Plant Growth Promotion Capacity Research Articles

Spatial and Temporal Shifts of Endophytic Bacteria in Conifer Seedlings of Abies religiosa (Kunth) Schltdl. & Cham.

Endophytes play an important role in plant development, survival, and establishment, but their temporal dynamics in young conifer plants are still largely unknown. In this study, the bacterial community was determined by metabarcoding of the 16S rRNA gene in the rhizoplane, roots, and aerial parts of 1- and 5-month-old seedlings of natural populations of Abies religiosa (Kunth) Schltdl. & Cham. In 1-month-old seedlings, Pseudomonas dominated aerial parts (relative abundance 71.6%) and roots (37.9%). However, the roots exhibited significantly higher bacterial species richness than the aerial parts, with the dissimilarity between these plant sections mostly explained by the loss of bacterial amplification sequence variants. After 5 months, Mucilaginibacter dominated in the rhizoplane (9.0%), Streptomyces in the roots (12.2%), and Pseudomonas in the aerial parts (18.1%). The bacterial richness and community structure differed significantly between the plant sections, and these variations were explained mostly by 1-for-1 substitution. The relative abundance of putative metabolic pathways significantly differed between the plant sections at both 1 and 5 months. All the dominant bacterial genera (e.g., Pseudomonas and Burkholderia-Caballeronia-Paraburkholderia) have been reported to have plant growth-promoting capacities and/or antagonism against pathogens, but what defines their role for plant development has still to be determined. This investigation improves our understanding of the early plant-bacteria interactions essential for natural regeneration of A. religiosa forest.

Enhancing the growth and zinc efficiency of cherry tomatoes by inoculating endophytic bacteria from zinc hyperaccumulator, Sedum alfredi Hance

Microbial biofertilizers, an innovative and eco-friendly strategy for nutritional improvement, could be utilized for zinc (Zn) efficiency and biofortification. Endophytic bacteria with Zn solubilizing and plant growth-promoting capacities were isolated from hyperaccumulator S. alfredi, and their impacts on the growth and Zn efficiency of cherry tomatoes under Zn-deficiency conditions were investigated. The results showed that most endophytic bacteria promoted the growth of tomato seedlings, while only specific endophytic bacteria played a role in Zn accumulation. Among them, inoculation with SaBA1, SaPS2, SaEN1, and SaPA1 not only significantly increased the fresh weight (FW) of tomato's young leaves by 3.1 to 3.6 times, but also raised the soil available Zn concentration by 0.58–0.92 mg/kg and improved the Zn accumulation in young leaves by 18 % to 30 %. The results indicated that these four bacteria could activate the availability of soil Zn, regulate the distribution of Zn in plants, and enable tomatoes to maintain healthy growth and development under Zn-deficiency conditions. This revealed their immense potential in enhancing crop yield, improving crop Zn nutrition, and boosting crop Zn efficiency.

Alone as effective as together: AMF and Trichoderma inoculation boost maize performance but differentially shape soil and rhizosphere microbiota

AbstractIntroductionInoculation of plants with beneficial microorganisms may improve plant performance yet suffers from efficacy variability. A solution might be the combined application of different inoculants as consortium. The objective of the present study was to evaluate the effects of single or combined inoculation of Trichoderma harzianum, strain TGFG411, and a consortium of arbuscular mycorrhizal fungi (AMF) on plant growth, and native microbial communities (here bacteria/archaea, fungi and AMF) in root‐associated soil (RAS) and rhizosphere (RH), that is, soil loosely or tightly attached to the roots, respectively.Materials and MethodsA greenhouse experiment was carried out with non‐sterile agricultural soil and the model crop maize, which was single inoculated with either TGFG411 or AMF or received a combined inoculation of TGFG411 + AMF. Control plants received only water. Seven weeks after the second AMF inoculation, the plant growth promotion capacity of the inoculants was measured based on shoot and root parameters. Furthermore, RAS and RH microbiota (fungi including AMF, bacteria and archaea) were assessed via a combination of different cultivation‐dependent, microscopic and DNA‐based methods.ResultsAfter 7 weeks of maize growth, both single and combined inoculation of AMF and TGFG411 enhanced shoot dry weight and led to a significant reduction in root biomass. The TGFG411 strain successfully established in the soil. However, no definite evidence for the establishment of the inoculated AMF was found. Single or combined inoculation of TGFG411 and AMF modified the composition of total bacterial in the RH, whereas modulated total fungal communities in the RAS.ConclusionThe combined inoculation did not result in a significant improvement of plant performance compared with single inoculation likely due to optimal nutrient supply. However, samples receiving the combined inoculation exhibited a distinct modulation of the native RAS/RH microbiota, which may influence the inoculant efficacy under less favourable conditions.

