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Walnut (Juglans regia) is an important nut tree species in the world, whereas walnut trees often face inadequate phosphorus (P) levels of soil, negatively limiting its growth and yield. Arbuscular mycorrhizal fungi (AMF) can colonize walnut roots, but whether and how AMF promotes walnut growth, physiological activities, and P acquisition is unclear. The present study aimed to evaluate the effects of Diversispora spurca on plant growth, chlorophyll component concentrations, leaf gas exchange, sugar and P concentrations, and expression of purple acid phosphatase (PAP) and phosphate transporter (PT) genes in leaves of J. regia var. Liaohe 1 seedling under moderate (100 μmol/L P) and low P (1 μmol/L P) levels conditions. Three months after inoculation, the root mycorrhizal colonization rate and soil hyphal length were 45.6−53.2% and 18.7−39.9 cm/g soil, respectively, and low P treatment significantly increased both root mycorrhizal colonization rate and soil hyphal length. Low P levels inhibited plant growth (height, stem diameter, and total biomass) and leaf gas exchange (photosynthetic rate, transpiration rate and stomatal conductance), while AMF colonization significantly increased these variables at moderate and low P levels. Low P treatment limited the level of chlorophyll a, but AMF colonization did not significantly affect the level of chlorophyll components, independent on soil P levels. AMF colonization also increased leaf glucose at appropriate P levels and leaf fructose at low P levels than non-AMF treatment. AMF colonization significantly increased leaf P concentration by 21.0−26.2% than non-AMF colonization at low and moderate P levels. Low P treatment reduced the expression of leaf JrPAP10, JrPAP12, and JrPT3;2 in the inoculated plants, whereas AMF colonization up-regulated the expression of leaf JrPAP10, JrPAP12, and JrPT3;2 at moderate P levels, although AMF did not significantly alter the expression of JrPAPs and JrPTs at low P levels. It is concluded that AMF improved plant growth, leaf gas exchange, and P acquisition of walnut seedlings at different P levels, where mycorrhizal promotion of P acquisition was dominated by direct mycorrhizal involvement in P uptake at low P levels, while up-regulation of host PAPs and PTs expressions at moderate P levels.

Multiple functions of glomalin released by arbuscular mycorrhizal fungi are well-recognized, whereas the role of exogenous glomalins including easily extractable glomalin-related soil protein (EE-GRSP) and difficultly extractable glomalin-related soil protein (DE-GRSP) is unexplored for plant responses. Our study was carried out to assess the effects of exogenous EE-GRSP and DE-GRSP at varying strengths on plant growth and chlorophyll concentration of trifoliate orange (Poncirus trifoliata) seedlings, along with changes in root nutrient acquisition, auxin content, auxin-related enzyme and transporter protein gene expression, and element contents of purified GRSP. Sixteen weeks later, exogenous GRSP displayed differential effects on plant growth (height, stem diameter, leaf number, and biomass production): the increase by EE-GRSP and the decrease by DE-GRSP. The best positive effect on plant growth occurred at exogenous EE-GRSP at ½ strength. Similarly, the GRSP application also differently affected total chlorophyll content, root morphology (total length, surface area, and volume), and root N, P, and K content: positive effect by EE-GRSP and negative effect by DE-GRSP. Exogenous EE-GRSP accumulated more indoleacetic acid (IAA) in roots, which was associated with the upregulated expression of root auxin synthetic enzyme genes (PtTAA1, PtYUC3, and PtYUC4) and auxin influx transporter protein genes (PtLAX1, PtLAX2, and PtLAX3). On the other hand, exogenous DE-GRSP inhibited root IAA and indolebutyric acid (IBA) content, associated with the downregulated expression of root PtTAA1, PtLAX1, and PtLAX3. Root IAA positively correlated with root PtTAA1, PtYUC3, PtYUC4, PtLAX1, and PtLAX3 expression. Purified EE-GRSP and DE-GRSP showed similar element composition but varied in part element (C, O, P, Ca, Cu, Mn, Zn, Fe, and Mo) concentration. It concluded that exogenous GRSP triggered differential effects on growth response, and the effect was associated with the element content of pure GRSP and the change in auxins and root morphology. EE-GRSP displays a promise as a plant growth biostimulant in citriculture.

