I wrote this as a small summary from the Literature Review I completed in my third year titled “Friend or Foe: Plant Perception of Bacterial Pathogens and Symbionts in Immunity and Symbiosis”. This is a lot less technical and I focus on conceptual differences/similarities as opposed to specific interactions! I’ve added links to free-full-text reviews where I can (and studies where I specify), but feel free to message me for institutional access – that goes for anything, not just articles specific to this topic.
Both plants and animals encounter a plethora of microbes in their lifetimes. Some of these microbes can cause disease, some are neutral, while others can develop a beneficial relationship with an organism. This relationship is known as a symbiosis, or symbiotic relationship. An example I have previously touched on is the Hawaiian bobtail squid and the bacteria Vibrio fischeri – to hunt at night the squid camoflauges against starlit waters by emitting light and preventing a shadow being cast onto the sea floor. This bioluminescence is thanks to a chemical reaction in the bacterial colonies that reside on the squid’s “light organ”. Another popular example is the gut bacteria found in humans, that have suggested roles in human digestion, metabolism and even brain activity. These bacteria are specific and have evolved to live alongside humans – in contrast to (for example) Salmonella, which when ingested will invade the intestines and cause the symptoms of food posioning.
Perhaps a surprise to some, plants have an immune system that can identify between those bacteria that are harmful and those that are not. This system depends on the ability for each cell to detect and defend, unlike human immunity which is dependent on a circulatory and lymphatic system with specialised immune cells. Plants have immune receptors located on cell membranes that can recognise common bacterial structures, and can trigger a downstream response that can result cell death of infected tissues. This would render symbiosis (a relationship with a beneficial bacteria) an incredibly difficult venture. Perception of a bacteria by a plant is crucial turning point that can lead to either an immune response or a symbiotic one. Here, I divide perception into two factors: the elicitor, i.e. the bacteria, and the plant cell surface receptor. For perception to occur, a plant cell requires specific receptors that can bind (thus identify) bacterial components. Both have key roles in distringuishing friend from foe.
Symbiotic bacteria can evade and suppress the immune response
Plants have developed detection systems that can identify key bacterial components that are clearly foreign to the plant kingdom. These components are called elicitors, since they elicit the immune response. For example, some bacteria have flagella, mostly made of a protein called flagellin, which was the first elicitor to be discovered in 1999. By dissecting this protein into smaller peptides, researchers found a stretch of 22 amino acids at one end of the protein that was responsible for its own detection in plants, called flg22. Later, plant membrane receptors were found that bind to this protein, signalling to the rest of the cell that flagellin (aka a bacterium) has been detected.
This non-specific detection system is used against other bacterial proteins as well as peptidoglycan, which is found in bacterial cell walls. This would potentially put symbiotic bacteria at risk of eliciting an immune response, however elicitors found on these bacteria often have modifications that render them silent – plant receptors, for example, fail to detect flagellin from the symbiont Sinorhizobium meliloti. The flagellin from this beneficial bacteria has a divergent amino acid sequence that enables it to hide from plant detection – it does not bind to typical flagellin receptors and does not trigger an immune response. Potent pathogens have been found to modify their elicitors as well – Agrobacterium flagellin has a divergent sequence of flg22 to evade the flagellin detection system.
Plants have a combination of different receptors and co-receptors that act together on the cell surface prepared to detect bacterial molecules. Receptors are protein complexes that recognise the elicitor and upon binding, will change its structure. This change causes subsequent reactions that eventually lead to relevant changes in the cell, that often results in the expression of defense genes. Because of this elicitor-receptor binding, receptors are specific in their nature – flagellin will have a different receptor to the protein EF-Tu, which has a different receptor to peptidoglycan. As mentioned above, symbiotic bacteria (and potent pathogens) can modify their elicitors so they evade detection by these receptors. An interesting exception is EF-Tu from symbiotic bacteria, which shares the same conserved motif and can induce the immune responses. This is because the symbiont plant host lacks a detection system for EF-Tu; it does not have the EF-Tu receptor (limited to the Brassicaceae plant family), so the protein will fail to elicit an immune response. This would suggest symbiotic bacteria have evolved to evade plant immunity. Symbiont bacteria witnessed no selective force (in the form of a detection system) in order to change its EF-Tu structure. Interestingly, in a study that transformed a host plant to express a known EF-Tu receptor, symbiotic bacteria still did not induce immune signalling, suggesting the bacteria has roles in suppressing the immune system.
In order for symbiosis to be established, the plant immune system needs to be suppressed. Studies have shown host plants exposed to both pathogens and symbionts, will prioritise the immune response before the symbiotic response. This increased immunity as a result (in the form of salicylic acid, jasmonic acid and ethylene) has a negative impact on the ability for a symbiont to establish itself. Symbiotic bacteria can produce chemicals that suppress plant defence mechanisms. Some pathogens can also produce these, called effectors, to suppress the immune response, and are often specific to the plant-bacteria interaction. Plant immunity involves many steps and these effectors can target different parts of this pathway, by blocking receptors, mimicing plant molecules, and inhibiting messenger proteins.
Symbiotic bacteria induce the symbiotic response
Symbiotic bacteria both evade and suppress plant immunity, much like an efficient pathogen. A third part of this puzzle are the signals released by symbionts to activate symbiosis in plants. Rhizobacteria are a group of symbiotic plant bacteria that reside in the root nodules of leguminous plants, and work to fix atmospheric nitrogen into organic molecules for the plant to use. A host plant releases flavonoids that trigger the expression of nodulation genes, which in turn results in the release of Nod-factors (nodulation factors) that signal to the plant to begin the formation of a nodule. The recognition of Nod-factors is focussed locally – only root hair cells behind the root tip will have the receptors necessary for Nod-factor recognition.
Rhizobacteria and their host legumes have co-evolved to limit the immune response and develop symbiosis. Nod factors can still transiently trigger immune responses, probably because they are structurally very similar to peptidoglycan and chitin (that do trigger immunity), also highlighting how crucial immune suppression is in symbiosis.
To summarise…
In order for a plant to recognise bacteria, it requires specific detection mechanisms in the form of cell surface receptors. These receptors can recognsie and bind to general bacterial structures as well as molecules that are specific to that bacteria. Because of this selective pressure, some bacteria have evolved to change its own structures to avoid being recognised. In interactions where plant immunity is still a problem, the bacteria will suppress the immune response by releasing effectors. Up until this point, symbiotic bacteria and some pathogenic bacteria are very similar – in its simplest form both are trying to avoid being detected to infect the plant. Symbionts and these pathogens differ when we consider Nod-factors, specific symbiotic signals whose release is triggered by host plant chemicals. This will directly signal to the plant that a symbiont is invading, and nodulation (the process of forming root nodules, where symbiotic bacteria will reside) begins.
All three mechanisms of invasion (evading immunity, suppressing immunity and inducing symbiosis) work side by side to aid a plant in recognising a “friend from foe”. Pathogens, such as Agrobacterium tumefaciens, that can utilise these mechanisms in a similar way tend to be incredibly successful at their job.
Related reviews to check out
Zipfel and Oldroyd, 2017. “Plant signalling in symbiosis and immunity”. A great review comparing the signalling systems between symbiosis and immunity, focussing on receptors, co-receptors, calcium signalling and symbiosis regulation.
Janeway and Medzitov, 2002. “Plant innate immunity: An updated insight into defense mechanism”. On how plant innate immunity works.
Jones and Dangl, 2006. “The plant immune system”
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