Physiological and Morphological Responses of Okra (Abelmoschus esculentus L.) to Rhizoglomus irregulare Inoculation under Ample Water and Drought Stress Conditions Are Cultivar Dependent

Okra is an important crop species for smallholder farmers in many tropical and subtropical regions of the world. Its interaction with mycorrhiza has been rarely studied, and little is known about its mycorrhizal dependency, especially under drought stress. In a glasshouse experiment, we investigated the effect of Arbuscular Mycorrhiza Fungi (AMF) inoculation on growth, evapotranspiration, mineral nutrition and root morphology of five okra cultivars under ample water and drought stress conditions. ‘Khartoumia’, ‘HSD6719’, ‘HSD7058’, ‘Sarah’ and ‘Clemson Spineless’-cultivars commonly used by farmers in Sudan were chosen for their geographical, morphological and breeding background variations. The plants were either inoculated with R. irregulare or mock-inoculated. Seven weeks after seeding, the soil–water content was either maintained at 20% w/w or reduced to 10% w/w to impose drought stress. Drought stress resulted in plant P deficiency and decreased shoot dry biomass (DB), especially in HSD7058 and Clemson Spineless (69% and 56% decrease in shoot DB, in the respective cultivars). Plant inoculation with AMF greatly enhanced the shoot total content of P and the total DB in all treatments. The mycorrhizal dependency (MD) -the degree of total plant DB change associated with AM colonization- differed among the cultivars, irrespective of the irrigation treatment. Key determinants of MD were the root phenotype traits. Khartoumia (with the highest MD) had the lowest root DB, root-to-shoot ratio, and specific root length (SRL). Meanwhile, HSD6719 (with the lowest MD) had the highest respective root traits. Moreover, our data suggest a relationship between breeding background and MD. The improved cultivar Khartoumia showed the highest MD compared with the wild-type Sarah and the HSD7058 and HSD6719 landraces (higher MD by 46%, 17% and 32%, respectively). Interestingly, the drought-affected HSD7058 and Clemson Spineless exhibited higher MD (by 27% and 15%, respectively) under water-deficiency compared to ample water conditions. In conclusion, the mediation of drought stress in the okra plant species by AMF inoculation is cultivar dependent. The presence of AMF propagules in the field soil might be important for increasing yield production of high MD and drought susceptible cultivars, especially under drought/low P environments.

Arbuscular Mycorrhizal Fungi Regulate MAPK Pathway Genes Expression and Enhance Drought Tolerance of Populus Simonii × P. Nigra Seedlings

Background: Arbuscular mycorrhizal fungi (AMF) form a symbiotic relationship with host plants, which can promote plants to absorb more water and nutrients, and thus improve the stress resistance of plants. Our study aimed to investigate the effects of Rhizophagus irregularis on Populus simonii × P. nigra seedlings under drought stress. Results: The experiment was a completely random design with two water conditions (well-watered or drought stress) and two AMF treatments (inoculated with or without R. irregularis). Our results showed that mycorrhizal seedlings performed less oxidative damage and stronger tolerance of drought, which recorded higher photosynthesis and less concentrations of Malondialdehyde (MDA), H2O2, and proline under drought stress versus non-mycorrhizal seedlings. Under drought stress, AMF inoculation reduced soluble sugar concentration in leaves but promoted its accumulation in roots. The superoxide dismutase (SOD) activity in leaves and roots, and catalase (CAT) activity in roots of mycorrhizal seedlings were lower than non-mycorrhizal seedlings, but CAT activity in leaves of mycorrhizal seedlings was higher than non-mycorrhizal seedlings under drought stress. Drought stress and AMF inoculation both induced the expressions of MAPKs of P. simonii × P. nigra, but the expression patterns of MAPKs under four treatments were obviously different.Conclusions: Overall, our results demonstrated that mycorrhizal seedlings had less oxidative damage and stronger tolerance to drought. MAPKs expressions of P. simonii×P. nigra (PsnMAPKs) were induced by drought stress and AMF inoculation, and the expression patterns of PsnMAPKs in response to drought stress were different between mycorrhizal and non-mycorrhizal seedlings. Non-mycorrhizal seedlings may be adapted to drought by up-regulating MAPKs expressions leading to stomatal closure. Drought stress decreased serval PsnMAPKs expressions induced by AMF inoculation, which may be associated with mycorrhizal colonization.

