Functional traits shape plant-plant interactions and recruitment in a hotspot of woody plant diversity.

Understanding and predicting recruitment in species-rich plant communities requires identifying functional determinants of both density-independent performance and interactions. In a common-garden field experiment with 25 species of the woody plant genus Protea, we varied the initial spatial and taxonomic arrangement of seedlings and followed their survival and growth during recruitment. Neighbourhood models quantified how six key functional traits affect density-independent performance, interaction effects and responses. Trait-based neighbourhood models accurately predicted individual survival and growth from the initial spatial and functional composition of species-rich experimental communities. Functional variation among species caused substantial variation in density-independent survival and growth that was not correlated with interaction effects and responses. Interactions were spatially restricted but had important, predominantly competitive, effects on recruitment. Traits increasing the acquisition of limiting resources (water for survival and soil P for growth) mediated trade-offs between interaction effects and responses. Moreover, resprouting species had higher survival but reduced growth, likely reinforcing the survival-growth trade-off in adult plants. Resource acquisition of juvenile plants shapes Protea community dynamics with acquisitive species with strong competitive effects suffering more from competition. Together with functional determinants of density-independent performance, this makes recruitment remarkably predictable, which is critical for efficient restoration and near-term ecological forecasts of species-rich communities.

Tissue-Specific Hormone Signalling and Defence Gene Induction in an In Vitro Assembly of the Rapeseed Verticillium Pathosystem.

Priming plants with beneficial microbes can establish rapid and robust resistance against numerous pathogens. Here, compelling evidence is provided that the treatment of rapeseed plants with Trichoderma harzianum OMG16 and Bacillus velezensis FZB42 induces defence activation against Verticillium longisporum infection. The relative expressions of the JA biosynthesis genes LOX2 and OPR3, the ET biosynthesis genes ACS2 and ACO4 and the SA biosynthesis and signalling genes ICS1 and PR1 were analysed separately in leaf, stem and root tissues using qRT-PCR. To successfully colonize rapeseed roots, the V. longisporum strain 43 pathogen suppressed the biosynthesis of JA, ET and SA hormones in non-primed plants. Priming led to fast and strong systemic responses of JA, ET and SA biosynthesis and signalling gene expression in each leaf, stem and root tissue. Moreover, the quantification of plant hormones via UHPLC-MS analysis revealed a 1.7- and 2.6-fold increase in endogenous JA and SA in shoots of primed plants, respectively. In roots, endogenous JA and SA levels increased up to 3.9- and 2.3-fold in Vl43-infected primed plants compared to non-primed plants, respectively. Taken together, these data indicate that microbial priming stimulates rapeseed defence responses against Verticillium infection and presumably transduces defence signals from the root to the upper parts of the plant via phytohormone signalling.

Synergistic Effects of a Root-Endophytic Trichoderma Fungus and Bacillus on Early Root Colonization and Defense Activation Against Verticillium longisporum in Rapeseed.

Rhizosphere-competent microbes often interact with plant roots and exhibit beneficial effects on plant performance. Numerous bacterial and fungal isolates are able to prime host plants for fast adaptive responses against pathogen attacks. Combined action of fungi and bacteria may lead to synergisms exceeding effects of single strains. Individual beneficial fungi and bacteria have been extensively studied in Arabidopsis thaliana, but little is known about their concerted actions in the Brassicaceae. Here, an in-vitro system with oilseed rape (Brassica napus) was established. Roots of two different cultivars were inoculated with well-characterized fungal (Trichoderma harzianum OMG16) and bacterial (Bacillus velezensis FZB42) isolates alone or in combination. Microscopic analysis confirmed that OMG16 hyphae entered root hairs through root hair tips and formed distinct intracellular structures. Quantitative PCR revealed that root colonization of OMG16 increased up to 10-fold in the presence of FZB42. Relative transcript levels of the ethylene- and jasmonic acid-responsive genes PDF1.2, ERF2, and AOC3 were recorded in leaves by quantitative reverse transcription PCR to measure induced systemic resistance in tissues distant from the roots. Combined action of OMG16 and FZB42 induced transcript abundances more efficiently than single inoculation. Importantly, microbial priming reduced Verticillium longisporum root infection in rapeseed by approximately 100-fold compared with nonprimed plants. Priming also led to faster and stronger systemic responses of the defense genes PDF1.2, ERF2, AOC3, and VSP2.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Rhizosphere pH and cation-anion balance determine the exudation of nitrification inhibitor 3-epi-brachialactone suggesting release via secondary transport.

