Download Plant pathology Pathogen resistance (Lecture 13-14)

Plant pathology
Pathogen resistance (Lecture 13-14)
Aspects of plant pathology
1)
2)
3)
4)
causes.
The living entities and the environmental conditions that cause disease in plants
The mechanisms by which these factors produce disease in plants
The interactions between the disease-causing agents and the diseased plant
The methods of preventing or controlling disease and alleviating the damage it
Disease is any disruption (biotic or abiotic) of the normal metabolism of a plant.
Pathogen – an agent that induces disease.
How do pathogens cause disease?
1) Weakening the host by continually absorbing food from the host cells for their own use.
Pythium, mistletoe,
2) Killing or disturbing the metabolism of host cells through toxins, enzymes, or growth-regulating
substances they secrete. Foolish disease of rice, Gibberella fujikuroi; Bacterial soft rots, Erwinia
carotovora
3) Blocking the transportation of food, mineral nutrients, and water through the conductive tissues.
Verticillium wilt, Fusarium wilt.
4) Consuming the contents of the host cells upon contact. nematodes.
Significance of plant pathology.
*8000 fungi in North America cause 80,000 different diseases.
*180 species of bacteria
*500 different viruses cause plant diseases
*150 different nematodes
For example - tomato is attacked by 80 fungi, 11 bacteria, 16 viruses, and a number of nematodes.
Diagnosis of Plant Disease
1.Diseases caused by parasitic higher plantsEx. dodder, mistletoe, witchweed - look for the presence of the parasitic higher plant.
2. Diseases caused by nematodesLook for nematode in the root zone and root tissue of the plant.
Look also for knots (Root knot nematode).
ID nematode on the basis of the stylet and morphology of the nematode.
The stylet is a hard, sharp spear used for feeding.
3. Diseases caused by fungi and bacteriaBacteria- Based on symptoms, the appearance of a large number of bacteria in the area, and
the absence of any other pathogens.
Bacteria are very small (1-2 um) and are difficult to ID based on morphology.
Use selective media and reinoculation.
Immunodiagonstic techniques have also been developed for many plant pathogenic
bacteria.
4. Diseases caused by phytoplasmasDiseases caused by phytoplasmas appear as stunting of plants, yellowing or reddening of
leaves, proliferation of shoots and roots, abnormal flowers, and eventual decline and death
of the plant.
Phytoplasmas are small, wall-less bacteria that live in the phloem of the hosts, invisible.
Therefore, diagnosis is based on symptoms, graft transmission, insect vectors, electron
microscopy, sensitivity to certain antibiotics (Tetracycline but not penicillin), sensitivity to
high temperatures, and PCR.
5. Diseases caused by viruses and viroids- ID by symptoms-mosaics, streaking, epinasty, curling,
stunting, discoloration.
Parasitism and disease development terminology
1.Parasite- an organism that lives on or in some other organism and obtains its food from the latter.
2. Symbiosis- both host and non-host benefit. Mycorrhizal fungi, Rhizobium bacteria (nitrogen
fixers).
3. Pathogenicity- The ability of the parasite to interfere with one or more of the essential functions
of the plant.
4. Biotrophs, obligate parasites- They can grow and reproduce in nature only in living hosts. e.g.
rusts, powdery mildew, downy mildew, viruses, viroids, phytoplasmas, nematodes, fastidious
bacteria, protozoa. - Obligate parasite doesn't kill the host directly, but redirects the nutrients to
itself.
5. Nonobligate parasites- Parasites that can live on either living or dead hosts and on various
nutrient media.
Most fungi and bacteria. The non-obligate parasite usually kills the plant cells with enzymes or a
toxin and then digest the host cells. The non-obligate lives like a saprophyte in a living host.
6. Facultative saprophytes- Parasites that can live on dead organic matter, but live most of their life
cycle on a living host.
7. Facultative parasites- Parasites that live most of their lives on dead organic matter but under
certain environmental conditions, can attack living tissue and become parasitic.
8. Saprophyte- organisms that lives on dead organic tissue.
9. Virulence-the degree of pathogenicity of a given pathogen. Often a quantitative scale when
comparing isolates in a population.
(scale) Virulence (10)-------------------------Avirulence (1)
Pathogenicity is a qualitative scale- either a pathogen or not.
10. Aggressiveness- a quantitative measure of the severity of disease over time in a pathogen
population. The one that develops faster is the most aggressive.
11. Host range - plants that a parasite can infect . The host range is usually broader for a nonobligate parasite, including a larger number of different species, plant parts, and plant ages.
Stages of the disease development
The disease cycle- involves the changes in the plant and the plant's symptoms as well as those in
the pathogen.
