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Chapter_ 5 ecology and disease cycle.ppt

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Chapter_ 5 ecology and disease cycle.ppt

  1. 1. Ecology and Bacterial Disease cycle  Bacteria: diverse in their ability to affect plant health, their genotypic and phenotypic characteristics, and their phylogeny  Typically members of complex microbial communities, with only a few establishing pure clonal populations within a plant  Many plant-associated microbes, even those that comprise only a small proportion of a community, can have functions that are of agricultural or environmental importance  Plants offer a wide range of habitats that support microbial growth
  2. 2. Ecology of Bacteria 2.1.1. The spermosphere  The spermosphere is the zone that is influenced by a seed; it often extends 1 to 10 mm from the seed surface.   Nutrients that support microorganisms are released when the seed imbibes, with nutrient release being greatest from the embryo end, i.e., the end closest to the emerging radicle, and from seeds that are cracked or damaged.  Bacteria that establish populations on seeds can colonize roots as they emerge
  3. 3. Ecology of Bacteria…continued 2.1.2. The rhizosphere  The rhizosphere is the zone that is influenced by the root, the soil adhering to the root  Root growth changes the physical and chemical properties of the soil, including the mineral and organic content, the water potential, the pH, and the salinity  The rhizosphere is nutrient rich. This region contains root exudates (low molecular weight (MW) compounds released during normal root cell metabolism), root secretions (low and high MW compounds that are synthesized for secretion), and compounds released by plant cell lysis. 
  4. 4. Ecology of Bacteria…continued 2.1.2. The rhizosphere…continued  A major component of root secretions is mucilage, which contains hydrated polysaccharides, organic acids, vitamins, and amino acids and thus is an excellent substrate for microbial growth  Mucilage binds water and thus helps to form a well-hydrated environment for the roots and rhizosphere microorganisms.  During root growth, the root cap continuously sloughs off cells, called peripheral or border cells, that secrete large amounts of mucilage to lubricate the roots, and after living for about 3 weeks, die and lyse.
  5. 5. Ecology of Bacteria…continued 2.1.3. The phyllosphere  The external regions of the above-ground parts of plants, including leaves, stems, blossoms, and fruits, are collectively referred to as the phyllosphere.  Leaves are the dominant tissue in the phyllosphere based on the surface area available for colonization.  The waxy plant cuticle ensures that water loss occurs primarily through the stomata, an adaptation that probably evolved to allow plants to live on land and that has major implications for the leaf surface microflora 
  6. 6. Ecology of Bacteria…continued 2.1.3. The phyllosphere…continued  Unlike the rhizosphere, the phyllospehre is subject to large and rapid fluctuations in temperature, solar radiation, and water availability,  These changes in environmental conditions are somewhat buffered by the boundary layer, an air layer that surrounds the leaf.
  7. 7. Ecology of Bacteria…continued 2.1.3. The phyllosphere…continued  These changes in environmental conditions are somewhat buffered by the boundary layer, an air layer that surrounds the leaf.  In contrast to the rhizosphere, which often supports at least 109 bacteria per gram of root, leaf surfaces typically support fewer than 107 bacteria per gram of leaf,  Although these community sizes vary greatly with plantspecies, physiology, age, and environmental conditions.
  8. 8. Ecology of Bacteria…continued 2.1.3. The phyllosphere…continued  Plant-derived nutrients on leaves probably originate from mesophyll and epidermal cell exudates leaking onto the surface as well as lysates from wounds and broken trichomes  The distribution of phyllosphere nutrients is highly heterogeneous as would be predicted for those originating from leaf damage or exogenous sources such as the honeydew of visiting insects  Blossoms and fruits offer unique habitats for microbial growth. Blossoms are short- lived, contain sugar-rich nectar (e.g., 10-50% sucrose), and are attractive to insects, which are excellent vectors for bacteria.  Fruits generally have a thick cuticle on their epidermis, which probably minimizes the leakage of water and nutrients to the fruit surface.
