Quantitative proteomic analysis of the interaction between the endophytic plant-growth-promoting bacterium Gluconacetobacter diazotrophicus and sugarcane

Fine-Tuning of Arabidopsis thaliana Response to Endophytic Colonization by Gluconacetobacter diazotrophicus PAL5 Revealed by Transcriptomic Analysis

Gluconacetobacter diazotrophicus is a diazotrophic endophytic bacterium that promotes the growth and development of several plant species. However, the molecular mechanisms activated during plant response to this bacterium remain unclear. Here, we used the RNA-seq approach to understand better the effect of G. diazotrophicus PAL5 on the transcriptome of shoot and root tissues of Arabidopsis thaliana. G. diazotrophicus colonized A. thaliana roots and promoted growth, increasing leaf area and biomass. The transcriptomic analysis revealed several differentially expressed genes (DEGs) between inoculated and non-inoculated plants in the shoot and root tissues. A higher number of DEGs were up-regulated in roots compared to shoots. Genes up-regulated in both shoot and root tissues were associated with nitrogen metabolism, production of glucosinolates and flavonoids, receptor kinases, and transcription factors. In contrast, the main groups of down-regulated genes were associated with pathogenesis-related proteins and heat-shock proteins in both shoot and root tissues. Genes encoding enzymes involved in cell wall biogenesis and modification were down-regulated in shoots and up-regulated in roots. In contrast, genes associated with ROS detoxification were up-regulated in shoots and down-regulated in roots. These results highlight the fine-tuning of the transcriptional regulation of A. thaliana in response to colonization by G. diazotrophicus PAL5.

Exploring the potential of endophyte-plant interactions for improving crop sustainable yields in a changing climate

Climate change poses a major threat to global food security, significantly reducing crop yields as cause of abiotic stresses, and for boosting the spread of new and old pathogens and pests. Sustainable crop management as a route to mitigation poses the challenge of recruiting an array of solutions and tools for the new aims. Among these, the deployment of positive interactions between the micro-biotic components of agroecosystems and plants can play a highly significant role, as part of the agro-ecological revolution. Endophytic microorganisms have emerged as a promising solution to tackle this challenge. Among these, Arbuscular Mycorrhizal Fungi (AMF) and endophytic bacteria and fungi have demonstrated their potential to alleviate abiotic stresses such as drought and heat stress, as well as the impacts of biotic stresses. They can enhance crop yields in a sustainable way also by other mechanisms, such as improving the nutrient uptake, or by direct effects on plant physiology. In this review we summarize and update on the main types of endophytes, we highlight several studies that demonstrate their efficacy in improving sustainable yields and explore possible avenues for implementing crop-microbiota interactions. The mechanisms underlying these interactions are highly complex and require a comprehensive understanding. For this reason, omic technologies such as genomics, transcriptomics, proteomics, and metabolomics have been employed to unravel, by a higher level of information, the complex network of interactions between plants and microorganisms. Therefore, we also discuss the various omic approaches and techniques that have been used so far to study plant-endophyte interactions.

Harnessing root exudates for plant microbiome engineering and stress resistance in plants

A wide range of abiotic and biotic stresses adversely affect plant's growth and production. Under stress, one of the main responses of plants is the modulation of exudates excreted in the rhizosphere, which consequently leads to alterations in the resident microbiota. Thus, the exudates discharged into the rhizospheric environment play a preponderant role in the association and formation of plant-microbe interactions. In this review, we aimed to provide a synthesis of the latest and most pertinent literature on the diverse biochemical and structural compositions of plant root exudates. Also, this work investigates into their multifaceted role in microbial nutrition and intricate signaling processes within the rhizosphere, which includes quorum-sensing molecules. Specifically, it explores the contributions of low molecular weight compounds, such as carbohydrates, phenolics, organic acids, amino acids, and secondary metabolites, as well as the significance of high molecular weight compounds, including proteins and polysaccharides. It also discusses the state-of-the-art omics strategies that unveil the vital role of root exudates in plant-microbiome interactions, including defense against pathogens like nematodes and fungi. We propose multiple challenges and perspectives, including exploiting plant root exudates for host-mediated microbiome engineering. In this discourse, root exudates and their derived interactions with the rhizospheric microbiota should receive greater attention due to their positive influence on plant health and stress mitigation.

Wild Wheat Rhizosphere-Associated Plant Growth-Promoting Bacteria Exudates: Effect on Root Development in Modern Wheat and Composition

Diazotrophic bacteria isolated from the rhizosphere of a wild wheat ancestor, grown from its refuge area in the Fertile Crescent, were found to be efficient Plant Growth-Promoting Rhizobacteria (PGPR), upon interaction with an elite wheat cultivar. In nitrogen-starved plants, they increased the amount of nitrogen in the seed crop (per plant) by about twofold. A bacterial growth medium was developed to investigate the effects of bacterial exudates on root development in the elite cultivar, and to analyze the exo-metabolomes and exo-proteomes. Altered root development was observed, with distinct responses depending on the strain, for instance, with respect to root hair development. A first conclusion from these results is that the ability of wheat to establish effective beneficial interactions with PGPRs does not appear to have undergone systematic deep reprogramming during domestication. Exo-metabolome analysis revealed a complex set of secondary metabolites, including nutrient ion chelators, cyclopeptides that could act as phytohormone mimetics, and quorum sensing molecules having inter-kingdom signaling properties. The exo-proteome-comprised strain-specific enzymes, and structural proteins belonging to outer-membrane vesicles, are likely to sequester metabolites in their lumen. Thus, the methodological processes we have developed to collect and analyze bacterial exudates have revealed that PGPRs constitutively exude a highly complex set of metabolites; this is likely to allow numerous mechanisms to simultaneously contribute to plant growth promotion, and thereby to also broaden the spectra of plant genotypes (species and accessions/cultivars) with which beneficial interactions can occur.

Endophytes and their potential in biotic stress management and crop production

Biotic stress is caused by harmful microbes that prevent plants from growing normally and also having numerous negative effects on agriculture crops globally. Many biotic factors such as bacteria, fungi, virus, weeds, insects, and nematodes are the major constrains of stress that tends to increase the reactive oxygen species that affect the physiological and molecular functioning of plants and also led to the decrease in crop productivity. Bacterial and fungal endophytes are the solution to overcome the tasks faced with conventional farming, and these are environment friendly microbial commodities that colonize in plant tissues without causing any damage. Endophytes play an important role in host fitness, uptake of nutrients, synthesis of phytohormone and diminish the injury triggered by pathogens via antibiosis, production of lytic enzymes, secondary metabolites, and hormone activation. They are also reported to help plants in coping with biotic stress, improving crops and soil health, respectively. Therefore, usage of endophytes as biofertilizers and biocontrol agent have developed an eco-friendly substitute to destructive chemicals for plant development and also in mitigation of biotic stress. Thus, this review highlighted the potential role of endophytes as biofertilizers, biocontrol agent, and in mitigation of biotic stress for maintenance of plant development and soil health for sustainable agriculture.