Genetic diversity, plant growth promotion potential, and antimicrobial activity of culturable endophytic actinobacteria isolated from Aconitum carmichaelii Debeaux.

This study evaluated the phylogenetic diversity, plant growth promotion capacity, antifungal activity, and biocontrol potential of culturable actinobacterial endophytes isolated from the medicinal plant Aconitum carmichaelii Debeaux. Isolation of actinobacteria from healthy A. carmichaelii plants was carried out on six different media. Full-length 16S rRNA gene was amplified by PCR from the genomic DNA of each strain. Indole-3-acetic acid and siderophore production were quantitatively assessed by the Salkowski and Chrome Azurol S methods, respectively. Rice seeds germination and seedling growth were employed to evaluate plant growth promotion capacities of candidate strains. Dual-culture assay and pot experiments were performed to investigate the antifungal and biocontrol potential of isolates. We obtained 129 actinobacterial isolates from A. carmichaelii, and they belonged to 49 species in 7 genera. These strains exhibited diverse plant growth promotion ability, among which one strain significantly enhanced rice seeds germination, while 31 strains significantly facilitated rice seedling growth. SWUST-123 showed strong antifungal activity against four pathogens in vitro and was most compatible with Qingchuan cultivar. SWUST-123 reduced around 40% of southern blight disease occurrence compared to blank control treatment. . Aconitum carmichaelii harbored genetically diverse actinobacterial endophytes exhibiting diverse plant growth promotion and antifungal potential, some of which can be served as good candidates for biofertilizers and biocontrol agents.

Lettuce growth improvement by Azospirillum argentinense and fluorescent Pseudomonas co-inoculation depends on strain compatibility

Azospirillum and fluorescent Pseudomonas are plant-associated bacteria used for inoculant formulation worldwide, both alone and in combination. Dual inoculation of these two bacterial groups is a common practice that is not usually accompanied by an assessment of strain compatibility. Here we sought to investigate to what extent Azospirillum and Pseudomonas strain compatibility influences their plant-growth promoting activity. Several strain combinations were analyzed for in vitro compatibility when growing in dual-species biofilms. The combinations A. argentinense Az39 + P. fluorescens A506 and A. argentinense Az39 + P. protegens Pf-5 were selected to investigate their plant growth-promotion capacity in lettuce, in comparison to single strains. When co-inoculated, Az39 negatively affected the early root colonization of P. fluorescens A506 but extended its survival in the roots for a longer period. Plant analysis at 35 days after inoculation showed that Az39 and A506 act synergistically to promote lettuce growth, increasing the measured parameters when compared to uninoculated plants and outperforming the rest of the bacterial treatments. In contrast, the Az39 + Pf-5 combination did not modify plant growth, indicating that these strains interfere in vivo to mutually impair their individual plant growth-promoting effects. Similarly, synergism or interference between these strain combinations was observed when plant yield was analysed at harvest time. We conclude that strain compatibility is an absolute requirement to achieve increased plant growth promotion by co-inoculation of beneficial strains. Dual inoculation of different bacterial species without making a compatibility study may impede the plant-stimulatory potential of individual strains, in turn leading to poor performance of the inoculant.

