Synergistic interactions between the ACC deaminase-producing bacterium Pseudomonas putida UW4 and the AM fungus Gigaspora rosea positively affect cucumber plant growth

Bacterial diversity and composition on the rinds of specific melon cultivars and hybrids from across different growing regions in the United States

The goal of this study was to characterize the bacterial diversity on different melon varieties grown in different regions of the US, and determine the influence that region, rind netting, and variety of melon has on the composition of the melon microbiome. Assessing the bacterial diversity of the microbiome on the melon rind can identify antagonistic and protagonistic bacteria for foodborne pathogens and spoilage organisms to improve melon safety, prolong shelf-life, and/or improve overall plant health. Bacterial community composition of melons (n = 603) grown in seven locations over a four-year period were used for 16S rRNA gene amplicon sequencing and analysis to identify bacterial diversity and constituents. Statistically significant differences in alpha diversity based on the rind netting and growing region (p < 0.01) were found among the melon samples. Principal Coordinate Analysis based on the Bray-Curtis dissimilarity distance matrix found that the melon bacterial communities clustered more by region rather than melon variety (R2 value: 0.09 & R2 value: 0.02 respectively). Taxonomic profiling among the growing regions found Enterobacteriaceae, Bacillaceae, Microbacteriaceae, and Pseudomonadaceae present on the different melon rinds at an abundance of ≥ 0.1%, but no specific core microbiome was found for netted melons. However, a core of Pseudomonadaceae, Bacillaceae, and Exiguobacteraceae were found for non-netted melons. The results of this study indicate that bacterial diversity is driven more by the region that the melons were grown in compared to rind netting or melon type. Establishing the foundation for regional differences could improve melon safety, shelf-life, and quality as well as the consumers' health.

Growth-promoting bacteria and arbuscular mycorrhizal fungus enhance maize tolerance to saline stress

Climate change intensifies soil salinization and jeopardizes the development of crops worldwide. The accumulation of salts in plant tissue activates the defense system and triggers ethylene production thus restricting cell division. We hypothesize that the inoculation of plant growth-promoting bacteria (PGPB) producing ACC (1-aminocyclopropane-1-carboxylate) deaminase favors the development of arbuscular mycorrhizal fungi (AMF), promoting the growth of maize plants under saline stress. We investigated the efficacy of individual inoculation of PGPB, which produce ACC deaminase, as well as the co-inoculation of PGPB with Rhizophagus clarus on maize plant growth subjected to saline stress. The isolates were acquired from the bulk and rhizospheric soil of Mimosa bimucronata (DC.) Kuntze in a temporary pond located in Pernambuco State, Brazil. In the first greenhouse experiment, 10 halophilic PGPB were inoculated into maize at 0, 40 and 80 mM of NaCl, and in the second experiment, the PGPB that showed the best performance were co-inoculated with R. clarus in maize under the same conditions as in the first experiment. Individual PGPB inoculation benefited the number of leaves, stem diameter, root and shoot dry mass, and the photosynthetic pigments. Inoculation with PGPB 28-10 Pseudarthrobacter enclensis, 24-1 P. enclensis and 52 P. chlorophenolicus increased the chlorophyll a content by 138%, 171%, and 324% at 0, 40 and 80 mM NaCl, respectively, comparing to the non-inoculated control. We also highlight that the inoculation of PGPB 28-10, 28-7 Arthrobacter sp. and 52 increased the content of chlorophyll b by 72%, 98%, and 280% and carotenoids by 82%, 98%, and 290% at 0, 40 and 80 mM of NaCl, respectively. Co-inoculation with PGPB 28-7, 46-1 Leclercia tamurae, 70 Artrobacter sp., and 79-1 Micrococcus endophyticus significantly increased the rate of mycorrhizal colonization by roughly 50%. Furthermore, co-inoculation promoted a decrease in the accumulation of Na and K extracted from plant tissue, with an increase in salt concentration, from 40 mM to 80 mM, also favoring the establishment and development of R. clarus. In addition, co-inoculation of these PGPB with R. clarus promoted maize growth and increased plant biomass through osmoregulation and protection of the photosynthetic apparatus. The tripartite symbiosis (plant-fungus-bacterium) is likely to reprogram metabolic pathways that improve maize growth and crop yield, suggesting that the AMF-PGPB consortium can minimize damages caused by saline stress.

Fertilization of Microbial Composts: A Technology for Improving Stress Resilience in Plants

Microbial compost plays a crucial role in improving soil health, soil fertility, and plant biomass. These biofertilizers, based on microorganisms, offer numerous benefits such as enhanced nutrient acquisition (N, P, and K), production of hydrogen cyanide (HCN), and control of pathogens through induced systematic resistance. Additionally, they promote the production of phytohormones, siderophore, vitamins, protective enzymes, and antibiotics, further contributing to soil sustainability and optimal agricultural productivity. The escalating generation of organic waste from farm operations poses significant threats to the environment and soil fertility. Simultaneously, the excessive utilization of chemical fertilizers to achieve high crop yields results in detrimental impacts on soil structure and fertility. To address these challenges, a sustainable agriculture system that ensures enhanced soil fertility and minimal ecological impact is imperative. Microbial composts, developed by incorporating characterized plant-growth-promoting bacteria or fungal strains into compost derived from agricultural waste, offer a promising solution. These biofertilizers, with selected microbial strains capable of thriving in compost, offer an eco-friendly, cost-effective, and sustainable alternative for agricultural practices. In this review article, we explore the potential of microbial composts as a viable strategy for improving plant growth and environmental safety. By harnessing the benefits of microorganisms in compost, we can pave the way for sustainable agriculture and foster a healthier relationship between soil, plants, and the environment.

Ethylene, ACC, and the Plant Growth-Promoting Enzyme ACC Deaminase

Here, a brief summary of the biosynthesis of 1-aminocyclopropane-1-carboxylate (ACC) and ethylene in plants, as well as overviews of how ACC and ethylene act as signaling molecules in plants, is presented. Next, how the bacterial enzyme ACC deaminase cleaves plant-produced ACC and thereby decreases or prevents the ethylene or ACC modulation of plant gene expression is considered. A detailed model of ACC deaminase functioning, including the role of indoleacetic acid (IAA), is presented. Given that ACC is a signaling molecule under some circumstances, this suggests that ACC, which appears to have evolved prior to ethylene, may have been a major signaling molecule in primitive plants prior to the evolution of ethylene and ethylene signaling. Due to their involvement in stimulating ethylene production, the role of D-amino acids in plants is then considered. The enzyme D-cysteine desulfhydrase, which is structurally very similar to ACC deaminase, is briefly discussed and the possibility that ACC deaminase arose as a variant of D-cysteine desulfhydrase is suggested.

Dynamic succession of microbial compost communities and functions during Pleurotus ostreatus mushroom cropping on a short composting substrate

Cultivating oyster mushrooms (Pleurotus ostreatus), a typical primary decomposer of lignocellulose, on a short composting substrate is a novel procedure which possesses energy conserves, reduced the chance of infection by competitive species, shorter production duration and achieved high production efficiency. However, the microbiome and microbial metabolic functions in the composting substrate during the mushroom cropping is unknown. In the present study, the contents of hemicellulose, cellulose and lignin and the activities of protease, laccase and cellulase were evaluated in the corncob short composting substrate from before oyster mushroom spawning to first flush fructification; meanwhile the changes in the microbiome and microbial metabolic functions were surveyed by using metagenomic sequencing. Results showed that the hemicellulose, cellulose and lignin in the short composting substrate were decomposed of 42.76, 34.01, and 30.18%, respectively, during the oyster mushroom cropping process. In addition, the contents of hemicellulose, cellulose and lignin in the composting substrate were reduced rapidly and negatively correlated with the abundance of the Actinobacteria phylum. The activities of protease, laccase and cellulase fastly increased in the period of before oyster mushroom spawning to full colonization and were positively correlated to the abundance of Actinobacteria phylum. The total abundance of bacteria domain gradually decreased by only approximately 15%, while the abundance of Actinobacteria phylum increased by 68% and was positively correlated with that of oyster mushroom. The abundance of oyster mushroom increased by 50 times from spawning to first flush fructification. The dominant genera, all in the order of Actinomycetales, were Cellulosimicrobium, Mycobacterium, Streptomyces and Saccharomonospora. The total abundance of genes with functions annotated in the Clusters of Orthologous Groups of proteins (COG) for Bacteria and Archaea and Kyoto Encyclopedia of Genes and Genomes (KEGG) database for all three life domains was positively correlated.

