Ch 6 - Genetics of Plant Disease

1. Genetics review
   
   
   • Prokaryotes (bacteria and mollicutes)
     
     
   • Eukaryotes
     
     
   • Genetic information is coded for in a linear fashion, and copied or transcribed in a directional manner
     
     
   • four possible bases in DNA:
     
     A = adenine
     T = thymine
     G = guanine
     C = cytosine
     
     
     gene is a stretch of DNA (100-150 triplets) -------> transcription -------> ribosomes, tRNA, and mRNA -------> translation -------> protein
   
   
   
2. Genes and Disease
   
   
   • Pathogens capable of inducing disease usually do so on one or a few related plant species.
     
     
   • Definitions from J.G. Manners.1982. Principles of plant Pathology. Cambridge University Press, Cambridge.
     
     
   • pathogen -
     
     
   • virulence -
     
     
   • Most plants are resistant to most pathogens (non-host); susceptible (host) plants have a specific genetic composition that conditions susceptibility - the degree of susceptibility is governed by the virulence in the pathogen and the resistance on the host plant
   
   
   
3. Incorporation of resistance in crop plants - and selection for virulence in pathogen populations
   
   
   • host -
     
     
   • pathogen -
     
     
   • genetic recombination-
     
     
   • selection pressure -
     
     
   • de novo synthesis (mutations) - when individuals reproduce asexually there is a reduction in the frequency and degree of variability among the progeny
     
     
     • spontaneous mutation rate is 1 x 10^-5 to 1 x 10^-8
       
       
     • stripe rust - 200,000 spores per pustule
       • 20 pustules per plant
       • 1 million plants per acre (irrigated wheat)
       
       
       
     • potential number of mutations = ?
     
     
     
   • 10⁶ plants x 20 pustules/plant x 200,000 spores x mutation rate of 1 x 10^-6 = 4,000,000 CHANCE mutations alone
   
   
   
4. Mechanisms of Variability
   
   
   1. Mutation -
      
      
      • substitution -
        
        
      • deletion -
        
        
      • inversion -
        
        
      • amplification to multiple copies
        
        
      • transposable elements - movable, mobile DNA segments that are excised and reinserted randomly into the genome
        
        
      • most mutations are recessive and require genetic recombination for expression
        
        
      • cytoplasmic inheritance - genetic mutations in cellular organelles does occur, but does not follow the traditional rules of Mendelian inheritance
      
      
      
   2. Recombination - primarily through sexual reproduction (plants, fungi, nematodes) as a result of genetic crossovers during meiosis
      
      
      • haploid (1N) nuclei -------> zygote (2N) -------> meiosis produces new haploid cells
        
        
      • recombination can also occur in mitotic cell division esp.of fungi
      
      
      
   3. Specialized Mechanisms of Producing Variability in Pathogens
      
      
      • Sexual-like Processes in Fungi
        
        
        1. heterokaryosis - the condition in which, as a result of fertilization or anastimosis, cells of fungal hyphae (or parts of hyphae) contain two genetically distinct types of nuclei
           
           
           • hyphae are then called dikaryons, are in a dikaryotic condition -
             
             
           • anastomosis -
             
             
           • dikaryon vs heterokaryons
           
           
           
        2. parasexualism - the process by which genetic recombination occurs in fungal heterokaryons
           
           
        3. heteroploidy - the existence of cells, tissues, or whole organisms with numbers of chromosomes that are different from the normal 1N or 2N complement of that organism
           
           
           • haploids, diploids, triploids, or tetraploids
             
             
           • aneuploids - having one, two, three or more extra chromosomes from the normal euploid number
             
             an organism whose somatic chromosome number is not an even multiple of the haploid number - R.W. Allard. 1960. Principles of Plant Breeding. John Wiley & Sons. USA.
             
             
           • ploidy level can affect growth rate, spore size, rate of spore production, hyphal color, enzyme activities, and pathogenicity
             
             
           • the normally haploid Verticilliun albo-atrum (causes wilt in cotton) loses pathogenicity when it becomes diploid
        
        
        
      • Sexual-like Processes in Bacteria (and other mollicutes?)
        
        
        • Figure 6-5 in text
          
          
        • Conjugation -
          
          
        • Transformation -
          
          
        • Transduction -
          
          horizontal gene transfer
        
        
        
      • Genetic Recombination in Viruses
        
        Inoculation of a single plant with two strains of the same virus results in genetic recombination
        
        
      • Loss of pathogen virulence in culture
        
        Prolonged growth in culture or the passing of pathogens through several different hosts can result in attentuation - partial or complete loss of virulence
      
      
      
   4. Stages of Variation in Pathogens
      
      
      • Species - made of a population of an organism that retains certain morphological and phenotypic characteristics in common
        
        
      • Puccinia graminis -
        
        
      • individuals that are capable of infecting certain host species are designated as formae specialis
        
