Biotechnology and the potential of rhizosphere microorganisms in agricultural practices

Objective: To perform a brief introspective regarding the biotechnological management of microorganisms in the rhizosphere, its implementation in bioprocesses, and its practical application in field. Design/methodology/approach: Bibliographic review regarding the beneficial effects of Arbuscular Mycorrhizal Fungi (AMF) and Plant Growth-Promoting Rhizobacteria (PGPR), which can be applied in bioformulations. Results: There are numerous documented applications of AMF and PGPR —both on laboratory and industrial scale for bioformulation production— aimed to improve crop yield and to provide resistance against abiotic stress and pests. Non-conventional uses are also shown in non-agricultural areas. Study limitations/implications: AMF and PGPR are widely recognized in agriculture due to their inherent ability to compete in harsh conditions within ecosystems, metabolism versatility, and production of secondary metabolites that enable beneficial interactions with plants and other microorganisms. However, industrial production of AMF presents challenges, as a result of their obligate biotrophs condition and a lack of compatibility with traditional bioprocesses. Findings/conclusions: The knowledge generated throughout rhizosphere research should be applied in the industry, in order to extend its use in agriculture.

Bacterial volatile-mediated suppression of root-knot nematode (Meloidogyne incognita)

Root-knot nematodes (Meloidogyne spp.) are obligate plant parasites that cause severe economic losses to agricultural crops worldwide. Due to serious health and environmental concerns related to the use of chemical nematicides, the development of efficient alternatives is of great importance. Biological control through exploiting the potential of rhizosphere microorganisms is currently accepted as an important approach for pest management in sustainable agriculture. In our research, during screening of rhizosphere bacteria against the root-knot nematodes Meloidogyne incognita, Ochrobactrum pseudogrignonense strain NC1 from the rhizosphere of healthy tomatoes showed strong nematode inhibition. A volatile nematicidal assay showed that the cell-free fermentation filtrate in the first-row wells of 12-well tissue culture plates caused M. incognita juvenile mortality in the second-row wells. Gas chromatography-mass spectrometry (GC-MS) analysis revealed that dimethyl disulfide (DMDS) and benzaldehyde were the main volatile compounds produced by strain NC1. The nematicidal activity of these compounds indicated that the LC50 against the M. incognita juveniles in the second-row wells and the fourth-row wells were 23.4 μmol/mL and 30.7 μmol/mL for DMDS and 4.7 μmol/mL and 15.2 μmol/mL for benzaldehyde, respectively. A greenhouse trial using O. pseudogrignonense strain NC1 provided management efficiencies of root-knot nematodes of 88 to 100% compared with the untreated control. This study demonstrated that nematode-induced root-gall suppression mediated by the bacterial volatiles DMDS and benzaldehyde presents a new opportunity for root-knot nematode management.

Agroecosystem Stability In Spring Wheat Crops With The Application Of Fertilizers And Microbial Biological Products

The formation of spring wheat biomass on sod-podzolic soil is carried out mainly due to soil nitrogen, the share of which reaches 1/3 of the total removal of the element when using mineral fertilizers. Inoculation of spring wheat seeds with biologics of rhizosphere microorganisms increases the nitrogen content of fertilizers to 7.3%, increases its immobilization by 5.9-6.7% and reduces losses by 7.4-13.9%. The stability of the agroecosystem is characterized by nitrogen flows. During the growing season of spring wheat with a hydrothermal coefficient of 1.55-1.72, the amount of mineralized nitrogen (mineralization (M)), depending on fertilizers, reaches 9.4-11.1 g/m2, while the reimobilized nitrogen (reimobilization (RI)) – 2.2-3.1 g/m2, net-mineralized (net-mineralization (N-M)) – 6.8 - 8.0 g/m2. The use of nitrogen fertilizers and biological products leads the agroecosystem to the resistance mode (the maximum permissible level of exposure) (RI : M = 27-28%, N-M : RI = 2.5-2.7).

Rhizosphere Microbiome: The Emerging Barrier in Plant-Pathogen Interactions

In the ecosystem, microbiome widely exists in soil, animals, and plants. With the rapid development of computational biology, sequencing technology and omics analysis, the important role of soil beneficial microbial community is being revealed. In this review, we mainly summarized the roles of rhizosphere microbiome, revealing its complex and pervasive nature contributing to the largely invisible interaction with plants. The manipulated beneficial microorganisms function as an indirect layer of the plant immune system by acting as a barrier to pathogen invasion or inducing plant systemic resistance. Specifically, plant could change and recruit beneficial microbial communities through root-type-specific metabolic properties, and positively shape their rhizosphere microorganisms in response to pathogen invasion. Meanwhile, plants and beneficial microbes exhibit the abilities to avoid excessive immune responses for their reciprocal symbiosis. Substantial lines of evidence show pathogens might utilize secreting proteins/effectors to overcome the emerging peripheral barrier for their advantage in turn. Overall, beneficial microbial communities in rhizosphere are involved in plant–pathogen interactions, and its power and potential are being explored and explained with the aim to effectively increase plant growth and productivity.

Characterization of Variovorax Strain C6d Isolated from the Algae-bacteria Consortia

The rhizosphere microorganisms can form beneficial, pathogenic, or neutral relationships. These relationships can promote plant growth and productivity. Among them, a number of Variovorax isolates from the rhizosphere were isolated. Bacteria Variovorax strain C6d (AB552893) was isolated from the non axenic culture of Chlorella spp., C6. The cell was Gram-negative, motile, non-spore-forming, short and rod-shaped (0.5-1.0x1.5-2.0µm). Colonies were in white colour after 7 days on 10-fold diluted Nutrient Broth. The strain was able to tolerate NaCL to 1.0% but not to 4.0% of NaCl. It grew quite well at temperatures ranging from 10°C to 37°C, yet did not show any growth at 4°C and 42°C. The dominant isoprenoid quinone was ubiquinone 8 (Q8). The major fatty acid composition of this strain was summed feature 3, 16:0 and 18:1:w7. The DNA G+C content of strain C6d was 70.4 mol%.

Response of Sugarcane Rhizosphere Bacterial Community to Drought Stress

Sugarcane is an important sugar and energy crop, and its yield is greatly affected by drought. Although a large number of studies have shown that rhizosphere microorganisms can help improve the adaptability of plants to biotic or abiotic stresses, there is a lack of studies on the adaptability of sugarcane rhizosphere microbial communities to host plants. Therefore, we conducted drought stress treatment and normal irrigation treatment on three sugarcane varieties GT21, GT31, and GT42 widely cultivated in Guangxi. Using 16S rDNA sequencing technology to analyze the changes in abundance of the sugarcane rhizosphere bacterial community under different treatments, combined with the determination of soil enzyme activity, soil nutrient content, and sugarcane physiological characteristics, we explored the sugarcane rhizosphere bacterial community response to drought stress. In addition, we used the structural equation model to verify the response path of sugarcane rhizosphere bacteria. The results show that the bacterial community structure in the rhizosphere of sugarcane is stable under normal water conditions. The change in the bacterial community structure under drought stress has a 25.2% correlation with the drought adaptability of sugarcane, but the correlation with drought stress is as high as 42.17%. The changes in abundance of rhizosphere bacteria under drought stress are mainly concentrated in the phylum Rhizobiales and Streptomycetales. This change is directly related to the physiological state of the host plant under drought stress, soil available phosphorus, soil urease and soil acid protease. We investigated the response species of rhizosphere microorganisms and their response pathways under drought stress, providing a scientific basis for rhizosphere microorganisms to assist host plants to improve drought adaptability.