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How Pathogens Attack Plants. A healthy plant is a community of cells built in a fortress-like fashion.The plant parts that pathogens encounter are either cellulose or cuticle often covered in a wax.The pathogen must penetrate these surfaces to gain access to the nutrient containing tissues. How Pathogens Attack Plants.
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1. CHAPTER 3PST 2340PLANT PATHOLOGY How Pathogens Attack Plants
2. How Pathogens Attack Plants A healthy plant is a community of cells built in a fortress-like fashion.
The plant parts that pathogens encounter are either cellulose or cuticle often covered in a wax.
The pathogen must penetrate these surfaces to gain access to the nutrient containing tissues
3. How Pathogens Attack Plants The cell contents may require break down before it can be used
The plant may react to the invasion by producing barriers and substances to interfere with the advancement of the pathogen
The pathogen must be able to enter, obtain nutrients and neutralize resistance
4. How Pathogens Attack Plants In trees the defense mechanism is called C.O.D.I.T
Compartmentalization
Of
Decay
In
Trees
5. How Pathogens Attack Plants The tree adds dense layers of cells surrounding the invading pathogen
Much study has been done about this
This can be seen in the sample as concentric rings around the decayed area
The rings vary in density but the density seems to be the way plants wall of the pathogen
6. How pathogens attack plants The pathogen accomplish these activities through secretions of chemical substances that affect components or metabolic mechanisms of the host
Penetration and invasion is aided or entirely the result of mechanical force exerted by the pathogen
7. Mechanical Forces Exerted by Pathogens on Host Tissues Only some fungi, parasitic plants, and nematodes seem to be able to apply mechanical pressure to the plant surface
The amount of pressure varies with the pre-softening of the plant by enzymatic action
Fungi and parasitic plant must first adhere to the host
8. Mechanical Forces Exerted by Pathogens on Host Tissues The attachment to plants seems to be an intermolecular force developing between the plant and the pathogen
In some cases an adhesion pad forms from the spore when it comes in contact with a wet surface
Some fungal spores carry adhesive substances
9. Mechanical Forces Exerted by Pathogens on Host Tissues After contact, the tip of a hyphae forms a bulblike tip that flattens into an adpressorium
A fine growing point called a penetration peg arises and advances through the cuticle
In some fungi penetration occurs only if a pigment accumulates in the appressorial wall.
10. Mechanical Forces Exerted by Pathogens on Host Tissues The pigment causes the accumulation of solutes which causes water to accumulate and cause an increase in turgor pressure
If the underlying cells are soft, penetration is easy
If the underlying cells are hard then penetration is difficult
11. Mechanical Forces Exerted by Pathogens on Host Tissues If the force at the growing point is more than the adhesive force, then the two separate
Most penetration is assisted by tissue softening enzymes
When the penetration tube enters the plant it is small
After penetration it enlarges
12. Mechanical Forces Exerted by Pathogens on Host Tissues Nematodes penetrate using a stylus to thrust back and forth to exert mechanical force
The stylet is in the body, the nematode lips act as suction cups to attach to the plant
The head end is perpendicular to the plant surface, the rear moves back and forth
The stylet is thrust into the plant
13. Mechanical Forces Exerted by Pathogens on Host Tissues Once a fungus or nematode enters, it secretes enzymes which allow further penetration easier
Once inside, more mechanical pressure is produced during fruitation which cause the pathogen to burst back out where fruiting bodies form the next generation
14. Chemical Weapons of Pathogens Although mechanical force is used to penetrate plants, biochemical reactions
Take place between host and pathogen
The main secretions are;
Enzymes
Toxins
Growth regulators
And Polysaccharides
