Sensitive approach and future perspectives in microscopic patterns of mycorrhizal roots

Models of mycorrhizal colonization patterns and strategies induced by biostimulator treatments in Zea mays roots

Agronomic inputs and technologies, especially fertilizers, act on the evolution of the symbiotic partnership between arbuscular mycorrhizal fungi and cultivated plants. The use of the MycoPatt method for the assessment of mycorrhizas in maize roots leads to the extraction of large parameter databases with an increased resolution over the colonization mechanism. The application of a biostimulator treatment on plants acted toward a reduction of root permissiveness for mycorrhizas. The phenomenon was noticeable through an increased colonization variability that overlapped with plant nutritional needs. The annual characteristic of the plant was highlighted by the simultaneous presence of arbuscules and vesicles, with a high share of arbuscules in the advanced phenophases. Colonized root parts presented numerous arbuscule-dominated areas in all phenophases, which indicated a continuous formation of these structures and an intense nutrient transfer between partners. Mycorrhizal maps showed the slowing effect of the biostimulators on colonization, with one phenophase delay in the case of biostimulated plants compared to the ones without biostimulators. The forecast models presented gradual colonization in plants without biostimulators, with the expansion of new hyphal networks. The use of biostimulators on plants exhibited a lower permissiveness for new colonization areas, and the mechanism relies on hyphae developed in the former phenophases.

Arbuscular Mycorrhizal Fungi and Fertilization Influence Yield, Growth and Root Colonization of Different Tomato Genotype

Arbuscular mycorrhizal fungi (AMF) are beneficial for plant development and help absorb water and minerals from the soil. The symbiosis between these fungi and plant roots is extremely important and could limit crop dependence on fertilizers. The aim of this study was to evaluate the influence of AMF on tomatoes (Solanum lycopersicum L.), based on important agronomic traits of vegetative biomass, production, and fruits. The experiment was conducted in high tunnels, using 12 tomato genotypes under three different treatments: T1, control, without fertilizer and mycorrhizae colonization; T2, fertigation, without mycorrhizae colonization; and T3, arbuscular mycorrhizal fungi (AMF), seedling roots being inoculated with specialized soil-borne fungi. Plant growth, yield and fruit parameters indicated better results under mycorrhizal treatment. Root colonization with fungi varied significantly depending on the treatment and genotype, with a variation of 6.0-80.3% for frequency and 2.6-24.6% for intensity. For a majority of characteristics, the mycorrhization (T3) induced significant differences compared with the T1 and T2 treatments. In addition, AMF treatment induced a different response among the genotypes. Among the elements analyzed in the soil, significant differences were observed in phosphorous levels between planting the seedlings and after tomato harvesting and clearing of the plants. The results suggest that reducing fertilizers and promoting the symbiotic relationships of plants with soil microorganisms may have beneficial consequences for tomato crops.

Methods for assessing the quality of AM fungal bio-fertilizer: Retrospect and future directions

In the recent past, the mass production of arbuscular mycorrhizal (AM) fungi has bloomed into a large biofertilizer industry. Due to their obligate symbiotic nature, these fungi are propagated on living roots in substrate-based pot cultures and RiTDNA in in vitro or root organ culture systems. The quality assessment of AM inocula remains critical for the production and efficacy evaluation of AM fungi. The vigour of AM inocula are assessed through microscopic methods such as inoculum potential, infectivity potential/infection units, most probable number (MPN) and spore density. These methods marginally depend on the researcher's skill. The signature lipids specific to AM fungi, e.g. 16:1ω5cis ester-linked, phospholipid, and neutral lipid fatty acids provide more robustness and reproducibility. The quantitative real-time PCR of AM fungal taxa specific primers and probes analyzing gene copy number is also increasingly used. This article intends to sensitize AM fungal researchers and inoculum manufacturers to various methods of assessing the quality of AM inocula addressing their merits and demerits. This will help AM producers to fulfil the regulatory requirements ensuring the supply of high-quality AM inocula to end-users, and tap a new dimension of AM research in the commercial production of AM fungi and its application in sustainable plant production systems.

