Advances in Microbial Toxin Research and Its Biotechnological Exploitation

Potential Biotechnological Strategies for the Cleanup of Heavy Metals and Metalloids
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Figure 3. The impact of environmental stress challenges on plant-microbe interactions. Ideas based on Zolla et al. Improving the ability of soil microorganisms for the biocontrol of pathogens diseases, pests…. Defense priming is the preconditioning of immunity induced by microbial colonization, fundamental for an efficient protection against pathogens.

Priming also acts systemically on distant parts of the root and shoots thereby inducing systemic resistance ISR to protect efficiently plants against both roots and foliar pathogens Selosse et al. The AM symbiosis protects plants against deleterious organisms including microbial pathogens, herbivorous insects and parasitic plants. AM colonization can prime plant immunity by boosting the plant ability to respond to pathogen attack, where jasmonic acid JA plays a key role.

The priming effects of MAMPs rely on that they activate the JA signaling pathway, which regulates the inducible plant defenses Pozo et al. Indeed, in the plant hormone signaling crosstalk, which regulates plant defense and development in microbe-plant-insect interactions, JA results in the main hormone at switching from growth to defense responses Pangesti et al.

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Because the effectiveness of biocontrol practices is affected by the prevailing environmental conditions, biocontrol-related research has to envisage the challenge of finding appropriate screening procedures to select microorganisms able to be highly effective under the current changing scenarios. Understanding the impact of the environment on the biocontrol agent performance will help to predict the resulting output and to develop effective combinations of antagonistic microorganisms.


A major challenge in rhizosphere biotechnology is to exploit the prophylactic ability of AM fungi in association with antagonist microorganisms. The final aim is to find out an enhanced capacity for bioprotection achieved by the combination of the mechanisms used by each organism individually.

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Recent research on agricultural weed control is revealing strategies focused on the initial steps in the host-parasite interaction. Actually, parasitic weeds are difficult to control because most of their life cycle occurs underground. The strigolactones, a new class of plant hormones are signaling molecules, which stimulate germination of root parasitic plant seeds. Besides, it has been shown that strigolactones are involved in root colonization by the AM fungi. The possible applicability of the AM symbiosis in weed control, based on AM activities regulating plant production of strigolactones, as an agricultural practice in the context of sustainability issues, has been discussed recently Jung et al.

Improving the ability of soil microorganisms for alleviating the negative effects of osmotic stressors drought, salinity. The level of aridity in many land areas of the world has increased progressively rising thereby drought and salinity problems.

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Compared with plants, algae exhibit higher productivities and theoretically could give to fold higher yields of oil per acre, although such capacities have not yet been achieved on a commercial scale Greenwell et al. Ensure the controllers in their appointment salah; they are highly! To be successful for phytoremediation purposes plants must be capable to thrive in polluted environments while their associated microorganisms have also to be adapted to the presence of contaminants Pongrac et al. A more recent study reported improved heavy metal biosorption capacities for E. Biochemistry and Molecular Biology. Overexpression of ATP sulfurylase in indian mustard leads to increased selenate uptake, reduction, and tolerance. Spence, C.

To cope with such osmotic stressors plants must develop a number of adaptation mechanisms including mainly a fine regulation of their water uptake capacity and transpiration rates, and the activation of the antioxidant machinery to overcome the overproduction of reactive oxygen species ROS caused by the stress. The related available information has been discussed recently Dimkpa et al. The two general mechanisms maintaining water and ROS balance may be ameliorated by both the establishment of the AM symbiosis and by inoculation with PGPR, which act through diverse specific mechanisms.

Such microbial activities result in a better regulation of plant water status and contribute to increase plant resistance to osmotic stress conditions. Finally, the improved water uptake capacity of microbiologically inoculated plants allows them to have higher transpiration rates and hence higher photosynthetic rates under conditions of water deficit. Particular attention is receiving the role played by AM fungi and other rhizosphere microorganisms to improve plant water status based on the improvement of root hydraulic conductance, which ultimately depends on aquaporin functioning Aroca et al.