15. Chemical Weapons of Pathogens These substances vary in importance in pathogenicity and from one disease to another
In soft rots, enzymes are important
In crown gall its growth regulators
Helminthosporium of Victoria Oats its toxins
Some produce compounds that suppress disease response
16. Chemical Weapons of Pathogens All except viruses and viroids produce enzymes, growth regulators and polysaccharides
Viruses and viroids are not known to produce substances themselves but induce the plant to do it
Natural selection has caused the survival of the pathogens that use these substances
17. Chemical Weapons of Pathogens Enzymes break down structural components of cells
Toxins act directly on protoplast components or interfere with their function
Growth regulators exert hormonal effects
Polysaccharides interfere with vascular translocation of water and may be toxic
18. Enzymes in plant disease Enzymes are large protein molecules that catalyze all interrelated reactions in a living cell
Each enzyme is coded for a specific gene
Some are always present
Some are produced only when needed
Some enzymes exist in several forms known as isozymes
19. Enzymatic Degradation of Cell Wall Substances Arial plant parts are cuticle and cellulose
Root cell walls are only cellulose
Cuticle consists of cutin impregnated with wax
Protein and lignin may also be found in epidermal call walls
Penetration is brought about by breakdown of cell walls
20. Enzymatic Degradation of Cell Wall Substances Cell walls consists of cellulose, pectins, hemicelluloses, structural proteins, and in the middle lamella, pectins
Disintegration involves the breakdown of lignin
This is brought about by one or more enzymes
21. Cuticular wax Often found as granular or rod-like projections or continuous layers
Some pathogens produce an enzyme that can degrade waxes
Fungi and parasitic plants use mechanical force
22. Cutin Cutin is the main component of the cuticle
The upper part is mixed with waxes
The lower part is mixed with pectins and cellulose
It is an insoluble polyster of C16 and C18 hydroxy fatty acids
The enzyme cutinase break down cutin and release monomers and oligomers
(single molecules & small groups of mol)
23. Cutin Fungi constantly produce small amount of cutinase which on contact with cutin releases small amounts of monomers
These enter the cell, trigger 1000x more cutinase
Studies have shown that cutinase is needed to invade through plant cells
24. Pectic substances Pectic substances constitute the main part of the middle lamella, the cement that holds cells together
They also make up a large part of the cell wall as a amophorus gel
Pectic substances are polysacharides consisting of chains of galacturonan molecules, rhamnose molecules, small side chains of galacturonan, & 5 C sugars
25. Pectic substances Enzymes that degrade pectic substances are called pectinases or pectolytic enzymes
Pectin methyl esterases remove small branches off pectin chains, altering the solubility
Chain splitting pectinases, polygalactuonases add a water molecule
26. Pectic substances This is then split by a pectin lyases
The whole process is complex
But it exists. I dont expect you to memorize each process but you should know generally how it works
27. Cellulose Is a polysccharide consisting of chains of glucose molecules held together by H bonds.
It is in the cell walls as microfibrils, the re- bar of the plant world
The spaces between are filled with pectins and hemicelluloses
The may be broken down by enzymes
28. Cellulose But are often broken down by oxidative systems such as activated oxygen and hydroxyl radicals
There are several enymes that break down cellulose called cellulases
These enzymes can be produced by fungi, bacteria, nematodes and parasitic plants
The enzymes release sugars as food
29. Hemicelluloses Are complex mixtures of polysaccharide polymers
They consist of xyloglucan, glucomannans, galactomannans, arbinogalactans and others
Hemicellulases are the enzyme that break them down
They may also break down by O &OH
30. Lignin Is found in the middle lamella and strengthens plants
Lignins are degraded by a large number of microorganisms
There are 500 species, mostly basidiomycetes, capable of decomposing wood.
