Rhizophagus irregularis, the model fungus in arbuscular mycorrhiza research, forms dimorphic spores

TraitAM, a global spore trait database for arbuscular mycorrhizal fungi

Knowledge regarding organismal traits supports a better understanding of the relationship between form and function and can be used to predict the consequences of environmental stressors on ecological and evolutionary processes. Most plants on Earth form symbioses with mycorrhizal fungi, but our ability to make trait-based inferences for these fungi is limited due to a lack of publicly available trait data. Here, we present TraitAM, a comprehensive database of multiple spore traits for all described species of the most common group of mycorrhizal fungi, the arbuscular mycorrhizal (AM) fungi (subphylum Glomeromycotina). Trait data for 344 species were mined from original species descriptions and used to calculate newly developed fungal trait metrics that can be employed to explore both intra- and inter-specific variation in traits. TraitAM also includes an updated phylogenetic tree that can be used to conduct phylogenetically-informed multivariate analyses of AM fungal traits. TraitAM will aid our further understanding of the biology, ecology, and evolution of these globally widespread, symbiotic fungi.

Enhancing consistency in arbuscular mycorrhizal trait-based research to improve predictions of function

Arbuscular mycorrhizal (AM) fungi (phylum Glomeromycota) are obligate symbionts with plants influencing plant health, soil a(biotic) processes, and ecosystem functioning. Despite advancements in molecular techniques, understanding the role of AM fungal communities on a(biotic) processes based on AM fungal taxonomy remains challenging. This review advocates for a standardized trait-based framework to elucidate the life-history traits of AM fungi, focusing on their roles in three dimensions: host plants, soil, and AM fungal ecology. We define morphological, physiological, and genetic key traits, explore their functional roles and propose methodologies for their consistent measurement, enabling cross-study comparisons towards improved predictability of ecological function. We aim for this review to lay the groundwork for establishing a baseline of AM fungal trait responses under varying environmental conditions. Furthermore, we emphasize the need to include underrepresented taxa in research and utilize advances in machine learning and microphotography for data standardization.

Metabolomics analysis reveals crucial effects of arbuscular mycorrhizal fungi on the metabolism of quality compounds in shoots and roots of Camellia sinensis L

Arbuscular mycorrhizal fungi (AMF) are known as plants' mutualists to enhance plant growth, but their impact on the quality-related metabolites in Camellia sinensis still needs to be studied. In this study, the 2-year-old potted C. sinensis cv. 'Longjing 43' was inoculated with AMF Rhizophagus irregularis to examine the effect of AMF colonization for 3 months on plant growth, photosynthesis, and changes in metabolomics and associated gene expression in the shoots and roots of tea plants. The results showed that AMF not only promoted the growth of tea plants but also significantly up-regulated the total contents of flavonoids and free amino acids, especially the anthocyanins, flavanols, GABA, and arginine. Consistently, the expression of genes such as F3H, DFR, LAR, ANR, UFGT, GDH, and GS in tea shoots was induced by AMF. Further studies found that transcription factors MYBs and HY5, as well as phytohormone strigolactones, were induced by AMF, which may participate in the regulatory mechanism controlling the metabolism of tea-quality compounds. These findings revealed regulatory mechanisms through which AMF affected tea quality and provided a theoretical basis for the application of AMF in tea gardens to improve the economic value and health benefits of tea.

The microbiome orchestrates contaminant low-dose phytostimulation

Our understanding of the physiological mechanisms of the plant hormetic response to countless environmental contaminants is rapidly advancing. However, the microbiome is a critical determinant of plant responses to stressors, thus possibly influencing hormetic responses. Here, we review the otherwise neglected role of microbes in shaping plant stimulation by subtoxic concentrations of contaminants and vice versa. Numerous contaminants at subtoxic levels enhance microorganisms and proliferate symbionts, such as mycorrhizae and other plant beneficial microbes, leading to both direct and indirect improvements in plant physiological performance. Microbial symbiosis facilitates nutrient uptake by plants, indicating an important contribution of symbionts to phytostimulation under subtoxic contamination. We also discuss the mechanisms and implications of the stimulation of plant-microbe systems by subtoxic contaminants.

The costs and benefits of symbiotic interactions: variable effects of rhizobia and arbuscular mycorrhizae on Vigna radiata accessions

BACKGROUND

The symbiosis among plants, rhizobia, and arbuscular mycorrhizal fungi (AMF) is one of the most well-known symbiotic relationships in nature. However, it is still unclear how bilateral/tripartite symbiosis works under resource-limited conditions and the diverse genetic backgrounds of the host.

