Plant Defense Mechanisms
Constitutive plant defense responses

Are also called pre-existing or passive defense mechanism mediated by Cutin, waxes, Suberin hydrophobic materials that coat plant surfaces exposed to air prevents water loss and blocks pathogen entry and are high levels of toxic or feeding deterrent compounds. Always present and often species-specific and may exist as stored or conjugated compounds. Most secondary metabolites as defense are constitutive but some are also induced.

Pre-existing (passive or constitutive) structural defense mechanisms

Constitutive structural defense is achieved by constituents of cutin, waxes, and Suberin

• Cutin

o A macromolecule, a polymer consisting of many long-chain fatty acids (16:0 and 18:1 fatty acids with OH)

o Component of cuticle, increase resistance; however, its thickness is not always correlated with resistance

• Waxes

o Not macromolecules, but complex mixtures of long-chain acyl lipids.

o Forms a water-repellent surface and thereby prevent the formation of a film of water on which pathogens might be deposited and germinate (fungi) or multiply (bacteria).

• Suberin

o Polymer whose structure is poorly understood.

o A principle component of outer cell walls of all underground organs and is associated with the cork cells of the periderm, the tissue that forms the outer bark of stems and roots during secondary growth of woody plants.

o Suberin also forms at sites of leaf abscission and in areas damaged by disease or wounding.

Pre-existing (passive or constitutive) structural defense mechanisms is the first line of defence in plants is present in its surface. Several characters of the plants surface function as barriers to penetration which pathogen must breach to enter the host. The pathogens enter the plant host by penetrating the epidermis along with cuticle and cuticular wax and number of natural openings existing before the onset of the pathogenesis can obstruct penetration. If the pathogen succeeds in penetration; it encounters pre-existing internal structural barriers. The external and internal structural barriers existing before pathogen attack are also called pre-existing defence structures or passive/static or anit-infection structures.

Wax and cuticle

The cuticle covers the epidermal cells of plants and consists of pectin layer, a cutinized layer and a wax layer. Cutin is composed of fatty acids. Waxes are mixture of long chain aliphatic compounds which prevent the retention of water on plant surface essential for spore germination. A negative charge usually develops on leaf surfaces due to fatty acids. This condition repels air-bone spore / propogules. Only few pathogens are known to dissolve cutin enzymatically. Examples: Monilinia fructicola penetrates cuticle of cherry leaves but not of Gingko biloba leaves; the latter contains abundant cutin than the former. F. solani f sp. Pisi produces the enzyme cutinase production by specific antibodies and inhibitors.

Epidermal layer

Epidermis is the first layer of living host cells that comes in contact with attacking microbes. The toughness of epidermis is due to the polymers of cellulose, hemicelluloses, lignin mineral substances, polymerized organic compounds, suberin etc. Potato tubers resistant to Pythium debaryanum contain higher fibre. Silicon accumulation in epidemal walls provides resistance against fungal attack. Suberization of epidermis confers protection against plant Xanthomonas axonopodis pv. Citri because of broad cuticular lips covering the stomata. A functional defence mechanism has been observed in some varieties (cv-Hope) in which stomata open late in the day when moisture on leaf surface has dried and the infection tunes have become non-functional.

Hydathodes

Hydathodes are natural openings on the edges of leaves and serve to excrete excess water from the interior. They are easy entry pints of bacterial pathogens such as black rot of cabbage. Similar to hydathodes are the nectarthodes in inflorescence of many plants. They secrete sugary nectar and this serves as barrier to those organisms that cannot tolerate this condition and thus, can enter through nectarines. Leaf hairs on leaves and on nectarines also resist entry of pathogens. High hairlines of leaves and pods in chickpea is resistant character against Ascpchyta rabei. Groundnut varieties showing resistance to Cercospora leaf spots have thick epidemis-cum cuticle and compact paslisade layer, few and smaller stomata and high frequency or trochomes on the abaxial surface of leaf.

Lenticels

Lenticels are opening in outer walls involved in gaseous exchange. They are weak points in defence unless the cork cells within them are suberized. After suberizatoin and periderm formation, lenticels are more resistant to invasion by pathogens.

Pre-existing biochemical defence

Plants liberate different chemicals, which interfere with activities of the pathogen and pathogenesis, thereby preventing or reduce infection. These chemicals and the biochemical conditions that develop may act either directly through toxic or lytic effect on the invader or indirectly through stimulating antagonistic plant surface microflora. The compounds pre-existing in plants as constitutive antibiotics and those, which are formed in response to wounds as wounds antibiotics.

Release of anti-microbial compounds

Plants while growing and developing release gases as well as organic substances, from leaves and roots (leaf and root exudates), containing sugars, amino acid, organic acids, enzymes, glycoside etc. These materials have profound effect on the nature of surrounding environment, particularly the phyllosphere, rhiizosphere microflora and fauna.