Biological nitrification inhibition (BNI) of Brachiaria humidicola has been attributed to nitrification-inhibiting fusicoccanes, most prominently 3-epi-brachialactone. However, its release mechanism from B. humidicola roots remains elusive. Two hydroponic experiments were performed to investigate the role of rhizosphere pH and nutritional N form in regulating 3-epi-brachialactone release by B. humidicola and verify the underlying release pathway. Low rhizosphere pH and NH4 + nutrition promoted 3-epi-brachialactone exudation. However, the substitution of NH4 + by K+ revealed that the NH4 + effect was not founded in a direct physiological response to the N form but was related to the cation-anion balance during nutrient uptake. Release of 3-epi-brachialactone correlated with the transmembrane proton gradient ΔpH and NH4 + uptake (R2 = 0.92 for high ~6.8 and R2 = 0.84 for low ~4.2 trap solution pH). This corroborated the release of 3-epi-brachialactone through secondary transport, with the proton motive force (ΔP) defining transport rates across the plasma membrane. It was concluded that 3-epi-brachialactone release cannot be conceptualized as a regulated response to soil pH or NH4 + availability, but merely as the result of associated changes in ΔP.

Loss of LaMATE impairs isoflavonoid release from cluster roots of phosphorus-deficient white lupin.

White lupin (Lupinus albus L.) forms brush-like root structures called cluster roots under phosphorus-deficient conditions. Clusters secrete citrate and other organic compounds to mobilize sparingly soluble soil phosphates. In the context of aluminum toxicity tolerance mechanisms in other species, citrate is released via a subgroup of MATE/DTX proteins (multidrug and toxic compound extrusion/detoxification). White lupin contains 56 MATE/DTX genes. Many of these are closely related to gene orthologs with known substrates in other species. LaMATE is a marker gene for functional, mature clusters and is, together with its close homolog LaMATE3, a candidate for the citrate release. Both were highest expressed in mature clusters and when expressed in oocytes, induced inward-rectifying currents that were likely carried by endogenous channels. No citrate efflux was associated with LaMATE and LaMATE3 expression in oocytes. Furthermore, citrate secretion was largely unaffected in P-deficient composite mutant plants with genome-edited or RNAi-silenced LaMATE in roots. Moderately lower concentrations of citrate and malate in the root tissue and consequently less organic acid anion secretion and lower malate in the xylem sap were identified. Interestingly, however, less genistein was consistently found in mutant exudates, opening the possibility that LaMATE is involved in isoflavonoid release.

LaALMT1 mediates malate release from phosphorus-deficient white lupin root tips and metal root to shoot translocation.

Under phosphorus (P) deficiency, Lupinus albus (white lupin) releases large amounts of organic acid anions from specialized root structures, so-called cluster or proteoid roots, to mobilize and acquire sparingly soluble phosphates from a restricted soil volume. The molecular mechanisms underlying this release and its regulation are, however, poorly understood. Here, we identified a gene belonging to the aluminium (Al)-activated malate transporter (ALMT) family that specifically contributes to malate, but not citrate release. This gene, LaALMT1, was most prominently expressed in the root apices under P deficiency, including those of cluster roots and was also detected in the root stele. Contrary to several ALMT homologs in other species, the expression was not stimulated, but moderately repressed by Al. Aluminium-independent malate currents were recorded from the plasma membrane localized LaALMT1 expressed in Xenopus oocytes. In composite lupins with transgenic roots, LaALMT1 was efficiently mutated by CRISPR-Cas9, leading to diminished malate efflux and lower xylem sap malate concentrations. When grown in an alkaline P-deficient soil, mutant shoot phosphate concentrations were similar, but iron and potassium concentrations were diminished in old leaves, suggesting a role for ALMT1 in metal root to shoot translocation, a function that was also supported by growth in hydroponics.

