Key indicators for renewal and reconstruction of perennial trees soil: Microorganisms and phloridzin

Dopamine Alleviates Phloridzin Toxicity in Apple by Modifying Rhizosphere Bacterial Community Structure and Function

Phloridzin significantly influences apple plant growth, development, and resistance to environmental stresses by engaging in various metabolic processes. Its excessive accumulation in soil, attributed to continuous monoculture practices, not only inhibits plant growth but also disrupts the rhizosphere microbial community. This study aims to explore the remedial effects of dopamine, a known antioxidant and stress resistance modulator in plants, on the adverse impacts of phloridzin stress in apple. Through hydroponic and pot experiments, it was demonstrated that dopamine significantly mitigates the growth inhibition caused by phloridzin stress in apple by reducing reactive oxygen species levels and enhancing photosynthesis and nitrogen transport. Additionally, dopamine reduced phloridzin concentrations in both the rhizosphere and roots. Furthermore, dopamine positively influences the structure of the rhizosphere microbial community, enriching beneficial microbes associated with nitrogen cycling. It increases the potential for soil nitrogen degradation and fixation by upregulating the abundance of ureC, GDH, and nifH, as revealed by metagenomic analysis. This aids in alleviating phloridzin stress. The study reveals dopamine's pivotal roles in modulating rhizosphere ecology under phloridzin stress and suggests its potential in sustainable apple cultivation practices to counter ARD and enhance productivity.

Apple-arbuscular mycorrhizal symbiosis confers resistance to Fusarium solani by inducing defense response and elevating nitrogen absorption

Fusarium solani exerts detrimental effects on plant growth, which is one of the reasons for the incidence of apple replant disease. Arbuscular mycorrhizal fungi (AMF) enhance plant resistance to Fusarium wilt; however, the mechanism remains poorly understood. Therefore, the present study investigated the symbiosis between apple and AMF and explored the physiology, especially nitrate metabolism, antioxidant defense, and photosynthetic performance, when infected by F. solani. The experiment was carried out with four treatments, namely -AMF - F. solani, -AMF + F. solani, -AMF + F. solani, and + AMF + F. solani. In this study, the -AMF + F. solani treatment increased the activity of enzymes associated with nitrogen metabolism, such as the nitrate and nitrite reductases, in the apple root system. The +AMF + F. solani treatment showed higher antioxidant enzyme activities than the -AMF + F. solani by F. solani infection. The apple seedlings of the +AMF + F. solani treatment decreased reactive oxygen accumulation and reduced the oxidative damages triggered by F. solani infection. The improvement in antioxidant capacity due to the +AMF + F. solani treatment was closely associated with the upregulation of genes related to the antioxidant system. The F. solani infection greatly damaged the photosynthetic process, while the +AMF + F. solani treatment significantly improved it compared to the -AMF + F. solani treatment. In conclusion, the study demonstrated that the apple-AMF symbiosis plays an active role in regulating the resistance against F. solani infection by enhancing defense response and nitrogen metabolism.

Characterization of Bacillus amyloliquefaciens BA-4 and its biocontrol potential against Fusarium-related apple replant disease

Apple replant disease (ARD), caused by Fusarium pathogens, is a formidable threat to the renewal of apple varieties in China, necessitating the development of effective and sustainable control strategies. In this study, the bacterial strain BA-4 was isolated from the rhizosphere soil of healthy apple trees in a replanted orchard, demonstrating a broad-spectrum antifungal activity against five crucial apple fungal pathogens. Based on its morphology, physiological and biochemical traits, utilization of carbon sources, and Gram stain, strain BA-4 was tentatively identified as Bacillus amyloliquefaciens. Phylogenetic analysis using 16S rDNA and gyrB genes conclusively identified BA-4 as B. amyloliquefaciens. In-depth investigations into B. amyloliquefaciens BA-4 revealed that the strain possesses the capacity to could secrete cell wall degrading enzymes (protease and cellulase), produce molecules analogous to indole-3-acetic acid (IAA) and siderophores, and solubilize phosphorus and potassium. The diverse attributes observed in B. amyloliquefaciens BA-4 underscore its potential as a versatile microorganism with multifaceted benefits for both plant well-being and soil fertility. The extracellular metabolites produced by BA-4 displayed a robust inhibitory effect on Fusarium hyphal growth and spore germination, inducing irregular swelling, atrophy, and abnormal branching of fungal hyphae. In greenhouse experiments, BA-4 markedly reduced the disease index of Fusarium-related ARD, exhibiting protective and therapeutic efficiencies exceeding 80% and 50%, respectively. Moreover, BA-4 demonstrated plant-promoting abilities on both bean and Malus robusta Rehd. (MR) seedlings, leading to increased plant height and primary root length. Field experiments further validated the biocontrol effectiveness of BA-4, demonstrating its ability to mitigate ARD symptoms in MR seedlings with a notable 33.34% reduction in mortality rate and improved biomass. Additionally, BA-4 demonstrates robust and stable colonization capabilities in apple rhizosphere soil, particularly within the 10-20 cm soil layer, which indicates that it has long-term effectiveness potential in field conditions. Overall, B. amyloliquefaciens BA-4 emerges as a promising biocontrol agent with broad-spectrum antagonistic capabilities, positive effects on plant growth, and strong colonization abilities for the sustainable management of ARD in apple cultivation.

