In this article we will discuss about the structural and biochemical defense mechanisms in plants
I. Structural Defense:
In plants some structures are already present to defend the attack while in others, the structures to defend the host develops after the infection. In this way, structural defense can be characterised as (A) Preexisting defense structures and (B) Defense structures developed after the attack of the pathogen.
(A) Preexisting Defense Structures:
(i) Cuticular Wax:
Wax-mixtures of long chain aliphatic compounds get deposited on the cuticular surface of some plants. Deposition of wax on the cuticular surface is thought to play a defensive role by forming a hydrophobic surface where water is repelled.
As a result, the pathogen does not get sufficient water to germinate or multiply. In addition, a negative charge usually develops on the leaf surface due to the presence of fatty acids – the main component of cuticle. The negative charge prevents/reduces the chance of infection by many pathogens.
(ii) Cuticle Thickness:
The thickness of cuticle is most important for those which try to enter the host through the leaf surface. The cuticle thickness obstructs the path of pathogen. In addition, a thick cuticle checks the exit of the pathogen from inside the host, thus reducing the secondary infection.
(iii) Structure of Epidermal Cell Walls:
Tough and thick outer walls of epidermal cells may directly prevent the entry of the pathogen completely or make the entry difficult. The presence or absence of lignin and silicic acid in the cell walls may show variation in resistance to penetration of the pathogen.
Most outer walls of epidermal cells of rice plants are lignified and are seldom penetrated by blast disease of rice pathogen. In resistant varieties of potato tubers (resistant to Pythium debaryanum) the epidermal cells contain higher fibre content than the susceptible ones.
(iv) Structure of Natural openings:
Structure of natural openings like stomata lenticels etc. also decide the fate of the entry of the pathogen. In Szincum variety of citrus, the stomata are small and possess very narrow openings surrounded by broad lipped raised structures which prevent entry of water drops containing citrus canker bacterium.
In the same way, the size and internal structures of lenticels may play a defensive role against the pathogens. Varieties having small lenticels in the apple fruits prevent the entry of the pathogen while those having large openings easily allow the pathogen to enter.
Nectaries provide openings in the epidermis and may play a defensive role due to high osmotic concentration of the nectar. In resistant varieties of apple, presence of abundant hairs in the nectaries acts as a defense mechanism while susceptible varieties are devoid of abundant hairs.
Internal Defense Structures:
There are many preexisting internal defense structures inside the plant that prevent the entry of pathogen beyond these structures. In some plants, cell walls of certain tissues become thick and tough due to environmental conditions and this makes the advance of the pathogen quite difficult.
In case of stems of cereal crops, vascular bundles or extended areas of sclerenchyma cells checks the progress of rust pathogen. Leaf veins effectively obstruct the spread of pathogen like the angular leaf spot pathogen.
(B) Defense Structures Developed after the Attack of the Pathogen:
After the pathogen has successfully managed to overcome the preexisting defense mechanisms of the host, it invades the cells and tissues of the host.
In order to check the further invasion by the pathogen, the host plants develop some structures/mechanisms which may be defense reactions in the cytoplasm, cell wall defense structures, defense structures developed by the tissues and ultimately the death of the invaded cell i.e. necrosis. These will be briefly discussed here.
(i) Defense Reactions in the Cytoplasm:
The cytoplasm of the invaded cell surrounds the hyphae of the pathogen and the nucleus of the host cell gets stretched to break into two. In some host cells, the cytoplasm and the nucleus of the infected cells enlarge.
The cytoplasm becomes granular and dense and develops granular particles. These result in the disintegration of the pathogen mycelium and thus the invasion stops. Such cytoplasmic defence mechanisms can be seen in weak pathogens like Annillaria and some mycorrhizal fungi.
(ii) Cell Wall Defense Structures:
Cell wall defense structures are of limited help to the host. These include morphological changes in the cell wall of the host.
Three types of cell wall defense structures are generally observed:
(i) Cell walls thicken in response to the pathogen by producing a cellulose material, thus preventing the entry of the pathogen
(ii) The outer layer of cell walls of the parenchyma cells in contact with invading bacterial cells produce an amorphous fibrillar material that traps the bacteria thus preventing them to multiply and
(iii) Callose papillae get deposited on the inner layers of the cell walls due to invasion by fungal pathogens.
In raw cases, the hyphal tips of the infecting fungal pathogen penetrating the cell wall and thereafter growing into the cell lumen get enveloped by callose material that, later become infused with phenolics forming a sheath around the hyphae.
(iii) Defense Structures Developed by the Tissues:
The following four developments take place in the tissues after penetration:
(a) Gum Deposition:
Plants produce a variety of gummy substances around lesions or spots as a result of infection. These gummy substances inhibit the progress of the pathogen. The gummy substances are commonly produced in stone fruits.
