Dominant Groups of Potentially Active Bacteria Shared by Barley Seeds become Less Abundant in Root Associated Microbiome

Structure and Diversity of Endophytic Bacteria in Maize Seeds and Germinating Roots

Seed endophytes in maize, which facilitate the transmission of microorganisms from one plant generation to the next, may play a crucial role in plant protection and growth promotion. This study aimed to investigate the effects of various maize varieties on the communities of endophytic bacteria in seeds and germinating roots. This study utilized Illumina high-throughput sequencing technology to examine the structural and diversity differences of endophytic bacterial communities within seed maize (BY1507), silage maize (QQ446), and wild maize (Teosinte) in both seeds and germinating roots. The results showed that 416 bacterial genera were detected, with Pantoea, Lachnospiraceae, Pararhizobium, Enterobacteriaceae, Stenotrophomonas, and Pseudonocardia being the most prevalent (relative abundance > 10%) at the genus level. No significant difference was observed in diversity indices (Chao1, ACE, Shannon, and Simpson) of seed endophytes among BY1507, QQ446, and Teosinte. The Shannon and Simpson indices for the germinating root endophyte from the wild variety (Teosinte) were significantly higher than the domesticated varieties (BY1507 and QQ446). PCoA revealed a notable overlap in the endophytic bacterial communities from the seeds of BY1507, QQ446, and Teosinte. Yet, clustering patterns were found. Co-occurrence network analysis showed that BY1507, QQ446, and Teosinte share a notable proportion of shared endophytic bacteria (>30%) between the seeds and germinating roots. This investigation elucidates the characteristics of endophytic microbial communities of seeds and germinating roots with seed maize, silage maize, and wild maize, offering data for future research on the physiological ecological adaptation of these endophytic microbial communities.

Seed bacterial microbiota in post-submergence tolerant and sensitive barley genotypes

Flooding is a predominant abiotic stress for cultivated plants, including barley. This cereal crop shows a large adaptability to different environmental conditions, suggesting the presence of key traits to tolerate adverse conditions. During germination, genetic variations account for dissimilarities in flooding tolerance. However, differences in the seed microbiota may also contribute to tolerance/sensitivity during seedling establishment. This work investigated differences in microbiome among the grains of barley accessions. Two barley phenotypes were compared, each either tolerant or sensitive to a short submergence period followed by a recovery. The study used a metataxonomic analysis based on 16S ribosomal RNA gene sequencing and subsequent functional prediction. Our results support the hypothesis that bacterial microbiota inhabiting the barley seeds are different between sensitive and tolerant barley accessions, which harbour specific bacterial phyla and families. Finally, bacteria detected in tolerant barley accessions show a peculiar functional enrichment that suggests a possible connection with successful germination and seedling establishment.

Seeds Act as Vectors for Antibiotic Resistance Gene Dissemination in a Soil-Plant Continuum

Though the evidence for antibiotic resistance spread via plant microbiome is mounting, studies regarding antibiotic resistome in the plant seed, a reproductive organ and important food resource, are still in their infancy. This study investigated the effects of long-term organic fertilization on seed bacterial endophytes, resistome, and their intergenerational transfer in the microcosm. A total of 99 antibiotic resistance genes (ARGs) and 26 mobile genetic elements (MGEs) were detected by high-throughput quantitative PCR. The amount of organic fertilizer applied was positively correlated to the number and relative abundance of seed-associated ARGs and MGEs. Moreover, the transmission of ARGs from the rhizosphere to the seed was mainly mediated by the shared bacteria and MGEs. Notably, the rhizosphere of progeny seedlings derived from seeds harboring abundant ARGs was found to have a higher relative abundance of ARGs. Using structural equation models, we further revealed that seed resistome and MGEs were key factors affecting the ARGs in the progeny rhizosphere, implying the seed was a potential resistome reservoir for rhizosphere soil. This study highlights the overlooked role of seed endophytes in the dissemination of resistome in the soil-plant continuum, and more attention should be paid to plant seeds as vectors of ARGs within the "One-Health" framework.

QTL for induced resistance against leaf rust in barley

Leaf rust caused by Puccinia hordei is one of the major diseases of barley (Hordeum vulgare L.) leading to yield losses up to 60%. Even though, resistance genes Rph1 to Rph28 are known, most of these are already overcome. In this context, priming may promote enhanced resistance to P. hordei. Several bacterial communities such as the soil bacterium Ensifer (syn. Sinorhizobium) meliloti are reported to induce resistance by priming. During quorum sensing in populations of gram negative bacteria, they produce N-acyl homoserine-lactones (AHL), which induce resistance in plants in a species- and genotype-specific manner. Therefore, the present study aims to detect genotypic differences in the response of barley to AHL, followed by the identification of genomic regions involved in priming efficiency of barley. A diverse set of 198 spring barley accessions was treated with a repaired E. meliloti natural mutant strain expR+ch producing a substantial amount of AHL and a transformed E. meliloti strain carrying the lactonase gene attM from Agrobacterium tumefaciens. For P. hordei resistance the diseased leaf area and the infection type were scored 12 dpi (days post-inoculation), and the corresponding relative infection and priming efficiency were calculated. Results revealed significant effects (p<0.001) of the bacterial treatment indicating a positive effect of priming on resistance to P. hordei. In a genome-wide association study (GWAS), based on the observed phenotypic differences and 493,846 filtered SNPs derived from the Illumina 9k iSelect chip, genotyping by sequencing (GBS), and exome capture data, 11 quantitative trait loci (QTL) were identified with a hot spot on the short arm of the barley chromosome 6H, associated to improved resistance to P. hordei after priming with E. meliloti expR+ch. Genes in these QTL regions represent promising candidates for future research on the mechanisms of plant-microbe interactions.

Single Seed Microbiota: Assembly and Transmission from Parent Plant to Seedling

The seed acts as the primary inoculum source for the plant microbiota. Understanding the processes involved in its assembly and dynamics during germination and seedling emergence has the potential to allow for the improvement of crop establishment. Changes in the bacterial community structure were tracked in 1,000 individual seeds that were collected throughout seed developments of beans and radishes. Seeds were associated with a dominant bacterial taxon that represented more than 75% of all reads. The identity of this taxon was highly variable between the plants and within the seeds of the same plant. We identified selection as the main ecological process governing the succession of dominant taxa during seed filling and maturation. In a second step, we evaluated the seedling transmission of seed-borne taxa in 160 individual plants. While the initial bacterial abundance on seeds was not a good predictor of seedling transmission, the identities of the seed-borne taxa modified the phenotypes of seedlings. Overall, this work revealed that individual seeds are colonized by a few bacterial taxa of highly variable identity, which appears to be important for the early stages of plant development. IMPORTANCE Seeds are key components of plant fitness and are central to the sustainability of the agri-food system. Both the seed quality for food consumption and the seed vigor in agricultural settings can be influenced by the seed microbiota. Understanding the ecological processes involved in seed microbiota assembly will inform future practices for promoting the presence of important seed microorganisms for plant health and productivity. Our results highlighted that seeds were associated with one dominant bacterial taxon of variable taxonomic identity. This variety of dominant taxa was due to (i) spatial heterogeneity between and within plants and (ii) primary succession during seed development. According to neutral models, selection was the main driver of microbial community assembly for both plant species.

