III. MUTUALISTIC SYMBIOSES Mutualistic symbioses are long-term relationships with two or more organisms working together in a way that all benefit. These associations can be classified as obligate or facultative. In this section we will briefly describe what is currently known about a few globally important relationships that are often the foundation for ecosystem and community health. Human understanding of these complex relationships is growing rapidly, and the data suggest that we are far from fully comprehending any of them. However, what is being elucidated continues to amaze and inspire researchers toward improving human technology, as the complexities in creation far surpass human engineering. For example, a study by U. C. Berkeley finds that sorghum manipulates soil conditions to promote a beneficial change in microbes on their roots during drought. Over a four month experiment the composition of the microbial community drastically shifted as conditions went from wet to drought. With the onset of drought, sorghum roots released an increased range of carbohydrates and amino acids, as well as secondary metabolites which may include antimicrobials and reactive oxygen species into the soil. The normally dominant microbes (Proteobacteria, Bacteroidetes, and Verrucomicrobia) are poorly suited to these changed conditions and their populations rapidly decrease. Streptomyces strains of Actinobacteria are normally present in small numbers but are particularly suited to both the new root products and drought conditions and take over primary colonization of the root system. Sorghum detects the recolonization by Actinobacteria and, through a yet to be identified mechanism, adjusts metabolism again so there is an increase in relative root-to-shoot resource allocation that is correlated with increased root biomass and deeper roots under drought stress. Researchers suggest that these interactions are the result of cross talk between sorghum cultivars and microbes (Hunter 2018). The complex communication and authentication operating by sophisticated genetic protocols, through soil media over a range of moisture conditions, is consistent with a model of interface design. Mutual symbioses highlighted are mycorrhizal networks and nitrogen fixation. A. Mycorrhizal networks The name mycorrhiza comes from the Greek meaning fungus-root. The term refers to how fungal hyphae connect and interact with plant roots, often forming mutual symbioses. The fungal symbionts improve nutrient uptake and enhance water transfer, seedling development, soil formation, resistance to pathogens, stress resistance, and plant community establishment and in turn, the plant provides crucial carbohydrates (Bonfante and Genre 2010; Hennigan 2009B; Pandey et al. 2019). Hyphae of the mycorrhiza can extend farther than their host plant roots and connect to other mycorrhizal networks forming the wood-wide-web, which is responsible for horizontal nutrient movement throughout the community (Anca and Bonfante 2009; Bonfante and Genre 2010). Two major types of mycorrhizal fungi are ectomycorrhizae (ECM) and endomycorrhizae (EM), and they differ in how they colonize the plant root. Fungal taxa of ECM include species from clades Basidiomycota or Ascomycota and their hyphae connect and grow on the outside of tree and shrub roots forming a Hartig net interface where nutrients are exchanged (Anca and Bonfante 2009; Bonfante and Genre 2010; Plett and Stuart 2020). Fungal taxa of EM include species from Glomeromycota where their hyphae penetrate the root cortex of forest trees/shrubs, along with some herbaceous plants, and enter cells forming vesicles. This type of mycorrhizae includes Arbuscular (AM), Ericaceous, and Orchidaceous endomycorrhiza (Bonfante and Genre 2010). We will only discuss arbuscular mycorrhizal symbiosis because it is the most common and well-researched type of mycorrhizal association. Arbuscular Mycorrhizae (AM) form symbioses with most terrestrial plants (Bonfante and Genre 2010; Chen et al. 2018; Rosendahl 2008). Most of these plants could live without AM fungi, but there are positive benefits to the relationship as the fungal partner increases the plant’s ability to resist pathogens, uptake nutrients, and tolerate stress (Bonfante and Genre 2010; Chen et al. 2018). All AM fungi benefit from the association with plants because of the carbon they receive from them (Walder and van der Heijden 2015). Glomeromycota fungi are characterized by their ability to reach the inner cortex of plant roots and form branched arbuscules which are the symbiotic interfaces for nutrient exchange (Bonfante and Genre 2010; Chen et al. 2018). These interfaces are key to understanding how these organisms interact and operate with each other. In AM symbiosis, the role of the plant-fungal interface is to exchange both signals and nutrients (Roth and Paszkowski 2017). Important molecules involved in signaling are plant metabolites that initiate and maintain AM symbiosis. The metabolome is reprogrammed by the fungus to promote colonization and allows the fungus to obtain carbon from the plant (Kaur and Suseela 2020). The metabolome includes primary metabolites (sugars, organic acids) and secondary metabolites (alkaloids, flavonoids), and changes in the concentration of these compounds depend on the environment, plant species, or fungus species (Machiani et al. 2022). The reprogramming of the plant metabolome can affect the type of relationship exhibited between the plant and the fungus (Kaur and Suseela 2020). Because the fungus has morphological similarities to that of a pathogen, the host plant recognizes self from non-self (authentication) but goes into a mode with highly restrictive exchanges. As with animals, plants have an immune system—which constitutes the interface system—and, thus, authentication is required (Sanabria et al. 2010). As seen in fungus/plant mutualisms, the protocols of interface systems facilitate a balance between the plant and fungal nutrient needs (Kogel et al. 2006). To have long lasting, mutualistic interactions, mutual protocols facilitate the plant cell’s acceptance of the fungal hyphae, and this starts with plant receptor-kinase-mediated transmembrane signaling that recognizes the plant as a cooperative organism (Kogel et al. 2006). The incredibly tight design of the protocols is demonstrated when the plants produce salicylic acid to suppress fungal colonization, but the fungus produces compounds that the plant detects and stops acid production so that the fungus can colonize the plant (Kuar and Suseela 2020). The plant also reorganizes cellular organelles to accommodate fungal structures (Kogel et al. 2006; Bonfante and Genre 2010). MacLean et al. (2017) discuss three stages for how arbuscular myHENNIGAN, GULIUZZA, INGLE, and LANSDELL Interface systems model in key global symbiotic relationships 2023 ICC 231
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