Impact of Pesticides on Plant`s Immune System

Researchers at the Boyce Thompson Institute for Plant Research at Cornell University identified the previously elusive signal, they were methyl salicylate, by the composition of aspirin-like, which brings the plant’s immune system ready for more intense work.

Head of Research Associate Professor of Plant Pathology Daniel F. Klessig said: “Finally we have identified the signal that is spreading from the place of infection, activating the protection of the whole plant and the enzymes that regulate the level of this signal, and perhaps we will change the sign, which will increase the ability of plants to protect themselves”. Scientists point out that the higher the protective properties of the plant, the fewer pesticides are needed.

Understanding the plant’s immune system

Plants are naturally exposed to various pathogenic microbes, such as bacteria, fungi, and viruses. Unlike people who can physically avoid infections, plants are immobile. Therefore, every cell of the plant has to be protected from attack. Plants have a multi-level immune system that helps them fight these microbes. It works in the same way as the human immune system. Plants detect pathogens by recognizing microbial “patterns.” These are unique characteristics of the microbe (bacterial flagella, I think) that the plant has evolved to identify itself as an “n-no.” We can equate this ability to recognizing human body antigens, which induces an immune response. Unfortunately, pathogens continuously evolve to evade recognition, usually by shielding or disguising these samples. This ability allows them to colonize plant cells before it can establish an effective immune response.

Congenital and gained

Reasons for resilience vary and, depending on their nature, secrete innate and gained immunity. Congenital immunity is inherited but can change under the influence of many factors (plant condition, the aggression of the pathogen, environmental conditions). Plants with innate immunity have different resistance to infection, i.e., introducing the pathogen. Sometimes, an obstacle to the penetration of the infectious element is the properties of the plant, inherent only in it and existing regardless of the presence or absence of the pathogen.\nSuch immunity is called passive. In other cases, the ability to withstand infection is manifested by the rapid reaction of the plant only at the moment of introducing the pathogen. We call such immunity active. The degree of passive immunity of different plants depends on the characteristics of the external and internal structure of tissues, and their physiological and biochemical properties.

Physical barrier

The thickness of the cuticle (the thin film covering the epidermis) is essential in resistance to diseases whose pathogens penetrate through the cuticle layer. The thicker it is, the higher the impediment to infection penetration. For example, Tunberga barberry and b. elongated (oblong) barberry are more resistant to powdery mildew and rust than b—common barberry, which has a thinner cuticle.

Less susceptible to rust are tea-hybrid, tea-grass, and wicker roses, which have thicker leaf cuticles than other species. Thanks to thicker skin, the puckered rose is highly resistant (close to absolute) to powdery mildew. The same factor is also responsible for the age-related resistance of hardwoods to this disease. As we age, the thickness of the cuticle increases, and the susceptibility of leaves and shoots to the disease decreases accordingly. Thus, leaves and stalks of root shoots are strongly affected by powdery mildew and dry out, while the mother plant practically does not suffer from the disease. Dense pubescence and waxy plaque on the affected organs not only serve as a physical barrier to infection but also prevent the moisture necessary for infection from reaching them. Thus, the blue needles of the spruce covered with wax are more resistant to rust and shunt. It also explains the lower susceptibility of Japanese larch to the shuttle (meriose).

Several pathogens penetrate plant tissues through the mouths and lentils, which are the natural gateway to infection. The small number and size of these holes reduce the possibility of the pathogen penetrating the tissues. Often, the resistance of plants to disease may depend on the thickness of cell walls. Thus, many years of research have shown that the least susceptible to the root sponge (Heterobasidion annosum) pine species, distinguished by thickened cell walls in wood. The resistance of woody plants to disease may be affected by the crown’s habitus. In open-worked scattered crowns, which are well lit and ventilated, a drier microclimate is created that is unfavorable for pathogen development. For example, forms of poplar with a dense pyramidal crown are more severely affected by brown CytoSport (Cytospora chrysosperma) and discosporium (Discosporium populeum) necroses.

Protective reactions

Active reactions of plants to the introduction of pathogens are manifested in the formation of protective necrosis, the emergence of antitoxins, and anti-enzymes. Protective necroses are dead tissue areas in which pathogens with high parasitic activity cannot develop and die. This reaction occurs when leaves are affected by rust and powdery mildew.  Antitoxic and antienzymatic reactions are expressed in the formation of phytoalexins, activation of oxidative enzymes, and the structure of mechanical barriers to pathogen penetration. Phytoalexins are substances that are formed in plant tissues only during the penetration of the pathogen and can delay or completely suppress its development.

Many plants resistant to this or that disease produce reductive enzymes that not only reduce the activity of the enzymes released by the pathogen or completely suppress their formation but also contribute to the synthesis of substances needed to restore cells and tissues destroyed by the pathogen.

Reduced susceptibility of woody plants to disease may be associated with the formation of mechanical barriers that prevent the spread of the pathogen, localizing it. For example, the reaction of hardwood trees to introducing autumn (Armillaria mellea) is manifested in the formation of cork tissue on the border between healthy and mushroom-destroyed tissue.