The three metabolic pathways for carbohydrates, amino acids and energy were the primary enrichment pathways in KEGG pathway, accounting for more than 30% of all pathways, during the mushroom cropping in which the glycine metabolic pathway, carbon fixation pathways in prokaryotes and methane metabolism were all dominated by bacteria. The genes of cellulolytic enzymes, hemicellulolytic enzymes, laccase, chitinolytic enzymes, peptidoglycanlytic enzymes and ammonia assimilation enzymes with abundances from 0.28 to 0.24%, 0.05 to 0.02%, 0.02 to 0.01%, 0.14 to 0.08%, 0.39 to 0.16%, and 0.13 to 0.12% during the mushroom cropping identified in the Evolutionary Genealogy of Genes: Non-supervised Orthologous Groups (eggNOG) database for all three life domains were all aligned to COG database. These results indicated that bacteria, especially Actinomycetales, were the main metabolism participants in the short composting substrate during the oyster mushroom cropping. The relationship between oyster mushrooms and bacteria was cooperative, Actinomycetales were oyster mushroom growth promoting bacteria (OMGPB).

Factors controlling the effects of mutualistic bacteria on plants associated with fungi

Plants interacting with mutualistic fungi (MF) or antagonistic fungi (AF) can form associations with bacteria. We assessed whether the performance gain conferred by mutualistic bacteria to fungal-associated plants is affected by the interaction between symbiont traits, type of bacterial-protective traits against AF and abiotic/biotic stresses. Results showed that (A) performance gain conferred by bacteria to MF-associated plants was greater when symbionts promoted distinct rather than similar plant functions, (B) bacterial-based alleviation of the AF's negative effect on plants was independent of the type of protective trait, (C) bacteria promoted a greater performance of symbiotic plants in presence of biotic, but not abiotic, stress compared to stress-free situations. The plant performance gain was not affected by any fungal-bacterial trait combination but optimised when bacteria conferred resistance traits in biotic stress situations. The effects of bacteria on fungal-associated plants were controlled by the interaction between the symbionts' functional traits and the relationship between bacterial traits and abiotic/biotic stresses.

Current Techniques to Study Beneficial Plant-Microbe Interactions

Many different experimental approaches have been applied to elaborate and study the beneficial interactions between soil bacteria and plants. Some of these methods focus on changes to the plant and others are directed towards assessing the physiology and biochemistry of the beneficial plant growth-promoting bacteria (PGPB). Here, we provide an overview of some of the current techniques that have been employed to study the interaction of plants with PGPB. These techniques include the study of plant microbiomes; the use of DNA genome sequencing to understand the genes encoded by PGPB; the use of transcriptomics, proteomics, and metabolomics to study PGPB and plant gene expression; genome editing of PGPB; encapsulation of PGPB inoculants prior to their use to treat plants; imaging of plants and PGPB; PGPB nitrogenase assays; and the use of specialized growth chambers for growing and monitoring bacterially treated plants.

Soil Microbial Network Complexity Varies With pH as a Continuum, Not a Threshold, Across the North China Plain

There has been little study on the biogeographical patterns of microbial co-occurrence, especially in agricultural soils. Here we investigated the biogeographical patterns and major drivers of co-occurrence network topological structure, and the relative abundance of keystone taxa for soil bacterial and fungal communities using high-throughput sequencing on a set of 90 samples across a 1,092 km transect in wheat fields of the North China Plain (NCP). We found that pH was the most important environmental factor driving network topology and relative abundance of keystone taxa. For the metacommunity composed of both bacteria and fungi, and for the bacterial community alone, lower soil pH was associated with a more complex microbial network. However, the network for fungi showed no strong trend with soil pH. In addition, keystone taxa abundance was positively correlated with ecosystem function and stability, and best explained by pH. Our results present new perspectives on impacts of pH on soil microbial network structure across large scales in agricultural environments. This improved knowledge of community processes provides a step toward understanding of functioning and stability of agricultural ecosystems.

Soil Inoculation With Beneficial Microbes Buffers Negative Drought Effects on Biomass, Nutrients, and Water Relations of Common Myrtle

Common myrtle (Myrtus communis L.) occurs in (semi-)arid areas of the Palearctic region where climate change, over-exploitation, and habitat destruction imperil its existence. The evergreen shrub is of great economic and ecological importance due to its pharmaceutical value, ornamental use, and its role in urban greening and habitat restoration initiatives. Under greenhouse conditions, we investigated the effect of soil inoculation with arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) on biomass allocation, water relations, and nutritional status of drought-stressed myrtle seedlings. Single and dual AMF (Funneliformis mosseae and Rhizophagus irregularis) and PGPR (Pseudomonas fluorescens and P. putida) soil inoculations were applied to myrtle seedlings growing under different soil water regimes (100, 60, and 30% of field capacity) for 6 months using a full factorial, completely randomized design. AMF and PGPR treatments, especially dual inoculations, alleviated negative drought effects on biomass and morpho-physiological traits, except for water-use efficiency, which peaked under severe drought conditions. Under the greatest soil water deficit, dual inoculations promoted leaf biomass (104%-108%), root biomass (56%-73%), mesophyll conductance (58%), and relative water content (1.4-fold) compared to non-inoculated controls. Particularly, dual AMF and PGPR inoculations stimulated nutrient dynamics in roots (N: 138%-151%, P: 176%-181%, K: 112%-114%, Ca: 124%-136%, and Mg: 130%-140%) and leaves (N: 101%-107%, P: 143%-149%, K: 83%-84%, Ca: 98%-107%, and Mg: 102%-106%). Our findings highlight soil inoculations with beneficial microbes as a cost-effective way to produce highly drought resistant seedling stock which is vital for restoring natural myrtle habitats and for future-proofing myrtle crop systems.

The Costs and Benefits of Plant-Arbuscular Mycorrhizal Fungal Interactions

The symbiotic interaction between plants and arbuscular mycorrhizal (AM) fungi is often perceived as beneficial for both partners, though a large ecological literature highlights the context dependency of this interaction. Changes in abiotic variables, such as nutrient availability, can drive the interaction along the mutualism-parasitism continuum with variable outcomes for plant growth and fitness. However, AM fungi can benefit plants in more ways than improved phosphorus nutrition and plant growth. For example, AM fungi can promote abiotic and biotic stress tolerance even when considered parasitic from a nutrient provision perspective. Other than being obligate biotrophs, very little is known about the benefits AM fungi gain from plants. In this review, we utilize both molecular biology and ecological approaches to expand our understanding of the plant-AM fungal interaction across disciplines.

