Revealing the role of Plant Growth Promoting Rhizobacteria in suppressive soils against Fusarium oxysporum f.sp. cubense based on metagenomic analysis

Sesame bacterial wilt significantly alters rhizosphere soil bacterial community structure, function, and metabolites in continuous cropping systems

Bacterial wilt is the leading disease of sesame and alters the bacterial community composition, function, and metabolism of sesame rhizosphere soil. However, its pattern of change is unclear. Here, the purpose of this study was to investigate how these communities respond to three differing severities of bacterial wilt in mature continuously cropped sesame plants by metagenomic and metabolomic techniques, namely, absence (WH), moderate (WD5), and severe (WD9) wilt. The results indicated that bacterial wilt could significantly change the bacterial community structure in the rhizosphere soil of continuously cropped sesame plants. The biomarker species with significant differences will also change with increasing disease severity. In particular, the gene expression levels of Ralstonia solanacearum in the WD9 and WD5 treatments increased by 25.29% and 33.61%, respectively, compared to those in the WH treatment (4.35 log10 copies g-1). The occurrence of bacterial wilt significantly altered the functions of the bacterial community in rhizosphere soil. KEEG and CAZy functional annotations revealed that the number of significantly different functions in WH was greater than that in WD5 and WD9. Bacterial wilt significantly affected the relative content of metabolites, especially acids, in the rhizosphere soil, and compared with those in the rhizosphere soil from WH, 10 acids (including S-adenosylmethionine, N-acetylleucine, and desaminotyrosine, etc.) in the rhizosphere soil from WD5 or WD9 significantly increased. In comparison, the changes in the other 10 acids (including hypotaurine, erucic acid, and 6-hydroxynicotinic acid, etc.) were reversed. The occurrence of bacterial wilt also significantly inhibited metabolic pathways such as ABC transporter and amino acid biosynthesis pathways in rhizosphere soil and had a significant impact on two key enzymes (1.1.1.11 and 2.6.1.44). In conclusion, sesame bacterial wilt significantly alters the rhizosphere soil bacterial community structure, function, and metabolites. This study enhances the understanding of sesame bacterial wilt mechanisms and lays the groundwork for future prevention and control strategies against this disease.

Microbial diversity in soils suppressive to Fusarium diseases

Fusarium species are cosmopolitan soil phytopathogens from the division Ascomycota, which produce mycotoxins and cause significant economic losses of crop plants. However, soils suppressive to Fusarium diseases are known to occur, and recent knowledge on microbial diversity in these soils has shed new lights on phytoprotection effects. In this review, we synthesize current knowledge on soils suppressive to Fusarium diseases and the role of their rhizosphere microbiota in phytoprotection. This is an important issue, as disease does not develop significantly in suppressive soils even though pathogenic Fusarium and susceptible host plant are present, and weather conditions are suitable for disease. Soils suppressive to Fusarium diseases are documented in different regions of the world. They contain biocontrol microorganisms, which act by inducing plants' resistance to the pathogen, competing with or inhibiting the pathogen, or parasitizing the pathogen. In particular, some of the Bacillus, Pseudomonas, Paenibacillus and Streptomyces species are involved in plant protection from Fusarium diseases. Besides specific bacterial populations involved in disease suppression, next-generation sequencing and ecological networks have largely contributed to the understanding of microbial communities in soils suppressive or not to Fusarium diseases, revealing different microbial community patterns and differences for a notable number of taxa, according to the Fusarium pathosystem, the host plant and the origin of the soil. Agricultural practices can significantly influence soil suppressiveness to Fusarium diseases by influencing soil microbiota ecology. Research on microbial modes of action and diversity in suppressive soils should help guide the development of effective farming practices for Fusarium disease management in sustainable agriculture.

