Soil-Plant-Microbe Interactions in Stressed Agriculture Management: A Review

Mechanisms underlying the protective effects of beneficial fungi against plant diseases

Fusarium wilt disease is a growing problem in agriculture systems. Application of bio-fertilizers containing beneficial microbes represents a promising disease control strategy. However, the mechanisms underlying disease suppression remain elusive. Here, in order to assess the importance of direct antagonism and modified soil microbiota on suppression of Fusarium wilt disease, we conducted a pot experiment with chemical, organic and biologically enhanced fertilizers, we tracked the impact of those fertilizer amendments on disease incidence, and measured the pathogen density and changes in soil microbiota. Alterations in bacterial abundance and community structure after bio-fertilizers application were determined to be key factors in constraining the pathogen, Fusarium oxysporum. In particular, bio-fertilizer application increased the abundance of indigenous microbial groups with reported antifungal activity, such as Lysobacter spp., which could play a keystone role in controlling this pathogen. The microbes introduced in the bio-fertilizer treatments (e.g. Bacillus and Trichoderma spp.) induced suppressiveness via alteration of the soil microbiome rather than direct pathogen inhibition. These results contrast with the commonly held paradigm of disease suppression using beneficial microbes and open up new perspectives for the promotion of soil health. In addition to seeking antagonistic microbes based on their direct inhibitory activity, disease suppression may also be achieved by introducing keystone species that reshape soil microbiome structure and function.

Bio-fertilizer application induces soil suppressiveness against Fusarium wilt disease by reshaping the soil microbiome

Agriculture plays an important role in a country's economy. The sector is challenged by many stresses, which led to huge loss in plant productivity worldwide. The ever-increasing population, rapid urbanization with shrinking agricultural lands, dramatic change in climatic conditions, and extensive use of agrochemicals in agricultural practices that caused environmental disturbances confront mankind of escalating problems of food security and sustainability in agriculture. Escalating environmental problems and global hunger have led to the development and adoption of genetic engineering and other conventional plant breeding approaches in developing stress-tolerant varieties of crops. However, these approaches have drawn flaws in their adoption as the process of generating tolerant varieties takes months to years in bringing the technology from the lab to the field. Under such scenario, sustainable and climate-smart agricultural practices that avail bacterial usage open the avenues in fulfilling the incessant demand for food for the global population. Ensuring stability on economic fronts, bacteria minimizes plant salt uptake by trapping ions in their exopolysaccharide matrix besides checking the expression of Na+/H+ and high-affinity potassium transporters. Herein we describe information on salinity stress and its effect on plant health as well as strategies adopted by plant growth-promoting rhizobacteria (PGPR) in helping plants to overcome salinity stress and in mitigating loss in overall plant productivity. It is believed that acquisition of advanced knowledge of plant-beneficial PGPR will help in devising strategies for sustainable, environment-friendly, and climate-smart agricultural technologies for adoption in agriculture to overcome the constrained environmental conditions.

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Mechanistic Insights of the Interaction of Plant Growth-Promoting Rhizobacteria (PGPR) With Plant Roots Toward Enhancing Plant Productivity by Alleviating Salinity Stress.

Almost one-third of crop yields are lost every year due to microbial alterations and diseases. The main control strategy to limit these losses is the use of an array of chemicals active against spoilage and unwanted pathogenic microorganisms. Their massive use has led to extensive environmental pollution, human poisoning and a variety of diseases. An emerging alternative to this chemical approach is the use of microbial biocontrol agents. Biopesticides have been used with success in several fields, but a better understanding of their mode of action is necessary to better control their activity and increase their use. Very few studies have considered that biofilms are the preferred mode of life of microorganisms in the target agricultural biotopes. Increasing evidence shows that the spatial organization of microbial communities on crop surfaces may drive important bioprotection mechanisms. The aim of this review is to summarize the evidence of biofilm formation by biocontrol agents on crops and discuss how this surface-associated mode of life may influence their biology and interactions with other microorganisms and the host and, finally, their overall beneficial activity.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/1751-7915.12693

Should the biofilm mode of life be taken into consideration for microbial biocontrol agents

Biological control of tobacco bacterial wilt using Trichoderma harzianum amended bioorganic fertilizer and the arbuscular mycorrhizal fungi Glomus mosseae