Stages
1. Inoculation
2. Penetration
3. Establishment of infection
4. Colonization (invasion)
5. Growth and reproduction of the pathogen
6. Dissemination of the pathogen
7. Survival of the pathogen in the absence of the host (overwintering or oversummering of the
pathogen).
Inoculation
Inoculation- when the pathogen comes in contact with the host. Inoculum- the pathogen (s) that
come into contact with the host.
Examples of inoculum- in fungi, spores, sclerotia, mycelium.
Individuals of bacteria, viruses, viroids, phytoplasmas.
In nematodes- adult, larvae, eggs.
Penetration
A.Germination of Spores and seeds
B. Attachment of Pathogen to host
Fungi, bacteria, and parasitic higher plants must first become attached.
They have on their surface polysaccharides, glycoproteins, polymers of hexosamines, and
fibrillar materials, when moistened becomes sticky and helps the plant to adhere.
C. Recognition between host and pathogen
D. Penetration-the act.
1.Natural openings-stomata, lenticels, hydathodes, nectarthodes. Bacteria, fungi,
nematodes.
2. Wounds- fruits and vegetables. Fungi, Bacteria, viruses (Vectored)
3. Direct penetration- enzymatic or pressure. Fungi, parasitic higher plants, nematodes.
Establishment of infection
Infection- the process by which pathogens establish contact with the susceptible cells or tissues of
the host and procure nutrients form them.
During infection pathogens grow or multiply, or both, within the plant tissues and invade and
colonize the plant to a lesser or greater extent.
-one result of infections is symptoms. In most cases, symptoms occur within two weeks of initial
invasion.
-Incubation period- the time between inoculation and the appearance of disease symptoms.
Colonization (invasion)
-Invasion-
Fungi - subcuticular (black spot on rose, apple scab), surface of the plant and also send haustoria
into the epidermis (powdery mildew, downy mildew),
intracellular or intercellular (rusts), xylem tissue (Fusarium, Verticillium).
Bacteria invade intercellularly until the cell wall breaks down and then the bacteria grows
intracellularly.
Viruses, viroids, and phytoplasmas invade intracellularly,
nematodes invade intercellularly in most cases.
-Colonization- by fungi- can grow throughout the plant and then produce spores by the millions.
Bacteria- divide every 20 to 30 minutes.
Fastidious bacteria and mycoplasmas reproduce much slower than bacteria and are usually in
lower numbers in the plant.
Viruses and Viroids-reproduce in the individual cells. 10 million virus particles per cell.
Nematodes- female lays about 300-600 eggs, about 1/2 are females. Two to 12 generations
produces per year. Each generation increases the number of nematodes in the soil by 100 fold.
Dissemination - spread of pathogen inoculum.
Almost all dissemination of pathogens that is responsible for plant disease outbreaks, is carried out
passively by such agents as air, water, insects, certain other animals, and humans.
Air - most of these spores do not contact a susceptible hosts.
Rusts occur at several thousand meters above infected fields and can be carried for miles.
Water- Important in disseminating pathogens in three ways:
1.Bacteria, nematodes, and spores, sclerotia and mycelial fragments of fungi present in the soil are
disseminated by rain or irrigation water that moves on the surface re through the soil.
2. All bacteria and the spores of many fungi are exuded in a sticky liquid and depend for their
dissemination on rain or irrigation water, which either washes them downward or splashes them in
all directions.
3. Raindrops or drops from overhead irrigation pick up the fungal spores and any bacteria present
in the air and wash them downward where some of them may land on susceptible plants.
*Water dissemination is more efficient in that the pathogens land on an already wet surface and
can move or germinate immediately.
-Insects, mites, nematodes, aphids and leafhoppers are primary vectors for viruses.
Leafhoppers are the main vectors for mycoplasmas and fastidious bacteria.
The Dutch elm disease also depends on a bug. In these vectored diseases, the pathogen is
completely dependent on the vector.
*Very efficient method of transmission.
Survival
Methods of survival
1:-fungi- perennial plants, mycelium in infected tissues
•Spores on fallen , infected leaves or fruits.
•Annual plants- mycelium in infected plant debris, as resting spores, sclerotia, on seeds,
tubers.
•Some are soil inhabitants- able to survive indefinitely as saprophytes (Pythium, Fusarium,
Rhizoctonia).
•Soil transients- are specialized parasites that generally live in close association with their
host but may survive in the soil for relatively short periods of time.
Rusts overwintering on plants grown at warmer temperatures and move from them to crops
grown in colder climates as temperature allows.
Rust go from annual to perennial and overwinters in the perennial.