Although these substances are ideal nutrients for microbes and help in germination and growth of several saprophytes and parasites number of inhibitory substances is also present in these exudates. Theses inhibitory substances directly affect the microorganism, or encourage certain groups to dominate the environment and function as antagonists of the pathogen.

Inhibitors present in the plant cells

In many host-parasite interactions, pre-existing toxic substances in the cells form the basis of resistance. In resistant variety these substances life in abundance while in susceptible variety they may be less or completely absent. Several phenollic compounds, tannins and some fatty acid like compounds such as dienes pre-exisitg in high concentrations in cells have been implicated for the resistance or young tissues to parasitic fungi such as Botrytis. Many such compounds are potent inhibitors of many hydrolytic enzymes. Several other types of preformed compounds such as saponins (glycosylaled steroidal or triterpenoid compound) tomatine in tomato and avenacin in oats, have antifungal membranolytic activity.

The fungal pathogens which lack enzymes (saponinases) that breakdown the saponins are prevented from infecting the host. Several preformed plant proteins have been reported to act as inhibitors of pathogen proteinases or of hydrolytic enzymes. Similarly lactins (proteins that bind to certain sugars) cause lyses and growth inhibition of many fungi. Plants surface cells also contain variable amounts of hydrolytic enzyymes such as glucanases and chitinases, which may cause breakdown of pathogen cell wall components.

Production of essential factors

Recognition factors

The first step in infection process is the cell-to-cell communication between host and pathogens. Plants of species or varieties may not be infected by pathogen if their surface cells lack specific recognition factors. If the pathogen does not recognize the plant as one of its hosts it may not adhere to the host surface or it may not produce infection substances such as enzymes, or structures (appresoria, haustoria). These recognition molecules are of various types ofoligosaccharides and polysaccharides and glycoproteins.

Host receptors and sites for toxins

In many host parasite interactions the pathogen produces host specific toxins, which are responsible for symptoms and disease development. The molecules of toxin are supposed to attach to specific sensitive sites or receptors in the cell. Only the plants that have such sensitive sites become diseased

Essential nutrients and growth factors

The fact that many facultative saprophytes and most of the obligate parasites are host specific and sometimes are so specialized that they can grow and reproduce only on certain varieties of those species suggests that for these pathogens the essential nutrients and growth factors are available only in these hosts. Absence of these nutrients and stimulus make the other varieties and species unsuitable hosts.

Induced or active Defence mechanism

Plants have to face the wide variety of pathogens standing at a place thus pre-existing (structural and biochemical) defence mechanism in plants exists which help plants in warding-off most of microbes as non-pathogens but it does not seems to be sufficient. The induced or active defence mechanism in plants may operate at different levels

o Biochemical defence

o Defence at cellular level

o Defences at tissue level

The activation or induction of defence mechanism may be both specific and non-specific. Several structural changes are induced by a range of biotic or abiotic elicitors. These defence mechanisms prevent further colonization or spread of pathogen. Active defence in plants involves cellular defences that rely upon preformed surveillance systems are encoded by resistance genes. The receptor-proteins are located in cell membrane to detect the pathogen or factor translocated by pathogens. The ability of plant to mount an active defence response is under genomic control. Disease occurs when

• Pre-exisiting defence mechanism are not enough to check the entry of pathogen

• A pathogen avoids timely eliciting active defence system in plant tissue or habits active defence response by secreting metabolic toxins.

Induced structural defence mechanisms

Induced histological defence which operates even after the establishment of infection in plant cells, the host defence system tries to create barriers for further colonization of tissues. This may be at various levels.

• Lignification

o Lignified cell wall provide effective barrier to hyphal penetration. They also act as impermeable barrier for free movement of nutrient causing starvation of pathogen.

o Following are examples; Radish: Peronospora parasitica, Alternaria japonica; Potato: Phytophtora infestans; Wheat: Septoria nodorum; Cucumber: Cladosporium cucumerium, Colletorichum lagenarium and Carrot: Botrytis cineria

• Suberization

o In several plants the infected cells are surrounded by suberized cells. Thus, isolating them from healthy tissue.

• Corky layer formation

o Corky layer formation is a part of natural healing system of plants e.g. common scab of potato and rot of sweet potato are good examples.

• Abscission layers

o It is a gap between host cell layers and devices for dropping off older leaves and mature fruits.

o Plant may use this for defence mechanism also i.e. to drop off infected or invaded plant tissue or parts, along with pathogen and shot holes in leaves of fruit trees is a common feature

• Tyloses

o The tyloses are formed by protrusion of xylem parachymatous cell walls, through pits, into xylem vessels.

o The size and number of tyloses physically block the vessel.

o The tyloses are inductively formed much ahead of infection, thus blocking the spread of pathogen.

o It suggests biochemical elicitors and movement of tyloses inducing factor (TIF) up the stem e.g. Sweet potato: Fusarium oxysporum

• Gum deposition

o The gums and vascular gels quickly accumulate and fill the intercellular spacis or within the cell surroundings the infection thread and haustoria, which may starve or die.

