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Sustainable farming of horticultural plants has been the focus of research during the last decade, paying significant attention to alarming weather extremities and climate change, as well as the pressure of biotic stressors on crops. Microbial biostimulants, including plant growth-promoting bacteria (PGPB) and arbuscular mycorrhizal fungi (AMF), have been proven to increase plant growth via both direct and indirect processes, as well as to increase the availability and uptake of nutrients, boosting soil quality, increasing plants' tolerance to abiotic stress and increasing the overall quality attributes of various horticultural crops (e.g., vegetables, fruit, herbs). The positive effects of microbial biostimulants have been confirmed so far, mostly through symbiotic interactions in the plant–soil–microbes ecosystem, which are considered a biological tool to increase quality parameters of various horticultural crops as well as to decrease soil degradation. However, more research is needed to address future challenges of crop production through revealing the mechanisms of action and identifying response patterns of crops to various microbial products. The present review aims to present the most up-to-date results regarding the practical applications of microbial biostimulants in horticultural species, including case studies of successful paradigms for the most important microbial genera of PGPB and AMF. Moreover, the mechanisms of the actions are briefly described while future remarks are also discussed, aiming to suggest further needs to be addressed for the successful establishment of microbial biostimulants in sustainable horticultural crop production. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) form mutualistic symbiotic relationships with many land plants and play a key role in nitrogen (N) acquisition. NO3−-N and NH4+-N are the main sources of soil mineral N, but how extraradical mycelial transfer affects the different N forms and levels available to tomato plants is not clear. In the present study, we set up hyphal compartments (HCs) to study the efficiency of N transfer from the extramycelium to tomato plants treated with different N forms and levels of fertilization. Labeled 15NO3−-N or 15NH4+-N was placed in hyphal compartments under high and low N application levels. 15N accumulation in shoots and the expression of LeNRT2.3, LeAMT1.1, and LeAMT1.2 in the roots of tomato were measured. According to our results, both 15NO3−-N and 15NH4+-N were transported via extraradical mycelia to the shoots of plants. 15N accumulation in shoots was similar, regardless of the N form, while a higher 15N concentration was found in shoots with low N application. Compared with the control, inoculation with AMF significantly increased the expression of LeAMT1.1 under high N and LeNRT2.3 under low N. The expression of LeAMT1.1 under high N was significantly increased when NO3—N was added, while the expression of LeNRT2.3 was significantly increased when NH4+-N was added under low N. Taken together, our results suggest that the N transfer by extraradical mycelia is crucial for the acquisition of both NO3−-N and NH4+-N by the tomato plant; however, partial N accumulation in plant tissue is more important with N deficiency compared with a higher N supply. The expression of N transporters was influenced by both the form and level of N supply. [ABSTRACT FROM AUTHOR]

To improve water and nutrient use efficiencies some strategies have been proposed, such as the use of mulching techniques or arbuscular mycorrhizal fungi (AMF) inoculation. To gain insights into the interaction between the use of hydromulch and AMF inoculation on plant growth and productivity, escarole plants (Cichorium endivia, L.) were inoculated with the AMF Rhizophagus irregularis and grown with non-inoculated plants under different soil cover treatments: ecological hydromulching based on the substrate of mushroom cultivation (MS), low-density black polyethylene (PE), and non-covered soil (BS). AMF inoculation or the use of mulching alone, but especially their interaction, increased the plant growth. The growth improvement observed in AMF-inoculated escarole plants grown under hydromulching conditions was mainly associated with the upgrading of nitrogen and phosphorous use efficiency through the regulation of the hormonal balance. Both hydromulching and AMF inoculation were found to increase the active gibberellins (GAs) and cytokinins (CKs), resulting in a positive correlation between these hormones and the growth-related parameters. In contrast, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and abscisic acid (ABA) decreased in AMF-inoculated plants and especially in those grown with the MS treatment. This study demonstrates that there exists a positive interaction between AMF and hydromulching which enhances the growth of escarole plants by improving nutrient use efficiency and hormonal balance. [ABSTRACT FROM AUTHOR]