Isolation of Saccharibacillus WB17 strain from wheat bran phyllosphere and genomic insight into the cellulolytic and hemicellulolytic complex of the Saccharibacillus genus

The microorganisms living on the phyllosphere (the aerial part of the plants) are in contact with the lignocellulosic plant cell wall and might have a lignocellulolytic potential. We isolated a Saccharibacillus strain (Saccharibacillus WB17) from wheat bran phyllosphere and its cellulolytic and hemicellulolytic potential was investigated during growth onto wheat bran. Five other type strains from that genus selected from databases were also cultivated onto wheat bran and glucose. Studying the chemical composition of wheat bran residues by FTIR after growth of the six strains showed an important attack of the stretching C-O vibrations assigned to polysaccharides for all the strains, whereas the C = O bond/esterified carboxyl groups were not impacted. The genomic content of the strains showed that they harbored several CAZymes (comprised between 196 and 276) and possessed four of the fifth modules reflecting the presence of a high diversity of enzymes families. Xylanase and amylase activities were the most active enzymes with values reaching more than 4746 ± 1400 mIU/mg protein for the xylanase activity in case of Saccharibacillus deserti KCTC 33693 T and 452 ± 110 mIU/mg protein for the amylase activity of Saccharibacillus WB17. The total enzymatic activities obtained was not correlated to the total abundance of CAZyme along that genus. The Saccharibacillus strains harbor also some promising proteins in the GH30 and GH109 modules with potential arabinofuranosidase and oxidoreductase activities. Overall, the genus Saccharibacillus and more specifically the Saccharibacillus WB17 strain represent biological tools of interest for further biotechnological applications.

Australian native Glycine clandestina seed microbiota hosts a more diverse bacterial community than the domesticated soybean Glycine max

BACKGROUND

Plant microbiome composition has been demonstrated to change during the domestication of wild plants and it is suggested that this has resulted in loss of plant beneficial microbes. Recently, the seed microbiome of native plants was demonstrated to harbour a more diverse microbiota and shared a common core microbiome with modern cultivars. In this study the composition of the seed-associated bacteria of Glycine clandestina is compared to seed-associated bacteria of Glycine max (soybean).

RESULTS

The seed microbiome of the native legume Glycine clandestina (crop wild relative; cwr) was more diverse than that of the domesticated Glycine max and was dominated by the bacterial class Gammaproteobacteria. Both the plant species (cwr vs domesticated) and individual seed accessions were identified as the main driver for this diversity and composition of the microbiota of all Glycine seed lots, with the effect of factor "plant species" exceeded that of "geographical location". A core microbiome was identified between the two Glycine species. A high percentage of the Glycine microbiome was unculturable [G. clandestina (80.8%) and G. max (75.5%)] with only bacteria of a high relative abundance being culturable under the conditions of this study.

CONCLUSION

Our results provided novel insights into the structure and diversity of the native Glycine clandestina seed microbiome and how it compares to that of the domesticated crop Glycine max. Beyond that, it also increased our knowledge of the key microbial taxa associated with the core Glycine spp. microbiome, both wild and domesticated. The investigation of this commonality and diversity is a valuable and essential tool in understanding the use of native Glycine spp. for the discovery of new microbes that would be of benefit to domesticated Glycine max cultivars or any other economically important crops. This study has isolated microbes from a crop wild relative that are now available for testing in G. max for beneficial phenotypes.

Deciphering the core seed endo-bacteriome of the highland barley in Tibet plateau

Highland barley (Hordeum vulgare var. nudum (L.) Hook.f., qingke) has unique physical and chemical properties and good potential for industrial applications. As the only crop that can be grown at high altitudes of 4200-4500 m, qingke is well adapted to extreme habitats at high altitudes. In this study, we analysed the seed bacterial community of 58 genotypes of qingke grown in different regions of Tibet, including qingke landraces, modern cultivars, and winter barley varieties, and characterised endophytic bacterial communities in seeds from different sources and the core endo-bacteriome of qingke. This study aim to provide a reference for the application of seed endophytes as biological inoculants for sustainable agricultural production and for considering microbe-plant interactions in breeding strategies. A total of 174 qingke seed samples from five main agricultural regions in Tibet were collected and subjected to investigation of endophytic endo-bacteriome using high-throughput sequencing and bioinformatics approaches. The phyla of endophytic bacteria in qingke seeds from different sources were similar; however, the relative proportions of each phylum were different. Different environmental conditions, growth strategies, and modern breeding processes have significantly changed the community structure of endophytic bacteria in seeds, among which the growth strategy has a greater impact on the diversity of endophytic bacteria in seeds. Seeds from different sources have conserved beneficial core endo-bacteriome. The core endo-bacteriome of qingke seeds dominated by Enterobacteriaceae may maintain qingke growth by promoting plant growth and assisting plants in resisting pests and diseases. This study reveals the core endo-bacteriome of qingke seeds and provides a basis for exploiting the endophytic endo-bacteriome of qingke seeds.

Dynamic changes in the endophytic bacterial community during maturation of Amorphophallus muelleri seeds

The seed microbiota is considered to be the starting point of the accumulation of plant microbiota, which is conducive to the preservation and germination of seeds and the establishment and development of seedlings. Our understanding of the colonization and migration dynamics of microbial taxa during seed development and maturation is still limited. This study used 16S rRNA high-throughput sequencing to investigate the dynamic changes in the composition and diversity of the endophytic bacterial community during maturation of Amorphophallus muelleri seeds. The results showed that as seeds matured (green to red), the Shannon index of their endophytic bacterial community first decreased and then increased, and the ACE and Chao1 indices of the endophytic bacterial community decreased gradually. The Shannon, ACE, and Chao1 indices of the endophytic bacterial community in the seed coat first decreased and then increased. Principal coordinate analysis of the bacterial communities revealed that the seed coat at different maturity stages showed significantly distinct bacterial communities and formed different clusters according to maturity stage. The bacterial communities of green and red seeds showed a clear separation, but they both overlapped with those of yellow seeds, indicating that some core taxa were present throughout seed maturation, but their relative abundance was dynamically changing. As the seeds grew more mature, the relative abundance of some bacterial communities with plant growth-promoting traits and others correlated with plant resistance (e.g., Burkholderia-Caballeronia-Paraburkholderia, Bacillus, Pseudomonas, Bradyrhizobium, Streptomyces) tended to increase and peaked in fully mature seeds and seed coats. The endophytic bacterial community of A. muelleri seeds seems to be driven by the seed maturation state, which can provide a theoretical basis for a comprehensive understanding of the assembly process of the microbial community during A. muelleri seed maturation.

Stochastic Inoculum, Biotic Filtering and Species-Specific Seed Transmission Shape the Rare Microbiome of Plants

