Spatio-temporal Responses of Arabidopsis Leaves in Photosynthetic Performance and Metabolite Contents to Burkholderia phytofirmans PsJN

Plant-Growth-Promoting Rhizobacteria Modulate Carbohydrate Metabolism in Connection with Host Plant Defense Mechanism

Plant-growth-promoting rhizobacteria (PGPR) could potentially enhance photosynthesis and benefit plant growth by improving soil nutrient uptake and affecting plant hormone balance. Several recent studies have unveiled a correlation between alterations in photosynthesis and host plant resistance levels. Photosynthesis provides materials and energy for plant growth and immune defense and affects defense-related signaling pathways. Photosynthetic organelles, which could be strengthened by PGPR inoculation, are key centers for defense signal biosynthesis and transmission. Although endophytic PGPRs metabolize plant photosynthates, they can increase soluble sugar levels and alternate sugar type and distribution. Soluble sugars clearly support plant growth and can act as secondary messengers under stressed conditions. Overall, carbohydrate metabolism modifications induced by PGPR may also play a key role in improving plant resistance. We provide a concise overview of current knowledge regarding PGPR-induced modulation in carbohydrate metabolism under both pathogen-infected and pathogen-free conditions. We highlight PGPR application as a cost-saving strategy amidst unpredictable pathogen pressures.

Paraburkholderia phytofirmans PsJN colonization of rice endosphere triggers an atypical transcriptomic response compared to rice native Burkholderia s.l. endophytes

The plant microbiome has recently emerged as a reservoir for the development of sustainable alternatives to chemical fertilizers and pesticides. However, the response of plants to beneficial microbes emerges as a critical issue to understand the molecular basis of plant-microbiota interactions. In this study, we combined root colonization, phenotypic and transcriptomic analyses to unravel the commonalities and specificities of the response of rice to closely related Burkholderia s.l. endophytes. In general, these results indicate that a rice-non-native Burkholderia s.l. strain, Paraburkholderia phytofirmans PsJN, is able to colonize the root endosphere while eliciting a markedly different response compared to rice-native Burkholderia s.l. strains. This demonstrates the variability of plant response to microbes from different hosts of origin. The most striking finding of the investigation was that a much more conserved response to the three endophytes used in this study is elicited in leaves compared to roots. In addition, transcriptional regulation of genes related to secondary metabolism, immunity, and phytohormones appear to be markers of strain-specific responses. Future studies need to investigate whether these findings can be extrapolated to other plant models and beneficial microbes to further advance the potential of microbiome-based solutions for crop production.

Bradyrhizobium diazoefficiens USDA110 Nodulation of Aeschynomene afraspera Is Associated with Atypical Terminal Bacteroid Differentiation and Suboptimal Symbiotic Efficiency

Legume-rhizobium symbiosis is a major ecological process in the nitrogen cycle, responsible for the main input of fixed nitrogen into the biosphere. The efficiency of this symbiosis relies on the coevolution of the partners.

Legume plants can form root organs called nodules where they house intracellular symbiotic rhizobium bacteria. Within nodule cells, rhizobia differentiate into bacteroids, which fix nitrogen for the benefit of the plant. Depending on the combination of host plants and rhizobial strains, the output of rhizobium-legume interactions varies from nonfixing associations to symbioses that are highly beneficial for the plant. Bradyrhizobium diazoefficiens USDA110 was isolated as a soybean symbiont, but it can also establish a functional symbiotic interaction with Aeschynomene afraspera. In contrast to soybean, A. afraspera triggers terminal bacteroid differentiation, a process involving bacterial cell elongation, polyploidy, and increased membrane permeability, leading to a loss of bacterial viability while plants increase their symbiotic benefit. A combination of plant metabolomics, bacterial proteomics, and transcriptomics along with cytological analyses were used to study the physiology of USDA110 bacteroids in these two host plants. We show that USDA110 establishes a poorly efficient symbiosis with A. afraspera despite the full activation of the bacterial symbiotic program. We found molecular signatures of high levels of stress in A. afraspera bacteroids, whereas those of terminal bacteroid differentiation were only partially activated. Finally, we show that in A. afraspera, USDA110 bacteroids undergo atypical terminal differentiation hallmarked by the disconnection of the canonical features of this process. This study pinpoints how a rhizobium strain can adapt its physiology to a new host and cope with terminal differentiation when it did not coevolve with such a host.

