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AimsDifferent drivers are known to shape rhizosphere microbiome assembly. How soil texture (Texture) and presence or lack of root hairs (Root Hair) of plants affect the rhizosphere microbiome assembly and soil potential extracellular enzyme activities (EEA) at defined rooting depth (Depth) is still a knowledge gap. We investigated effects of these drivers on microbial assembly in rhizosphere and on potential EEA in root-affected soil of maize.MethodsSamples were taken from three depths of root hair defective mutant rth3 and wild-type WT maize planted on loam and sand in soil columns after 22 days. Rhizosphere bacterial, archaeal, fungal and cercozoan communities were analysed by sequencing of 16S rRNA gene, ITS and 18S rRNA gene fragments. Soil potential EEA of ß-glucosidase, acid phosphatase and chitinase were estimated using fluorogenic substrates.ResultsThe bacterial, archaeal and cercozoan alpha- and beta-diversities were significantly and strongly altered by Texture, followed by Depth and Root Hair. Texture and Depth had a small impact on fungal assembly, and only fungal beta-diversity was significantly affected. Significant impacts by Depth and Root Hair on beta-diversity and relative abundances at taxonomic levels of bacteria, archaea, fungi and cercozoa were dependent on Texture. Likewise, the patterns of potential EEA followed the trends of microbial communities, and the potential EEA correlated with the relative abundances of several taxa.ConclusionsTexture was the strongest driver of rhizosphere microbiome and of soil potential EEA, followed by Depth and Root Hair, similarly to findings in maize root architecture and plant gene expression studies. Aims: Different drivers are known to shape rhizosphere microbiome assembly. How soil texture (Texture) and presence or lack of root hairs (Root Hair) of plants affect the rhizosphere microbiome assembly and soil potential extracellular enzyme activities (EEA) at defined rooting depth (Depth) is still a knowledge gap. We investigated effects of these drivers on microbial assembly in rhizosphere and on potential EEA in root-affected soil of maize. Methods: Samples were taken from three depths of root hair defective mutant rth3 and wild-type WT maize planted on loam and sand in soil columns after 22 days. Rhizosphere bacterial, archaeal, fungal and cercozoan communities were analysed by sequencing of 16S rRNA gene, ITS and 18S rRNA gene fragments. Soil potential EEA of ß-glucosidase, acid phosphatase and chitinase were estimated using fluorogenic substrates. Results: The bacterial, archaeal and cercozoan alpha- and beta-diversities were significantly and strongly altered by Texture, followed by Depth and Root Hair. Texture and Depth had a small impact on fungal assembly, and only fungal beta-diversity was significantly affected. Significant impacts by Depth and Root Hair on beta-diversity and relative abundances at taxonomic levels of bacteria, archaea, fungi and cercozoa were dependent on Texture. Likewise, the patterns of potential EEA followed the trends of microbial communities, and the potential EEA correlated with the relative abundances of several taxa. Conclusions: Texture was the strongest driver of rhizosphere microbiome and of soil potential EEA, followed by Depth and Root Hair, similarly to findings in maize root architecture and plant gene expression studies.

Beneficial interactions with microorganisms are pivotal for crop performance and resilience. However, it remains unclear how heritable the microbiome is with respect to the host plant genotype and to what extent host genetic mechanisms can modulate plant-microbe interactions in the face of environmental stress. Here, we surveyed 3,168 root and rhizosphere microbiome samples from 129 accessions of locally adapted Zea mays, sourced from diverse habitats and grown under control and different stress conditions. We quantified treatment and host genotype effects on the microbiome. Plant genotype and source environment were predictive of microbiome abundance. Genome wide association analysis identified host genetic variants linked to both rhizosphere microbiome abundance and source environment. We identified transposon insertions in a candidate gene linked to both the abundance of a keystone microbe Massilia and source total soil nitrogen, finding specific Massilia alone can contribute to root development, biomass production and nitrogen resilience. We conclude that locally adapted maize varieties exert patterns of genetic control on their root and rhizosphere microbiomes that follow variation in their home environments, consistent with a role in tolerance to prevailing stress.