A plant’s health and productivity is influenced by its associated microbes. Although the common/core microbiome is often thought to be the most influential, significant numbers of rare or uncommon microbes (e.g., specialized endosymbionts) may also play an important role in the health and productivity of certain plants in certain environments. To help identify rare/specialized bacteria and fungi in the most important angiosperm plants, we contrasted microbiomes of the seeds, spermospheres, shoots, roots and rhizospheres of Arabidopsis, Brachypodium, maize, wheat, sugarcane, rice, tomato, coffee, common bean, cassava, soybean, switchgrass, sunflower, Brachiaria, barley, sorghum and pea. Plants were grown inside sealed jars on sterile sand or farm soil. Seeds and spermospheres contained some uncommon bacteria and many fungi, suggesting at least some of the rare microbiome is vertically transmitted. About 95% and 86% of fungal and bacterial diversity inside plants was uncommon; however, judging by read abundance, uncommon fungal cells are about half of the mycobiome, while uncommon bacterial cells make up less than 11% of the microbiome. Uncommon-seed-transmitted microbiomes consisted mostly of Proteobacteria, Firmicutes, Bacteriodetes, Ascomycetes and Basidiomycetes, which most heavily colonized shoots, to a lesser extent roots, and least of all, rhizospheres. Soil served as a more diverse source of rare microbes than seeds, replacing or excluding the majority of the uncommon-seed-transmitted microbiome. With the rarest microbes, their colonization pattern could either be the result of stringent biotic filtering by most plants, or uneven/stochastic inoculum distribution in seeds or soil. Several strong plant–microbe associations were observed, such as seed transmission to shoots, roots and/or rhizospheres of Sarocladium zeae (maize), Penicillium (pea and Phaseolus), and Curvularia (sugarcane), while robust bacterial colonization from cassava field soil occurred with the cyanobacteria Leptolyngbya into Arabidopsis and Panicum roots, and Streptomyces into cassava roots. Some abundant microbes such as Sakaguchia in rice shoots or Vermispora in Arabidopsis roots appeared in no other samples, suggesting that they were infrequent, stochastically deposited propagules from either soil or seed (impossible to know based on the available data). Future experiments with culturing and cross-inoculation of these microbes between plants may help us better understand host preferences and their role in plant productivity, perhaps leading to their use in crop microbiome engineering and enhancement of agricultural production.

Assembly of root-associated N2O-reducing communities of annual crops is governed by selection for nosZ clade I over clade II

The rhizosphere is a hotspot for denitrification. The nitrous oxide (N₂O) reductase among denitrifiers and nondenitrifying N₂O reducers is the only known N₂O sink in the biosphere. We hypothesized that the composition of root-associated N₂O-reducing communities when establishing on annual crops depend on soil type and plant species, but that assembly processes are independent of these factors and differ between nosZ clades I and II. Using a pot experiment with barley and sunflower and two soils, we analyzed the abundance, composition, and diversity of soil and root-associated N₂O reducing communities by qPCR and amplicon sequencing of nosZ. Clade I was more abundant on roots compared to soil, while clade II showed the opposite. In barley, this pattern coincided with N₂O availability, determined as potential N₂O production rates, but for sunflower no N₂O production was detected in the root compartment. Root and soil nosZ communities differed in composition and phylogeny-based community analyses indicated that assembly of root-associated N₂O reducers was driven by the interaction between plant and soil type, with inferred competition being more influential than habitat selection. Selection between clades I and II in the root/soil interface is suggested, which may have functional consequences since most clade I microorganisms can produce N₂O.

The Role of Soil Microbial Diversity in the Conservation of Native Seed Bacterial Microbiomes

Research into understanding the structure, composition and vertical transmission of crop seed microbiomes has intensified, although there is much less research into the seed microbiomes of crop wild relatives. Our previous study showed that the standard seed storage procedures (e.g., seed drying and storage temperature) can influence the seed microbiome of domesticated Glycine max. In this study, we characterized the seed microbiota of Glycine clandestina, a perennial wild relative of soybean (G. max (L.) Merr.) to expand our understanding about the effect of other storage procedures such as the periodic regeneration of seed stocks to bulk up seed numbers and secure viability on the seed microbiome of said seed. The G. clandestina microbiota was analysed from Generation 1 (G1) and Generation 2 (G2) seed and from mature plant organs grown in two different soil treatments T (treatment [native soil + potting mix]) and C (control [potting mix only]). Our dataset showed that soil microbiota had a strong influence on next generation seed microbiota, with an increased contribution of root microbiota by 90% and seed transmissibility by 36.3% in G2 (T) seed. Interestingly, the G2 seed microbiota primarily consisted of an initially low abundance of taxa present in G1 seed. Overall, our results indicate that seed regeneration can affect the seed microbiome composition and using native soil from the location of the source plant can enhance the conservation of the native seed microbiota.

Assembly and potential transmission of the lens culinaris seed microbiome

Soil is an important source of bacteria and fungi for the plant, but seeds can also provide microbial inocula through heritable or stochastic assembly. Seed-associated microbial communities can potentially interact with the host plant through multiple generations. Here, we assessed the impact of two different soil types on the seed microbiome assembly of seven lentil (Lens culinaris) genotypes under environmentally controlled conditions and examined the vertical transmission of bacterial communities from seed to seed across two generations. Bulk soil microbiomes and seed microbiomes were characterized using high-throughput amplicon sequencing of the bacterial 16S rRNA gene. Our results revealed that bacterial communities in the two soils differed significantly and that bacterial communities associated with seeds were significantly impacted by genotype (15%) in one of the soils. Co-occurrence of amplicon sequence variants (ASVs) between generations suggests members of the genera Cutibacterium, Methylobacterium, Sphingomonas, Streptococcus, and Tepidimonas are transmitted and preserved in lentil genotypes irrespective of the soil in which they were grown. Increasing our knowledge of how microbial communities carried by seeds are assembled, transmitted, and preserved offers a promising way for future breeding programs to consider microbial communities when selecting for more resilient and productive cultivars.

Seed-Transmitted Bacteria and Fungi Dominate Juvenile Plant Microbiomes

Plant microbiomes play an important role in agricultural productivity, but there is still much to learn about their provenance, diversity, and organization. In order to study the role of vertical transmission in establishing the bacterial and fungal populations of juvenile plants, we used high-throughput sequencing to survey the microbiomes of seeds, spermospheres, rhizospheres, roots, and shoots of the monocot crops maize (B73), rice (Nipponbare), switchgrass (Alamo), Brachiaria decumbens, wheat, sugarcane, barley, and sorghum; the dicot crops tomato (Heinz 1706), coffee (Geisha), common bean (G19833), cassava, soybean, pea, and sunflower; and the model plants Arabidopsis thaliana (Columbia-0) and Brachypodium distachyon (Bd21). Unsterilized seeds were planted in either sterile sand or farm soil inside hermetically sealed jars, and after as much as 60 days of growth, DNA was extracted to allow for amplicon sequence-based profiling of the bacterial and fungal populations that developed. Seeds of most plants were dominated by Proteobacteria and Ascomycetes, with all containing operational taxonomic units (OTUs) belonging to Pantoea and Enterobacter. All spermospheres also contained DNA belonging to Pseudomonas, Bacillus, and Fusarium. Despite having only seeds as a source of inoculum, all plants grown on sterile sand in sealed jars nevertheless developed rhizospheres, endospheres, and phyllospheres dominated by shared Proteobacteria and diverse fungi. Compared to sterile sand-grown seedlings, growth on soil added new microbial diversity to the plant, especially to rhizospheres; however, all 63 seed-transmitted bacterial OTUs were still present, and the most abundant bacteria (Pantoea, Enterobacter, Pseudomonas, Klebsiella, and Massilia) were the same dominant seed-transmitted microbes observed in sterile sand-grown plants. While most plant mycobiome diversity was observed to come from soil, judging by read abundance, the dominant fungi (Fusarium and Alternaria) were also vertically transmitted. Seed-transmitted fungi and bacteria appear to make up the majority of juvenile crop plant microbial populations by abundance, and based on occupancy, there seems to be a pan-angiosperm seed-transmitted core bacterial microbiome. Further study of these seed-transmitted microbes will be important to understand their role in plant growth and health, as well as their fate during the plant life cycle and may lead to innovations for agricultural inoculant development.