(b) Abcission Layers:
Abscission layers are usually formed to separate the ripe fruits and old leaves from the plant. But in some stone fruit trees, these layers develop in their young leaves in response to infection by several fungi, bacteria or viruses. An abscission layer is a gap formed between two circular layers of cells surrounding the point of infection.
This gap is created by the dissolution of one or two layers of the middle lamella, one or two layers of cells surrounding the infected loci resulting in the infected locus becoming unsupported, shrivels, dies and falls down along with the pathogen. Abscission layer formation protects the healthy leaf tissue from the attack of the pathogen.
Tyloses are out growths of protoplasts of adjacent live parenchyma cells protruding into xylem vessels through pits under stress or in response to attack by the vascular pathogens. Their development blocks the Xylem vessels, obstructing the flow of water and resulting in the development of wilt symptoms.
However, tyloses are formed in some resistant plants ahead of infection and the prevent the plant from being attacked.
(D) Formation of Layers:
Some pathogens like certain bacteria, some fungi and even some viruses and nematodes stimulate the host to form multilayered cork cells in response to infection, these develop as a result of stimulation of host cells by substances secreted by thus, pathogen.
These layers inhibit the further invasion by the pathogen and also block the flow of toxic substances secreted by the pathogen. Cork layers also stop the flow of nutrients of the host thus also depriving the pathogen of the nutrients.
Examples of cork layer formation as a result of infection are: soft not of potato caused by Rhizopus sp., potato tuber disease caused by Rhizoctonia sp., Scab of potato caused by Streptomyces scabies and necrotic lesions on tobacco caused by tobacco mosaic virus.
IV. Necrosis or Hypersensitive Type of Defense:
Necrosis or hypersensitive type of defense is another defense mechanism adopted by some pathogens like Synchytrium endobioticum causing wart disease of potato, Phytophthora infestans causing late blight disease of potato and Pyricularia oryzae causing blast of rice etc.
In such diseases, the host nucleus moves toward the pathogen when the latter comes in contact with the protoplasm of the host. The nucleus soon disintegrates into brown granules which first accumulate around the pathogen, later dispersing throughout the host cytoplasm.
Soon the cell membrane swells and finally the cell bursts and dies. These cause the pathogen nucleus to disintegrate into a homogenous mass and its cytoplasm dense. As a result, the pathogen fails to grow beyond the necrotic or dead cells and the further growth of the pathogen is stopped.
II. Biochemical Defense:
Although structural defense mechanisms do prevent the attack of the pathogen, the defense mechanism also includes the chemical substances produced in the plant cells before or after the infection.
It has now been established that biochemical defense mechanisms play more important role than the structural defense mechanisms. This has been supplemented by the fact that many pathogens entering non host plants naturally or artificially inoculated fail to cause infections in absence of any structural barriers.
This does suggest that chemical defense mechanisms rather than structural mechanisms are responsible for resistance in plants against certain pathogens.
(A) Preexisting Biochemical Defense:
(i) Inhibitors Released in the Prepenetration Stage:
Plant generally exudes organic substance through above ground parts (phyllosphere) and roots (rhizosphere). Some of the compounds released by some plants are known to have an inhibitory effect on certain pathogens during the prepenetration stage.
For example fungistatic chemicals released by tomato and sugar beet prevent the germination of Botrytis and Cercospora. Presence of phenolics like protocatechuic acid and catechol in scales of red onion variety inhibit the germination of conidia of Colletotrichum circinans on the surface of red onion.
Inhibitors present in high concentrations in the plant cells also play an important role in defense of plants. Presence of several phenolics, tannins and some fatty acid like compounds such as dienes in cells of young fruits, leaves or seeds afford them resistance to Botrytis.
The tubers of resistance vars of potato against potato scab disease contain higher concentrations of chlorogenic acid around the lenticels and tubers than the susceptible vars. Several other compounds like saponin tomatin in tomato and avinacin in oats have antifungal activity. Some enzymes like glucanases and chitinases present in cells of some plants may break down the cell wall components of pathogens.
(ii) Lack of nutrients essential for the pathogen is another preexisting biochemical defense mechanism. Plant varieties or species which do not produce any of the chemicals essential for the growth of pathogen may act as resistant variety.
For example, a substance present in seedling varieties susceptible to Rhizoctonia initiates hyphae cushion formation from which the fungus sends penetration hyphae inside the host plants. When this substance is not present, hyphal cushions are not formed and the infection does not occur.
(iii) Absence of Common Antigen in Host plant:
It is now clear that the presence of a common protein (antigen) in both the pathogen and host determines diseases occurrence in the host. But if the antigen is present in the host and absent in the host or vice-versa, it makes the host resistant to the pathogen.