Study of the plant growth-promoting capacity of Pseudomonas putida 1046 in a model plant system

Plant Growth Promoting Rhizobacteria (PGPR) represent a microbial community that exerts growth-promoting capabilities in plants by various mechanisms. Among the PGPR genera, Pseudomonas spp. deserves special attention. It is due to its characteristic traits, like the production of phytohormones and siderophores, solubilization of minerals and phosphates, and plant protection from biotic and abiotic stress. These PGPR properties depend on the microorganism and its plant counterpart. The use of microbial strains as bioinoculants must consider the physiological and economic aspects of the process, and the plant growth stimulating effect has to be checked and proved. This study aimed to explore the PGP capacity of Pseudomonas putida 1046 strain in a model plant system of the economically important corn culture (Zea mays). The effect of the strain’s metabolic status on the plant germination capacity was evaluated. Bacterial cultures, grown 16 h and 48 h, were explored for the treatment of the corn seeds at three experimental concentrations: 0.1, 0.2, and 0.4%, and monitoring of their germination capacity through the growth indicators length of the radicle, length of the coleoptile, and the number of lateral roots. The data obtained outline the positive effect of Pseudomonas putida 1046 on the germination capacity of corn when applied at 0.2% concentration. The in vitro treatment of the model plants with 0.2% suspension resulted in a 22.87%–28.33% increase in the length of the radicle, a 35.96%–49.56% increase in the length of the coleoptile, and a 5.41–16.67% increase in the number of the lateral roots. High values of the vigour index (2125 for 16 h and 2721 for 48 h culture) were also registered. The strain’s ability to produce siderophores of hydroximate type and exhibit phosphate solubilizing activity is proved. The optimal treatment parameters of the corn seeds comprise the application of 0.2% suspension of 16 h grown Pseudomonas putida 1046 strain for five days.

Taxonomic Study of Three Novel Paenibacillus Species with Cold-Adapted Plant Growth-Promoting Capacities Isolated from Root of Larix gmelinii.

Exploration of the novel species of the genus Paenibacillus with plant-growth promoting characteristics at the low-temperature environment is of great significance for the development of psychrotolerant biofertilizer in forestry and agriculture. During the course of isolation of root endophytes of Larix gmelinii in the island frozen soil, three strains designated as T3-5-0-4, N1-5-1-14 and N5-1-1-5 were isolated. The three strains showed plant growth-promoting properties at the low temperature, such as phosphate solubilization, indole-3-acetic acid biosynthesis and siderophore production. According to pairwise sequence analyses of the 16S rRNA genes, the three strains represent putatively novel taxa within the genus Paenibacillus. The strains have typical chemotaxonomic characteristics of the genus Paenibacillus by having meso-diaminopimelic acid as diagnostic diamino acid, anteiso-C15:0 as the predominant fatty acid and MK-7 as the predominant menaquinone. The polar lipid profiles of all strains contained diphosphatidylglycerol, phosphatidylglycerol, and phosphatidylethanolamine. The sizes of the genomes of the stains ranged from 5.66 to 9.07 Mb and the associated G+C contents ranged from 37.9% to 44.7%. Polyphasic taxonomic study including determination of genome relatedness indices revealed that the strains are representatives of three novel species in the genus Paenibacillus. Consequently, isolates T3-5-0-4, N1-5-1-14 and N5-1-1-5 are proposed as novel species for which the names of Paenibacillus endoradicis sp. nov. (CFCC15691T = KCTC43441T), Paenibacillus radicibacter sp. nov, (CFCC15694T = KCTC43442T) and Paenibacillus radicis sp. nov. (CFCC15710T = KCTC43173T), respectively. Moreover, analysis for biosynthetic genes showed that the strains have potential for plant growth-promoting characteristics, plant rhizospheres colonization and low-temperature adaption, most of which are consistent with the results of the bioactivity test.

Insights on strain 115 plant growth-promoting bacteria traits and its contribution in lead stress alleviation in pea (Pisum sativum L.) plants.