The treasure inside barley seeds: microbial diversity and plant beneficial bacteria

BACKGROUND

Bacteria associated with plants can enhance the plants' growth and resistance against phytopathogens. Today, growers aim to reduce the use of mineral fertilizers and pesticides. Since phytopathogens cause severe yield losses in crop production systems, biological alternatives gain more attention. Plant and also seed endophytes have the potential to influence the plant, especially seed-borne bacteria may express their beneficiary impact at initial plant developmental stages. In the current study, we assessed the endophytic seed microbiome of seven genetically diverse barley accessions by 16S rRNA gene amplicon sequencing and verified the in vitro plant beneficial potential of isolated seed endophytes. Furthermore, we investigated the impact of the barley genotype and its seed microbiome on the rhizosphere microbiome at an early growth stage by 16S rRNA gene amplicon sequencing.

RESULTS

The plant genotype displayed a significant impact on the microbiota in both barley seed and rhizosphere. Consequently, the microbial alpha- and beta-diversity of the endophytic seed microbiome was highly influenced by the genotype. Interestingly, no correlation was observed between the endophytic seed microbiome and the single nucleotide polymorphisms of the seven genotypes. Unclassified members of Enterobacteriaceae were by far most dominant. Other abundant genera in the seed microbiome belonged to Curtobacterium, Paenibacillus, Pantoea, Sanguibacter and Saccharibacillus. Endophytes isolated from barley seeds were affiliated to dominant genera of the core seed microbiome, based on their 16S rRNA gene sequence. Most of these endophytic isolates produced in vitro plant beneficial secondary metabolites known to induce plant resistance.

CONCLUSION

Although barley accessions representing high genetic diversity displayed a genotype-dependent endophytic seed microbiome, a core seed microbiome with high relative abundances was identified. Endophytic isolates were affiliated to members of the core seed microbiome and many of them showed plant beneficial properties. We propose therefore that new breeding strategies should consider genotypes with high abundance of beneficial microbes.

Transmission of Seed and Soil Microbiota to Seedling

The seed microbial community constitutes an initial inoculum for plant microbiota assembly. Still, the persistence of seed microbiota when seeds encounter soil during plant emergence and early growth is barely documented. We characterized the encounter event of seed and soil microbiota and how it structured seedling bacterial and fungal communities by using amplicon sequencing. We performed eight contrasting encounter events to identify drivers influencing seedling microbiota assembly. To do so, four contrasting seed lots of two Brassica napus genotypes were sown in two soils whose microbial diversity levels were manipulated by serial dilution and recolonization. Seedling root and stem microbiota were influenced by soil but not by initial seed microbiota composition or by plant genotype. A strong selection on the seed and soil communities occurred during microbiota assembly, with only 8% to 32% of soil taxa and 0.8% to 1.4% of seed-borne taxa colonizing seedlings. The recruitment of seedling microbiota came mainly from soil (35% to 72% of diversity) and not from seeds (0.3% to 15%). Soil microbiota transmission success was higher for the bacterial community than for the fungal community. Interestingly, seedling microbiota was primarily composed of initially rare taxa (from seed, soil, or unknown origin) and intermediate-abundance soil taxa.

IMPORTANCE Seed microbiota can have a crucial role for crop installation by modulating dormancy, germination, seedling development, and recruitment of plant symbionts. Little knowledge is available on the fraction of the plant microbiota that is acquired through seeds. We characterize the encounter between seed and soil communities and how they colonize the seedling together. Transmission success and seedling community assemblage can be influenced by the variation of initial microbial pools, i.e., plant genotype and cropping year for seeds and diversity level for soils. Despite a supposed resident advantage of the seed microbiota, we show that transmission success is in favor of the soil microbiota. Our results also suggest that successful plant-microbiome engineering based on native seed or soil microbiota must include rare taxa.

Seed-borne, endospheric and rhizospheric core microbiota as predictors of plant functional traits across rice cultivars are dominated by deterministic processes

A host-plant and its associated microbiota depend on one another. However, the assembly process and the functioning of host-associated microbiota are poorly understood. Herein, rice was used as model plant to investigate the assemblage of bacterial microbiota, including those in the seed, root endosphere and rhizosphere. We also assessed the degree to which endosphere and rhizosphere communities were influenced by vertical transmission through seed and identified the core microbes that potentially associated with plant phenotypic properties. Plant microhabitat, rather than subspecies type, was the major driver shaping plant-associated bacterial microbiota. Deterministic processes were primarily responsible for community assembly in all microhabitats. The influence of vertical transmission from seed to root-associated bacterial communities appeared to be quite weak (endosphere) or even absent (rhizosphere). A core microbial community composed of 15 generalist species persisted across different microhabitats and represented key connectors in networks. Host-plant functional traits were linked to the relative abundance of these generalist core microbes and could be predicted from them using machine learning algorithms. Overall, bacterial microbiota is assembled by host-plant interactions in a deterministic-based manner. This study enhances our understanding of the driving mechanisms and associations of microbiota in various plant microhabitats and provides new perspectives to improve plant performance.

Temporal dynamics of bacterial communities during seed development and maturation

Seed microbiota acts as a starting point for the assembly of the plant microbiota and contributes to successful plant establishment. To date, the order and timing of microbial taxa immigration during seed development and maturation remained unknown. We investigated the temporal dynamics of seed bacterial communities in bean and radish. A high phylogenetic turnover was observed for both plant species with few taxa associated with all seed developmental stages. Greater heterogeneity in communities structure within each stage was observed for radish. While, about one-third of radish seed bacterial taxa were detected in buds, flowers and fruits, very few taxa seem to be transmitted by the floral route in bean. In the latter species, bacterial populations belonging to the P. fluorescens species complex were found either in buds, flowers and fruits or in seeds. The relative phylogenetic proximity of these bacterial populations combined with their habitat specificity led us to explore the genetic determinants involved in successful seed transmission in bean. Comparative genomic analyses of representatives bacterial strains revealed dozens of coding sequences specifically associated with seed-transmitted strains. This study provided a first glimpse on processes involved in seed microbiota assembly, which could be used for designing plant-beneficial microbial consortia.

Quality of Immature and Mature Pepper (Capsicum annuum L.) Seeds in Relation to Bio-Priming with Endophytic Pseudomonas and Bacillus spp

Fruit maturity for seed production can occur at various times because of the continual flowering of pepper plants. Accordingly, seeds with different maturity are acquired as the fruits are collected in a single harvest. Immature seeds obtained in this harvest may lead to a decrease in the quality of seed lots. Therefore, this research aimed to evaluate the influence of four different endophytic bacteria strains (Pseudomonas fluorescens strain L5b, Pseudomonas gessardii strain L13, Bacillus subtilis strain Bs1 and Bacillus mojavensis strain ApBm) on germination and seedling vigor of immature and mature bell pepper seeds. To obtain seeds with different maturity levels, fruits were collected 45–49 days after flowering for immature seeds and 65–69 days for mature seeds. The effectiveness of these bacteria strains was examined by coating seeds with four different endophytic bacteria strains separately. Additionally, to see the activity of endophytic bacteria more clearly, a mock treatment with sterile water was added to the experiment as a control (+) group. Bio-priming (especially strain Bs1 and L13) improved germination and seedling emergence characteristics of both immature and mature seed lots compared to control groups (p < 0.05). The results demonstrate that bio-priming with beneficial endophytic bacteria can be used to stimulate the quality of both immature and mature seeds from the pepper.

