Variability in Plant Pathogens Notes

Variability in Plant Pathogens

Learning Objectives

• Discuss the significance of variability in plant pathogens.
• Describe the different mechanisms of variation in plant pathogens.
• Discuss the gene-for-gene concept and its role in disease resistance.

General Concepts

• Plant pathogenic microorganisms, like other organisms, continually undergo changes.
• Shorter generation time and larger number of reproductive units formed in each generation increase the chances of producing genetic changes over time.
• Bacteria and fungi can grow and multiply rapidly.
  • Certain bacterial populations can double every 20 minutes under favorable conditions.
  • Fungi can form millions of spores within a few days.
• Changes in pathogens may involve an increase or decrease in pathogenicity, leading to the formation of new races capable of attacking new host varieties, or the loss of virulence in some races.

Definitions

• Biotype: A "population of life forms that are identical in all inheritable traits", making it genetically homogeneous. Hybridization between different biotypes results in a new group.
• Pathovar: A strain or group of strains at the infrasubspecies level in bacterial plant pathogens, characterized by identical or similar pathogenicity, symptoms/signs, and host range.
• Pathogenic Race: Made up of one or more biotypes with morphologically identical members. Different races attack different cultivars or host varieties.
• Formae Speciales: Special form based on the ability to attack different genera of crop plants. Example: Puccinia graminis.
  • Puccinia graminis f. sp. tritici attacks wheat only.
  • Puccinia graminis f. sp. avenae attacks oat only.
  • Each forma specialis may contain different races that attack different varieties of the host.
• Taxonomic Hierarchy: Species → Genus → Family → Order → Class

Factors Enhancing Pathogenic Race Development

• Absence of susceptible varieties.
• Presence of resistant varieties.
• Sexual reproduction of the pathogen.
• Obligate parasitism.
• Narrow host range.
  • These factors stimulate or "force" an existing race to change in order to survive.

Mechanisms of Variation

• Microorganisms undergo genetic changes through hybridization, mutation, and various sexual-like and asexual processes.

Variation in Fungi

• Mechanisms include mutation, heterokaryosis, parasexual process, cytoplasmic variation, and sexual reproduction.

Heterokaryosis

• Presence of different nuclei in the same mycelium.
• Each nucleus is independent, but the microorganism's behavior and phenotype are controlled by the genes present and their proportion.
• Heterokaryosis results from:
  • Anastomosis (fusion of genetically different hyphae).
  • Mutation in a multinucleate homokaryotic mycelium.
  • Fusion of haploid nuclei to form diploid nuclei, followed by their multiplication and spread among haploid nuclei.
• Nuclei in a heterokaryon are separated upon the formation of uninucleate asexual spores.
• Heterokaryosis produces new fungal strains or races with changes in pathogenicity.

Parasexual Process

• Observed in Aspergillus nidulans by Pontecorvo and Roper.
• Found in several plant pathogenic fungi like Fusarium oxysporum, Ustilago maydis, and Puccinia graminis.
• Mimics sexual reproduction as genetic recombination occurs, but it is mitotic recombination within vegetative heterokaryotic hyphae.
• Process:
  • Formation of a heterokaryon.
  • Karyogamy (diploidization of nuclei).
  • Multiplication of diploid nuclei.
  • Restoration to the haploid state.
    • Haploidization involves nondisjunction and the formation of deficient aneuploid nuclei, which undergo chromosome loss until a haploid nucleus is formed.
    • Mitotic crossing over at chromosome replication results in genetically different nuclei after segregation.

Mutation

• A discontinuous heritable change of the genetic material that may arise spontaneously or through mutagenic agents.
• Results from:
  • Structural changes in single sites or loci of DNA (point mutation) due to base pair substitutions during DNA replication.
  • Structural alterations in chromosomes caused by rearrangement or loss of chromosome segments (segmental mutation).
  • Abnormalities during meiosis and/or mitosis resulting in polyploids and aneuploids (genome mutation).
• Mutants may exhibit increased pathogenicity or resistance to commercial fungicides.

