For decades, rhizobia were thought to be the only nitrogen-fixing inhabitants of legume nodules, and biases in culture techniques prolonged this belief. However, other bacteria, which are not typical ...rhizobia, are often detected within nodules obtained from soil, thus revealing the existence of a phytomicrobiome where the interaction among the individuals is not only complex, but also likely to affect the behavior and fitness of the host plant. Many of these nonrhizobial bacteria are nitrogen fixers, and some also induce nitrogen-fixing nodules on legume roots. Even more striking is the incredibly diverse population of bacteria residing within nodules that elicit neither nodulation nor nitrogen fixation. Yet, this community exists within the nodule, albeit clearly out-numbered by nitrogen-fixing rhizobia. Few studies of the function of these nodule-associated bacteria in nodules have been performed, and to date, it is not known whether their presence in nodules is biologically important or not. Do they confer any benefits to the Rhizobium-legume nitrogen-fixing symbiosis, or are they parasites/saprophytes, contaminants, or commensals? In this review, we highlight the lesser-known bacteria that dwell within nitrogen-fixing nodules and discuss their possible role in this enclosed community as well as any likely benefits to the host plant or to the rhizobial inhabitants of the nodule. Although many of these nodule inhabitants are not capable of nitrogen fixation, they have the potential to enhance legume survival especially under conditions of environmental stress. This knowledge will be useful in defining strategies to employ these bacteria as bioinoculants by themselves or combined with rhizobia. Such an approach will enhance rhizobial performance or persistence as well as decrease the usage of chemical fertilizers and pesticides.
is a complex genus of ascomycete fungi that consists of plant pathogens of agricultural relevance. Controlling
infection in crops that leads to substantial yield losses is challenging. These economic ...losses along with environmental and human health concerns over the usage of chemicals in attaining disease control are shifting focus toward the use of biocontrol agents for effective control of phytopathogenic
spp. In the present study, an analysis of the plant-growth promoting (PGP) and biocontrol attributes of four bacilli (
30N-5,
11,
237, and
30VD-1) has been conducted. The production of cellulase, xylanase, pectinase, and chitinase in functional assays was studied, followed by
gene analysis of the PGP-related and biocontrol-associated genes. Of all the bacilli included in this study,
30VD-1 (30VD-1) demonstrated the most effective antagonism against
spp. under
conditions. Additionally, 100 μg/ml of the crude 1-butanol extract of 30VD-1's cell-free culture filtrate caused about 40% inhibition in radial growth of
spp. Pea seed bacterization with 30VD-1 led to considerable reduction in wilt severity in plants with about 35% increase in dry plant biomass over uninoculated plants growing in
-infested soil. Phase contrast microscopy demonstrated distortions and abnormal swellings in
hyphae on co-culturing with 30VD-1. The results suggest a multivariate mode of antagonism of 30VD-1 against phytopathogenic
spp., by producing chitinase, volatiles, and other antifungal molecules, the characterization of which is underway.
Micromonospora is a Gram positive bacterium that can be isolated from nitrogen fixing nodules from healthy leguminous plants, where they could be beneficial to the plant. Their plant growth promoting ...activity in legume and non-legume plants has been previously demonstrated. The present study explores the ability of Micromonospora strains to control fungal pathogens and to stimulate plant immunity. Micromonospora strains isolated from surface sterilized nodules of alfalfa showed in vitro antifungal activity against several pathogenic fungi. Moreover, root inoculation of tomato plants with these Micromonospora strains effectively reduced leaf infection by the fungal pathogen Botrytis cinerea, despite spatial separation between both microorganisms. This induced systemic resistance, confirmed in different tomato cultivars, is long lasting. Gene expression analyses evidenced that Micromonospora stimulates the plant capacity to activate defense mechanisms upon pathogen attack. The defensive response of tomato plants inoculated with Micromonospora spp. differs from that of non-inoculated plants, showing a stronger induction of jasmonate-regulated defenses when the plant is challenged with a pathogen. The hypothesis of jasmonates playing a key role in this defense priming effect was confirmed using defense-impaired tomato mutants, since the JA-deficient line def1 was unable to display a long term induced resistance upon Micromonospora spp. inoculation. In conclusion, nodule isolated Micromonospora strains should be considered excellent candidates as biocontrol agents as they combine both direct antifungal activity against plant pathogens and the ability to prime plant immunity.