Brown rot fungi degrade but cant use it
31. Lignin White rot fungi secrete enzymes which enables them to use it
Some ascomycetes and bacteria also secrete the enzyme ligninase
32. Cell wall structural proteins The are 5 classes of structural proteins
Extensins = 5%
Proline-rich proteins, PRPs
Glycine-rich proteins, GRPs
Solanaceous lectins
Arbinogalactan proteins, AGPs
These collect in response to fungal attack
This is the CODIT response system
33. Enzymatic degradation of substances contained in plant cells As the pathogens degrade the contents of cells, they utilize them as food
Sugars and amino acids are small enough to be used as is but others must be broken down into smaller units by additional enzymes
34. Starch, lipids Is the main reserve polysaccharide
It is broken down by amylases
Lipids are fats and oils, waxes, phospholipids, and glycolipids
They are broken down by lipases, phospholipases and so on
The breakdown often serves as a defense signal to the plant
35. Microbial toxins Living plants are a complex system of biochemical reactions
Disturbance of these process will cause a development of disease
Toxins are one of the substance produced by pathogens that will cause disruption
Toxins are produced by bacteria and fungi exist in many forms and may affect a few or many species
36. Microbial toxins Toxins injure the host by
Affecting the permeability of membranes
Or by inactivating or inhibiting the plants enzymatic reactions
37. Wide range toxins Several toxic substances produced by phytopathogenic microorganisms have been shown to produce all or part of the disease syndrome
They are called non-host-specific or
Non-host-selective toxins
Tabtoxin and phaseolotoxin inhibit host enzymes
38. Wide range toxins This leads to an increase of toxic substrates or depletion of needed compounds
Some affect the cellular transport system especially H+/K+
Tagetitoxin inhibits the transcription of cell organelles such as chloroplasts
Cercosporin is a photosensitizing agent
39. Tabotoxin Produced by Pseudomonas syringae
Causes wildfire disease of tobacco
Other strains cause disease of beans, soybeans, oats, maize, and coffee
The symptom is a dead spot surrounded by a yellow halo
Weaker strains produce the spot but no halo
40. Tabotoxin Tabotoxin as such is not toxic but becomes hydrolized and releases taboxinine
It disables the enzyme glutamine synthetase
Which leads to depleted glutamine
Which accumulates toxic levels of ammonia
41. Tabotoxin This uncouples photosynthesis and photorespiration and destroys the thylakoid membrane of chloroplasts which causes chlorosis and death
This reduces the ability of the plant to resist bacteria
This may become a secondary infection and so on
42. Phaseolotoxin Produced by pseudomonas syringae sp.
The cause of halo blight of beans and other legumes
It is a modified substance that cleaves the peptid bonds and releases alanine, arginine, and phosphosulfinylornithine
43. Tentoxin Produced by fungus alternaria alternata
Causes chlorosis in seedlings of many species
It binds to and inactivates a protein involved in energy transfer into chloroplasts
Inhibits light-dependant phosphorylation of ADP and ATP
The stress reduces resistance to disease
44. Non-host specific toxins Fumaric acid from Rhizopus spp,
Oxalic acid from Sclerototium, Sclerotina spp., & Cryphonectria parasitica
Alteraric acid, alternariol, and zinniol from Alteraria spp.
Cerato-ulmin from Ceratocystis ulmi (Dutch elm disease)
Fusicococcin from Fusicoccum amygdali
45. Non-host specific toxins Pyricularin from Pyricularia oryzae in rice blast disease
Fusaric acid & lycomarasmin from Fusarium oxysporum in tomato wilt
Coronatine from Pseudomonas syringae pv. Atropurpurea, affectin grasses
Syringomycin by Ps. Syringae pv. Syringae
Syringotoxin by Ps. Syringae pv. tagetis
46. Host specific toxins Fungi ; Cochliobolus
Alternaria
Periconia
Phyllosticta
Corynespora
Hypoxylon
Bacteria; Pseudomonas Xanthomonas
47. Victorin, or HV toxin Victorin, produced by fungus Cochliobolus victoriae appeared in 1945 on Victoria oats and related oats
Victoria oats contains a gene for resistance to the disease crown rust
Cochliobolus victoria infects the basal portion of susceptible oats, produces a toxin causes leaf blight and death
All other oat species are immune
48. Victorin, or HV toxin The primary target of the toxin is the cell plasma membrane where victorin seems to bind to several proteins
49. T-Toxin (Cochliobolus heterostrophus Race T Toxin) (Helminthosporium)
The cause of southern corn leaf blight
Appeared in U.S. in 1968
By 1970 attacked all corn with Texas male sterile cytoplasm
Corn with normal protoplasm is resistant
Controlled by a gene acts on mitochondria
Inhibits ATP synthesis
50. HC- ToxinCochliobolus carbonum Causes leaf spot disease in maize
Host specific, only affect specific lines
Resistant corn has a gene (Hm1) that reduces and detoxifies the toxin
Susceptible corn lines dont have the gene
51. AM-ToxinAlternaria alternaria Alternaria leaf spot of apple
Toxin is a mixture of three forms
Extremely selective as to variety
Causes plasma cells to crease and lose electrolytes
Also causes loss of chlorophyll
52. Other Host-Specific Toxins HS-toxin by C sacchari = sugarcane
ACL-toxin by Alternaia citri = lemon
ACT-toxin same =tangerine
AL-toxin by A.a lycopersici = tomato
AF-toxin on strawberry
AK-toxin on Jap. Pear
AT-toxin on tobacco
53. Other Host-Specific Toxins PC-toxin by Perconia circinata = sorghum
PM-toxin in corn
CC-toxin by corynespora cassiicola on tomato
Species rather than host specific
Hypoxylon mammatum on poplar
Perenophora teres on barley
54. Growth Regulators in Plant Disease Plant growth is regulated by hormones called growth regulators
Auxins
Gibberellins
Cytokinins
Ethylene and growth inhibitors also play a roll
Present in very small amounts
55. Growth Regulators in Plant Disease Appear to act by promoting synthesis of RNA molecules which control plant growth
Plant pathogens may produce same regulators as plants
May stimulate or retard growth
Cause plant imbalance that results in abnormal responses such as broom
56. Auxins IAA indole-3-actetic acid
Move from young growth to old
Constantly destroyed by IAA oxidase
Required for cell elongation and differentiation , affect permeability of membranes