Induced cellular defence

Involves cellular defence structures, i.e. changes in cell walls and have only a limited role in defence and commonly observed examples include

o Carhohydrate apposition - synthesis of secondary wall and papillae formation

o Callose deposition - hyphal sheathing just outside plasma lemma around the haustorium which delays contact of pathogen Phytophythora infestans with host cells.

o Formation of structural proteins

o Induced cytoplasmic defence that present last line of host defence and may effective against slow growing pathogens, weak parasites or some symbiotic relationship.

Induced biochemical changes

The induced biochemical changes in host plants are the last line of host defence and may condition a plant or plant tissue from susceptible to resistant to immune status as per their genetic potential. The role of biochemical factor in host defence is based on the following four attributes

• The substance is associated with protection against disease at the site where protection occurs.

• The substance can be isolated from the host showing protection against the disease.

• Introduction of isolated substance to the appropriate susceptible host confers protection.

• The nature of protection so induced resembles that of the natural agents of a resistant plant.

The induced biochemical changes in host plants include

• Production of toxic substances

o Rapid production/suitable modifications and/or/ accumulation of chemicals toxic to pathogen upto effective concentrations is an important component of overall active defence strategy of plants.

o Slow production or accumulation or low levels of similar chemicals have reported in susceptible host plants also.

• Synthesis of phenolic compounds

o The phenolic compounds, chlorogenic acid caffeic acid and ixidation products of floretin, hydroquinone hydroxyquionones and phytoalexins are main toxi chemical produced to inhibit pathogen or its activities.

o Some of these are performed toxic chemicals while others may be de novo synthesized or modified to more toxic forms.

o The enzymes involved in chemical pathways are present in host cell which are pre-existing defense mechanisms.

• Synthesis of phytoalexins

o Most common response of plants to stress, biotic (phytoalexins/insects) or abiotic (wounding), is the production and accumulation of substrates that can inhibit the growth and activities of the biotic factors or may help in healing process.

o Phytoalexins are antibiotics produced in plant pathogens interactions or as result reposnse to injury or other psychological simulation.

• Synthesis of new proteins

o Post-infectional changes in host cells involve production and modification of large number of proteins both structural and enzymatic, which have important role in defence mechanism.

o The enzymes are required for various synthetic pathways, normal or modified for production of resistance related substances. In addition, phenol-oxidizing enzymes have vital role. The influence of these changes may be confined to infection site or nearby cells.

o Increased synthesis and activity of phenyl ammonia lyase (PAL) has been reported in several bacterial and viral pathogens in resistant reaction which plays key role in syntheses of phenols, phytoalexins and lignin.

o The effectiveness of resistance depends on speed and amount of synthesized products and their movements to neighboring healthy tissues to create defensive barriers.

• Inactivation of enzymes and toxins

o The necrotrophs and hemihiotrophs employ more of these substances from causing those tissue damage as compared to speacialized obligate parasites.

o The defence strategy of resistant plants, through activity of phenols, tannins and protein as enzymes inhibitors. The phenolics are not anti-fungal but make pathogen ineffective by neutralizing their enzymes.

o In immature grape fruits catechol-tannin is known to inhibit enzymes produced by Botrytis cinerea.

o Toxins are known to be involved in pathogenesis to various editents (pathotoxins/vivotoxins). The resistance to toxins, in host is resistance to pathogens achieved by detoxification or lack of receptor sites for these toxins

• Alteration of biosynthetic pathway

o The pose inflectional metabolism of host tissue is altered stress physiology to cope with the advancing activities of pathogen and new enzymes are produced in an effort to synthesize defence related substances.

o Most of these compounds are formed through Shikmic acid pathway and modified acetate pathway.

o Respiration in diseased tissue is invariably increased; a part of glycolysis is replaced by pentose pathway, which yields four carbon compounds are formed through Shikmic acid pathway and modified acetate pathway.

o It is possible that in early stages of infection the gene regulation of host cell is influenced and some specific genes.

Active defense to pathogens

Induction of host resistance, structural or biochemical seems to be universal in plants. Active defense responses against all classes of pathogens (fungi, bacteria, viruses and nematodes) exists. Active defense response may lead to incompatible host-pathogen interaction. The biochemical defense response are achieved by use of secondary metabolites and they differ from primary metabolites in distributions. Particular secondary metabolites are often found in only one plant species or related groups, whereas primary metabolites are found throughout the plant kingdom. They defend plants against herbivores and pathogens and have three ecological functions:

• Protect from against herbivores or pathogens • Attractants for pollinators and seed-dispersing animals • Agents of plant-plant competition and plant-microbe symbiosis