Scientists asked a highly virulent microbe, what made you this tough? Here is the answer. Microorganisms including fungi, bacteria, viruses, molecules as well as nematodes can become pathogenic to plants. The ability to cause a disease or pathogenicity depends on the virulence of the pathogen, host resistance and environmental conditions. Pathogenesis helps pathogens to invade plant tissues to obtain nutrients for their survival and reproduction. Nature has given pathogens their “level of virulence” in this survival-based interaction that requires for the infection while plants were given their “range of defense”.
Back in the evolutionary time scale, plants and their special parts have evolved to provide at least room and board to many microorganisms, enabling them to fulfill their functions. As a result of this long-lasting process, some microbes might have evolved to interfere with the physiological function of plants due to the mutations favoured by natural selection. Then plants obviously might have had to react through responsive mutations related to their defense against those microbes.
Plants protect themselves as a result of molecular, cellular and tissue responses. These defenses can be either structural or chemical. Plants might have first tried their best to safeguard themselves using basic physical and chemical barriers as a form of passive defense. This can be pre-existing structural barriers such as wax, hairs, cuticle, epidermis, the orientation of leaves and the nature of stomata. Plants are well armed with pre-existing chemical defenses as well. They release inhibitors such as fungal toxic exudates to inhibit spore germination and some of these inhibitors are present in plant cells, for example, phenolics, tannins, saponins and lectins. Moreover, some plants lack receptors for toxins and some lack essential substances for pathogens.
Plants have further developed another level of defense called active defense, literally the immune system of plants. Plants can detect the presence of infectious agents through signals. These signalling molecules perceived by plant cells to induce defense responses are called elicitors. Elicitors can be specific or nonspecific. Nonspecific elicitors such as pathogen-associated molecular patterns (PAMPs) have a kind of evolutionary stability. These elicitors can induce defense in a wide range of host species. They are originated from pathogenic organisms. These are sensed through pattern recognition receptors (PRRs) on the surface of the plant cells, which would trigger the basal resistance called PAMP triggered immunity (PTI), but still, there could be better fighters!
The pathogens which successfully overcome basal resistance would produce effector proteins, which could inhibit signalling pathways mediated by basal immunity. Thus, they become specific elicitors, if the host plant manages to recognize them. Specific elicitors are produced by specialized pathogenic strains that match the receptors of the specific host cultivar. These effector proteins are encoded by avirulence genes (Avr genes) of pathogens. If the host plant has specific receptors to bind to those effectors, then effector-triggered immunity (ETI) will occur within the plant. The resistance genes (R genes) are required to encode for these receptors, and not all hosts can produce such genes.
Figure 1: PTI and ETI (Hypersensitive response)
This never-ending race between pathogen and their host plants along the evolutionary pathway was proven through the ‘gene for gene concept’ by H.H. Flor in 1956. According to the theory, the host plants and their pathogens have evolved together. It further states that any change in the pathogen virulence is balanced by the evolutionary change in host resistance and this dynamic equilibrium is maintained by the step wise evolution. So, isn’t it a never-ending fight? Each opponent makes the other stronger in this battle while walking along the evolutionary pathway.
References:
https://oajournals.fupress.net/index.php/pm/article/view/5543
https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1009175
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https://www.britannica.com/list/botanical-barbarity-9-plant-defense mechanisms
Figure 1:
https://media.springernature.com/original/springer-static/image/chp%3A10.1007%2F978-981-10-2854-0_12/MediaObjects/417622_1_En_12_Fig1_HTML.gif