Beneficial Plant-microbe Interactions Research Articles

Piriformospora indica mycorrhization increases grain yield by accelerating early development of barley plants

Root colonization by the basidiomycete fungus Piriformospora indica induces host plant tolerance against abiotic and biotic stress, and enhances growth and yield. As P. indica has a broad host range, it has been established as a model system to study beneficial plant–microbe interactions. Moreover, its properties led to the assumption that P. indica shows potential for application in crop plant production. Therefore, possible mechanisms of P. indica improving host plant yield were tested in outdoor experiments: Induction of higher grain yield in barley was independent of elevated pathogen levels and independent of different phosphate fertilization levels. In contrast to the arbuscular mycorrhiza fungus Glomus mosseae total phosphate contents of host plant roots and shoots were not significantly affected by P. indica. Analysis of plant development and yield parameters indicated that positive effects of P. indica on grain yield are due to accelerated growth of barley plants early in development.

Chitooligosaccharide sensing and downstream signaling: contrasted outcomes in pathogenic and beneficial plant–microbe interactions

In plants, short chitin oligosaccharides and chitosan fragments (collectively referred to as chitooligosaccharides) are well-known elicitors that trigger defense gene expression, synthesis of antimicrobial compounds, and cell wall strengthening. Recent findings have shed new light on chitin-sensing mechanisms and downstream activation of intracellular signaling networks that mediate plant defense responses. Interestingly, chitin receptors possess several lysin motif domains that are also found in several legume Nod factor receptors. Nod factors are chitin-related molecules produced by nitrogen-fixing rhizobia to induce root nodulation. The fact that chitin and Nod factor receptors share structural similarity suggests an evolutionary conserved relationship between mechanisms enabling recognition of both deleterious and beneficial microorganisms. Here, we will present an update on molecular events involved in chitooligosaccharide sensing and downstream signaling pathways in plants and will discuss how structurally related signals may lead to such contrasted outcomes during plant-microbe interactions.

Plant–microbe interactions promoting plant growth and health: perspectives for controlled use of microorganisms in agriculture

Plant-associated microorganisms fulfill important functions for plant growth and health. Direct plant growth promotion by microbes is based on improved nutrient acquisition and hormonal stimulation. Diverse mechanisms are involved in the suppression of plant pathogens, which is often indirectly connected with plant growth. Whereas members of the bacterial genera Azospirillum and Rhizobium are well-studied examples for plant growth promotion, Bacillus, Pseudomonas, Serratia, Stenotrophomonas, and Streptomyces and the fungal genera Ampelomyces, Coniothyrium, and Trichoderma are model organisms to demonstrate influence on plant health. Based on these beneficial plant-microbe interactions, it is possible to develop microbial inoculants for use in agricultural biotechnology. Dependent on their mode of action and effects, these products can be used as biofertilizers, plant strengtheners, phytostimulators, and biopesticides. There is a strong growing market for microbial inoculants worldwide with an annual growth rate of approximately 10%. The use of genomic technologies leads to products with more predictable and consistent effects. The future success of the biological control industry will benefit from interdisciplinary research, e.g., on mass production, formulation, interactions, and signaling with the environment, as well as on innovative business management, product marketing, and education. Altogether, the use of microorganisms and the exploitation of beneficial plant-microbe interactions offer promising and environmentally friendly strategies for conventional and organic agriculture worldwide.

Recovery of useful traits from isolates inhabiting an agricultural soil cultivated with herbicide-resistant poplars

The aim of this study was to investigate the culturable bacteria living in soil cultivated with Basta-tolerant transgenic white poplars (Populus alba L. 'Villafranca'). Plate Count Agar medium containing phosphinothricin, the active component of Basta, was used to isolate the herbicide-resistant bacteria (HRB). No significant changes in the size of the soil microbial flora following herbicide treatment were observed. The characterization of HRB isolates by 16S rDNA-based taxonomy revealed a predominance of Pseudomonas and Bacillus species. The screening carried out on soil samples allowed for the recovery of isolates with useful properties for biotechnological and agronomical purposes, particularly in relation to root development. Among the tested isolates, only HRB-1b, HRB-1c, and HRB-7 showed remarkable swarming ability, a valuable trait supporting the beneficial plant-microbe interactions. HRB-1c was also characterized by consistent production of indoleacetic acid (17.8 +/- 0.09 microg x mL-1 x (OD600 unit)-1), and it was able to stimulate the in vitro growth of Villafranca explants. Since novel tools are constantly required to enhance productivity of perennial species and to expand their use for practical purposes, the availability of bacteria that support tree growth, such as the HRB-1c isolate, represents a significant advantage.