Perspective of ACC-deaminase producing bacteria in stress agriculture

The 1-aminocyclopropane-1-carboxylate deaminase (ACCD) enzyme plays an important role in stress alleviation of both biotic and abiotic stressors in plants and thereby enhances their growth under harsh environmental conditions. In-depth analysis of AcdS gene encoding for ACC deaminase reveals its presence in diverse microorganisms including bacteria and fungi. Particularly, plant growth-promoting bacteria (PGPB) containing ACCD supports plant growth by modulating the level of 'stress ethylene' and cleaving its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) into α-ketobutyrate and ammonia, enabling PGPB to utilize ACC as a carbon and nitrogen source. The reduced synthesis of ethylene in plants further relieves the ethylene inhibition of plant growth and development, and improves plant resistance to various stressors. Therefore, the dual role of microbial ACCD makes it a cost-effective and eco-friendly biocatalyst for sustainable agricultural productions. The inducible ACCD encoding gene AcdS is differentially regulated by varying environmental conditions. Successful generation of transgenic plants with microbial AcdS gene enhanced biotic and abiotic stress tolerance in plants. In the present review, we discuss the importance of ACCD-producing PGPB for their ability to reduce ethylene production and the promotion of plant growth under stress conditions. We also highlighted the development of transgenic plants by overexpressing bacterial AcdS gene to improve their performance under stress conditions.

Pseudomonas 1-Aminocyclopropane-1-carboxylate (ACC) Deaminase and Its Role in Beneficial Plant-Microbe Interactions

The expression of the enzyme 1-aminocylopropane-1-carboxylate (ACC) deaminase, and the consequent modulation of plant ACC and ethylene concentrations, is one of the most important features of plant-associated bacteria. By decreasing plant ACC and ethylene concentrations, ACC deaminase-producing bacteria can overcome some of the deleterious effects of inhibitory levels of ACC and ethylene in various aspects of plant-microbe interactions, as well as plant growth and development (especially under stressful conditions). As a result, the acdS gene, encoding ACC deaminase, is often prevalent and positively selected in the microbiome of plants. Several members of the genus Pseudomonas are widely prevalent in the microbiome of plants worldwide. Due to its adaptation to a plant-associated lifestyle many Pseudomonas strains are of great interest for the development of novel sustainable agricultural and biotechnological solutions, especially those presenting ACC deaminase activity. This manuscript discusses several aspects of ACC deaminase and its role in the increased plant growth promotion, plant protection against abiotic and biotic stress and promotion of the rhizobial nodulation process by Pseudomonas. Knowledge regarding the properties and actions of ACC deaminase-producing Pseudomonas is key for a better understanding of plant-microbe interactions and the selection of highly effective strains for various applications in agriculture and biotechnology.

Differential strategies of two species of arbuscular mycorrhizal fungi in the protection of maize plants grown in chromium-contaminated soils

Chromium (Cr) is a nonessential element for plants that is extremely toxic at high concentrations. Zea mays L. is a species of plant that has developed adaptive mechanisms to increase its tolerance and absorption capacity for this metal. One effective mechanism is to form associations with arbuscular mycorrhizal fungi (AMF), which help the plant cope with stress from heavy metals such as Cr. However, it is still not clear which species of AMF are more efficient as bioremediating agents for plants of agricultural importance. Here, we evaluated the effect of Gigaspora gigantea and Rhizophagus irregularis as protective agents of maize plants in soils contaminated with Cr at concentrations of 0, 48.5, 97 and 194 mg kg^-1 under greenhouse conditions. Maize tolerance to Cr was corroborated, as well as increased absorption of this element by plants associated with both species of AMF. G. gigantea caused an increase in nitrogen content and greater translocation of Cr to the aerial part of the plant; R. irregularis registered an increase in the formation of arbuscules and vesicles with increasing metal concentration and greater retention of Cr in the roots of the plants. Based on these results, we can conclude that the analyzed species of fungi use different strategies, with similar effectiveness, to enhance the absorption capacity of Cr by the plant and influence the differential deposition of this metal in various parts of the plant.

Contribution of Arbuscular Mycorrhizal Fungi, Phosphate-Solubilizing Bacteria, and Silicon to P Uptake by Plant

Phosphorus (P) availability is usually low in soils around the globe. Most soils have a deficiency of available P; if they are not fertilized, they will not be able to satisfy the P requirement of plants. P fertilization is generally recommended to manage soil P deficiency; however, the low efficacy of P fertilizers in acidic and in calcareous soils restricts P availability. Moreover, the overuse of P fertilizers is a cause of significant environmental concerns. However, the use of arbuscular mycorrhizal fungi (AMF), phosphate-solubilizing bacteria (PSB), and the addition of silicon (Si) are effective and economical ways to improve the availability and efficacy of P. In this review the contributions of Si, PSB, and AMF in improving the P availability is discussed. Based on what is known about them, the combined strategy of using Si along with AMF and PSB may be highly useful in improving the P availability and as a result, its uptake by plants compared to using either of them alone. A better understanding how the two microorganism groups and Si interact is crucial to preserving soil fertility and improving the economic and environmental sustainability of crop production in P deficient soils. This review summarizes and discusses the current knowledge concerning the interactions among AMF, PSB, and Si in enhancing P availability and its uptake by plants in sustainable agriculture.

Mycorrhizal-Bacterial Amelioration of Plant Abiotic and Biotic Stress

Soil microbiota plays an important role in the sustainable production of the different types of agrosystems. Among the members of the plant microbiota, mycorrhizal fungi (MF) and plant growth-promoting bacteria (PGPB) interact in rhizospheric environments leading to additive and/or synergistic effects on plant growth and heath. In this manuscript, the main mechanisms used by MF and PGPB to facilitate plant growth are reviewed, including the improvement of nutrient uptake, and the reduction of ethylene levels or biocontrol of potential pathogens, under both normal and stressful conditions due to abiotic or biotic factors. Finally, it is necessary to expand both research and field use of bioinoculants based on these components and take advantage of their beneficial interactions with plants to alleviate plant stress and improve plant growth and production to satisfy the demand for food for an ever-increasing human population.

Hormones as go-betweens in plant microbiome assembly

The interaction of plants with complex microbial communities is the result of co-evolution over millions of years and contributed to plant transition and adaptation to land. The ability of plants to be an essential part of complex and highly dynamic ecosystems is dependent on their interaction with diverse microbial communities. Plant microbiota can support, and even enable, the diverse functions of plants and are crucial in sustaining plant fitness under often rapidly changing environments. The composition and diversity of microbiota differs between plant and soil compartments. It indicates that microbial communities in these compartments are not static but are adjusted by the environment as well as inter-microbial and plant-microbe communication. Hormones take a crucial role in contributing to the assembly of plant microbiomes, and plants and microbes often employ the same hormones with completely different intentions. Here, the function of hormones as go-betweens between plants and microbes to influence the shape of plant microbial communities is discussed. The versatility of plant and microbe-derived hormones essentially contributes to the creation of habitats that are the origin of diversity and, thus, multifunctionality of plants, their microbiota and ultimately ecosystems.

Tailoring plant-associated microbial inoculants in agriculture: a roadmap for successful application

Plants are now recognized as metaorganisms which are composed of a host plant associated with a multitude of microbes that provide the host plant with a variety of essential functions to adapt to the local environment. Recent research showed the remarkable importance and range of microbial partners for enhancing the growth and health of plants. However, plant-microbe holobionts are influenced by many different factors, generating complex interactive systems. In this review, we summarize insights from this emerging field, highlighting the factors that contribute to the recruitment, selection, enrichment, and dynamic interactions of plant-associated microbiota. We then propose a roadmap for synthetic community application with the aim of establishing sustainable agricultural systems that use microbial communities to enhance the productivity and health of plants independently of chemical fertilizers and pesticides. Considering global warming and climate change, we suggest that desert plants can serve as a suitable pool of potentially beneficial microbes to maintain plant growth under abiotic stress conditions. Finally, we propose a framework for advancing the application of microbial inoculants in agriculture.