Noninvasive brain stimulation in rehabilitation of hemispatial neglect after stroke.

BACKGROUND: Noninvasive brain stimulation can modulate neural processing within the motor cortex and thereby might be beneficial in the rehabilitation of hemispatial neglect after stroke. METHODS: We review the pertinent literature regarding the use of transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation in order to facilitate recovery of hemispatial neglect after stroke. RESULTS: Twenty controlled trials (including 443 stroke patients) matched our inclusion criteria. Methodology and results of each study are presented in a comparative approach. Current data seem to indicate a better efficiency of repetitive transcranial magnetic stimulation, compared to tDCS to ameliorate hemispatial neglect after stroke. CONCLUSIONS: Noninvasive brain stimulation has the potential to facilitate recovery of hemispatial neglect after stroke, but until today, there are not enough data to claim its routine use.

Estimating the importance of maize root hairs in low phosphorus conditions and under drought.

BACKGROUND AND AIMS: Root hairs are single-cell extensions of the epidermis that face into the soil and increase the root-soil contact surface. Root hairs enlarge the rhizosphere radially and are very important for taking up water and sparingly soluble nutrients, such as the poorly soil-mobile phosphate. In order to quantify the importance of root hairs for maize, a mutant and the corresponding wild type were compared. METHODS: The rth2 maize mutant with very short root hairs was assayed for growth and phosphorus (P) acquisition in a slightly alkaline soil with low P and limited water supply in the absence of mycorrhization and with ample P supply. KEY RESULTS: Root and shoot growth was additively impaired under P deficiency and drought. Internal P concentrations declined with reduced water and P supply, whereas micronutrients (iron, zinc) were little affected. The very short root hairs in rth2 did not affect internal P concentrations, but the P content of juvenile plants was halved under combined stress. The rth2 plants had more fine roots and increased specific root length, but P mobilization traits (root organic carbon and phosphatase exudation) differed little. CONCLUSIONS: The results confirm the importance of root hairs for maize P uptake and content, but not for internal P concentrations. Furthermore, the performance of root hair mutants may be biased by secondary effects, such as altered root growth.

The LaCEP1 peptide modulates cluster root morphology in Lupinus albus.

White lupin cluster roots are specialized brush-like root structures that are formed in some species under phosphorus (P)-deficient conditions. They intensely secrete protons and organic acid anions for solubilization and acquisition of sparingly soluble phosphates. Phytohormones and sucrose modulate cluster root number, but the molecular mechanisms of cluster root formation have been elusive. Here, a novel peptide phytohormone was identified that affects cluster root development. It belongs to the C-TERMINALLY-ENCODED PEPTIDE (CEP) family. Members of that family arrest root growth and modulate branching in model species. LaCEP1 was highly expressed in the pre-emergence zone of clusters. Over-expression of the gene encoding the LaCEP1 propeptide resulted in moderate inhibition of cluster root formation. The primary and lateral root lengths of lupin were little affected by the overexpression, but LaCEP1 reduced cluster rootlet and root hair elongation. Addition of a 15-mer core peptide derived from LaCEP1 similarly altered root morphology and modified cluster activity, suggesting that a core sequence of the propeptide is functionally sufficient. Stable overexpression in Arabidopsis confirmed the LaCEP1 function in root growth inhibition across species. Taken together, the root inhibitory effects of the LaCEP1 phytohormone suggest a role as of a regulatory module involved in cluster root development in white lupin.

A re-assessment of sucrose signaling involved in cluster-root formation and function in phosphate-deficient white lupin (Lupinus albus).