Differentially expressed and new non-protein-coding genes from a Carica papaya root transcriptome survey

This study surveyed the root transcriptome of papaya ( Carica papaya L.) to identify tissue-regulated genes for promoter isolation. A cDNA library constructed from C. papaya roots was screened using three different methods, and isolated clones were evaluated for expression in roots, leaves, hypocotyls, flowers, and fruit. Homology based functional classification revealed cDNAs for genes associated with defense, beneficial plant–microbe interaction, abiotic stress, and plant development. Homologs associated with plant defense included a hypersensitive-induced response protein, a pathogen-inducible tyrosine-rich hydroxyproline-rich glycoprotein, and two unique peroxidases that have predominate root and hypocotyl-root transcript abundance. A basic leucine zipper protein homologous to the developmental regulatory liguleless2 maize gene was identified and found to exhibit root-specific expression. Of particular interest were three differentially expressed non-protein-coding RNAs (npcRNAs) that lack obvious open reading frames and are predicted to form stable, complex secondary structures. One npcRNA was found to have specific hypocotyl and root expression. Another exhibits moderate expression in flowers and, interestingly, was found to have three alternative splice variants, two of which appear to be derived from a sequential splicing pathway. The novel npcRNAs have no database sequence homology and are proposed to be new genes. A fourth npcRNA contains a 21 bp sub-region within a 2.7 kb transcript with complementary downstream sequence that is identical to the microRNA MIR162a, suggesting that this is a large, primary microRNA (pri-miRNA) transcript. This is one of only a few pri-miRNA sequences available for a MIR162a ortholog, and it can be used to investigate pri-miRNA evolution and function. The genes with tissue-regulated expression identified here will provide regulatory elements for bioengineering papaya, while the novel non-coding transcripts are candidates for further investigation to explore gene function and miRNA evolution.

Establishment of Azotobacter on plant roots: chemotactic response, development and analysis of root exudates of cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.)

Biofertilizers contribute in N(2) fixation, P solubilization, phytohormone production and thus enhance plant growth. Beneficial plant-microbe interactions and the stability and effectiveness of biofertilizer depend upon the establishment of bacterial strains in the rhizosphere of the plant. This interaction depends upon many factors, one of them being plant exudates. Root exudates are composed of small organic molecules like carbonic acids, amino acids or sugars etc., which are released into the soil and bacteria can be attracted towards these exudates due to chemotaxis. The chemotactic behaviour of Azotobacter strains was studied using cotton (Desi HD 123 and American H 1098) and wheat (WH 711) seedlings and the root exudates of these two plants were chemically characterized. Analysis of the root exudates revealed the presence of sugars and simple polysaccharides (glucose), amino acids (glutamate, lysine) and organic acids (citric acid, succinic acid, maleic acid, malonic acid). Differences between cotton cultivars in root exudates were observed which influenced chemotactic response in Azotobacter. These results indicate colonization with rhizobacteria which implies that optimal symbionts, on the sides of both plant cultivar and bioinoculant bacteria can lead to better plant growth under cultivation conditions.

Effect of bacteria isolated from composts and macrofauna on sorghum growth and mycorrhizal colonization

Beneficial plant–microbe interactions in the rhizosphere are primary determinants of plant health and soil fertility. The effect of combined inoculation of plant growth-promoting bacteria, Bacillus circulans EB 35, Serratia marcescens EB 67 and Pseudomonas sp. CDB 35 and arbascular mycorrhizal fungi, Glomus spp. on sorghum growth and mycorrhizal colonization was investigated. Plant growth observations taken at 45 days after sowing (DAS) revealed that all the three strains applied along with arbascular mycorrhizae (AM) improved plant biomass from 17 to 20% and mycorrhizal colonization from 25 to 35%. Further studies at 90 DAS also showed improvement in plant growth parameters measured. It was apparent that all the three strains stimulated plant and root growth in combination with AM and infection of sorghum roots with mycorrhizae at 45 DAS was equal to or even greater than the AM + rock phosphate (RP) inoculation at 90 DAS. This shows the possible reduction of AM culturing period to 45 days compared to its 3-month culturing in the pot cultures.