Sugarcane Root Transcriptome Analysis Revealed the Role of Plant Hormones in the Colonization of an Endophytic Diazotroph

Some sugarcane germplasms can absorb higher amounts of nitrogen via atmospheric nitrogen fixation through the bacterial diazotrophs. Most endophytic diazotrophs usually penetrate through the root, colonize inside the plant, and fix the nitrogen. To assess the plant’s bacterial association during root colonization, strain GXS16 was tagged with a plasmid-bear green fluorescent protein (GFP) gene. The results demonstrated that the strain can colonize roots all the way to the maturation zone. The strain GXS16 showed maximum nitrogenase enzyme activity at pH 8 and 30°C, and nitrogenase activity is less affected by different carbon sources. Further, strain GXS16 colonization response was investigated through plant hormones analysis and RNAseq. The results showed that the bacterial colonization gradually increased with time, and the H₂O₂ and malondialdehyde (MDA) content significantly increased at 1 day after inoculation. There were no substantial changes noticed in proline content, and the ethylene content was detected initially, but it decreased with time. The abscisic acid (ABA) content showed significant increases of 91.9, 43.9, and 18.7%, but conversely, the gibberellin (GA₃) content decreased by 12.9, 28.5, and 45.2% at 1, 3, and 5 days after inoculation, respectively. The GXS16 inoculation significantly increased the activities of catalase (CAT), superoxide dismutase (SOD), polyphenol oxidase (PPO), ascorbate peroxidase (APX), and glutathione reductase (GR) at different timepoint. In contrast, the peroxisome (POD) activity had no changes detected during the treatment. In the case of RNAseq analysis, 2437, 6678, and 4568 differentially expressed genes (DEGs) were identified from 1, 3, and 5 days inoculated root samples, and 601 DEGs were shared in all samples. The number or the expression diversity of DEGs related to ethylene was much higher than that of ABA or GA, which indicated the critical role of ethylene in regulating the sugarcane roots response to GXS16 inoculation.

The Multifunctions and Future Prospects of Endophytes and Their Metabolites in Plant Disease Management

Endophytes represent a ubiquitous and magical world in plants. Almost all plant species studied by different researchers have been found to harbor one or more endophytes, which protect host plants from pathogen invasion and from adverse environmental conditions. They produce various metabolites that can directly inhibit the growth of pathogens and even promote the growth and development of the host plants. In this review, we focus on the biological control of plant diseases, aiming to elucidate the contribution and key roles of endophytes and their metabolites in this field with the latest research information. Metabolites synthesized by endophytes are part of plant disease management, and the application of endophyte metabolites to induce plant resistance is very promising. Furthermore, multi-omics should be more fully utilized in plant–microbe research, especially in mining novel bioactive metabolites. We believe that the utilization of endophytes and their metabolites for plant disease management is a meaningful and promising research direction that can lead to new breakthroughs in the development of more effective and ecosystem-friendly insecticides and fungicides in modern agriculture.

Discovery of potential protein biomarkers associated with sugarcane white leaf disease susceptibility using a comparative proteomic approach

Sugarcane white leaf disease (SCWLD) is caused by phytoplasma, a serious sugarcane phytoplasma pathogen, which causes significant decreases in crop yield and sugar quality. The identification of proteins involved in the defense mechanism against SCWLD phytoplasma may help towards the development of varieties resistant to SCWLD. We investigated the proteomes of four sugarcane varieties with different levels of susceptibility to SCWLD phytoplasma infection, namely K88-92 and K95-84 (high), KK3 (moderate), and UT1 (low) by quantitative label-free nano-liquid chromatography-tandem mass spectrometry (nano LC-MS/MS). A total of 248 proteins were identified and compared among the four sugarcane varieties. Two potential candidate protein biomarkers for reduced susceptibility to SCWLD phytoplasma were identified as proteins detected only in UT1. The functions of these proteins are associated with protein folding, metal ion binding, and oxidoreductase. The candidate biomarkers could be useful for further study of the sugarcane defense mechanism against SCWLD phytoplasma, and in molecular and conventional breeding strategies for variety improvement.

Combination of osmotic stress and sugar stress response mechanisms is essential for Gluconacetobacter diazotrophicus tolerance to high-sucrose environments

Sugar-rich environments represent an important challenge for microorganisms. The osmotic and molecular imbalances resulting from this condition severely limit microbial metabolism and growth. Gluconacetobacter diazotrophicus is one of the most sugar-tolerant prokaryotes, able to grow in the presence of sucrose concentrations up to 30%. However, the mechanisms that control its tolerance to such conditions remain poorly exploited. The present work investigated the key mechanisms of tolerance to high sugar in G. diazotrophicus. Comparative proteomics was applied to investigate the main functional pathways regulated in G. diazotrophicus when cultivated in the presence of high sucrose. Among 191 proteins regulated by high sucrose, regulatory pathways related to sugar metabolism, nutrient uptake, compatible solute synthesis, amino acid metabolism, and proteolytic system were highlighted. The role of these pathways on high-sucrose tolerance was investigated by mutagenesis analysis, which revealed that the knockout mutants zwf::Tn5 (sugar metabolism), tbdr::Tn5 (nutrient uptake), mtlK::Tn5 (compatible solute synthesis), pepN::Tn5 (proteolytic system), metH::Tn5 (amino acid metabolism), and ilvD::Tn5 (amino acid metabolism) became more sensitive to high sucrose. Together, our results identified mechanisms involved in response to high sugar in G. diazotrophicus, shedding light on the combination of osmotolerance and sugar-tolerance mechanisms. KEY POINTS: • G. diazotrophicus intensifies glycolysis to metabolize the excess of sugar. • G. diazotrophicus turns down the uptake of nutrients in response to high sugar. • G. diazotrophicus requires amino acid availability to resist high sugar.

Herbaspirillum seropedicae strain HRC54 expression profile in response to sugarcane apoplastic fluid

Bacterial transcriptome profiling in the presence of plant fluids or extracts during microbial growth may provide relevant information on plant-bacteria interactions. Here, RNA sequencing (RNA-Seq) was used to determine the transcriptomic profile of Herbaspirillum seropedicae strain HRC54 at the early stages of response to sugarcane apoplastic fluid. Differentially expressed gene (DEG) analysis was performed using the DESeq2 and edgeR packages, followed by functional annotation using Blast2GO and gene ontology enrichment analysis using the COG and KEGG databases. After 2 h of sugarcane apoplastic fluid addition to the H. seropedicae HRC54 culture, respectively, 44 and 45 genes were upregulated and downregulated. These genes were enriched in bacterial metabolism (e.g., oxidoreductase and transferase), ABC transporters, motility, secretion systems, and signal transduction. RNA-Seq expression profiles of 12 genes identified in data analyses were verified by RT-qPCR. The results suggested that H. seropedicae HRC54 recognized sugarcane apoplastic fluid as the host signal, and some DEGs were closely involved at the early stages of the establishment of plant-bacteria interactions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02848-y.