Garden rose production involves field culture for rootstock production and cultivation of grafted plants. When roses are replanted at the same site, reduced growth, stunting and discoloration of roots are observed. This phenomenon is well-known as rose replant disease (RRD), which results in considerable economic losses. Until now, the causes of RRD are largely unknown and measures to overcome RRD are lacking. In contrast to apple, research on replant disease in roses is scarce. In this study, we have carried out a greenhouse pot experiment, using either untreated or gamma irradiated RRD soils from two sites. Slow release fertilizer was added to the soils before planting seedlings of the rootstock Rosa corymbifera ‘Laxa’. After eight weeks, shoot and root growth were recorded, roots were examined by detailed microscopic analyses and secondary metabolites were analyzed by gas chromatography-mass spectrometry. Rhizosphere samples were taken and their respective microbial communities were analyzed by amplicon sequencing of the 16S rRNA gene for bacteria and archaea as well as the ITS2 and cox2 region for fungi and oomycetes, respectively. Finally, segments of surface-disinfected roots were placed on 523 medium and outgrowing endophytic bacteria were isolated and identified. For both soils, significantly higher shoot and root biomass were observed for plants growing in irradiated compared to untreated soils. Roots were darkened, deformed and finally damaged in their outer cell layers when growing in untreated RRD soil. This corresponded to higher concentrations of two catechin derivatives and gallic acid in roots in this RRD soil compared to roots in the irradiated variants. Twenty-six endophytic bacterial isolates were obtained from roots that were affiliated to 15 different bacterial genera. Rhizosphere microbial community compositions not only differed significantly between soils of the two sites, but also between treatments (untreated versus gamma irradiated) for bacteria, fungi and oomycetes. The identification of genera differing in relative abundance in the different soils and treatments will provide a deeper insight in causal agents of RRD as well as antagonists or beneficials. Future analyses should include different rootstock species in order to identify RRD tolerant germplasm.

Plants interact with the rhizosphere microbiome via root exudates that consist of numerous metabolites serving as energy or carbon sources for microbial growth and as modulators of the uncountable rhizosphere interactions. The rhizosphere microbiome plays also an important role in plant health, growth, and productivity. Different drivers are known to shape the rhizosphere microbiome, but limited investigation exists whether there is a spatial variability in the microbiome along the root system (depth). The present study aimed to assess effects of potentially different drivers such as soil substrates, soil compartments (rhizosphere, and bulk soil), depths, and plant genotypes on bacterial/archaeal communities associated with two maize genotypes, root hair defective mutant (rth3), and the corresponding wild-type (WT). Experiments using maize genotypes rth3 and WT, grown on soil substrates loam, and sand under growth chamber, and field conditions were performed. Under growth camber conditions, the rhizosphere samples were harvested at twenty-two days after sowing the maize seeds from three different soil depths at 4.5 – 6.1 (GD1), 9.0 – 10.6 (GD2), and 13.5 – 15.1 (GD3) cm from soil surface. Under field conditions, analyses were carried out using both rhizosphere, and bulk soil samples taken at three developmental growth stages BBCH14, -19, and -59 of the maize plants; each from two depths at (0 – 20) FD1, and FD2 (20 - 40) cm from soil surface, except the BBCH14 (only samples from D1 were available). Bacterial/archaeal communities were analyzed by MiSeq Illumina sequencing of 16S rRNA gene fragments amplified from total community DNAs.Under growth chamber conditions, we observed shifts in bacterial/archaeal diversity of maize rhizosphere at different depths as plant genotype- and soil substrate-dependent effects. Depth-dependent effects of maize rhizosphere (rth3/WT) on bacterial/archaeal compositions displayed high differences between GD1, and the GD3 on both soil substrates. The relative abundances of the bacterial phylum Proteobacteria were significantly higher at GD3 than GD1 for both plant genotypes on sand, but not on loam. Overall, the factor soil substrate was the strongest driver of bacterial/archaeal maize rhizosphere, followed by depth, and maize genotype.Under field conditions, depths affected the rhizosphere bacterial/archaeal diversity only at the BBCH59 for WT grown on sand. Lower bacterial/archaeal diversity in soil substrates sand than loam was observed at both FD1 and FD2 in the rhizosphere, but not in bulk soil at all developmental growth stages of maize. The bacterial/archaeal diversity of both maize genotypes was not affected by developmental growth stages of maize on both soil substrates, and soil compartments. Depth gradients of bacterial/archaeal community composition in rhizosphere, and bulk soil displayed at BBCH59 on both soil substrates, and they were relatively higher on sand than loam (rhizosphere). Differences in relative abundances of the bacterial phyla Proteobacteria, and Actinobacteria between soil compartments, developmental growth stages of maize were observed mainly at FD1. Overall, factor soil compartment is the strongest driver of bacterial/archaeal communities followed by soil substrates, developmental growth stages and sampling depths for maize grown under field conditions.

The effects of biofumigation with Brassica juncea ‘Terra Plus’ and Raphanus sativus ‘Defender’ in comparison to Basamid on apple plant growth and on soil microbial communities were studied at three sites affected by replant disease under field conditions. Apple rootstocks were planted on differently treated plots to evaluate the effect of the treatments on plant growth under field and greenhouse conditions. The glucosinolates in biofumigant plant organs and their breakdown products in soils were determined. Denaturing gradient gel electrophoresis fingerprints were performed with 16S rRNA gene and ITS fragments amplified from total community DNA extracted from different soils. The highest glucosinolate concentrations were found in inflorescences of both biofumigant plant species with no differences between sites. The most abundant degradation product in soil biofumigated with B. juncea was 2-propenyl isothiocyanate, while in soil treated with R. sativus only 4-(methylthio)-3-butenyl isothiocyanate was detected. Effects of biofumigation were recorded to be stronger on fungi than on bacteria. Growth of apple rootstocks was positively affected by the treatments in a site-dependent manner. The effects of biofumigation evaluated by the apple plant growth were site-dependent and might result from suppression of soil-borne pests and pathogens, changes in soil microbial community compositions, and additional nutrients from the incorporated biomass.

