When biologists learn about immune systems, we usually hear only about humans. Recently I have been studying about plant immune system for one of my courses, and have found it very interesting. In wilderness plants are constantly exposed to all kinds of microbes and diseases, but they rarely get sick, because their immune system has evolved to literally fight the pathogens that may infect them.
As in mammals, plants possess physical barriers that protect them from the entry of pathogens. In fact, plants have been smarter than us in this manner: each cell has a complex layer of polymers, strategically located surrounding it: the cell wall (which animal cells lack). One important component of the innermost layer of cell walls is lignin, because it is made of monomers hard to degrade that bond to each other randomly. Because of the random bonding, pathogens will be facing different polymeric structures every time, hence it is going to be difficult for them find a mechanisms to brake all the many random bonds between monomers and therefore infect the plant.
If pathogens can overcome the cell wall, plants will recognize them using specialized proteins- similar to antibodies in humans. Once plants recognize pathogens, they will trigger a signal cascade to activate what is called inducible immune system or basal response that is based on the release of molecules that will fight the pathogen and prevent infection. Nonetheless, pathogens always fight back. Many bacteria possess what is called the type III secretion system, a smart device that allows them to introduce a stylet-like structure in to the plant cells. Throughout the structure, bacteria will leak in molecules that will interfere with the basal response, hence they will be able to survive and take over the plant.
But plants can still fight back and stop infection. They have also evolved to produce intracellular molecules that will recognize the proteins injected through the type three secretion system. These proteins, called “R” or resistance proteins will trigger again the inducible immune system, and if the infection is severe they will recur to one extreme strategy: suicide. This is a phenomenon called programmed cell death, and it is very useful, as plants will kill infected cells to stop the spreading of infection and prevent it to take over the whole organism.
This is only a glimpse of plant immune system. More can be found in this review paper by Dodds & Rathjen (2010), and in this other review by Barros and collaborators (2015) (By the way, one the collaborators is my teacher on this topic. Very cool huh?)
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