Paenibacillus polymyxa, a Jack of all trades

The bacterium Paenibacillus polymyxa is found naturally in diverse niches. Microbiome analyses have revealed enrichment in the genus Paenibacillus in soils under different adverse conditions, which is often accompanied by improved growth conditions for residing plants. Furthermore, Paenibacillus is a member of the core microbiome of several agriculturally important crops, making its close association with plants an interesting research topic. This review covers the versatile interaction possibilities of P. polymyxa with plants and its applicability in industry and agriculture. Thanks to its array of produced compounds and traits, P. polymyxa is likely an efficient plant growth-promoting bacterium, with the potential of biofertilization, biocontrol and protection against abiotic stresses. By contrast, cases of phytotoxicity of P. polymyxa have been described as well, in which growth conditions seem to play a key role. Because of its adjustable character, we propose this bacterial species as an outstanding model for future studies on host-microbe communications and on the manner how the environment can influence these interactions.

Domestication affects the composition, diversity, and co-occurrence of the cereal seed microbiota

Introduction

The seed-associated microbiome has a strong influence on plant ecology, fitness, and productivity. Plant microbiota could be exploited for a more responsible crop management in sustainable agriculture. However, the relationships between seed microbiota and hosts related to the changes from ancestor species to breeded crops still remain poor understood.

Objectives

Our aims were i) to understand the effect of cereal domestication on seed endophytes in terms of diversity, structure and co-occurrence, by comparing four cereal crops and the respective ancestor species; ii) to test the phylogenetic coherence between cereals and their seed microbiota (clue of co-evolution).

Methods

We investigated the seed microbiota of four cereal crops (Triticum aestivum, Triticum monococcum, Triticum durum, and Hordeum vulgare), along with their respective ancestors (Aegilops tauschii, Triticum baeoticum, Triticum dicoccoides, and Hordeum spontaneum, respectively) using 16S rRNA gene metabarcoding, Randomly Amplified Polymorphic DNA (RAPD) profiling of host plants and co-evolution analysis.

Results

The diversity of seed microbiota was generally higher in cultivated cereals than in wild ancestors, suggesting that domestication lead to a bacterial diversification. On the other hand, more microbe-microbe interactions were detected in wild species, indicating a better-structured, mature community. Typical human-associated taxa, such as Cutibacterium, dominated in cultivated cereals, suggesting an interkingdom transfers of microbes from human to plants during domestication. Co-evolution analysis revealed a significant phylogenetic congruence between seed endophytes and host plants, indicating clues of co-evolution between hosts and seed-associated microbes during domestication.

Conclusion

This study demonstrates a diversification of the seed microbiome as a consequence of domestication, and provides clues of co-evolution between cereals and their seed microbiota. This knowledge is useful to develop effective strategies of microbiome exploitation for sustainable agriculture.

A novel PCR-clamping assay reducing plant host DNA amplification significantly improves prokaryotic endo-microbiome community characterization

Due to the sequence homology between the bacterial 16S rRNA gene and plant chloroplast and mitochondrial DNA, the taxonomic characterization of plant microbiome using amplicon-based high throughput sequencing often results in the overwhelming presence of plant-affiliated reads, preventing the thorough description of plant-associated microbial communities. In this work we developed a PCR blocking primer assay targeting the taxonomically informative V5-V6 region of the 16S rRNA gene in order to reduce plant DNA co-amplification, and increase diversity coverage of associated prokaryotic communities. Evaluation of our assay on the characterization of the prokaryotic endophytic communities of Zea mays, Pinus taeda and Spartina alternifora leaves led to significantly reducing the proportion of plant reads, yielded 20 times more prokaryotic reads and tripled the number of detected OTUs compared to a commonly used V5-V6 PCR protocol. To expand the application of our PCR-clamping assay across a wider taxonomic spectrum of plant hosts, we additionally provide an alignment of chloroplast and mitochondrial DNA sequences encompassing more than 200 terrestrial plant families as a supporting tool for customizing our blocking primers.

Seed-Derived Microbial Colonization of Wild Emmer and Domesticated Bread Wheat (Triticum dicoccoides and T. aestivum) Seedlings Shows Pronounced Differences in Overall Diversity and Composition

The composition of the plant microbiota may be altered by ecological and evolutionary changes in the host population. Seed-associated microbiota, expected to be largely vertically transferred, have the potential to coadapt with their host over generations. Strong directional selection and changes in the genetic composition of plants during domestication and cultivation may have impacted the assembly and transmission of seed-associated microbiota. Nonetheless, the effect of plant speciation and domestication on the composition of these microbes is poorly understood. Here, we have investigated the composition of bacteria and fungi associated with the wild emmer wheat (Triticum dicoccoides) and domesticated bread wheat (Triticum aestivum). We show that vertically transmitted bacteria, but not fungi, of domesticated bread wheat species T. aestivum are less diverse and more inconsistent among individual plants compared to those of the wild emmer wheat species T. dicoccoides. We propagated wheat seeds under sterile conditions to characterize the colonization of seedlings by seed-associated microbes. Hereby, we show markedly different community compositions and diversities of leaf and root colonizers of the domesticated bread wheat compared to the wild emmer wheat. By propagating the wild emmer wheat and domesticated bread wheat in two different soils, we furthermore reveal a small effect of plant genotype on microbiota assembly. Our results suggest that domestication and prolonged breeding have impacted the vertically transferred bacteria, but only to a lesser extent have affected the soil-derived microbiota of bread wheat.IMPORTANCE Genetic and physiological changes associated with plant domestication have been studied for many crop species. Still little is known about the impact of domestication on the plant-associated microbiota. In this study, we analyze the seed-associated and soil-derived bacterial and fungal microbiota of domesticated bread wheat and wild emmer wheat. We show a significant difference in the seed-associated, but not soil-derived, bacterial communities of the wheat species. Interestingly, we find less pronounced effects on the fungal communities. Overall, this study provides novel insight into the diversity of vertically transmitted microbiota of wheat and thereby contributes to our understanding of wheat as a "metaorganism." Insight into the wheat microbiota is of fundamental importance for the development of improved crops.

Response of Barley Plants to Drought Might Be Associated with the Recruiting of Soil-Borne Endophytes

Mechanisms used by plants to respond to water limitation have been extensively studied. However, even though the inoculation of beneficial microbes has been shown to improve plant performance under drought stress, the inherent role of soil microbes on plant response has been less considered. In the present work, we assessed the importance of the soil microbiome for the growth of barley plants under drought stress. Plant growth was not significantly affected by the disturbance of the soil microbiome under regular watering. However, after drought stress, we observed a significant reduction in plant biomass, particularly of the root system. Plants grown in the soil with disturbed microbiome were significantly more affected by drought and did not recover two weeks after re-watering. These effects were accompanied by changes in the composition of endophytic fungal and bacterial communities. Under natural conditions, soil-derived plant endophytes were major colonizers of plant roots, such as Glycomyces and Fusarium, whereas, for plants grown in the soil with disturbed microbiome seed-born bacterial endophytes, e.g., Pantoea, Erwinia, and unclassified Pseudomonaceae and fungal genera normally associated with pathogenesis, such as Gibberella and Gaeumannomyces were observed. Therefore, the role of the composition of the indigenous soil microbiota should be considered in future approaches to develop management strategies to make plants more resistant towards abiotic stress, such as drought.

