CHAPTER 3 PST 2340 PLANT PATHOLOGY

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 don�t 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 can�t 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, PRP�s Glycine-rich proteins, GRP�s Solanaceous lectins Arbinogalactan proteins, AGP�s 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 plant�s 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 don�t 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)