IMPORTANCE Legume-rhizobium symbiosis is a major ecological process in the nitrogen cycle, responsible for the main input of fixed nitrogen into the biosphere. The efficiency of this symbiosis relies on the coevolution of the partners. Some, but not all, legume plants optimize their return on investment in the symbiosis by imposing on their microsymbionts a terminal differentiation program that increases their symbiotic efficiency but imposes a high level of stress and drastically reduces their viability. We combined multi-omics with physiological analyses to show that the symbiotic couple formed by Bradyrhizobium diazoefficiens USDA110 and Aeschynomene afraspera, in which the host and symbiont did not evolve together, is functional but displays a low symbiotic efficiency associated with a disconnection of terminal bacteroid differentiation features.

Impact of Paraburkholderia phytofirmans PsJN on Grapevine Phenolic Metabolism

Phenolic compounds are implied in plant-microorganisms interaction and may be induced in response to plant growth-promoting rhizobacteria (PGPRs). Among PGPR, the beneficial bacterium Paraburkholderia phytofirmans PsJN was previously described to stimulate the growth of plants and to induce a better adaptation to both abiotic and biotic stresses. This study aimed to investigate the impact of PsJN on grapevine secondary metabolism. For this purpose, gene expression (qRT-PCR) and profiling of plant secondary metabolites (UHPLC-UV/DAD-MS QTOF) from both grapevine root and leaves were compared between non-bacterized and PsJN-bacterized grapevine plantlets. Our results showed that PsJN induced locally (roots) and systemically (leaves) an overexpression of PAL and STS and specifically in leaves the overexpression of all the genes implied in phenylpropanoid and flavonoid pathways. Moreover, the metabolomic approach revealed that relative amounts of 32 and 17 compounds in roots and leaves, respectively, were significantly modified by PsJN. Once identified to be accumulated in response to PsJN by the metabolomic approach, antifungal properties of purified molecules were validated in vitro for their antifungal effect on Botrytis cinerea spore germination. Taking together, our findings on the impact of PsJN on phenolic metabolism allowed us to identify a supplementary biocontrol mechanism developed by this PGPR to induce plant resistance against pathogens.

Transcriptional alterations reveal Bacillus amyloliquefaciens-rice cooperation under salt stress

The Bacillus amyloliquefaciens-SN13 and model crop rice (Oryza sativa) were chosen to understand the complex regulatory networks that govern plant-PGPR interaction under salt stress. During stress, inoculation with SN13 significantly increased biomass, relative water content, proline and total soluble sugar in rice while decreased lipid peroxidation and electrolyte leakage. Extensive alterations in gene expression were also observed in rice root transcriptome under stress in the presence of SN13. Rhizobacteria induced changes in expression of a considerable number of photosynthesis, hormone, and stress-responsive genes, in addition to cell-wall and lipid metabolism-related genes under salt stress as compared to salt stress or SN13 inoculation alone, indicating its potential role in reducing the harmful effects of salinity. To validate RNA-seq data, qRT-PCR was performed for selected differentially expressed genes representing various functional categories including metabolism, regulation, stress-response, and transporters. Results indicate qualitative and quantitative differences between roots responses to SN13 under stressed and unstressed conditions. Functional expressions of OsNAM and OsGRAM in yeast showed enhanced tolerance to various abiotic stresses, indicating crucial SN13-rice interaction in imparting beneficial effects under stress. This is first detailed report on understanding molecular mechanism underlying beneficial plant-microbe interaction in any economically important model crop plant under abiotic stress.

From Intracellular Bacteria to Differentiated Bacteroids: Transcriptome and Metabolome Analysis in Aeschynomene Nodules Using the Bradyrhizobium sp. Strain ORS285 bclA Mutant