For example, varieties of linseed which have an antigen common to their pathogen are susceptible to the disease rust of linseed caused by Melampsora lini.
In contrast, the absence of antigen in linseed varieties but occurring in the pathogen are resistant to the pathogen. Another example is leaf spot disease of cotton caused by Xanthomonas campestris pv. malvacearum.
(B) Post-Infection-Biochemical Defense Mechanism:
In order to sight infections caused by pathogens or injuries caused by any other means, the plant cells and tissues produce by synthesis many substances (chemicals) which inhibit the growth of causal organism.
These substances are generally produced around the site of infection or injury with the main aim at overcoming the problem.
Some such important chemicals are described below:
(i) Phenolic Compounds:
These are the most common compounds produced by plants in response to injury or infection. The synthesis of phenolic compounds takes place either through “acetic acid pathway” or “Shikimic acid pathway”.
Some common phenolic compounds toxic to pathogens are chlorgenic acid, caffeic acid and ferulic acid. These phenolic compounds are produced at a much faster rate in resistant varieties than in susceptible varieties.
Probably that the combined effect of all phenolics present is responsible for inhibiting the growth of the infection.
Phytoalexins are toxic antimicrobial substances synthesized ‘de novo’ in the plants in response to injury, infectious agents or their products and physiological stimuli. The term phytoalexin was first used by the two phytopathologists Muller and Borger (1940) for fungi static compounds produced by plants in response to mechanical or chemical injury or infection.
All phytoalexins are lipophilic compounds and were first detected after a study of late blight of potato caused by Phytophthora infestans. Phytoalexins are believed to be synthesized in living cells but surprisingly necrosis follows very quickly.
According to Bill (1981), peak concentration of phytoalexins almost always coincides with necrosis. Although the exact mechanism of production of phytoalexin has not been properly understood, it is considered that a metabolite of the host plant interacts with specific receptor on the pathogen’s membrane resulting in the secretion of “phytoalexin elicitor” which enters the host plant cells and stimulates the phytoalexin synthesis.
Phytoalexins are considered to stop the growth of pathogens by altering the plasma membrane and inhibiting the oxidative phosphorylation.
Phytoalexins have been identified in a wide variety of species of plants such as Soyabean, Potato, sweet potato, barley, carrot, cotton etc. are being investigated. Some common phytoalexins are Ipomeamarone, Orchinol, Pistatin, Phaseolin, Medicarpin, Rishitin, Isocoumarin, ‘Gossypol’ Cicerin, Glyceolin, Capisidiol etc.
The following Table gives a list of phytoalexins, chemical nature the host and the pathogens in response to which these are produced:
(iii) Substances Produced in Host to Resist Enzymes Produced by Pathogen:
Some hosts produce chemicals which neutralise the enzymes produced by pathogen, thus defending the host. Therefore these substances help plants to defend themselves from the attack of the pathogen.
In bean plants, infection with Rhizoctonia solani causes necrosis. In resistant bean varieties, the entry of pathogen causes the separation of methyl group from methylated pectic substances and forms polyvalent cations of pectic salts which contain calcium.
The calcium ions accumulate in infected as well as neighbouring healthy tissues and because of the calcium accumulation, the pathogen fails to disintegrate middle lamella by its polygalacturonase enzymes. These are known to dissolve the middle lamella of healthy tissue in susceptible varieties.
(iv) Detoxification of Pathogen Toxins and Enzymes:
In some cases, the plants produce chemicals which deactivate the toxins produced by the pathogens. For example, Pyricularia oryzae which causes blast disease of rice produces Picolinic acid and pyricularin as toxins.
Although resistant varieties convert these toxins into N-methyl picolininic acid pyrecularin into other compounds, the susceptible varieties do get affected by these toxins. Similarly in case of cotton and tomato wilts, the toxin fusaric acid produced by the pathogen gets converted into non-toxic N-methyl-fusaric acid amide in resistant varieties.
As in case of detoxification of toxins, the toxic enzymes produced by the pathogen is deactivated by phenolic compounds or their oxidation products. Some varieties of cider apple are resistant to brown not disease caused by Sclereotiniafructigena.
It may be because of the resistant varieties producing pheolic oxidation products which inactivate the pectinolytic enzymes produced by the pathogen.
(v) Biochemical Alterations:
It has been observed that infection of the host by the pathogen brings about biochemical changes in the host which may prove toxic to the pathogenic microorganisms and cause resistance to the pathogen. Production of certain new enzymes and other compounds are synthesized and accumulated in higher concentration. This may also add to the resistance of the plant by being toxic to pathogenic microorganisms.