Molecular-based strategies to exploit Pseudomonas biocontrol strains for environmental biotechnology applications

Exploitation of beneficial plant-microbe interactions in the rhizosphere can result in the promotion of plant health and have significant implications for low input sustainable agriculture applications such as biocontrol. Bacteria such as Bacillus and Pseudomonas, and fungi such as Trichoderma, have been developed as commercial biocontrol products. Registration of microbial inocualants as biocontrol agents in either the European Union or the United States requires production of extensive dossiers covering efficacy, safety and risk assessment. Despite the fact that a number of Pseudomonas biocontrol products have been marketed there are still some limitations hampering the development of this technology for widespread use in agriculture. Although many strains show good performance in specific trials, this is often not translated into consistent, effective biocontrol in diverse field situations. Advances in 'Omics' technology and the publication of complete genome sequences of a number of plant-associative bacterial strains, has facilitated investigations into the molecular basis underpinning the establishment of beneficial plant-microbe interactions in the rhizosphere. The understanding of these molecular signalling processes and the functions they regulate is fundamental to promoting beneficial microbe-plant interactions, to overcome existing limitations and to designing improved strategies for the development of novel Pseudmonas biocontrol inoculant consortia.

Transcriptional snapshots provide insights into the molecular basis of arbuscular mycorrhiza in the model legume Medicago truncatula.

The arbuscular mycorrhizal (AM) association between terrestrial plants and soil fungi of the phylum Glomeromycota is the most widespread beneficial plant-microbe interaction on earth. In the course of the symbiosis, fungal hyphae colonise plant roots and supply limiting nutrients, in particular phosphorus, in exchange for carbon compounds. Owing to the obligate biotrophy of mycorrhizal fungi and the lack of genetic systems to study them, targeted molecular studies on AM symbioses proved to be difficult. With the emergence of plant genomics and the selection of suitable models, an application of untargeted expression profiling experiments became possible. In the model legume Medicago truncatula, high-throughput expressed sequence tag (EST)-sequencing in conjunction with in silico and experimental transcriptome profiling provided transcriptional snapshots that together defined the global genetic program activated during AM. Owing to an asynchronous development of the symbiosis, several hundred genes found to be activated during the symbiosis cannot be easily correlated with symbiotic structures, but the expression of selected genes has been extended to the cellular level to correlate gene expression with specific stages of AM development. These approaches identified marker genes for the AM symbiosis and provided the first insights into the molecular basis of gene expression regulation during AM.

Association of Piriformospora indica with Arabidopsis thaliana roots represents a novel system to study beneficial plant–microbe interactions and involves early plant protein modifications in the endoplasmic reticulum and at the plasma membrane

Piriformospora indica, an endophytic fungus of the Sebacinaceae family, colonizes the roots of a wide variety of plant species and promotes their growth, in a manner similar to arbuscular mycorrhizal fungi. The results of the present study demonstrate that the fungus interacts also with the non‐mycorrhizal host Arabidopsis thaliana and promotes its growth. The interaction is detectable by the appearance of a strong autofluorescence in the roots, followed by the colonization of root cells by fungal hyphae and the generation of chlamydospores. Promotion of root growth was detectable even before noticeable root colonization. Membrane‐associated proteins from control roots and roots after cultivation with P. indica were separated by two‐dimensional gel‐electrophoresis and identified by electrospray ionization mass spectrometry and tandem mass spectrometry. Differences were found in the expression of glucosidase II, beta‐glucosidase PYK10, two glutathione‐S‐transferases and several so‐far uncharacterized proteins. Based on conserved domains present in the latter proteins their possible roles in plant–microbe interaction are predicted. Taken together, the present results suggest that the interaction of Arabidopsis thaliana with P. indica is a powerful model system to study beneficial plant–microbe interaction at the molecular level. Furthermore, the successful accommodation of the fungus in the root cells is preceded by protein modifications in the endoplasmatic reticulum as well as at the plasma membrane of the host.

Rhizoremediation: a beneficial plant-microbe interaction.

Worldwide, contamination of soil and ground water is a severe problem. The negative effects of pollutants on the environment and on human health are diverse and depend on the nature of the pollution. The search for alternative methods for excavation and incineration to clean polluted sites resulted in the application of bioremediation techniques. In this review, we describe some generally accepted bioremediation tools and subsequently focus on the combination of two approaches, phytoremediation and bioaugmentation, resulting in rhizoremediation. During rhizoremediation, exudates derived from the plant can help to stimulate the survival and action of bacteria, which subsequently results in a more efficient degradation of pollutants. The root system of plants can help to spread bacteria through soil and help to penetrate otherwise impermeable soil layers. The inoculation of pollutant-degrading bacteria on plant seed can be an important additive to improve the efficiency of phytoremediation or bioaugmentation.

Phenazines and their role in biocontrol by Pseudomonas bacteria.