Bacterial endophytes: Molecular interactions with their hosts

Plant growth promotion has been found associated with plants on the surface (epiphytic), inside (endophytic), or close to the plant roots (rhizospheric). Endophytic bacteria mainly have been researched for their beneficial activities in terms of nutrient availability, plant growth hormones, and control of soil-borne and systemic pathogens. Molecular communications leading to these interactions between plants and endophytic bacteria are now being unrevealed using multidisciplinary approaches with advanced techniques such as metagenomics, metaproteomics, metatranscriptomics, metaproteogenomic, microRNAs, microarray, chips as well as the comparison of complete genome sequences. More than 400 genes in both the genomes of host plant and bacterial endophyte are up- or downregulated for the establishment of endophytism and plant growth-promoting activity. The involvement of more than 20 genes for endophytism, about 50 genes for direct plant growth promotion, about 25 genes for biocontrol activity, and about 10 genes for mitigation of different stresses has been identified in various bacterial endophytes. This review summarizes the progress that has been made in recent years by these modern techniques and approaches.

The Endophytic Microbiome as a Hotspot of Synergistic Interactions, with Prospects of Plant Growth Promotion

The plant root is the primary site of interaction between plants and associated microorganisms and constitutes the main components of plant microbiomes that impact crop production. The endophytic bacteria in the root zone have an important role in plant growth promotion. Diverse microbial communities inhabit plant root tissues, and they directly or indirectly promote plant growth by inhibiting the growth of plant pathogens, producing various secondary metabolites. Mechanisms of plant growth promotion and response of root endophytic microorganisms for their survival and colonization in the host plants are the result of complex plant-microbe interactions. Endophytic microorganisms also assist the host to sustain different biotic and abiotic stresses. Better insights are emerging for the endophyte, such as host plant interactions due to advancements in 'omic' technologies, which facilitate the exploration of genes that are responsible for plant tissue colonization. Consequently, this is informative to envisage putative functions and metabolic processes crucial for endophytic adaptations. Detection of cell signaling molecules between host plants and identification of compounds synthesized by root endophytes are effective means for their utilization in the agriculture sector as biofertilizers. In addition, it is interesting that the endophytic microorganism colonization impacts the relative abundance of indigenous microbial communities and suppresses the deleterious microorganisms in plant tissues. Natural products released by endophytes act as biocontrol agents and inhibit pathogen growth. The symbiosis of endophytic bacteria and arbuscular mycorrhizal fungi (AMF) affects plant symbiotic signaling pathways and root colonization patterns and phytohormone synthesis. In this review, the potential of the root endophytic community, colonization, and role in the improvement of plant growth has been explained in the light of intricate plant-microbe interactions.

Insights into the early stages of plant-endophytic bacteria interaction

The plant holobiont is a complex entity composed of the plant and the organisms that live in and on it including its microbiota. The plant microbiota includes, among other microorganisms, bacterial endophytes, which are bacteria that can invade living plant tissues without causing symptoms of disease. The interaction between the endophytic bacterial microbiota and their plant host has profound influences on their fitness and depends on biotic and abiotic factors. For these interactions to be established, the bacteria have to be present at the right time, in the right place either colonizing the soil or the seed. In this review we summarize the current knowledge regarding the sources of the bacterial endophytic microbiome and the processes involved in the assemblage of the resulting community during the initial stages of plant development. The adaptations that allow the spatial approximation of soil- and seed-borne bacteria towards infection and colonization of the internal tissues of plants will be addressed in this review.

What Did We Learn From Plant Growth-Promoting Rhizobacteria (PGPR)-Grass Associations Studies Through Proteomic and Metabolomic Approaches?

Plant growth stimulation by microorganisms that interact in a mutually beneficial manner remains poorly understood. Understanding the nature of plant-bacteria interactions may open new routes for plant productivity enhancement, especially cereal crops consumed by humans. Proteomic and metabolomic analyses are particularly useful for elucidating these mechanisms. A complete depiction of these mechanisms will prompt researchers to develop more efficient plant-bacteria associations. The success of microorganisms as biofertilizers may replace the current massive use of chemical fertilizers, mitigating many environmental and economic issues. In this review, we discuss the recent advances and current state of the art in proteomics and metabolomics studies involving grass-bacteria associations. We also discuss essential subjects involved in the bacterial plant-growth promotion, such, nitrogen fixation, plant stress, defense responses, and siderophore production.

Arabidopsis thaliana exudates induce growth and proteomic changes in Gluconacetobacter diazotrophicus

Background

Plants interact with a variety of microorganisms during their life cycle, among which beneficial bacteria deserve special attention. Gluconacetobacter diazotrophicus is a beneficial bacterium able to fix nitrogen and promote plant growth. Despite its biotechnological potential, the mechanisms regulating the interaction between G. diazotrophicus and host plants remain unclear.

Methods

We analyzed the response of G. diazotrophicus to cocultivation with Arabidopsis thaliana seedlings. Bacterial growth in response to cocultivation and plant exudates was analyzed. Through comparative proteomic analysis, G. diazotrophicus proteins regulated during cocultivation were investigated. Finally, the role of some up-accumulated proteins in the response G. diazotrophicus to cocultivation was analyzed by reverse genetics, using insertion mutants.

Results

Our results revealed the induction of bacterial growth in response to cocultivation. Comparative proteomic analysis identified 450 bacterial proteins, with 39 up-accumulated, and 12 down-accumulated in response to cocultivation. Among the up-accumulated pathways, the metabolism of pentoses and protein synthesis were highlighted. Proteins potentially relevant to bacterial growth response such as ABC-F-Etta, ClpX, Zwf, MetE, AcnA, IlvC, and AccC were also increased. Reverse genetics analysis, using insertion mutants, revealed that the lack of ABC-F-Etta and AccC proteins severely affects G. diazotrophicus response to cocultivation. Our data demonstrated that specific mechanisms are activated in the bacterial response to plant exudates, indicating the essential role of “ribosomal activity” and “fatty acid biosynthesis” in such a process. This is the first study to demonstrate the participation of EttA and AccC proteins in plant-bacteria interactions, and open new perspectives for understanding the initial steps of such associations.