ACC deaminase in plant growth-promoting bacteria (PGPB): An efficient mechanism to counter salt stress in crops

Salinity in agricultural soil is a major problem around the world, with negative consequences for the growth and production of a wide range of crops. To counteract these harmful effects, plants sometimes have bacterial partners that contain the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, which acts by degrading ACC (the precursor of ethylene in all higher plants). The enzymatic activity of ACC deaminase results in the production of α-ketobutyrate and ammonia, which, by lowering ACC levels, prevents excessive increases in the synthesis of ethylene under various stress conditions and is one of the most efficient mechanisms to induce plant tolerance to salt stress. In the present review, recent works on the role of ACC deaminase are discussed alongside its importance in promoting plant growth under conditions of salt stress in endophytic and rhizospheric bacteria, with some emphasis on Bacillus species. In addition, the toxic effects of soil salinity on plants and microbial biodiversity are analysed. Recent findings on the synergetic functioning of ACC deaminase and other bacterial mechanisms of salt stress tolerance, such as trehalose accumulation, are also summarized. Finally, we discuss the various advantages of ACC deaminase-producing bacilli as bioinoculants to address the problem of salinity in agricultural soils.

Enhancing the 1-Aminocyclopropane-1-Carboxylate Metabolic Rate of Pseudomonas sp. UW4 Intensifies Chemotactic Rhizocompetence

1-aminocyclopropane-1-carboxylic acid (ACC) is a strong metabolism-dependent chemoattractant for the plant beneficial rhizobacterium Pseudomonas sp. UW4. It is unknown whether enhancing the metabolic rate of ACC can intensify the chemotaxis activity towards ACC and rhizocompetence. In this study, we selected four promoters to transcribe the UW4 ACC deaminase (AcdS) gene in the UW4 ΔAcdS mutant. PA is the UW4 AcdS gene promoter, PB20, PB10 and PB1 are synthetic promoters. The order of the AcdS gene expression level and AcdS activity of the four strains harboring the promoters were PB20 > PA > PB10 > PB1. Interestingly, the AcdS activity of the four strains and their parent strain UW4 was significantly positively correlated with their chemotactic activity towards ACC, rhizosphere colonization, roots elongation and dry weight promotion. The results released that enhancing the AcdS activity of PGPRenable them to achieve strong chemotactic responses to ACC, rhizocompetence and plant growth promotion.

The Effects of a Microorganisms-Based Commercial Product on the Morphological, Biochemical and Yield of Tomato Plants under Two Different Water Regimes

The practice of organic agriculture represents an essential requirement for conserving natural resources and for providing the food necessary for a growing population, on a sustainable basis. Tomatoes are considered to be one of the most important crops worldwide. In this context, the organic production of tomatoes should be taken into more consideration. The use of microorganisms-based commercial products is an alternative to chemical fertilizers. Anyway, the results of their use are still variable because of various factors. The aim of this study was to test the effect of inoculation with AMF, PGPR and fungi-based products (Rizotech plus^®) on the morphological (length of the plants), biochemical (lycopen, polyphenols, antioxidant activity), and number of fruits and yields of four tomato cultivars (Siriana F1, HTP F1, Minaret F1, Inima de Bou) in two different water regimes used for irrigation (200 m³ or 300 m³ of water/hectare) under a protected area. The results showed that the efficiency of Rizotech plus^® application is dependent on the cultivar and the amount of water used. Also, it was clearly demonstrated that the microorganism inoculation significantly increased the yield of Minaret F1, Siriana F1 and HTP F1 cultivars as compared to the uninoculated plants, regardless of the water amount used in the experiment. Moreover, it was observed that for the irrigation of all four cultivars, inoculated with Rizotech plus^®, a lower amount of water (200 m³·ha⁻¹) can be used to get the same length of plants, number of fruits and yield as in the case of a higher amount of water (300 m³·ha⁻¹). In the case of lycopene, polyphenols and antioxidant activity, the results varied with the cultivar and the water amount used. This study gives new information about the functionality and performance of the microorganisms from Rizotech plus^® product when applied to different tomato cultivars grown in a tunnel, in the condition of two different water regimes, contributing to a better characterization of it and maybe to a more efficient use in agriculture to achieve optimum results.

Promising bacterial genera for agricultural practices: An insight on plant growth-promoting properties and microbial safety aspects

In order to address the ever-increasing problem of the world's population food needs, the optimization of farming crops yield, the combat of iron deficiency in plants (chlorosis) and the elimination/reduction of crop pathogens are of key challenges to solve. Traditional ways of solving these problems are either unpractical on a large scale (e.g. use of manure) or are not environmental friendly (e.g. application of iron-synthetic fertilizers or indiscriminate use of pesticides). Therefore, the search for greener substitutes, such as the application of siderophores of bacterial source or the use of plant-growth promoting bacteria (PGPB), is presented as a very promising alternative to enhance yield of crops and performance. However, the use of microorganisms is not a risk-free solution and the potential biohazards associated with the utilization of bacteria in agriculture should be considered. The present work gives a current overview of the main mechanisms associated with the use of bacteria in the promotion of plant growth. The potentiality of several bacterial genera (Azotobacter, Azospirillum, Bacillus, Pantoea, Pseudomonas and Rhizobium) regarding to siderophore production capacity and other plant growth-promoting properties are presented. In addition, the field performance of these bacteria genera as well as the biosafety aspects related with their use for agricultural proposes are reviewed and discussed.

Seed Treatment with Trichoderma longibrachiatum T6 Promotes Wheat Seedling Growth under NaCl Stress Through Activating the Enzymatic and Nonenzymatic Antioxidant Defense Systems

Salt stress is one of the major abiotic stresses limiting crop growth and productivity worldwide. Species of Trichoderma are widely recognized for their bio-control abilities, but little information is regarding to the ability and mechanisms of their promoting plant growth and enhancing plant tolerance to different levels of salt stress. Hence, we determined (i) the role of Trichoderma longibrachiatum T6 (TL-6) in promoting wheat (Triticum aestivum L.) seed germination and seedling growth under different levels of salt stress, and (ii) the mechanisms responsible for the enhanced tolerance of wheat to salt stress by TL-6. Wheat seeds treated with or without TL-6 were grown under different levels of salt stress in controlled environmental conditions. As such, the TL-6 treatments promoted seed germination and increased the shoot and root weights of wheat seedlings under both non-stress and salt-stress conditions. Wheat seedlings with TL-6 treatments under different levels of NaCl stress increased proline content by an average of 11%, ascorbate 15%, and glutathione 28%; and decreased the contents of malondialdehyde (MDA) by an average of 19% and hydrogen peroxide (H₂O₂) 13%. The TL-6 treatments induced the transcriptional level of reactive oxygen species (ROS) scavenging enzymes, leading to the increases of glutathione s-transferase (GST) by an average of 17%, glutathione peroxidase (GPX) 16%, ascorbate peroxidase (APX) 17%, glutathione reductase (GR) 18%, dehydroascorbate reductase (DHAR) 5%. Our results indicate that the beneficial strain of TL-6 effectively scavenged ROS under NaCl stress through modulating the activity of ROS scavenging enzymes, regulating the transcriptional levels of ROS scavenging enzyme gene expression, and enhancing the nonenzymatic antioxidants in wheat seedling in response to salt stress. Our present study provides a new insight into the mechanisms of TL-6 can activate the enzymatic and nonenzymatic antioxidant defense systems and enhance wheat seedling tolerance to different levels of salt stress at physiological, biochemical and molecular levels.