A Story of Serendipities: From Phlorizin to Gliflozins

Diabetes has been acknowledged since ancient times. However, it was only during the late 1800s that we realized that the primary organ for blood glucose regulation was the pancreas. The 20th century witnessed insulin purification, which revolutionized the treatment of diabetes maigre; this was followed by the development of oral antidiabetic drugs. The sodium-glucose cotransporter 2 inhibitors or gliflozins are the latest class. Unique cardio- and renoprotective effects separate them from other oral antidiabetic drugs. Here, we present the history behind the development of these inhibitors, arguably the hottest and the most pleasant topic in nephrology. The first serendipity was Koninck and Stas (assistants to Prof. Van Mons, a renowned pomology expert); these researchers isolated a crystalline glycoside called phloridzin (phlorizin) from the bark of apple trees while working at their boss's nursery. Their discovery was published in German in 1835. The second serendipity, after a half century, was from Prof. von Mering, who decided to administer phlorizin to dogs. Oskar Minkowski initially observed polyuria than glucosuria. Insightfully, von Mering postulated that phlorizin affects kidneys. In 1887, they reported that phlorizin induced glucosuria in people with diabetes. The third serendipity was that phlorizin causes several gastrointestinal side effects and has poor oral bioavailability. The first phlorizin-based drug to enter trials was T-1095. The first clinically available gliflozin was dapagliflozin, receiving approval in Europe and the United States in 2012 and 2014, respectively. The 2015 EMPA-REG Outcome trial reported extremely satisfying results that no one expected. Subsequent trials and real-world data have resulted in changes in all impactful guidelines. The impact of these agents on heart failure and chronic kidney disease seems independent of their antidiabetic properties. More than 100 years after von Mering's original discovery, descendants of phlorizin are fast becoming the most inspiring medicine for the 21st century physician.

Chemical fumigants control apple replant disease: Microbial community structure-mediated inhibition of Fusarium and degradation of phenolic acids

Fusarium and phenolic acids in apple replant soil have deleterious effects on soil, which affects the growth of young replanted apple trees. Here, we studied the effects of different chemical fumigants (metham sodium, dazomet, calcium cyanamide, 1,3-dichloropropene, and methyl bromide) on Fusarium and phenolic acids in soil. The chemical fumigants disturbed the apple replant soil microbial community to different degrees in the order from highest to the lowest as methyl bromide > 1,3-dichloropropene > dazomet > metham sodium > calcium cyanamide. Compared with the control, the total numbers of Operational Taxonomic Unit (OTU) were 104.63 % and 9.38 % lower in the methyl bromide and calcium cyanamide treatments, respectively while the average contents of Fusarium were 88.04 % and 59.18% lower in these treatments, respectively. Higher disturbance degrees resulted in a slower recovery rate of the soil microbial community, which facilitated the transformation of the soil into a disease-suppressing state. During the recovery process, the roots recruited Streptomyces OTU2796 and Bacillus OTU2243, which alleviated Fusarium-induced stress via the synthesis of polyketones and macrolides. The roots also recruited Sphingomonas OTU3488, OTU5572, and OTU8147, which alleviated phenolic acid-induced stress through the degradation of benzoate and polycyclic aromatic hydrocarbons.