The differences and overlaps in the seed-resident microbiome of four Leguminous and three Gramineous forages

Given the important roles that seed-borne endophytes can play on their plant hosts, comprehensive studies of the bacterial and fungal communities of seeds are of great importance. In this study, we assessed the seed endophytes of three gramineous (Avena sativa, Elymus sibiricus and Elymus dahuricus) and four leguminous (Vicia villosa, Trifolium repens, Trifolium pretense and Medicago sativa) forages using high-throughput sequencing. In total, 1013 distinct bacterial operational taxonomic units (OTUs) and 922 fungal OTUs were detected, with bacteria and fungi per sample ranging from 240 to 425 and 261 to 463 respectively. These seven forages shared a high number of potentially beneficial taxa, including Bacillus, Pantoea, Candida and Helotiales, but the relative proportion of these taxa was different in each seed. Fungal communities were clustered more distinctively by host genotypes than bacterial. Some bacterial taxa may be involved in the recruitment of genera from the same phylum. Three Pantoea sp. and five Bacillus sp. were isolated from seeds, and all showed positive effects on Medicago sativa germination rate under salt stress, and of these, Bacillus subtilis Es-1 and Pantoea agglomerans Ed-3 performed best, but their influence was affected by the seed's microbiome. Rather than simply promoting host plant growth directly, some taxa may also participate in organizing the assembly of plant microbiomes which will influence seed response to biological factors. This study uses a new, high-throughput sequencing based strategy to identify beneficial strains and analyse the interactions between microorganisms and plants to maximize microbial functions in long-term agricultural practices.

Impact of seed-transmitted endophytic bacteria on intra- and inter-cultivar plant growth promotion modulated by certain sets of metabolites in rice crop

Exploring the beneficial interactions between plant and endophytes could be an effective strategy in the implementation of sustainable agricultural practices to enhance crop productivity. In this study, we aimed to evaluate holistically the plant growth promoting (PGP) abilities rendered by seed-transmitted endophytic bacteria isolated from in vitro grown calli of two rice cultivars. Nine bacterial endophytes, designated as PB001-PB009, were isolated and identified at the genus level through 16S rRNA gene sequence analysis. Biochemical investigations disclosed that they possess several PGP traits, such as phosphate solubilization, indole acetic acid biosynthesis, ammonia production, nitrogen fixation, amylase production and siderophore production. Results in gnotobiotic conditions revealed an increase in fresh weight, dry weight, root length and shoot length of seedlings germinated from endophyte-primed seeds than the control (uninoculated) set in a non-host and two host rice cultivars. In net house experiments, plants germinated from Micrococcus sp. PB001, Pseudomonas sp. PB002, Methylobacterium sp. PB005 and Methylorubrum sp. PB009 primed seeds showed an increase of upto 34.06 %, 38.77 %, 182.87 %, 16.59 % and 33.52 % in chlorophyll content, number of tillers/plant, number of grains/plant, grain size and grain weight, respectively than control plant sets in the non-host rice cultivar, further validating inter-cultivar PGP abilities of these endophytes. Metabolite profiling unfolded the abundance of few metabolites that are involved in pathways associated with PGP traits, in seedlings germinated from the endophyte-primed seeds. Together, the study documents the effect of seed-transmitted endophytic bacteria on intra- and inter-cultivar PGP by modulating certain sets of metabolites in rice plant, and is promising in developing bioinoculant formulations employing these selected endophytes for enhancement of rice productivity.

Pioneer trees of Betula pendula at a red gypsum landfill harbour specific structure and composition of root-associated microbial communities

The study of root-associated microbial communities is important to understand the natural processes involved in plant recolonisation at degraded areas. Root associated bacterial and fungal communities of woody species colonising a red gypsum landfill (a metal-enriched environment) were characterised through metabarcoding. Among trees naturally growing on the landfill, Betula pendula is the only tree species in the centre of the area, whereas companion tree species such as Populus nigra, P. tremula and Salix purpurea were present on the edges. The bacterial community was dominated by Proteobacteria (38%), Actinobacteria (35%) and Bacteroidetes (20%) and the most abundant bacterial OTU belonged to the family Streptomycetaceae. The fungal community was dominated by Ascomycota (60%) and Basidiomycota (30%) and the most abundant family was Pyronemataceae. Analysis of similarities, heatmap and hierarchical cluster analysis showed that B. pendula grown in the centre of the landfill harboured a specific microbial community, which was unique and different, not only from other tree species (Populus or Salix spp.), but also from other B. pendula growing at the edges. Our findings on relevant indicator OTUs associated to the birches located in the centre of the landfill (such as Otu00716 Catellatospora sp. (family Micromonosporaceae, phylum Actinobacteria) or Otu4_35502 Russula sp. (family Russulaceae, phylum Basidiomycota)) may have important implications for the successful revegetation of these harsh environments using microbial-based phytostabilisation approaches.

Effects of continuous cropping of sugar beet (Beta vulgaris L.) on its endophytic and soil bacterial community by high-throughput sequencing

Purpose

As a major sugar crop, sugar beet (Beta vulgaris L.) plays an important role in both sugar industry and feed products. Soil, acts as the substrate for plant growth, provides not only nutrients to plants but also a habitat for soil microorganisms. High soil fertility and good micro-ecological environment are basic requirements for obtaining high-yield and high-sugar sugar beets. This study aimed at exploring the effects of continuous cropping of sugar beet on its endophytic, soil bacterial community structures, and diversity.

Methods

Using high-throughput sequencing technology which is based on Illumina Hiseq 2500 platform, the seeds of sugar beet (sample S), non-continuous cropping sugar beet (sample Bn) with its rhizosphere soil (sample Sr), and planting soil (sample Sn), continuous cropping sugar beet (sample Bc) with its planting soil (sample Sc), were collected as research materials.

Result

The results showed that the bacterial communities and diversity in each sample exhibited different OTU richness; 67.9% and 63.8% of total endophytic OTUs from samples Bc and Bn shared with their planting soil samples Sc and Sn, while sharing 36.4% and 31.8% of total OTUs with their seed sample S. Pseudarthrobacter and Bacillus as the two major groups coexisted among all samples, and other shared groups belonged to Achromobacter, Sphingomonas, Novosphingobium, Terribacillus, Planococcus, Paracoccus, Nesterenkonia, Halomonas, and Nocardioides. Genera, including Pantoea, Pseudomonas, Stenotrophomonas, Weissella, Leuconostoc, and Acinetobacter, were detected in each sugar beet sample but not in their corresponding soil sample. In this study, the bacterial community structures and soil compositions have significantly changed before and after continuous cropping; however, the effects of continuous cropping on endophytic bacteria of sugar beet were not statistically significant.

Conclusion

This study would provide a scientific basis and reference information for in-depth research on correlations between continuous cropping and micro-ecological environment of sugar beet plant.

Analogous wheat root rhizosphere microbial successions in field and greenhouse trials in the presence of biocontrol agents Paenibacillus peoriae SP9 and Streptomyces fulvissimus FU14

Two Pythium-infested soils were used to compare the wheat root and rhizosphere soil microbial communities from plants grown in the field or in greenhouse trials and their stability in the presence of biocontrol agents. Bacteria showed the highest diversity at early stages of wheat growth in both field and greenhouse trials, while fungal diversity increased later on, at 12 weeks of the crop cycle. The microbial communities were stable in roots and rhizosphere samples across both soil types used in this study. Such stability was also observed irrespective of the cultivation system (field or greenhouse) or addition of biocontrol coatings to wheat seeds to control Pythium disease (in this study soil infected with Pythium sp. clade F was tested). In greenhouse plant roots, Archaeorhizomyces, Debaryomyces, Delftia, and unclassified Pseudeurotiaceae were significantly reduced when compared to plant roots obtained from the field trials. Some operational taxonomic units (OTUs) represented genetic determinants clearly transmitted vertically by seed endophytes (specific OTUs were found in plant roots) and the plant microbiota was enriched over time by OTUs from the rhizosphere soil. This study provided key information regarding the microbial communities associated with wheat roots and rhizosphere soils at different stages of plant growth and the role that Paenibacillus and Streptomyces strains play as biocontrol agents in supporting plant growth in infested soils.

