The role of root exudates in rhizosphere interactions with plants and other organisms

Root-secreted allelochemical in the noxious weed Phragmites australis deploys a reactive oxygen species response and microtubule assembly disruption to execute rhizotoxicity

Phragmites australis is considered the most invasive plant in marsh and wetland communities in the eastern United States. Although allelopathy has been considered as a possible displacing mechanism in P. australis, there has been minimal success in characterizing the responsible allelochemical. We tested the occurrence of root-derived allelopathy in the invasiveness of P. australis. To this end, root exudates of two P. australis genotypes, BB (native) and P38 (an exotic) were tested for phytotoxicity on different plant species. The treatment of the susceptible plants with P. australis root exudates resulted in acute rhizotoxicity. It is interesting to note that the root exudates of P38 were more effective in causing root death in susceptible plants compared to the native BB exudates. The active ingredient in the P. australis exudates was identified as 3,4,5-trihydroxybenzoic acid (gallic acid). We tested the phytotoxic efficacy of gallic acid on various plant systems, including the model plant Arabidopsis thaliana. Most tested plants succumbed to the gallic acid treatment with the exception of P. australis itself. Mechanistically, gallic acid treatment generated elevated levels of reactive oxygen species (ROS) in the treated plant roots. Furthermore, the triggered ROS mediated the disruption of the root architecture of the susceptible plants by damaging the microtubule assembly. The study also highlights the persistence of the exuded gallic acid in P. australis's rhizosphere and its inhibitory effects against A. thaliana in the soil. In addition, gallic acid demonstrated an inhibitory effect on Spartina alterniflora, one of the salt marsh species it successfully invades.

High proportions of nonpathogenic Streptomyces are associated with common scab-resistant potato lines and less severe disease

Streptomyces isolates were obtained from potato tubers with common scab lesions from 2 fields over a 3 year period in Minnesota and a 5 year period in Maine. Isolates were obtained from different potato cultivars or breeding lines and types of scab lesions. A majority of isolates could be classified as putative pathogens based on the presence of genes for biosynthesis of the pathogenicity determinant, thaxtomin, but large numbers of streptomycetes lacking genes for thaxtomin biosynthesis (presumably nonpathogenic) were also recovered. Most Streptomyces isolates recovered from raised and pitted lesions were pathogens, whereas mostly nonpathogenic isolates were recovered from unblemished potato skin or nonscab lesions. Fewer pathogenic than nonpathogenic isolates were recovered from the most resistant potato lines. The proportion and diversity of nonpathogenic isolates recovered was higher in Maine than in Minnesota. The association between greater numbers of nonpathogenic Streptomyces and less severe common scab suggests that the interaction between plant genotype and Streptomyces microbial community is important in determining the severity of common scab on potato, and emphasizes the role of complex interactions between plants and microbial populations on and near plant roots in plant disease outcomes.

Flavonoids and strigolactones in root exudates as signals in symbiotic and pathogenic plant-fungus interactions

Secondary plant compounds are important signals in several symbiotic and pathogenic plant-microbe interactions. The present review is limited to two groups of secondary plant compounds, flavonoids and strigolactones, which have been reported in root exudates. Data on flavonoids as signaling compounds are available from several symbiotic and pathogenic plant-microbe interactions, whereas only recently initial data on the role of strigolactones as plant signals in the arbuscular mycorrhizal symbiosis have been reported. Data from other plant-microbe interactions and strigolactones are not available yet. In the present article we are focusing on flavonoids in plant-fungal interactions such as the arbuscular mycorrhizal (AM) association and the signaling between different Fusarium species and plants. Moreover the role of strigolactones in the AM association is discussed and new data on the effect of strigolactones on fungi, apart from arbuscular mycorrhizal fungi (AMF), are provided.

Rhizosphere communication of plants, parasitic plants and AM fungi

Plants use an array of secondary metabolites to defend themselves against harmful organisms and to attract others that are beneficial. However, the attraction of beneficial organisms could also lead to abuse by malevolent organisms. An exciting example of such abuse is the relationship between plants, beneficial mutualistic arbuscular mycorrhizal fungi and harmful parasitic plants. Signalling molecules called strigolactones, which are secreted by plant roots in low concentrations, induce the growth of both obligate biotrophs. Here, we review the importance of strigolactones for these two interactions and discuss possible developments that should further clarify the role of these signalling molecules in rhizosphere processes.