Allium fistulosum L. Alleviates Apple Replant Disease by Suppressing Fusarium solani

Fusarium solani has often been isolated from replanted apple roots, suggesting that it is associated with apple replant disease. The mechanism underlying the ability of the mixed cropping of apple trees with Allium fistulosum L. to alleviate apple replant disease remains unclear. The aim of this study was to determine the pathogenicity of the Fusarium solani isolate HBH 08 isolated from diseased roots and the effect of A. fistulosum L. and its root secretions on Fusarium solani isolate HBH 08 and apple seedings. The field experiment showed that A. fistulosum L. not only significantly reduced the amount of the Fusarium solani isolate HBH 08 in replanted soil but also increased the biomass of the grafted apple seedlings. The GC-MS analysis indicated that dimethyl disulphide and diallyl disulphide were active molecules in the root exudates of A. fistulosum L. They inhibited the growth of the Fusarium solani isolate HBH 08 mycelium and decreased the number of spores germinated. In addition, these compounds reduced the amount of the Fusarium solani isolate HBH 08 under replanted conditions and promoted the growth of grafted apple seedlings. Overall, mixed cropping with A. fistulosum L. might be an effective approach for cultivating apple trees and controlling apple replant disease.

Controlled-Release Diammonium Phosphate Alleviates Apple Replant Disease: An Integrated Analysis of Soil Properties, Plant Growth, and the Soil Microbiome

Exogenous application of nitrogen (N) and phosphate (P) has been demonstrated to alleviate apple replant disease (ARD). Yet, the effect of controlled-release diammonium phosphate (C-DAP), which continuously supply N and P for ARD control, is still poorly understood. Applying C-DAP markedly alleviated the typical symptoms of ARD. C-DAP maintained soil N and P at relatively high and stable levels during the entire growth period of the replanted seedlings, thus, limiting the copy number of the four key pathogenic Fusarium species that cause ARD. Particularly, continuously supplying N and P by C-DAP established a higher fungal diversity than that of conventional diammonium phosphate and induced the fungal community to be more similar to fumigated soil. The positive effect of C-DAP originated from the synergistic effects of regulating microorganisms and enhancing the resistance of the plant caused by a continuous nutrient supply. These findings provide a new perspective in the management of soil-borne diseases.

Autotoxin affects the rhizosphere microbial community structure by influencing the secretory characteristics of grapevine roots

Autotoxins secreted by roots into the soil can trigger rhizosphere microecological imbalances and affect root secretory properties resulting in conditions such as replanting disease. However, information on the effect of autotoxins on root secretion characteristics and regulation of the composition of rhizosphere microorganisms by altered root exudates is limited. In this study, autotoxin ρ-hydroxybenzoic acid (4-HBA) was added to the soil of potted grapevine seedlings, CO₂ pulse-labeling, and DNA stable isotope probing were used to track the rhizosphere microbiome that assimilates root exudates. Bacterial and fungal microbiomes that assimilated plant-derived carbon were identified by high-throughput sequencing. Results showed that 4-HBA treatment altered bacterial and fungal communities in ¹³C-labeled organisms, with a lower abundance of beneficial bacteria (e.g., Gemmatimonas, Streptomyces, and Bacillus) and a higher abundance of potential pathogen fungi (e.g., Fusarium, Neocosmospora, Gibberella, and Fusicolla) by changing the composition of root exudates. The exogenous addition of upregulated compound mixtures of root exudates reduced the abundance of beneficial bacterial Bacillus and increased the abundance of potential pathogen fungi Gibberella. These results suggest that 4-HBA can alter root secretion properties and altered root exudates may enrich certain potential pathogens and reduce certain beneficial bacteria, thereby unbalancing the structure of the rhizosphere microbial community.

Toward a holistic view of orchard ecosystem dynamics: A comprehensive review of the multiple factors governing development or suppression of apple replant disease

Replant diseases are a common occurrence in perennial cropping systems. In apple, progress toward the development of a universally effective disease management strategy, beyond the use of broad-spectrum soil fumigants, is impeded by inconsistencies in defining replant disease etiology. A preponderance of evidence attributes apple replant disease to plant-induced changes in the soil microbiome including the proliferation of soilborne plant pathogens. Findings from alternative studies suggest that the contribution of abiotic factors, such as the accumulation of phenolic detritus from previous orchard plantings, may play a part as well. Engineering of the resident soil microbiome using resource-based strategies is demonstrating potential to limit activity of replant pathogens and improve productivity in newly established orchards. An understanding of factors promoting the assembly of a disease-suppressive soil microbiome along with consideration of host factors that confer disease tolerance or resistance is imperative to the developing a more holistic view of orchard ecosystem dynamics. Here, we review the literature concerning the transition of orchard soil from a healthy state to a replant disease-conducive state. Included in the scope of this review are studies on the influence of soil type and geography on the apple replant pathogen complex. Furthermore, several tolerance and innate resistance mechanisms that have been described in apple to date, including the role of root chemistry/exudates are discussed. Finally, the interplay between apple rootstock genotype and key resource-based strategies which have been shown to “reshape” the plant holobiont in favor of a more prophylactic or disease-suppressive state is highlighted.