Apart from substrate functions, a signaling role of sucrose in root growth regulation is well established. This raised the question whether sucrose signals might also be involved in formation of cluster-roots (CRs) under phosphate (Pi) limitation, mediating exudation of phosphorus (P)-mobilizing root exudates, e.g. in Lupinus albus and members of the Proteaceae. Earlier studies demonstrated that CR formation in L. albus was mimicked to some extent by external application of high sucrose concentrations (25 mM) in the presence of extremely high P supply (1-10 mM), usually suppressing CR formation. In this study, we re-addressed this question using an axenic hydroponic culture system with normal P supply (0.1 mM) and a range of sucrose applications (0.25-25 mM). The 2.5 mM sucrose concentration was comparable with internal sucrose levels in the zone of CR initiation in first-order laterals of P-deficient plants (3.4 mM) and induced the same CR morphology. Similar to earlier studies, high sucrose concentrations (25 mM) resulted in root thickening and inhibition of root elongation, associated with a 10-fold increase of the internal sucrose level. The sucrose analog palatinose and a combination of glucose/fructose failed to stimulate CR formation under P-sufficient conditions, demonstrating a signal function of sucrose and excluding osmotic or carbon source effects. In contrast to earlier findings, sucrose was able to induce CR formation but had no effect on CR functioning with respect to citrate exudation, in vitro activity and expression of genes encoding phosphoenolpyruvate carboxylase, secretory acid phosphatase and MATE transporters, mediating P-mobilizing functions of CRs.

The regulatory network of cluster-root function and development in phosphate-deficient white lupin (Lupinus albus) identified by transcriptome sequencing.

Lupinus albus serves as model plant for root-induced mobilization of sparingly soluble soil phosphates via the formation of cluster-roots (CRs) that mediate secretion of protons, citrate, phenolics and acid phosphatases (APases). This study employed next-generation sequencing to investigate the molecular mechanisms behind these complex adaptive responses at the transcriptome level. We compared different stages of CR development, including pre-emergent (PE), juvenile (JU) and the mature (MA) stages. The results confirmed that the primary metabolism underwent significant modifications during CR maturation, promoting the biosynthesis of organic acids, as had been deduced from physiological studies. Citrate catabolism was downregulated, associated with citrate accumulation in MA clusters. Upregulation of the phenylpropanoid pathway reflected the accumulation of phenolics. Specific transcript expression of ALMT and MATE transporter genes correlated with the exudation of citrate and flavonoids. The expression of transcripts related to nucleotide degradation and APases in MA clusters coincided with the re-mobilization and hydrolysis of organic phosphate resources. Most interestingly, hormone-related gene expression suggested a central role of ethylene during CR maturation. This was associated with the upregulation of the iron (Fe)-deficiency regulated network that mediates ethylene-induced expression of Fe-deficiency responses in other species. Finally, transcripts related to abscisic acid and jasmonic acid were upregulated in MA clusters, while auxin- and brassinosteroid-related genes and cytokinin receptors were most strongly expressed during CR initiation. Key regulations proposed by the RNA-seq data were confirmed by quantitative real-time polymerase chain reaction (RT-qPCR) and some physiological analyses. A model for the gene network regulating CR development and function is presented.

Protective potential of selected microbial and non-microbial biostimulants against Zymoseptoria tritici leaf blotch in winter wheat as affected by the form of N supply.