AgNO3 Sterilizes Grains of Barley (Hordeum vulgare) without Inhibiting Germination-A Necessary Tool for Plant-Microbiome Research

To understand and manipulate the interactions between plants and microorganisms, sterile seeds are a necessity. The seed microbiome (inside and surface microorganisms) is unknown for most plant species and seed-borne microorganisms can persist and transfer to the seedling and rhizosphere, thereby obscuring the effects that purposely introduced microorganisms have on plants. This necessitates that these unidentified, seed-borne microorganisms are removed before seeds are used for studies on plant–microbiome interactions. Unfortunately, there is no single, standardized protocol for seed sterilization, hampering progress in experimental plant growth promotion and our study shows that commonly applied sterilization protocols for barley grains using H₂O₂, NaClO, and AgNO₃ yielded insufficient sterilization. We therefore developed a sterilization protocol with AgNO₃ by testing several concentrations of AgNO₃ and added two additional steps: Soaking the grains in water before the sterilization and rinsing with salt water (1% (w/w) NaCl) after the sterilization. The most efficient sterilization protocol was to soak the grains, sterilize with 10% (w/w) AgNO₃, and to rinse with salt water. By following those three steps, 97% of the grains had no culturable, viable microorganism after 21 days based on microscopic inspection. The protocol left small quantities of AgNO₃ residue on the grain, maintained germination percentage similar to unsterilized grains, and plant biomass was unaltered. Hence, our protocol using AgNO₃ can be used successfully for experiments on plant–microbiome interactions.

Bioinformatics analysis of endophytic bacteria related to berberine in the Chinese medicinal plant Coptis teeta Wall

Endophytic microorganisms absorb nutrients and prevent pathogen damage, supporting healthy plant growth. However, the relationship between endophytic bacteria and berberine synthesis in the medicinal plant Coptis teeta Wall. remains unclear. Herein, we explored the community composition of endophytic bacteria related to berberine in roots, stems, and leaves of wild-type and cultivated C. teeta. Endophytic bacterial communities were analyzed by 16S rRNA sequencing, and berberine content in roots was analyzed by high-performance liquid chromatography. Proteobacteria, Actinobacteria, and Bacteroidetes were the major phyla, and Mycobacterium, Salmonella, Nocardioides, Burkholderia-Paraburkholderia, and Rhizobium were the dominant genera in root, stem, and leaf tissues. Root berberine content was positively correlated with total N, total P, total K, and available K in rhizosphere soil. In addition, root berberine content was positively correlated with Microbacterium and norank_f_7B-8, whereas soil total K was positively correlated with Microbacterium and Burkholderia-Paraburkholderia in roots. Our results demonstrated a clear correlation between dominant endophytic bacteria and berberine synthesis in C. teeta. The findings are useful for the promotion of berberine production in C. teeta via manipulation of endophytic bacteria.

Hydrogen Peroxide Metabolism in Interkingdom Interaction Between Bacteria and Wheat Seeds and Seedlings

In endophytes, the abundance of genes coding for enzymes processing reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂), argues for a crucial role of ROS metabolism in plant-microbe interaction for plant colonization. Here, we studied H₂O₂ metabolism of bread wheat (Triticum aestivum L.) seeds and their microbiota during germination and early seedling growth, the most vulnerable stages in the plant life cycle. Treatment with hot steam diminished the seed microbiota, and these seeds produced less extracellular H₂O₂ than untreated seeds. Using a culture-dependent approach, Pantoea and Pseudomonas genera were the most abundant epiphytes of dry untreated seeds. Incubating intact seedlings from hot steam-treated seeds with Pantoea strains triggered H₂O₂ production, whereas Pseudomonas strains dampened H₂O₂ levels, attributable to higher catalase activities. The genus Pantoea was much less represented among seedling endophytes than genus Pseudomonas, with other endophytic genera, including Bacillus and Paenibacillus, also possessing high catalase activities. Overall, our results show that certain bacteria of the seed microbiota are able to modulate the extracellular redox environment during germination and early seedling growth, and high catalase activity is proposed as a key trait of seed endophytes.

Investigating the endophytic bacterial diversity and community structures in seeds of genetically related maize (Zea mays L.) genotypes

This research aimed to investigate the composition and diversity of endophytic bacterial community in seeds of four hybrid maize and their parental lines, which was used to reveal the potential relationship and association of endophytic bacteria between maize genotypes and their genetic relevance. High-throughput sequencing (HTS) technology showed that a total of 1419 OTUs (46.6%) were parental lines unique and 1052 OTUs (34.5%) were hybrid varieties unique, with only 575 core OTUs revealed in all the samples. Most OTUs belonged to Proteobacteria. Enterobacter (23.2%), Shigella (21.2%), Pseudomonas (15.8%) and Achromobacter (10.1%) were the major genera; the bacterial community composition and diversity of endophytic bacteria were inconsistent among different seed genotypes. Based on principal component analysis (PCA), the results referred that the endophytic composition of hybrid sample showed obvious correlation with their female parental lines, and in 'Jingke968' and 'MC738' with the same female line the endophytic community was more similar than other hybrid samples. This was the first ever use of HTS technology for investigating the endophytic bacterial diversity and community structures in seeds of genetically related maize genotypes, it was shown that, there were core microbes shared among all genotypes of seed samples, and the female parental line was more significant to impact on the composition of their hybrid seeds than male parental line. This study would provide scientific clues for the future research on the vertical transmission of endophytes among maize generations.

Discussion paper: Sustainable increase of crop production through improved technical strategies, breeding and adapted management - A European perspective

During the next decade it will be necessary to develop novel combinations of management strategies to sustainably increase crop production and soil resilience. Improving agricultural productivity, while conserving and enhancing biotic and abiotic resources, is an essential requirement to increase global food production on a sustainable basis. The role of farmers in increasing agricultural productivity growth sustainably will be crucial. Farmers are at the center of any process of change involving natural resources and for this reason they need to be encouraged and guided, through appropriate incentives and governance practices, to conserve natural ecosystems and their biodiversity, and minimize the negative impact agriculture can have on the environment. Farmers and stakeholders need to revise traditional approaches not as productive as the modern approaches but more friendly with natural and environmental ecosystems values as well as emerging novel tools and approaches addressing precise farming, organic amendments, lowered water consumption, integrated pest control and beneficial plant-microbe interactions. While practical solutions are developing, science based recommendations for crop rotations, breeding and harvest/postharvest strategies leading to environmentally sound and pollinator friendly production and better life in rural areas have to be provided.

Differences in resource use lead to coexistence of seed-transmitted microbial populations

Seeds are involved in the vertical transmission of microorganisms in plants and act as reservoirs for the plant microbiome. They could serve as carriers of pathogens, making the study of microbial interactions on seeds important in the emergence of plant diseases. We studied the influence of biological disturbances caused by seed transmission of two phytopathogenic agents, Alternaria brassicicola Abra43 (Abra43) and Xanthomonas campestris pv. campestris 8004 (Xcc8004), on the structure and function of radish seed microbial assemblages, as well as the nutritional overlap between Xcc8004 and the seed microbiome, to find seed microbial residents capable of outcompeting this pathogen. According to taxonomic and functional inference performed on metagenomics reads, no shift in structure and function of the seed microbiome was observed following Abra43 and Xcc8004 transmission. This lack of impact derives from a limited overlap in nutritional resources between Xcc8004 and the major bacterial populations of radish seeds. However, two native seed-associated bacterial strains belonging to Stenotrophomonas rhizophila displayed a high overlap with Xcc8004 regarding the use of resources; they might therefore limit its transmission. The strategy we used may serve as a foundation for the selection of seed indigenous bacterial strains that could limit seed transmission of pathogens.

