ghum to exchange products and information fit the description of biological interfaces (e.g., authentication, protocols, and media) as first described by Guliuzza and Sherwin (2016). Mutualism between plants and arbuscular fungi is extremely complex and involves constant communication and signaling between organisms. However, this mutualism can change to parasitism caused in part from issues such as disrupted authentication (signal exchange), degraded protocols (unequal nutrient exchange) or unfavorable environmental media conditions. B. Nitrogen fixation Approximately 78% of earth’s atmosphere consists of molecular dinitrogen (N2). In this form, its stability and non-toxicity make it a useful way to store it in the atmosphere. Though nitrogen is required by all organisms for the biosynthesis of life-requiring biomolecules such as DNA, RNA, ATP, and proteins, atmospheric dinitrogen is not usable and must be transformed (fixed) into nitrogen compounds organisms can use. Therefore, nitrogen fixation is a globally important process where dinitrogen is converted into usable ammonia (NH3) or other important nitrogen compounds. Zuill and Standish (2007) suggested that the biospheric nitrogen cycle system had irreducibly interdependent (complex) properties, evidence of an intelligent Creator. This is because the intricate and global process is mostly accomplished by a diverse suite of organism relationships, with a small percentage produced by lightning (Hill et al. 1980). The rhizosphere of terrestrial plants is that region of soil that surrounds plant roots. It is a rich ecosystem composed of a plethora of diverse organisms. One of those groups are diazotrophs and they are designed to fix nitrogen. Diazotrophs include several phyla from both Eubacteria and Archaea domains and are further subdivided into root nodule bacteria and plant growth promoting rhizobacteria (Mus et al. 2016). In turn, plants have a diversity of ways they can associate with diazotrophs and work together for the benefit of their relationship and for the community. The proximity of plant roots and diazotrophs determine the type of association, and their proximity has been delineated into three major categories; free-living interactions through soil/ water media, endophytic where diazotrophs are able to enter plant tissue intercellular spaces, and endosymbiotic where diazotrophs are taken into a plant cell and housed within plant synthesized membranes (Mus et al. 2016). Along with these types of relationships, there is evidence that nitrogen fixing associations also occur within fungi and termites (Mullins et al. 2021; Thanganathan and Hasan 2018). 1. Free-living diazotrophs There is a diversity of free-living diazotrophic prokaryotes in the global rhizosphere. They can be facultative or obligate anaerobes or aerobes and consist of taxa that include proteobacteria, (a recently delineated and controversial phylum of Gram-negative bacteria) and Cyanobacteria (National Laboratories of Medicine 2021; Mus et al. 2016). Because these are not classified as symbiotic relationships, and space prohibits detailed discussion, they do fall under relationships where protocols do not require attachment (Hennigan et al. 2022). Mus et al. (2016) describe plant roots secreting chemicals into the soil which are detected by plant growth promoting rhizobacteria (PGPR; e.g., proteobacteria). Once detected, these bacteria move toward the roots and when they get into proximity, they have nitrogenase enzymes to transform dinitrogen into metabolically usable NH3 or NH4 + which then enriches the soil and improves the health of the rhizosphere and greater community (Mus et al. 2016; Steenhoudt and Vanderleyden 2000). Hennigan et al. (2022) emphasize that outcomes like increased soil fertility are the result of each autonomous organism’s authentication capability of identifying self from non-self, protocols for non-attachment interactions, and relational functioning in soil and growth promoting chemical media. The intelligence required for both plant and PGPR to communicate and respond to one another, for the good of the community, is consistent with an infinitely wise and knowledgeable Designer. 2. Intercellular endophytes Many diazotrophic bacteria (e.g., Azoarcus, Herbaspirillum, Gluconacetobacter, and Nostoc) can enter plant tissue through openings such as lenticels, stomata, or root cracks without causing damage and tripping a defense response (Mus et al. 2016). For this reason, they are classified as endophytes. Endophytes are ubiquitous and can be facultative or obligate, mutualistic, commensal, or parasitic, and have been identified in all plant species to date (Mus et al. 2016; Hardoim et al. 2015). Nitrogen fixing cyanobacteria (Nostoc sp.) are also associated with some bryophytes (e.g., mosses and liverworts), fungi, lichens, and cycads (palm-like gymnosperms), and do not form root nodules (de Vries and de Vries 2018; Mus et al. 2016). Details differ depending on species associations, but general patterns can be described across diverse associations. For example, relationships begin with highly complex communication via authentication in soil or water media. Plants can attract nitrogen-fixing symbionts with flavonoids and/or a diversity of polysaccharides that function as signal molecules that symbionts detect, recognize, and respond to in a variety of ways (de Vries and de Vries 2018). Not all signal molecules have been identified and in the case of Nostoc relationships, signal molecules are also called hormogonia-inducing factors (HIF). HIF are detected in soil or water media and transform Nostoc non-motile vegetative cells into motile cells called hormogonia. Hormogonia communicate with other plant molecular signals and move toward the plant by gliding over wet surfaces or viscous substrates. When hormogonia reach the plant host, they differentiate again into unique, thick-walled cells called heterocysts and enter plant tissue. Research into structural defects on the surface of polysaccharide signal molecules suggests that some of these symbiotic associations fail because these defects prohibit authentication (Mus et al. 2016). This is consistent with Hennigan et al. (2022) when they discussed that failure of one or more interface components would result in deleterious effects on symbiotic associations. de Vries and de Vries (2018) outline the intricate components needed once symbionts enter the plant where each symbiont is tightly controlling their respective interfaces. Several protocols have been described for how they do this and they are regulated both transcriptionally and at a protein level where proteins regulate other proteins, undergo modification, and/or use available non-protein cofactors that assist with reactions. Nitrogen fixing genes (nif) can be regulated by nitrogen fixing proteins such as nifA. For example, the protocols for the nitrogenase enzyme complex alone are intricate. Two HENNIGAN, GULIUZZA, INGLE, and LANSDELL Interface systems model in key global symbiotic relationships 2023 ICC 233
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