Advanced Research on Rhizosphere Microorganisms: Plant–Microbial Interactions and Sustainable Agriculture

Dear Colleagues,

Rhizosphere microorganisms are important organisms for biocontrol and plant growth promotion. The concept of rhizosphere was first proposed by German microbiologist Lorenz Hiltner in 1904, showing the relationship between bacteria and plants, and it was illustrated as the soil around the roots of plants influenced by root growth. Rhizosphere microorganisms are one of the main complex microbial communities on the earth with rich diversity. Important microbiomes can be divided into belowground and aboveground. The plant microbiota, its habitats, inhabitants, genomes, and surrounding environmental conditions are known as the plant microbiome. The interactions between microorganisms and plant roots have direct effects on the soil and the rhizosphere region, which is the home of diverse microorganisms. The plant microbiome includes a variety of microbes such as archaea, protists, fungi, bacteria, and viruses, which reside inside or on their host plants. The microbial population can also promote plant development and respond to a wide range of environmental conditions. Rhizosphere microorganisms refer to the diverse array of microscopic lifeforms that inhabit the rhizosphere, the narrow region of soil that is directly influenced by root secretions, and the associated soil microorganisms. The region is characterized by a high level of biological activity due to the presence of substances secreted by roots, such as organic acids, amino acids, sugars, and different secondary metabolites. These components are considered as nutrients for microorganisms, fostering a rich and dynamic microbial community around plant roots. Organisms found in the rhizosphere include fungi, bacteria, nematodes, oomycetes, algae, protozoa, archaea, viruses, and arthropods. In recent year, scholars and researchers around the world have started to intensively examine the dynamics of structure and community as well as the various functions of fungal and bacterial communities associated with plant roots. The highly diverse plant-associated microbial communities are affected by abiotic and biotic constraints changing with space and time. The diversity in the rhizosphere microbial community is also affected by various host-related parameters such as the genotype of plant, the host plant species, the interactions among microbes that span from facultative to antagonistic, the richness of plant community, the root length, and different traits of the plant-like growth rate. In terms of fungal phyla, the rhizosphere is dominated by Basidiomycota and Ascomycota, which are the most common taxonomical phyla in soil; moreover, the rhizosphere is dominated by prokaryotic phyla including Acidobacteria, Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. It is also known that healthy and asymptomatic herbs and plants can maintain complex relationships with their rhizosphere microbiota and support considerable plant performance. On the other side, plants locally affect the activity and composition of their rhizosphere microbiome by changing oxygen availability, soil structure, and soil pH, and by providing an energy source via carbon-rich exudates. The Special Issue focuses on roles and functions of different types of microbes and their interactions in different agricultural and horticultural crops within the framework of sustainable crop management. It also considers both direct and indirect mechanisms, aiming to gather critical and important information regarding the positive effects of rhizosphere microorganisms on plant growth and crop yield, as well as their impacts on the quality of the final product. Furthermore, the main limitations of these practices as well as the future prospects of rhizosphere microorganisms research in sustainable agriculture will be presented. In this Special Issue, we also highlight the mechanisms of plant–microbe interactions, manipulation, modulation, and inoculation strategies, and their effects on crop growth, pathogen control, and qualitative parameter improvement. Moreover, this Special Issue aims to study the molecular mechanisms underlying the composition of root exudates under nutrient deficiency, different microbes, and their influences on alleviating nutrient scarcity effectively.

Dr. Mohamad Hesam Shahrajabian
Dr. Spyridon A. Petropoulos
Dr. Wenli Sun
Guest Editors

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• algae
• arbuscular mycorrhizal fungi
• Asteraceae
• mineral nutrition
• nematodes
• nitrogen deposition
• plant growth-promoting rhizobacteria
• rhizobacteria
• root exudates
• secondary metabolites
• signaling
• soil bacterial community
• soil microorganisms