A preliminary examination of bacterial, archaeal, and fungal communities inhabiting different rhizocompartments of tomato plants under real-world environments

Plant microbiota is a key determinant of plant health and productivity. The composition and structure of plant microbiota varies according to plant tissue and compartment, which are specific habitats for microbial colonization. To investigate the structural composition of the microbiome associated with tomato roots under natural systems, we characterized the bacterial, archaeal, and fungal communities of three belowground compartments (rhizosphere, endosphere, and bulk soil) of tomato plants collected from 23 greenhouses in 7 geographic locations of South Korea. The microbial diversity and structure varied by rhizocompartment, with the most distinctive community features found in the endosphere. The bacterial and fungal communities in the bulk soil and rhizosphere were correlated with soil physicochemical properties, such as pH, electrical conductivity, and exchangeable cation levels, while this trend was not evident in the endosphere samples. A small number of core bacterial operational taxonomic units (OTUs) present in all samples from the rhizosphere and endosphere represented more than 60% of the total relative abundance. Among these core microbes, OTUs belonging to the genera Acidovorax, Enterobacter, Pseudomonas, Rhizobium, Streptomyces, and Variovorax, members of which are known to have beneficial effects on plant growth, were more relatively abundant in the endosphere samples. A co-occurrence network analysis indicated that the microbial community in the rhizosphere had a larger and more complex network than those in the bulk soil and endosphere. The analysis also identified keystone taxa that might play important roles in microbe-microbe interactions in the community. Additionally, profiling of predicted gene functions identified many genes associated with membrane transport in the endospheric and rhizospheric communities. Overall, the data presented here provide preliminary insight into bacterial, archaeal, and fungal phylogeny, functionality, and interactions in the rhizocompartments of tomato roots under real-world environments.

1-Aminocyclopropane-1-Carboxylate: A Novel and Strong Chemoattractant for the Plant Beneficial Rhizobacterium Pseudomonas putida UW4

Plant growth-promoting rhizobacteria (PGPR) and fungi-bacterial biofilms are both important biofertilizer inoculants for sustainable agriculture. However, the strongest chemoattractant for bacteria to colonize the rhizosphere and mycelia is not clear. Coincidentally, almost all the PGPRs possess 1-aminocyclopropane-1-carboxylate (ACC) deaminase (AcdS) and can utilize ACC as the sole nitrogen source. Here, we found that ACC was a novel, metabolic dependent and methyl-accepting chemoreceptor-involved chemoattractant for Pseudomonas putida UW4. The chemotactic response of UW4 to ACC is significantly greater than that to the amino acids and organic acids identified in the plant root and fungal hyphal exudates. The colonization counts of the UW4 acdS or cheR deletion mutants in the wheat rhizosphere and on Agaricus bisporus mycelia were reduced one magnitude compared with those of UW4. The colonization counts of UW4 on A. bisporus antisense ACC oxidase mycelia with a high ACC production significantly increased compared with A. bisporus, followed by the UW4 cheR complementary strain and the ethylene chemoreceptor gene-deletion mutant. The colonization counts of the UW4 strains on A. bisporus acdS⁺ mycelia with a low ACC production decreased significantly compared with A. bisporus wild type. These results suggested that ACC and not ethylene should be the strongest chemoattractant for the PGPR that contain AcdS.

Effects of biochar on the ecological performance of a subtropical landfill

Landfills commonly occupy large areas of land that may be ecologically important. Ecological restoration of landfill cover is a necessary approach to rebuild sustainable habitats. However, unfavourable soil conditions and invasion by exotic plants in certain regions hinder the restoration. In this study, the effects of biochar as a soil amendment on the restoration of a landfill cover were investigated under field condition. Topsoils of a landfill cover in the subtropical region (Shenzhen, China) were mixed with 0, 5 and 10% (v/v) of biochar. Soil pH, electronic conductivity, organic matter, total organic carbon, water content, total N and total P were enhanced by biochar amendment. After nine months of self-succession, plant productivity, species richness and diversity were enhanced by biochar. The structures of soil bacterial and arbuscular mycorrhizal (AM) fungal communities were changed, and species richness and diversity were moderately promoted. Enhanced plant growth and diversity were probably attributed to a number of enhanced bacterial functions related to nutrient cycling including aerobic ammonia oxidation, aerobic nitrite oxidation, nitrification, sulphur respiration, nitrate respiration, nitrogen respiration, ureolysis, chemoheterotrophy and fermentation. The higher abundances of bacteria Streptomyces sp. and Pseudomonas sp. in biochar treatments potentially enhanced the AM fungal diversity. The bacterial diversity was more related to the soil properties, especially pH, than AM fungi. Continuous monitoring is necessary to track the changes of species composition and ecological functions over time. This is the first comprehensive study on the effects of biochar on the ecological performance of a man-made ecosystem. In addition to agricultural application, biochar can be used for restoring degraded lands.

Impact of Beneficial Microorganisms on Strawberry Growth, Fruit Production, Nutritional Quality, and Volatilome

Arbuscular mycorrhizal fungi (AMF) colonize the roots of most terrestrial plant species, improving plant growth, nutrient uptake and biotic/abiotic stress resistance and tolerance. Similarly, plant growth promoting bacteria (PGPB) enhance plant fitness and production. In this study, three different AMF (Funneliformis mosseae, Septoglomus viscosum, and Rhizophagus irregularis) were used in combination with three different strains of Pseudomonas sp. (19Fv1t, 5Vm1K and Pf4) to inoculate plantlets of Fragaria × ananassa var. Eliana F1. The effects of the different fungus/bacterium combinations were assessed on plant growth parameters, fruit production and quality, including health-promoting compounds. Inoculated and uninoculated plants were maintained in a greenhouse for 4 months and irrigated with a nutrient solution at two different phosphate levels. The number of flowers and fruits were recorded weekly. At harvest, fresh and dry weights of roots and shoots, mycorrhizal colonization and concentration of leaf photosynthetic pigments were measured in each plant. The following fruit parameters were recorded: pH, titratable acids, concentration of organic acids, soluble sugars, ascorbic acids, and anthocyanidins; volatile and elemental composition were also evaluated. Data were statistically analyzed by ANOVA and PCA/PCA-DA. Mycorrhizal colonization was higher in plants inoculated with R. irregularis, followed by F. mosseae and S. viscosum. In general, AMF mostly affected the parameters associated with the vegetative portion of the plant, while PGPB were especially relevant for fruit yield and quality. The plant physiological status was differentially affected by inoculations, resulting in enhanced root and shoot biomass. Inoculation with Pf4 bacterial strain increased flower and fruit production per plant and malic acid content in fruits, while decreased the pH value, regardless of the used fungus. Inoculations affected fruit nutritional quality, increasing sugar and anthocyanin concentrations, and modulated pH, malic acid, volatile compounds and elements. In the present study, we show for the first time that strawberry fruit concentration of some elements and/or volatiles can be affected by the presence of specific beneficial soil microorganisms. In addition, our results indicated that it is possible to select the best plant-microorganism combination for field applications, and improving fruit production and quality, also in terms of health promoting properties.