Soil bacteria called rhizobia trigger the formation of root nodules on legume plants. The rhizobia infect these symbiotic organs and adopt an intracellular lifestyle within the nodule cells, where they differentiate into nitrogen-fixing bacteroids. Several legume lineages force their symbionts into an extreme cellular differentiation, comprising cell enlargement and genome endoreduplication. The antimicrobial peptide transporter BclA is a major determinant of this process in Bradyrhizobium sp. strain ORS285, a symbiont of Aeschynomene spp. In the absence of BclA, the bacteria proceed until the intracellular infection of nodule cells, but they cannot differentiate into enlarged polyploid and functional bacteroids. Thus, the bclA nodule bacteria constitute an intermediate stage between the free-living soil bacteria and the nitrogen-fixing bacteroids. Metabolomics on whole nodules of Aeschynomene afraspera and Aeschynomene indica infected with the wild type or the bclA mutant revealed 47 metabolites that differentially accumulated concomitantly with bacteroid differentiation. Bacterial transcriptome analysis of these nodules demonstrated that the intracellular settling of the rhizobia in the symbiotic nodule cells is accompanied by a first transcriptome switch involving several hundred upregulated and downregulated genes and a second switch accompanying the bacteroid differentiation, involving fewer genes but ones that are expressed to extremely elevated levels. The transcriptomes further suggested a dynamic role for oxygen and redox regulation of gene expression during nodule formation and a nonsymbiotic function of BclA. Together, our data uncover the metabolic and gene expression changes that accompany the transition from intracellular bacteria into differentiated nitrogen-fixing bacteroids.IMPORTANCE Legume-rhizobium symbiosis is a major ecological process, fueling the biogeochemical nitrogen cycle with reduced nitrogen. It also represents a promising strategy to reduce the use of chemical nitrogen fertilizers in agriculture, thereby improving its sustainability. This interaction leads to the intracellular accommodation of rhizobia within plant cells of symbiotic organs, where they differentiate into nitrogen-fixing bacteroids. In specific legume clades, this differentiation process requires the bacterial transporter BclA to counteract antimicrobial peptides produced by the host. Transcriptome analysis of Bradyrhizobium wild-type and bclA mutant bacteria in culture and in symbiosis with Aeschynomene host plants dissected the bacterial transcriptional response in distinct phases and highlighted functions of the transporter in the free-living stage of the bacterial life cycle.

Isolation, identification and plant growth promotion ability of endophytic bacteria associated with lupine root nodule grown in Tunisian soil

The present study aims to characterize nodule endophytic bacteria of spontaneous lupine plants regarding their diversity and their plant growth promoting (PGP) traits. The potential of PGPR inoculation was investigated to improve white lupine growth across controlled, semi-natural and field conditions. Lupinus luteus and Lupinus angustifolius nodules were shown inhabited by a large diversity of endophytes. Several endophytes harbor numerous plant growth promotion traits such as phosphates solubilization, siderophores production and 1-aminocyclopropane-1-carboxylate deaminase activity. In vivo analysis confirmed the plant growth promotion ability of two strains (Paenibacillus glycanilyticus LJ121 and Pseudomonas brenneri LJ215) in both sterilized and semi-natural conditions. Under field conditions, the co-inoculation of lupine by these strains increased shoot N content and grain yield by 25% and 36%, respectively. These two strains Paenibacillus glycanilyticus LJ121 and Pseudomonas brenneri LJ215 are effective plant growth-promoting bacteria and they may be used to develop an eco-friendly biofertilizer to boost white lupine productivity.

On a Cold Night: Transcriptomics of Grapevine Flower Unveils Signal Transduction and Impacted Metabolism

Low temperature is a critical environmental factor limiting plant productivity, especially in northern vineyards. To clarify the impact of this stress on grapevine flower, we used the Vitis array based on Roche-NimbleGen technology to investigate the gene expression of flowers submitted to a cold night. Our objectives were to identify modifications in the transcript levels after stress and during recovery. Consequently, our results confirmed some mechanisms known in grapes or other plants in response to cold stress, notably, (1) the pivotal role of calcium/calmodulin-mediated signaling; (2) the over-expression of sugar transporters and some genes involved in plant defense (especially in carbon metabolism), and (3) the down-regulation of genes encoding galactinol synthase (GOLS), pectate lyases, or polygalacturonases. We also identified some mechanisms not yet known to be involved in the response to cold stress, i.e., (1) the up-regulation of genes encoding G-type lectin S-receptor-like serine threonine-protein kinase, pathogen recognition receptor (PRR5), or heat-shock factors among others; (2) the down-regulation of Myeloblastosis (MYB)-related transcription factors and the Constans-like zinc finger family; and (3) the down-regulation of some genes encoding Pathogen-Related (PR)-proteins. Taken together, our results revealed interesting features and potentially valuable traits associated with stress responses in the grapevine flower. From a long-term perspective, our study provides useful starting points for future investigation.