Arbuscular Mycorrhizal Fungus Alters Root System Architecture in Camellia sinensis L. as Revealed by RNA-Seq Analysis

Arbuscular mycorrhizal fungus (AMF), forming symbiosis with most terrestrial plants, strongly modulates root system architecture (RSA), which is the main characteristic of root in soil, to improve plant growth and development. So far, the studies of AMF on tea plant seedlings are few and the relevant molecular mechanism is not deciphered. In this study, the 6-month-old cutting seedlings of tea plant cultivar “Wancha No.4” were inoculated with an AMF isolate, Rhizophagus intraradices BGC JX04B and harvested after 6 months of growth. The indexes of RSA and sugar contents in root were determined. The transcriptome data in root tips of mycorrhizal and non-mycorrhizal cutting seedlings were obtained by RNA-sequence (Seq) analysis. The results showed that AMF significantly decreased plant growth, but increased the sucrose content in root and the higher classes of lateral root (LR) formation (third and fourth LR). We identified 2047 differentially expressed genes (DEGs) based on the transcriptome data, and DEGs involved in metabolisms of phosphorus (42 DEGs), sugar (39), lipid (67), and plant hormones (39) were excavated out. Variation partitioning analysis showed all these four categories modulated the RSA. In phosphorus (P) metabolism, the phosphate transport and release (DEGs related to purple acid phosphatase) were promoted by AMF inoculation, while DEGs of sugar transport protein in sugar metabolism were downregulated. Lipid metabolism might not be responsible for root branching but for AMF propagation. With respect to phytohormones, DEGs of auxin (13), ethylene (14), and abscisic acid (5) were extensively affected by AMF inoculation, especially for auxin and ethylene. The further partial least squares structural equation modeling analysis indicated that pathways of P metabolism and auxin, as well as the direct way of AMF inoculation, were of the most important in AMF promoting root branching, while ethylene performed a negative role. Overall, our data revealed the alterations of genome-wide gene expression in tea plant roots after inoculation with AMF and provided a molecular basis for the regulatory mechanism of RSA (mainly root branching) changes induced by AMF.

Interactions between Biochar and Compost Treatment and Mycorrhizal Fungi to Improve the Qualitative Properties of a Calcareous Soil under Rhizobox Conditions

Most calcareous soils have relatively low levels of organic matter. To evaluate the effect of pruning waste biochar (PWB) and pruning waste compost (PWC) combined with arbuscular mycorrhizal fungi (AMF) on the biological indices, a rhizobox study on wheat using a completely randomized design was conducted under greenhouse conditions. The studied factors included the source of organic material (PWB, PWC, and control), the microbial inoculation (+AMF or −AMF), and the zone (rhizosphere and non-rhizosphere soil). At the end of the plant growth period, organic carbon (OC), microbial biomass carbon (MBC), microbial biomass phosphorous (MBP), microbial respiration (BR), substrate-induced respiration (SIR), and alkaline (ALP) and acid (ACP) phosphatase enzyme activities in the rhizosphere and non-rhizosphere soils were determined. Simultaneous application of a source of organic matter and AMF inoculation significantly increased the OC and biological indices of soil relative to those observed when applying organic matter without AMF inoculation. Additionally, MBC, MBP, ACP, and ALP enzymes activities in the rhizosphere zone were significantly higher than in the non-rhizosphere. AMF increased BR and SIR levels in the rhizosphere by 13.06% and 7.95% compared to those in the non-rhizosphere, respectively. It can be concluded that PWC and PWB can improve soil biological properties by increasing microbial activity.