Isolation and characterization of a novel 1-aminocyclopropane-1-carboxylate (ACC) deaminase producing plant growth promoting marine Gammaproteobacteria from crops grown in brackish environments. Proposal for Pokkaliibacter plantistimulans gen. nov., sp. nov., Balneatrichaceae fam. nov. in the order Oceanospirillales and an emended description of the genus Balneatrix

Three novel strains namely, L1E11^T, L1E4 and 228 were isolated as part of an ongoing study on 1-aminocyclopropane-1-carboxylate (ACC) deaminase expressing rhizobacteria from crops cultivated in saline affected coastal agro-ecosystems of Kerala, India. The novel strains were positive for many properties that are beneficial to plant growth including ACC deaminase (ACCd) activity that ranged from 1.87±0.27 to 2.88±0.71μmol of α-ketobutyrate/hr/mg of total protein. Presence of other traits such as biofilm formation, siderophore production, phosphate solubilisation, utilisation of root derived compounds and ability to colonise host roots indicates its plant-associated life style. In complement, the genomic data reveals gene features for higher adaptation to plant-associated environments. In-planta assays showed that L1E11^T can promote and protect pokkali rice plants from 200mM NaCl stress. Phylogenetic, chemotaxonomic, phenotypic and genomic characterisation indicates that the novel strains belong to a novel genus and species of the order Oceanospirillales for which the names Pokkaliibacter gen. nov., and Pokkaliibacter plantistimulans sp. nov., are proposed with L1E11^T (=DSM 28732^T=MCC 2992^T) as the type strain. Further, on the basis of low 16S rRNA sequence similarity, phylogenetic divergence, source of isolation and few differences in the phenotypic properties against its nearest taxon, a new family Balneatrichaceae fam. nov., is proposed to accommodate the two genera Balneatrix and Pokkaliibacter gen.nov. with Balneatrix as the type genus. An emended description of the genus Balneatrix is also presented.

Spore associated bacteria regulates maize root K+/Na+ ion homeostasis to promote salinity tolerance during arbuscular mycorrhizal symbiosis

BACKGROUND

The interaction between arbuscular mycorrhizal fungi (AMF) and AMF spore associated bacteria (SAB) were previously found to improve mycorrhizal symbiotic efficiency under saline stress, however, the information about the molecular basis of this interaction remain unknown. Therefore, the present study aimed to investigate the response of maize plants to co-inoculation of AMF and SAB under salinity stress.

RESULTS

The co-inoculation of AMF and SAB significantly improved plant dry weight, nutrient content of shoot and root tissues under 25 or 50 mM NaCl. Importantly, co-inoculation significantly reduced the accumulation of proline in shoots and Na+ in roots. Co-inoculated maize plants also exhibited high K+/Na+ ratios in roots at 25 mM NaCl concentration. Mycorrhizal colonization significantly positively altered the expression of ZmAKT2, ZmSOS1, and ZmSKOR genes, to maintain K+ and Na+ ion homeostasis. Confocal laser scanning microscope (CLSM) view showed that SAB were able to move and localize into inter- and intracellular spaces of maize roots and were closely associated with the spore outer hyaline layer.

CONCLUSION

These new findings indicate that co-inoculation of AMF and SAB effectively alleviates the detrimental effects of salinity through regulation of SOS pathway gene expression and K+/Na+ homeostasis to improve maize plant growth.

Ethylene and 1-Aminocyclopropane-1-carboxylate (ACC) in Plant-Bacterial Interactions

Ethylene and its precursor 1-aminocyclopropane-1-carboxylate (ACC) actively participate in plant developmental, defense and symbiotic programs. In this sense, ethylene and ACC play a central role in the regulation of bacterial colonization (rhizospheric, endophytic, and phyllospheric) by the modulation of plant immune responses and symbiotic programs, as well as by modulating several developmental processes, such as root elongation. Plant-associated bacterial communities impact plant growth and development, both negatively (pathogens) and positively (plant-growth promoting and symbiotic bacteria). Some members of the plant-associated bacterial community possess the ability to modulate plant ACC and ethylene levels and, subsequently, modify plant defense responses, symbiotic programs and overall plant development. In this work, we review and discuss the role of ethylene and ACC in several aspects of plant-bacterial interactions. Understanding the impact of ethylene and ACC in both the plant host and its associated bacterial community is key to the development of new strategies aimed at increased plant growth and protection.

Mixtures of Plant-Growth-Promoting Rhizobacteria Enhance Biological Control of Multiple Plant Diseases and Plant-Growth Promotion in the Presence of Pathogens

Several studies have shown that mixtures of plant-growth-promoting rhizobacteria (PGPR) could enhance biological control activity for multiple plant diseases through the mechanisms of induced systemic resistance or antagonism. Prior experiments showed that four individual PGPR strains-AP69 (Bacillus altitudinis), AP197 (B. velezensis), AP199 (B. velezensis), and AP298 (B. velezensis)-had broad-spectrum biocontrol activity via antagonism in growth chambers against two foliar bacterial pathogens (Xanthomonas axonopodis pv. vesicatoria and Pseudomonas syringae pv. tomato) and one of two tested soilborne fungal pathogens (Rhizoctonia solani and Pythium ultimum). Based on these findings, the overall hypothesis of this study was that a mixture of two individual PGPR strains would exhibit better overall biocontrol and plant-growth promotion than the individual PGPR strains. Two separate greenhouse experiments were conducted. In each experiment, two individual PGPR strains and their mixtures were tested for biological control of three different diseases and for plant-growth promotion in the presence of the pathogens. The results demonstrated that the two individual PGPR strains and their mixtures exhibited both biological control of multiple plant diseases and plant-growth promotion. Overall, the levels of disease suppression and growth promotion were greater with mixtures than with individual PGPR strains.

Biofertilizers: a potential approach for sustainable agriculture development

The worldwide increase in human population raises a big threat to the food security of each people as the land for agriculture is limited and even getting reduced with time. Therefore, it is essential that agricultural productivity should be enhanced significantly within the next few decades to meet the large demand of food by emerging population. Not to mention, too much dependence on chemical fertilizers for more crop productions inevitably damages both environmental ecology and human health with great severity. Exploitation of microbes as biofertilizers is considered to some extent an alternative to chemical fertilizers in agricultural sector due to their extensive potentiality in enhancing crop production and food safety. It has been observed that some microorganisms including plant growth promoting bacteria, fungi, Cyanobacteria, etc. have showed biofertilizer-like activities in the agricultural sector. Extensive works on biofertilizers have revealed their capability of providing required nutrients to the crop in sufficient amounts that resulted in the enhancement of crop yield. The present review elucidates various mechanisms that have been exerted by biofertilizers in order to promote plant growth and also provides protection against different plant pathogens. The aim of this review is to discuss the important roles and applications of biofertilizers in different sectors including agriculture, bioremediation, and ecology.

Rhizosphere bacteria containing 1-aminocyclopropane-1- carboxylate deaminase increase growth and photosynthesis of pea plants under salt stress by limiting Na+ accumulation

Although plant salt tolerance has been improved by soil inoculation with rhizobacteria containing the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase (which metabolises ACC, the immediate precursor of the phytohormone ethylene), it is not always clear whether ion homeostasis and plant water relations are affected. When pea (Pisum sativum L. cv. Alderman) was grown with 70 and 130mM NaCl, the ACC-deaminase containing rhizobacterium Variovorax paradoxus 5C-2 increased total biomass by 25 and 54% respectively. Nutrient flow modelling showed that V. paradoxus 5C-2 increased K uptake and root to shoot K flow, but decreased Na flow and increased Na deposition in roots. Thus, shoot K+:Na+ ratio increased following V. paradoxus 5C-2 inoculation. At 70 and 130mM NaCl, rhizobacterial inoculation decreased stomatal resistance by 14 and 31% and decreased xylem balancing pressure by 7 and 21% respectively. Furthermore, rhizobacterial inoculation improved photosynthetic efficiency (Fv/Fm) by 12 and 19% and increased maximal electron transport rate (ETR) by 18 and 22% at 70 and 130mM NaCl respectively. Thus V. paradoxus 5C-2 mitigates salt stress by improving water relations, ion homeostasis and photosynthesis of pea plants, and may provide an economic means of promoting growth of plants exposed to salt stress.