Biological Control of Pierce's Disease of Grape by an Endophytic Bacterium

Effective preventive measures and therapies are lacking for control of Pierce's disease of grape caused by the xylem-colonizing bacterium Xylella fastidiosa responsible for serious losses in grape production. In this study we explored the potential for endophytic bacteria to alter the disease process. While most endophytic bacteria found within grape did not grow or multiply when inoculated into mature grape vines, Paraburkholderia phytofirmans strain PsJN achieved population sizes as large as 10⁶ cells/g and moved 1 m or more within 4 weeks after inoculation into vines. While X. fastidiosa achieved large population sizes and moved extensively in grape when inoculated alone, few viable cells were recovered from plants in which it was co-inoculated with strain PsJN and the incidence of leaves exhibiting scorching symptoms typical of Pierce's disease was consistently greatly reduced from that in control plants. Suppression of disease symptoms occurred not only when strain PsJN was co-inoculated with the pathogen by puncturing stems in the same site in plants, but also when inoculated at the same time but at different sites in the plant. Large population sizes of strain PsJN could be established in both leaf lamina and petioles by topical application of cell suspensions in 0.2% of an organo-silicon surfactant conferring low surface tension, and such treatments were as effective as direct puncture inoculations of this biocontrol strain in reducing disease severity. While inoculation of strain PsJN into plants by either method at the same time as or even 4 weeks after that of the pathogen resulted in large reductions in disease severity, much less disease control was conferred by inoculation of PsJN 4 weeks prior to that of the pathogen. The expression of grapevine PR1 and ETR1 within 3 weeks of inoculation was substantially higher in plants inoculated with both X. fastidiosa and strain PsJN compared with that in plants inoculated only with the pathogen or strain PsJN, suggesting that this biological control agent reduces disease by priming expression of innate disease resistance pathways in plants that otherwise would have exhibited minimal responses to the pathogen. Strain PsJN thus appears highly efficacious for the control of Pierce's disease when used as an eradicant treatment that can be easily made even by spray application.

Impacts of Paraburkholderia phytofirmans Strain PsJN on Tomato (Lycopersicon esculentum L.) Under High Temperature

Abnormal temperatures induce physiological and biochemical changes resulting in the loss of yield. The present study investigates the impact of the PsJN strain of Paraburkholderia phytofirmans on tomato (Lycopersicon esculentum Mill.) in response to heat stress (32°C). The results of this work showed that bacterial inoculation with P. phytofirmans strain PsJN increased tomato growth parameters such as chlorophyll content and gas exchange at both normal and high temperatures (25 and 32°C). At normal temperature (25°C), the rate of photosynthesis and the photosystem II activity increased with significant accumulations of sugars, total amino acids, proline, and malate in the bacterized tomato plants, demonstrating that the PsJN strain had a positive effect on plant growth. However, the amount of sucrose, total amino acids, proline, and malate were significantly affected in tomato leaves at 32°C compared to that at 25°C. Changes in photosynthesis and chlorophyll fluorescence showed that the bacterized tomato plants were well acclimated at 32°C. These results reinforce the current knowledge about the PsJN strain of P. phytofirmans and highlight in particular its ability to alleviate the harmful effects of high temperatures by stimulating the growth and tolerance of tomato plants.

Associative bacteria influence maize (Zea mays L.) growth, physiology and root anatomy under different nitrogen levels

Despite the great diversity of plant growth-promoting bacteria (PGPB) with potential to partially replace the use of N fertilisers in agriculture, few PGPB have been explored for the production of commercial inoculants, reinforcing the importance of identifying positive plant-bacteria interactions. Aiming to better understand the influence of PGPB inoculation in plant development, two PGPB species with distant phylogenetic relationship were inoculated in maize. Maize seeds were inoculated with Bacillus sp. or Azospirillum brasilense. After germination, the plants were subjected to two N treatments: full (N+) and limiting (N-) N supply. Then, anatomical, biometric and physiological analyses were performed. Both PGPB species modified the anatomical pattern of roots, as verified by the higher metaxylem vessel element (MVE) number. Bacillus sp. also increased the MVE area in maize roots. Under N+ conditions, both PGPB decreased leaf protein content and led to development of shorter roots; however, Bacillus sp. increased root and shoot dry weight, whereas A. brasilense increased photosynthesis rate and leaf nitrate content. In plants subjected to N limitation (N-), photosynthesis rate and photosystem II efficiency increased in maize inoculated with Bacillus sp., whilst A. brasilense contained higher ammonium, amino acids and total soluble sugars in leaves, compared to the control. Plant developmental and metabolical patterns were switched by the inoculation, regardless of the inoculant bacterium used, producing similar as well as distinct modifications to the parameters studied. These results indicate that even non-diazotrophic inoculant strains can improve the plant N status as result of the morpho-anatomical and physiological modifications produced by the PGPB.

