Rhizosphere Of Trees Research Articles

Exploring the potentialities of an improved triglyceride 16:1ω5 extraction technique for quantitative estimation of arbuscular mycorrhizal fungi in the rhizosphere of olive trees

A method combining solid-phase extraction (SPE) on silica (Si) and aminopropyl-bonded silica (NH2) columns has been applied for the separation of phospholipids (PL), glycolipids (GL), neutral lipids (NL) and triglyceride fatty acids from the spores of Rhizophagus irregularis DAOM 197198 and in olive rhizosphere with and without arbuscular mycorrhizal (AM) fungi. Chromatography on the Si-column resulted in the separation of NL, GL and PL. Chromatography on the NH2- column led to the separation of TG from NL, previously coeluted on the Si-column. The fatty acid composition of TG of Rhizophagus irregularis spores obtained using this method showed higher amounts of 16:1ω5 (40%). The TGFA 16:1ω5 was present in higher levels in rhizosphere with AM fungi than in rhizosphere without AM fungi. This Si/NH2 method provided a rapid separation of TGFA 16:1ω5, commonly found in living cells, and will be useful to quantify the biomass of AM fungi in soil.

Plant Growth Promoting (PGP) Performances and Diversity of Bacterial Species Isolated from Olive (Olea europaea L.) Rhizosphere in Arid and Semi-arid Regions of Morocco

Plant Growth Promoting Rhizobacteria (PGPR) play an essential role in enhancing plant growth, health and yield. In this study, we describe the isolation of PGPR associated with the olive tree (Olea europaea L.) grown in three Moroccan regions of Zouala, Errachidia, and Essaouira. In these regions, we isolated 95 PGPRs from rhizosphere of Olive trees, 78% of them were characterized by their tolerance to a salinity of 4-11%. We also found that 39% of these PGPRs were phosphate solubilizing bacteria (PSB) with a solubilization ability greater than 100 µg/mL. In fact, Pantoea agglomerans (MRC_ZO_17) and Enterobacter ludwigii (MRC_ZO_97), showed the highest phosphate solubilization rates of ~450 µg/mL and ~196 µg/mL, respectively. In addition to their ability to solubilize phosphate, various isolates had the ability to produce Indole-3-acetic acid (IAA). For instance, E. ludwigii (MRC_ZO_97) had an IAA production of ~60.48 g/mL. In the region of Zouala, characterized by relatively higher salinity and lower rate of organic matter, Firmicutes isolates account for 87% of the isolated rhizobacteria. Interestingly, we found that the olive tree-associated PGPRs vary significantly between the three sampled regions. Several rhizobacteria isolated in this study are excellent candidates for formulation as bioinocula for plants in saline soils.

Albocycline Is the Main Bioactive Antifungal Compound Produced by Streptomyces sp. OR6 against Verticillium dahliae.

Verticillium wilt is a soil-borne fungal disease that affects olive trees (Olea europaea) and poses a serious threat to their cultivation. The causal agent of this disease is Verticillium dahliae, a pathogen that is difficult to control with conventional methods. Therefore, there is a need to explore alternative strategies for the management of Verticillium wilt. In this study, we aimed to isolate and characterize actinobacteria from the rhizosphere of olive trees that could act as potential biocontrol agents against V. dahliae. We selected a Streptomyces sp. OR6 strain based on its in vitro antifungal activity and its ability to suppress the pathogen growth in soil samples. We identified the main active compound produced by this strain as albocycline, a macrolide polyketide with known antibacterial properties and some antifungal activity. Albocycline was able to efficiently suppress the germination of conidiospores. To our knowledge, this is the first report of albocycline as an effective agent against V. dahliae. Our results suggest that Streptomyces sp. OR6, or other albocycline-producing strains, could be used as a promising tool for the biological control of Verticillium wilt.