Plant roots can be colonized by many symbiotic fungi, whereas it is unclear whether and how symbiotic fungi including arbuscular mycorrhizal fungi and endophytic fungi promote phosphorus (P) uptake in Camellia oleifera plants. The objective of the present study was to analyze the effect of inoculation with a culturable endophytic fungus (Piriformospora indica), three arbuscular mycorrhizal fungi (Funneliformis mosseae, Diversispora versiformis, and Rhizophagus intraradices), and mixture of F. mosseae, D. versiformis and R. intraradices on plant growth, root architecture, soil Olsen-P, soil phosphatase activities, leaf and root P concentrations, and phosphate transporter gene expressions, in order to explore the potential and mechanism of these symbiotic fungi on P acquisition. All the symbiotic fungi colonized roots of C. oleifera after 16 weeks, with P. indica showing the best effect on fungal colonization. All the symbiotic fungi significantly increased acid, neutral, and total phosphatase activities in the soil, accompanied with an elevation of soil Olsen-P, of which P. indica presented the best effect. All symbiotic fungal treatments, except D. versiformis, significantly promoted plant growth, coupled with an increase in root total length, area, and volume. Symbiotic fungi almost up-regulated root CoPHO1-3 expressions as well as leaf CoPHO1-1, CoPHO1-3, and CoPHT1;4 expressions. Correlation analysis showed that P concentrations in leaves and roots were significantly positively correlated with root morphological variables (length, volume, and surface area) and soil acid, neutral and total phosphatase activities. It is concluded that symbiotic fungi, especially P. indica, played an important role in P uptake of C. oleifera plants through regulating root architecture, part plant phosphate transporter gene expressions and soil phosphatase activities.

Camellia is a genus of evergreen shrubs or trees, such as C. japonica, C. sinensis, C. oleifera, etc. A group of beneficial soil microorganisms, arbuscular mycorrhizal fungi (AMF), inhabit the rhizosphere of these Camellia spp. A total of eight genera of Acaulospora, Entrophospora, Funneliformis, Gigaspora, Glomus, Pacispora, Scutellospora, and Sclerocystis were found to be associated with Camellia plants with Glomus and/or Acaulospora being most abundant. These mycorrhizal fungi can colonize the roots of Camellia spp. and thus form arbuscular mycorrhizal symbionts. AMF is an important partner of Camellia spp. in the field of physiological activities. Studies indicated that AMF inoculation has been shown to promote plant growth, improve nutrient acquisition and nutritional quality, and increase resistance to drought, salinity and heavy metal contamination in potted Camellia. This review thus provides a comprehensive overview of AMF species occurring in the rhizosphere of Camellia spp. and summarizes the variation in root AMF colonization rate as well as the environmental factors and soil nutrients affecting root colonization. The paper also reviews the effects of AMF on plant growth response, nutrient acquisition, food quality, and stress tolerance of Camellia spp.

Glomalin-related soil protein (GRSP) is a specific glycoprotein secreted into the soil by hyphae and spores of arbuscular mycorrhizal fungi that have many potential functions. It is not clear whether exogenous GRSP has an effect on plant growth and soil properties or whether the effects are related to the type of GRSP used. In this study, trifoliate orange (Poncirus trifoliata L. Raf.) seedlings were used to analyze the effects of easily extractable GRSP (EE-GRSP) and difficultly extractable GRSP (DE-GRSP) at a quarter-, half-, and full-strength concentration on shoot and root biomass as well as soil properties The results showed that, at different strengths, exogenous EE-GRSR significantly increased shoot and root biomass compared to the control, which displayed the most significant effects from the half-strength EE-GRSP. In contrast, DE-GRSP, at various strengths, significantly reduced shoot and root biomass. Furthermore, the application of exogenous EE-GRSP stimulated soil water-stable aggregate (WSA) content at 2–4 mm and 0.5–1 mm sizes, while DE-GRSP strongly reduced WSA content at the 2–4 mm, 1–2 mm, 0.5–1 mm, and 0.25–0.5 mm sizes, consequently leading to an increase or decrease in the WSA stability, according to the mean weight diameter. However, exogenous EE-GRSP decreased soil pH and DE-GRSP increased it, which was related to WSA stability. Exogenous EE-GRSP almost significantly increased soil acidic, neutral, and alkaline phosphatase activity at different strengths, while exogenous DE-GRSP, also at different strengths, significantly inhibited soil acidic phosphatase activity. The application of both exogenous EE-GRSP and DE-GRSP increased the organic carbon content of the soil. This study concluded that exogenous GRSP exerted differential effects on plant biomass and soil properties, and EE-GRSP can be considered as a soil stimulant for use in citrus plants. To our knowledge, this is the first report on the negative effects of exogenous DE-GRSP on plant biomass and soil properties.