The present study aims to characterize the plant growth-promoting bacterial traits of Bacillus simplex (strain 115). This bacterium was inoculated in hydroponically conditions to improve pea (Pisum sativum L.) growth submitted to lead (Pb) toxicity. Root nodulation system was developed enough in 23-day-old plants attesting the interaction between the two organisms. In addition to its phosphate solubilization and siderophore production traits that reached 303.8μgPmL-1 and 49.6 psu respectively, the Bacillus strain 115 exhibited Pb bio-sorption ability. Inoculation of Pb-stressed pea with strain 115 showed roots and shoots biomass recovery (+ 70% and + 61%, respectively). Similarly, water and protein contents were increased in Pb-treated plants after bacterial inoculation. In the presence of strain 115, Pb relative toxicity level decreased (-39.3% compared to Pb stress only). Moreover, catalase and superoxide dismutase activities were upregulated in Pb-exposed plants (+ 56% and + 51%, respectively). After inoculation with strain 115, catalase and superoxide dismutase activities were restored by -38% and -44% respectively. Simultaneously, oxidant stress indicator (H2O2 and 4-hydroxynonenal) and osmo-regulators (proline and glycine-betaine) contents as well as lipoxygenase activity decreased significantly in Pb-treated plants after Bacillus strain's inoculation. Taken together, the results give some evidences for the plant growth-promoting capacity of strain 115 in helping alleviation of Pb stress.

Insights into Genomic Evolution and the Potential Genetic Basis of Klebsiella variicola subsp. variicola ZH07 Reveal Its Potential for Plant Growth Promotion and Autotoxin Degradation.

The accumulation of autotoxins in soil causes continuous cropping obstacle stress in crops, and the bioremediation of autotoxins by microorganisms is an efficient process. In this study, strain ZH07 was isolated from the peanut rhizosphere and was found to be utilizing multiple autotoxins as its carbon sources. Based on its genomic characteristics and a phylogenetic analysis, ZH07 represents a member of Klebsiella variicola subsp. variicola. A comparative genomic analysis exhibited evolutionary dynamics exhibited by mobile genetic elements (MGEs), strain-specific genes, potential horizontal genes, and evolutionary constraints driven by purifying selection, which facilitated its genomic adaptation to rhizosphere soil. Genome mining revealed the potential genomic properties associated with plant growth promotion, such as nitrogen fixation, indole acetic acid synthesis, phosphonate solubilization and assimilation, siderophore production, and secondary metabolite synthesis. Moreover, abundant genes putatively responsible for the biodegradation of aromatic xenobiotics, including benzoic acid, cinnamic acid, vanillic acid, protocatechuic acid, phenylacetic acid, and p-hydroxybenzoic acid were also observed in the ZH07 genome. Compared to autotoxin stress alone, the combination of ZH07 application promoted peanut germination and seedling growth. Our analysis revealed the genetic adaptation of ZH07 to the rhizosphere environment and the potential genetic basis and effectiveness of the isolate to serve as a plant growth stimulator. IMPORTANCE Continuous cropping obstacles reduce the production and quality of agricultural products, and the application of rhizosphere beneficial microbes is an important strategy. Strain ZH07 showed autotoxin-degrading and plant growth-promoting capacities. The objectives of this study were to characterize its genomic evolution and the potential genetic basis of the autotoxin degradation and plant growth promotion. ZH07 represents a member of Klebsiella variicola subsp. variicola, based on genomic and phylogenetic analyses. Its genomic components have undergone different degrees of purifying selection, and the disparity in the evolutionary rate may be associated with its niche adaptation. A systematic analysis of the ZH07 genome identified the potential genetic basis that contributes to plant growth promotion and to aromatic xenobiotic biodegradation. This study demonstrates that plant growth-promoting rhizobacteria (PGPR) play important roles in autotoxin biodegradation and can be used as biofertilizers to enhance the growth of peanuts in response to continuous cropping obstacle stress.

Bioprospecting Soil Bacteria from Arid Zones to Increase Plant Tolerance to Drought: Growth and Biochemical Status of Maize Inoculated with Plant Growth-Promoting Bacteria Isolated from Sal Island, Cape Verde.

Climate change and anthropogenic activities are responsible for extensive crop yield losses, with negative impact on global agricultural production. The occurrence of extreme weather events such as drought is a big challenge for agriculture, negatively impacting crops. Thus, methodologies reducing crop dependence on water will be a great advantage. Plant roots are colonized by soil bacteria, that can establish beneficial associations with plants, increasing crop productivity and plant tolerance to abiotic stresses. The aim of this study was to promote plant growth and to increase crop tolerance to drought by inoculation with osmotolerant bacterial strains. For that, bacteria were isolated from plants growing in Sal Island (Cape Verde) and identified. The osmotolerance and plant-growth promotion (PGP) abilities of the strains were determined. A maize seed cultivar tolerant to drought was inoculated with the strains evidencing best PGP capacity and osmo-tolerance. Results evidenced the ability of some bacterial strains increasing the development and inducing osmotolerance in plants. These results evidence the potential of osmotolerant bacteria to further increase the level of tolerance of maize varieties tolerant to drought, decreasing the dependence of this crop on irrigation, and open new perspectives to growth maize in drought affected areas and to use water more efficiently.