The ability of two indigenous bacteria isolated from Moroccan olive tree to promote the growth of olive seedlings in the presence of the pathogen Verticillium dahliae

ABSTRACT The rhizosphere of olive trees (Olea europaea L.) is a source of bacteria with a high potential for biocontrol and plant growth promotion. In the present study, two native olive tree gram-negative bacteria were isolated from the rhizosphere of century-old trees in the region of Zouala (south-eastern Morocco), known for its arid climate and saline soils. The strains Pantoea agglomerans and Enterobacter ludwigii exhibited high tolerance to high PEG and NaCl concentrations, and significant plant growth-promoting (PGP) potential in vitro. The two strains efficiently solubilised phosphate in its two forms (TCP and RP), and Mica as a source of potassium and produced IAA. The two strains exhibited in vitro inhibition ability against Verticillium dahliae with partial inhibition of the growth of the mycelium by the strain P. agglomerans MRC_ZO_17. Greenhouse experiments showed that olive tree seedlings treated with the E. ludwigii MRC_ZO_97 strain significantly improved growth (dry weight) in the presence and absence of the pathogen compared to untreated plants, while the P. agglomerans MRC_ZO_17 strain showed significant effects on seedling growth only in the presence of the pathogen V. dahliae OV1 defoliating pathotype. Both strains showed high exopolysaccharides (EPS) production and good biofilm formation, making them excellent plant growth-promoting rhizobacteria (PGPRs) and Biological Control Agents (BCAs) with great potential to survive in soil under stress conditions.

Effect of microbiologically enriched fertilizers on soil microorganisms in the rhizosphere of apple trees

Effect of microbiologically enriched fertilizers on soil microorganisms in the rhizosphere of apple trees

Correlation of fruit tree rhizosphere soils with entomopathogenic fungi

AbstractEntomopathogenic fungi are microorganisms that control the density of host insects in nature; they are being studied as environmentally friendly alternatives to chemical insecticides for controlling insect pests. The main habitat of entomopathogenic fungi is soil, and the correlation between the distribution of entomopathogenic fungi and the physicochemical characteristics of soils planted with different trees, including vine (outdoor, greenhouse, and greenhouse shine musket), apple, peach, and pear, were analyzed. The entomopathogenic fungi of the genera Beauveria, Metarhizium, and Purpureocillium investigated in this study were all found in soil samples from vine‐greenhouse, apple, and peach trees. Purpureocillium and Beauveria abundances were positively correlated with soil properties; however, Metarhizium abundances were not correlated with soil properties. The Metarhizium isolates discovered in this study showed pathogenicity to cotton aphids (an agricultural pest) and can be employed as sources for biological studies in the future. This study provides data on the diversity and abundance of entomopathogenic fungi related with soil properties, as well as the molecular, biological, and insecticidal characteristics of Metarhizium isolates.

Diversity of Endomycorrhizal Fungi in Argan Forest Stands: Implications for the Success of Reforestation Programs

Over the last few decades, argan trees (Argania spinosa L.) skeels have faced harsh ecological conditions and anthropogenic pressure, leading to a dramatic decline in surface and density of cultivation. Nowadays, most techniques used to regenerate argan trees have failed. Arbuscular mycorrhizal fungi (AMF) are root symbionts that increase plant resistance to biotic and abiotic stresses during transplantation. The exploration of these symbiotic fungi from different soils of argan stands is the starting point for the selection and production of high-performance organisms adapted to the reforestation sites. The objective of this study is to investigate the composition of the AMF community associated with the argan tree rhizosphere. Forty adult argan trees were sampled in eight forest sites representative of the distribution and genetic diversity of argan forest stands. Five sub-samples of rhizospheric soil were taken around each tree. Our results revealed the presence of different AMF structures (i.e., hyphae, vesicles/and arbuscules) in root samples. Based on morphological characterization, six genera of AMF spores were identified with a dominance of the genera Septoglomus (34%). In addition, soil organic matter and phosphorus concentrations showed a highly significant correlation with AMF spore density. The chi-square test showed a highly significant dependence of the distribution of genera on the site conditions of forest stands. These AMF could be tested and used during the inoculation of argan seedlings in forest nurseries for the success of restoration and reforestation programs, as well as for the development and sustainable improvement of this agroforestry system.

Unleashing the Potential of Bacterial Isolates from Apple Tree Rhizosphere for Biocontrol of Monilinia laxa: A Promising Approach for Combatting Brown Rot Disease.