Various rhizosphere bacteria are potential (micro)biological pesticides which are able to protect plants against diseases and improve plant yield. Knowledge of the molecular mechanisms that govern these beneficial plant-microbe interactions enables optimization, enhancement and identification of potential synergistic effects in plant protection. The production of antifungal metabolites, induction of systemic resistance, and the ability to compete efficiently with other resident rhizobacteria are considered to be important prerequisites for the optimal performance of biocontrol agents. Intriguing aspects in the molecular mechanisms of these processes have been discovered recently. Phenazines and phloroglucinols are major determinants of biological control of soilborne plant pathogens by various strains of fluorescent Pseudomonas spp. This review focuses on the current state of knowledge on biocontrol by phenazine-producing Pseudomonas strains and the action, biosynthesis, and regulation mechanisms of the production of microbial phenazines.

The contribution of arbuscular mycorrhizal fungi in sustainable maintenance of plant health and soil fertility

Beneficial plant–microbe interactions in the rhizosphere are primary determinants of plant health and soil fertility. Arbuscular mycorrhizas are the most important microbial symbioses for the majority of plants and, under conditions of P-limitation, influence plant community development, nutrient uptake, water relations and above-ground productivity. They also act as bioprotectants against pathogens and toxic stresses. This review discusses the mechanism by which these benefits are conferred through abiotic and biotic interactions in the rhizosphere. Attention is paid to the conservation of biodiversity in arbuscular mycorrhizal fungi (AMF). Examples are provided in which the ecology of AMF has been taken into account and has had an impact in landscape regeneration, horticulture, alleviation of desertification and in the bioremediation of contaminated soils. It is vital that soil scientists and agriculturalists pay due attention to the management of AMF in any schemes to increase, restore or maintain soil fertility.

Effects of Salicylic Acid on the Development and Root Nodulation of Soybean Seedlings

Salicylic acid (SA) is recognized as an endogenous regulator of plant metabolism, mainly involved in induction of systemic acquired resistance (SAR). Exogenous SA can also induce a SAR reaction and SAR gene expression. Excessive SAR‐related activity can be an overall cost to the plant in terms of energy and materials expended unnecessarily. Elicitation of plant defence responses might also block beneficial plant‐microbe interactions and result in negative effects on plant growth. The objective of this study was to investigate the effects of SA concentration (5, 1, 0.5, 0.1 and 0 m m) on soybean seedling growth and nodulation by watering of soybean seedling roots or soaking of seedling leaves with SA solutions. It was found that 5 m m SA had negative effects on soybean seedling development, but other concentrations of SA did not affect the development of soybean seedlings. In addition, there were no negative effects on seedling development due to SA soaking of seedling leaves. Soybean seedling growth in sterile soil was reduced due to repressed nitrogen uptake following addition of 5 m m SA, indicating that some concentrations of SA can alter the N nutrition of seedlings. A model is presented that ties SA to nodule formation and plant growth.

Piriformospora indica, a cultivable plant-growth-promoting root endophyte

Piriformospora indica (Hymenomycetes, Basidiomycota) is a newly described cultivable endophyte that colonizes roots. Inoculation with the fungus and application of fungal culture filtrate promotes plant growth and biomass production. Due to its ease of culture, this fungus provides a model organism for the study of beneficial plant-microbe interactions and a new tool for improving plant production systems.

Genetic basis in plants for interactions with disease-suppressive bacteria.

Plant health depends, in part, on associations with disease-suppressive microflora, but little is known about the role of plant genes in establishing such associations. Identifying such genes will contribute to understanding the basis for plant health in natural communities and to new strategies to reduce dependence on pesticides in agriculture. To assess the role of the plant host in disease suppression, we used a genetic mapping population of tomato to evaluate the efficacy of the biocontrol agent Bacillus cereus against the seed pathogen Pythium torulosum. We detected significant phenotypic variation among recombinant inbred lines that comprise the mapping population for resistance to P. torulosum, disease suppression by B. cereus, and growth of B. cereus on the seed. Genetic analysis revealed that three quantitative trait loci (QTL) associated with disease suppression by B. cereus explained 38% of the phenotypic variation among the recombinant inbred lines. In two cases, QTL for disease suppression by B. cereus map to the same locations as QTL for other traits, suggesting that the host effect on biocontrol is mediated by different mechanisms. The discovery of a genetic basis in the host for interactions with a biocontrol agent suggests new opportunities to exploit natural genetic variation in host species to enhance our understanding of beneficial plant-microbe interactions and develop ecologically sound strategies for disease control in agriculture.