plant-growth regulators, improve environmental stress tolerance of their host, induce host defenses and systemic resistance against pests or pathogens, or be pathogenic. Parasites consume carbon from the plant without providing any benefit, or providing too little benefit relative to what they cost in carbon, thereby compromising host fitness. Symbionts may be biotrophic (subsisting off of living tissue) or necrotrophic (subsisting off of dead tissue). Mutualist-parasite continuum While some microbes may be purely mutualistic or parasitic, many may behave one way or the other depending on the host species with which it is associated, environmental conditions, and host health. A host’s immune response controls symbiont infection and growth rates. If a host’s immune response is not able to control a particular microbial species, or if host immunity is compromised, the microbe-plant relationship will likely reside somewhere nearer the parasitic side of the mutualist-parasite continuum. Similarly, high nutrients can push some microbes into parasitic behavior, encouraging unchecked growth at a time when symbionts are no longer needed to aid with nutrient acquisition. Composition Roots are colonized by fungi, bacteria and archaea. Because they are multicellular, fungi can extend hyphae from nutrient exchange organs within host cells into the surrounding rhizosphere and bulk soil. Fungi that extend beyond the root surface and engage in nutrient-carbon exchange with the plant host are commonly considered to be mycorrhizal, but external hyphae can also include other endophytic fungi. Mycorrhizal fungi can extend a great distance into bulk soil, thereby increasing the root system’s reach and surface area, enabling mycorrhizal fungi to acquire a large percentage of its host plant’s nutrients. In some ecosystems, up to 80% of plant nitrogen and 90% of plant phosphorus is acquired by mycorrhizal fungi. In return, plants may allocate ~20-40% of their carbon to mycorrhizae. Fungi Mycorrhizae Mycorrhizal (from greek) literally means “fungus roots” and defines symbiotic interaction between plants and fungus. Fungi are important to decompose and recycle organic material, however the boundaries between pathogenic and symbiotic lifestyles of fungi are not always clear-cut. Most of the time the association is symbiotic with fungus improving acquisition of nutrients and water from soil or increasing stress tolerance and fungus benefiting from carbohydrates produced by plant. Mycorrhizae include a broad variety of root-fungi interactions characterized by mode of colonization. Essentially all plants form mycorrhizal associations, and there is evidence that some mycorrhizae transport carbon and other nutrients not just from soil to