AAP1 transports uncharged amino acids into roots of Arabidopsis

Amino acids are available to plants in some soils in significant amounts, and plants frequently make use of these nitrogen sources. The goal of this study was to identify transporters involved in the uptake of amino acids into root cells. Based on the fact that high concentrations of amino acids inhibit plant growth, we hypothesized that mutants tolerating toxic levels of amino acids might be deficient in the uptake of amino acids from the environment. To test this hypothesis, we employed a forward genetic screen for Arabidopsis thaliana mutants tolerating toxic concentrations of amino acids in the media. We identified an Arabidopsis mutant that is deficient in the amino acid permease 1 (AAP1, At1g58360) and resistant to 10 mm phenylalanine and a range of other amino acids. The transporter was localized to the plasma membrane of root epidermal cells, root hairs, and throughout the root tip of Arabidopsis. Feeding experiments with [(14)C]-labeled neutral, acidic and basic amino acids showed significantly reduced uptake of amino acids in the mutant, underscoring that increased tolerance of aap1 to high levels of amino acids is coupled with reduced uptake by the root. The growth and uptake studies identified glutamate, histidine and neutral amino acids, including phenylalanine, as physiological substrates for AAP1, whereas aspartate, lysine and arginine are not. We also demonstrate that AAP1 imports amino acids into root cells when these are supplied at ecologically relevant concentrations. Together, our data indicate an important role of AAP1 for efficient use of nitrogen sources present in the rhizosphere.

Multiscale responses of microbial life to spatial distance and environmental heterogeneity in a patchy ecosystem

Spatial distance (SD) and environmental heterogeneity (EH) are currently thought to represent major factors shaping genetic variation and population abundance, but their relative importance is still poorly understood. Because EH varies at multiple spatial scales, so too are microbial variables expected to vary. The determination of SD x EH interactions at multiple scales is, however, not a trivial exercise, especially when one examines their effects on microbial abundance and genomic similarities. Here we assessed those interactions at all scales perceptible in a patchy environment composed of known plant species and of heterogeneous soil physical and chemical parameters. For free-living, soil-borne Burkholderia ambifaria, genomic similarities responded to most of the spatial scales that the experimental sampling scheme could reveal, despite limited dispersal of the individuals. Species abundance and community composition were, however, responding to much smaller scales more consistent with local responses to EH. Our results suggest that whole-genome similarities may reflect the simultaneous effects of both SD and EH in microbial populations, but the pure effects of each factor only contributed to < 2% of the total genetic variation. The large amount of unexplained variation that remains after considering most environmental, spatial, and biological interactions is then posited to be the result of noise introduced by unmeasured environmental and spatial variability, sampling effects, and neutral ecological drift.

Phylloplane proteins: emerging defenses at the aerial frontline?

The phylloplane, or leaf surface, is an interkingdom crossroads between plants and microorganisms, and secretion of antimicrobial biochemicals to aerial surfaces is thought to be one defensive strategy by which plants deter potential pathogens. Secondary metabolites on leaf surfaces are well documented but antimicrobial phylloplane proteins have only recently been identified. In this review, we describe the physical structures and biochemicals of the phylloplane and briefly discuss protein-based surface defenses of animals. We also review the emerging evidence pertaining to antimicrobial phylloplane proteins and mechanisms by which proteins can be released to the phylloplane, including biosynthesis (e.g. phylloplanins) by specific trichomes and delivery in guttation fluid from hydathodes. Future research should lead to exciting advances in our understanding of the phylloplane and to useful biotechnological interventions.

Extracellular proteins in pea root tip and border cell exudates

Newly generated plant tissue is inherently sensitive to infection. Yet, when pea (Pisum sativum) roots are inoculated with the pea pathogen, Nectria haematococca, most newly generated root tips remain uninfected even though most roots develop lesions just behind the tip in the region of elongation. The resistance mechanism is unknown but is correlated spatially with the presence of border cells on the cap periphery. Previously, an array of >100 extracellular proteins was found to be released while border cell separation proceeds. Here we report that protein secretion from pea root caps is induced in correlation with border cell separation. When this root cap secretome was proteolytically degraded during inoculation of pea roots with N. haematococca, the percentage of infected root tips increased from 4% +/- 3% to 100%. In control experiments, protease treatment of conidia or roots had no effect on growth and development of the fungus or the plant. A complex of >100 extracellular proteins was confirmed, by multidimensional protein identification technology, to comprise the root cap secretome. In addition to defense-related and signaling enzymes known to be present in the plant apoplast were ribosomal proteins, 14-3-3 proteins, and others typically associated with intracellular localization but recently shown to be extracellular components of microbial biofilms. We conclude that the root cap, long known to release a high molecular weight polysaccharide mucilage and thousands of living cells into the incipient rhizosphere, also secretes a complex mixture of proteins that appear to function in protection of the root tip from infection.