In the present study, Funneliformis mosseae (FM), Claroideoglomus etunicatum (CE), and Acaulospora foveata (AF) were inoculated to hybrid maize (Zea mays L. cv. CP888®). Upregulation of nitrogen levels were dependent on the type of mycorrhiza (AMF). Photosynthetic efficiency (Fv/Fm) and water content in FM- and AF-inoculated plants were elevated, resulting in promotion of leaf area and shoot biomass. N content in the shoot and root tissues of the FM-inoculated plants increased by 21% and 30% over the control. A positive correlation between biochemical, physiological, and morphological parameters using Pearson's coefficient was demonstrated. A decline in lipid peroxidation was noticed in the FM-inoculated plants. In addition, we investigated the potential of N fertilizer application in combination with FM inoculation in maize plants. The FM-inoculated plants with organic O_LT, a chicken manure fertilizer, increased N content in the host shoots by 73% over the control, leading to improved Fv/Fm as a physiological adaptation strategy. The FM and the O_LT on the regulation of the N enhancement and photosynthetic efficiency of the hybrid maize should further be validated in field trials in different environments for sustainability. [ABSTRACT FROM AUTHOR]

In acidic soils with manganese (Mn) toxicity, arbuscular mycorrhizal fungi (AMF) can improve plant host growth by enhancing nutrition and protecting against environmental stress. The intact extraradical mycelium (ERM) of AMF is able to survive Mediterranean summer conditions and provide an earlier colonization of winter crops. This study evaluated if summer season conditions hindered the beneficial effects of wheat colonization by the intact ERM associated with a native plant, in acidic soil. Wheat was grown in soil with intact or disrupted ERM associated with Ornithopus compressus (ORN), developed for 7 or 24 weeks, to simulate ERM summer survival. The activity of antioxidant enzymes was determined, and the quantitative analysis of Mn and macronutrients was performed by inductively coupled plasma mass spectrometry (ICP-MS), in wheat shoots and respective subcellular fractions. Wheat colonization by intact ERM decreased shoot Mn concentration but increased the proportion of Mn in the apoplast. Overall, antioxidant enzymatic activity decreased but the proportion of Mn-superoxide dismutase activity over the remaining isoforms increased, suggesting its important role in the AMF-mediated mitigation of Mn toxicity. Summer conditions did not substantially reduce the benefits provided by ORN ERM. A no-till strategy allied to the development of native microbiota can contribute to the sustainable optimization of acidic soil use. [ABSTRACT FROM AUTHOR]

Grapevine (Vitis vinifera L.), widely used for berry and wine production, is highly susceptible to the pathogenic oomycete Plasmopara viticola, the etiological agent of grapevine downy mildew disease. The method commonly used to prevent and control P. viticola infection relies on multiple applications of chemical fungicides. However, with European Union goals to lower the usage of such chemicals in viticulture there is a need to develop new and more sustainable strategies. The use of beneficial microorganisms with biocontrol capabilities, such as the arbuscular mycorrhizal fungi (AMF), has been pointed out as a viable alternative. With this study, we intended to investigate the effect of AMF colonization on the expression of P. viticola effectors during infection of grapevine. Grapevine plants were inoculated with the AMF Rhizophagus irregularis and, after mycorrhizae development, plants were infected with P. viticola. The expression of P. viticola RxLR effectors was analyzed by real-time PCR (qPCR) during the first hours of interaction. Results show that pre-mycorrhizal inoculation of grapevine alters the expression of several P. viticola effectors; namely, PvRxLR28, which presented decreased expression in mycorrhizal plants at the two time points post-infection tested. These results suggest that the pre-inoculation of grapevine with AMF could interfere with the pathogen's ability to infect grapevine by modulation of pathogenicity effectors expression, supporting the hypothesis that AMF can be used to increase plant resistance to pathogens and promote more sustainable agriculture practices, particularly in viticulture. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) are determinant for the performance of plant communities and for the functionality of terrestrial ecosystems. In natural ecosystems, grazing can have a major impact on mycorrhizal fungi and consequently on plant growth. The objective of this study was to evaluate the statements referred above in Mediterranean arid areas in Tunisia. Root samples and rhizosphere soils of five dominant herbaceous plants were studied at six distinct arid sites differing on soil proprieties and grazing intensity. At each site, chemical and dynamic properties of the soil were characterized as well as the AMF colonization intensity and the soil functionality. Results showed that the mycorrhizal frequency and intensity and spore density, varied between plants in the same site and, for each plant, between sites and evidenced a positive effect of mycorrhized plants on soil microbial activity. Grazing and soil properties strongly affected AMF composition and the soil microbial and biochemical dynamics, which presented the lowest values at the sites with the highest grazing intensities. In conclusion, these results demonstrate that AMF improve soil biological properties, supporting the hypothesis that mycorrhiza and grazing compete for plant photosynthates, and highlight the importance of mycorrhizal symbiosis towards soil functionality under arid conditions. [ABSTRACT FROM AUTHOR]