Endophytes have the ability to improve plant nutrition alongside their agronomic performance, among which arbuscular mycorrhizal fungi provide the most benefits to their host. Previously, we reported for the first time that an arbuscular mycorrhizal-like fungus Piriformospora indica had the ability to colonize roots of trifoliate orange (Poncirus trifoliata) and conferred positive effects on nutrient acquisition. Present study showed the changes in fatty acids and sugars to unravel the physiological and symbiotic association of trifoliate orange with P. indica and an arbuscular mycorrhizal fungus, Funneliformis mosseae singly or in combination. All the endophytic fungi collectively increased fructose, glucose, and sucrose content in leaves and roots, along with a relatively higher increase with P. indica inoculation than with F. mosseae alone or dual inoculation. Treatment with P. indica increased the concentration of part unsaturated fatty acids such as C18:3N6, C20:2, C20:3N6, C20:4N6, C20:3N3, C20:5N3, C22:1N9, and C24:1. Additionally, P. indica induced the increase in the concentration of part saturated fatty acids such as C6:0, C8:0, C13:0, C14:0, and C24:0. F. mosseae hardly changed the content of fatty acids, except for increase in C14:0 and C20:5N3. Double inoculation only reduced the C21:0, C10:0, C12:0, C18:3N3, and C18:1 content and increased the C20:5N3 content. These endophytic fungi up-regulated the root PtFAD2, PtFAD6, PtΔ9, and PtΔ15 gene expression level, coupled with a higher expression of PtFAD2 and PtΔ9 by P. indica than by F. mosseae. It was concluded that P. indica exhibited a stronger response, for sugars and fatty acids, than F. mosseae on trifoliate orange. Such results also reveal the Pi (an in vitro culturable fungus) as a bio-stimulator applying to citriculture.

Arbuscular mycorrhizal fungi (AMF) are well known for their important physiological implications on diversified host plants, while the information on AMF diversity and its relationship with soil properties of Camellia oleifera is yet not fully understood. In the proposed study, high-throughput sequencing of small subunit ribosomal RNA was performed to analyze the AMF diversity of the rhizosphere and endosphere of 20-year-old C. oleifera Xianglin in the field at Wuhan (China) and their relationship with soil physico-chemical properties. As high as 30.73–41.68% of the roots of C. oleifera were colonized by indigenous AMF with a spore density of 66–111 spores/10 g soil. The surface soil (0–20 cm) showed significantly higher root fungal colonization, spore density, soil hyphal length, and easily extractable glomalin-related soil protein content than the sub-surface soil (20–40 cm). Soil pH value, available K, and NO3−-N content affected the root and soil mycorrhizal development, whilst soil pH proved to be the most influential soil property governing their variability. A total of 467 OTUs associated with AMF were detected from the endosphere and rhizosphere, representing 10 genera and 138 species, of which 295 OTUs and 9 genera were jointly observed. The genus Glomus displayed maximum relative abundance (>86%) in both endosphere and rhizosphere. Scutellospora was detected in the endosphere, but absent in the rhizosphere. The endosphere recorded a relatively higher number of OTUs and alpha diversity indices (Shannon, Simpson, and PD index) of AMF than rhizosphere. Our study, hence, revealed that C. oleifera in fields was mainly colonized by Glomus, coupled with comparatively greater AMF diversity in the endosphere than in the rhizosphere, governed predominantly by soil pH, NO3−-N content, and available K content.