Introduction: The production of high-quality food for the growing world population on the one hand and the reduction of chemical-synthetic pesticides on the other hand represents a major challenge for agriculture worldwide. The effectiveness of a combination of microbial and non-microbial biostimulants (BSs) with various nitrogen (N) forms in pathogen defense is discussed as a promising, but still poorly understood bio-based alternative for crop protection. Methods: For this reason, nitrate and stabilized ammonium fertilizer both combined with a consortium of Pseudomonas brassicacearum, Bacillus amyloliquefaciens, and Trichoderma harzianum as soil treatment or with a mixture of seaweed extract (Ascophyllum nodosum) together with chitosan-amended micronutrient fertilizer as foliar spray application were compared under controlled greenhouse conditions. Furthermore, a combination of microbial and different non-microbial BSs (seaweed extracts + chitosan) and micronutrients with nitrate or with stabilized ammonium fertilizer was tested under field conditions to improve nutrient availability, promote plant growth, and suppress Zymoseptoria tritici (Zt) in winter wheat. Results and discussion: While plant-protective effects against Zt by the microbial consortium application could be observed particularly under ammonium fertilization, the application of seaweed extract-chitosan mixture expressed plant defense against Zt more strongly under nitrate fertilization. In the field trial, the combination of microbial consortium with the seaweed extract-chitosan mixture together with micronutrients zinc (Zn) and manganese (Mn) showed positive effects against Zt under ammonium fertilization, associated with increased levels of defense metabolites. Furthermore, the additional input of Zn and copper (Cu) from the chitosan application improved the micronutrient status by minimizing the risk of Zn and Cu deficiency under controlled and field conditions. The use of BSs and the inoculation of Zt did not show any effects on plant growth and yield neither under controlled greenhouse conditions nor in the field. Summarized, microbial and non-microbial BSs separately applied or even combined together as one treatment did not influence plant growth or yield but made a positive contribution to an N form-dependent promotion of pathogen defense.

Long-term conservation tillage with reduced nitrogen fertilization intensity can improve winter wheat health via positive plant-microorganism feedback in the rhizosphere.

Microbiome-based solutions are regarded key for sustainable agroecosystems. However, it is unclear how agricultural practices affect the rhizosphere microbiome, plant-microorganism interactions and crop performance under field conditions. Therefore, we installed root observation windows in a winter wheat field cultivated either under long-term mouldboard plough (MP) or cultivator tillage (CT). Each tillage practice was also compared at two nitrogen (N) fertilization intensities, intensive (recommended N-supply with pesticides/growth regulators) or extensive (reduced N-supply, no fungicides/growth regulators). Shoot biomass, root exudates and rhizosphere metabolites, physiological stress indicators, and gene expression were analyzed together with the rhizosphere microbiome (bacterial/archaeal 16S rRNA gene, fungal ITS amplicon, and shotgun metagenome sequencing) shortly before flowering. Compared to MP, the rhizosphere of CT winter wheat contained more primary and secondary metabolites, especially benzoxazinoid derivatives. Potential copiotrophic and plant-beneficial taxa (e.g. Bacillus, Devosia, and Trichoderma) as well as functional genes (e.g. siderophore production, trehalose synthase, and ACC deaminase) were enriched in the CT rhizosphere, suggesting that tillage affected belowground plant-microorganism interactions. In addition, physiological stress markers were suppressed in CT winter wheat compared to MP. In summary, tillage practice was a major driver of crop performance, root deposits, and rhizosphere microbiome interactions, while the N-fertilization intensity was also relevant, but less important.

Effectiveness of bio-effectors on maize, wheat and tomato performance and phosphorus acquisition from greenhouse to field scales in Europe and Israel: a meta-analysis.

Biostimulants (Bio-effectors, BEs) comprise plant growth-promoting microorganisms and active natural substances that promote plant nutrient-acquisition, stress resilience, growth, crop quality and yield. Unfortunately, the effectiveness of BEs, particularly under field conditions, appears highly variable and poorly quantified. Using random model meta-analyses tools, we summarize the effects of 107 BE treatments on the performance of major crops, mainly conducted within the EU-funded project BIOFECTOR with a focus on phosphorus (P) nutrition, over five years. Our analyses comprised 94 controlled pot and 47 field experiments under different geoclimatic conditions, with variable stress levels across European countries and Israel. The results show an average growth/yield increase by 9.3% (n=945), with substantial differences between crops (tomato > maize > wheat) and growth conditions (controlled nursery + field (Seed germination and nursery under controlled conditions and young plants transplanted to the field) > controlled > field). Average crop growth responses were independent of BE type, P fertilizer type, soil pH and plant-available soil P (water-P, Olsen-P or Calcium acetate lactate-P). BE effectiveness profited from manure and other organic fertilizers, increasing soil pH and presence of abiotic stresses (cold, drought/heat or salinity). Systematic meta-studies based on published literature commonly face the inherent problem of publication bias where the most suspected form is the selective publication of statistically significant results. In this meta-analysis, however, the results obtained from all experiments within the project are included. Therefore, it is free of publication bias. In contrast to reviews of published literature, our unique study design is based on a common standardized protocol which applies to all experiments conducted within the project to reduce sources of variability. Based on data of crop growth, yield and P acquisition, we conclude that application of BEs can save fertilizer resources in the future, but the efficiency of BE application depends on cropping systems and environments.