Chitin is an important component of the exteriors of insects and fungi. Upon degradation of chitin by a number of organisms, severe damage and even death may occur in pathogens and pests whose ...external surfaces contain this polymer. Currently, chemical fungicides and insecticides are the major means of controlling these disease-causing agents. However, due to the potential harm that these chemicals cause to the environment and to human and animal health, new strategies are being developed to replace or reduce the use of fungal- and pest-killing compounds in agriculture. In this context, chitinolytic microorganisms are likely to play an important role as biocontrol agents and pathogen antagonists and may also function in the control of postharvest rot. In this review, we discuss the literature concerning chitin and the basic knowledge of chitin-degrading enzymes, and also describe the biocontrol effects of chitinolytic microorganisms and their potential use as more sustainable pesticides and fungicides in the field.
After a forest wildfire, the microbial communities have a transient alteration in their composition. The role of the soil microbial community in the recovery of an ecosystem following such an event ...remains poorly understood. Thus, it is necessary to understand the plant-microbe interactions that occur in burned soils. By high-throughput sequencing, we identified the main bacterial taxa of burnt holm-oak rhizosphere, then we obtained an isolate collection of the most abundant genus and its growth promoting activities were characterised. 16S rRNA amplicon sequencing showed that the genus Arthrobacter comprised more than 21% of the total community. 55 Arthrobacter strains were isolated and characterized using RAPDs and sequencing of the almost complete 16S rRNA gene. Our results indicate that isolated Arthrobacter strains present a very high genetic diversity, and they could play an important ecological role in interaction with the host plant by enhancing aerial growth. Most of the selected strains exhibited a great ability to degrade organic polymers in vitro as well as possibly presenting a direct mechanism for plant growth promotion. All the above data suggests that Arthrobacter can be considered as an excellent PGP rhizobacterium that may play an important role in the recovery of burned holm-oak forests.
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The Green Revolution developed new crop varieties, which greatly improved food security worldwide. However, the growth of these plants relied heavily on chemical fertilizers and pesticides, which ...have led to an overuse of synthetic fertilizers, insecticides, and herbicides with serious environmental consequences and negative effects on human health. Environmentally friendly plant-growth-promoting methods to replace our current reliance on synthetic chemicals and to develop more sustainable agricultural practices to offset the damage caused by many agrochemicals are proposed herein. The increased use of bioinoculants, which consist of microorganisms that establish synergies with target crops and influence production and yield by enhancing plant growth, controlling disease, and providing critical mineral nutrients, is a potential solution. The microorganisms found in bioinoculants are often bacteria or fungi that reside within either external or internal plant microbiomes. However, before they can be used routinely in agriculture, these microbes must be confirmed as nonpathogenic strains that promote plant growth and survival. In this article, besides describing approaches for discovering plant-growth-promoting bacteria in various environments, including phytomicrobiomes and soils, we also discuss methods to evaluate their safety for the environment and for human health.
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Microbial biofertilizers are becoming an effective tool for sustainable agriculture by means of the reduction of the use of chemical fertilizers. However, the knowledge of each specific ...plant–microorganism interaction is essential for a correct application. In this study, we analyzed the in vitro plant‐growth‐promotion mechanisms of a Rhizobium leguminosarum strain named PEPV16 isolated from Phaseolus vulgaris nodules. This strain was able to produce siderophores and indole acetic acid and to solubilize phosphate. Confocal microscopy showed that this strain was able to colonize the roots of two horticultural crops, Lactuca sativa L. (lettuce) and Daucus carota L. (carrot). Strain PEPV16 was also able to promote the plant growth of both plant species increasing the dry matter of shoots and roots of lettuce and carrots, respectively, as well as to increase the uptake of N and P in the edible parts of both plant species. These data confirmed the suitability of Rhizobium as biofertilizer for nonlegumes.