Regulating Root Fungal Community Using Mortierella alpina for Fusarium oxysporum Resistance in Panax ginseng

Plant-associated microbes play important roles in plant health and disease. Mortierella is often found in the plant rhizosphere, and its possible functions are not well known, especially in medical plants. Mortierella alpina isolated from ginseng soil was used to investigate its effects on plant disease. The promoting properties and interactions with rhizospheric microorganisms were investigated in a medium. Further, a pot experiment was conducted to explore its effects on ginseng root rot disease. Physicochemical properties, high-throughput sequencing, network co-occurrence, distance-based redundancy analysis (db-RDA), and correlation analysis were used to evaluate their effects on the root rot pathogen. The results showed that Mortierella alpina YW25 had a high indoleacetic acid production capacity, and the maximum yield was 141.37 mg/L at 4 days. The growth of M. alpina YW25 was inhibited by some probiotics (Bacillus, Streptomyces, Brevibacterium, Trichoderma, etc.) and potential pathogens (Cladosporium, Aspergillus, etc.), but it did not show sensitivity to the soil-borne pathogen Fusarium oxysporum. Pot experiments showed that M. alpina could significantly alleviate the diseases caused by F. oxysporum, and increased the available nitrogen and phosphorus content in rhizosphere soil. In addition, it enhanced the activities of soil sucrase and acid phosphatase. High-throughput results showed that the inoculation of M. alpina with F. oxysporum changed the microbial community structure of ginseng, stimulated the plant to recruit more plant growth-promoting bacteria, and constructed a more stable microbial network of ginseng root. In this study, we found and proved the potential of M. alpina as a biocontrol agent against F. oxysporum, providing a new idea for controlling soil-borne diseases of ginseng by regulating rhizosphere microorganisms.

The Phlorizin-Degrading Bacillus licheniformis XNRB-3 Mediates Soil Microorganisms to Alleviate Apple Replant Disease

In this study, an endophytic phlorizin-degrading Bacillus licheniformis XNRB-3 was isolated from the root tissue of healthy apple trees, and its control effect on apple replant disease (ARD) and how it alleviates the pathogen pressure via changes in soil microbiomes were studied. The addition of strain XNRB-3 in Fusarium infested soils significantly reduced the number of pathogens in the soil, thus resulting in a lower disease incidence, and the relative control effect on Fusarium oxysporum reached the highest of 66.11%. The fermentation broth can also protect the roots of the plants from Fusarium oxysporum, Fusarium moniliforme, Fusarium proliferatum, and Fusarium solani infection. These antagonistic effects were further validated using an in vitro assay in which the pathogen control was related to growth and spore germination inhibition via directly secreted antimicrobial substances and indirectly affecting the growth of pathogens. The secreted antimicrobial substances were identified using gas chromatography-mass spectrometry (GC-MS) technology. Among them, alpha-bisabolol and 2,4-di-tert-butylphenol had significant inhibitory effects on many planted pathogenic fungi. Butanedioic acid, monomethyl ester, and dibutyl phthalate promoted root development of Arabidopsis plants. Strain XNRB-3 has multifarious plant growth promoting traits and antagonistic potential. In pot and field experiments, the addition of strain XNRB-3 significantly promoted the growth of plants, and the activity of enzymes related to disease resistance [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)] was also significantly enhanced. It also reduced the abundance of four species of Fusarium and the content of phenolic acids in the rhizosphere soil, improved soil microbial community structure and nutritional conditions, and increased soil microbial diversity and activity, as well as the soil enzyme activity. The above results indicated that B. licheniformis XNRB-3 could be developed into a promising biocontrol and plant-growth-promoting agent.