  9. 9. Ecology of Bacteria…continued 2.1.4. Endophytic sites  Endophytic sites include any region internal to the plant epidermis, although the vascular system is usually considered separately.  Endophytic microorganisms are usually found within the intercellular, or apoplastic, spaces.  Intercellular air spaces comprise a significant fraction of the tissue inside roots and leaves.  For example, the spaces between the cortical root cells can comprise as much as 30% of the root volume, and those between the mesophyll leaf cells can comprise as much as 70% of the leaf volume.
  10. 10. Ecology of Bacteria…continued 2.1.4. Endophytic site…continued  Microorganisms that reach these intercellular regions must contend with plant defense responses, which are triggered when bacteria are in close proximity to the plant cells  Bacteria in endophytic sites may access nutrients and water more easily than those on the surface, particularly if the plant cells lyse or leak nutrients, as occurs during pathogenesis  Bacteria in endophytic sites may be buffered from the environmental fluctuations characteristic of the phyllosphere and from the intense competition for nutrients characteristic of the rhizosphere.
  11. 11. Ecology of Bacteria…continued 2.1.4. Endophytic site…continued  Bacterial entry into plants occurs at sites of epidermal damage, of lateral root or radicle emergence, through natural openings such as stomata and lenticels (pores for gas exchange), hydathodes (water pores), nectarthodes (openings in the nectary of blossoms), and into progency plants through infected seeds.  Some symbiotic bacteria have evolved sophisticated entry mechanisms that include directing the plant to form a channel, called an infection thread, that promotes bacterial penetration into the plant tissue.
  12. 12. Ecology of Bacteria…continued 2.1.5. Vascular tissue  The two elements of the vascular system, the xylem and the phloem, offer distinct habitats for microbial colonization.  Xylem vessels function in the transport of water and minerals and thus contain a highly dilute solution of minerals and simple organic compounds.  The vessels consist of dead cells that do not contain cytoplasm, lignified secondary cell walls, and lateral wall openings, called pits, with membranes that must be crossed to leave the xylem. 
  13. 13. Ecology of Bacteria…continued 2.1.5. Vascular tissue… continued.  Some bacterial xylem colonists can live on only the nutrients in the xylem; these fastidious organisms exhibit complex requirements for growth indicative of a high level of adaptation to this habitat  Others live on nutrients that are released following destruction of the xylem vessel walls  In contrast, the phloem functions in the transport of sugars from the leaves to the rest of the plant and contains a concentrated sucrose solution (15-30%).
  14. 14. Ecology of Bacteria…continued 2.1.5. Vascular tissue… continued  The phloem consists of living cells, including sieve tube elements and their associated companion cells, which load sucrose into the sieve tubes.  The plates between adjacent sieve elements have pores with a diameter of 1 to 15 μm, which is large enough to allow bacterial passage.  Bacteria that can colonize the phloem are highly adapted to this habitat, as evidenced by the low cultivation rate of these organisms and the unique presence of phytoplasmas, or cell wall-less bacteria, in the phloem.  Phloem colonists, all of which are believed to be pathogens, are also unique among plant-associated bacteria in being located intracellularly.
  15. 15. Ecology of Bacteria…continued 2.1.5. Vascular tissue… continued.  Bacteria gain access to the xylem tissue through a variety of routes, including through stomata or hydathodes that lead to the open ends of xylem vessels in leaves, through the sites of lateral root emergence in roots, through xylem-feeding insects and wounds, and via active dissolution of the xylem vessel walls.  In contrast, bacteria gain access to the phloem primarily by transmission from phloem-feeding insects or by cultivation practices such as grafting, which involves cutting stems.
