Host Pathogen Interaction PDF

# Host Pathogen Interaction ## All Pathogens are parasites but all parasites are not pathogens. - A parasite is defined by Thrower (1966) as "an organism or virus existing in an intimate association with another living organism from which it derives an essential part of the materials for its exi...

# Host Pathogen Interaction ## All Pathogens are parasites but all parasites are not pathogens. - A parasite is defined by Thrower (1966) as "an organism or virus existing in an intimate association with another living organism from which it derives an essential part of the materials for its existance". - A pathogen, on the other hand, has been defined according to a guide to the use of terms in plant pathology prepared by the Federation of British Plant Pathologists (1973) as "an organism or virus capable of causing disease in a particular host or range of hosts". - While dealing with the plant diseases, an interaction between the disease inciting organism and the host is conventionally termed as "host parasite interaction". - Though the term host parasite interaction carries almost the same meaning, it should be more appropriate to call "host pathogen interaction". ## Inoculum - Host pathogen interaction begins as soon as the infecting propagule or inoculum makes the intial contact with the host heading towards development of disease or pathogenesis and expression of the disease in the form of symptoms. - After landing on the host, the pathogen generally initiates the process of infection, and from this stage the actual pathogenesis begins. - The part of the pathogen coming in contact with the host to cause infection, is called inoculum. - The inoculum exists in the form of virus particles, bacterial cells, active mycelium (Rhizoctonia), dormant mycelium (Ustilago tritici), Sclerotia (Claviceps) and various kinds of spores. - The inoculum may cause successful infection only when the following conditions are fulfilled: - The host should be susceptible and have disease proneness. - The pathogen should be aggressive, capable of fast multiplication and have proper inoculum potential. - The environmental conditions should be favorable for the pathogen but not for the host. ## Inoculum Potential - Out of all the factors, inoculum potential is the most important deciding factor to ensure successful infection. - The term 'inoculum potential' was coined by Horsfall (1932) and defined as "the number of infecting particles present in the environment of the infected host."." - Since then the concept of inoculum potential has undergone a great change due to involvement of many other factors such as the type of pathogen, effect of environment, susceptibility of the host, genetic capacity to multiply rapidly, availability of nutrients, etc. - Garrett (1956) redefined the inoculum potential as "the energy-of-a pathogen available for infection of a host at the surface of the host organ to be infected." - According to this definition the energy available to the pathogen from density of inoculum, available nutrients to the propagules, the environmental factors, genetic capacity, plant exudates, plant residues, etc., is decisive in the establishment of infection. - Furthermore, many other plant pathologists modified the definition and included many other factors. - Since the inoculum potential included so many factors, a large number of plant pathologists are of the opinion that the term should exclude all other factors except the density of inoculum. - Thus, Withelm (1966) finally concluded that the inoculum potential should be used to indicate inoculum density (i.e., the mass or number of propagules present on the host surface). - All other factors such as host susceptibility, influence of environment, availability of nutrients, etc., should be included under a separate term 'disease potential'. ## Pathogenesis - Pathogenesis is the sequence of progress in disease development from the initial contact between a pathogen and its host to the completion of disease symptoms. - It depends on many external as well as internal factors including - (i) Genetic responses of the plant to the activity of the pathogen i.e., susceptibility and resistance of the host); (ii) Environmental factors such as temperature, humidity, light, nutrition, pH, chemicals, plant exudates and the other micro organisms present on host surface; (iii) Age and vigour of host plant; (2) Aggressiveness of the pathogen (i.e., inoculum potential, disease potential, etc.) and many other factors. - The events of pathogenesis can be studied under the following heads: - Prepenetration - Penetration - Post penetration ## Prepenetration - This stage includes the