Effect of Pathogen on Plant Physiology - PDF

Effect of pathogen on the Physiology of host plant Dr. J. Joel Gnanadoss  Plant physiology is the science which deals with the life processes of plants, or it deals with the functions of cells, tissues, organs or the plant as a whole.  Plant disease is an outcome of interaction between host a...

Effect of pathogen on the Physiology of host plant Dr. J. Joel Gnanadoss  Plant physiology is the science which deals with the life processes of plants, or it deals with the functions of cells, tissues, organs or the plant as a whole.  Plant disease is an outcome of interaction between host and pathogen.  Interaction begins with physical contact and effects remain localised until host tissues have been penetrated.  Depending on pathogen and the plant organ and tissue they infect it interfere with the different physiological functions of the plant.  Harmful effects of infection on physiology of the host are the main cause of symptom expression, damage and loss. Some harmful effects are- i) Disintegration of tissues ii) Effect on growth due to hormonal imbalance iii) Reduction in photosynthesis iv) Abnormal respiration  Pathogens infecting flowers, interfere with the seed production of plants.  Root pathogens affect translocation resulting in its wilting and death of plant.  Foliar pathogen- Infect the part of the leaves ,stem ,petiole and other aerial parts. Destroy chlorophyll formation leading to reduced photosynthesis Effects growth and yield of the plants Changes in the cell biology- Striking increase in protoplasm. Movement of nucleus at site of penetration Granulation of cytoplasmic particles. Cell contents becomes yellow and finally dark brown leading to death of cell.  Photosynthesis  Respiration  Translocation of water and nutrients  Transpiration  Permeability of the cell membrane  Transcription and Translation  Photosynthesis is the basic function of green plants.  It transform light energy into chemical energy, which is utilized in all cell activities. Chlorophyll  6CO2+6H20 Light C6H12O6+ 6O 2  Any interference of pathogen with photosynthesis result in plant diseased condition.  Once the pathogen is established in the host colony it redirects the host nutrients for their own use. This condition is known as metabolic sink.  Effect on photosynthesis  In leaf spots, blight, and other diseases in which there is destruction of leaf tissue e.g., in cereal rusts and fungal leaf spots, bacterial leaf spots, viral mosaics, yellowing and stunting diseases, or in defoliations, photosynthesis is reduced.  Chlorosis, necrosis and reduced growth and yield.  Reduce amount of photosynthetic surface  Chlorophyll degeneration  Produce toxins that inhibit enzyme Eg; tentoxin, tabtoxin  Vascular pathogens reduced the chlorophyll and photosynthesis rate.  Stomata remain partially closed Leaf spot of barley by Leaf spot of egg plant by Wheat leaf rust caused by Mosaic of mung bean Rhynchosporium sp. Cercospora melongenae Puccinia recondita by mung bean mosaic virus Angular leaf spot of cucurbits Yellowing and stunting of Leaf spot caused by Alternaria by Pseudomonas syringae rice by Tungro virus sp  Respiration is the process by which the cells, through enzymatic oxidation of organic materials(energy rich carbohydrates and fatty acids), produces energy for various activities and carbon skeletons.  In plants the process of respiration takes place in two major steps: 1.Glycolysis or EMP pathway 2.The Terminal phase GLUCOSE GLYCOLYSIS OR COMMON TO BOTH AEROBIC AND EMP PATHWAY ANAEROBIC RESPIRATION PYRUVIC ACID ANAEROBIC RESIRATION AEROBBIC RESPIRATION Acetyl CoA C2H5OH+CO2+ENERGY CO2+H2O+ENERGY  Infected plant shows increased respiration rate and slight rise in temperature.  Affected tissue use up their reserve carbohydrates faster than the healthy tissue.  Increase in respiration starts soon after inoculation and rises to a maximum rate coincident with the sporulation of a fungal pathogen and then declines to normal or subnormal level.  In resistant plants the rate of increase in respiration is very rapid but declines soon while in susceptible plants respiration rises slowly but lasts for longer time.  Increase in plant metabolism and protoplasmic streaming.  Increased respiration has been noticed in cereal rusts, powdery mildews, blast of rice, late blight of potato and many other diseases.  Levels of many enzymes associated with respiratory processes is increased in the diseased plants.  Accumulation and oxidation of phenols also increases with increase in respiration.  Diseased plants carry out more fermentation than the healthy plants. Blast of rice by Pyricularia grisea Late blight of potato by Phytopthora infestens Powdery mildew of soybean by Microsphaera diffusa  EFFECT ON UPTAKE AND TRANSLOCATION OF WATER AND NUTRIENTS XYLEM Water and minerals absorbed by Roots Translocated through the stem xylem vessel to vascular bundle of petioles and leaf veins then enter into the leaf cells Most of the water evaporates and diffuses into the atmosphere via the stomata Results in water tension in the xylem Pulls the water column upward Stimulates additional uptake  Pathogens Interfere with Normal Translocation Process Flow of nutrients or water blocked Disease in affected cells/tissues Inability to carry out functions Shortage of their products Disease in entire plant Pathogens Impair Translocation of Water and Inorganic Nutrients 1. Root damage – Damping-off fungi –ex-Pythium sp., Rhizoctonia sp. root-rot fungi, bacteria, nematodes and some viruses. Mechanisms of damage- i) Cause direct injury to roots ii) Inhibit root hair production iii) Alter permeability of root cells 2. Xylem gets destroyed –i) Rot or canker pathogens – E.g-Sclerotium rolfsii ii) Gall formation – E.g-Agrobacterium tumefaciens, Plasmodiophora brassicae, root knot nematode(Meloidogyne sp.) 3. Xylem gets clogged –i) Growth of vascular wilt pathogens E.g- Ceratocystis, Ophiostoma, Fusarium, Verticillium, Ralstonia ii) Secretions of tylose by plant in response to the pathogen. Blight of pepper by Phytopthora sp. Wilt to tomato by Ralstonia solanacearum Seedling damping off by Pythium sp. Wilt of tomato by Fusarium sp.  