A Halotolerant Bacterium Bacillus licheniformis HSW-16 Augments Induced Systemic Tolerance to Salt Stress in Wheat Plant (Triticum aestivum)

Certain plant growth promoting bacteria can protect associated plants from harmful effects of salinity. We report the isolation and characterization of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase bacterium Bacillus licheniformis HSW-16 capable of ameliorating salt (NaCl) stress in wheat plants. The bacterium was isolated from the water of Sambhar salt lake, Rajasthan, India. The presence of ACC deaminase activity was confirmed by enzyme assay and analysis of AcdS gene, a structural gene for ACC deaminase. Inoculation of B. licheniformis HSW-16 protected wheat plants from growth inhibition caused by NaCl and increased plant growth (6-38%) in terms of root length, shoot length, fresh weight, and dry weight. Ionic analysis of plant samples showed that the bacterial inoculation decreased the accumulation of Na⁺ content (51%), and increased K⁺ (68%), and Ca²⁺ content (32%) in plants at different concentration of NaCl. It suggested that bacterial inoculation protected plants from the effect of NaCl by decreasing the level of Na⁺ in plants. Production of exopolysaccharide by the B. licheniformis HSW-16 can also protect from Na⁺ by binding this ion. Moreover, application of test isolate resulted in an increase in certain osmolytes such as total soluble sugar, total protein content, and a decrease in malondialdehyde content, illustrating their role in the protection of plants. The ability of B. licheniformis HSW-16 to colonize plant root surface was examined by staining the bacterium with acridine orange followed by fluorescence microscopy and polymerase chain reaction-based DNA finger printing analysis. These results suggested that B. licheniformis HSW-16 could be used as a bioinoculant to improve the productivity of plants growing under salt stress.

Copper-resistant bacteria reduces oxidative stress and uptake of copper in lentil plants: potential for bacterial bioremediation

For effective microbe-assisted bioremediation, metal-resistant plant growth-promoting bacteria (PGPB) must facilitate plant growth by restricting excess metal uptake in plants, leading to prevent its bio-amplification in the ecosystem. The aims of our study were to isolate and characterize copper (Cu)-resistant PGPB from waste water receiving contaminated soil. In addition, we investigated the phytotoxic effect of copper on the lentil plants inoculated with copper-resistant bacteria Providencia vermicola, grown in copper-contaminated soil. Copper-resistant P. vermicola showed multiple plant growth promoting characteristics, when used as a seed inoculant. It protected the lentil plants from copper toxicity with a considerable increase in root and shoot length, plant dry weight and leaf area. A notable increase in different gas exchange characteristics such as A, E, C i , g s , and A/E, as well as increase in N and P accumulation were also recorded in inoculated plants as compared to un-inoculated copper stressed plants. In addition, leaf chlorophyll content, root nodulation, number of pods, 1,000 seed weight were also higher in inoculated plants as compared with non-inoculated ones. Anti-oxidative defense mechanism improved significantly via elevated expression of reactive oxygen species -scavenging enzymes including ascorbate peroxidase, superoxide dismutase, catalase, and guaiacol peroxidase with alternate decrease in malondialdehyde and H2O2 contents, reduced electrolyte leakage, proline, and total phenolic contents suggesting that inoculation of P. vermicola triggered heavy metals stress-related defense pathways under copper stress. Overall, the results demonstrated that the P. vermicola seed inoculation confer heavy metal stress tolerance in lentil plant which can be used as a potent biotechnological tool to cope with the problems of copper pollution in crop plants for better yield.

Effect of arbuscular mycorrhizal and bacterial inocula on nitrate concentration in mesocosms simulating a wastewater treatment system relying on phytodepuration

High nitrogen concentration in wastewaters requires treatments to prevent the risks of eutrophication in rivers, lakes and coastal waters. The use of constructed wetlands is one of the possible approaches to lower nitrate concentration in wastewaters. Beyond supporting the growth of the bacteria operating denitrification, plants can directly take up nitrogen. Since plant roots interact with a number of soil microorganisms, in the present work we report the monitoring of nitrate concentration in macrocosms with four different levels of added nitrate (0, 30, 60 and 90 mg l(-1)), using Phragmites australis, inoculated with bacteria or arbuscular mycorrhizal fungi, to assess whether the use of such inocula could improve wastewater denitrification. Higher potassium nitrate concentration increased plant growth and inoculation with arbuscular mycorrhizal fungi or bacteria resulted in larger plants with more developed root systems. In the case of plants inoculated with arbuscular mycorrhizal fungi, a faster decrease of nitrate concentration was observed, while the N%/C% ratio of the plants of the different treatments remained similar. At 90 mg l(-1) of added nitrate, only mycorrhizal plants were able to decrease nitrate concentration to the limits prescribed by the Italian law. These data suggest that mycorrhizal and microbial inoculation can be an additional tool to improve the efficiency of denitrification in the treatment of wastewaters via constructed wetlands.

Bacterial Modulation of Plant Ethylene Levels

A focus on the mechanisms by which ACC deaminase-containing bacteria facilitate plant growth.Bacteria that produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, when present either on the surface of plant roots (rhizospheric) or within plant tissues (endophytic), play an active role in modulating ethylene levels in plants. This enzyme activity facilitates plant growth especially in the presence of various environmental stresses. Thus, plant growth-promoting bacteria that express ACC deaminase activity protect plants from growth inhibition by flooding and anoxia, drought, high salt, the presence of fungal and bacterial pathogens, nematodes, and the presence of metals and organic contaminants. Bacteria that express ACC deaminase activity also decrease the rate of flower wilting, promote the rooting of cuttings, and facilitate the nodulation of legumes. Here, the mechanisms behind bacterial ACC deaminase facilitation of plant growth and development are discussed, and numerous examples of the use of bacteria with this activity are summarized.

Heterologous expression of ACC deaminase from Trichoderma asperellum improves the growth performance of Arabidopsis thaliana under normal and salt stress conditions

Transgenic Arabidopsis thaliana plants expressing the 1-aminocyclopropane-1-carboxylate deaminase gene (ACCD) of Trichoderma asperellum ACCC30536 (TaACCD) were created and their growth performance was assessed under normal and salt stress conditions. In order to characterize their growth, root length, root number, fresh weight (FW), relative water content (RWC), seed production, and seed number were measured. Under normal growing condition, all growth parameters except for dry weight (DW) of the transgenic plants increased significantly compared to WT plants. Furthermore, the transgenic line also exhibited higher tolerance and faster growth than WT plants in the presence of 150 mM NaCl. The increased salt stress tolerance of the transgenic plants is attributed to a greater RWC, root weight, root length, root number and FW under salt stress, and to reduced reactive oxygen species (ROS) level, cell death and electrolyte leakage compared to WT plants. The reduction in ROS levels could be explained by increased activity of several antioxidant enzymes, including peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT). Thus, we propose that heterologous expression of TaACCD could be used to improve salt stress tolerance in plants.

The ARGOS gene family functions in a negative feedback loop to desensitize plants to ethylene

BACKGROUND

Ethylene plays critical roles in plant growth and development, including the regulation of cell expansion, senescence, and the response to biotic and abiotic stresses. Elements of the initial signal transduction pathway have been determined, but we are still defining regulatory mechanisms by which the sensitivity of plants to ethylene is modulated.

RESULTS

We report here that members of the ARGOS gene family of Arabidopsis, previously implicated in the regulation of plant growth and biomass, function as negative feedback regulators of ethylene signaling. Expression of all four members of the ARGOS family is induced by ethylene, but this induction is blocked in ethylene-insensitive mutants. The dose dependence for ethylene induction varies among the ARGOS family members, suggesting that they could modulate responses across a range of ethylene concentrations. GFP-fusions of ARGOS and ARL localize to the endoplasmic reticulum, the same subcellular location as the ethylene receptors and other initial components of the ethylene signaling pathway. Seedlings with increased expression of ARGOS family members exhibit reduced ethylene sensitivity based on physiological and molecular responses.

