More than one hundred years have passed since the development of the first microbial inoculant for plants. Nowadays, the use of microbial inoculants in agriculture is spread worldwide for different ...crops and carrying different microorganisms. In the last decades, impressive progress has been achieved in the production, commercialization and use of inoculants. Nowadays, farmers are more receptive to the use of inoculants mainly because high-quality products and multi-purpose elite strains are available at the market, improving yields at low cost in comparison to chemical fertilizers. In the context of a more sustainable agriculture, microbial inoculants also help to mitigate environmental impacts caused by agrochemicals. Challenges rely on the production of microbial inoculants for a broader range of crops, and the expansion of the inoculated area worldwide, in addition to the search for innovative microbial solutions in areas subjected to increasing episodes of environmental stresses. In this review, we explore the world market for inoculants, showing which bacteria are prominent as inoculants in different countries, and we discuss the main research strategies that might contribute to improve the use of microbial inoculants in agriculture.
The genus
Azospirillum
comprises plant-growth-promoting bacteria (PGPB), which have been broadly studied. The benefits to plants by inoculation with
Azospirillum
have been primarily attributed to its ...capacity to fix atmospheric nitrogen, but also to its capacity to synthesize phytohormones, in particular indole-3-acetic acid. Recently, an increasing number of studies has attributed an important role of
Azospirillum
in conferring to plants tolerance of abiotic and biotic stresses, which may be mediated by phytohormones acting as signaling molecules. Tolerance of biotic stresses is controlled by mechanisms of induced systemic resistance, mediated by increased levels of phytohormones in the jasmonic acid/ethylene pathway, independent of salicylic acid (SA), whereas in the systemic acquired resistance—a mechanism previously studied with phytopathogens—it is controlled by intermediate levels of SA. Both mechanisms are related to the NPR1 protein, acting as a co-activator in the induction of defense genes.
Azospirillum
can also promote plant growth by mechanisms of tolerance of abiotic stresses, named as induced systemic tolerance, mediated by antioxidants, osmotic adjustment, production of phytohormones, and defense strategies such as the expression of pathogenesis-related genes. The study of the mechanisms triggered by
Azospirillum
in plants can help in the search for more-sustainable agricultural practices and possibly reveal the use of PGPB as a major strategy to mitigate the effects of biotic and abiotic stresses on agricultural productivity.
ABSTRACT For decades, researchers around the world search for strategies aiming at higher sustainability in agriculture. The microbial inoculants or biofertilizers are biotechnological products used ...for different purposes, the main one being to totally or partially replace chemical fertilizers, with an emphasis on N-fertilizers, reducing costs of production and decreasing the contamination of the soil, water, and atmosphere. Depending on the microorganism and the inoculated crop, inoculants can also induce plant protection to abiotic and biotic stresses and positively modify their physiology. Although inoculation studies and the use of inoculants by farmers date more than a century ago, they have gained more notoriety in the past decade. Brazil has a long tradition in the use of rhizobial inoculants, especially for the soybean crop, but it was only in 2009 that the first commercial inoculant carrying the plant-growth-promoting Azospirillum brasilense strains Ab-V5 (=CNPSo 2083) and Ab-V6 (=CNPSo 2084), identified by our research group, reached the market. One decade after the release of these two strains, 10.5 million doses were commercialized for grasses, including corn, wheat, rice, and pastures of brachiarias, and co-inoculation of legumes, such as soybean and common bean. Several research groups in Brazil presented impressive results of increases in root growth, biomass production, grain yield, uptake of nutrients and water, and increased tolerance to abiotic stresses due to the inoculation with Ab-V5 and Ab-V6. In this review, we gathered the results obtained so far in one decade with these two strains in several grasses and legume crops, confirming their versatility and indicating that with convincing, reliable, and consistent results, the Brazilian farmers are receptive to the adoption of new sustainable technologies based on microorganisms.
