Why can't plant grafting lead to rejection?

From the perspective of evolution, the immune system of natural organisms can be divided into two sets. One is an earlier, more primitive, but also more common immune system, called the innate immune system (IIS), which is called "innate immune system" in Chinese and exists in all living things on earth. The other set appeared relatively late. The adaptive immune system (AIS) only exists in vertebrates with mandibles, which is called "acquired immune system" in Chinese. That is to say, vertebrates have two immune systems, IIS and AIS, while plants have only one innate immune system (IIS). There should be applause here. )

Plants and vertebrates adopt different immune systems, which can be understood as different adaptation strategies adopted by plants and vertebrates in the face of the pressure of natural selection.

The accuracy of innate immune recognition is relatively low, but the response is fast and the attack range is wide. The so-called cannon hits mosquitoes. The first advantage of this immune defense mechanism is its fast reaction speed, because the "weapons" used by innate immunity are generally chemical killers, and the synthesis and decomposition speed of these chemicals can be calculated in hours or even minutes. The so-called kill you three thousand minutes. Moreover, these chemicals destroy the "lifeblood" of invasive pathogens (such as metabolic pathway, nutrient absorption pathway, division and reproduction mechanism, etc. ). Because of this, the side effects of innate immune response are also obvious, which are the "lifeblood" of pathogens and the lifeblood of plants. After all, life on earth is homologous, and many metabolic pathways, nutrient absorption patterns and fission and reproduction patterns are similar to each other. If you destroy these mechanisms, you will inevitably hurt yourself by mistake, so plants will wither or wither when they get sick, that is, killing one thousand enemies and losing five hundred. In order to prevent the disease from spreading on plants, you should either kill yourself or break your arm. This sounds bad for the survival of plants. What should you do? It doesn't matter, plants have an additional talent, that is, the ability to regenerate. I lost a leaf, withered a branch and grew back. IIS, the original defense mechanism, has no memory. Plants grow new leaves. The next time they encounter the same disease, they will self-harm again and grow new leaves. Anyway, water, carbon dioxide and minerals are free to use. 18, oh.

Another "benefit" of the low recognition accuracy of plant innate immune system is that because the protection mechanism is so rough, the threshold for triggering this protection mechanism is relatively high, so that plants will not start to hurt themselves crazily as soon as they encounter any foreign body ... This non-self compatibility is called tolerance in immunology. Because the immune system of plants is only IIS, the recognition accuracy is rough, and it is not easy to distinguish who is self and who is non-self (including foreign bodies, pathogens and allogeneic transplantation) in some cases, so if plants think that foreign bodies look immune compatible with themselves and the phenotype of the other person looks like themselves or their relatives, then they can accept each other to play happily together ... It is also mentioned in other answers. Plants cannot be grafted casually. The closer the genetic relationship, the higher the grafting success rate. However, if they are not the same species at all, it is difficult to survive. This is because the more closely related biological immune phenotypes are similar, the more the recipient's immune system will be judged as "friendly", resulting in tolerance, which is also the result of the balance between "attack" and "tolerance" of immune system recognition mechanism.