Monilinia laxa, a notorious fungal pathogen responsible for the devastating brown rot disease afflicting apples, wreaks havoc in both orchards and storage facilities, precipitating substantial economic losses. Currently, chemical methods represent the primary means of controlling this pathogen in warehouses. However, this study sought to explore an alternative approach by harnessing the biocontrol potential of bacterial isolates against brown rot in apple trees. A total of 72 bacterial isolates were successfully obtained from the apple tree rhizosphere and subjected to initial screening via co-cultivation with the pathogen. Notably, eight bacterial isolates demonstrated remarkable efficacy, reducing the mycelial growth of the pathogen from 68.75 to 9.25%. These isolates were subsequently characterized based on phenotypic traits, biochemical properties, and 16S rRNA gene amplification. Furthermore, we investigated these isolates' production capacity with respect to two enzymes, namely, protease and chitinase, and evaluated their efficacy in disease control. Through phenotypic, biochemical, and 16S rRNA gene-sequencing analyses, the bacterial isolates were identified as Serratia marcescens, Bacillus cereus, Bacillus sp., Staphylococcus succinus, and Pseudomonas baetica. In dual culture assays incorporating M. laxa, S. marcescens and S. succinus exhibited the most potent degree of mycelial growth inhibition, achieving 68.75 and 9.25% reductions, respectively. All the bacterial isolates displayed significant chitinase and protease activities. Quantitative assessment of chitinase activity revealed the highest levels in strains AP5 and AP13, with values of 1.47 and 1.36 U/mL, respectively. Similarly, AP13 and AP6 exhibited the highest protease activity, with maximal enzyme production levels reaching 1.3 and 1.2 U/mL, respectively. In apple disease control assays, S. marcescens and S. succinus strains exhibited disease severity values of 12.34% and 61.66% (DS), respectively, highlighting their contrasting efficacy in mitigating disease infecting apple fruits. These findings underscore the immense potential of the selected bacterial strains with regard to serving as biocontrol agents for combatting brown rot disease in apple trees, thus paving the way for sustainable and eco-friendly alternatives to chemical interventions.

Improve tea quality by enhancing the stability of bacterial community interaction and nitrogen-fixing function in pecan-tea interplanted plantation

Using the 16 S rDNA high-throughput sequencing, the effects of interplanted pecan (Carya illinoinensis) in a typical tea (Camellia sinensis) plantation on soil bacterial species composition and marker species were comparatively analyzed to determine whether nested pecan increases soil microbial diversity in the tea tree rhizosphere. We explored the effects of the underlying mechanisms of this complex ecosystem on tea quality by determining soil physicochemical properties using two types of planting modes: intercropping with pecan versus tea tree monoculture. Our results showed that Rhizobium (Allorhizobium, Neorhizobium, Pararhizobium, and Rhizobium), and Enterobacter, had pollution-degrading effects and were enriched in the soil bacterial communities of the pecan interplanted model. There was a significant enrichment of prebiotic functional bacteria, such as Pedosphaeraceae and Coriobateriaceae_UCG-002, which stimulate plant growth and improve disease resistance, while the functions of Chloroflexi and Firmicutes, which are dominant denitrifying bacteria in the soil, were inhibited. The average degree of soil microbial co-occurrence network increased from 13.40 to 18.17, and the number of nodes in the network increased from 117 to 166, indicating that the network increased in complexity and decreased in mutual exclusivity, and the complex interactions among bacterial populations tended to be stable. The intercropping mode reduced the phenol/ammonia ratio in tea from 2.37 to 1.38, improving the tea quality. Therefore, the soil microbial community composition under this complex model promotes material cycling in microecosystems and effectively reduces the transmission risk of soil-borne diseases. The soil microbial community structure of the tea rhizosphere could be re-balanced and shifted toward a more favorable tea quality formation by the introduction of pecan into tea plantations.

