolerans, B. vallismortis, and Enterococcus mundtii have plant growth promoting properties and can improve soils contaminated with heavy metals [25].
Rhizobacteria produce siderophores that promote the uptake of trace elements by the rhizosphere and the production of organic and inorganic acids, thereby affecting the bioavailability of trace elements and plant-induced systemic tolerance to limit metal accumulation in the crops [26].
Siderophore-producing bacteria actually supply plants with nutrients, especially iron. The benefits of combining metal-tolerant siderophore-producing bacteria with plants in order to remove metals from contaminated soils are demonstrated. Perhaps the bacterial root microbiota stimulated by secreted coumarins is an essential mediator of plant adaptation to iron-deficient soils [27,28].
A number of isolated rhizosphere bacteria from the mine tailings containing high concentrations of heavy metals capable of stimulating plant growth showed the greatest capacity for nitrogen fixation and produced indolylacetic acid, gibberellins, siderophores, and lytic enzymes [29]. An improvement in plant growth with the presence of heavy metals in the soil is possible due to the bacterial enzyme 1-aminocyclopropane-1-carboxylate deaminase. It was revealed that the inoculation of plants with the bacteria-producers of 1-aminocyclopropane-1-carboxylate deaminase had a positive effect on their growth under stress [30].
Therefore, plant growth-promoting bacteria play a key role in growth regulation through the synthesis of phytohormones, plant nutrient uptake, and the relief of abiotic and biotic stress, which helps plants to tolerate high concentrations of heavy metals [31].
This entry is adapted from the peer-reviewed paper 10.3390/d15020175
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