Comparative proteomics reveals essential mechanisms for osmotolerance in Gluconacetobacter diazotrophicus

Plant growth-promoting bacteria are a promising alternative to improve agricultural sustainability. Gluconacetobacter diazotrophicus is an osmotolerant bacterium able to colonize several plant species, including sugarcane, coffee, and rice. Despite its biotechnological potential, the mechanisms controlling such osmotolerance remain unclear. The present study investigated the key mechanisms of resistance to osmotic stress in G. diazotrophicus. The molecular pathways regulated by the stress were investigated by comparative proteomics, and proteins essential for resistance were identified by knock-out mutagenesis. Proteomics analysis led to identify regulatory pathways for osmotic adjustment, de novo saturated fatty acids biosynthesis, and uptake of nutrients. The mutagenesis analysis showed that the lack of AccC protein, an essential component of de novo fatty acid biosynthesis, severely affected G. diazotrophicus resistance to osmotic stress. Additionally, knock-out mutants for nutrients uptake (Δtbdr and ΔoprB) and compatible solutes synthesis (ΔmtlK and ΔotsA) became more sensitive to osmotic stress. Together, our results identified specific genes and mechanisms regulated by osmotic stress in an osmotolerant bacterium, shedding light on the essential role of cell envelope and extracytoplasmic proteins for osmotolerance.

Deciphering plant-microbe crosstalk through proteomics studies

Proteomic approaches are being used to elucidate a better discretion of interactions occurring between host, pathogen, and/or beneficial microorganisms at the molecular level. Application of proteomic techniques, unravel pathogenicity, stress-related, and antioxidant proteins expressed amid plant-microbe interactions and good information have been generated. It is being perceived that a fine regulation of protein expression takes place for effective pathogen recognition, induction of resistance, and maintenance of host integrity. However, our knowledge of molecular plant-microbe interactions is still incomplete and inconsequential. This review aims to provide insight into numerous ways used for proteomic investigation including peptide/protein identification, separation, and quantification during host defense response. Here, we highlight the current progress in proteomics of defense responses elicited by bacterial, fungal, and viral pathogens in plants along with which the proteome level changes induced by beneficial microorganisms are also discussed.

Transcriptome Profiles Reveal the Growth-Promoting Mechanisms of Paenibacillus polymyxa YC0136 on Tobacco (Nicotiana tabacum L.)

Paenibacillus polymyxa is an important member of the plant growth-promoting rhizobacteria. P. polymyxa YC0136 inoculation had beneficial effect on growth promotion and biological control of tobacco (Nicotiana tabacum L.) under field conditions. This study aimed to reveal the growth-promoting mechanisms of strain YC0136. In growth-promotion assays, tobacco plant height was increased by 8.42% and 8.25% at 60 and 90 days, respectively, after inoculation with strain YC0136. Strain YC0136 also promoted the accumulation of tobacco biomass in varying degrees. Following inoculation with strain YC0136, 3,525 and 4,368 tobacco genes were up-regulated and down-regulated, respectively. Strain YC0136 induced the expression of plant hormone-related genes in tobacco, including auxin, cytokinin, and gibberellin, as well as transcription factors related to stress resistance such as WRKY and MYB. In addition, strain YC0136 induced the up-regulation of genes in the phenylpropanoid biosynthesis pathway by 1.51-4.59 times. Interaction with tobacco also induced gene expression changes in strain YC0136, with 286 and 223 genes up-regulated and down-regulated, respectively. Tobacco interaction induced up-regulation of the ilvB gene related to auxin biosynthesis in strain YC0136 by 1.72 times and induced expression of some nutrient transport genes. This study contributes to our understanding of the growth-promoting mechanisms of strain YC0136 on tobacco and provides a theoretical basis for the application of P. polymyxa YC0136 as a biological fertilizer.

Deciphering the Omics of Plant-Microbe Interaction: Perspectives and New Insights

Introduction

Plants do not grow in isolation, rather they are hosts to a variety of microbes in their natural environments. While, few thrive in the plants for their own benefit, others may have a direct impact on plants in a symbiotic manner. Unraveling plant-microbe interactions is a critical component in recognizing the positive and negative impacts of microbes on plants. Also, by affecting the environment around plants, microbes may indirectly influence plants. The progress in sequencing technologies in the genomics era and several omics tools has accelerated in biological science. Studying the complex nature of plant-microbe interactions can offer several strategies to increase the productivity of plants in an environmentally friendly manner by providing better insights. This review brings forward the recent works performed in building omics strategies that decipher the interactions between plant-microbiome. At the same time, it further explores other associated mutually beneficial aspects of plant-microbe interactions such as plant growth promotion, nitrogen fixation, stress suppressions in crops and bioremediation; as well as provides better insights on metabolic interactions between microbes and plants through omics approaches. It also aims to explore advances in the study of Arabidopsis as an important avenue to serve as a baseline tool to create models that help in scrutinizing various factors that contribute to the elaborate relationship between plants and microbes. Causal relationships between plants and microbes can be established through systematic gnotobiotic experimental studies to test hypotheses on biologically derived interactions.

Conclusion

This review will cover recent advances in the study of plant-microbe interactions keeping in view the advantages of these interactions in improving nutrient uptake and plant health.

Plants endophytes: unveiling hidden agenda for bioprospecting toward sustainable agriculture

Endophytic microbes are present in nearly all of the plant species known to date but how they enter and flourish inside a host plant and display multiple benefits like plant growth promotion (PGP), biodegradation, and stress alleviation are still unexplored. Until now, the majority of the research has been conducted assuming that the host-endophyte interaction is analogous to the PGP microbes, although, studies related to the mechanisms of their infection, colonization as well as conferring important traits to the plants are limited. It would be fascinating to explore the role of these endophytic microbes in host gene expression, metabolism, and the modulation of phenotypic traits, under abiotic and biotic stress conditions. In this review, we critically focused on the following areas: (i) endophytic lifestyle and the mechanism of their entry into plant tissues, (ii) how endophytes modulate the immune system of plants and affect the genotypic and phenotypic expression of host plants under abiotic and biotic stress condition, and (iii) the role of omics and other integrated genomic approaches in unraveling complex host-endophyte signaling crosstalk. Furthermore, we discussed their role in phytoremediation of heavy metal stress and whole genomic analysis based on an understanding of different metabolic pathways these endophytes utilize to combat stress.

Plant-Microbe Symbiosis: What Has Proteomics Taught Us?

