Editorial

Abstract

The relative importance of beneficial fungi to global ecosystems is immense. Largely due to their filamentous nature, parasitic and mutualistic fungi form a subterranean ‘neural network’ that bridges signalling between them and other organisms such as host plants. While pathogenic fungi deservedly garner notoriety due to their detrimental effects on plant growth and development, mutualistic fungi are known to positively influence plant processes such as photosynthesis, mineral and water uptake, growth, and resistance to pathogens. In exchange for these benefits, fungi enhance their own nutritional strategy and reproductive success. This Special Issue focuses on the frontier of beneficial fungal–plant interactions. We seek to highlight recent results of novel studies contributing to our understanding of beneficial fungi in this important context. Due to recent technological advancements in plant and fungal molecular biology and genomics, innovative tools have been deployed to unravel some of the key mechanisms in plant–fungal interactions. Our growing knowledge of these interactions will lead to the development of viable strategies to improve plant health, sustainable agricultural systems and the broader global environment. Arbuscular mycorrhizal fungi (AMF) are the most widespread fungal symbionts of plants, being associated with more than 80% of current land plants (Smith, S. E. & Read, D. J. Mycorrhizal Symbiosis Academic Press, 2008). All AMF belong to the phylum Glomeromycota, and so far, appear to be unculturable in the absence of a host; as such, they are referred to as obligate biotrophs. In this issue, Chen et al. describe for the first time, AMF enhancing the resistance of the invasive plant, Wedelia trilobata, to a common phytopathogen. Understanding the influence of AMF on an invasive species’ tolerance to challenge by a pathogen represents an important step towards mitigating the impact of invasive plants by identifying factors enabling them to expand their niches. In another related report in this issue, Li et al. sought to define alfalfa’s (Medicago sativa) response to Phoma medicaginis infection, the colonization of AMF and subsequent effect on plant–pathogen interactions. The authors conducted a genome-wide transcriptomic profile of alfalfa plants infected with P. medicaginis and inoculated with AMF (Rhizophagus intraradices). They found specific genes related to disease resistance to be differentially expressed after pathogen infection and AMF colonization. The results support the hypothesis that, in many pathosystems, AMF inoculation may bolster the host’s transcriptomic response of key resistance genes to pathogen challenge. This issue also contains insightful reviews such as the article from Harman et al., who focused on endophytic fungi in the genus Trichoderma, documenting recent findings describing the ability of these fungi to enhance plant photosynthetic capabilities and to boost resistance to abiotic stresses. Endophytic fungi are highly diverse and their associations with land plants are ubiquitous. Some are known to produce highly varied bioactive compounds while others have been shown to be pathogenic to insects. Several ascomycetous insect pathogenic fungi, including species in the genera Beauveria and Metarhizium, are also plant root symbionts/endophytes and are termed endophytic insect pathogenic fungi (EIPF). In this issue, Hu and Bidochka reviewed studies surrounding the ability of EIPF to confer benefits to plant hosts. Interactions between EIPF and their plant hosts result in the translocation of insect-derived nitrogen from infected insect cadavers in exchange for nutrient-rich plant exudates benefitting EIPF. The ability of EIPF to infect insects as well as colonize plant roots involves secretion of signalling factors produced by both host and fungus, modulation of plant defence responses and nutrient exchange. The review by Hu and Bidochka summarizes the current knowledge of these critical and complex interactions and highlights the remaining gaps in our understanding. To date, studies on fungi and their beneficial interactions with plants have brought to light many interesting aspects of these mutualistic and oftentimes clandestine associations. Recent studies have revealed the complexities and intricacies of these interactions, permitting these organisms to thrive as partners in different environments, and to withstand biotic and abiotic stresses. The different contributions presented in this issue underscore some of the recent advancements in this ever-evolving field of study. This new knowledge will undoubtedly serve as a foundation upon which future investigations will expand the current understanding of ecosystem function and the impact of fungi on plant health and development.