Testing plant growth promoting microorganisms in the field - a proposal for standards.

In the European Union and worldwide there are a burgeoning markets for plant growth promoting microorganisms (PGPM) and other biological agents as soil improvers, bio-fertilizers, plant bio-stimulants, and biological control agents or bio-pesticides. Microbial agents have a major share in this development. The use of such products is often advertised with the promise of contributing to sustainable agricultural practices by increasing crop growth and yield and offering an alternative or substitute to decrease the dependency of agriculture on hazardeous agrochemicals. In contrast to registered microbial plant protection products, PGPM that are marketed in the EU as soil improvers or plant biostimulants, are not strictly required to have proven minimum efficacy levels under field conditions. Manufacturers only have to ensure that these products do not pose unacceptable risks to human, animal or plant health, safety or the environment. Uniform guidelines comparable to the EPPO - standards (European and Mediterranean Plant Protection Organisation) to test the efficacy in field trials are not available. This paper attempts to fill the gap. It proposes guidelines for PGPM field trial design and implementation, as well as recommendations for the type and scope of data collection and evaluation. Selected research papers from literature were evaluated to analyze, whether and to what extent the requirements are already met. The majority of the papers had a clear experimental design followed by proper data evaluation. Frequent deficiencies were the low number of tested environments and crop species, insufficient site and agronomic management description and missing data on soil humidity and temperature. Using the suggested standards is assumed to increase the expressive power of tested microbial products.

Beneficial microbial consortium improves winter rye performance by modulating bacterial communities in the rhizosphere and enhancing plant nutrient acquisition.

The beneficial effect of microbial consortium application on plants is strongly affected by soil conditions, which are influenced by farming practices. The establishment of microbial inoculants in the rhizosphere is a prerequisite for successful plant-microorganism interactions. This study investigated whether a consortium of beneficial microorganisms establishes in the rhizosphere of a winter crop during the vegetation period, including the winter growing season. In addition, we aimed for a better understanding of its effect on plant performance under different farming practices. Winter rye plants grown in a long-time field trial under conventional or organic farming practices were inoculated after plant emergence in autumn with a microbial consortium containing Pseudomonas sp. (RU47), Bacillus atrophaeus (ABi03) and Trichoderma harzianum (OMG16). The density of the microbial inoculants in the rhizosphere and root-associated soil was quantified in autumn and the following spring. Furthermore, the influence of the consortium on plant performance and on the rhizosphere bacterial community assembly was investigated using a multidisciplinary approach. Selective plating showed a high colonization density of individual microorganisms of the consortium in the rhizosphere and root-associated soil of winter rye throughout its early growth cycle. 16S rRNA gene amplicon sequencing showed that the farming practice affected mainly the rhizosphere bacterial communities in autumn and spring. However, the microbial consortium inoculated altered also the bacterial community composition at each sampling time point, especially at the beginning of the new growing season in spring. Inoculation of winter rye with the microbial consortium significantly improved the plant nutrient status and performance especially under organic farming. In summary, the microbial consortium showed sufficient efficacy throughout vegetation dormancy when inoculated in autumn and contributed to better plant performance, indicating the potential of microbe-based solutions in organic farming where nutrient availability is limited.

Ammonium fertilization increases the susceptibility to fungal leaf and root pathogens in winter wheat.