Relationship of nematodes in natural and disturbed land with physicochemical properties in Magoebaskloof, Limpopo Province, South Africa

AbstractMagoebaskloof is an evergreen mountainous landscape covered with Afromontane forests. During a survey on soil nematode diversity, the soil samples were collected from the rhizosphere of a forest tree (no cultivation over 50 years), grassland, prepared kiwi land, oak tree, kiwifruits trees (37-year-old), and 8-year-old kiwifruit in the Limpopo Province, South Africa. Overall, results showed that a total of 30 nematode genera were found to be associated with the surveyed plant species. The frequency of occurrence showed Ditylenchus (100%), Acrobeloides (100%), and Nanidorus (100%), followed by Meloidogyne (83%) were the most distributed nematodes. The study of the relationship between nematodes with physicochemical properties in the soil using Pearson correlation revealed that clay percentage of the soil has a positive correlation with Pseudacrobeles (r = 0.684), Acrobeloides (r = 0.658) and Meloidogyne (r = 0.668) species. In contrast, clay percentage had a strong negative correlation (r = -0.900) with Filenchus. Soil pH showed no strong correlation with the nematodes. The principal component analysis placed kiwifruits and forest soil in two groups, indicating the biodiversity dynamics among the two locations. In conclusion, forest trees and grasslands showed a high diversity of free-living than disturbed land of kiwifruits. Additionally, plant-parasitic nematodes numbered more in the rhizosphere of kiwifruit trees. Our results suggest that the soil nematodes, especially free-living bacterivores, may mediate the effects of ecosystem disturbance on soil health.

Improvement of phenolic acid autotoxicity in tea plantations by Pseudomonas fluorescens ZL22

Accumulation of phenolic acids, such as p-hydroxybenzoic acid (PHBA), 3,4 dihydroxybenzoic acid (PA), and cinnamic acid (CA) causes a decline in tea plantation soil quality. Bacterial strains that can balance phenolic acid autotoxicity (PAA) in tea tree rhizosphere soil are used to improve tea plantation soil. In this study, the effects of Pseudomonas fluorescens ZL22 on soil restoration and PAA regulation in tea plantations were investigated. ZL22 carries a complete pathway for degrading PHBA and PA to acetyl coenzyme A. ZL22 can colonise and reduce PHBA by 96% and PA by 98% in tea rhizosphere soil within 30 days. The cooccurrence of ZL22 and low CA levels further promotes lettuce seed growth and substantially increases tea production. ZL22 effectively regulates PAA to a safe level in rhizospheric soil, alleviating the inhibition of microbiota by PAA, increases the abundance of genera associated with soil N, C, and S cycling, and creates optimum pH (approximately 4.2) and organic carbon (approximately 25 g/kg), and available N (approximately 62 mg/kg) contents for secondary metabolite accumulation in tea leaves. The application of P. fluorescens ZL22 controls PAA, which synergistically improves plant growth and soil nutrition, thereby promoting tea production and quality.

Assessment of Potent Phosphate-Solubilizing Bacteria Isolated from the Olive Tree Rhizosphere Grown on Phosphate Sludge and Their Effect on Common Bean Growth

One of the most restricting macronutrients for crop yield worldwide is phosphorus, which is often less than 1% of the total amount contained in soils. To promote its bioavailability, there are microorganisms considered vital that can solubilize and mineralize their pools in soils. In the current study, we assessed the potential of phosphate-solubilizing bacteria (PSB), isolated from the olive tree rhizosphere grown on phosphate sludge, to solubilize phosphate, promote plant growth, and tolerate abiotic stresses. Based on 16S rRNA gene sequencing, twenty-four PSB strains were identified and retained for in vitro analysis. According to the results, all the strains were able to solubilize the Moroccan rock phosphate, with soluble phosphate concentrations ranged from 5.34 µg/mL to 227 µg/mL. Furthermore, the majority of the strains are thermo-tolerant and halotolerant. Nonetheless, only five strains produced indole acetic acid. Regarding biocontrol potentialities, several PSB strains were characterized by producing hydrogen cyanide and hydrolytic enzymes (cellulase, and chitinase), of which three strains identified as Pseudomonas moraviensis, Bacillus cereus, and Bacillus aryabhattai, with a remarkable multi-trait combination were selected for antagonism and co-inoculation tests. The findings revealed that these PSB strains significantly inhibited Fusarium oxysporum (17.65%–62.35%) and Verticillium dahliae (52.35%–66.87%) and promoted common bean growth. The consortium of the three strains showed the best results by significantly increasing both plant height and tap root length and dry biomass compared to individual inoculation. The PSB selected from the olive tree rhizosphere growing on phosphate sludge have the potential to be useful as biofertilizer and biocontrol agents for attaining sustainable food crop production.

