Clase 3 by HC11121318410

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									              Modelo gen por gen
           La reaccion hipersensitiva.
                      8 de Julio 2009


Leonardo De La Fuente
Department of Entomology and Plant Pathology
Auburn University
Auburn, Alabama, USA
                              Gene-for-gene concept
• For each gene conferring virulence to a
  pathogen, there is a corresponding gene in
  the host conferring resistance
                                            Resistance or susceptibility genes in plant
Virulence or avirulence
genes in the pathogen




                                          R (resistant) Dominant   r (susceptible) recessive
                          A (avirulent)           AR (-)                     Ar (+)
                          Dominant
                          a (virulent)            aR (+)                     ar (+)
                          Recessive


                                  (+) Susceptible, compatible reaction: disease
                                  (-) Resistant, incompatible reaction: no infection
        Gene-for-gene theory

• H.H. Flor. USA. 1955.
• Working with flax and
  flax rust (Melampsora
  lini)
• North Dakota State
  University (Fargo, ND)
 Implications of gene-for-gene theory
• Recognition avrA product (elicitor) with R
  gene (receptor) cause Hypersensitive
  Reaction (HR)

• Breeder can add new R genes in host that
  recognize avr in pathogen

• Why pathogens wants “avirulence” genes? Avr
  genes are virulence genes that when
  recognized by R gene trigger avirulence
    The Hypersensitive Reaction (HR)
• Localized, induced cell
  defense in host plant at
  the site of infection by a
  pathogen
• Cascade of defense
  responses by affected
  and surrounding cells.
  Release of toxic
  compounds
• Death of plant cells to
  stop pathogen
                               Plant Pathology. G.N. Agrios. 2005
                  HR history
• First described with bacteria by Klement et al.,
  1964
• Developed inoculation technique enabling
  infiltration
• Inoculated 22 Pseudomonas species or pvs in
  tobacco (control P. syringae pv. tabaci
  (compatible) and 5 saprophytic species
• Concentration about 106 cells/ml induce tissue
  necrosis 12-24h
• High concentrations: 108, 109 cells/ml induce
  weak chlorosis with saprophytic species
                     Artificial HR
• Infiltrate bacteria into leaf of non-host plant
• Large sector become water-soaked first, then
  necrotic, collapse after 8-12h

• Bacteria injected are trapped in necrotic lesions
   and kill rapidly
• Occur when: - virulent bacteria into nonhost or
   resistant variety
  - avirulent strain into susceptible cultivar
      Interactions plant-bacteria

• Bacteria in plant surface: no interactions with
  epidermal cells under cuticle.
• Bacteria needs to enter into intercellular
  spaces (flagella, vascular system)
• Saprophytic bacteria: do not cause HR nor
  pathogenesis. Do not multiply inside plant
  cells (rarely 10% increase in P. fluorescens)
                 Role of EPS
• Intercellular space: air in natural conditions
• Plant cell surface: water film
• Bacteria absorb water by EPS
• Pseudomonas and Xanthomonas typically
  cause water soaked lesions (high water
  content in intercellular spaces)
       Multiplication of bacteria
P. syringae pv. phaseolicola inoculate 103/cm2:
  • In beans after 5 days: 107 CFU/cm2
  • In cherry after 5 days: 105 CFU/cm2

Doubling time:
 • Host/pathogenic bacteria: 2.3h
 • Host/non pathogenic bacteria: 5.7h
 • Non host/pathogenic bacteria: 9.7h
    Living bacteria required for HR
• Need RNA and protein synthesis
• Ercolani (1970): use histidine-requiring mutant of P. s.
  pv. syringae in non-host tomato
• HR develops only after histidine injection
• Increase in RNA synthesis (increase uptake Uridine-H3)
• “Induction period”: needs living bacteria only at the
  beginning (varies between 30min to 4h)
• Detected by adding antibiotics at different time
  intervals
• Time needed for hrp gene expression
              Events during HR
• Rapid burst of ROS (Reactive Oxygen Species)
• Increase oxidative reactions
• Increase ion movements through membranes (K +, H+)
• Membrane disruption
• Cellular compartmentalization
• Strengthen cell walls (cross-linking phenolic
  compounds with cell wall components)
• Activation of protein kinases
• Antimicrobial substances (Phytoalexins)
• PR proteins (Chitinases)
      HR events




Plant Pathology. G.N. Agrios. 2005
   Signal transduction between
pathogenicity and resistance genes
Regulation by network of interconnecting signaling
  pathways:

•   Salicylic acid (SA)
•   Jasmonic acid (JA)
•   Ethylene (ET)
•   Nitric Oxide (NO)

• Exogenous applications of SA, JA, ET or NO results in
  increased resistance
Overview cascade of reactions:
 Systemic acquired resistance




                     Vlot et al., 2008
   Overview cascade of reactions
• Salicylic acid (SA) reacts with plant proteins
  (catalase, ascorbate peroxidase) with
  antioxidant activity
• SA triggers PR genes, activate JA, ET pathways
  leading to defense
• NO increase during inoculation of resistant
  plants. Induce expression of PR-1 and early
  defense gene PAL (Phenyl Alanine Lyase)
   Overview cascade of reactions
• NO inhibits activity of enzymes aconitase,
  catalase and ascorbate peroxidase.
• SA in some plants required for maintenance of
  SAR
• HR based on interplay and mutual positive
  feedback of ROI (reactive oxygen intermediates)
  and SA-dependent signals
• ROI and NO collaborate to initiate HR
• Balance between H2O2 and NO is required for HR
Systemic acquired resistance (SAR)
• Secondary resistance induced after HR to avirulent
  pathogen.
• Long-lasting, broad spectrum disease resistance
• Spread systemically
• Signal generated after 4-6h. Salicylic acid (SA) detected
  in phloem. After 24h all the plant has SA
• Plants transformed with nahG (salicylate hydroxylase)
  cannot accumulate SA and cannot express SAR
• PAL (involved in SA synthesis) suppressed in plants,
  more susceptible to infection
• External application of SA induces resistance to disease
           Programmed cell death
• HR
• Interaction elicitor:receptor leads to ion flux. Results in
  alkalinization of apoplast, ROI, NO, signaling cascades
• Activate signals that will trigger SAR at distal parts of the
  plant
• Extent of cell death: one or tens
• SA, JA, ET, NO regulates this and also cell growth
• Lesion simulating disease allele (lsd1). Mutant of this gene
  are unable to stop spread of cell death once it started.
• Cell death however does not spread beyond the treated
  leaf.
        Programmed cell death
• Lesions form as accumulation of extracellular
  superoxide (O2-), therefore death is initiated
• Leads to superoxide formation in live
  neighboring cells, which leads to further
  superoxide and runaway cell death
• Necrotrophic fungi (B. cinerea) mimics HR
  signal: release H2O2 or O2- (SOD converts it to
  H2O2): stimulates HR
                 Summary
• SAR equivalent to “Immunization” of plants

• Not known how plants integrate signals by
  different pathways into specific responses

• Defense response depends on cross talk
  among SA, JA, ET and NO signal pathways

								
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