Nitrogen (N) fertilization is indispensable for high yields in agriculture due to its central role in plant growth and fitness. Different N forms affect plant defense against foliar pathogens and may alter soil-plant-microbe interactions. To date, however, the complex relationships between N forms and host defense are poorly understood. For this purpose, nitrate, ammonium, and cyanamide were compared in greenhouse pot trials with the aim to suppress two important fungal wheat pathogens Blumeria graminis f. sp. tritici (Bgt) and Gaeumannomyces graminis f. sp. tritici (Ggt). Wheat inoculated with the foliar pathogen Bgt was comparatively up to 80% less infested when fertilized with nitrate or cyanamide than with ammonium. Likewise, soil inoculation with the fungal pathogen Ggt revealed a 38% higher percentage of take-all infected roots in ammonium-fertilized plants. The bacterial rhizosphere microbiome was little affected by the N form, whereas the fungal community composition and structure were shaped by the different N fertilization, as revealed from metabarcoding data. Importantly, we observed a higher abundance of fungal pathogenic taxa in the ammonium-fertilized treatment compared to the other N treatments. Taken together, our findings demonstrated the critical role of fertilized N forms for host-pathogen interactions and wheat rhizosphere microbiome assemblage, which are relevant for plant fitness and performance.

Long-Term Fertilization Strategy Impacts Rhizoctonia solani-Microbe Interactions in Soil and Rhizosphere and Defense Responses in Lettuce.

The long-term effects of agricultural management such as different fertilization strategies on soil microbiota and soil suppressiveness against plant pathogens are crucial. Therefore, the suppressiveness of soils differing in fertilization history was assessed using two Rhizoctonia solani isolates and their respective host plants (lettuce, sugar beet) in pot experiments. Further, the effects of fertilization history and the pathogen R. solani AG1-IB on the bulk soil, root-associated soil and rhizosphere microbiota of lettuce were analyzed based on amplicon sequencing of the 16S rRNA gene and ITS2 region. Organic fertilization history supported the spread of the soil-borne pathogens compared to long-term mineral fertilization. The fertilization strategy affected bacterial and fungal community composition in the root-associated soil and rhizosphere, respectively, but only the fungal community shifted in response to the inoculated pathogen. The potential plant-beneficial genus Talaromyces was enriched in the rhizosphere by organic fertilization and presence of the pathogen. Moreover, increased expression levels of defense-related genes in shoots of lettuce were observed in the soil with organic fertilization history, both in the absence and presence of the pathogen. This may reflect the enrichment of potential plant-beneficial microorganisms in the rhizosphere, but also pathogen infestation. However, enhanced defense responses resulted in retarded plant growth in the presence of R. solani (plant growth/defense tradeoff).

Role of Benzoic Acid and Lettucenin A in the Defense Response of Lettuce against Soil-Borne Pathogens.

Soil-borne pathogens can severely limit plant productivity. Induced defense responses are plant strategies to counteract pathogen-related damage and yield loss. In this study, we hypothesized that benzoic acid and lettucenin A are involved as defense compounds against Rhizoctonia solani and Olpidium virulentus in lettuce. To address this hypothesis, we conducted growth chamber experiments using hydroponics, peat culture substrate and soil culture in pots and minirhizotrons. Benzoic acid was identified as root exudate released from lettuce plants upon pathogen infection, with pre-accumulation of benzoic acid esters in the root tissue. The amounts were sufficient to inhibit hyphal growth of R. solani in vitro (30%), to mitigate growth retardation (51%) and damage of fine roots (130%) in lettuce plants caused by R. solani, but were not able to overcome plant growth suppression induced by Olpidium infection. Additionally, lettucenin A was identified as major phytoalexin, with local accumulation in affected plant tissues upon infection with pathogens or chemical elicitation (CuSO4) and detected in trace amounts in root exudates. The results suggest a two-stage defense mechanism with pathogen-induced benzoic acid exudation initially located in the rhizosphere followed by accumulation of lettucenin A locally restricted to affected root and leaf tissues.