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Background: Plant microbiota contributes to plant growth and health, including enhancing plant resistance to various diseases. Despite remarkable progress in understanding diseases resistance in plants, the precise role of rhizosphere microbiota in enhancing watermelon resistance against soil-borne diseases remains unclear. Here, we constructed a synthetic community (SynCom) of 16 core bacterial strains obtained from the rhizosphere of grafted watermelon plants. We further simplified SynCom and investigated the role of bacteria with synergistic interactions in promoting plant growth through a simple synthetic community. Results: Our results demonstrated that the SynCom significantly enhanced the growth and disease resistance of ungrafted watermelon grown in non-sterile soil. Furthermore, analysis of the amplicon and metagenome data revealed the pivotal role of Pseudomonas in enhancing plant health, as evidenced by a significant increase in the relative abundance and biofilm-forming pathways of Pseudomonas post-SynCom inoculation. Based on in vitro co-culture experiments and bacterial metabolomic analysis, we selected Pseudomonas along with seven other members of the SynCom that exhibited synergistic effects with Pseudomonas. It enabled us to further refine the initially constructed SynCom into a simplified SynCom comprising the eight selected bacterial species. Notably, the plant-promoting effects of simplified SynCom were similar to those of the initial SynCom. Furthermore, the simplified SynCom protected plants through synergistic effects of bacteria. Conclusions: Our findings suggest that the SynCom proliferate in the rhizosphere and mitigate soil-borne diseases through microbial synergistic interactions, highlighting the potential of synergistic effects between microorganisms in enhancing plant health. This study provides a novel insight into using the functional SynCom as a promising solution for sustainable agriculture. 6UoEKpA3MAKccxe2Gy9v1z Video Abstract [ABSTRACT FROM AUTHOR]

Rare bacteria in rhizospheres from natural watermelon vs chimeric watermelon were investigated to understand the rare rhizobacteria assembling processes along with plant development and effects of rare species on functional stability. Over 80% of the total OTUs were defined as rare taxa (i.e., transient, permanent, and conditionally rare) in the rhizosphere. Among these three rarities, transient rare taxa possessed the highest richness, while the conditionally rarity displayed the largest variations along growth stages and exhibited the greatest deterministic process for assembling. Pairs of conditionally rare taxa with high asynchrony and similar functional potentials were identified in the rhizobacterial communities. This suggests the functional stability of the rhizosphere through functional redundancy. The number of pairs within the rhizosphere of chimeric plant was ~ two fold higher than that of natural watermelon, which illustrated that chimeric plants can stabilize rhizospheric functions. [ABSTRACT FROM AUTHOR]

Microbial composition and functions in the rhizosphere—an important microbial hotspot—are among the most fascinating yet elusive topics in microbial ecology. We used 557 pairs of published 16S rDNA amplicon sequences from the bulk soils and rhizosphere in different ecosystems around the world to generalize bacterial characteristics with respect to community diversity, composition, and functions. The rhizosphere selects microorganisms from bulk soil to function as a seed bank, reducing microbial diversity. The rhizosphere is enriched in Bacteroidetes, Proteobacteria, and other copiotrophs. Highly modular but unstable bacterial networks in the rhizosphere (common for r-strategists) reflect the interactions and adaptations of microorganisms to dynamic conditions. Dormancy strategies in the rhizosphere are dominated by toxin–antitoxin systems, while sporulation is common in bulk soils. Functional predictions showed that genes involved in organic compound conversion, nitrogen fixation, and denitrification were strongly enriched in the rhizosphere (11–182%), while genes involved in nitrification were strongly depleted. Understanding soil microbiota dynamics is key the development of soil-based sustainable agriculture and conservation strategies. This meta-analysis shows that bulk soil functions as a seed bank for the rhizosphere, which encompasses a rich microbiota adapted to dynamic conditions in hotpots. [ABSTRACT FROM AUTHOR]

Summary: 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. [ABSTRACT FROM AUTHOR]

Grafting is a basic technique which is widely used to increase yield and enhance biotic and abiotic stress tolerance in plant production. The diversity and interactions of rhizobacterial assemblages shaped by grafting are important for the growth of their hosts but remain poorly understood. To test the hypothesis that plant grafting shapes complexity and co-occurrence of rhizobacterial assemblage, four types of plants, including ungrafted bottle gourd (B), ungrafted watermelon (W), grafted watermelon with bottle gourd rootstock (W/B), and grafted bottle gourd with watermelon rootstock (B/W), were cultivated in two soil types in a greenhouse, and the rhizosphere bacterial communities were analyzed by 16S rRNA gene high-throughput sequencing. Both the soil type and grafting significantly influenced the bacterial community composition. Grafting increased bacterial within-sample diversity in both soils. Core enriched operational taxonomic units (OTUs) in the W/B rhizosphere compared with the other three treatments (B, W, and B/W) were mainly affiliated with Alphaproteobacteria, Deltaproteobacteria, and Bacteroidetes, which are likely related to methanol oxidation, methylotrophy, fermentation, and ureolysis. Co-occurrence network analysis proved that grafting increased network complexity, including the number of nodes, edges, and modules. Moreover, grafting strengthened the structural robustness of the network in the rhizosphere, while ungrafted watermelon had the lowest network robustness. Homogeneous selection played a predominant role in bacterial community assembly, and the contribution of dispersal limitation was increased in grafted watermelon with bottle gourd rootstock. Grafting increased the diversity and transformed the network topology of the bacterial community, which indicated that grafting could improve species coexistence in the watermelon rhizosphere. [ABSTRACT FROM AUTHOR]