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Genome analysis of fourteen mimosoid and four papilionoid beta-rhizobia together with fourteen reference alpha-rhizobia for both nodulation (nod) and nitrogen-fixing (nif/fix) genes has shown ...phylogenetic congruence between 16S rRNA/MLSA (combined 16S rRNA gene sequencing and multilocus sequence analysis) and nif/fix genes, indicating a free-living diazotrophic ancestry of the beta-rhizobia. However, deeper genomic analysis revealed a complex symbiosis acquisition history in the beta-rhizobia that clearly separates the mimosoid and papilionoid nodulating groups. Mimosoid-nodulating beta-rhizobia have nod genes tightly clustered in the nodBCIJHASU operon, whereas papilionoid-nodulating Burkholderia have nodUSDABC and nodIJ genes, although their arrangement is not canonical because the nod genes are subdivided by the insertion of nif and other genes. Furthermore, the papilionoid Burkholderia spp. contain duplications of several nod and nif genes. The Burkholderia nifHDKEN and fixABC genes are very closely related to those found in free-living diazotrophs. In contrast, nifA is highly divergent between both groups, but the papilionoid species nifA is more similar to alpha-rhizobia nifA than to other groups. Surprisingly, for all Burkholderia, the fixNOQP and fixGHIS genes required for cbb3 cytochrome oxidase production and assembly are missing. In contrast, symbiotic Cupriavidus strains have fixNOQPGHIS genes, revealing a divergence in the evolution of two distinct electron transport chains required for nitrogen fixation within the beta-rhizobia.
Corn (
Zea mays
L.) is not only an important food source, but also has numerous uses, including for biofuels, fillers for cosmetics, glues, and so on. The amount of corn grown in the U.S. has ...significantly increased since the 1960’s and with it, the demand for synthetic fertilizers and pesticides/fungicides to enhance its production. However, the downside of the continuous use of these products, especially N and P fertilizers, has been an increase in N
2
O emissions and other greenhouse gases into the atmosphere as well as run-off into waterways that fuel pollution and algal blooms. These approaches to agriculture, especially if exacerbated by climate change, will result in decreased soil health as well as human health. We searched for microbes from arid, native environments that are not being used for agriculture because we reasoned that indigenous microbes from such soils could promote plant growth and help restore degraded soils. Employing cultivation-dependent methods to isolate bacteria from the Negev Desert in Israel, we tested the effects of several microbial isolates on corn in both greenhouse and small field studies. One strain,
Dietzia cinnamea
55, originally identified as
Planomicrobium chinense
, significantly enhanced corn growth over the uninoculated control in both greenhouse and outside garden experiments. We sequenced and analyzed the genome of this bacterial species to elucidate some of the mechanisms whereby
D. cinnamea
55 promoted plant growth. In addition, to ensure the biosafety of this previously unknown plant growth promoting bacterial (PGPB) strain as a potential bioinoculant, we tested the survival and growth of
Caenorhabditis elegans
and
Galleria mellonella
(two animal virulence tests) as well as plants in response to
D. cinnamea
55 inoculation. We also looked for genes for potential virulence determinants as well as for growth promotion.
Biotic interactions can improve agricultural productivity without costly and environmentally challenging inputs. Micromonospora strains have recently been reported as natural endophytes of legume ...nodules but their significance for plant development and productivity has not yet been established. The aim of this study was to determine the diversity and function of Micromonospora isolated from Medicago sativa root nodules. Micromonospora-like strains from field alfalfa nodules were characterized by BOX-PCR fingerprinting and 16S rRNA gene sequencing. The ecological role of the interaction of the 15 selected representative Micromonospora strains was tested in M. sativa. Nodulation, plant growth and nutrition parameters were analyzed. Alfalfa nodules naturally contain abundant and highly diverse populations of Micromonospora, both at the intra- and at interspecific level. Selected Micromonospora isolates significantly increase the nodulation of alfalfa by Ensifer meliloti 1021 and also the efficiency of the plant for nitrogen nutrition. Moreover, they promote aerial growth, the shoot-to-root ratio, and raise the level of essential nutrients. Our results indicate that Micromonospora acts as a Rhizobia Helper Bacteria (RHB) agent and has probiotic effects, promoting plant growth and increasing nutrition efficiency. Its ecological role, biotechnological potential and advantages as a plant probiotic bacterium (PPB) are also discussed.
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