Crown gall of apple by Root galls by Meloidogyne sp. Agrobacterium tumefacience  PHLOEM  Carbohydrates produced in leaf cells move through plasmodesmata into adjoining phloem elements  Move down phloem sieve tubes  Move into non-photosynthetic cells or into storage organs  Nutrients are removed from ‘Circulation’.  How Can Pathogens Impair Translocation in the Phloem?  Pathogen attacks and destroys phloem elements interferring with downward translocation of nutrients.  Starch accumulation in the leaves is a result of degeneration of the phloem of infected plants (leaf curling viruses).  Obligate fungal parasites (eg: Rust and Mildew) – Respiration increases – photosynthesis decreases.  Viruses – Viruses that cause leaf-curling e.g. tomato leaf curl virus and some yellows diseases cause death of phloem  Some viruses inhibit enzymes that break down starch into smaller, trans-locatable molecules. Transpiration is the process of water movement through the plant and its evaporation from aerial parts, such as leaves, stems and flowers. Effects of pathogens on transpiration  Disintegration of cuticle  Increased transpiration  Disfunctioning in permeability of leaf cells  Collapse of vessels or formation of tyloses  Powdery mildew- epidermal cells invaded by the fungus  Rust- leaf surface get exposed due to rupture of epidermis cause unrestricted loss of water  Blight- number of active and healthy cells reduced, transport of water to leaves by xylem is reduced  Wheat leaf rust by Powdery mildew of grapes by Puccinia recontida Uncinula necator Tomato leaf blight by Septoria sp. Scab of apple by Venturia inequalis  Cell Membrane Double layer of lipid molecules Protein molecules embedded – Parts usually protrude on one or both sides of lipid bilayer Membranes are selective barriers – Allow entry of substances the cell needs – Inhibit passage out of the cell Cell wall keeps large molecules away from membrane Small water-soluble molecules such as ions, sugars and amino acids flow through or are pumped through membrane channels (proteins)  Effect on cell membrane  Disruption of the cell membrane alters permeability – uncontrollable loss of useful substances and entry of harmful substances.  Membrane permeability is often the first detectable response of cells to infection  Leakage of electrolytes o Unknown if membrane leakage is primary or secondary effect. o If direct attack, pathogens probably use one of the following strategies Stimulate membrane-bound enzymes that pump H+ in and K+ out Interfere with maintenance and repair of membrane’s fluid film Degrade the lipid or protein components of membrane E.g.- i. Fusicoccin produced by Fusicoccum amygdali-activates membrane ATPase and disrupts solute fluxes. ii. Sclerotium rolfsii and Botrytis cinerea-produces lipid degrading enzymes. iii. Rhizoctonia solani-degrade protein  Transcription – Copying coding on DNA onto messenger RNA Translation – Use of messenger RNA as a template to produce proteins  Disturbances of any of the processes may affect expression of genes and cause drastic, unfavourable changes in structure and function of affected cells.  EFFECT ON TRANSCRIPTION  Some pathogens change composition, structure or function of chromatin associated with cell DNA  Virus uses host cell’s nucleotides and machinery to make its own RNA  Sometimes increase in activity of enzymes that break down RNA  Pathogen may be forcing plant to produce new kinds of enzymes not produced in healthy plants  Higher levels of RNA in infected plants, especially resistant ones probably due to increased synthesis of substances involved in defence mechanisms.  In biotrophic infections(e.g. Plasmodiophora brassicae,cabbage; Fusarium solani,pea; powdery mildew, barley; rust disease,wheat) host nuclei and nucleoli increases in size. EFFECT ON TRANSLATION  Higher energy needs – Increased activity in enzymes associated with respiration  Increased production of phenols or oxidation of phenolic compounds – Used in defence reactions  Resistant plants have increased protein synthesis in first few minutes of infection – Levels remain high up to 2 to 20 hours after inoculation  CONCLUSION  Human and animal survival depend on plant growth.  Agriculture, the world’s biggest business, is based on ability of plants to grow and convert simple raw material to complex substances in order to suit human needs.  The biochemical pathways in a plant are very highly integrated an inter- dependent and any disruption represents a loss of control by the host.  Even small and localised changes may have drastic consequences to the plant as a whole.  Pathogen effects plants by inducing changes in its physiological functions that leads to different types of symptoms and diseases.  This leads to the reduction in quality and quantity of plant products.  So, it directly or indirectly effects our crop economy.