Factors of passive resistan-ce
Wang Jinsheng
The inherent disease resistance mechanism of plants. The structural and chemical characteristics of plants that hinder pathogen infection, but do not include the defense responses that occur after the plant is infected by pathogens.
Structural factors
Structural components of plant cell walls, such as cuticle, wax, suberin, lignin and other special substances and stomata structure.
The stratum corneum
It is composed of a water-insoluble biopolyester membrane - cutin embedded in hydrophobic wax. It is located on the surface of epidermal cells and is close to the pectin layer of the cell wall. . Keratin is a polyester composed of C16 and C18 hydroxyl groups and epoxy fatty acids. The former is dihydroxypalmitic acid, while the latter is mainly ω-hydroxyoleic acid, ω-hydroxy-9, 10-epoxy stearic acid, 9, 10, 18 trihydroxystearic acid and another double bond at position C-12. analogues. In most plant organs, cutin is composed of the above monomers, while in rapidly growing organs it is mainly composed of C16 family units. The monomers are mainly connected through primary alcohol ester bonds, and a few are connected through secondary alcohol ester bonds. The thickness of the cuticle is generally 0.5 to 14 microns, and the thickness of the stomata and mesophyll cell gaps is generally 0.15 to 1.0 microns. The disease-resistant role of the cuticle lies first in its role as a chemical and physical barrier preventing fungal spores from germinating and penetrating leaf tissue. The thickness of the stratum corneum is related to its ability to resist fungal penetration of the tissue. The cuticle of some plants, such as chrysanthemums and tobacco, contains substances that inhibit the growth of fungi. In addition, the hydrophobicity of the cuticle prevents the formation of a water film on the plant surface, making it difficult for fungal spores in water droplets to remain on the leaf surface. At the same time, the cuticle can also prevent the loss of intracellular nutrients out of the cells, which is not conducive to the colonization of pathogens on the plant surface (Figure 1).
Figure 1 Surface cross-section of tomato varieties resistant to Botrytis cinerea (A) and susceptible varieties (B) (the cuticle is painted black) (quoted from Ainsworth, Oyler and Read, 1938)
Waxy
Distributed in the outermost layer of the above-ground tissues and organs of plants, it is uneven and has protrusions of various shapes. The ingredients are a variety of non-polar C20~C32 aliphatic compounds. Its function is to prevent water from evaporating from the leaf surface and pathogens from adhering to the leaf surface.
Suberin
Mainly distributed in the epidermal tissues of roots, stems, branches and tubers of plants. Similar to cutin, it is a polymer of hydroxy fatty acids. The suberized cells also contain wax, so they are also a hydrophobic environment. Suberin plays a barrier role in preventing bacterial infection. The suberization of the cell wall on the wound surface not only prevents the wound from expanding, but also forms a barrier that prevents germs from invading from the wound. During the process of pathogenic fungi penetrating the suberin layer, it has been found that some can produce cutinase to hydrolyze suberin at the ester chain site. Certain compounds produced during hydrolysis, such as epoxy acid compounds, have antifungal effects.
Lignin
is distributed in the middle layer, primary wall and secondary wall of xylem, and is also found in sclerenchyma, bundle sheath cells and surface cells. It is a polymer of aromatic phenolic substances, formed by dehydrogenation polymerization of three hydroxycinnamic alcohols (sinacohol, coumaryl alcohol and coniferyl alcohol) under the action of peroxidase. Increased cell wall strength due to the deposition of lignin between cellulose microfibrils. It can interfere with the occurrence and development of pathogen infection and disease in many ways. ① The blocking effect of lignin interferes with the transport of water and nutrients in plants to pathogens and the movement of mycotoxins and degradative enzymes to healthy plant cells and tissues. ②Due to the lignification of germ tube or hyphae tip cells and host plant tissues and the inactivation effect of low molecular weight lignin phenolic precursors on some fungal metabolites, the normal growth of fungi is interfered with until the invasion stops. ③ Because plant lignified cells slow down the expansion of pathogens in plant tissues, plants have enough time to synthesize and accumulate phytodefensins, which promotes the localization of fungal development and the formation of local lesions. ④ Lignification interferes with the movement of viruses between cells, causing local lesions on some hosts.
Silicon
is a component of the lignified tissue of plant epidermis.
In soil, silicon exists in the form of H4SiO4. After absorption by plants, it is deposited around cells or in cell walls in the form of amorphous hydrate SiO·2H2O. Silicon can increase the strength of plant tissues and organs and reduce transpiration. In terms of enhancing plant disease resistance, it has been proven that silicon content in soil is significantly correlated with rice resistance to rice blast.
Divalent ions
Ca2+ and Mg2+ are related to plant disease resistance to a certain extent. Ca2+, as a cross-linking agent of cell wall structure, is related to plant resistance to R solania, and it also has an inhibitory effect on enzymes secreted by pathogenic bacteria.
Chemical factors
Including natural antibiotics in plants such as esters, alkaloids and hydrolases (Figure 2).
Figure 2 Several passive resistance chemical factors in plants
A. Catechol (R=H), protocatechuic acid (R=CO2); B. Pinocin; C. (1) Tulipin A; (2) Tulipin B (Glc=glucose); D. Ranunculus (glucose); E. Ovenin; F. Berberine