Seedling growth promotion and potential biocontrol against phytopathogenic Fusarium by native rhizospheric Pseudomonas spp. strains from Amarillo Zamorano maize landrace

Maize is one of the most important staple crops around the word. In Mexico, Los Altos de Jalisco is an agricultural region with the highest production of maize with rainfed irrigation, an agronomic management based on the use of no longer sustainable agrochemicals, and sowing of coated seeds of different commercial origin. Nowadays, the microbiota associated to the rhizosphere of native landraces is considered a reservoir of microbial resources with the potential to promote plant growth and control diseases. With the aim to find ecofriendly and cost-effective alternatives that can reduce the use of agrochemicals at seed germination and seedling growth of maize crop in Los Altos de Jalisco, this study focused on the search of fluorescent Pseudomonas strains as germination and seedlings growth promoters and biocontrol agents of fusariosis in maize, and highlight the importance of Mexican maize landraces as natural reservoirs of plant growth-promoting microorganisms. Samples of rhizospheric soil and seeds were collected from ten maize landraces and a pure line. Pseudomonas and Fusarium strains were isolated and selected by its antagonistic and pathogenic potential, respectively. Pseudomonas strains were phylogenetically identified by a 16S rRNA gene analysis, and Fusarium strains by the phylogenetic analysis of ITS region. The potential of Pseudomonas strains to promote seed germination and seedling/plant growth was explored through metabolic profiles and bioassays to evaluate bacteria-plant-fungus interactions. A total of 57 Fusarium spp. strains were grouped into four different Fusarium species complexes and 14 fluorescent Pseudomonas strains were classified into four Pseudomonas fluorescens subgroups. Pseudomonas strains displayed plant growth promotion and biocontrol capacity by increasing 38%–66% the dry weight of maize seedlings (p < 0.05), 23%–63% growth-inhibition of different Fusarium strains in dual cultures, and a range of 4.5–20 inhibition score based on a rate scale when inoculated in seeds. Pseudomonas spp. metabolic profiles were diverse, but all the strains proved to be innocuous, and Pseudomonas sp. 4L, 10L and 11L showed desirable enzymatic capacities from an agrobiotechnological approach. Additionally, Pseudomonas sp. 4L increased germination and seedling growth of maize, by increasing 20–51% the number of germinated seeds, 16–38% the length of seedling aerial part, 30–140% seedling length of roots, 15–32% seedling fresh weight, and 21–33% seedling dry weight, and reduced 75–96% the damage generated by Fusarium spp. on naturally infected seeds. This study revealed that Pseudomonas sp. 4L and 11L are regional promising ecofriendly and cost-effective alternatives as adjuvants in the nutritional and phytosanitary management at seed germination and seedling growth of maize crop in Los Altos de Jalisco region, and highlight the importance of the rhizosphere microbiome of Mexican maize landrace as natural reservoir of plant growth-promoting microorganisms.

Suppression of Macrophomina root rot, Fusarium wilt and growth promotion of some pulses by antagonistic rhizobacteria