Mycorrhizal Inoculation and Chemical Fertilizer Interactions in Pineapple under Field Conditions

Excessive inorganic fertilizers applied to pineapple crops in Mexico cause the progressive degradation and pollution of soils in the short- and long-term, and they also increase production costs. An alternative to reduce excessive fertilization is its partial substitution by nutrition and growth enhancing arbuscular mycorrhizal fungi (AMF). The goal of this research was to compare the effect of AMF inoculation combined with different fertilizer doses and full chemical fertilization on pineapple yield variables in a commercial plantation. We used a randomized block design with six treatments: a non-inoculated control with 100% chemical fertilization, and five treatments with AMF inoculation and fertilization doses of 0%, 25%, 50%, 75%, and 100% chemical fertilization. There were four replicates of each treatment containing 30 plants in each experimental unit (plot). We measured the dry weight of the D-leaf 9 months after planting, and the root mycorrhizal colonization percentage, yield, and fruit quality after 18 months. Mycorrhizal inoculation equated to 100% chemical fertilization already when combined with 25% fertilization and surpassed it when combined with 50% fertilization in most of the yield variables measured. The fruit mass and organoleptic variables were significantly higher in mycorrhizal plants with 50% fertilization than in the non-inoculated control and the treatments inoculated with AMF and combined with 0%, 25%, 75%, and 100% of a dose of chemical fertilizer. Inoculation with mycorrhizal fungi in the field could reduce chemical fertilizer application by 50%, with no yield loss and with improved fruit quality.

Unraveling the Interaction between Arbuscular Mycorrhizal Fungi and Camellia Plants

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.

Effect of arbuscular mycorrhizal fungi on the physiological functioning of maize under zinc-deficient soils

Zinc (Zn) deficiency can severely inhibit plant growth, yield, and enzymatic activities. Zn plays a vital role in various enzymatic activities in plants. Arbuscular mycorrhizal fungi (AMF) play a crucial role in improving the plant’s Zn nutrition and mitigating Zn stress effects on plants. The current study was conducted to compare the response of inoculated and non-inoculated maize (YH 1898) in the presence of different levels of zinc under greenhouse conditions under a Zn deficient condition. There were two mycorrhizal levels (i.e., M + with mycorrhizae, M- without mycorrhizae) and five Zn levels (i.e., 0, 1.5, 3, 6, and 12 mg kg-1), with three replicates following completely randomized design. At the vegetative stage (before tillering), biochemical, physiological, and agronomic attributes were measured. The results showed that maize plants previously inoculated with AMF had higher gaseous exchange traits, i.e., a higher stomatal conductance rate, favoring an increased photosynthetic rate. Improvement in antioxidant enzyme activity was also observed in inoculated compared to non-inoculated maize plants. Moreover, AMF inoculation also played a beneficial role in nutrients availability and its uptake by plants. Higher Zn12 (12 mg Zn kg-1 soil) treatment accumulated a higher Zn concentration in soil, root, and shoot in AMF-inoculated than in non-inoculated maize plants. These results are consistent with mycorrhizal symbiosis beneficial role for maize physiological functioning in Zn deficient soil conditions. Additionally, AMF inoculation mitigated the stress conditions and assisted nutrient uptake by maize.

Growth, physiological, and biochemical responses of thyme (Thymus vulgaris L.) to the application of arbuscular mycorrhizal fungi under cadmium stress conditions