Response of soil nutrients, enzyme activities, and fungal communities to biochar availability in the rhizosphere of mountainous apple trees

Response of soil nutrients, enzyme activities, and fungal communities to biochar availability in the rhizosphere of mountainous apple trees

Grapefruit Root and Rhizosphere Responses to Varying Planting Densities, Fertilizer Concentrations and Application Methods.

Huanglongbing (HLB) disease has caused a severe decline in citrus production globally over the past decade. There is a need for improved nutrient regimens to better manage the productivity of HLB-affected trees, as current guidelines are based on healthy trees. The aim of this study was to evaluate the effects of different fertilizer application methods and rates with different planting densities on HLB-affected citrus root and soil health. Plant material consisted of 'Ray Ruby' (Citrus × paradisi) grapefruit trees grafted on 'Kuharske' citrange (Citrus × sinensis × Citrus trifoliata). The study consisted of 4 foliar fertilizer treatments, which included 0×, 1.5×, 3× and 6× the University of Florida Institute of Food and Agriculture (UF/IFAS) recommended guidelines for B, Mn and Zn. Additionally, 2 ground-applied fertilizer treatments were used, specifically controlled-release fertilizer (CRF1): 12-3-14 + B, Fe, Mn and Zn micronutrients at 1× UF/IFAS recommendation, and (CRF2): 12-3-14 + 2× Mg + 3× B, Fe, Mn and Zn micronutrients, with micronutrients applied as sulfur-coated products. The planting densities implemented were low (300 trees ha-1), medium (440 trees ha-1) and high (975 trees ha-1). The CRF fertilizer resulted in greater soil nutrient concentrations through all of the time sampling points, with significant differences in soil Zn and Mn. Grapefruit treated with ground-applied CRF2 and 3× foliar fertilizers resulted in the greatest bacterial alpha and beta diversity in the rhizosphere. Significantly greater abundances of Rhizobiales and Vicinamibacterales were found in the grapefruit rhizosphere of trees treated with 0× UF/IFAS foliar fertilizer compared to higher doses of foliar fertilizers.

Exploring the Biocontrol Efficacy of Trichoderma spp. against Rigidoporus microporus, the Causal Agent of White Root Rot Disease in Rubber Trees (Hevea brasiliensis).

Rigidoporus microporus, which causes white root rot disease (WRD) in Hevea brasiliensis, is a looming threat to rubber plantation in Malaysia. The current study was conducted to determine and evaluate the efficiency of fungal antagonists (Ascomycota) against R. microporus in rubber trees under laboratory and nursery conditions. A total of 35 fungal isolates established from the rubber tree rhizosphere soil were assessed for their antagonism against R. microporus by the dual culture technique. Trichoderma isolates can inhibit the radial growth of R. microporus by 75% or more in the dual culture test. Strains of T. asperellum, T. koningiopsis, T. spirale, and T. reesei were selected to assess the metabolites involved in their antifungal activity. Results indicated that T. asperellum exhibited an inhibitory effect against R. microporus in both volatile and non-volatile metabolite tests. All Trichoderma isolates were then tested for their ability in producing hydrolytic enzymes such as chitinase, cellulase and glucanase, indole acetic acid (IAA), siderophores production, and phosphate solubilization. From the positive results of the biochemical assays, T. asperellum and T. spirale were selected as the biocontrol candidates to be further tested in vivo against R. microporus. The nursery assessments revealed that rubber tree clone RRIM600 pretreated with only T. asperellum or with the combination of T. asperellum and T. spirale was able to reduce the disease severity index (DSI) and exert higher suppression of R. microporus compared to other pretreated samples, with the average DSI below 30%. Collectively, the present study demonstrates that T. asperellum represents a potential biocontrol agent that should be further explored to control R. microporus infection on rubber trees.

Microbiome diversity, composition and assembly in a California citrus orchard.