2. bacteria-same as fungi. Many overwinter in insect vector.
3. viruses-in living plant tissue such as the tops and roots of perennial plants, the vegetative
propagating organs, and in the seeds of some hosts. Some viruses overwinter in their vectors. TMV
(cigarettes).
4. Nematodes-as eggs in the soil and in plant roots .
5. Parasitic plants- survive either as seeds or as their vegetative form on their host.
How pathogens attack plants
PATHOGEN
HOST PLANT
1. Penetration with pressure
Biochemical defense (Phytoalexins,
hypersensitive response, phenols)
2. Penetration with enzymes
Intricate cell wall and cuticle
Suberization, antifungal enzymes
(Chitinases, B-glucanases).
Deactivation of pathogen enzymes
3. Toxins
Detoxification by enzymes
4. Growth regulators and
general virulence factors
Biochemical defense (Phytoalexins,
hypersensitive response, phenols)
Intricate cell wall and cuticle
Suberization
Conversion of the cell walls into cork tissue by development of suberin; -- commonly taking place
in exposed tissues, as when a callus forms over a wound.
Suberized cell walls are impervious to water.
Mechanical forces exerted by pathogens on host tissues
For a pathogen to exist it must enter the plant, obtain nutrients from the plant and neutralize the
plant defenses.
1.What plant parasites enter by mechanical pressure?? some fungi, parasitic higher (flowering
plants) plants, and nematodes.
2. The parasite first adheres to the plant surface.
The fungus forms an appressorium-a flattened bulb like structure. This increases the area of
adherence between the two organisms.
From the appressorium a penetration peg forms.
Chemical weapons of pathogens
The effects caused by pathogens in plants are largely chemical in nature.
Main groups:
Enzymes (most common)
Toxins (2nd most common)
Growth regulators (3rd most common)
Polysaccharides (least common)
All plant pathogens can produce these compounds except for the viruses and viroids.
But viruses and viroids can stimulate the plant to produce these compounds.
Enzymatic degradation of cell wall substances
a.cuticular wax- no pathogens are known that produce enzymes that can degrade waxes. Fungi
and parasitic higher plants apparently penetrate wax layers by means of mechanical force alone.
b.Cutin- an insoluble polyester of mostly branched derivatives of C16 and C18 hydroxy fatty acids.
c.Cutinases - esterases - they break the ester linkages between cutin molecules and release
monomers as well as oligomers.
Many fungi and at least one bacterium (Streptomyces scabies) have been shown to produce
cutinases.
c. Pectic substances - constitute the main component of the middle lamella.
Pectinases-pectolytic enzymes
d. Cellulose- the spaces between microfibrils and between micelles or cellulose chains within the
microfibrils may be filled with pectins and hemicellulose and probably some lignin at maturation.
Cellulase- C1-attacks native cellulose by cleaving cross linkages between chains.
C2-breaks the chains down farther.
CX-breaks the chains down into disaccharide cellobiose.
ß-glucosidase- breaks disaccharide into glucose.
e. Hemicelluloses-complex mixtures of polysaccharide polymers. They are a primary component
of the cell wall and may also be in the middle lamella and the secondary wall.
*many different kinds of hemicellulases-xylanase, galactanase, glucanase, arabinase, mannase.
f. Lignin - found in the middle lamella, in the cell wall of xylem vessels, and in the fibers that
strengthen plants.
Structure of lignin-phenylpropanoid.
*Only about 500 species of fungi can degrade lignin. These are almost all basidiomycetes.
The largest group is called the white rot fungi and they produce ligninases.
Enzymatic degradation of substances contained in plant cells
Proteinases- proteins
Amylases- starch
Lipases, phospholipases - oils and fats, membranes.
Mechanisms of resistance
Resistance of individual plant genotypes may from a number of mechanisms:
(I)Preformed chemical inhibitors;
(II)Induced chemical inhibitors;
(III)Structural barriers;
(IV)Rapid necrosis with systemic acquired resistance.
Preformed chemical inhibitors
A. Pre-existing biochemical defensesResistance of green tomato fruits to the fungus Fusarium.
Green tomato contain a steroidal glycoalkaloid, tomatine, and are resistant to Fusarium.
Mutant Fusarium strain can grow on the high level of this alkaloid.
*Example-Onion smudge disease caused by Colletotrichum circinans.
Resistant onion varieties have red scales and contain the phenolic compound protocatechuic acid
and catechol. These substances cause inhibition of C. circinans germination. White scale onions
are susceptible.
B. Defense through lack of essential factors
1. Lack of recognition between host and pathogen.
Host cell surface may lack recognition factors.