Bacteria belonging to the genus Bradyrhizobium are capable of establishing symbiotic relationships with a broad range of plants belonging to the three subfamilies of the family Leguminosae ( = ...Fabaceae), with the formation of specialized structures on the roots called nodules, where fixation of atmospheric nitrogen takes place. Symbiosis is under the control of finely tuned expression of common and host-specific nodulation genes and also of genes related to the assembly and activity of the nitrogenase, which, in Bradyrhizobium strains investigated so far, are clustered in a symbiotic island. Information about the diversity of these genes is essential to improve our current poor understanding of their origin, spread and maintenance and, in this study, we provide information on 40 Bradyrhizobium strains, mostly of tropical origin. For the nodulation trait, common (nodA), Bradyrhizobium-specific (nodY/K) and host-specific (nodZ) nodulation genes were studied, whereas for fixation ability, the diversity of nifH was investigated. In general, clustering of strains in all nod and nifH trees was similar and the Bradyrhizobium group could be clearly separated from other rhizobial genera. However, the congruence of nod and nif genes with ribosomal and housekeeping genes was low. nodA and nodY/K were not detected in three strains by amplification or hybridization with probes using Bradyrhizobium japonicum and Bradyrhizobium elkanii type strains, indicating the high diversity of these genes or that strains other than photosynthetic Bradyrhizobium must have alternative mechanisms to initiate the process of nodulation. For a large group of strains, the high diversity of nod genes (with an emphasis on nodZ), the low relationship between nod genes and the host legume, and some evidence of horizontal gene transfer might indicate strategies to increase host range. On the other hand, in a group of five symbionts of Acacia mearnsii, the high congruence between nod and ribosomal/housekeeping genes, in addition to shorter nodY/K sequences and the absence of nodZ, highlights a co-evolution process. Additionally, in a group of B. japonicum strains that were symbionts of soybean, vertical transfer seemed to represent the main genetic event. In conclusion, clustering of nodA and nifH gives additional support to the theory of monophyletic origin of the symbiotic genes in Bradyrhizobium and, in addition to the analysis of nodY/K and nodZ, indicates spread and maintenance of nod and nif genes through both vertical and horizontal transmission, apparently with the dominance of one or other of these events in some groups of strains.
We assessed the effects of co-inoculation with
Azospirillum brasilense
or the application of its exudates via seeds or leaf spray on root morphological traits and nodulation, as well as on shoot ...development and grain yield of soybean inoculated with
Bradyrhizobium
. Two experiments were performed in sterile substrate under greenhouse, and two were performed at field conditions in sandy soils in a crop season with episodes of drought. The treatments in the greenhouse experiments comprised the non-inoculated control, sole inoculation with
Bradyrhizobium
, co-inoculation of
Bradyrhizobium
and
A. brasilense
, and inoculation of
Bradyrhizobium
with the application of
Azospirillum
exudates via seeds or leaf spray. Field treatments included non-inoculated controls without and with N-fertilizer and inoculation of
Bradyrhizobium
and co-inoculation with
A. brasilense
. Plants were assessed for root weight, total and specific lengths, volume, diameter, tissue density, branches number, root-hair length and incidence, nodule number and dry weight, shoot dry weight and N content, and grain yield. Co-inoculation of
Bradyrhizobium
and
A. brasilense
and seed application of
A. brasilense
exudates increased the number of root branches and nodules compared with the sole inoculation of
Bradyrhizobium
. However, leaf spray application of exudates was less effective. Co-inoculation also increased specific root length, root length density in soil, root-hair incidence and length, and total N content in shoot, altogether resulting in increases in grain yield. Co-inoculation of soybean with
Bradyrhizobium
and
A. brasilense
improved several root morphological traits, increasing the plant capacity to overcome moderate drought stress episodes in sandy soils, allowing to reach higher yields.
strains Ab-V5 and Ab-V6 are largely used in commercial inoculants for grasses and legumes in Brazil. Their genomes were estimated at 6,934,595 and 7,197,196 bp, respectively, and encompass genes ...related to nitrogen fixation, synthesis of phytohormones, and environmental adaptation. Although the strains differ in phenotypic properties, their genomes are highly similar.
Azospirillum
spp. are plant-growth-promoting bacteria used worldwide as inoculants for a variety of crops. Among the beneficial mechanisms associated with
Azospirillum
inoculation, emphasis has been ...given to the biological nitrogen fixation process and to the synthesis of phytohormones. In Brazil, the application of inoculants containing
A. brasilense
strains Ab-V5 and Ab-V6 to cereals is exponentially growing and in this study we investigated the effects of maize inoculation with these two strains applied on seeds or by leaf spray at the V2.5 stage growth—a strategy to relieve incompatibility with pesticides used for seed treatment. We also investigate the effects of spraying the metabolites of these two strains at V2.5. Maize growth was promoted by the inoculation of bacteria and their metabolites. When applied via foliar spray, although
A. brasilense
survival on leaves was confirmed by confocal microscopy and cell recovery, few cells were detected after 24 h, indicating that the effects of bacterial leaf spray might also be related to their metabolites. The major molecules detected in the supernatants of both strains were indole-3-acetic acid, indole-3-ethanol, indole-3-lactic acid and salicylic acid. RT-PCR of genes related to oxidative stress (
APX1
,
APX2
,
CAT1
,
SOD2
,
SOD4
) and plant defense (pathogenesis-related
PR1, prp2
and
prp4
) was evaluated on maize leaves and roots. Differences were observed according to the gene, plant tissue, strain and method of application, but, in general, inoculation with
Azospirillum
resulted in up-regulation of oxidative stress genes in leaves and down-regulation in roots; contrarily, in general,
PR
genes were down-regulated in leaves and up-regulated in roots. Emphasis should be given to the application of metabolites, especially of Ab-V5 + Ab-V6 that in general resulted in the highest up-regulation of oxidative-stress and
PR
genes both in leaves and in roots. We hypothesize that the benefits of inoculation of
Azospirillum
on seeds or by leaf spray, as well as of leaf spraying of
Azospirillum
metabolites, are strongly correlated with the synthesis of phytohormones and by eliciting genes related to plant-stress tolerance and defense against pathogens.
