Plants-Microbe Interactions

Plants-Microbe Interactions

1. Plants defend themselves

   • Structural defenses
     • Pre-existing defense structures
       • Wax and cuticle covering epidermal cells
       • Lignified cell walls
       • Stomata characteristics
     • Induced defense structures
       • Cork layers
       • Tyloses form in xylem vessels
         • limit colonization by vascular wilt pathogens
       • Wall appositions(papillae)
         • form in response to pathogens attempting to penetrate cells walls
         • response to pathogens attempting to penetrate cell walls
       • Hypersensitive Response (HR)
         • Rapid death of host cells at the infection site
   • Biochemical defenses
     • Pre-existing biochemical defenses
       • Proinhibitins – constitutive antimicrobial compounds
       • Fungitoxic exudates from leaves or roots
     • Induced biochemical defenses
       • Pathogenesis=related proteins (PR)
         • Toxic substances that slow down or stop the advance of the pathogen into host tissues
         • b-1,3-glucanases, chitinases
       • Phenolic compounds
         • Accumulate faster rate after infection, Toxic to pathogens
         • Chlorogenic acid, caffeic acid
       • Phytoalexins
         • Antimicrobial substances produced by healthy cells adjacent to necrotic cells
         • Inhibit pathogen growth
         • pisatin (pea), glyceollin (soybean), rishitin (potato)
       • Phenol-oxidizing enzymes
         • Oxidize phenolic compounds to quinones (more toxic)
         • polyphenol oxidases, peroxidases

2. Fungi cause disease

   • Mechanical forces
     • Some pathogens use physical force to penetrate plant tissues. Pathogens have evolved many mechanisms that enable them to colonize plant tissues and cause disease.
     • Fungi
       • Exp: Rust Fungi, Colletotrichum
       • Use mechanical pressure to penetrate the host surface-appressoria and haustoria of fungi
       • Produce tissue-degrading enzymes
     • Nematodes
       • Penetration through stylet
       • Second stage larvae of the root-knot nematode in the host root tissue
   • Chemical weapons
     • Pathogens produce a wide range of chemical factors which contribute to their ability to cause disease.
     • Enzymes
       • Cutinases
         • Fungal- Cochliobolus heterostrophus, Rhizoctonia solani
         • Rot
         • Plantation crops, perennial tree
         • Dissolve cutin in plant cuticle
       • Ligninases
         • Dissolve lignin
         • Plantation crops, perennial tree
         • Bacteria -Pseudomonas fluorescens
         • dissolve lignin (middle lamella & secondary cell wall of xylem vessels), mostly produced by basidiomycetes (white rot fungi)
       • Cellulases
         • Dissolve Cellulose (plant cell wall)
         • Rot
         • Plantation, crops, perrenial tree
         • Cellulomonas flavigena &Terendinibacter turnerae,
       • Pectinases
         • Dissolve pectic substance (middle lamella)
         • Soft rot
         • Fruits and vegetables
         • Erwinia spp.
     • Toxins
       • Damage host tissues and aid in disease development. Injure host cells by affecting the permeability of cell membrane or inactivating or inhibit enzymes.
       • Non host specific toxins
         • substance which is toxic to both host and non-host plants.
         • Cercosporin
           • Cercospora sp
           • leaf spot and blight diseases
           • Toxic due to generation of activated oxygen species
         • Phaseolotoxin
           • Bacterium Pseudomonas syringae pv. phaseolicola
           • Halo blight of bean & legumes
           • chlorosis in both hosts and non-hosts
       • Host-specific toxins
         • substance which is toxic only to the hosts of pathogen with little or no activity on non-susceptible plants
         • Victorin
           • Fungus Cochlibolus victoriae
           • Leaf blight on oat variety Victoria
           • Damages cell walls structure causing loss pf electrolytes from cells
         • T-toxin
           • Bipolaris maydis race T
           • affecting only corn with Texas Male- Sterile cyto plasm
           • causes southern corn leaf blight
     • Plant growth regulators
       • Auxins
         • Indole-3-acetic acid (IAA)
         • High conc. changes normal to tunour cells
       • Gibberelin
         • Fungi Gibberella fujikouri
         • produces gibberelin in infected tissues
         • Cause- plant grow abnormally tall and fall over
       • Cytokinins
         • crown galls, clibroot galls
         • High conc. causes development of galls
         • Zeatin, isopentenyl adenosine (IPA)
       • Abscisic acid
         • High conc. cause stunted growth
         • mosaic diseases
     • Polysaccharides
       • wilt disease
       • causes a mechanical blockage of vascular bundes
   • Disruption in
     • Translocation of water and nurients
       • due to disruption in roots & vascular system.
     • Photosynthesis
     • Respiration
       • higher rate due to disruption in ATP synthesis.
     • Permeability of membrane cells
     • Transpiration
       • higher rate due to loss of water.