interaction of host and pathogen before penetration (or entry) of pathogen into the host. - In case of viruses and bacteria where the pathogen as a whole acts as infecting propagule (inoculum), there is no evident prepenetration stage. The viruses enter into the host cells by the activity of their vectors or by some mechanical means. The bacteria also enter into the host cells passively through natural openings without pre-penetration activity. - The fungal pathogens, on the other hand, produce various kinds of inoculum such as dormant mycelium, active mycelium, sclerotia, conidia, zoospores, sporangiospores, chlamydospores, oidia, ascospores, basidiospores, uredospores, aeciospores, etc. - Thus, the prepenetration stage is important in case of fungal pathogens. In fungi, the prepenetration stage involves two stages - (i) spore germination and (ii) growth of germ tube. ### Germination of propagules: - The most usual forms of fungal propagules are different kinds of spores, (viz., conidia, sporangia, zoospores, oidia, chlamydospores, smut spores, ascospores, oospores, basidiospores, uredospores, aeciospores, etc.). - Besides, the inoculum may exist in the form of dormant mycelium, active mycelium, sclerotia, etc. - Germination of propagules on the host surface involves a change from resting stage or low metabolic stage to high metabolic active stage. This change requires an adequate energy source. Some propagules are thin-walled and store less food material. They germinate without resting period. - Others are thick-walled and store more food materials. They can survive under unfavorable conditions and germinate after a resting period. - There are many environmental, chemical and biological factors which either stimulate or inhibit the spore germination. - Some of these factors include moisture, temperature, light, pH, oxygen, carbon dioxide, host exudates, residues and many biological factors. - The microorganisms present around the rhizosphere and phyllosphere also influence the spore germination by antagonism, competition and microparasitism. - The chemicals secreted from rhizosphere and phyllosphere also influence the germination of propagules. - For example, the root exudate of turnip stimulates the germination of oospores of Pythium manitatum chemicals around the roots of alfa alfa stimulate the germination of sclerotia of Sclerotium roffsii. - On the other hand, malic acid secreted from the leaves of several varieties of gram inhibit the germination of Mycosphorella spores. - The waxy leaf surfaces of apple and other plants also inhibit germination of many fungal spores. ### Growth of germ tube: - Most of the fungal propagules germinate to produce germ tubes (e.g., conidia, zoospores, sporangiospores, oidia, uredospores, etc.). - The germ tube elongates over the surface of host and either enters directly into the host or proliferates to produce mycelial hyphae. - In case of Rhizoctonia solani, the fungal hyphae grow on the host surface to form an infection cushion (Fig. 6.1) probably to achieve proper numerical and chemical strength to cause infection. The infection cushion then gives rise many appresoria and infection pegs which result multiple infection. - In case of Armillaria mellea, the fungal hyphae aggregate in rope like strands, called rhizomorphs. - These structures then cause infection. - In case the germ tubes enter into the host, the tip of germ tube usually forms an appresorium from which the infection threads arise and enter into the host. - Growth of germ tube and formation of appresorium are influenced by the environmental factors, type of plant surface, chemical substances secreted by the plant organs and the microorganisms of rhizosphere and phyllosphere. - It is also indicated that some germ tubes are negatively phototropic and, therefore, move away from light into the interior of host. - The chemicals secreted from the rhizosphere either inhibit or promote the growth of germ tube whereas they are either not secreted from the phyllosphere due o presence of cuticle or they are washed away by rain and dew. ## Penetration - Actual entry of pathogen into the host is called penetration. - If the environmental factors are favorable and the host is susceptible, the germ tube or any other form of infecting propagule enters into the host either by - - indirect penetration or by - indirect penetration ### Indirect penetration: - Indirect penetration occurs through natural openings such as stomata, lenticels, hydathodes, etc., or through artificial openings such as wounds, insect punctures, etc. - **Indirect penetration through natural openings: ** Stomata, lenticels, nectarthodes, hydathodes, etc. are the natural openings