Contrasting preferences of arbuscular mycorrhizal and dark septate fungi colonizing boreal and subarctic Avenella flexuosa

Melanized root-associated fungi: key players in plant-soil systems

Melanized root-associated fungi are a group of fungi that produce melanized structures and form root associations, including different mycorrhizal and endophytic symbioses with plants. They are pervasive across terrestrial ecosystems and play an important role in the prevailing soil carbon (C) and nutrient cycling syndromes through direct and indirect mechanisms, where they may strongly modulate plant-microbe interactions and structure root and soil microbiomes. Furthermore, melanized root-associated fungi can confer on plants an enhanced ability to tolerate abiotic and biotic stressors such as drought, extreme temperatures, heavy metals, and pathogen attacks. We propose that melanized root-associated fungi are a cohesive and ecologically relevant grouping that can be an indicator of plant-soil system functioning, and considering them will advance research on plant-soil interactions.

©Relationship between endophytic fungal diversity and colonization and soil factors of cultured blueberry roots in Guizhou Province, Southwest China

Dark septate endophytes (DSEs) inhabit plant roots and soil in ecosystems and host plants worldwide. DSE colonization is influenced by cultivars, soil factors, and specific habitat conditions. The regular diversity of DSEs in blueberries in Guizhou, China, is still unclear. In this study, four cultivars (Gardenblue, Powderblue, O'Neal, and Legacy) in three areas (Gaopo, Majiang, and Fenggang) in Guizhou were used to identify DSEs by morphological and molecular biological methods and to clarify the relationship between DSE diversity and DSE colonization and soil factors of cultivated blueberries in Guizhou. The DSEs isolated from cultivated blueberry roots in 3 areas in Guizhou Province were different, belonging to 17 genera, and the dominant genera were Penicillium, Phialocephala, and Thozetella. DSEs isolated from Majiang belonged to 12 genera and 16 species, those from Gaopo belonged to 7 genera and 15 species, and those from Fenggang belonged to 5 genera and 7 species. Among the different blueberry varieties, 11 genera were isolated from O'Neal, 12 genera were isolated from Powderblue, 11 genera were isolated from Legacy and 13 genera were isolated from Gardenblue. Coniochaeta is endemic to O'Neal, Chaetomium and Curvularia are endemic to Powderblue, and Thielavia is endemic to Legacy. Correlation analysis showed that DSE diversity was significantly correlated with DSE colonization and soil factors.

Inter-Kingdom Networks of Canola Microbiome Reveal Bradyrhizobium as Keystone Species and Underline the Importance of Bulk Soil in Microbial Studies to Enhance Canola Production

The subterranean microbiota of plants is of great importance for plant growth and health, as root-associated microbes can perform crucial ecological functions. As the microbial environment of roots is extremely diverse, identifying keystone microorganisms in plant roots, rhizosphere, and bulk soil is a necessary step towards understanding the network of influence within the microbial community associated with roots and enhancing its beneficial elements. To target these hot spots of microbial interaction, we used inter-kingdom network analysis on the canola growth phase of a long-term cropping system diversification experiment conducted at four locations in the Canadian Prairies. Our aims were to verify whether bacterial and fungal communities of canola roots, rhizosphere, and bulk soil are related and influenced by diversification of the crop rotation system; to determine whether there are common or specific core fungi and bacteria in the roots, rhizosphere, and bulk soil under canola grown in different environments and with different levels of cropping system diversification; and to identify hub taxa at the inter-kingdom level that could play an important ecological role in the microbiota of canola. Our results showed that fungi were influenced by crop diversification, which was not the case on bacteria. We found no core microbiota in canola roots but identified three core fungi in the rhizosphere, one core mycobiota in the bulk soil, and one core bacterium shared by the rhizosphere and bulk soil. We identified two bacterial and one fungal hub taxa in the inter-kingdom networks of the canola rhizosphere, and one bacterial and two fungal hub taxa in the bulk soil. Among these inter-kingdom hub taxa, Bradyrhizobium sp. and Mortierella sp. are particularly influential on the microbial community and the plant. To our knowledge, this is the first inter-kingdom network analysis utilized to identify hot spots of interaction in canola microbial communities.