Beneficial microbes have a positive impact on the productivity and fitness of the host plant. A better understanding of the biological impacts and underlying mechanisms by which the host derives these benefits will help to address concerns around global food production and security. The recent development of omics-based technologies has broadened our understanding of the molecular aspects of beneficial plant-microbe symbiosis. Specifically, proteomics has led to the identification and characterization of several novel symbiosis-specific and symbiosis-related proteins and post-translational modifications that play a critical role in mediating symbiotic plant-microbe interactions and have helped assess the underlying molecular aspects of the symbiotic relationship. Integration of proteomic data with other "omics" data can provide valuable information to assess hypotheses regarding the underlying mechanism of symbiosis and help define the factors affecting the outcome of symbiosis. Herein, an update is provided on the current and potential applications of symbiosis-based "omic" approaches to dissect different aspects of symbiotic plant interactions. The application of proteomics, metaproteomics, and secretomics as enabling approaches for the functional analysis of plant-associated microbial communities is also discussed.

Anatomical and Biochemical Changes Induced by Gluconacetobacter diazotrophicus Stand Up for Arabidopsis thaliana Seedlings From Ralstonia solanacearum Infection

Nowadays, fertilization and pest control are carried out using chemical compounds that contaminate soil and deteriorate human health. Plant growth promoting bacteria endophytes (PGPBEs), are a well-studied group of bacteria that offers benefits to the host plant, such as phytostimulation, biofertilization, and protection against other microorganisms. The study of Gluconacetobacter diazotrophicus-which belongs to PGPBEs-aids the development of alternative strategies of an integrated approach for crop management practices. Ralstonia solanacearum is responsible for bacterial wilt disease. This phytopathogen is of great interest worldwide due to the enormous economic losses it causes. In this study the action of G. diazotrophicus as a growth promoting bacterium in Arabidopsis thaliana seedlings is analyzed, evaluating the antagonistic mechanisms of this beneficial endophytic bacterium during biotic stress produced by R. solanacearum. Effective colonization of G. diazotrophicus was determined through bacterial counting assays, evaluation of anatomical and growth parameters, and pigments quantification. Biocontrol assays were carried out with Ralstonia pseudosolanacearum GMI1000 model strain and R. solanacearum A21 a recently isolated strain. Inoculation of A. thaliana (Col 0) with G. diazotrophicus Pal 5 triggers a set of biochemical and structural changes in roots, stems, and leaves of seedlings. Discrete callose deposits as papillae were observed at specific sites of root hairs, trichomes, and leaf tissue. Upon R. pseudosolanacearum GMI1000 infection, endophyte-treated plants demonstrated being induced for defense through an augmented callose deposition at root hairs and leaves compared with the non-endophyte-treated controls. The endophytic bacterium appears to be able to prime callose response. Roots and stems cross sections showed that integrity of all tissues was preserved in endophyte-treated plants infected with R. solanacearum A21. The mechanisms of resistance elicited by the plant after inoculation with the endophyte would be greater lignification and sclerosis in tissues and reinforcement of the cell wall through the deposition of callose. As a consequence of this priming in plant defense response, viable phytopathogenic bacteria counting were considerably fewer in endophyte-inoculated plants than in not-inoculated controls. Our results indicate that G. diazotrophicus colonizes A. thaliana plants performing a protective role against the phytopathogenic bacterium R. solanacearum promoting the activation of plant defense system.

Are we there yet? The long walk towards the development of efficient symbiotic associations between nitrogen-fixing bacteria and non-leguminous crops

Nitrogen is an essential element of life, and nitrogen availability often limits crop yields. Since the Green Revolution, massive amounts of synthetic nitrogen fertilizers have been produced from atmospheric nitrogen and natural gas, threatening the sustainability of global food production and degrading the environment. There is a need for alternative means of bringing nitrogen to crops, and taking greater advantage of biological nitrogen fixation seems a logical option. Legumes are used in most cropping systems around the world because of the nitrogen-fixing symbiosis with rhizobia. However, the world's three major cereal crops-rice, wheat, and maize-do not associate with rhizobia. In this review, we will survey how genetic approaches in rhizobia and their legume hosts allowed tremendous progress in understanding the molecular mechanisms controlling root nodule symbioses, and how this knowledge paves the way for engineering such associations in non-legume crops. We will also discuss challenges in bringing these systems into the field and how they can be surmounted by interdisciplinary collaborations between synthetic biologists, microbiologists, plant biologists, breeders, agronomists, and policymakers.

Characterizing endophytic competence and plant growth promotion of bacterial endophytes inhabiting the seed endosphere of Rice

Background

Rice (Oryza sativa L. ssp. indica) seeds as plant microbiome present both an opportunity and a challenge to colonizing bacterial community living in close association with plants. Nevertheless, the roles and activities of bacterial endophytes remain largely unexplored and insights into plant-microbe interaction are compounded by its complexity. In this study, putative functions or physiological properties associated with bacterial endophytic nature were assessed. Also, endophytic roles in plant growth and germination that may allow them to be selectively chosen by plants were also studied.

Results

The cultivable seed endophytes were dominated by Proteobacteria particularly class Gammaproteobacteria. Highly identical type strains were isolated from the seed endosphere regardless of the rice host’s physiological tolerance to salinity. Among the type strains, Flavobacterium sp., Microbacterium sp. and Xanthomonas sp. were isolated from the salt-sensitive and salt-tolerant cultivars. PCA-Biplot ordination also showed that specific type strains isolated from different rice cultivars have distinguishing similar characteristics. Flavobacterium sp. strains are phosphate solubilizers and indole-3-acetic acid producers with high tolerance to salinity and osmotic stress. Pseudomonas strains are characterized as high siderophore producers while Microbacterium sp. and Xanthomonas sp. strains have very high pectinase and cellulase activity. Among the physiological traits of the seed endophytes, bacterial pectinase and cellulase activity are positively correlated as well as salt and osmotic tolerance. Overall characterization shows that majority of the isolates could survive in 4–8% salt concentration as well as in 0.6 M and 1.2 M sucrose solution. The activities of catalase, pectinase and cellulase were also observed in almost all of the isolates indicating the importance of these characteristics for survival and colonization into the seed endosphere. Seed bacterial endophytes also showed promising plant growth promoting activities including hormone modulation, nitrogen fixation, siderophore production and phosphate solubilization.

Conclusion

Though many of the isolates possess similar PGP and endophytic physiological traits, this study shows some prominent and distinguishing traits among bacterial groups indicating key determinants for their success as endophytes in the rice seed endosphere. Rice seeds are also inhabited by bacterial endophytes that promote growth during early seedling development.

Electronic supplementary material

The online version of this article (10.1186/s12866-017-1117-0) contains supplementary material, which is available to authorized users.