Effects of Bacillus amyloliquefaciens QSB-6 on the Growth of Replanted Apple Trees and the Soil Microbial Environment

Apple replant disease (ARD), caused largely by soil-borne fungal pathogens, has seriously hindered the development of the apple industry. The use of antagonistic microorganisms has been confirmed as a low-cost and environmentally friendly means of controlling ARD. In the present study, we assessed the effects of Bacillus amyloliquefaciens QSB-6 on the growth of replanted apple saplings and the soil microbial environment under field conditions, thus providing a theoretical basis for the successful use of microbial biocontrol agents. Four treatments were implemented in three apple orchards: untreated replant soil (CK1), methyl bromide fumigation (CK2), blank carrier treatment (T1), and QSB-6 bacterial fertilizer treatment (T2). The plant height, ground diameter, and branch length of apple saplings treated with T2 in three replanted apple orchards were significantly higher than that of the CK1 treatment. Compared with the other treatments, T2 significantly increased the number of soil bacteria, the proportion of actinomycetes, and the activities of soil enzymes. By contrast, compared with the CK1 treatments, the phenolic acid content, the number of fungi, and the abundance of Fusarium oxysporum, Fusarium moniliforme, Fusarium proliferatum, and Fusarium solani in the soil were significantly reduced. PCoA and cluster analysis showed that soil inoculation with strain QSB-6 significantly decreased the Mcintosh and Brillouin index of soil fungi and increased the diversity of soil bacteria in T2 relative to CK1. The soil bacterial community structure in T2 was different from the other treatments, and the soil fungal communities of T2 and CK2 were similar. In summary, QSB-6 bacterial fertilizer shows promise as a potential bio-inoculum for the control of ARD.

Nematode-Microbe Complexes in Soils Replanted with Apple

Apple replant disease is a severe problem in orchards and tree nurseries. Evidence for the involvement of a nematode–microbe disease complex was reported. To search for this complex, plots with a history of apple replanting, and control plots cultivated for the first time with apple were sampled in two fields in two years. Shoot weight drastically decreased with each replanting. Amplicon sequencing of the nematode community and co-extracted fungal and bacterial communities revealed significant differences between replanted and control plots. Free-living nematodes of the genera Aphelenchus and Cephalenchus and an unidentified Dorylaimida were associated with replanted plots, as indicated by linear discriminant analysis effect size. Among the co-extracted fungi and bacteria, Mortierella and Methylotenera were most indicative of replanting. Some genera, mostly Rhabditis, Streptomyces and a fungus belonging to the Chaetomiaceae indicated healthy control plots. Isolating and investigating the putative disease complexes will help to understand and alleviate stress-induced root damage of apple in replanted soil.

Integrative Physiological, Transcriptional, and Metabolic Analyses Provide Insights Into Response Mechanisms of Prunus persica to Autotoxicity Stress

Autotoxicity is known as a critical factor in replanting problem that reduces land utilization and creates economic losses. Benzoic acid (BA) is identified as a major autotoxin in peach replant problem, and causes stunted seedling growth or even death. However, the physiological and molecular mechanisms of peach response to BA stress remain elusive. Here, we comprehensively studied the morphophysiological, transcriptional, and metabolic responses of peach plants to BA toxicity. Results showed that BA stress inhibited peach seedlings growth, decreased chlorophyll contents and fluorescence levels, as well as disturbed mineral metabolism. The contents of hydrogen peroxide, superoxide anion, and malondialdehyde, as well as the total antioxidant capacity, were significantly increased under BA stress. A total of 6,319 differentially expressed genes (DEGs) were identified after BA stress, of which the DEGs related to photosynthesis, redox, and ion metabolism were greatly changed; meanwhile, numerous stress-responsive genes (HSPs, GSTs, GR, and ABC transporters) and transcription factors (MYB, AP2/ERF, NAC, bHLH, and WRKY) were noticeably altered under BA stress. BA induced metabolic reprogramming, and 74 differentially accumulated metabolites, including amino acids and derivatives, fatty acids, organic acids, sugars, and sugar alcohols, were identified in BA-stressed roots. Furthermore, an integrated analysis of genes and metabolites indicated that most of the co-mapped KEGG pathways were enriched in amino acid and carbohydrate metabolism, which implied a disturbed carbon and nitrogen metabolism after BA stress. The findings would be insightful in elucidating the mechanisms of plant response to autotoxicity stress, and help guide crops in alleviating replant problem.