The present study aimed to isolate Bacillus and Streptomyces species to control Macrophomina phaseolina and Fusarium oxysporum f. sp. udum that cause root rot and wilt diseases in pulses in general and pigeon pea, respectively. Bacillus subtilis BRBac4-1, B. subtilis BRBac24-2 and Bacillus siamensis BRBac21-1, Streptomyces cavourensis BRAcB10-1 and Streptomyces griseofuscus BRAcB11-2 showed potential antagonistic activity against Macrophomina phaseolina and Fusarium oxysporum f. sp. udum, in a dual culture assay. These rhizobacteria were found to possess multiple plant growth-promoting traits besides producing -lytic enzymes, non-volatile metabolites (iturin and surfactin), and volatile organic carbon compounds (disulphide, azithromycin, 1-butanol, 2-methyl, and dimethyl disulphide) with antifungal activities which were detected in culture filtrate through GC-MS and LC-MS analyses. A pot culture experiment was conducted to test plant growth promotion and disease suppression capacity of the selected rhizobacteria in green gram, black gram, and red gram. In a pot culture experiment, combined inoculation of B. siamensis BRBac21-1 plus S. cavourensis BRAcB10-1 rendered the plant more resistant to fungal infection and further development through the induction of systemic defense in host tissue by improving the status of different defense enzymes (phenylalanine ammonia lyase, polyphenol oxidase, and peroxidase). Furthermore, these two bacteria significantly reduced disease severity, and, promoted shoot and root growth in all the tested crops. In conclusion, B. siamensis BRBac21-1 and S. cavourensis BRAcB10-1 displayed the potential antifungal activities and promoted plant growth and the combined bio-formulation of these two rhizobacteria may be used to manage wilt and root rot pathogens in pulses after field evaluation.

In vitro bioprospecting of rhizospheric fungi associated to cactus (Opuntia cochenillifera) to plant growth promotion

This study aimed to evaluate in vitro the plant growth promotion capacity of rhizospheric fungi from Opuntia cochenillifera. The isolates were submitted to phosphate solubilization, acetic acid-3-indole (AIA) production, antagonism against phytopathogens, and growth under water activity. All test results were subjected to statistical analysis. The studied isolates were able to solubilize phosphate, being F04 (Aspergillus sp.) and F05 (Coprinellus radians) the ones that presented the highest solubilization indices. As to produce AIA, F02 (Penicillium sp.) and F07 (Aspergillus sp.) were the most promising. All isolates could inhibit the growth of Colletotrichum sp. and Fusarium sp. Water restriction provided mycelial growth for all isolates. Therefore, these characteristics confer characteristics as growth promoters in plants.

Bioprospecting of Plant Growth-Promoting Traits of Pseudomonas sp. Strain C3 Isolated from the Atacama Desert: Molecular and Culture-Based Analysis

Soil microorganisms that inhabit extreme environments have unique metabolic capacities and/or physical structures that allow them to survive in oligotrophic conditions. The bioprospecting of unknown bacteria in the context of current advances in genome mining is fundamental for the discovery of natural products with novel properties or applications. In this study, the plant growth-promoting and biocontrol traits of a Pseudomonas isolated from soil associated with plants from the Atacama Desert were characterized by whole-genome sequencing and in vitro assays. A high-quality genome draft of Pseudomonas sp. isolate C3 was obtained. An automated biosynthetic gene cluster analysis using antiSMASH 6.0 revealed the presence of a cluster of genes for the biosynthesis, regulation, and transport of the metabolite 2,4-diacetylphloroglucinol, which showed a high protein sequence identity (&gt;89%) with a validated orthologous gene cluster from another Pseudomonas. In addition, via an in vitro assay, the biocontrol activity of Pseudomonas sp. isolate C3 against Botrytis cinerea, Monilinia fructicola, Phytium sp., Alternaria sp., Geotrichum candidum, and Fusarium oxysporum was corroborated. Finally, through KofamKOALA, the presence of genes involved in different metabolic pathways of plant growth-promoting traits was identified, which was corroborated by in vitro assays. This study provides information obtained from genomic analyses and culture tools on a bacterial isolate from the Atacama Desert characterized by plant growth-promoting capacities and biocontrol activity.

Bacteria in (vermi)composted organic wastes mostly survive when applied to an arable soil cultivated with wheat (Triticum sp. L.).