Herbaspirillum rubrisubalbicans, a mild pathogen impairs growth of rice by augmenting ethylene levels

Herbaspirillum rubrisubalbicans decreases growth of rice. Inoculation of rice with H. rubrisubalbicans increased the ACCO mRNA levels and ethylene production. The H. rubrisubalbicans rice interactions were further characterized by proteomic approach. Herbaspirillum rubrisubalbicans is a well-known growth-promoting rhizobacteria that can also act as a mild phyto-pathogen. During colonisation of rice, RT-qPCR analyses showed that H. rubrisubalbicans up-regulates the methionine recycling pathway as well as phyto-siderophore synthesis genes. mRNA levels of ACC oxidase and ethylene levels also increased in rice roots but inoculation with H. rubrisubalbicans impaired growth of the rice plant. A proteomic approach was used to identify proteins specifically modulated by H. rubrisubalbicans in rice and amongst the differentially expressed proteins a V-ATPase and a 14-3-3 protein were down-regulated. Several proteins of H. rubrisubalbicans were identified, including the type VI secretion system effector Hcp1, suggesting that protein secretion play a role colonisation in rice. Finally, the alkyl hydroperoxide reductase, a primary scavenger of endogenous hydrogen peroxide was also identified. Monitoring the levels of reactive oxygen species in the epiphytic bacteria by flow cytometry revealed that H. rubrisubalbicans is subjected to oxidative stress, suggesting that the alkyl hydroperoxide reductase is an important regulator of redox homeostasis in plant-bacteria interactions.

Quorum-Sensing Systems in the Plant Growth-Promoting Bacterium Paraburkholderia phytofirmans PsJN Exhibit Cross-Regulation and Are Involved in Biofilm Formation

Quorum-sensing systems play important roles in host colonization and host establishment of Burkholderiales species. Beneficial Paraburkholderia species share a conserved quorum-sensing (QS) system, designated BraI/R, that controls different phenotypes. In this context, the plant growth-promoting bacterium Paraburkholderia phytofirmans PsJN possesses two different homoserine lactone QS systems BpI.1/R.1 and BpI.2/R.2 (BraI/R-like QS system). The BpI.1/R.1 QS system was previously reported to be important to colonize and produce beneficial effects in Arabidopsis thaliana plants. Here, we analyzed the temporal variations of the QS gene transcript levels in the wild-type strain colonizing plant roots. The gene expression patterns showed relevant differences in both QS systems compared with the wild-type strain in the unplanted control treatment. The gene expression data were used to reconstruct a regulatory network model of QS systems in P. phytofirmans PsJN, using a Boolean network model. Also, we examined the phenotypic traits and transcript levels of genes involved in QS systems, using P. phytofirmans mutants in homoserine lactone synthases genes. We observed that the BpI.1/R.1 QS system regulates biofilm formation production in strain PsJN and this phenotype was associated with the lower expression of a specific extracytoplasmic function sigma factor ecf26.1 gene (implicated in biofilm formation) in the bpI.1 mutant strain.

Metacoder: An R package for visualization and manipulation of community taxonomic diversity data

Community-level data, the type generated by an increasing number of metabarcoding studies, is often graphed as stacked bar charts or pie graphs that use color to represent taxa. These graph types do not convey the hierarchical structure of taxonomic classifications and are limited by the use of color for categories. As an alternative, we developed metacoder, an R package for easily parsing, manipulating, and graphing publication-ready plots of hierarchical data. Metacoder includes a dynamic and flexible function that can parse most text-based formats that contain taxonomic classifications, taxon names, taxon identifiers, or sequence identifiers. Metacoder can then subset, sample, and order this parsed data using a set of intuitive functions that take into account the hierarchical nature of the data. Finally, an extremely flexible plotting function enables quantitative representation of up to 4 arbitrary statistics simultaneously in a tree format by mapping statistics to the color and size of tree nodes and edges. Metacoder also allows exploration of barcode primer bias by integrating functions to run digital PCR. Although it has been designed for data from metabarcoding research, metacoder can easily be applied to any data that has a hierarchical component such as gene ontology or geographic location data. Our package complements currently available tools for community analysis and is provided open source with an extensive online user manual.

Comparative proteomics reveals that central metabolism changes are associated with resistance against Sporisorium scitamineum in sugarcane

BACKGROUND

Sugarcane smut, which is caused by Sporisorium scitamineum, has been threatening global sugarcane production. Breeding smut resistant sugarcane varieties has been proven to be the most effective method of controlling this particular disease. However, a lack of genome information of sugarcane has hindered the development of genome-assisted resistance breeding programs. Furthermore, the molecular basis of sugarcane response to S. scitamineum infection at the proteome level was incomplete and combining proteomic and transcriptional analysis has not yet been conducted.

RESULTS

We identified 273 and 341 differentially expressed proteins in sugarcane smut-resistant (Yacheng05-179) and susceptible (ROC22) genotypes at 48 h after inoculation with S. scitamineum by employing an isobaric tag for relative and absolute quantification (iTRAQ). The proteome quantitative data were then validated by multiple reaction monitoring (MRM). The integrative analysis showed that the correlations between the quantitative proteins and the corresponding genes that was obtained in our previous transcriptome study were poor, which were 0.1502 and 0.2466 in Yacheng05-179 and ROC22, respectively, thereby revealing a post-transcriptional event during Yacheng05-179-S. scitamineum incompatible interaction and ROC22-S. scitamineum compatible interaction. Most differentially expressed proteins were closely related to sugarcane smut resistance such as beta-1,3-glucanase, peroxidase, pathogenesis-related protein 1 (PR1), endo-1,4-beta-xylanase, heat shock protein, and lectin. Ethylene and gibberellic acid pathways, phenylpropanoid metabolism and PRs, such as PR1, PR2, PR5 and PR14, were more active in Yacheng05-179, which suggested of their possible roles in sugarcane smut resistance. However, calcium signaling, reactive oxygen species, nitric oxide, and abscisic acid pathways in Yacheng05-179 were repressed by S. scitamineum and might not be crucial for defense against this particular pathogen.

CONCLUSIONS

These results indicated complex resistance-related events in sugarcane-S. scitamineum interaction, and provided novel insights into the molecular mechanism underlying the response of sugarcane to S. scitamineum infection.

Sugarcane proteomics: An update on current status, challenges, and future prospects

Sugarcane is one of the most important commercial crops cultivated worldwide for the production of crystal sugar, ethanol, and other related by-products. Unlike other comparable monocots like sorghum, maize, and rice, sugarcane genome by virtue of its polyploidy nature remains yet to be fully deciphered. Proteomics-an established complementary tool to genomics is at its infancy in sugarcane as compared to the other monocots. However, with the surge in genomics research accomplished by next-generation sequencing platforms, sugarcane proteomics has gained momentum. This review summarizes the available literature from 1970 to 2014, which ensures a comprehensive coverage on sugarcane proteomics-a topic first of its kind to be reviewed. We herewith compiled substantial contributions in different areas of sugarcane proteomics, which include abiotic and biotic stresses, cell wall, organelle, and structural proteomics. The past decade has witnessed a paradigm shift in the pace with which sugarcane proteomics is progressing, as evident by the number of research publications. In addition to extensively reviewing the progress made thus far, we intend to highlight the scope in sugarcane proteomics, with an aspiration to instigate focused research on sugarcane to harness its full potential for the human welfare.

