Deciphering Differences in Microbial Community Diversity between Clubroot-Diseased and Healthy Soils

Microbial Community Structure and Diversity of Endophytic Bacteria and Fungi in the Healthy and Diseased Roots of Angelica sinensis, and Identification of Pathogens Causing Root Rot

Angelica sinensis (Oliv.) Diels is an important traditional Chinese herbal medicine, and its main medicinal part is the root. In recent years, root rot has become one of the bottlenecks hindering the healthy and green development of Angelica cultivation due to the inappropriate application of chemical fertilizers, pesticides, plant growth regulators, and continuous cropping. In this study, high-throughput sequencing technology was adopted to reveal the differences in the community structure and diversity of endophytic bacteria and fungi in the roots of healthy and diseased A. sinensis. The results showed that the diversity index of endophytic bacterial communities was significantly higher in healthy root than in diseased Angelica root systems. There was a significant difference in endophytic fungal community diversity only at the m1 sampling site. There was a significant difference in the β-diversity of bacterial communities, but not of fungi. In terms of community composition, Proteobacteria was the dominant phylum of bacteria, and Sphingobium and Pseudomonas were the dominant genera; Ascomycota and Basidiomycota were the dominant phyla of fungi, and Plectosphaerella, Paraphoma, and Fusarium were the dominant genera. In addition, the relative abundance of the genera Sphingobium and Pseudomonas was higher in healthy roots, while Fusarium was higher in diseased samples. Among the five pathogens isolated from diseased root, four strains were Fusarium sp., and one was Paraphoma chrysanthemicola, which is reported for the first time. Our findings indicate that the endophyte community structure of A. sinensis infected with root rot changed significantly compared with healthy plants, and Fusarium is an important pathogenic factor, which provides a valuable microbiological basis for the targeted biocontrol of Angelica root rot.

Trichoderma brevicompactum 6311: Prevention and Control of Phytophthora capsici and Its Growth-Promoting Effect

Pepper Phytophthora blight caused by Phytophthora capsici results in substantial losses in global pepper cultivation. The use of biocontrol agents with the dual functions of disease suppression and crop growth promotion is a green and sustainable way of managing this pathogen. In this study, six biocontrol strains of Trichoderma with high antagonistic activity against P. capsici were isolated and screened from the rhizosphere soil of healthy peppers undergoing long-term continuous cultivation. Morphological identification and molecular biological identification revealed that strains 2213 and 2221 were T. harzianum, strains 5111, 6311, and 6321 were T. brevicompactum, and strain 7111 was T. virens. The results showed that T. brevicompactum 6311 had the greatest inhibitory effect against P. capsici. The inhibition rate of 6311 on the mycelial growth of P. capsici was 82.22% in a double-culture test, whereas it reached 100% in a fermentation liquid culture test. Meanwhile, the pepper fruit tests showed that 6311 was 29% effective against P. capsici on pepper, and a potting test demonstrated that the preventive and controlling effect of 6311 on pepper epidemics triggered by P. capsici was 55.56%. The growth-promoting effect, germination potential, germination rate, radicle-embryonic axis length, germination index, and fresh weight of peppers cultured in the 6311 fermentation broth were significantly increased compared with the results for the control group. Scanning electron microscopy revealed that 6311 achieved the parasitism of P. capsici, producing siderophores and the growth hormone indoleacetic acid (IAA) to achieve disease-suppressive and growth-promoting functions. Transcriptomic results indicated that genes encoding proteins involved in plant disease resistance, namely flavanone 3-hydroxylase (F3H) and growth transcription factor (AUX22), were generally upregulated after the application of 6311. This study demonstrated that 6311 exhibits significant bioprotective and growth-promoting functions.

Characterization of rhizosphere bacterial communities in oilseed rape cultivars with different susceptibility to Plasmodiophora brassicae infection

Rhizosphere microbiomes are constantly mobilized during plant-pathogen interactions, and this, in turn, affects their interactions. However, few studies have examined the activities of rhizosphere microbiomes in plants with different susceptibilities to soil-borne pathogens, especially those that cause clubroot disease. In this study, we compared the rhizosphere bacterial community in response to infection of Plasmodiophora brassicae among the four different clubroot susceptibility cultivars of oilseed rape (Brassica napus). Our results revealed obvious differences in the responses of rhizosphere bacterial community to the P. brassicae infection between the four cultivars of oilseed rape. Several bacterial genera that are associated with the nitrogen cycle, including Limnobacter, Thiobacillus, Anaeromyxobacter, Nitrosomonas, Tumebacillus, and Halomonas, showed significantly different changes between susceptible and resistant cultivars in the presence of P. brassicae infection. Moreover, increased connectedness and robustness were exhibited in the rhizosphere bacterial community co-occurrence network in clubroot-susceptible cultivars that were infected with P. brassicae, while only slight changes were observed in clubroot-resistant cultivars. Metagenomic analysis of microbial metabolism also indicated differences in the rhizosphere bacterial community between susceptible and resistant cultivars that were infected with P. brassicae. Functional analysis of the nitrogen cycle showed that genes related to nitrification (nxrB) were upregulated in susceptible cultivars, while genes related to assimilatory nitrate reduction (nasA, narB, and nirA) were upregulated in resistant cultivars that were infected with P. brassicae. These findings indicate that the synthesis and assimilation process of NO3 - content were promoted in susceptible and resistant cultivars, respectively. Our study revealed differences in the characteristics of rhizosphere bacterial communities in response to P. brassicae infection between clubroot-susceptible and clubroot-resistant cultivars as well as the potential impact of these differences on the plant-P. brassicae interaction.