Plant association with dark septate endophytes: When the going gets tough (and stressful), the tough fungi get going

Dark septate endophytes (DSEs) comprise a diverse and ubiquitous group of fungal generalists with broad habitat niches that robustly colonize the roots of plants in stressful environments. DSEs possess adaptation strategies that determine their high tolerance to heavy metal (HM) contamination, drought, and salinity. Most DSEs developed efficient melanin-dependent and melanin-independent mechanisms of HM detoxification and osmoprotection, including intracellular immobilization and extracellular efflux of HMs and excess ions, and the scavenging of reactive oxygen species. DSEs form mutualistic relationship with plants according to the hypothesis of "habitat-adapted associations", supporting the survival of their hosts under stressful conditions. As saprophytes, DSEs mineralize a complex soil substrate improving plants' nutrition and physiological parameters. They can protect the host plant from HMs by limiting HM accumulation in plant tissues and causing their sequestration in root cell walls as insoluble compounds, preventing further HM translocation to shoots. The presence of DSE in drought-affected plants can substantially ameliorate the physiology and architecture of root systems, improving their hydraulic properties. Plant growth-promoting features, supported by the versatility and easy culturing of DSEs, determine their high potential to enhance phytoremediation and revegetation projects for HM-contaminated, saline, and desertic lands reclamation.

Dark septate endophytes: mutualism from by-products?

Plant roots are abundantly colonized by dark septate endophytic (DSE) fungi in virtually all ecosystems. DSE fungi are functionally heterogeneous and their relationships with plants range from antagonistic to mutualistic. Here, we consider the role of by-product benefits in DSE and other root-fungal symbioses. We compared host investments against symbiont-derived benefits for the host plant and categorized these benefits as by-products or benefits requiring reciprocal investment from the host. By-product benefits may provide the variability required for the evolution of invested mutualisms between the host and symbiont. We suggest that DSE could be considered as 'a by-product mutualist transitional phase' in the evolution of cooperative mycorrhizal symbionts from saprotrophic fungi.

Ecology and potential functions of plant-associated microbial communities in cold environments

Complex microbial communities are associated with plants and can improve their resilience under harsh environmental conditions. In particular, plants and their associated communities have developed complex adaptation strategies against cold stress. Although changes in plant-associated microbial community structure have been analysed in different cold regions, scarce information is available on possible common taxonomic and functional features of microbial communities across cold environments. In this review, we discuss recent advances in taxonomic and functional characterization of plant-associated microbial communities in three main cold regions, such as alpine, Arctic and Antarctica environments. Culture-independent and culture-dependent approaches are analysed, in order to highlight the main factors affecting the taxonomic structure of plant-associated communities in cold environments. Moreover, biotechnological applications of plant-associated microorganisms from cold environments are proposed for agriculture, industry and medicine, according to biological functions and cold adaptation strategies of bacteria and fungi. Although further functional studies may improve our knowledge, the existing literature suggest that plants growing in cold environments harbor complex, host-specific and cold-adapted microbial communities, which may play key functional roles in plant growth and survival under cold conditions.

Dual inoculation of dark septate endophytes and Trichoderma viride drives plant performance and rhizosphere microbiome adaptations of Astragalus mongholicus to drought