CONCLUSIONS

These results support a model in which the ARGOS gene family functions as part of a negative feedback circuit to desensitize the plant to ethylene, thereby expanding the range of ethylene concentrations to which the plant can respond. These results also indicate that the effects of the ARGOS gene family on plant growth and biomass are mediated through effects on ethylene signal transduction.

The interactions of bacteria with fungi in soil: emerging concepts

In this chapter, we review the existing literature on bacterial-fungal interactions in soil, exploring the role fungi may play for soil bacteria as providers of hospitable niches. A focus is placed on the mycosphere, i.e., the narrow zone of influence of fungal hyphae on the external soil milieu, in which hypha-associated bacterial cells dwell. Evidence is brought forward for the contention that the hyphae of both mycorrhizal and saprotrophic fungi serve as providers of ecological opportunities in a grossly carbon-limited soil, as a result of their release of carbonaceous compounds next to the provision of a colonizable surface. Soil bacteria of particular nature are postulated to have adapted to such selection pressures, evolving to the extent that they acquired capabilities that allow them to thrive in the novel habitat created by the emerging fungal hyphae. The mechanisms involved in the interactions and the modes of genetic adaptation of the mycosphere dwellers are discussed, with an emphasis on one key mycosphere-adapted bacterium, Burkholderia terrae BS001. In this discussion, we interrogate the positive interactions between soil fungi and bacteria, and refrain from considering negative interactions.

Influence of Pseudomonas aeruginosa as PGPR on oxidative stress tolerance in wheat under Zn stress

Plant Growth Promoting Rhizobacteria (PGPR), whose role is still underestimated, plays an important (or perhaps essential) role in improving plant growth. The comprehensive understanding of bacterial plant growth promoting mechanism helps to get sustainable agriculture production under biotic and abiotic stresses. In the present study, plant growth promoting (PGP) bacterial strain Pseudomonas aeruginosa having maximum inhibitory concentration of 1500mg kg(-1) against Zn was isolated from arable land, irrigated with industrial effluent and evaluated to determine it bioremediation potential. The study was mainly focused on plant biomass production, nutrient uptake and oxidative stress tolerance in relation to the activities of antioxidative enzymes and the content of non-enzymatic antioxidants. The oxidative stress tolerance was measured by estimating the MDA accumulation as well as H2O2 production in wheat plants under Zn (1000mg kg(-1)) stress and inoculation of soil with Zn resistant Pseudomonas aeruginosa. Zn in rooting medium reduced the plant growth, leaf photosynthetic pigments as well as uptake of N and P. However, content of MDA and H2O2 increased at higher concentration of Zn. Inoculation of P. aeruginosa improved the uptake of P and N in wheat plants with an increase in leaf chlorophyll, total soluble protein and plant biomass production. Analysis of plant root and shoot disclosed that Zn concentration was significantly lowered in P. aeruginosa inoculated zinc stressed plants as compare to the plants grown under Zn stress only. The amelioration of adverse effects of Zn stress on biomass production due to P. aeruginosa inoculation was related with enhanced antioxidative enzyme activities (SOD, POD and CAT), and the contents of non-enzymatic components such as ascorbic acid and total phenolics (TPC) as compare to Zn-treated plants. The up-gradation in antioxidative defense mechanism, resulted a reduction in H2O2 and MDA content due to the scavenging of ROS by antioxidants. It was concluded that P. aeruginosa is an ideal candidate for bioremediation and wheat growth promotion against Zn-induced oxidative stress by improving the availability of necessary nutrient, eliciting antioxidant defense system as well as by lowering the Zn metal uptake.

Maize development and grain quality are differentially affected by mycorrhizal fungi and a growth-promoting pseudomonad in the field

Arbuscular mycorrhizal (AM) fungi and plant growth-promoting bacteria (PGPB) can increase the growth and yield of major crops, and improve the quality of fruits and leaves. However, little is known about their impact on seed composition. Plants were inoculated with AM fungi and/or the bacterial strain Pseudomonas fluorescens Pf4 and harvested after 7 months of growth in open-field conditions. Plant growth parameters were measured (biomass, length and circumference of spikes, number of grains per cob, grain yield, and grain size) and protein, lipid, and starch content in grains were determined. Plant growth and yield were increased by inoculation with the microorganisms. Moreover, spikes and grains of inoculated plants were bigger than those produced by uninoculated plants. Regarding grain composition, the bacterial strain increased grain starch content, especially the digestible components, whereas AM fungi-enhanced protein, especially zein, content. Plant inoculation with the fluorescent pseudomonad and mycorrhizal fungi resulted in additive effects on grain composition. Overall, results showed that the bacterial strain and the AM fungi promoted maize growth cultivated in field conditions and differentially affected the grain nutritional content. Consequently, targeted plant inoculation with beneficial microorganisms can lead to commodities fulfilling consumer and industrial requirements.

Regulation of root morphogenesis in arbuscular mycorrhizae: what role do fungal exudates, phosphate, sugars and hormones play in lateral root formation?

BACKGROUND

Arbuscular mycorrhizae (AMs) form a widespread root-fungus symbiosis that improves plant phosphate (Pi) acquisition and modifies the physiology and development of host plants. Increased branching is recognized as a general feature of AM roots, and has been interpreted as a means of increasing suitable sites for colonization. Fungal exudates, which are involved in the dialogue between AM fungi and their host during the pre-colonization phase, play a well-documented role in lateral root (LR) formation. In addition, the increased Pi content of AM plants, in relation to Pi-starved controls, as well as changes in the delivery of carbohydrates to the roots and modulation of phytohormone concentration, transport and sensitivity, are probably involved in increasing root system branching.

SCOPE

This review discusses the possible causes of increased branching in AM plants. The differential root responses to Pi, sugars and hormones of potential AM host species are also highlighted and discussed in comparison with those of the non-host Arabidopsis thaliana.

CONCLUSIONS

Fungal exudates are probably the main compounds regulating AM root morphogenesis during the first colonization steps, while a complex network of interactions governs root development in established AMs. Colonization and high Pi act synergistically to increase root branching, and sugar transport towards the arbusculated cells may contribute to LR formation. In addition, AM colonization and high Pi generally increase auxin and cytokinin and decrease ethylene and strigolactone levels. With the exception of cytokinins, which seem to regulate mainly the root:shoot biomass ratio, these hormones play a leading role in governing root morphogenesis, with strigolactones and ethylene blocking LR formation in the non-colonized, Pi-starved plants, and auxin inducing them in colonized plants, or in plants grown under high Pi conditions.

Bacteria with ACC deaminase can promote plant growth and help to feed the world

To feed all of the world's people, it is necessary to sustainably increase agricultural productivity. One way to do this is through the increased use of plant growth-promoting bacteria; recently, scientists have developed a more profound understanding of the mechanisms employed by these bacteria to facilitate plant growth. Here, it is argued that the ability of plant growth-promoting bacteria that produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase to lower plant ethylene levels, often a result of various stresses, is a key component in the efficacious functioning of these bacteria. The optimal functioning of these bacteria includes the synergistic interaction between ACC deaminase and both plant and bacterial auxin, indole-3-acetic acid (IAA). These bacteria not only directly promote plant growth, they also protect plants against flooding, drought, salt, flower wilting, metals, organic contaminants, and both bacterial and fungal pathogens. While a considerable amount of both basic and applied work remains to be done before ACC deaminase-producing plant growth-promoting bacteria become a mainstay of plant agriculture, the evidence indicates that with the expected shift from chemicals to soil bacteria, the world is on the verge of a major paradigm shift in plant agriculture.