RESULTS

Using a full factorial design, we manipulated mungbean accessions/subspecies, rhizobia, and AMF to test their effects on each other. Rhizobia functions as a typical facilitator by increasing plant nitrogen content, plant weight, chlorophyll content, and AMF colonization. In contrast, AMF resulted in a tradeoff in plants (reducing biomass for phosphorus acquisition) and behaved as a competitor in reducing rhizobia fitness (nodule weight). Plant genotype did not have a significant effect on AMF fitness, but different mungbean accessions had distinct rhizobia affinities. In contrast to previous studies, the positive relationship between plant and rhizobia fitness was attenuated in the presence of AMF, with wild mungbean being more responsive to the beneficial effect of rhizobia and attenuation by AMF.

CONCLUSIONS

We showed that this complex tripartite relationship does not unconditionally benefit all parties. Moreover, rhizobia species and host genetic background affect the symbiotic relationship significantly. This study provides a new opportunity to re-evaluate the relationships between legume plants and their symbiotic partners.

Arbuscular mycorrhizal diversity increases across a plant productivity gradient driven by soil nitrogen availability

Arbuscular mycorrhizal fungi (AMF) are widespread obligate symbionts of plants. This dynamic symbiosis plays a large role in successful plant performance, given that AMF help to ameliorate plant responses to abiotic and biotic stressors. Although the importance of this symbiosis is clear, less is known about what may be driving this symbiosis, the plant's need for nutrients or the excess of plant photosynthate being transferred to the AMF, information critical to assess the functionality of this relationship. Characterizing the AMF community along a natural plant productivity gradient is a first step in understanding how this symbiosis may vary across the landscape. We surveyed the AMF community diversity at 12 sites along a plant productivity gradient driven by soil nitrogen availability. We found that AMF diversity in soil environmental DNA significantly increased along with the growth of the host plants Acer rubrum and A. saccharum., a widespread tree genus. These increases also coincided with a natural soil inorganic N availability gradient. We hypothesize photosynthate from the increased tree growth is being allocated to the belowground AMF community, leading to an increase in diversity. These findings contribute to understanding this complex symbiosis through the lens of AMF turnover and suggest that a more diverse AMF community is associated with increased host-plant performance.

Building blocks toward sustainable biofertilizers: variation in arbuscular mycorrhizal spore germination when immobilized with diazotrophic bacteria in biodegradable hydrogel beads

AIM

We investigated whether there was interspecies and intraspecies variation in spore germination of 12 strains of arbuscular mycorrhizal fungi when co-entrapped with the diazotrophic plant growth-promoting bacteria, Azospirillum brasilense Sp7 in alginate hydrogel beads.

METHODS AND RESULTS

Twelve Rhizophagus irregularis, Rhizophagus intraradices, and Funneliformis mosseae strains were separately combined with a live culture of Azospirillum brasilense Sp7. Each fungal-bacterial consortia was supplemented with sodium alginate to a 2% concentration (v/v) and cross-linked in calcium chloride (2% w/v) to form biodegradable hydrogel beads. One hundred beads from each combination (total of 1200) were fixed in solidified modified Strullu and Romand media. Beads were observed for successful spore germination and bacterial growth over 14 days. In all cases, successful growth of A. brasilense was observed. For arbuscular mycorrhizal fungi, interspecies variation in spore germination was observed, with R. intraradices having the highest germination rate (64.3%), followed by R. irregularis (45.5%) and F. mosseae (40.3%). However, a difference in intraspecies germination was only observed among strains of R. irregularis and F. mosseae. Despite having varying levels of germination, even the strains with the lowest potential were still able to establish with the plant host Brachypodium distachyon in a model system.

CONCLUSIONS

Arbuscular mycorrhizal spore germination varied across strains when co-entrapped with a diazotrophic plant growth-promoting bacteria. This demonstrates that hydrogel beads containing a mixed consortium hold potential as a sustainable biofertilizer and that compatibility tests remain an important building block when aiming to create a hydrogel biofertilizer that encases a diversity of bacteria and fungi. Moving forward, further studies should be conducted to test the efficacy of these hydrogel biofertilizers on different crops across varying climatic conditions in order to optimize their potential.

Three new species of arbuscular mycorrhizal fungi (Glomeromycota) and Acaulospora gedanensis revised

Studies of the morphology and the 45S nuc rDNA phylogeny of three potentially undescribed arbuscular mycorrhizal fungi (phylum Glomeromycota) grown in cultures showed that one of these fungi is a new species of the genus Diversispora in the family Diversisporaceae; the other two fungi are new Scutellospora species in Scutellosporaceae. Diversispora vistulana sp. nov. came from maritime sand dunes of the Vistula Spit in northern Poland, and S. graeca sp. nov. and S. intraundulata sp. nov. originally inhabited the Mediterranean dunes of the Peloponnese Peninsula, Greece. In addition, the morphological description of spores of Acaulospora gedanensis, originally described in 1988, was emended based on newly found specimens, and the so far unknown phylogeny of this species was determined. The phylogenetic analyses of 45S sequences placed this species among Acaulospora species with atypical phenotypic and histochemical features of components of the two inner germinal walls.