Composting and vermicomposting are an environmentally friendly way to reduce pathogens in organic wastes and generate a valuable product that provides nutrients for crops. However, how the bacterial community structure changes during these different processes and if the bacteria applied with the (vermi)composted products survive in an arable cultivated soil is still largely unknown. In this study, we monitored how the bacterial community structure changed during conditioning, composting with and without Eisenia fetida, and when the end-product was applied to arable soil cultivated with wheat Triticum sp. L. The organic wastes used were biosolid, cow manure, and a mixture of both. Large changes occurred in the relative abundance of some of the most abundant bacterial genera during conditioning, but the changes were much smaller during composting or vermicomposting. The bacterial community structure was significantly different in the organic wastes during conditioning and (vermi)composting but adding E. fetida had no significant effect on it. Changes in the relative abundance of the bacterial groups in the (vermi)composted waste applied to the arable soil cultivated with wheat were small, suggesting that most survived even after 140days. As such, applying (vermi)composted organic wastes not only adds nutrients to a crop but also contributes to the survival of plant growth-promoting bacteria found in the (vermi)compost. However, putative human pathogens found in the biosolid also survived in the arable soil, and their relative abundance remained high but mixing the biosolid with cow manure reduced that risk. It was found that applying (vermi)composted organic wastes to an arable soil not only provides plant nutrients and adds bacteria with plant growth-promoting capacities, but some putative pathogens also survived.

Plant Growth-Promoting Rhizobacteria Modulate the Concentration of Bioactive Compounds in Tomato Fruits

Background: The application of microorganisms as bioestimulants in order to increase the yield and/or quality of agricultural products is becoming a widely used practice in many countries. In this work, five plant growth-promoting rhizobacteria (PGPR), isolated from cultivated rice paddy soils, were selected for their plant growth-promoting capacities (e.g., auxin synthesis, chitinase activity, phosphate solubilisation and siderophores production). Two different tomato cultivars were inoculated, Tres Cantos and cherry. Plants were grown under greenhouse conditions and different phenotypic characteristics were analysed at the time of harvesting. Results: Tres Cantos plants inoculated with PGPR produced less biomass but larger fruits. However, the photosynthetic rate was barely affected. Several antioxidant activities were upregulated in these plants, and no oxidative damage in terms of lipid peroxidation was observed. Finally, ripe fruits accumulated less sugar but, interestingly, more lycopene. By contrast, inoculation of cherry plants with PGPR had no effect on biomass, although photosynthesis was slightly affected, and the productivity was similar to the control plants. In addition, antioxidant activities were downregulated and a higher lipid peroxidation was detected. However, neither sugar nor lycopene accumulation was altered. Conclusion: These results support the use of microorganisms isolated from agricultural soils as interesting tools to manipulate the level of important bioactive molecules in plants. However, this effect seems to be very specific, even at the variety level, and deeper analyses are necessary to assess their use for specific applications.

Isolation, Identification, Biocontrol Activity, and Plant Growth Promoting Capability of a Superior Streptomyces tricolor Strain HM10.

Streptomyces is a genus with known biocontrol activity, producing a broad range of biologically active substances. Our goal was to isolate local Streptomyces species, evaluate their capacity to biocontrol the selected phytopathogens, and promote the plant growth via siderophore and indole acetic acid (IAA) production and phosphate solubilization. Eleven isolates were obtained from local soil samples in Saudi Arabia via the standard serial dilution method and identified morphologically by scanning electron microscope (SEM) and 16S rRNA amplicon sequencing. The biocontrol of phytopathogens was screened against known soil-borne fungi and bacteria. Plant growth promotion capacity was evaluated based on siderophore and IAA production and phosphate solubilization capacity. From eleven isolates obtained, one showed 99.77% homology with the type strain Streptomyces tricolor AS 4.1867, and was designated S. tricolor strain HM10. It showed aerial hyphae in SEM, growth inhibition of ten known phytopathogens in in vitro experiments, and the production of plant growth promoting compounds such as siderophores, IAA, and phosphate solubilization capacity. S. tricolor strain HM10 exhibited high antagonism against the fungi tested (i.e., Colletotrichum gloeosporides with an inhibition zone exceeding 18 mm), whereas the lowest antagonistic effect was against Alternaria solani (an inhibition zone equal to 8 mm). Furthermore, the most efficient siderophore production was recorded to strain HM8, followed by strain HM10 with 64 and 22.56 h/c (halo zone area/colony area), respectively. Concerning IAA production, Streptomyces strain HM10 was the most effective producer with a value of 273.02 μg/ml. An autochthonous strain S. tricolor HM10 should be an important biological agent to control phytopathogens and promote plant growth.