Dhar M. "Omics" Tools for Better Understanding the Plant-Endophyte Interactions

Endophytes, which mostly include bacteria, fungi and actinomycetes, are the endosymbionts that reside asymptomatically in plants for at least a part of their life cycle. They have emerged as a valuable source of novel metabolites, industrially important enzymes and as stress relievers of host plant, but still many aspects of endophytic biology are unknown. Functions of individual endophytes are the result of their continuous and complex interactions with the host plant as well as other members of the host microbiome. Understanding plant microbiomes as a system allows analysis and integration of these complex interactions. Modern genomic studies involving metaomics and comparative studies can prove to be helpful in unraveling the gray areas of endophytism. A deeper knowledge of the mechanism of host infestation and role of endophytes could be exploited to improve the agricultural management in terms of plant growth promotion, biocontrol and bioremediation. Genome sequencing, comparative genomics, microarray, next gen sequencing, metagenomics, metatranscriptomics are some of the techniques that are being used or can be used to unravel plant-endophyte relationship. The modern techniques and approaches need to be explored to study endophytes and their putative role in host plant ecology. This review highlights "omics" tools that can be explored for understanding the role of endophytes in the plant microbiome.

Plant growth-promoting bacterial endophytes

Bacterial endophytes ubiquitously colonize the internal tissues of plants, being found in nearly every plant worldwide. Some endophytes are able to promote the growth of plants. For those strains the mechanisms of plant growth-promotion known to be employed by bacterial endophytes are similar to the mechanisms used by rhizospheric bacteria, e.g., the acquisition of resources needed for plant growth and modulation of plant growth and development. Similar to rhizospheric plant growth-promoting bacteria, endophytic plant growth-promoting bacteria can act to facilitate plant growth in agriculture, horticulture and silviculture as well as in strategies for environmental cleanup (i.e., phytoremediation). Genome comparisons between bacterial endophytes and the genomes of rhizospheric plant growth-promoting bacteria are starting to unveil potential genetic factors involved in an endophytic lifestyle, which should facilitate a better understanding of the functioning of bacterial endophytes.

Understanding and engineering beneficial plant-microbe interactions: plant growth promotion in energy crops

Plant production systems globally must be optimized to produce stable high yields from limited land under changing and variable climates. Demands for food, animal feed, and feedstocks for bioenergy and biorefining applications, are increasing with population growth, urbanization and affluence. Low-input, sustainable, alternatives to petrochemical-derived fertilizers and pesticides are required to reduce input costs and maintain or increase yields, with potential biological solutions having an important role to play. In contrast to crops that have been bred for food, many bioenergy crops are largely undomesticated, and so there is an opportunity to harness beneficial plant-microbe relationships which may have been inadvertently lost through intensive crop breeding. Plant-microbe interactions span a wide range of relationships in which one or both of the organisms may have a beneficial, neutral or negative effect on the other partner. A relatively small number of beneficial plant-microbe interactions are well understood and already exploited; however, others remain understudied and represent an untapped reservoir for optimizing plant production. There may be near-term applications for bacterial strains as microbial biopesticides and biofertilizers to increase biomass yield from energy crops grown on land unsuitable for food production. Longer term aims involve the design of synthetic genetic circuits within and between the host and microbes to optimize plant production. A highly exciting prospect is that endosymbionts comprise a unique resource of reduced complexity microbial genomes with adaptive traits of great interest for a wide variety of applications.

Structure and functional characterization of pyruvate decarboxylase from Gluconacetobacter diazotrophicus

BACKGROUND

Bacterial pyruvate decarboxylases (PDC) are rare. Their role in ethanol production and in bacterially mediated ethanologenic processes has, however, ensured a continued and growing interest. PDCs from Zymomonas mobilis (ZmPDC), Zymobacter palmae (ZpPDC) and Sarcina ventriculi (SvPDC) have been characterized and ZmPDC has been produced successfully in a range of heterologous hosts. PDCs from the Acetobacteraceae and their role in metabolism have not been characterized to the same extent. Examples include Gluconobacter oxydans (GoPDC), G. diazotrophicus (GdPDC) and Acetobacter pasteutrianus (ApPDC). All of these organisms are of commercial importance.

RESULTS

This study reports the kinetic characterization and the crystal structure of a PDC from Gluconacetobacter diazotrophicus (GdPDC). Enzyme kinetic analysis indicates a high affinity for pyruvate (K M 0.06 mM at pH 5), high catalytic efficiencies (1.3 • 10(6) M(-1) • s(-1) at pH 5), pHopt of 5.5 and Topt at 45°C. The enzyme is not thermostable (T½ of 18 minutes at 60°C) and the calculated number of bonds between monomers and dimers do not give clear indications for the relatively lower thermostability compared to other PDCs. The structure is highly similar to those described for Z. mobilis (ZmPDC) and A. pasteurianus PDC (ApPDC) with a rmsd value of 0.57 Å for Cα when comparing GdPDC to that of ApPDC. Indole-3-pyruvate does not serve as a substrate for the enzyme. Structural differences occur in two loci, involving the regions Thr341 to Thr352 and Asn499 to Asp503.

CONCLUSIONS

This is the first study of the PDC from G. diazotrophicus (PAL5) and lays the groundwork for future research into its role in this endosymbiont. The crystal structure of GdPDC indicates the enzyme to be evolutionarily closely related to homologues from Z. mobilis and A. pasteurianus and suggests strong selective pressure to keep the enzyme characteristics in a narrow range. The pH optimum together with reduced thermostability likely reflect the host organisms niche and conditions under which these properties have been naturally selected for. The lack of activity on indole-3-pyruvate excludes this decarboxylase as the enzyme responsible for indole acetic acid production in G. diazotrophicus.

