key components that make up the enzyme complex are nitrogenase reductase and nitrogenase. Nitrogenase reductase consists of two identical building blocks (subunits) coded by nifH while nitrogenase is composed of two identical β subunits (β2) coded by nifK and two α subunits (α2) coded by nifD. Another protein, nifL, can interact with nifA, depending on environmental cues that are detected by sensors on the diazotroph and plant (Hennigan and Guliuzza 2019). Appropriate gene expression is also dependent on nifS and nifU proteins regulating fixation rates with the help of cofactors. For dinitrogen to be transformed into useful plant compounds other genomic/proteomic protocols are required for anaerobic conditions and high ATP rates. This is because oxygen inactivates nitrogenase and a lot of energy is required for these biochemical reactions. Research also suggests that sugars involved with cyanobacteria are not just signal molecules but are also important in providing a nitrogenase pathway through the pentose-phosphate biochemical pathway (de Vries and de Vries 2018; Mus et al. 2016). But, to produce high ATP rates, high oxygen concentrations are also needed. Therefore, these autonomous organisms must tightly regulate gene expression so that the proper chemical environment, monitoring of environmental variables (e.g., temperature, N2 concentration, O2 concentration, light, plant growth), efficient resource exchange, and exchange rates result in healthy functioning for one another. In Nostoc, the genomic and proteomic protocols are programmed to synthesize unique heterocyst cells that become the anaerobic chambers required for nitrogen fixing reactions. Requirements include; three cell walls for oxygen impermeability, nitrogenase production, retention of photosystem I for ATP production, dismantling of photosystem II so that oxygen production stops, upregulation of glycolytic enzymes, and synthesis of special proteins for scavenging remaining oxygen (de Vries and de Vries 2018). There is a unique endophytic association with aquatic ferns (Azolla sp.) and cyanobacteria (Nostoc sp.) worth highlighting. Similar authentication and genetic/proteomic protocols discussed above are still relevant and complex. However, rather than a continual attraction and interaction of Nostoc symbionts recruited from the environment, symbionts are inherited via vertical transfer and always found in Azolla leaf tissue (de Vries and de Vries 2018). Though much research is still needed to understand the Azolla/Nostoc nitrogen fixing relationship, de Vries and de Vries (2018) outline our understanding to date. As with all types of mutualistic symbiosis Nostoc and Azolla sp. require a highly complex and tightly controlled communication with one another because they undergo several complex stages of symbiosis that includes transferring inoculum sexually and asexually, but also control pathogens that can remove fixed nitrogen for themselves. Generation to generation vertical inheritance has provided a stable relationship between the two and has provided continuity in this relationship for 66 to 100 million years based on conventional dating. Humans have recognized Azolla spp. for thousands of years as a fertilizer in rice agriculture and recent data suggest this symbiosis can increase agricultural yield by 200% (de Vries and de Vries 2018). 3. Intracellular endosymbionts Fabaceae is considered the third largest flowering plant family and includes the legumes of the world (Roy et al. 2020). Many global members of this family enter the most complex and intricate nitrogen fixing symbiosis with microbes. They are known as nodule forming endosymbionts. These microbes proceed through a typical, yet to be fully understood, relationship progression, similar to what was described above, but also includes authentication and protocols for determining self from non-self (recognition), protocols for physical interaction between entities (penetration), protocols for diverse growth structures (e.g., cell division stimulation of nodules), and protocols for morphological/physiological changes and diversification in the endosymbiont (Mus et al. 2016, Roy et al. 2020). Nodule forming endosymbioses is one of the best studied relationships using model species such as the soybean (Glycine max) and common bean (Phaseolus vulgaris). The following description is a summary of our current understanding of these interactions. Roy et al. (2020) outline the following general steps for nitrogen fixing rhizobia to enter relationally tight interactions with leguminous plants. They include the importance of authentication where complex signaling mechanisms trigger gene expression in both partners so that partners can identify and determine relationship parameters while immune system responses exclude other microorganisms. This authentication process is a considerable undertaking because the soil medium is full of compatible and incompatible microorganisms and plants use complex signaling to distinguish them. Protocols are involved with coordinating bacterial access to epidermal and cortical cells, stimulating root cell mitosis and nodule formation, manufacturing thousands of symbiosomes that are specialized organelles that house bacteroids which are the anaerobic chambers for Nitrogen fixation, and modifying plant tissue for providing nutrients to bacteroids (Clúa et al. 2018; Roy et al. 2020). Early signaling molecules include legumes producing flavonoids into the soil medium. Rhizobia detect and recognize these signals and they synthesize lipochitooligosaccharides (Nod factors) which allows the plant to recognize the bacteria, which triggers the plant to synthesize welcome signals to the symbiont. Complex genetic protocols express thousands of genes which express large numbers of chemical signaling pathways, from both organisms, so that this partnership will be successful. These genetic protocols and signal pathways reorganize several key anatomical structures in the plant for a successful symbiosis. For example, root hairs, cytoskeleton for root hair reorientation, cell wall degradation, and membrane invagination are all required for microbial entrance into the plant and are tightly controlled by genetic and hormonal interactions modifying these structures that are still being illuminated (Clúa et el. 2018; Roy et al. 2020). Plant hormones such as cytokinins and auxins control nodule organogenesis depending on concentrations and ratio of the two. These parameters determine timing and location of nodule cell division (Roy et al. 2020). Soil media characteristics affect the efficiency of these relationships and include concentrations of water, phosphate, and sulfate. Metal ions in soil such as iron, molybdenum, zinc, and copper are crucial cofactors required in symbiotic nitrogen fixation metabolism. As limiting factors, they can directly or indirectly affect the process and cause negative effects for the bacteria, plants, or both. This, in turn, can affect how we view the symbiosis as mutual, commensal, or even HENNIGAN, GULIUZZA, INGLE, and LANSDELL Interface systems model in key global symbiotic relationships 2023 ICC 234
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