In this and my next post, I will revisit the idea of the plant-pathogen running game. This time, however, the focus will be on the pathogen side of the contest. In other words, I will describe the means by which pathogens try to keep up with the host. As it turns out, pathogens maintain large gene repertoires that allow them to either prevent changes in the host or respond to defense-associated events during infection.
Image: Collection of microbes that would love to infect plants, but probably can't! (Source: Author).
Previously, we have established that plant-pathogen interactions represent a running game of life and death and that plants:
(i) Have evolved inducible defenses to fend off microbes that overcome physical barriers.
(ii) Defense-associated proteins are often secreted to combat infection and do so by
(iii) Either acting on the pathogen cell itself (lytic enzymes) or countering the activity of pathogen proteins that promote virulence.
Besides the synthesis and secretion of anti-microbial proteins, however, plant immune responses also feature the production of Reactive Oxygen Species (ROS) such as water peroxide (some people use this to bleach their hair...) and secondary metabolites. The mobilization of secondary metabolites signifies a distinct switch of plant cells into a defensive state (Piasecka et al., 2015). The host immune response, therefore, causes a drastic change to the host extracellular environment, turning previously healthy and nutritionally rich tissues into a hostile space, loaded with toxic substances. The fact that pathogens persist despite these changes tells us that these microbes have acquired mechanisms to cope with the first lines of host defenses. The question is: what are these mechanisms?
On the most basic level, pathogens can employ several cellular strategies to deal with active defense responses. All of these scenarios rely on the assumption that plant-derived toxins act on specific cellular targets that enable the toxic effect. For example, if toxin-A would act on an enzyme that mediates cell wall synthesis (which it needs for growth), a pathogen could circumvent this compound by evolving an improved version that is insensitive and continues to engage in cell wall construction despite the toxin.
Alternatively and if target modification is not possible without incurring a penalty (e.g., loss of enzyme activity), the pathogen could try to reduce toxin levels inside the cell. Mechanisms to achieve this include:
1. sequestration of toxic compounds using intracellular transport & storage inside organelles
2. Conversion of Toxin-A into its harmless sibling compound-B.
3. Active export of the harmful chemicals out of the cell in an energy-dependent manner.
Despite the vast interest in the biology of plant pathogens, we know little about the mechanisms I have described above, in the context of plant-microbe interactions. Given the advent of genome-wide studies of eukaryotic pathogens, we have failed to consistently identify or pinpoint gene repertoires with clear roles in detoxification (for example, see work on the saponins). This observation contrasts success in the identification of factors that govern insensitivity to drugs or pesticides. I would thus suggest that either the contribution of detoxification needs to be properly and more intensely investigated. What we will find is that pathogens have devised ways to prevent the accumulation of substances that are detrimental to their fitness. It is important to note that this is a challenge every cell on this planet faces. How do I keep nasty chemicals at bay in an ever-changing environment?
Because of these commonalities, we can be confident that any discovery made in pathogens, may be applicable to other systems. That's how fundamental research often works.
To learn more about plant pathogens and their not so friendly hosts, stay tuned for more. In the next article of the Salads Under Siege series, I will focus on the suppression of inducible responses.
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