Nurseries producing apple and rose rootstock plants, apple orchards as well as rose production often experience replanting problems after several cultivations on the same site when a chemical soil disinfectant is not applied. The etiology of apple and rose replanting problems is most likely caused by soil-borne pathogen complex, defined as ‘replant disease (RD)’. RD symptoms are typically reduced shoot and root growth, a smaller leaf area, a significant decrease in plant biomass, yield and fruit quality and a shorter life span. In our previous study, we showed that RD symptoms were reduced when apple rootstock M106 were grown in RD soils treated either with the soil fumigant Basamid or after biofumigation by incorporating Brassica juncea or Raphanus sativus or by growing Tagetes under field conditions compared to untreated control soil. The present study aimed at identifying potential bacterial and fungal taxa that were affected by different soil treatments and linking bacterial and fungal responders to plant performance. Miseq® Illumina® sequencing of 16S rRNA gene fragments (bacteria) and ITS regions (fungi) amplified from total community DNA extracted from soil samples taken four weeks after treatments were performed. Soil properties and culture history of the two RD sites influenced greatly soil microbiomes. Several bacterial genera were identified that significantly increased in treated soils such as Arthrobacter (R. sativus, both sites), Curtobacterium (Basamid, both sites), Terrimonas (Basamid and R. sativus, site A) and Ferruginibacter (B. juncea, site K and R. sativus, site A) that were also significantly and positively correlated with growth of apple M106 plants. Only few fungal genera, such as Podospora, Monographella and Mucor, were significantly promoted in soils treated with B. juncea and R. sativus (both sites). The least pronounced changes were recorded for bacterial as well as fungal communities in the RD soils planted with Tagetes. The detection of bacterial and fungal genera that were significantly increased in relative abundance in response to the treatments and that were positively correlated with plant growth suggests that management of the soil microbial community could contribute to overcome the apple RD encountered in affected sites.

Replant problems or soil sickness are known phenomena but still unsolved. The aims of this study were (i) to set up a test system for detecting replant problems using in vitro propagated apple rootstocks (M26) based on different soil disinfection treatments and (ii) to explore the treatment effects on root morphology and soil microbial community structure. The bio-test involved soil with apple replant problems (apple sick) and healthy soil from an adjacent plot, both either untreated, or submitted to treatments of 50 and 100 °C, or the chemical soil disinfectant Basamid. Histological analyses of roots and denaturing gradient gel electrophoresis (DGGE) fingerprints in rhizosphere soil collected at the final evaluation were performed. After 10 weeks, shoot dry mass on apple sick soil was 79, 108 and 124 % higher for soil treated at 50 °C, 100 °C and with Basamid, respectively, compared to the untreated soil. Roots in untreated apple sick soil showed destroyed epidermal and cortical layers. DGGE fingerprints revealed treatment dependent differences in community composition and relative abundance of total bacteria, Bacillus, Pseudomonas and total fungi. The clear differences detected in soil microbial communities are the first steps towards a better understanding of the causes for apple replant problems.

Replant disease (RD) severely affects apple production in propagation tree nurseries and in fruit orchards worldwide. This study aimed to investigate the effects of soil disinfection treatments on plant growth and health in a biotest in two different RD soil types under greenhouse conditions and to link the plant growth status with the bacterial community composition at the time of plant sampling. In the biotest performed we observed that the aboveground growth of apple rootstock M26 plants after 8 weeks was improved in the two RD soils either treated at 50°C or with gamma irradiation compared to the untreated RD soils. Total community DNA was extracted from soil loosely adhering to the roots and quantitative real-time PCR revealed no pronounced differences in 16S rRNA gene copy numbers. 16S rRNA gene-based bacterial community analysis by denaturing gradient gel electrophoresis (DGGE) and 454-pyrosequencing revealed significant differences in the bacterial community composition even after 8 weeks of plant growth. In both soils, the treatments affected different phyla but only the relative abundance of Acidobacteria was reduced by both treatments. The genera Streptomyces, Bacillus, Paenibacillus, and Sphingomonas had a higher relative abundance in both heat treated soils, whereas the relative abundance of Mucilaginibacter, Devosia, and Rhodanobacter was increased in the gamma-irradiated soils and only the genus Phenylobacterium was increased in both treatments. The increased abundance of genera with potentially beneficial bacteria, i.e., potential degraders of phenolic compounds might have contributed to the improved plant growth in both treatments.