Impacts of Oak Mulch Amendments on Rhizosphere Microbiome of Citrus Trees Grown in Florida Flatwood Soils

Rhizosphere interactions are an understudied component of citrus production. This is even more important in Florida flatwood soils, which pose significant challenges in achieving sustainable and effective fruit production due to low natural fertility and organic matter. Citrus growers apply soil amendments, including oak mulch, to ameliorate their soil conditions. Thus, the aim of this research was to evaluate the effects of oak mulch on citrus nutrient uptake, soil characteristics, and rhizosphere composition. The plant material consisted of 'Valencia' sweet orange (Citrus × sinensis) trees grafted on 'US-812' (C. reticulata × C. trifoliata) rootstock. The experiment consisted of two treatments, which included trees treated with oak mulch (300 kg of mulch per plot) and a control. The soil and leaf nutrient contents, soil pH, cation exchange capacity, moisture, temperature, and rhizosphere bacterial compositions were examined over the course of one year (spring and fall 2021). During the spring samplings, the citrus trees treated with oak mulch resulted in significantly greater soil Zn and Mn contents, greater soil moisture, and greater rhizosphere bacterial diversity compared to the control, while during the fall samplings, only a greater soil moisture content was observed in the treated trees. The soil Zn and Mn content detected during the spring samplings correlated with the significant increases in the diversity of the rhizosphere bacterial community composition. Similarly, the reduced rates of leaching and evaporation (at the soil surface) of oak mulch applied to Florida sandy soils likely played a large role in the significant increase in moisture and nutrient retention.

Metapangenomics of wild and cultivated banana microbiome reveals a plethora of host-associated protective functions

BACKGROUND

Microbiomes are critical to plants, promoting growth, elevating stress tolerance, and expanding the plant's metabolic repertoire with novel defense pathways. However, generally microbiomes within plant tissues, which intimately interact with their hosts, remain poorly characterized. These endospheres have become a focus in banana (Musa spp.)-an important plant for study of microbiome-based disease protection. Banana is important to global food security, while also being critically threatened by pandemic diseases. Domestication and clonal propagation are thought to have depleted protective microbiomes, whereas wild relatives may hold promise for new microbiome-based biological controls. The goal was to compare metapangenomes enriched from 7 Musa genotypes, including wild and cultivated varieties grown in sympatry, to assess the host associations with root and leaf endosphere functional profiles.

RESULTS

Density gradients successfully generated culture-free microbial enrichment, dominated by bacteria, with all together 24,325 species or strains distinguished, and 1.7 million metagenomic scaffolds harboring 559,108 predicted gene clusters. About 20% of sequence reads did not match any taxon databases and ~ 62% of gene clusters could not be annotated to function. Most taxa and gene clusters were unshared between Musa genotypes. Root and corm tissues had significantly richer endosphere communities that were significantly different from leaf communities. Agrobacterium and Rhizobium were the most abundant in all samples while Chitinophagia and Actinomycetia were more abundant in roots and Flavobacteria in leaves. At the bacterial strain level, there were > 2000 taxa unique to each of M. acuminata (AAA genotype) and M. balbisiana (B-genotype), with the latter 'wild' relatives having richer taxa and functions. Gene ontology functional enrichment showed core beneficial functions aligned with those of other plants but also many specialized prospective beneficial functions not reported previously. Some gene clusters with plant-protective functions showed signatures of phylosymbiosis, suggesting long-standing associations or heritable microbiomes in Musa.

CONCLUSIONS

Metapangenomics revealed key taxa and protective functions that appeared to be driven by genotype, perhaps contributing to host resistance differences. The recovery of rich novel taxa and gene clusters provides a baseline dataset for future experiments in planta or in vivo bacterization or engineering of wild host endophytes.

The Golden Goal of Soil Management: Disease-Suppressive Soils

Disease-suppressive soils encompass specific plant-pathogen-microbial interactions and represent a rare example of an agroecosystem where soil conditions and microbiome together prevent the pathogen from causing disease. Such soils have the potential to serve as a model for characterizing soil pathogen-related aspects of soil health, but the mechanisms driving the establishment of suppressive soils vary and are often poorly characterized. Yet, they can serve as a resource for identifying markers for beneficial activities of soil microorganisms concerning pathogen prevention. Many recent studies have focused on the nature of disease-suppressive soils, but it has remained difficult to predict where and when they will occur. This review outlines current knowledge on the distribution of these soils, soil manipulations leading to pathogen suppression, and markers including bacterial and fungal diversity, enzymes, and secondary metabolites. The importance to consider soil legacy in research on the principles that define suppressive soils is also highlighted. The goal is to extend the context in which we understand, study, and use disease-suppressive soils by evaluating the relationships in which they occur and function. Finally, we suggest that disease-suppressive soils are critical not only for the development of indicators of soil health, but also for the exploration of general ecological principles about the surrounding landscape, effects of deeper layers of the soil profile, little studied soil organisms, and their interactions for future use in modern agriculture.