found in the host. - Most of the pathogens enter through these natural openings and cause infection. However, a pathogen may either penetrate in one way only or in more than one way. - The entry depends on susceptibility of host, favorable environmental conditions and the chemicals secreted from host. - The stomata are minute pores, each guarded by two kidneys shaped guard cells. They are usually present on leaves and green stems. - The penetration through Stomata requires the presence of a film of water which extends from the exterior through the stomatal aperture to the substomatal chamber. - While entering through stomata, most of the protoplasm of the germ tube migrate to the appresorium which develops just above the stomatal pore. Then the appresorium gives out an infection thread which passes through the aperture into the stomatal cavity. - The protoplasm of appresorium migrates into the tip of infection thread which swells to form a substomatal vesicle. The empty germ tube and appresorium are cut off by the formation of septa. Then the vesicle sends out thread like hyphae which make physical contact with the living cells of host. - **Entry of pathogen through lenticels, hydathodes, nectarthodes, etc. also follow the same general rule as through stomata.** - **Through stomata (Fig. 6.2):** Usually the fungi and bacteria enter through stomata. - **Examples of fungal pathogens are uredospores of Puccinia graminis tritici (causing black rust of wheat), Peronospora destructor infecting onion leaves, Phytophthora infecting fennel, Mycosphaerella musicola infecting banana leaves causing sighatoka disease, Cladosporium fulvum infecting tomato, etc.** - **Examples of parasitic bacteria entering through stomata are - Pseudomonas tabaci infecting tobacco, Erwinia amylovora infecting apples, Xanthomonas malvacearum infecting cotton, etc.** - **Through lenticels:** Examples of pathogens entering through lenticels are - Streptomyces scabies causing scab of potato, Oospora pustulans causing potato scab, Penicillium expansum causing apple rot, Nectria galligena causing apple canker, etc. - **Through hydathodes:** The pathogens which enter through hydathodes are Erwinia amylovora and Xanthomonas campestris. - **Through nectarthodes:** Erwinia amylovora enters through nectaries of apple and pear flowers. - **Indirect penetration through wounds:** Many types of wounds occur on the surfaces of plant parts. They are caused by worker's hands, tools, by animals, strong winds, storms, hails, friction between plant parts, etc. The infection occurring through wounds is called wound or trauma infection. - **Examples of pathogens which enter the host through wounds are - root rot fungus Thielaviopsis basicola, wilt fungus Fusarium solani F. ovophaseoli, wood rotting fungus Fomes annosus, fruit rotting fungus Rhizopus and Penicillium digitatum, etc.** - The bacteria - Erwinia tracheiphila (the causal organism of vascular wilt), Erwinia amylovora (the causal organism of fire blight of apple), Xanthomonas stewartii (the causal organism of bacterial wilt), etc., enter through wounds. The viruses which are transmitted by insect vectors enter into the host cells through wounds caused by insect bites. ### Direct penetration (Fig. 6.3): - Direct penetration of pathogen occurs through the cuticularized surface of epidermis or uncuticularized surfaces of root tips, root hairs, buds, seedlings, stigma, anther, nectaries, etc. - Direct penetration through the epidermis, which is covered by a well defined cuticle is more complicated process as the pathogen has to cross morphological, biological and chemical barriers. - The morphological barrier includes a protective layer of cutin, cuticular waxes following by a layer of pectin which gradually merges with cellulose cell wall. - The biological barriers includes the presence of microorganisms in the phyllosphere and rhizosphere which antagonise or compete with the penetrating pathogen. - The chemical barrier includes lack of suitable nutrients for the pathogen and presence of inhibitory toxic substances on the host surface. - These barriers are so strong that the viruses can not cross them because they have neither the physical force nor the enzyme system of their own. - Therefore, the viruses are usually penetrated indirectly through wounds or with the help of vectors. - They bacteria also do not possess the physical force to cross the mechanical barrier. They penetrate directly only by their chemical action. - The nematodes and fungi possess both the physical force as well as the capacity to cross these barriers. - Fungal pathogens penetrate directly through cuticularized epidermal wall by applying physical force and