Mycorrhizal Patterns in the Roots of Dominant Festuca rubra in a High-Natural-Value Grassland

Grassland ecosystems occupy significant areas worldwide and represent a reservoir for biodiversity. These areas are characterized by oligotrophic conditions that stimulate mycorrhizal symbiotic partnerships to meet nutritional requirements. In this study, we selected Festuca rubra for its dominance in the studied mountain grassland, based on the fact that grasses more easily accept a symbiotic partner. Quantification of the entire symbiosis process, both the degree of colonization and the presence of a fungal structure, was performed using the root mycorrhizal pattern method. Analysis of data normality indicated colonization frequency as the best parameter for assessing the entire mycorrhizal mechanism, with five equal levels, each of 20%. Most of the root samples showed an intensity of colonization between 0 and 20% and a maximum of arbuscules of about 5%. The colonization degree had an average value of 35%, which indicated a medium permissiveness of roots for mycorrhizal partners. Based on frequency regression models, the intensity of colonization presented high fluctuations at 50% frequency, while the arbuscule development potential was set to a maximum of 5% in mycorrhized areas. Arbuscules were limited due to the unbalanced and unequal root development and their colonizing hyphal networks. The general regression model indicated that only 20% of intra-radicular hyphae have the potential to form arbuscules. The colonization patterns of dominant species in mountain grasslands represent a necessary step for improved understanding of the symbiont strategies that sustain the stability and persistence of these species.

Divergence in Corn Mycorrhizal Colonization Patterns Due to Organic Treatment

Excessive application of chemical fertilizers and other agrochemicals can cause large imbalances in soils and agricultural ecosystems. In this context, mycorrhizae represent a viable solution to mitigate these negative effects. Arbuscular mycorrhizae are vital symbionts due to the multiple benefits they bring to both crops and the entire agroecosystem. The main purpose of this study was to observe whether differentiated fertilization has an influence on mycorrhizal colonization patterns in corn. Observed frequencies and intensities of colonization varied widely between phenophases and treatments, with 20% variation for frequency and 14% for intensity, which implies the constant development of both partners during the vegetation period. Arbuscules and vesicles were present in all development stages, but the overall mean was lower than 4% for arbuscules and 1% for vesicles in the analyzed root fragments. Intensity was highly correlated with frequency of colonization compared with arbuscules, where the coefficient was 0.54, and vesicles, with a coefficient of 0.16. Both PCA and NMDS provided good graphical solutions, with a high resolution due to explained variance and good spatial position of vectors. The use of mycorrhizal maps permits the full exploration of colonization patterns and fungal strategy, and the assessment of mycorrhizae-free areas. For the untreated variant, the strategy was oriented toward a longitudinal colonization followed by an irregular development of hyphae with multiple non-colonized areas. Treatment acts to stimulate the appearance of mycorrhizal spots, which further develop radially.

VBA-AMF: A VBA Program Based on the Magnified Intersections Method for Quantitative Recording of Root Colonization by Arbuscular Mycorrhizal Fungi

The extent of mycorrhizal fungi colonization is an important factor for determining the function of mycorrhizal fungi in fungi-host interaction, and quantifying the extent of mycorrhizal fungi colonization is a fundamental and essential task for researchers engaged in mycorrhizal studies. Intersect methods, such as the gridline intersect and magnified intersections methods, are accurate and objective, and are widely used to assess the colonization status of arbuscular mycorrhizal (AM) fungus. However, no convenient procedures or add-ins for Excel spreadsheets have been developed to simplify these methods. Here, we propose a procedure using the Visual Basic for Application (VBA) program in Excel that is based on the magnified intersections method, which we refer to as VBA-AMF (arbuscular mycorrhizal fungi). Time-saving and convenience are the two most prominent advantages of the VBA-AMF procedure, as it enables researchers to compute the colonization rate of AM fungi in roots, and consequently the extent of root colonization by AM fungi. VBA-AMF can also be modified to measure the status of other fungal colonizations in plant roots following the same strategy.