The citrus root and rhizosphere microbiomes have been relatively well described in the literature, especially in the context of Huanglonbing disease. Yet questions addressing the assembly of root microbial endophytes have remained unanswered. In the above ground tree tissues, leaves and stems have been the research focus point, while flush and flower microbiomes, two important tissues in the vegetative and reproductive cycles of the tree, are not well described. In this study, the fungal and bacterial taxa in five biocompartments (bulk soil, rhizosphere, root endosphere, flower and flush) of citrus trees grown in a single California orchard were profiled using an amplicon-based metagenomic Illumina sequencing approach. Trees with no observable signs of abiotic or biotic stresses were sampled for two consecutive years during the floral development phase. The rhizosphere was the most biodiverse compartment compared to bulk soil, root endosphere, flower and flush microbiomes. In addition, the belowground bacteriome was more diverse than the mycobiome. Microbial richness decreased significantly from the root exosphere to the endosphere and was overall low in the above ground tissues. Root endophytic microbial community composition shared strong similarities to the rhizosphere but also contained few taxa from above ground tissues. Our data indicated compartmentalization of the microbiome with distinct profiles between above and below ground microbial communities. However, several taxa were present across all compartments suggesting the existence of a core citrus microbiota. These findings highlight key microbial taxa that could be engineered as biopesticides and biofertilizers for citriculture.

Effect of fabric mulch ground covers on lemon trees rhizosphere microbiome in Florida flatwood soils

IntroductionThe rhizosphere community composition has been shown to provide benefits in nutrient acquisition and plant health. Rhizosphere communities can be heavily influenced by cultural practices in citrus production systems, providing benefits in soil and root health, and plant physiology. Florida growers are implementing new and more sustainable soil management practices, such as using fabric mulch ground covers to improve weed and pest control, while retaining soil moisture. Little is known regarding the influence of these ground covers on the rhizosphere microbial communities of lemon trees, especially under endemic Huanglongbing pressure. Understanding how lemon roots and the rhizosphere microbiome are influenced by fabric mulch ground covers can potentially lead to improved management practices. The aim of this research was to evaluate the effects of fabric mulch ground covers on lemon tree rhizosphere health.MethodsThe experiment was conducted in a commercial citrus grove located in Fort Pierce, FL, US. Plant material consisted of four-year old lemon trees cv. ‘Meyer’ (Citrus limon) grafted on ‘sour orange’ (Citrus × aurantium) rootstocks. The experiment consisted of two treatments, which included trees grown with fabric mulch ground covers and trees grown without fabric mulch ground covers. Rhizosphere DNA was extracted, sequenced, and analyzed to assess differences in diversity and composition (alpha and beta diversity) of rhizosphere bacteria among treatments.Results and DiscussionObtained results showed that lemon trees treated with fabric mulch ground covers had significantly greater rhizosphere bacterial diversity when compared to the uncovered trees. The presence of fabric mulch ground covers resulted in significantly greater soil Zn, soil Mn, soil temperatures and pH, potentially contributing towards the significant more diverse rhizosphere bacterial commu nity composition compared than those grown without fabric mulch ground covers.

Efficacy of native strains of entomopathogenic nematode, Heterorhabditis indica against the fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) from India

Abstract Background The fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is a notorious polyphagous pest that has recently invaded India in 2018. Within a short period, this pest has spread throughout the country, causing a significant damage to maize. In order to manage this menace, farmers rely on chemical insecticides, but due to concealed feeding nature of this pest insecticides remain ineffective in reaching the target site. In this context, the present study aimed to isolate, characterize and evaluate the reproductive potential of the native strains of entomopathogenic nematode, Heterorhabditis indica against larval instars of S. frugiperda under laboratory conditions. Results Two strains of H. indica isolated and characterized from the rhizosphere of mango and curry leaf trees were designated as CICR-HI-MN and CICR-HI-CL, respectively. Both strains, CICR-HI-CL and CICR-HI-MN showed differential mortality against 3rd (LC50 value of 21.65, 48.91 IJs/larva), 4th (LC50 value of 25.46, 52.36 IJs/larva) and 5th (LC50 value of 59.20, 71.04 IJs/larva) instar larvae of S. frugiperda, respectively. The instar-wise mortality of S. frugiperda showed that 3rd instar was relatively more susceptible compared to 4th and 5th instar larvae. In terms of efficacy, CICR-HI-CL was almost twice effective than CICR-HI-MN when LC50 and LC95 values of 3rd instar larvae were compared. However, the reproductive potential of both the strains was significantly higher against 5th instar than the 3rd and 4th instar larvae. Strain CICR-HI-CL was more effective in larval mortality, while nematode reproduction was higher in CICR-HI-MN. Conclusions Native strains of H. indica evaluated in the present study were found effective against larval instars of fall armyworm, S. frugiperda. Higher reproduction on 5th instar larvae demonstrates the ability of these strains to multiply, sustain and perpetuate on late instars of this polyphagous pest and may serve as viable option in integrated pest management program.