Increase of respiration, RNA, proteins enzymes, and structural proteins
57. Auxins Increased levels caused by fungi, bacteria, viruses, mollicutes, and nematodes i.e.
Ustilago maydis = corn smut
Plasmodiophora brassicae = cabbage
Phytophthora infestens = late blight potato
Gymnosporangium j-v = cedar apple rust
Fusarium oxysporum = banana wilt
Meloidogyne sp = root knot nematode
58. Auxins There are many effects of IAA
Required for cell elongation
Differentiation
Membrane permeability
Increases respiration
Promotes synthesis of messenger RNA
59. Auxins Plants infected by fungi, bacteria, viruses, mollicutes, and nematodes generally have an increase in IAA levels while a few show a decrease
60. Auxins Some increase IAA by inducing degradation of IAA oxidase, the enzyme that degrades IAA
Corn smut and stem rust of wheat
Pseudomonas solanacearum induces 100 x increase in IAA levels
61. Auxins Crown gall = Agrobacterium tumefaciens effects over 100 species
The bacteria attach to the outside
The bacteria do their own gene splice with the plant cells and transform normal cells to tumor cells which grow at a rapid rate
Once the cell is infected it divides on its own
62. Auxins The integrated T-DNA also contains genes for a substance called opines
Opines can be only used for food by the crown gall bacteria
In Pseudomonas savastanoi, knot disease of oleander, olive, privet
The more IAA a strain produces, the more severe the symptoms
63. Gibberellins Gibberellins are naturally produced by plants and several micro organisms
They normally promote growth in dwarf varieties
Promote flowering, stem and root elongation and growth of fruit
Gibberella fujikuroi causes foolish disease of rice
Induces IAA formation
64. Gibberellins Gibberellins can be applied to reverse dwarfing diseases in some plants
It is not known if the dwarfing of plants is caused by a reduction of gibberellins in diseased plants
65. Cytokinins Potent growth factors for cell growth and differentiation
Inhibit the breakdown of proteins and nucleic acids
Directs the flow of amino acids to areas of high cytokinin concentration
Act by turning genes on and off and;
By activating genes that were turned off
66. Cytokinins Cytokinin activity increases clubroot galls, crown root galls, smut, rust galls, and rust infected bean leaves
Partly responsible for several bacterial galls
Treating infected plants with kinetin before or shortly after inoculation with a virus reduces the number of lesions
67. Ethylene:CH2==CH2 Effects, naturally produced in plants;
Chlorosis
Leaf abscission
Epinasty
Stimulation of adventitious roots
Fruit ripening (never ship apples with bananas)
68. Ethylene:CH2==CH2 May play a role in increasing plant resistance to infection
Pseudomonas solanacearum, ethylene content increases with early yellowing of fruit
Implicated in leaf epinasty symptom of vascular and wilt syndromes
69. Polysaccharides The role of slimy polysaccharides is important in wilt diseases caused by pathogens that invade vascular systems
Causes mechanical blockage of vascular bundles when;
Combined with substances released by breakdown causes the blockage
70. Suppressors of Plant Defense Responses Some pathogens such as Puccinia graminis f.sp. Tritici (stem rust of wheat), and Mycosphaerells pinodes ( leaf spot on pea) produce suppressors that suppress defense responses in plants
Mycosphaerells suppressors reduce protein-pumping activity of the host cell membrane lowers ability to function and defend itself (last slide)