Aquaporins are membrane intrinsic proteins that allow for water and other small neutral molecules to pass across biological membranes following an osmotic gradient Chaumont and Tyerman, ; Li et al. This is fundamental because soil and rhizosphere microorganisms are key factors for plant survival under a changing environment where plants are going to be exposed to adversity on the oncoming years, as driven by the climatic change Duarte et al.

Improving the ability of so il microorganisms for the phytoremediation of contaminated soil. Plant-associated microorganisms, i.

Bacteria Toxins: Exotoxins, Endotoxins & Membrane-Damaging Toxins – Microbiology - Lecturio

AM fungi and bacteria can enhance plant abilities for the remediation phytoremediation of environments contaminated with heavy metals, radionuclides or organic xenobiotics including volatile organic compounds, oil derived alkanes or polycyclic aromatic hydrocarbons. To be successful for phytoremediation purposes plants must be capable to thrive in polluted environments while their associated microorganisms have also to be adapted to the presence of contaminants Pongrac et al. The mechanisms underlying the role of plant associated bacteria in phytoremediation of environments contaminated with HMs or organic xenobiotics, in general Germaine et al.


These mechanisms include improvement of plant growth, nutrient P and N supply, production of Fe-binding siderophores, plant hormones production, enhanced ACC-deaminase activity ethylene reductions , organic xenobiotic degradation, etc. Another mechanism for improving phytoremediation is the bio-augmentation of plant associated microbial communities based on horizontal gene transfer Germaine et al.

This mechanism, a challenge of future research, relies on that many resistance genes involved in HMs bioremediation processes are located in plasmids that can be transferred within the bacterial communities. The complete genomes of a number of plant-associated bacteria are becoming available. This, together with the genome sequence of diverse plant host, would facilitate establishing the molecular communications between plant and bacteria, a key step to provide new insights allowing for design improved strategies in phytoremediation Germaine et al.

The AM fungi have also evolved a series of strategies to restrict entry of toxic metal species and to keep intracellular metal homeostasis Ferrol et al. Specific metal transporters regulate cytosolic metal ion concentrations and translate the excess of metal within vacuoles, where they would cause less damage. AM fungi have also evolved mechanisms to fight against the oxidative stress produced by HMs or to repair the oxidative damage. Increased HM tolerance of mycorrhizal plants may be related to extensive changes in gene expression and protein synthesis induced by the symbiosis itself.

Understanding the key molecular determinants of metal homeostasis in AM fungi is challenging. To get some insights into the underlying mechanisms, a genome-wide analysis of HMs transporters was undertaken Tamayo et al. This in silico analysis allowed identification of 30 open reading frames in the R. The authors depict a comprehensive scheme of the mechanisms involved.

A current challenge is to characterize the functionally of these transporters and to identify their location and roles in the AM symbiosis. The main achievements resulting from these experiments were: i the target bacteria accumulated large amounts of HMs; ii co-inoculation enhanced plant establishment and growth, and lowered HM concentrations in plants, supporting a phytostabilization-based activity, while the total HM content in plant shoots was higher in dually-inoculated plants, suggesting a phytoextraction activity; and iii inoculated HM-adapted bacteria increased enzymatic activities and plant hormone production in the mycorrhizosphere.

Inoculation of autochthonous AM fungi and PGPR, together with the application of treated agrowaste residue, changed the bacterial community structure and enhanced phytoextraction to remediate HM contaminated soils. A challenging topic for future research is to realize the phytoremediation effects of mycorrhizosphere interactions under field conditions.

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Microbial toxins are secondary metabolites that accumulate in the organism and, Advances in Microbial Toxin Research and Its Biotechnological Exploitation. Advances in Microbial Toxin Research and Its Biotechnological Exploitation: Medicine & Health Science Books @

Engeneering the rhizosphere to encourage beneficial microbe establishment: a great challenge for the future. Diverse research approaches are currently addressed trying to ascertain whether the rhizosphere can be engineered to encourage beneficial organisms, while prevent presence of pathogens.

The related research topics offer many challenges because there are many gaps in our understanding on the ad hoc research strategies. Undoubtedly, getting biased rhizosphere opens new opportunities for future agricultural developments based in exploiting the beneficial microbial services to reduce the inputs of agrochemicals thereby reaching sustainable environmental and economical goals. Learning how plants shape microbial community structure in the rhizosphere. Current research is realizing that plants can structure their root-associated microbial communities, concerning both diversity and functions Achouak and Haichar, ; Hirsch et al.

