PLANT-MICROBE INTERACTIONS

PLANT-MICROBE INTERACTIONS

1. Objectives

   • How do pathogens cause disease?
   • How do plants defend themselves from pathogens?

2. How Pathogens Attack Plants

   • 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.
     • Example of pathogens
       • Fungi
         • Use mechanical pressure to penetrate the host surface
           • appressoria of fungi
           • haustoria of fungi
         • Produce cell wall-degrading enzymes
         • Example: Rust fungi, Colletotrichum
       • Nematodes
         • Penetration through stylet
         • Second stage larva 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.
     • Substances secreted by pathogens
       • Extracellular Polysaccharide production (EPS)
         • Important in wilt diseases caused by pathogens that invade the vascular system of the plant.
         • Causes a mechanical blockage of vascular bundles, thus initiate wilting.
       • Plant growth regulators
         • Auxins
           • Indole-3-acetic acid (IAA)
           • High conc. changes normal cells to tumour cells
         • Gibberelin
           • The fungus causes “foolish seedling” disease of rice
           • Fungus produces gibberellin in infected tissues
           • Causing the plants grow abnormally tall and fall over
           • Used to improve fruit set and size
           • Ex disease: Bakanea disease
         • Cytokinins
           • Zeatin, isopentenyl adenosine (IPA)
           • High conc. causes development of galls
           • E.g.: crown galls, clubroot galls
         • Abscisic acid
           • High conc. causes stunted growth
           • E.g.: mosaic diseases
       • Toxins
         • Many pathogens produce toxins which damage host tissues and aid in disease development.
         • Injure host cells by
           • affecting the permeability of cell membrane
           • Inactivating or inhibitning enzymes
         • Toxin production
           • Non host-specific toxins (substance which is toxic to both host and non-host plants)
             • Phaseolotoxin
               • Produced by the bacterium Pseudomonas syringae pv. phaseolicola
               • Halo blight of bean & legumes
               • Causes chlorosis in both hosts and non- hosts.
             • Cercosporin
               • Produced by Cercospora sp.
               • Causes leaf spot & blight diseases
               • toxic due to generation of activated oxygen species
           • Host-specific toxins (substance which is toxic only to the hosts of the pathogen with little or no activity on non- susceptible plants)
             • Victorin
               • Produced by the fungus Cochliobolus victoriae
               • Causes leaf blight on oat variety Victoria
               • Damages cell wall structure causing loss of electrolytes from cells
             • T-toxin
               • Bipolaris maydis race T produces a host-specific toxin affecting only corn with Texas Male-Sterile cytoplasm.
               • Causes southern corn leaf blight
       • Cell wall degrading enzymes (CWDE)
         • PECTINASE
           • dissolve pectic substances
           • mostly produced by soft-rot pathogens
           • degrade: pectin, a polysaccharide found in plant cell walls.
           • mainly in fruits and vegetables
           • pathogen
             • fungi : Aspergillus niger.
         • CELLULASE
           • degrade cellulose (plant cell wall)
           • mostly produced by rot pathogen
           • mainly in plantation crops, perennials tree
           • pathogen
             • fungi, bacteria, and protozoans that catalyze cellulolysis
               • Ex: cellulomonas flavigena, terendinibacter turnerae
         • CUTINASE
           • mostly produced by rot pathogen
           • mainly in plantation crops, perennials tree
           • pathogen
             • fungi, bacteria
               • Ex : venturia inaequalis, Cochliobolus heterostrophus, Rhizoctonia solani
           • degrade: cutin (polyester composed of hydroxy and hydroxyepoxy fatty acids)
         • LIGNINASE
           • degrade : dissolve lignin (middle lamella & secondary cell wall of xylem vessel
           • mainly in plantation crops, perennials tree
           • mostly produced by rot pathogen
           • pathogen
             • fungi and bacteria that catalyze the breakdown of lignin
               • Ex: pseudomonas fluorescens, Fusarium solani

3. Effect of pathogen infection distrup in

   • Photosynthesis
   • Translocation of water & nutrients
   • Transpiration
   • Respiration
   • Permeability of membrane cells

4. Resistance mechanisms

   • Structural defenses
     • Pre-existing defense structures
       • Thick layers of wax and cuticle covering epidermal cells
       • Lignified cell walls
       • Stomata characteristics - smaller stomata
     • Induced defense structures
       • Cork layers
       • Tyloses form in xylem vessels to limit colonization by vascular wilt pathogens
       • Wall appositions (papillae)
   • Biochemical defenses
     • Pre-existing biochemical defenses
       • Proinhibitins – constitutive antimicrobial compounds
       • Fungitoxic exudates from leaves or roots
       • E.g.: protocatechuic acid & catechol (phenolic compounds) produced by onion inhibit the germination of conidia of the fungus Collectotrichum circinans
     • Induced biochemical defenses
       • Pathogenesis-related proteins (PR)
       • Phenolic compounds
       • Phenol-oxidizing enzymes
       • Phytoalexins