Microbial communities in tree root-compartment niches under Cd and Zn pollution: Structure, assembly process and co-occurrence relationship

Woody plants have showed great potential in remediating severely contaminated soils by heavy metals (HMs) due to their cost-efficient and ecologically friendly trait. It is believed the root-associated microbiota plays a vital role in phytoremediation for HMs. However, the ecological process controlling the assembly and composition of tree root-associated microbial communities under HMs stress remains poorly understood. Herein, we profiled the bulk soil, rhizosphere and endosphere microbial communities of trees growing in heavily Cd and Zn polluted soil. The microbiota was gradually filtered from bulk soil to the tree roots and was selectively enriched in roots with specific taxa, such as Proteobacteria and Ascomycota. The microbial community assembly along the soil-root continuum was mainly controlled by deterministic processes from bulk soil to the endosphere, with the normalized stochasticity ratio (NST) indices of 67.16–31.05 % and 30.37–15.02 % for bacteria and fungi, respectively. Plant selection pressure sequentially increased from bulk soil to rhizosphere to endosphere, with the reduced bacterial alpha diversity accompanying the consequently reduced complexity of the co-occurrence network. Together, the findings provide new evidence for horizontal transmission of endophytic microbiome from soil to the host, which can shed light on the future screening and application of microbial-assisted phytoremediation.

Soil organic carbon sourcing variance in the rhizosphere vs. non-rhizosphere of two mycorrhizal tree species

Soil organic carbon (SOC) plays a central role in ecosystem carbon sequestration and climate change mitigation, and its stability and dynamics are related to sourcing from microbial vs. plant residues. However, SOC sourcing and its regulating mechanisms remain poorly understood in soil's most bioactive compartment, the rhizosphere, which may differ from non-rhizosphere and under different mycorrhizal tree species. To fill the knowledge gap, here we collect the rhizosphere and non-rhizosphere soils under an arbuscular mycorrhizal (AM; Castanopsis eyrie) vs. an ectomycorrhizal (ECM) tree species (Pinus massoniana) of varied tree diameters (i.e., ages) in the Gutianshan subtropical forest of China. Plant and microbial residual components are quantified by lignin phenols and amino sugars, respectively. Coupled with the measurements of soil, microbial community and plant litter properties, we assess potential mechanisms (i.e., saprotrophic bacteria competition, microbial necromass recycling/reuse, and substrate quality control) influencing the distribution of plant and microbial residues in the rhizosphere vs. non-rhizosphere. We show that lignin phenols are more concentrated in rhizosphere than non-rhizosphere SOC, especially under the ECM trees showing inhibited saprotrophic decomposition induced by competition between ECM fungi and (saprophytic) bacteria. Amino sugars are also more concentrated in the rhizosphere of ECM trees due to ECM fungal contribution, but not under AM trees exhibiting reduced fungal necromass stability partially reflected by low biomass-normalized necromass accumulation coefficients in the rhizosphere. As a result, ratios of amino sugars to lignin phenols are relatively lower in the rhizosphere than non-rhizosphere under AM tree, challenging the presumed microbial dominance in rhizosphere carbon accumulation. These results highlight differences in and controls on rhizosphere SOC sourcing related to different mycorrhizal tree species, providing new information on the mechanisms regulating soil carbon dynamics in root-soil systems.