Particularly, Achouak and Haichar used the stable isotope probing SIP together with fingerprinting approaches as a molecular detection tool to analyze the impact of the plant species on their rhizosphere microbiome.

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They confirmed the differential impact of each target plant species on the genetic and functional diversity of the plant-associated bacterial communities. Therefore, such ability of the plants for shaping microbial communities in their rhizospheres appears as a new opportunity for linking structure and function of the root-microbiome related to nutrient supply and plant protection.

Carbon compounds and signal molecules from root exudates are the main drivers of plant specific effects on rhizosphere bacteria and their proteomes. Actually, the identity and quality of rhizodeposits varies from plant to plant thereby attracting a specific set of bacteria to the rhizosphere and providing them with a selective pressure to stimulate bacteria to compete and persist Hirsch et al. Harnessing the rhizosphere microbial communities through agricultural managements.

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In a comprehensive analysis of the available experimental evidence, combined with theoretical models, Bakker et al. In some cases, the ideas are based on speculations but these have a reasonable feasibility in the nearest future. An example of harnessing the rhizosphere microbiome derives from the existence of plant-microbe co-adaptation, involving a shared evolution history of interactions between plants and soil microbiome.

In a co-adapted rhizosphere, pathogens can be present but their activities are controlled. In contrast, when an agricultural plant species is moved to other parts of the world, as happens with the current agricultural global exchange, the plant will grow in association with a microbiome, which has not shared evolutionary history. The target crop will associate with an un-adapted microbiome, which does not constrain pathogen establishment and therefore becoming susceptible to diseases. According to Bakker et al. One of these alternate paths relies on develop plants able to shape their microbiome by targeting particular taxa for specific functions i.

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N 2 -fixation, P-mobilization, biocontrol, etc. The other approach is based on develop plants able to shape their microbiome for broad characteristics related to promotion of plant growth and health. All in all, in the nearest future it appears that the more feasible approach to enhance beneficial microbial services in agriculture is the direct manipulation of the soil microbiome.

Particularly, a target aim is to reconstruct a minimal rhizosphere microbiome able to provide a maximized benefit to plant at a minimal photosynthetic cost Raaijmakers, A challenging strategy which offers opportunities to enable plants to recruit microorganisms targeted for specific functions, is that aimed at engineering nitrogen-fixing cereals Rogers and Oldroyd, ; Oldroyd and Dixon, ; Venkateshwaran, The "biased rhizosphere" concept is based on the possibility of provoking the production by the plant of special compounds that can be catabolized only by target beneficial bacteria introduced as inoculants, e.

PGPR Savka et al. These authors have revised the biased rhizosphere concept and provide pioneering insights on its origin and significance. The origin of the biased rhizosphere concept derived from an analysis of the interactions between rhizobia and plant generated by rhizopine-like molecules and Agrobacterium and plant generated by opine-like molecules , specific compounds able to foster such interactions. Savka et al. For example, the time-course effect of host rhizosphere chemistry can be monitored in studying microbial community structure and metagenomics.

Future studies have to be undertaken to find specific metabolite-plant species-microbe combinations. Deciphering the biotic and abiotic plant factors that shape the plant-associated microbiome through biasing the rhizosphere offers many challenges that current research is trying to envisage. According to Savka et al. A combination of all of these approaches can improve our understanding on how to enhance the competitiveness and persistence of bacteria in the biased rhizosphere to finally improve plant health and agro-ecosystem productivity.

Exploiting the interactions between soil microbial communities and crops is a relevant approach to increase food production for the growing world population at the lowest environmental costs, in the current scenario of global change. Essentially, there are two major strategies for managing the soil microbiome, these are being based either on the development of microbial inoculants or on the manipulation of naturally existing microbial populations.

Both rhizosphere bacteria PGPR and fungi, either saprophytic or endophytic symbionts with special reference to N 2 -fixing rhizobia and AM fungi are protagonists of applied microbial biotechnology in agriculture. Particular emphasis is being paid to formulation, quality control and modes of application of microbial inoculants.