Thyme (Thymus vulgaris L.) is one of the most important medicinal plants used in various pharmaceutical, osmotic, health, and food industries. Arbuscular mycorrhizal fungi (AMF) symbiosis is viewed as one of the several methods to improve growth under heavy metals stress. To investigate the effects of cadmium (Cd) and AMF bio-fertilizers on the growth and morpho-physiological characteristics of thyme, a greenhouse experiment was performed in three replications. Experimental treatments included Cd at three levels 0, 75, and 150 mg/kg of soil and AMF at three levels without inoculation, inoculation with Funneliformis etunicatum, and Funneliformis mosseae. Cadmium stressed plant showed reduced plant height, number of leaves, stem fresh and dry weight, and root fresh and dry weight while AMF inoculation enhanced the increased means of these traits considerably. Inoculation with F. mosseae also ameliorated the Cd stress (150 mg/kg) induced reduction in plant height, number of leaves, and stem and root dry weight by 13.41%, 8.42%, 30.3%, and 22.2%, respectively. Cadmium stress reduced membrane stability index while AMF inoculation enhanced membrane stability index considerably. An increase in soluble carbohydrate and proline content was observed due to Cd stress and AMF inoculation caused a further increase in these two metabolite contents ensuring better growth under Cd stressed conditions. Results indicated that F. mosseae had a higher efficiency in increasing morphological traits and improving physiological characteristics than F. etunicatum. Overall, AMF inoculation, especially F. mosseae significant ameliorative potential for Cd toxicity in thyme plants.

Mycorrhizal inoculation and treatment with biochar and compost from pruning waste improve the qualitative properties of a calcareous soil under wheat cultivation

Most calcareous soils have relatively low levels of organic matter. To address this issue and improve the qualitative properties of calcareous soils, soils can be treated with mycorrhizal fungi and/or exogenous organic material such as biochar or compost derived from tree pruning waste. To evaluate the effect of pruning waste biochar (PWB) and pruning waste compost (PWC) derived from apple and grape trees combined with arbuscular mycorrhizal fungi (AMF) on the biological indices of calcareous soils, a rhizobox study on wheat plants using a completely randomized design was conducted under greenhouse conditions. The studied factors included the source of the type of organic material applied (PWB, PWC, and control), the nature of the microbial inoculation (inoculation with AMF or no inoculation), and the zone to which the treatments were applied (rhizosphere and non-rhizosphere soil). At the end of the plant growth period, organic carbon (OC), microbial biomass carbon (MBC), microbial biomass phosphorous (MBP), microbial respiration (BR), substrate-induced respiration (SIR), alkaline (ALP), acid (ACP) phosphatase enzyme activities in the rhizosphere and non-rhizosphere soils, and root mycorrhizal colonization were determined. Simultaneous application of a source of organic matter and AMF inoculation significantly increased the OC and biological indices of soil relative to those observed when applying organic matter without AMF inoculation. Additionally, MBC, MBP, ACP and ALP - enzymes activities in the rhizosphere zone were significantly higher than in the non-rhizosphere. AMF increased BR and SIR levels in the rhizosphere by 13.06% and 7.95% compared to non-rhizosphere, respectively. It can be concluded that in calcareous soils with low organic carbon contents, organic amendments such as PWC and PWB can improve soil biological properties by increasing microbial activity and changing the properties of the rhizosphere.

Agronomic Biofortification of Garlic through Selenium and Arbuscular Mycorrhizal Fungi Application

Garlic has a strong ability of selenium (Se) accumulation and is one of the best target crops for Se biofortification. Arbuscular mycorrhizal fungi (AMF) inoculation might enhance the nutritional qualities and the absorption ability of exogenous Se in plants. However, little is known about the exogenous Se application and AMF inoculation on garlic. Here, we evaluated the effects of different concentrations of exogenous Se on the growth, nutritional quality, and selenium enrichment of garlic. The results demonstrated that significantly higher Se content of garlic bulb was found in exogenous Se treated plants, and the Se accumulation was improved with the increasing of Se supply. Low application of exogenous Se appreciably improved the yield and the contents of soluble sugar and allicin in garlic bulbs, but the opposite was observed at high Se concentration. Furthermore, AMF inoculation significantly reduced the inhibition effect of high concentration Se on garlic. AMF supply was effective in improving the growth and nutritional indicators of garlic, which promoted the exogenous Se utilization rate when combined with 10 mg/L exogenous Se treatment. The results will provide a more theoretical basis for the production of high-quality selenium enrichment garlic.