Ecological importance of plant bacteria

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. 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. 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. 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. 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. 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. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.


16. 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%).

17. 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.

18. 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.

19. 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)

22. 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

23. 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.

24. 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.

25. 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

26. 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.

27. 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.

28. 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.

29. 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

30. 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.

31. 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

32. 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.


33. 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.

34. 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.

35. 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.

36. 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

37. Survival of Plant Pathogenic Bacteria
 Plant dependent survival
 Survival by latent infection:
 Survival in Seeds and Planting material
 Survival as residents
 Saprophytic Survival