  16. 16. Bacterial Disease–Cycle  In infectious diseases a series of events occurs in succession and leads to the development and perpetuation of the disease and the pathogen  Disease-cycle refers to the appearance, development and perputation of the disease as a function of the pathogen  It involves changes in the plant and its symptoms.  The primary events are inoculation, penetration, establishment of infection, colonization (invasion), growth and reproduction of the pathogen, dissemination (spread), and survival of the pathogen in the absence of the host (over-wintering or over- summering)
  17. 17. Bacterial Disease–Cycle …continued  The inocula of phytopathogenic bacteria are whole individual. It consist millions of bacterial cells. The individual bacterial cell is a propagule  The sources of bacterial inoculum are either inside the plant or outside  It includes infected debris, infested soil, infested seed, transplants, tubers or other propagative organs  The outside sources are nearby plants or fields. E.g., perennial weeds, alternate hosts.  Fastidious bacteria produce their inoculum within the plants; such an inoculum almost never reaches the plant surface in nature. It can not by itself escape from one plant and spread to another
  18. 18. Bacterial Disease–Cycle …continued Penetration:  Bacteria penetrate plant surfaces through wounds and natural openings (stoma, hydathode, nectorthode, lenticles).  Fastidious bacteria enter through wounds made by vectors.  The wounds utilized by bacteria may be fresh or old and may consist of lacerated or killed tissue.  Laceration or death of tissues are the result of environmental factors such as wind breakage and hail; animal feeding – insects and large animals; cultural practices-pruning, transplanting, harvesting; self – inflicted injuries-deaf scars, wounds or lesions caused by other organism.
  19. 19. Bacterial Disease–Cycle …continued Penetration … continued  Bacteria penetrating through wounds multiply in the wound sap or in a film of rain or dew water present on the wound.  Bacteria present in a film of water over a stoma can easily swim through the stoma.  Hydathodes are more or less permanently open pores at the margins and tips of leaves.  They are connected to the veins and screte droplets of liquid containing various nutrients.  Some bacteria use these pores as a means of entry into leaves. Some bacteria also enter blossoms through the nectarthodes or nectarines, which are similar to hydathodes.  Lenticels are openings on fruits, stems, and tubes that are filled with loosely connected cells to allow passage of air.
  20. 20. Bacterial Disease–Cycle …continued Infection  The process by which pathogens establish contract with the susceptible cells or tissues of the host and procure nutrient from them.  During infection pathogens grow or multiply within the plant tissues and invade and colonize the plant to a less or or greater extent  Thus, invasion of the plant tissues by the pathogen, and growth and reproduction of the pathogen (colonization/ in or on infected tissue are two concurrent stages of disease development within the stage of infection.  Successful infections result in the appearance of symptoms: discoloration, malformation or necrotic areas on the host plant. Symptoms may change continuously from the moment of their appearance until the entire plant dies, or they may develop
  21. 21. Bacterial Disease–Cycle …continued Recognition  Recognition between host and bacterial pathogens involves many biochemical substances, structures and pathways.  These may include specific host signal compounds or structures, or specific pathogen elicitor molecules.  This shall be discussed under host pathogen interaction and pathogencity factors.  Host components acting as signals for activation of pathogens are: fatty acid of the plant cuticle that activate production of the cutinase enzyme which breaks down cutin, galacturonan molecules of pectin which stimulate production of pectin lyase enzymes.
  22. 22. Bacterial Disease–Cycle …continued Invasion:  various pathogens invade hosts in different ways and to different extents.  Bacteria invade tissues intercellularly, although when parts of the cell wall dissolve, bacteria also grow intracellularly.  Bacteria causing vascular wilts invade the xylem vessels. Fastidious bacteria invade tissues by moving from cell to cell intracellularly.  Many infections are local, that is, they involve a single cell, a few cells, or a small area of the plant.  These infections may remain localized throughout the growing season, or they may enlarge slightly or very slowly.  All infections caused by fastidious bacteria are systemic, the pathogen from one initial point spreads and invades most ro all susceptible cells and tissues throughout the plant.
  23. 23. Bacterial Disease–Cycle …continued Growth and reproduction of the pathogen (colonization):  Bacteria invade and infect new tissues within the plant by reproducing at a rapid rate and increasing their number in the infected tissues.  The progeny move through cells on their own power, through phloem (fastidious bacteria) or xylem (vascular wilt bacteria) plant pathogenic bacteria reproduce by fission, in which one mature individual splits into two equal, smaller individuals.  The pathogen reproduce between or within host cells, and come to the host surface only through wounds, cracks, stomata, and so on.