One Way to Achieve Germination: Common Molecular Mechanism Induced by Ethylene and After-Ripening in Sunflower Seeds

Dormancy is an adaptive trait that blocks seed germination until the environmental conditions become favorable for subsequent vegetative plant growth. Seed dormancy is defined as the inability to germinate in favorable conditions. Dormancy is alleviated during after-ripening, a dry storage period, during which dormant (D) seeds unable to germinate become non-dormant (ND), able to germinate in a wide range of environmental conditions. The treatment of dormant seeds with ethylene (D/ET) promotes seed germination, and abscisic acid (ABA) treatment reduces non-dormant (ND/ABA) seed germination in sunflowers (Helianthus annuus). Metabolomic and transcriptomic studies have been performed during imbibition to compare germinating seeds (ND and D/ET) and low-germinating seeds (D and ND/ABA). A PCA analysis of the metabolites content showed that imbibition did not trigger a significant change during the first hours (3 and 15 h). The metabolic changes associated with germination capacity occurred at 24 h and were related to hexoses, as their content was higher in ND and D/ET and was reduced by ABA treatment. At the transcriptional level, a large number of genes were altered oppositely in germinating, compared to the low-germinating seeds. The metabolomic and transcriptomic results were integrated in the interpretation of the processes involved in germination. Our results show that ethylene treatment triggers molecular changes comparable to that of after-ripening treatment, concerning sugar metabolism and ABA signaling inhibition.

Molecular Insights into the Involvement of a Never Ripe Receptor in the Interaction Between Two Beneficial Soil Bacteria and Tomato Plants Under Well-Watered and Drought Conditions

Management of plant growth-promoting bacteria (PGPB) can be implemented to deal with sustainable intensification of agriculture. Ethylene is an essential component for plant growth and development and in response to drought. However, little is known about the effects of bacterial inoculation on ethylene transduction pathway. Thus, the present study sought to establish whether ethylene perception is critical for growth induction by two different PGPB strains under drought conditions and the analysis of bacterial effects on ethylene production and gene expression in tomatoes (Solanum lycopersicum). The ethylene-insensitive never ripe (nr) and its isogenic wild-type (wt) cv. Pearson line were inoculated with either Bacillus megaterium or Enterobacter sp. strain C7 and grown until the attainment of maturity under both well-watered and drought conditions. Ethylene perception is crucial for B. megaterium. However, it is not of prime importance for Enterobacter sp. strain C7 PGPB activity under drought conditions. Both PGPB decreased the expression of ethylene-related genes in wt plants, resulting in stress alleviation, while only B. megaterium induced their expression in nr plants. Furthermore, PGPB inoculation affected transcriptomic profile dependency on strain, genotype, and drought. Ethylene sensitivity determines plant interaction with PGPB strains. Enterobacter sp. strain C7 could modulate amino-acid metabolism, while nr mutation causes a partially functional interaction with B. megaterium, resulting in higher oxidative stress and loss of PGPB activity.

Proteomic analysis reveals that tomato interaction with plant growth promoting bacteria is highly determined by ethylene perception

Feeding an increasing global population as well as reducing environmental impact of crops is the challenge for the sustainable intensification of agriculture. Plant-growth-promoting bacteria (PGPB) management could represent a suitable method but elucidation of their action mechanisms is essential for a proper and effective utilization. Furthermore, ethylene is involved in growth and response to environmental stimuli but little is known about the implication of ethylene perception in PGPB activity. The ethylene-insensitive tomato never ripe and its isogenic wild-type cv. Pearson lines inoculated with Bacillus megaterium or Enterobacter sp. C7 strains were grown until mature stage to analyze growth promotion, and bacterial inoculation effects on root proteomic profiles. Enterobacter C7 promoted growth in both plant genotypes, meanwhile Bacillus megaterium PGPB activity was only noticed in wt plants. Moreover, PGPB inoculation affected proteomic profile in a strain- and genotype-dependent manner modifying levels of stress-related and interaction proteins, and showing bacterial inoculation effects on antioxidant content and phosphorus acquisition capacity. Ethylene perception is essential for properly recognition of Bacillus megaterium and growth promotion mediated in part by increased levels of reduced glutathione. In contrast, Enterobacter C7 inoculation improves phosphorus nutrition keeping plants on growth independently of ethylene sensitivity.