Towards Sustainable Bioinoculants: A Fermentation Strategy for High Cell Density Cultivation of Paraburkholderia sp. SOS3, a Plant Growth-Promoting Bacterium Isolated in Queensland, Australia

Paraburkholderia sp. SOS3 is a plant growth-promoting bacterium (PGPB) that displays pleiotropic effects and has the potential to be applied at a large scale across several agronomically important crops. The use of SOS3 is a suitable option to reduce the use of chemical fertilisers. While the benefits of SOS3 have been demonstrated in vitro, its potential applications at large scale are limited due to low biomass yield in current batch culture systems. Here, we developed a strategy for high-cell density cultivation of SOS3 in instrumented bioreactors, moving from low-biomass yield in a complex medium to high-biomass yield in a semi-defined medium. We achieved a 40-fold increase in biomass production, achieving cell densities of up to 11 g/L (OD600 = 40). This result was achieved when SOS3 was cultivated using a fed-batch strategy. Biomass productivity, initially 0.02 g/L/h in batch cultures, was improved 12-fold, reaching 0.24 g/L/h during fed-batch cultures. The biomass yield was also improved 10-fold from 0.07 to 0.71 gbiomass/gsolids. Analysis of the fermentation profile of SOS3 indicated minimal production of by-products and accumulation of polyhydroxybutyrate (PHB) during the exponential growth phase associated with nitrogen limitation in the medium. By implementing proteomics analysis in fed-batch cultures, we identified the expression of four metabolic pathways associated with growth-promoting effects, which may be used as a qualitative parameter to guarantee the efficacy of SOS3 when used as a bioinoculant. Ultimately, we confirmed that the high-cell density cultures maintained their plant growth-promoting capacity when tested in sorghum and maize under glasshouse conditions.

Genome sequencing and analysis of plant growth-promoting attributes from Leclercia adecarboxylata.

Plant growth-promoting bacteria are ecological alternatives for fertilization, mainly for gramineous. Since plant x bacteria interaction is genotype and strain dependent, searching for new strains may contribute to the development of new biofertilizers. We aim to characterize plant growth-promoting capacity of Leclercia adecarboxylata strain Palotina, formerly isolated by our group in corn. A single isolated colony was taken and its genome was sequenced using Illumina technology. The whole genome was compared to other Leclercia adecarboxylata strains, and their biological and growth-promoting traits, such as P solubilization and auxin production, were tested. Following that, a 4.8 Mb genome of L. adecarboxylata strain Palotina was assembled and the functional annotation was carried out. This paper is the first to report the genes associated with plant growth promotion demonstrating in vitro indole acid production by this strain. These results project the endophyte as a potential biofertilizer for further commercial exploitation.

Coffee Microbiota and Its Potential Use in Sustainable Crop Management. A Review

Intensive coffee production is accompanied by several environmental issues, including soil degradation, biodiversity loss, and pollution due to the wide use of agrochemical inputs and wastes generated by processing. In addition, climate change is expected to decrease the suitability of cultivated areas while potentially increasing the distribution and impact of pests and diseases. In this context, the coffee microbiota has been increasingly studied over the past decades in order to improve the sustainability of the coffee production. Therefore, coffee associated microorganisms have been isolated and characterized in order to highlight their useful characteristics and study their potential use as sustainable alternatives to agrochemical inputs. Indeed, several microorganisms (including bacteria and fungi) are able to display plant growth-promoting capacities and/or biocontrol abilities toward coffee pests and diseases. Despite that numerous studies emphasized the potential of coffee-associated microorganisms under controlled environments, the present review highlights the lack of confirmation of such beneficial effects under field conditions. Nowadays, next-generation sequencing technologies allow to study coffee associated microorganisms with a metabarcoding/metagenomic approach. This strategy, which does not require cultivating microorganisms, now provides a deeper insight in the coffee-associated microbial communities and their implication not only in the coffee plant fitness but also in the quality of the final product. The present review aims at (i) providing an extensive description of coffee microbiota diversity both at the farming and processing levels, (ii) identifying the “coffee core microbiota,” (iii) making an overview of microbiota ability to promote coffee plant growth and to control its pests and diseases, and (iv) highlighting the microbiota potential to improve coffee quality and waste management sustainability.