Gluconacetobacter diazotrophicus PAL5 possesses an active quorum sensing regulatory system

The endophytic bacterium Gluconacetobacter diazotrophicus colonizes a broad range of host plants. Its plant growth-promoting capability is related to the capacity to perform biological nitrogen fixation, the biosynthesis of siderophores, antimicrobial substances and the solubilization of mineral nutrients. Colonization of and survival in these endophytic niche requires a complex regulatory network. Among these, quorum sensing systems (QS) are signaling mechanisms involved in the control of several genes related to microbial interactions, host colonization and stress survival. G. diazotrophicus PAL5 possesses a QS composed of a luxR and a luxI homolog, and produces eight molecules from the AHL family as QS signals. In this report data are provided showing that glucose concentration modifies the relative levels of these signal molecules. The activity of G. diazotrophicus PAL5 QS is also altered in presence of other carbon sources and under saline stress conditions. Inactivation of the QS system of G. diazotrophicus PAL5 by means of a quorum quenching strategy allowed the identification of extracellular and intracellular proteins under the control of this regulatory mechanism.

Integrated metabolomic and proteomic approaches dissect the effect of metal-resistant bacteria on maize biomass and copper uptake

Marginal soils arise due to various industrial and agricultural practices reducing crop productivity. Pseudomonas sp. TLC 6-6.5-4 is a free-living multiple-metal-resistant plant-growth-promoting bacteria (PGPB) isolated from Torch Lake sediment that promotes maize growth and nutrient uptake. In this study, we examined both PGPB-soil and PGPB-plant interactions. PGPB inoculation resulted in significant increase in maize biomass. Soil inoculation before sowing seeds and coating seeds with the PGPB resulted in higher copper uptake by maize compared to other methods. The PGPB-soil interaction improved phosphorus uptake by maize and led to significant decrease in organic bound copper in marginal soil and a notable increase in exchangeable copper. PGPB improved soil health based on soil enzyme activities. Metabolomic analysis of maize revealed that PGPB inoculation upregulated photosynthesis, hormone biosynthesis, and tricarboxylic acid cycle metabolites. Proteomic analysis identified upregulation of proteins related to plant development and stress response. Further, the activity of antioxidant enzymes and total phenolics decreased in plants grown in marginal soil suggesting alleviation of metal stress in presence of PGPB. The ability of PGPB to modulate interconnected biochemical pathways could be exploited to increase crop productivity in marginal soils, phytoremediation of metal contaminated soils, and organic agriculture.

Root ethylene signalling is involved in Miscanthus sinensis growth promotion by the bacterial endophyte Herbaspirillum frisingense GSF30(T)

The bacterial endophyte Herbaspirillum frisingense GSF30(T) is a colonizer of several grasses grown in temperate climates, including the highly nitrogen-efficient perennial energy grass Miscanthus. Inoculation of Miscanthus sinensis seedlings with H. frisingense promoted root and shoot growth but had only a minor impact on nutrient concentrations. The bacterium affected the root architecture and increased fine-root structures. Although H. frisingense has the genetic requirements to fix nitrogen, only minor changes in nitrogen concentrations were observed. Herbaspirillum agglomerates were identified primarily in the root apoplast but also in the shoots. The short-term (3h) and long-term (3 weeks) transcriptomic responses of the plant to bacterial inoculation revealed that H. frisingense induced rapid changes in plant hormone signalling, most prominent in jasmonate signalling. Ethylene signalling pathways were also affected and persisted after 3 weeks in the root. Growth stimulation of the root by the ethylene precursor 1-aminocyclopropane 1-carboxylic acid was dose dependent and was affected by H. frisingense inoculation. Minor changes in the proteome were identified after 3 weeks. This study suggests that H. frisingense improves plant growth by modulating plant hormone signalling pathways and provides a framework to understand the beneficial effects of diazotrophic plant-growth-promoting bacteria, such as H. frisingense, on the biomass grass Miscanthus.

The bacterial superoxide dismutase and glutathione reductase are crucial for endophytic colonization of rice roots by Gluconacetobacter diazotrophicus PAL5

Gluconacetobacter diazotrophicus is an aerobic diazotrophic plant-growth-promoting bacterium isolated from different gramineous plants. We showed that reactive oxygen species (ROS) were produced at early stages of rice root colonization, a typical plant defense response against pathogens. The transcription of the pathogen-related-10 gene of the jasmonic acid (JA) pathway but not of the PR-1 gene of the salicylic acid pathway was activated by the endophytic colonization of rice roots by G. diazotrophicus strain PAL5. Quantitative polymerase chain reaction analyses showed that, at early stages of colonization, the bacteria upregulated the transcript levels of ROS-detoxifying genes such as superoxide dismutase (SOD) and glutathione reductase (GR). To proof the role of ROS-scavenging enzymes in the colonization and interaction process, transposon insertion mutants of the SOD and GR genes of strain PAL5 were constructed. The SOD and GR mutants were unable to efficiently colonize the roots, indicated by the decrease of tightly root-associated bacterial cell counts and endophytic colonization and by fluorescence in situ hybridization analysis. Interestingly, the mutants did not induce the PR-10 of the JA-pathway, probably due to the inability of endophytic colonization. Thus, ROS-scavenging enzymes of G. diazotrophicus strain PAL5 play an important role in the endophytic colonization of rice plants.

Untapped mutualistic paradigms linking host plant and endophytic fungal production of similar bioactive secondary metabolites

The biosynthetic potential of endophytic fungi has gained impetus in recent times owing to the continual discovery of fungal endophytes capable of synthesizing plant compounds. However, the sustained production of the desired plant compounds has not yet been achieved using endophytes. It is thus imperative to investigate the diverse interactions that endophytes have with coexisting endophytes, host plants, insect pests, and other specific herbivores. The precise role of these associations on the endophytic production of host plant compounds is mostly overlooked and open to future discoveries. Here, highlighted are the implications of the poorly investigated links and molecular mechanisms that might trigger similar chemical responses in both plants and endophytes. Elucidating such connections can not only enhance the understanding of evolution of complex defense mechanisms in plants and associated organisms, but also help in the sustained production of plant compounds using endophytes harbored within them.

The chequered history of the development and use of simultaneous equations for the accurate determination of chlorophylls a and b

Over the last half century, the most frequently used assay for chlorophylls in higher plants and green algae, the Arnon assay [Arnon DI (1949) Plant Physiol 24: 1-15], employed simultaneous equations for determining the concentrations of chlorophylls a and b in aqueous 80% acetone extracts of chlorophyllous plant and algal materials. These equations, however, were developed using extinction coefficients for chlorophylls a and b derived from early inaccurate spectrophotometric data. Thus, Arnon's equations give inaccurate chlorophyll a and b determinations and, therefore, inaccurate chlorophyll a/b ratios, which are always low. This paper describes how the ratios are increasingly and alarmingly low as the proportion of chlorophyll a increases. Accurate extinction coefficients for chlorophylls a and b, and the more reliable simultaneous equations derived from them, have been published subsequently by many research groups; these new post-Arnon equations, however, have been ignored by many researchers. This Minireview records the history of the development of accurate simultaneous equations and some difficulties and anomalies arising from the retention of Arnon's seriously flawed equations.