Microbiota Modulation in Blueberry Rhizosphere by Biocontrol Bacteria

Microbial interactions in agricultural soils can play important roles in the control of soil-borne phytopathogenic diseases. Yields from blueberry plantations from southern Spain have been impacted by the pathogenic fungus, Macrophomina phaseolina. The use of chemical fungicides has been the common method for preventing fungal infections, but due to their high environmental impact, legislation is increasingly restricting its use. Biocontrol alternatives based on the use of microorganisms is becoming increasingly important. Using the metabarcoding technique, fungi and bacteria were characterized (via 16S and ITS regions, respectively) from rhizosphere soils of healthy and dead blueberry plants infected by M. phaseolina, and which had undergone three different treatments: two biocontrol strategies—one of them a mix of Pseudomonas aeruginosa and Bacillus velezensis and the other one with Bacillus amyloliquefaciens—and a third treatment consisting of the application of a nutrient solution. The treatments produced changes in the bacterial microbiota and, to a lesser extent, in the fungi. The abundance of Fusarium was correlated with dead plants, likely favoring the infection by M. phaseolina. The presence of other microorganisms in the soil, such as the fungi Archaeorhizomyces or the bacteria Actinospica, were correlated with healthy plants and could promote their survival. The different genera detected between dead and healthy plants opens the possibility of studying new targets that can act against infection and identify potential microorganisms that can be used in biocontrol strategies.

Characterization of the Soil Bacterial Community from Selected Boxwood Gardens across the United States

In a recent study, we observed a rapid decline of the boxwood blight pathogen Calonectria pseudonaviculata (Cps) soil population in all surveyed gardens across the United States, and we speculated that these garden soils might be suppressive to Cps. This study aimed to characterize the soil bacterial community in these boxwood gardens. Soil samples were taken from one garden in California, Illinois, South Carolina, and Virginia and two in New York in early summer and late fall of 2017 and 2018. Soil DNA was extracted and its 16S rRNA amplicons were sequenced using the Nanopore MinION^® platform. These garden soils were consistently dominated by Rhizobiales and Burkholderiales, regardless of garden location and sampling time. These two orders contain many species or strains capable of pathogen suppression and plant fitness improvement. Overall, 66 bacterial taxa were identified in this study that are known to have strains with biological control activity (BCA) against plant pathogens. Among the most abundant were Pseudomonas spp. and Bacillus spp., which may have contributed to the Cps decline in these garden soils. This study highlights the importance of soil microorganisms in plant health and provides a new perspective on garden disease management using the soil microbiome.

Harnessing rhizobacteria to fulfil inter-linked nutrient dependency on soil and alleviate stresses in plants

Plant rhizo-microbiome comprises of complex microbial communities that colonizes at the interphase of plant roots and soil. Plant-growth-promoting rhizobacteria (PGPR) in the rhizosphere provides important ecosystem services ranging from release of essential nutrients for enhancing soil quality and improving plant health to imparting protection to plants against rising biotic and abiotic stresses. Hence, PGPR serve as restoring agents to rejuvenate soil health and mediate plant fitness in the facet of changing climate. Though, it is evident that nutrients availability in soil are managed through inter-linked mechanisms, how PGPR expediate these processes remain less recognized. Promising results of PGPR inoculation on plant growth are continually reported in controlled environmental conditions, however, their field application often fails due to competition with native microbiota and low colonization efficiency in roots. The development of highly efficient and smart bacterial synthetic communities by integrating bacterial ecological and genetic features provides better opportunities for successful inoculant formulations. This review provides an overview of the inter-play between nutrient availability and disease suppression governed by rhizobacteria in soil followed by the role of synthetic bacterial communities in developing efficient microbial inoculants. Moreover, an outlook on the beneficial activities of rhizobacteria in modifying soil characteristics to sustainably boost agroecosystem functioning is also provided.