This study focused on the legacy effects of 8-year application of N (in gradient of 0, 140, 280, 470, and 660 kg N ha−1 year−1) on the bacterial community diversity, interactions, and assembly processes in the wheat rhizosphere. The rhizosphere bacterial α-diversity increased with the rate of historical N input, while it did not change at N addition rates of over 280 kg N ha−1 year−1. Historical N input clearly shifted the rhizosphere bacterial community composition, and soils with more N input were more dissimilar to those without N input. The net relatedness index (NRI) and nearest taxon index (NTI) analysis revealed that the rhizosphere bacterial communities in most samples were phylogenetically clustered, and the treatments with high N (> 470 kg N ha−1 year−1) showed higher levels of clustering than those with low N (< 140 kg N ha−1 year−1), indicating more environmental selection stress in soil with higher historical N input. Increased co-occurrence network size and connectivity were accompanied by increased aboveground biomass of wheat. Overall, with the increase in historical N input, the resulting legacy effects forced the bacterial community in the rhizosphere to undergo higher environmental selection pressure, and indirectly affected the complexity of wheat rhizosphere assemblages during subsequent crop growth. [ABSTRACT FROM AUTHOR]

Abstract: The application method for a novel bioorganic fertilizer (BIO) was developed to improve its biocontrol efficacy of Fusarium wilt (Ling et al. 2010). However, its efficacy on controlling Fusarium wilt and the variations of microbial community after long-term application for watermelon production had not been elucidated. To clarify, a 4-years pot experiment of mono-cropping watermelon was conducted. The results revealed that though the disease incidences were increased in all treatments with the increase of continuous cropping years, the treatment of BIO application both in nursery and pot soil always maintained the lowest disease incidence. The real-time PCR results showed that the population of Paenibacillus polymyxa was decreased with continuous cropping years, but in all seasons, the treatment with BIO application both in nursery and pot soil had a highest population of P. polymyxa than the other treatments. On the other hand, the abundance of the pathogen FON was increased with the increase of continuous cropping years and the lowest rate of increase was found by BIO application in both nursery and pot soil. DGGE patterns showed that the bacterial diversity was weakened after mono-cropping of watermelon for 4 years, but the consecutive applications of BIO at nursery and transplanting stage resulted in the minimal change of bacterial diversity. More detailed differences on bacterial diversity between control and double application of BIO treatment after 4-years monoculture were analyzed by 454 pyrosequencing, which showed the dominant phyla found in both samples were Firmicutes, Proteobacteria and Actinobacteria, and the consecutive applications of BIO recruited more beneficial bacteria than control, such as Bacillus, Paenibacillus, Haliangium, Streptomyces. Overall, these results, to a certain extent, approved that the consecutive applications of BIO at nursery and transplanting stage could effectively suppress watermelon Fusarium wilt by regulating the rhizosphere bacterial diversity. These results could give some clues that how to regulate the soil microbial community to an appropriate level which can keep the plant healthy and thus control the soil-borne diseases. [Copyright &y& Elsevier]

Watermelon Fusarium wilt is one of the most severe soil-borne diseases caused by Fusarium oxysporum f. sp. niveum. In this study, the population of F. oxysporum was quickly monitored by real-time PCR and DNA array in watermelon Fusarium wilt infected soils treated with Paenibacillus polymyxa SQR21 enhanced bio-organic fertilizer (BIO) at the beginning of nursery growth and/or at the beginning of transplanting. The fungal community composition was investigated by molecular cloning and DGGE techniques. The real-time PCR results showed the F. oxysporum population in the rhizosphere soil decreased from 8.56 × 10 colony-forming units (cfu) g rhizosphere soil to 9.41 × 10 cfu g rhizosphere soil after BIO application and the DNA array detection signals of F. oxysporum population weakened. The difference between F. oxysporum abundance of BIO amended and not amended bulk soils was lower than 10 cfu g soil. DGGE profile indicated that BIO application changed the fungal community structure in the rhizosphere soils; the molecular cloning data revealed that consecutive applications of BIO at nursery and transplanting stages not only decreased Ascomycota and increased Basidiomycota abundance in the rhizosphere soil but also caused the apperance of unique fungal group which were not found in the control. The beneficial fungi Chaetomium sp. Aspergillus penicillioides were found in the BIO amended treatment, while some harmful fungi such as F. oxysporum, Rhizoctonia solani, and Fusarium solani were only detected in the control. Data from this study indicated that BIO application can control watermelon Fusarium wilt by suppressing the population of F. oxysporum and changing the fungal community structure in the rhizosphere soils. [ABSTRACT FROM AUTHOR]

Abstract: Paenibacillus polymyxa can serve as a biocontrol agent with a broad host range, but knowledge of the possible contribution of root exudates to its colonization of the rhizosphere remains limited. In this experiment, we identified several organic acids in the root exudates of watermelon. Chemotaxis and swarming assays were performed to investigate the ability of these organic acids to induce the motility of P. polymyxa SQR-21. Oxalic acid, malic acid and citric acid were present in the root exudates but only the intermediate products in tricarboxylic acid cycle, i.e., malic acid and citric acid, could significantly induce motility in P. polymyxa SQR-21. The maximal inducing ability was obtained with malic acid. Values for malic acid were 3.9 and 1.5 times higher than the control in the chemotaxis assay and the swarming assay, respectively. An in vitro experiment further confirmed that these intermediate products in tricarboxylic acid cycle could promote recruitment to P. polymyxa SQR-21, thereby increasing the population in the rhizosphere. In conclusion, some of the organic acids secreted by roots could play an important role in root colonization of SQR-21. This finding contributes to our understanding of the interactions of bacteria and plants under natural conditions. [Copyright &y& Elsevier]