Purpose
Brazil has 180 Mha of pastures, 86 Mha occupied with
Urochloa
spp. (syn.
Brachiaria
), and 70% in some level of degradation. Inoculation with plant-growth-promoting bacteria (PGPR) may ...represent an economic and environmental feasible strategy to improve pasture production.
Methods
Two greenhouse and seven field trials were performed to verify the effects of seed inoculation at sowing or leaf-spray inoculation in established pastures of
Urochloa
with elite strains of
Azospirillum brasilense
and
Pseudomonas fluorescens
. All plants received nutrients including 40 kg ha
−1
of N at sowing, and half of the treatments a supply of 40 kg ha
−1
of N 30 days after emergence.
Results
A. brasilense
increased shoot biomass by an average of 16.8% with both seed and leaf-spray inoculation, whereas
P. fluorescens
increased by 15.2 and 14.2%, respectively, always higher with the extra supply of N. Seed and leaf-spray inoculation with
A. brasilense
increased N content by an average of 11.7 and 20.7%, and K by 9.9 and 11.3%, respectively; for
P. fluorescens
average increases were of 33.3 and 36.6% for P, and of 10.6 and 13.6% for K, respectively. Benefits were mainly attributed to improvements in root architecture by the synthesis of phytohormones. Biological nitrogen fixation in
A. brasilense
, P acquisition (solubilization of phosphates and siderophores synthesis) and ACC-deaminase in
P. fluorescens
also contributed to plant growth and nutrient status.
Conclusion
Inoculation with elite PGPR strains translated into more fodder and improved nutritional value of feed for livestock, representing a promising and environmentally-friendly strategy for tropical pastures.
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•Determinate and indeterminate soybeans had similar responses to N fertilization.•N fertilizer had negative effects in nodule number and dry weight of soybeans.•Neither basal nor ...topdressing N fertilization improved soybean yield.•Bradyrhizobium inoculation is sufficient to supply all N needs of soybeans.
Soybean (Glycine max (L.) Merrill) new cultivars have high N demands to sustain increasing grain yields. One may ask whether these cultivars would need basal and/or topdressing N application as perhaps symbiotic N2 fixation would not catch up with the increased N needs. In this study, three field experiments were performed in three different ecoregions of Brazil to compare soybean with determinate (BRS-294-RR and BRS-295-RR) and indeterminate (Vmax-RR, BMX-Potência-RR and BRS 284) growth types. The experiments consisted of four treatments, all inoculated with efficient Bradyrhizobium strains, but with differences in N fertilizer (as urea) application: (T1) control without N fertilizer; (T2) basal application of 30kgNha−1 at sowing; (T3) topdressing of 50kgNha−1 at full flowering; (T4) 30kgNha−1 and 50kgNha−1 as basal and topdressing, respectively. None of the basal or topdressing treatments improved biomass production or grain yield. However, N fertilizer negatively affected nodule number and dry weight, pointing out a down-regulation in the symbiotic performance, without improving crop yields. Similar results were observed in cultivars of both growth types, indicating that biological nitrogen fixation can supply N needs of new soybean genotypes of both growth types.
Twenty years ago, the first members of the genus
Burkholderia
capable of nodulating and fixing N
2
during symbiosis with leguminous plants were reported. The discovery that β-proteobacteria could ...nodulate legumes represented a breakthrough event because, for over 100 years, it was thought that all rhizobia belonged exclusively to the α-Proteobacteria class. Over the past 20 years, efforts toward robust characterization of these bacteria with large-scale phylogenomic and taxonomic studies have led to the separation of clinically important and phytopathogenic members of
Burkholderia
from environmental ones, and the symbiotic nodulating species are now included in the genera
Paraburkholderia
and
Trinickia
.
Paraburkholderia
encompasses the vast majority of β-rhizobia and has been mostly found in South America and South Africa, presenting greater symbiotic affinity with native members of the families Mimosoideae and Papilionoideae, respectively. Being the main center of
Mimosa
spp. diversity, Brazil is also known as the center of symbiotic
Paraburkholderia
diversity. Of the 21 symbiotic
Paraburkholderia
species described to date, 11 have been isolated in Brazil, and others first isolated in different countries have also been found in this country. Additionally, besides the symbiotic N
2
-fixation capacity of some of its members,
Paraburkholderia
is considered rich in other beneficial interactions with plants and can promote growth through several direct and indirect mechanisms. Therefore, these bacteria can be considered biological resources employed as environmentally friendly alternatives that could reduce the agricultural dependence on agrochemical inputs.
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