A potted experiment was carried out to evaluate the effect of an arbuscular mycorrhizal fungus (AMF), Acaulospora scrobiculata, on peach seedlings grown in non-replant (NR) and replant (R) soils, to establish whether AMF inoculation alleviated soil replant disease through changes in physiological levels and relevant gene expression. After 15 weeks of mycorrhization, root mycorrhizal colonization was heavily inhibited by R treatment versus NR treatment. AMF plants under NR and R soil conditions displayed significantly higher total plant biomass than non-AMF plants. AMF inoculation significantly increased root sucrose and fructose concentrations and root catalase, peroxidase, polyphenol oxidase, and phenylalanine ammonialyase activities under R conditions. Likewise, salicylic acid, jasmonic acid, chitinase, total soluble phenol, and lignin concentrations in roots were significantly higher in AMF than in non-AMF seedlings grown in R soil. Over-expression of PpCHI, PpLOX1, PpLOX5, PpAOC3, PpAOC4, and PpOPR2 in roots was observed in AMF-inoculated seedlings, as compared to that of non-AMF-inoculated seedlings grown in R soils. Thus, mycorrhizal fungal inoculation conferred a greater tolerance to peach plants in R soil by stimulating antioxidant enzyme activities, disease-resistance substance levels, and the expression of relevant genes. [ABSTRACT FROM AUTHOR]

Mycorrhizas play a vital role in ecosystem function, diversity and productivity. However, mycorrhizas in tropical forests are considered to be a neglected area of research in contrast to the well-studied diversity patterns of macro organisms. To this end, soil samples from 0 to 30 cm in depth were collected from six or four elevations in a typical tropical forest of Mt. Jianfeng and Mt. Diaoluo in China. The arbuscular mycorrhizal fungal (AMF) diversity and community composition were explored among different elevations based on high-throughput barcoded sequencing on the Illumina MiSeq platform. Environmental variables of soil characteristics, and elevation on AMF community assembly were analyzed using canonical correspondence analysis. In total, 316 AMF operational taxonomic units (OTUs) were found to belong to four identified and one unclassified order, and Glomus was the most dominant genus in tropical forest. AMF communities or diversity did not clearly reflect local environmental conditions, the spatial distance between mountains and elevation. In total, 68% and 56% of taxa of AMF were observed on multiple elevations in Mt. Jianfeng and Mt. Diaoluo, respectively. Furthermore, 8.9% and 19% of OTUs were exhibited on all elevations in Mt. Jianfeng and Mt. Diaoluo, respectively. The AMF alpha diversity, richness and evenness were similar across the two surveyed tropical mountains. The influence of elevation showed no distinct role on the diversity of AMF taxa. Overall, AMF communities and diversity are abundant in tropical forests and with little influence of elevation in tropical forests of China. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal (AM) fungi (AMF) are widely known to form a symbiosis with most higher plants and enhance plant adaptation to a series of environmental stresses. Sweet sorghum (Sorghum bicolor (L.) Moench) is considered a promising alternative feedstock for bioalcohol production because of its sugar-rich stalk and high biomass. However, little is known of AMF benefit for biomass production and salt tolerance of sweet sorghum. Here, we investigated the effects of Acaulospora mellea ZZ on growth and salt tolerance in two sweet sorghum cultivars (Liaotian5 and Yajin2) under different NaCl addition levels (0, 0.5, 1, 2, and 3 g NaCl/kg soil). Results showed AMF colonized the two cultivars well under all NaCl addition levels. NaCl addition increased mycorrhizal colonization rates in Yajin2, but the effects on Liaotian5 ranged from stimulatory at 0.5 and 1 g/kg to insignificant at 2 g/kg, and even inhibitory at 3 g/kg. High NaCl addition levels produced negative effects on both AM and non-AM plants, leading to lower biomass production, poorer mineral nutrition (N, P, K), higher Na+ uptake, and lower soluble sugar content in leaves. Compared with non-AM plants, AM plants of both cultivars had improved plant biomass and mineral uptake, as well as higher K+/Na+ ratio, but only Yajin2 plants had a low shoot/root Na ratio. AM inoculation increased the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), and soluble sugar content in leaves. Overall, both cultivars benefited from mycorrhization, and Yajin2 with less salt tolerance showed higher mycorrhizal response. In conclusion, AMF could help to alleviate the negative effects caused by salinity, and thus showed potential in biomass production of sweet sorghum in saline soil. [ABSTRACT FROM AUTHOR]