2. Lack of host receptors and sensitive sites for toxins.
3. Lack of essential nutrients for the pathogen.
*Example-Erwinia carotovora p v. atroseptica is less severe on potatoes with low reducing sugar
content than in potatoes high in reducing sugars.
Induced chemical inhibitors
Defense through increased levels of phenolic compounds.
1. Phytoalexins- toxic substances produced in response to infection and mechanical injury.
Cells adjacent to those infected produce Pytoalexins.
Resistance occurs when the phytoalexin concentration reaches a high enough concentration to
restrict pathogen growth.
Phytoalexins do not occur in response to biotrophs.
Phytoalexins are stimulated by pathogen substances called elicitors.
Elicitors are usually part of the fungal cell wall (glucans, chitosan, glycoproteins, and
polysaccharides).
In a susceptible response the pathogen is thought to have suppressors.
*The amount of phytoalexin produced often correlates with the resistance of the host.
*What about the virulent pathogen? How do they respond?
Defense through increased levels of phenolic compounds.
2. Fungitoxic phenolics released from nontoxic phenolic complexes- nontoxic glycosidase
that cleaves phenol and then renders it toxic.
3. The role of phenol-oxidizing enzymes in disease resistance- Polyphenoloxidase (PPO)-;
usually higher in resistant plants than in susceptible plants. PPO oxidizes phenolics to
quinones (more toxic to microbes). Peroxidases
4. The role of induced synthesis of enzymes.
Chitinase, b-glucanases, PG-inhibitors.
5. Defense through inactivation of pathogen enzymes- caused by phenols and proteins.
6. Defense through release of fungitoxic cyanides from non-toxic complexes. Cyanidesreleased by decompartmentalized hydrolytic enzymes.
7. Defense through detoxification of the pathogen toxins.
A number of studies have shown that pathogen attack will induce a range of metabolic changes in
the host plant including the synthesis of pathogen-related proteins.
The fungal pathogen of pea, Fusarium solani produces an enzyme (demethylase) that can
detoxify the pea phytoalexin pisatin.
When the pathogen Cochliobolus hetetostrophus (non-pathogenic for pea) was transformed with
the gene encoding the Fusarium demethylase it became infectious for pea.
This indicates that single phytoalexin confers significant resistance to fungal pathogen in peas.
Structural barriers
A. Pre-existing defense structures
-wax, cuticle, was not only impede penetration but also repel water and therefore a moist surface
in which the pathogen can germinate.
Structural variation between genotypes can be a major determinant of resistance to pathogens.
The spores of fungal disease (loose smut) in barley are spread by wind.
They enter open flowers and infect the plant via stigma and ovary wall.
Barley is a naturally self-pollinating species (pollination occurs before the flowers open).
Major goal of breeders has been the development of a barley variety with closed flowers.
B. Defense structures formed in response to pathogen invasion
1. Histological defense structures*Cork layers-stop the physical advance of the fungus, stops nutrients from going to the pathogen
and stops toxins that may come from the pathogen.
*Abscission layers- often occurs in Prunus sp. e.g. Shot hole of cherry.
*Tyloses-overgrowths of the protoplasts of adjacent living parenchymatous cells which protrude
into xylem vessels through pits.
C. Cellular defense structures-morphological changes in the cell wall or changes derived
from the cell wall.
Example- Callose papillae-produced by cells within minutes after wounding and within 2-3 hrs
after inoculation with microbes.
The main function of papillae seems to be to repair cellular damage; sometimes they also seem to
prevent the pathogen from subsequently penetrating the cell.
D. Cytoplasmic defense*Example- Necrotic defense reaction: defense through hypersensitivity- The faster the host cell
dies after invasion, the more resistant to infection the plant seems to be.
Rapid necrosis and systemic acquired resistance
Hypersensitive response (HR) – an important active defense system. It is characterized by rapid
necrosis of the cells in the vicinity of the invading pathogen.
It includes two phases:
Firstly, the death of the surrounding cells may limit the growth of a pathogen;
Secondly, there is a broad physiological immunity, called systemic acquired resistance (SAR),
that results from the HR.
Disease control
The best means of disease control, particularly against biotrophic pathogens is resistant varieties.
Varieties that already have resistance to the pathogen(s) are least expensive, most effective, and
environmentally safest.
A. Genes and Disease
The pathogen has genes that code for virulence mechanism.
It is, therefore, the concurrent occurrence and interaction of specific genes for virulence in the
pathogen and of specific genes for susceptibility in the host that determine the initiation and
development of disease.
The gene or genes for virulence in the pathogen are usually specific for one or a few related kinds
of host plants. Also, the genes that make a host plant susceptible to a particular pathogen are
present only in that one host and possibly a few related kinds of host plants.