Decoding Wheat Endosphere-Rhizosphere Microbiomes in Rhizoctonia solani-Infested Soils Challenged by Streptomyces Biocontrol Agents

The endosphere and the rhizosphere are pertinent milieus with microbial communities that perturb the agronomic traits of crop plants through beneficial or detrimental interactions. In this study, we challenged these communities by adding Streptomyces biocontrol strains to wheat seeds in soils with severe Rhizoctonia solani infestation. Wheat plants were grown in a glasshouse standardized system, and the bacterial and fungal microbiomes of 233 samples of wheat roots (endosphere) and rhizosphere soils were monitored for 20 weeks, from seed to mature plant stage. The results showed highly dynamic and diverse microbial communities that changed over time, with Sphingomonas bacteria and Aspergillus, Dipodascus, and Trichoderma fungi increasing over time. Application of biocontrol Streptomyces strains promoted plant growth and maturation of wheat heads and modulated the root microbiome, decreasing Paenibacillus and increasing other bacterial and fungal OTUs. The soils with the highest levels of R. solani had increased reads of Thanatephorus (Rhizoctonia anamorph) and increased root disease levels and increased Balneimonas, Massilia, Pseudomonas, and unclassified Micrococcaceae. As we enter the era of biologically sustainable agriculture, it may be possible to reduce and limit the effects of serious fungal infestations by promoting a beneficial microbiome through the application of biocontrol agents during different periods of plant development.

Roots and Panicles of the C4 Model Grasses Setaria viridis (L). and S. pumila Host Distinct Bacterial Assemblages With Core Taxa Conserved Across Host Genotypes and Sampling Sites

Virtually all studied plant tissues are internally inhabited by endophytes. Due to their relevance for plant growth and health, bacterial microbiota of crop plants have been broadly studied. In plant microbiome research the root is the most frequently addressed environment, whereas the ecology of microbiota associated with reproductive organs still demands investigation. In this work, we chose the model grasses Setaria viridis and Setaria pumila to better understand the drivers shaping bacterial communities associated with panicles (representing a reproductive organ) as compared to those associated with roots. We collected wild individuals of both grass species from 20 different locations across Austria and investigated the bacterial assemblages within roots and ripe grain-harboring panicles by 16S rRNA gene-based Illumina sequencing. Furthermore, plant samples were subjected to genotyping by genetic diversity-focused Genotyping by Sequencing. Overall, roots hosted more diverse microbiota than panicles. Both the plant organ and sampling site significantly shaped the root and panicle-associated microbiota, whereas the host genotype only affected root communities. In terms of community structure, root-specific assemblages were highly diverse and consisted of conserved bacterial taxa. In contrast, panicle-specific communities were governed by Gammaproteobacteria, were less diverse and highly origin-dependent. Among OTUs found in both plant tissues, relative abundances of Gammaproteobacteria were higher in panicles, whereas Rhizobiales dominated root communities. We further identified core and non-core taxa within samples of both Setaria species. Non-core taxa included members of the Saccharibacteria and Legionelalles, while core communities encompassed eleven OTUs of seven bacterial orders, together with a set of ten panicle-enriched OTUs. These communities were widespread across root and panicle samples from all locations, hinting toward an evolved form of mutualism through potential vertical transmission of these taxa within Setaria species.

Functional Microbial Features Driving Community Assembly During Seed Germination and Emergence

Microbial interactions occurring on and around seeds are especially important for plant fitness since seed-borne microorganisms are the initial source of inoculum for the plant microbiota. In this study, we analyze structural and functional changes occurring within the plant microbiota at these early stages of the plant cycle, namely germination and emergence. To this purpose, we performed shotgun DNA sequencing of microbial assemblages associated to seeds, germinating seeds and seedlings of two plant species: bean and radish. We observed an enrichment of Enterobacteriales and Pseudomonadales during emergence and a set of functional traits linked to copiotrophy that could be responsible for this selection as a result of an increase of nutrient availability after germination. Representative bacterial isolates of taxa that are selected in seedlings showed indeed faster bacterial growth rate in comparison to seed-associated bacteria isolates. Finally, binning of metagenomics contigs results in the reconstruction of population genomes of the major bacterial taxa associated to the samples. Together, our results demonstrate that, although seed microbiota varied across plant species, nutrient availability during germination elicits changes of the composition of microbial communities by potentially selecting microbial groups with functional traits linked to copiotrophy. The data presented here represents the first attempts to empirically assess changes in the microbial community during plant emergence and moves us toward a more holistic understanding of the plant microbiome.

Consistent associations with beneficial bacteria in the seed endosphere of barley (Hordeum vulgare L.)

The importance of the plant microbiome for host fitness has led to the concept of the "plant holobiont". Seeds are reservoirs and vectors for beneficial microbes, which are very intimate partners of higher plants with the potential to connect plant generations. In this study, the endophytic seed microbiota of numerous barley samples, representing different cultivars, geographical sites and harvest years, was investigated. Cultivation-dependent and -independent analyses, microscopy, functional plate assays, greenhouse assays and functional prediction were used, with the aim of assessing the composition, stability and function of the barley seed endophytic bacterial microbiota. Associations were consistently detected in the seed endosphere with Paenibacillus, Pantoea and Pseudomonas spp., which were able to colonize the root with a notable rhizocompetence after seed germination. In greenhouse assays, enrichment with these bacteria promoted barley growth, improved mineral nutrition and induced resistance against the fungal pathogen Blumeria graminis. We demonstrated here that barley, an important crop plant, was consistently associated with beneficial bacteria inside the seeds. The results have relevant implications for plant microbiome ecology and for the holobiont concept, as well as opening up new possibilities for research and application of seed endophytes as bioinoculants in sustainable agriculture.

Core Microbiome of Medicinal Plant Salvia miltiorrhiza Seed: A Rich Reservoir of Beneficial Microbes for Secondary Metabolism?

Seed microbiome includes special endophytic or epiphytic microbial taxa associated with seeds, which affects seed germination, plant growth, and health. Here, we analyzed the core microbiome of 21 Salvia miltiorrhiza seeds from seven different geographic origins using 16S rDNA and ITS amplicon sequencing, followed by bioinformatics analysis. The whole bacterial microbiome was classified into 17 microbial phyla and 39 classes. Gammaproteobacteria (67.6%), Alphaproteobacteria (15.6%), Betaproteobacteria (2.6%), Sphingobacteria (5.0%), Bacilli (4.6%), and Actinobacteria (2.9%) belonged to the core bacterial microbiome. Dothideomycetes comprised 94% of core fungal microbiome in S. miltiorrhiza seeds, and another two dominant classes were Leotiomycetes (3.0%) and Tremellomycetes (2.0%). We found that terpenoid backbone biosynthesis, degradation of limonene, pinene, and geraniol, and prenyltransferases, were overrepresented in the core bacterial microbiome using phylogenetic examination of communities by reconstruction of unobserved states (PICRUSt) software. We also found that the bacterial genera Pantoea, Pseudomonas, and Sphingomonas were enriched core taxa and overlapped among S. miltiorrhiza, maize, bean, and rice, while a fungal genus, Alternaria, was shared within S. miltiorrhiza, bean, and Brassicaceae families. These findings highlight that seed-associated microbiomeis an important component of plant microbiomes, which may be a gene reservoir for secondary metabolism in medicinal plants.