The Ability of Different Tea Tree Germplasm Resources in South China to Aggregate Rhizosphere Soil Characteristic Fungi Affects Tea Quality

It is generally recognized that the quality differences in plant germplasm resources are genetically determined, and that only a good "pedigree" can have good quality. Ecological memory of plants and rhizosphere soil fungi provides a new perspective to understand this phenomenon. Here, we selected 45 tea tree germplasm resources and analyzed the rhizosphere soil fungi, nutrient content and tea quality. We found that the ecological memory of tea trees for soil fungi led to the recruitment and aggregation of dominant fungal populations that were similar across tea tree varieties, differing only in the number of fungi. We performed continuous simulation and validation to identify four characteristic fungal genera that determined the quality differences. Further analysis showed that the greater the recruitment and aggregation of Saitozyma and Archaeorhizomyces by tea trees, the greater the rejection of Chaetomium and Trechispora, the higher the available nutrient content in the soil and the better the tea quality. In summary, our study presents a new perspective, showing that ecological memory between tea trees and rhizosphere soil fungi leads to differences in plants' ability to recruit and aggregate characteristic fungi, which is one of the most important determinants of tea quality. The artificial inoculation of rhizosphere fungi may reconstruct the ecological memory of tea trees and substantially improve their quality.

Comparative Metagenomic Analysis Reveals Rhizosphere Microbiome Assembly and Functional Adaptation Changes Caused by Clubroot Disease in Chinese Cabbage

Clubroot is a major disease and severe threat to Chinese cabbage, and it is caused by the pathogen Plasmodiophora brassicae Woron. This pathogen is an obligate biotrophic protist and can persist in soil in the form of resting spores for more than 18 years, which can easily be transmitted through a number of agents, resulting in significant economic losses to global Chinese cabbage production. Rhizosphere microbiomes play fundamental roles in the occurrence and development of plant diseases. The changes in the rhizosphere microorganisms could reveal the severity of plant diseases and provide the basis for their control. Here, we studied the rhizosphere microbiota after clubroot disease infections with different severities by employing metagenomic sequencing, with the aim of exploring the relationships between plant health, rhizosphere microbial communities, and soil environments; then, we identified potential biomarker microbes of clubroot disease. The results showed that clubroot disease severity significantly affected the microbial community composition and structure of the rhizosphere soil, and microbial functions were also dramatically influenced by it. Four different microbes that had great potential in the biocontrol of clubroot disease were identified from the obtained results; they were the genera Pseudomonas, Gemmatimonas, Sphingomonas, and Nocardioides. Soil pH, organic matter contents, total nitrogen, and cation exchange capacity were the major environmental factors modulating plant microbiome assembly. In addition, microbial environmental information processing was extremely strengthened when the plant was subjected to pathogen invasion, but weakened when the disease became serious. In particular, oxidative phosphorylation and glycerol-1-phosphatase might have critical functions in enhancing Chinese cabbage's resistance to clubroot disease. This work revealed the interactions and potential mechanisms among Chinese cabbage, soil environmental factors, clubroot disease, and microbial community structure and functions, which may provide a novel foundation for further studies using microbiological or metabolic methods to develop disease-resistant cultivation technologies.

Association between Soil Physicochemical Properties and Bacterial Community Structure in Diverse Forest Ecosystems

Although the importance of the soil bacterial community for ecosystem functions has long been recognized, there is still a limited understanding of the associations between its community composition, structure, co-occurrence patterns, and soil physicochemical properties. The objectives of the present study were to explore the association between soil physicochemical properties and the composition, diversity, co-occurrence network topological features, and assembly mechanisms of the soil bacterial community. Four typical forest types from Liziping Nature Reserve, representing evergreen coniferous forest, deciduous coniferous forest, mixed conifer-broadleaf forest, and its secondary forest, were selected for this study. The soil bacterial community was analyzed using Illumina MiSeq sequencing of 16S rRNA genes. Nonmetric multidimensional scaling was used to illustrate the clustering of different samples based on Bray-Curtis distances. The associations between soil physicochemical properties and bacterial community structure were analyzed using the Mantel test. The interactions among bacterial taxa were visualized with a co-occurrence network, and the community assembly processes were quantified using the Beta Nearest Taxon Index (Beta-NTI). The dominant bacterial phyla across all forest soils were Proteobacteria (45.17%), Acidobacteria (21.73%), Actinobacteria (8.75%), and Chloroflexi (5.06%). Chao1 estimator of richness, observed ASVs, faith-phylogenetic diversity (faith-PD) index, and community composition were distinguishing features of the examined four forest types. The first two principal components of redundancy analysis explained 41.33% of the variation in the soil bacterial community, with total soil organic carbon, soil moisture, pH, total nitrogen, carbon/nitrogen (C/N), carbon/phosphorous (C/P), and nitrogen/phosphorous (N/P) being the main soil physicochemical properties shaping soil bacterial communities. The co-occurrence network structure in the mixed forest was more complex compared to that in pure forests. The Beta-NTI indicated that the bacterial community assembly of the four examined forest types was collaboratively influenced by deterministic and stochastic ecological processes.