Different Arabidopsis thaliana photosynthetic and defense responses to hemibiotrophic pathogen induced by local or distal inoculation of Burkholderia phytofirmans

Pathogen infection of plant results in modification of photosynthesis and defense mechanisms. Beneficial microorganisms are known to improve plant tolerance to stresses. Burkholderia phytofirmans PsJN (Bp), a beneficial endophytic bacterium, promotes growth of a wide range of plants and induces plant resistance against abiotic and biotic stresses such as coldness and infection by a necrotrophic pathogen. However, mechanisms underlying its role in plant tolerance towards (hemi)biotrophic invaders is still lacking. We thus decipher photosynthetic and defense responses during the interaction between Arabidopsis, Bp and the hemibiotrophic bacterium Pseudomonas syringae pv. tomato DC3000 (Pst). Different Bp inoculations allowed analyzes at both systemic and local levels. Despite no direct antibacterial action, our results showed that only local presence of Bp alleviates Pst growth in planta during the early stage of infection. Molecular investigations showed that seed inoculation of Bp, leading to a restricted presence in the root system, transiently primed PR1 expression after challenge with Pst but continuously primed PDF1.2 expression. Bacterization with Bp reduced Y(ND) but had no impact on PSII activity or RuBisCO accumulation. Pst infection caused an increase of Y(NA) and a decrease in ΦPSI, ETRI and in PSII activity, showed by a decrease in Fv/Fm, Y(NPQ), ΦPSII, and ETRII values. Inoculation with both bacteria did not display any variation in photosynthetic activity compared to plants inoculated with only Pst. Our findings indicated that the role of Bp here is not multifaceted, and relies only on priming of defense mechanisms but not on improving photosynthetic activity.

Tomato ethylene sensitivity determines interaction with plant growth-promoting bacteria

BACKGROUND AND AIMS

Plant growth-promoting bacteria (PGPB) are soil micro-organisms able to interact with plants and stimulate their growth, positively affecting plant physiology and development. Although ethylene plays a key role in plant growth, little is known about the involvement of ethylene sensitivity in bacterial inoculation effects on plant physiology. Thus, the present study was pursued to establish whether ethylene perception is critical for plant-bacteria interaction and growth induction by two different PGPB strains, and to assess the physiological effects of these strains in juvenile and mature tomato ( Solanum lycopersicum ) plants.

METHODS

An experiment was performed with the ethylene-insensitive tomato never ripe and its isogenic wild-type line in which these two strains were inoculated with either Bacillus megaterium or Enterobacter sp. C7. Plants were grown until juvenile and mature stages, when biomass, stomatal conductance, photosynthesis as well as nutritional, hormonal and metabolic statuses were analysed.

KEY RESULTS

Bacillus megaterium promoted growth only in mature wild type plants. However, Enterobacter C7 PGPB activity affected both wild-type and never ripe plants. Furthermore, PGPB inoculation affected physiological parameters and root metabolite levels in juvenile plants; meanwhile plant nutrition was highly dependent on ethylene sensitivity and was altered at the mature stage. Bacillus megaterium inoculation improved carbon assimilation in wild-type plants. However, insensitivity to ethylene compromised B. megaterium PGPB activity, affecting photosynthetic efficiency, plant nutrition and the root sugar content. Nevertheless, Enterobacter C7 inoculation modified the root amino acid content in addition to stomatal conductance and plant nutrition.

CONCLUSIONS

Insensitivity to ethylene severely impaired B. megaterium interaction with tomato plants, resulting in physiological modifications and loss of PGPB activity. In contrast, Enterobacter C7 inoculation stimulated growth independently of ethylene perception and improved nitrogen assimilation in ethylene-insensitive plants. Thus, ethylene sensitivity is a determinant for B. megaterium , but is not involved in Enterobacter C7 PGPB activity.