The colonization of rhizosphere by microorganisms is directly associated with bacterial growth, chemotaxis, biofilm formation, and the interaction with host plant root exudates. In this study, the responses of Ralstonia solanacearum to the root exudates from two tobacco cultivars (Hongda, susceptible; K326, resistant) were determined. The results showed that the population of R. solanacearum was much higher in the rhizosphere soil of Hongda than in the rhizosphere soil of K326, resulting in a higher disease index for the Hongda treatments (92.73 %). The attraction of R. solanacearum to Hongda root exudates (HRE) was stronger than the response to K326 root exudates (KRE). Four organic acids, oxalic acid, malic acid, citric acid, and succinic acid, from the root exudates were identified and subsequently evaluated. The amount of oxalic acid from HRE was significantly higher than that from KRE. The results also showed that oxalic acid could both significantly induce the chemotactic response and increase the biofilm biomass of R. solanacearum. Both malic acid and citric acid could significantly increase the chemotaxis ability in vitro and the recruitment of R. solanacearum to tobacco root under gnotobiotic conditions. Overall, these data suggested that the colonization of tobacco rhizosphere by pathogenic bacterial strains was influenced by the organic acids secreted from the roots. The results expand our understanding of the roles of root exudates in plant-microbe interactions and will be useful for screening and applying beneficial bacteria for better control of plant wilt diseases.

Root exudates from two tobacco cultivars affect colonization of Ralstonia solanacearum and the disease index

Bacillus amyloliquefaciens is a plant-beneficial Gram-positive bacterium involved in suppressing soil-borne pathogens through the secretion of secondary metabolites and high rhizosphere competence. Biofilm formation is regarded as a prerequisite for high rhizosphere competence. In this work, we show that plant extracts affect the chemotaxis and biofilm formation of B. amyloliquefaciens SQY 162 (SQY 162). All carbohydrates tested induced the chemotaxis and biofilm formation of the SQY 162 strain; however, the bacterial growth rate was not influenced by the addition of carbohydrates. A strong chemotactic response and biofilm formation of SQY 162 were both induced by pectin through stimulation of surfactin synthesis and transcriptional expression of biofilm formation related matrix genes. These results suggested that pectin might serve as an environmental factor in the stimulation of the biofilm formation of SQY 162. Furthermore, in pot experiments the surfactin production and the population of SQY 162 in the rhizosphere significantly increased with the addition of sucrose or pectin, whereas the abundance of the bacterial pathogen Ralstonia decreased. With increased production of secondary metabolites in the rhizosphere of tobacco by SQY 162 and improved colonization density of SQY 162 in the pectin treatment, the disease incidences of bacterial wilt were efficiently suppressed. The present study revealed that certain plant extracts might serve as energy sources or environmental cues for SQY 162 to enhance the population density on tobacco root and bio-control efficacy of tobacco bacterial wilt.

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Pectin Enhances Bio-Control Efficacy by Inducing Colonization and Secretion of Secondary Metabolites by Bacillus amyloliquefaciens SQY 162 in the Rhizosphere of Tobacco

The invention belongs to the technical field of agricultural intensive production, and provides an effective antagonistic strain SQR-7 capable of comprehensively preventing and controlling a continuous cropping tobacco bacterial wilt disease, microorganism organic fertilizer manufactured by the effective antagonistic strain, and the application of the effective antagonistic strain to the continuous cropping tobacco bacterial wilt disease prevention and control. The SQR-7 strain belongs to bacillus amyloliquefaciens and is collected at the common microorganism center of the China Committee for Culture Collection of Microorganisms on April 13th in 2013, and the strain collection number is CGMCC NO. 7497. The control rate of the microorganism organic fertilizer including the effective antagonistic strain to organisms with the tobacco bacterial wilt disease reaches 80%. Soil chemical amendments are combined to conduct pretreatment on soil, the continuous cropping soil is sterilized, the number of pathogenic bacteria is lowered, soil microflora is improved, the growth of the pathogenic bacteria is restrained, the growth of tobaccos is also promoted, and the disease resistance of the tobaccos is greatly improved.

Effective antagonistic strain for preventing and controlling continuous cropping tobacco bacterial wilt disease, microorganism organic fertilizer including effective antagonistic strain, and production method and application of effective antagonistic strain

Wen Shi

Papers

Biological control of tobacco bacterial wilt using Trichoderma harzianum amended bioorganic fertilizer and the arbuscular mycorrhizal fungi Glomus mosseae

Root exudates from two tobacco cultivars affect colonization of Ralstonia solanacearum and the disease index

Pectin Enhances Bio-Control Efficacy by Inducing Colonization and Secretion of Secondary Metabolites by Bacillus amyloliquefaciens SQY 162 in the Rhizosphere of Tobacco

Effective antagonistic strain for preventing and controlling continuous cropping tobacco bacterial wilt disease, microorganism organic fertilizer including effective antagonistic strain, and production method and application of effective antagonistic strain