Differences in the rhizosphere bacterial community of a transplastomic tobacco plant compared to its non-engineered counterpart

Cultivation-independent analyses were carried out to compare the bacterial community structure found in the rhizospheres of a transplastomic tobacco plant carrying the antibiotic resistance marker-gene aadA and its non-engineered parental line. PCR- and reverse transcriptase PCR-amplifications of 16S rRNA and their corresponding genes were carried out with primers targeting the domain Bacteria. The diversity of PCR-products amplified from total nucleic acids extracted from rhizospheres of 10-week-old plants, which had been grown in potting soil in the greenhouse, was visualized by genetic profiling using the single-strand conformation polymorphism (SSCP) technique. The SSCP profiles generated from DNA extracted with two different protocols, one including total RNA and the other only DNA, did not show any differences. The SSCP profiles amplified from RNA and DNA were also highly similar to each other, indicating that the dominant bacteria detected were metabolically active. High similarities were seen between the SSCP profiles from the transplastomic and the non-engineered plants, except for a single band that consistently occurred with samples from the non-engineered plants (six replicates), but not, or only weakly, with their engineered counterparts. DNA sequencing and database analysis revealed that the partial rRNA gene matched to a Flavobacterium sp. Other bands of the SSCP-profiles, related to Burkholderia and Bordetella were variable between individual plants but not affected by the transplastomic modification. Thus, the transplastomic modification caused a relative decline of a specific Flavobacterium population but not of other bacteria. Further studies including additional tobacco cultivars, soils and conditions of cultivation would be desirable, to elucidate the ecological importance of this difference.

Volatiles of bacterial antagonists inhibit mycelial growth of the plant pathogen Rhizoctonia solani

Bacterial antagonists are bacteria that negatively affect the growth of other organisms. Many antagonists inhibit the growth of fungi by various mechanisms, e.g., secretion of lytic enzymes, siderophores and antibiotics. Such inhibition of fungal growth may indirectly support plant growth. Here, we demonstrate that small organic volatile compounds (VOCs) emitted from bacterial antagonists negatively influence the mycelial growth of the soil-borne phytopathogenic fungus Rhizoctonia solani Kühn. Strong inhibitions (99-80%) under the test conditions were observed with Stenotrophomonas maltophilia R3089, Serratia plymuthica HRO-C48, Stenotrophomonas rhizophila P69, Serratia odorifera 4Rx13, Pseudomonas trivialis 3Re2-7, S. plymuthica 3Re4-18 and Bacillus subtilis B2g. Pseudomonas fluorescens L13-6-12 and Burkholderia cepacia 1S18 achieved 30% growth reduction. The VOC profiles of these antagonists, obtained through headspace collection and analysis on GC-MS, show different compositions and complexities ranging from 1 to almost 30 compounds. Most volatiles are species-specific, but overlapping volatile patterns were found for Serratia spp. and Pseudomonas spp. Many of the bacterial VOCs could not be identified for lack of match with mass-spectra of volatiles in the databases.

Can plant biochemistry contribute to understanding of invasion ecology?

Ecologists have long searched for an explanation as to why some plant invaders become much more dominant in their naturalized range than in their native range, and, accordingly, several non-exclusive ecological hypotheses have been proposed. Recently, a biochemical explanation was proposed--the "novel weapons hypothesis"--based on findings that Centaurea diffusa and Centaurea maculosa produce bioactive compounds (weapons) that are more active against naïve plant species in the introduced range than against co-evolved species in the native range. In this Opinion article, we revise and expand this biochemical hypothesis and discuss experimental and conceptual advances and limitations.

Phytotoxic Allelochemicals From Roots and Root Exudates of Leafy Spurge (Euphorbia esula L.)

Invasive plants are a widespread problem but the mechanisms used by these plants to become invasive are often unknown. The production of phytotoxic natural products by invasive weeds is one mechanism by which these species may become successful competitors. Here we conducted a bioactivity-driven fractionation of root extracts and exudates from the invasive plant leafy spurge (Euphorbia esula L.), and structurally characterized jatrophane diterpenes and ellagic acid derivatives. Ellagic acid derivatives and one of the jatrophane diterpenes, esulone A, have been previously reported from leafy spurge, but another of the jatrophane diterpenes, kasuinine B, has not. We show that these compounds are phytotoxic but affect plants in different ways, either inducing overall plant necrosis or reducing root branching and elongation.

First encounters--deployment of defence-related natural products by plants

Plant-derived natural products have important functions in ecological interactions. In some cases these compounds are deployed to sites of pathogen challenge by vesicle-mediated trafficking. Polar vesicle trafficking of natural products, proteins and other, as yet uncharacterized, cargo is emerging as a common theme in investigations of diverse disease resistance mechanisms in plants. Root-derived natural products can have marked effects on interactions between plants and soilborne organisms, for example by serving as signals for initiation of symbioses with rhizobia and mycorrhizal fungi. They may also contribute to competitiveness of invasive plant species by inhibiting the growth of neighbouring plants (allelopathy). Very little is known about the mechanisms of release of natural products from aerial plant parts or from roots, although there are likely to be commonalities in these processes. There is increasing evidence to indicate that pathogens and symbionts can manipulate plant endomembrane systems to suppress host defence responses and facilitate accommodation within plant cells. The relationship between secretory processes and plant interactions forms the focus of this review, which brings together different aspects of the deployment of defence-related natural products by plants.