User:Jeremyfdean/Plant symbiont distinction

Many plant species are able to form symbioses with fungal or bacterial organisms. These symbioses range from pathogenic to mutualistic relationships on an undefined scale. Mutualistic plant-microbe symbioses are critical to the success of many plant species, including several crop species. Conversely, pathogenic plant-microbe symbioses have caused significant agricultural and ecological effects. The mechanisms by which plants are able to distinguish between pathogenic and mutualistic microbes is a topic of current scientific inquiry.

Pathogen vs. Mutualist

Distinguishing pathogenic from mutualistic microbes is a challenge encountered by most plants, and efficient distinction is required for a plant species to thrive. Plant distinction between mutualists and pathogens is particularly challenging because plant-microbe relationships may change over time, and many plant-microbe symbioses are not purely pathogenic or mutualistic, but instead exist on a continuum. Environmental changes in abiotic factors, nutrient availability, and the presence of other organisms change the nature of these relationships. For instance, plant-endophyte relationships are typically considered to be mutualistic or neutral . However, plant-endophyte interactions can also be pathogenic. Certain microbial species, such as Chaetomium elatum, oscillate between pathogens and mutualists depending on resource availability and presence of intraspecific competitors. Moreover, it is also possible for mutualistic relationships to evolve into a pathogenic relationship if one of the species involved in the original mutualism engages in "cheating". For example, when resources are limited by soil factors or intraspecific competition, C. elatum becomes pathogenic.

Plant Response to Pathogens

Initial Response to Infection

Plants possess a variety of mechanisms that enable them to resist or dampen the effects of an infection. In fact, many plants initially treat mutualistic microbes as pathogens and only later are initial immune responses suppressed.

Detection and Response Coordination

Lotus japonicus

One method by which pathogenic microbes may infect a plant host is by secreting plant cell wall degrading enzymes (PCWDEs). PCWDEs degrade plant cell walls thereby allowing microbes easier access to the host plant. The degradation of the plant cell provides nutrients for the pathogen. Plants have evolved multiple mechanisms to detect the presence of PWCDEs and subsequently increase immune response. First, plants recognize pathogen-associated molecular patterns (PAMPs) that indicate PCWDE presence. Second, plants recognize damage-associated molecular patters (DAMPs), which indicate plant cell destruction. After pathogen recognition, plants may employ one or multiple immune responses. Upon PAMP or DAMP recognition by pattern recognition receptors (PRRs), plants coordinate immune responses using ethylene signaling.

Evasion of Detection

Beneficial microbes need to cope with host immune response. Many pathogens are capable of down-regulating plant immune response via various mechanisms. For instance, pathogens use small RNAs such as microRNAs to alter host post-transcriptional machinery responsible for immune responses. Plants also recognize specific peptide motifs in pathogenic microbes, specifically bacterial flagellar protein motifs. To prevent their detection, beneficial microbes have evolved differences in the structure of the protein recognized by the plant that illicit an immune response. With this structural change, microbes go undetected by the plant's immune system, and can prevail to eventually colonize the root of the host plant. One common peptide motif, flg22, exists in the flagella of many bacterial pathogens. In certain mutants of Lotus japonicus, flg22 presence negatively affects initial nodule formation in rhizobium-legume interactions.

Plant Response to Mutualists

Initial vs. Eventual Response

Plant response to a potential mutualist varies and there are multiple models of mutualism that can account for the different mutualistic relationships that may occur. Though plants typically respond to initial symbiotic infections as pathogenic, many mechanisms exist whereby plants eventually tolerate or even support the presence of microbial symbionts. For instance, clusters of genes that mount immune defenses were found to be downregulated during root nodulation stages in L. japonicus, thus allowing mutualistic resource exchange.

Small Signaling Proteins

Fungal hyphae mass

In a study of Laccaria bicolor, an ectomycorrhizal fungus, researchers discovered the presence of many small signaling proteins (SSPs) with no known function. These SSPs were primarily found in the proliferating hyphae colonizing host plant roots. In fact, 5 of the 20 most upregulated L. bicolor root tip transcripts coded for SSPs. It was hypothesized that these SSPs may manipulate host cell signaling and suppress defense pathways.

Maintenance of Mutualism

Counter-intuitively, plants must also possess robust immune functioning in order to maintain a balanced mutualistic plant-microbe relationship. By the expression of defense-related genes, plants can control the extent to which a mutualistic association develops, allowing only a certain number of infections to persist at a time. Results from a study using Arabidopsis mutants found that adequate secondary metabolites were required to restrict growth rates of various fungi that form plant-fungal endoophyte relationships. This growth restriction maintains a homeostatic, mutualistic relationship in which both plant and fungus benefit. Similarly, plants produce salicylic acid to modulate levels of root colonization by certain arbuscular mycorrhizal fungi. Oftentimes, beneficial microorganisms only become pathogenic when the host plant is susceptible and does not posses a strong immune system.

Cheaters and Sanctioning

On occasion, normally mutualistic rhizobia may "cheat" on their plant hosts by withholding nitrogen from the plant. Plants have several mechanisms by which they can sanction cheating microbes, including limiting carbon supply, limiting nodule oxygen supply, and attacking non-fixing microbes with acid hydrolases. It has been experimentally demonstrated that soybeans possess the ability to distinguish between mutualistic and cheating rhizobia, as well as containing mechanisms which allow them to punish the cheating rhizobia by decreasing oxygen supply to their nodules.