        P. graminis f. sp. tritci - infects wheat
        P. graminis f. sp. hordei - infects barley
        
        
      • races - variants in the formae specialis that will only infect certain varieties of the host plant
        
        
      • variants -
        
        
      • biotype - identical individuals produced asexually by the variant
   
   
   
5. Types of Plant Resistance to Pathogens
   
   
   1. Non-host resistance -
      
      
   2. Host-resistance -
      
      also called true resistance -
      
      
      1. horizontal resistance -
         
         synonyms:
         
         minor gene resistance -
         
         nondifferential resistance - the differences in resistance between varieties are usually not significantly different and incapable of differentiating between plant varieties
         
         
      2. Vertical resistance -
         
         synonyms:
         
         always controlled by one or a few R genes - control a major step in the recognition of the pathogen by the host plant and therefore play a major role in the expression of resistance
         
         types of resistance:
         
         pyramiding resistance genes -
         
         In general, incorporating monogenic, vertical, resistance on a base of horizontal resistance is optimum when breeding for resistance.
         
         
      3. Apparent resistance -
         
         disease escape - occurs when genetically susceptible plants do not become infected -
         
         is often entirely controlled by the environment managing cultural methods to increase disease escape:
         
         tolerance -
   
   
   
6. Genetics of Virulence in Pathogens and of Resistance in Host Plants
   
   Resistance/susceptibility of a plant species and pathogenicity of a pathogen species is complex and multigenic
   
   In some diseases, virulence/avirulence (pathogen) and inherited resistance (plant) CAN be regulated by single-gene interactions
   
   in plants - the presence of an R gene = resistance
   - the absence = susceptiblity
   
   in pathogens - the presence of an avirulence gene = no infection
   - the absence of an avirulence gene = infection (virulence)
   
   1. The Gene-for-Gene Concept
   
   The stepwise evolution of virulence and resistance where for each gene that confers virulence in the pathogen there is a corresponding gene in the host that confers resistance to the pathogen, and vice versa.
   
   Has been demonstrated in many fungal pathogens, as well as some diseases caused by bacteria, viruses, parasitic higher plants, and nematodes.
   
   Quadratic Check
   
   Host - resistance is dominant = R and r = "universal suscept"
   
   Pathogen - virulence is recessive = a and mutations are generally a switch from A to a
   
   With Host R and Pathogen A = resistant
   With Host R and Pathogen a = susceptible
   With Host r and Pathogen A = susceptible
   With Host r and Pathogen a = susceptible
   
   This gene-for-gene system was first postulated and identified in the 1950's by H.H. Flor, using the flax-rust (Melampsori lini) pathosystem
   
   
7. Breeding of Resistant Varieties
   
   Is actually, the most environmentally safe, and cheapest method to control disease in agricultural situations where genetic uniformity of a large number of individuals is required.
   
   Selection of crop plants initially was selection by the local growers for the highest yielding plants that were genetically adapted to each individual location and resistant to the local pathogens. Heterogeneity was maintained throughout a region.
   
   Breeding for characteristics that make a crop cultivatable also decreases genetic variablity:
   
   Incorporating Genetic Variability - or generating variablity
   
   obtaining genetic variability from wild related species
   
   generating mutations
   
   using genetic engineering - introducing spliced or engineered DNA segments
   
   tissue culture of somatic cells
   
   diploidization of haploid lines
   
   Sources of resistance:
   
   
8. Classical Breeding Techniques
   
   -the method depends on the type of pollination - open or self-pollinated
   
   Seed Selection - mass selection of seed from the most highly resistant plants surviving in the field
   
   Pedigree (Pure Line) Selection - individual highly resistant plants and their progenies are selected and propagated
   
   Recurrent Selection - a susceptible variety is crossed with a resistant variety. The progeny are planted and selected heavily each year for the desired characteristics.
   
   Backcrossing - selection of the resistant individuals that are then repeatedly crossed back to the agronomically desirable variety
   
   
9. Breeding for resistance using tissue culture and genetic engineering techniques
   
   Tissue culture - in itself generates mutant lines - (somatic), but also allows for the rapid propagation of resistant lines
   
   Isolation of Disease Resistant Mutants from Plant Cell Cultures
   somaclonal variants
   
   Production of Resistant Dihaploids from Haploid Plants
   
   • generation of haploid plants from pollen cells or megacells
     
   • treatment with colchicine results in diploidization
   
   
   Increasing resistance by protoplast fusion - by closely related or even non-closely related species
   
   Genetic transformation of Plant cells for disease resistance
   
   • small or large pieces of DNA are introduced into plant cells or protoplasts
     
   • success has been achieved with microprojectile bombardment and the Agrobacterium system
   
   
   
10. Advantages and Problems in Breeding Vertical or Horizontal Resistance
    
    Vertical resistance - complete resistance to some races of a pathogen, but not to all races
    
    Horizontal resistance - partial resistance equally effective against all pathogen races.
    
    Multilines -
    
    Pyramiding -