Cytoplasmic Variation

• The cytoplasm carries heredity determinants.
• Extrachromosomal particulate elements can multiply and be transmitted asexually through hyphal anastomosis or sexually.
• Include extrachromosomal factors in mitochondrial DNA, particulate plasmogenes in the cell's cytoplasm, and mycoviruses (mostly double-stranded RNA).
• Extrachromosomal hereditary elements significantly contribute to the variability of certain pathogens.
  • Resistance to mercury in Aspergillus glaucus and copper in yeasts are extrachromosomally inherited.
  • A mycovirus associated with Helminthosporium oryzae caused the fungus to produce a toxin responsible for abnormal root growth of infected rice seedlings.
• Southern corn leaf blight epidemic in the US in 1970 showed the role of cytoplasmic variation in plant susceptibility to disease.
  • The epidemic was due to the spread of the new Race T of Helminthosporium maydis.
  • cms-T cytoplasm in corn was very susceptible to this new race, while normal corn cytoplasms were resistant.
  • The susceptibility of plants with cms-T cytoplasm to race T of H. maydis is believed to be related to a pathotoxin formed by race T and its site of action on the plasmogenes.

Sexual Reproduction

• Results in hybridization, genetic recombination, and segregation.
• Hybridization requires the union of two different but compatible haploid nuclei.
• Nuclear fusion forms a diploid nucleus, which undergoes meiosis.
• During meiosis, segregation and recombination of genes occur.
• The resulting haploid nucleus may differ from either parent, receiving a varying combination of genes from each parent.
• Fungi that reproduce sexually produce variants through hybridization.

Variation in Bacteria

• Most variations are brought about by mutation and transformation.
  • Mutations may occur spontaneously, naturally, or through mutagens.
• Transformation: A sexual-like process where bacterial cells absorb and incorporate heritable genetic materials released by other bacteria.
  • The agent is believed to be DNA acquired from the growth filtrate of the donor bacteria.
• Other mechanisms include transduction and conjugation.
  • Transduction: A bacteriophage transfers genetic material from its former host bacterium to its next host.
  • Conjugation: Transfer of genetic material occurs when two compatible cells come into contact.

Variation in Nematodes

• Nematodes form new pathogenic races through hybridization and mutation.
• Pathogenicity varies according to the degree of resistance/susceptibility of the host plant.

Variation in Other Pathogens

• All pathogens reproducing sexually utilize hybridization as a means of variation.
• Recombination may occur if two or more virus strains are present in the same plant.
• Mutation is universal among organisms, including viruses and viroids.
• Variations are expressed through direct changes in pathogenicity, growth rate, longevity, host range, optimum temperature, or pH.

Genetics of Host-Parasite Interactions

• The development of infectious plant diseases depends on the resistance or susceptibility of the host plant and the virulence of the pathogen.
• The host and pathogen possess distinct genetic determinants.
• Plants have genes determining resistance or susceptibility, and pathogens have genes determining virulence.
• Various physiologic races of pathogens vary in their ability to attack a host variety, and different host cultivars vary in their response to a physiologic race.
• Plants and their pathogens have co-existed and evolved together in nature, maintaining a dynamic equilibrium of resistance and virulence for mutual survival.
• An unrestrained virulent pathogen would eliminate the host plant; a 'super-resistant' plant would make the pathogen extinct.

Gene-for-Gene Concept

• Explains the step-wise evolution of virulence and resistance.
• For each gene in the host that confers resistance, there is a corresponding gene in the pathogen that confers virulence.
• To infect a variety with resistance genes a, b, or c, the pathogen must have the corresponding virulence genes a, b, or c, respectively.
• This applies to diseases in which host resistance is simply inherited or involves only one or a few genes.
• When a new resistant host variety is developed, a new pathogen gene for virulence appears, leading to the breakdown of host resistance.
• Plant breeders introduce another gene for resistance in the host against the gene for virulence in the pathogen, and the cycle continues.