  24. 24. Bacterial Disease–Cycle …continued Growth and reproduction of the pathogen (colonization)… continued  Bacteria reproduce rapidly within infected tissues. Under optimum nutritional and environmental conditions (in culture) bacteria divide (double their numbers) every 20 to 30 minutes  Millions of bacteria may be present in a single drop of infected plant sap,  Fastidious bacteria appear to reproduce more slowly than typical bacteria.  Although fastidious bacteria spread systematically throughout the vascular system of the plant, they are present in relatively few xylem or phloem vessels, and the total number of these pathogens in infected plants is relatively small
  25. 25. Dissemination and dispersal of bacterial diseases  Plant pathogenic bacteria in diseased tissues are embedded in abundant polysaccharides from which individual bacteria can be separated and released into the air only with difficulty.  For effective dispersal, these bacteria need various vectors such as rain splash, wind, surface water, insects, and humans. Dispersal by water:  Dissemination by water is the primary means of dispersal for most bacterial plant pathogens. When lesions occupied by bacterial plant pathogens are wetted the polysaccharide matrix in which bacterial cells are embedded dissolved readily, and bacterial cells are released to the surface of the lesion.  Bacterial cells thus suspended in water can readily gain access into natural openings or wounds.
  26. 26. Dissemination and dispersal of bacterial diseases…continued Dispersal by rain splash:  During rainfall bacteria emerge from diseased tissues into surface water, providing a source of inoculum for as long the rain lasts. Water droplets carrying bacterial cells can be dispersed long distances during stormy rains as in a typhoon. Dispersal by irrigation Water  Irrigation water is an important mode of dissemination by bacterial diseases. This type of dissemination has been documented with R. solanacearum; X. campestris, X. campestris pv. Oryzae
  27. 27. Dissemination and dispersal of bacterial diseases…continued Dispersal by soil:  Plant pathogenic bacteria surviving in soil can be distantly disseminate through running surface water or agricultural practices, but dispersal by their own movement or by the help of insect or nematode vectors is restricted to a very short distance.  The typical soil-borne bacteria include A. tumefaciens, R. solanacearum, and E.carotovora subsp. Carotovora.  These bacteria can survive for a long time in soil in the absence of host plants.  Most other plant pathogenic bacteria are unable to survive in soil unless there are growing plants or un decomposed plant residues.  In these bacteria, un decomposed tissues of diseased plants buried in soil become a potential source of inoculum.  These bacteria can also survive in the rhizosphere of non-host plants and infect the lower leaves of host plants by rain splash. 
  28. 28. Dissemination and dispersal of bacterial diseases…continued Dispersal by seeds and planting material:  Many plant pathogenic bacteria are disseminated by internally infected or extermally infested seeds and vegetatively propagating materials such as tubers, bulbs, tuberous roots, rhizomes, cuttings, budwood, and small plants.  Seed borne diseases are characterized by long-distance dispersal, frequently across continent.  This mode of dissemination is of great practical importance, even if the rate of contamination is small in the bulked seed.  Under favourable environmental conditions, disease may readily spread from the diseased plants derived from a small number infested seeds to adjacent healthy plants and finally result in an epidemic.
  29. 29. Dissemination and dispersal of bacterial diseases…continued  Dispersal by insects:  All mycoplasma – like organisms (MLO) multiply in their leaf hopper and plant hopper vectors. Xylella fastidiosa are also transmitted by leaf hopper vector. These organisms are the parasites propagating both in insects and in plants and are disseminated by a persistent or circulative transmission but not through transovarial passage.
  30. 30. Dissemination and dispersal of bacterial diseases…continued Dispersal by farming practices:  Various cultural practices can increase the spread of diseases. Bacterial pathogens can be carried on infested agricultural implements and machinery during pruning, or disbudding.
  31. 31. Survival of Plant Pathogenic Bacteria  Plant pathogenic bacteria surviving in nature, even those in diseased plant tissues, are usually exposed to a variety of saprophytic micro organisms. To survive in coexistence with these micro organisms, plant pathogenic baceria are assumed to free themselves from competition and/or antagonism by entering in hypobiosis, by lying concealed in their own unique habitats, or by establishing a synergistic association with some other organisms. Generally plant Pathogenic survival depends on the following conditions  Plant dependent survival  Survival by latent infection:  Survival in Seeds and Planting material  Survival as residents  Saprophytic Survival
  32. 32. Survival of Plant Pathogenic Bacteria  Plant dependent survival  Survival by latent infection:  Survival in Seeds and Planting material  Survival as residents  Saprophytic Survival