Compatible (disease resulting) interaction
Why are some pathogens virulent to several hosts? Virulence genes are broad spectrum or Vgenes are very numerous.
Examples- Phymatotrichum omnivorum-- attacks 2000 different species
Puccinia graminis var. tritici --attacks wheat only
Fusarium oxysporum f. sp. lycopersici--attacks tomato only
Resistance is the rule, susceptibility the exception
Plants of a particular species also have resistance genes. These resistance genes may have
been bred out of a particular variety or may not exist to certain pathogens. In many cases
resistance genes are found in wild accessions.
Introducing a new resistance gene
A resistance gene confers resistance to the different races of a pathogen. However, it has been
observed that once a new resistance gene is in a plant population, new virulent races often occur.
*How did this new population of pathogens acquire the new gene for virulence?
-Genes already present in a very small number. Race T.
H. maydis
-mutation - nuclear and cytoplasmic
- Recombination of genetic material - P. capsici. Sexual reproduction
B. Stages of Variations in Pathogens
Species - the entire population of an organism on earth, for example a fungal pathogen, has
certain morphological characteristics in common and makes up the species of the pathogen.
Example: Puccinia graminis - cause of stem rust of cereals.
Some of the individuals within this species attack only wheat or barley or oats. These groups are
called varieties or special forms (formae specialist).
C. Types of Plant Resistance to Pathogens
Each kind of plant is a non-host to the vast majority of known plant pathogens.
Non-host are immune (non-host resistance).
Example: Elm tree resistant to Fusarium oxysporum f. sp. niveum
1.True resistance - Disease resistance that is genetically controlled by the presence of one, a
few, or many genes.
The host and pathogen are incompatible with one another due to chemical recognition or other
defense mechanisms.
1.1 Horizontal resistance- also called nonspecific, general, quantitative, adult-plant, field, or
durable resistance.
Many genes (multigene resistance) control horizontal resistance.
These genes appear to exert their influence by controlling the numerous steps of the
physiological processes in;
the plant that provide the materials and structures that make up the defense mechanisms of
the plant. e.g. cutin depth, plant architecture
•Affected by environment.
•Does not protect against infection but slows down disease development. There is usually
some degree of horizontal resistance.
1.2 Vertical resistance- very resistant to some races and very susceptible to others (specific,
qualitative, differential).
Characteristics of true resistance •usually controlled by one or a few genes. (monogenic or oligogenic)
•These genes apparently control a major step in the interaction of the pathogen with the
host plant and therefore play a major role in the expression of vertical resistance.
•Responds like an incompatible, HR, complete resistance
•limits initial inoculum.
•Genes in the nucleus generally control both horizontal and vertical resistance.
2. Apparent resistance- susceptible plants that do not become infected.
a. Disease escape-the 3 factors of disease (susceptible host, virulent pathogen, environment) do
not coincide.
Disease escape can be managed by:
using disease free seed,
vigorous seed,
proper soil,
planting date,
depth of sowing,
distance between plants in the fields,
proper crop rotation,
sanitation (rouging, pruning, etc.)
interplantings and multilines,
insect control.
Tolerance to disease is the ability o f plants to produce a good crop even when they are infected
with a pathogen.
•specific inheritable traits
•vigorous
•produce a better crop when not infected.
•The Gene- for-gene concept - for each gene that confers resistance in the host there is a
corresponding gene in the pathogen that confers virulence to the pathogen, and vice
versa.
•Operates in many diseases.
•Plant resistance is dominant
•Virulence in the pathogen is recessive.
Plant-pathogen interaction
A simple classification of the relationship between plant host and pathogen is:
Host (plant)
Pathogen
Virulent
Avirulent
Resistant
No disease
No disease
Susceptible
Disease
No disease
The basic principle behind these are that there is a gene product that inhibits an effect or product
of the viral infection.
One gene for infection, one gene to resist.
A few examples are the L6 gene for the Flax Rust virus, the N gene in tobacco for TMV
resistance, as well as the RPS2 gene in Arabidopsis for bacterial resistance.
These gene-for-gene resistance's operate on the basis of recognition of the infecting particle by
an interaction between the host resistance system, and the avirulence genes of the infecting virus.
This results in the activation of anti-pathogen molecules, or hypersensitive cell death pathways, in
the tissues around the point of infection.
The RPS2, and N gene products have domains for nucleotide binding, and repeats of leucine-rich
area. The postulated mechanism involves a signal transduction that is dependent on A\GTP,
responding to pathogen avirulence genes.
Part of the material adapted from
http://taipan.nmsu.edu/EPWS310