chemical actions. - The germ tube of fungal pathogen develops penetration hypha or appressorium, which exerts a strong mechanical pushing on the intact wall. - The pressure results stretching of the epidermis which becomes thin. Finally the infection peg pierces the delicate thin area. - Thus the penetration of cuticle is purely mechanical. - After passing through the cuticle, the infection hypha comes in contact with the pectic substances and the cellulose cell wall. - These are penetrated by enzymatic action of the infection hypha so that the cell wall becomes softened and penetration is done. - Thus the direct penetration is first mechanical and then chemical. - At the same time the pathogen has to neutralize several toxic substances secreted by the host. - **Examples of direct penetration are -** Erysiphe graminis causing powdery mildew of wheat, Phytophthora infestans causing late blight of potato, Ustilago sp. causing loose smut of wheat, covered smut of barley, sugarcane smut, bunt of wheat, Puccinia graminis on barberry, Albugo candida causing white rust of crucifers, etc. ## Post Penetration (Invasion) - After successful penetration, the pathogen proceeds further to establish proper infection. - This stage is called invasion. - It should be mentioned here that the penetration does not mean infection. - There are so many internal factors which interact or inhibit the progress of pathogen. - The pathogen gets established to cause disease only after overcoming all these internal barriers. - It is the interaction of host and pathogen that determines the type of reaction which follows. - Different types of pathogens invade the host in different ways and to different extents. - Some cause only local infections (localized) while others infect the entire plant body (systemic). - These infections can be categorized as follows:- ### Ectoparasites (Ectotrophic) : - The pathogen grows superficially. - The mycelium of pathogenic fungi grows on the surface of host and sends only haustoria into the epidermal cells. - Example, powdery mildews. ### Endoparasite with external mycelium: - The pathogen colonies inside the host and also produces external mycelium which enters the host repeatedly. - Examples, Corticium solani causing stem canker of potato, Ophiobolus graminis causes take all disease of wheat, etc. ### Subcuticular parasites: - The pathogen colonizes inbetween the cuticle and outer wall of epidermis of host. - Examples, Venturia inaequalis causing apple scab, Diplocarpon rosae causing black spot disease of rose. ### Parasites in parenchyma: - Most of the pathogens colonize in parenchymatous tissue of cortex, pith, mesophyll, etc. - Examples, Albugo, Phytophthora, Pythium, Puccinia, etc. ### Parasites in vascular tissues: - The pathogens causing vascular wilt diseases invade the xylem vessels and tracheids. - They grow and multiply in vascular tissues. - Example, Fusarium. ### Endobiotic: - Some unicellular pathogens grow and reproduce only within the host cells. - They are distributed to the daughter cells during cell division. - Examples, Synchytrium endobioticum causing wart disease of potato, Plasmodiophora brassicae causing finger and toe disease of crucifers, bacteria, viruses and mycoplasma. ### Systemic infection: - These are highly specialized pathogens which grow throughout the plant body and cause serious damage. - Examples, smuts, downy mildews, etc. - **After successful infection, the pathogen derives its nourishment from the host, starts multiplication and secretes some chemical substances to express the symptoms of the disease. ** - As stated earlier, the viruses do not require nutrients for their growth and activity. - They enter directly into the protoplasm of host cells, disturb the normal functioning of host nucleus and multiply themselves within host cells. - The malfunctioning of host nucleus results disease development. - Bacteria and fungi, on the other hand, require nutrition from host for their growth and reproduction. - The fungal pathogens may colonize in the intercellular spaces of host tissues and obtain the nourishment through the membrane of adjacent cells (e.g., Taphrina) penetrate into the host cells intracellularly. - Most of the obligate parasites, facultative saprophytes and facultative parasites colonize in the intercellular spaces of host tissues. - They send out special short branches which penetrate the host cells. - Within the host cells (usually parenchyma) the branches get modified into special types of absorbing organs, called haustoria. - The haustoria absorb nutrients from the host and supply it to the main body of pathogen. - These pathogens soon start reproduction and multiplication.