Rhizosphere microbiome adapts their structural compositions to water scarcity and have the potential to mitigate drought stress of plants. To unlock this potential, it is crucial to understand community responses to drought in the interplay between soil properties, water management and exogenous microbes interference. Inoculation with dark septate endophytes (DSE) (Acrocalymma vagum, Paraboeremia putaminum) and Trichoderma viride on Astragalus mongholicus grown in the non-sterile soil was exposed to drought. Rhizosphere microbiome were assessed by Illumina MiSeq sequencing of the 16S and ITS2 rRNA genes. Inoculation positively affected plant growth depending on DSE species and water regime. Ascomycota, Proteobacteria, Actinobacteria, Chloroflexi and Firmicutes were the dominant phyla. The effects of dual inoculation on bacterial community were greater than those on fungal community, and combination of P. putaminum and T. viride exerted a stronger impact on the microbiome under drought stress. The observed changes in soil factors caused by inoculation could be explained by the variations in microbiome composition. Rhizosphere microbiome mediated by inoculation exhibited distinct preferences for various growth parameters. These findings suggest that dual inoculation of DSE and T. viride enriched beneficial microbiota, altered soil nutrient status and might contribute to enhance the cultivation of medicinal plants in dryland agriculture.

A shift from arbuscular mycorrhizal to dark septate endophytic colonization in Deschampsia flexuosa roots occurs along primary successional gradient

Soil fungal community and dominant mycorrhizal types are known to shift along with plant community changes during primary succession. However, it is not well understood how and why root fungal symbionts and colonization types vary within the plant host when the host species is able to thrive both at young and at old successional stages with different light and nutrient resource availability. We asked (i) how root fungal colonization of Deschampsia flexuosa (Poaceae) by arbuscular mycorrhizal (AM) fungi and dark septate endophytes (DSE) changes along a postglacial primary successional land uplift gradient. As neighboring vegetation may play a role in root fungal colonization, we also asked (ii) whether removal of the dominant neighbor, Empetrum nigrum ssp. hermaphroditum (Ericaceae), affects root fungal colonization of Deschampsia. We also studied whether (iii) foliar carbon (C) and nitrogen (N) concentration of Deschampsia is related to successional changes along a land uplift gradient. AM colonization decreased (-50 %), DSE colonization increased (+200 %), and foliar C declined in Deschampsia along with increasing successional age, whereas foliar N was not affected. Empetrum removal did not affect AM colonization but increased DSE sclerotial colonization especially at older successional stages. The observed decrease in foliar C coincides with an increase in canopy closure along with increasing successional age. We suggest that the shift from an AM-dominated to a DSE-dominated root fungal community in Deschampsia along a land uplift successional gradient may be related to different nutritional benefits gained through these root fungal groups.

AM and DSE colonization of invasive plants in urban habitat: a study of Upper Silesia (southern Poland)

Interactions between invasive plants and root endophytes may contribute to the exploration of plant invasion causes. Twenty plant species of alien origin differing in invasiveness were studied in terms of status and typical structures of arbuscular mycorrhizal fungi and dark septate endophytes (DSE) in urban habitats in Silesia Upland (southern Poland). We observed that 75 % of investigated plant species were mycorrhizal. The arbuscular mycorrhiza (AM) of most plant species was of the Arum morphology. The nearly 100 % mycorrhizal frequency, high intensity of AM colonization within root cortex and the presence of arbuscules in all mycorrhizal plant species indicate that the investigated species are able to establish AM associations in the secondary range and urban habitats. DSE were present in all mycorrhizal and non-mycorrhizal species. The frequency of DSE was significantly lower in non-mycorrhizal group of plants, however, sclerotia of DSE were found mainly in the roots of non-mycorrhizal plant species. The group of species native to North America including three Solidago congeners have the highest values of all AM mycorrhization and DSE indices. Moreover, we observed that most mycorrhizal invasive species belonged to the family Asteraceae. In turn, representatives of Poaceae had the lowest values of AM mycorrhization. Nevertheless, quite high values of DSE frequency were also encountered in roots of Poaceae species. The high invasiveness of the representatives of the Asteraceae family from North America support theory that both taxonomic pattern, and the fact of root endophytes colonization contribute to invasion success. While, the taxa of Reynoutria also represent successful invaders but they are of Asiatic origin, non-mycorrhizal and weakly colonized by DSE fungi.