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Fungal relationships with plants Obligate and facultative parasitism/pathogenicity.

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Presentation on theme: "Fungal relationships with plants Obligate and facultative parasitism/pathogenicity."— Presentation transcript:

1 Fungal relationships with plants Obligate and facultative parasitism/pathogenicity

2 Why is plant pathology important? Food quantity Food quantity Food quality Food quality Agrinomic practices – tillage, pesticide use Agrinomic practices – tillage, pesticide use Diversity and stability of ecosystems Diversity and stability of ecosystems Beauty Beauty

3 Better understanding of disease processes – examples of use Disease control through life cycle management Disease control through life cycle management Puccinia graminis tritici on wheat and barberry Puccinia graminis tritici on wheat and barberry Orchard management practice in Venturia inaequalis Orchard management practice in Venturia inaequalis Optimizing pesticide application Optimizing pesticide application Disease forecasting Disease forecasting Pesticide design Pesticide design Breeding for resistance Breeding for resistance

4 Fungal pathogenicity on plants Plant pathogens can be biotrophic (rusts and smuts) or hemibiotrophic/necrotrophic (opportunistic) Plant pathogens can be biotrophic (rusts and smuts) or hemibiotrophic/necrotrophic (opportunistic) Necrotrophs can display high levels of host specificity, e. g. Magnaporthe grisea Necrotrophs can display high levels of host specificity, e. g. Magnaporthe grisea Saprotroph “Weak” Facultative parasite Necrotroph “Strong” Facultative parasite Biotroph Obligate parasite Increasing specializationIncreasing host range

5 Symptom types -- necrosis Blumeriella on plumAlternaria solani – early blight of potato

6 Symptom types – wilt Ophiostoma novo-ulmi Panama disease: Fusarium oxysporum f.sp. cubense

7 Symptom types – hormone induced Taphrina deformans Hypertrophy Hypertrophy auxin Synchytrium endobioticum

8 Symptom types – hormone induced Fusarium moniliforme = Gibberella fujikuroi gibberellins Etiolation Bakanae of rice

9 Symptom types – abscission Hemileia vastatrix rust.lbl.gov

10 Symptom types – sterilization Claviceps purpurea – ergot Ustilago maydis

11 Obligate parasites – e. g. Uredinales Rust fungi may have as many as five different spore-producing stages in their life cycles Heteroecism – e.g. wheat stem rust - two taxonomically different host plants in order to complete life cycle - ‘alternate’ host: stages (haploid) - primary host: stages (diploid) Autoecism – e.g. bean rust - entire life cycle completed on a single host species Microcyclic rusts ≤ 3 spore types

12 spermatia (n) insect transported to receptive hyphae (n) heterothallic aecia on barberry (n+n) aeciospores (n+n) airborne SUMMER urediniospores (n+n) airborne FALLFALL uredinia on grass from infection by aeciospores or urediniospores telia on grass teliospore on straw (n+n) WINTER karyogamy (2n) meiosis teliospore (2n) germinating on straw with promycelium and basidiospores (n) SPRING basidiospore (n) airborne spermagonia on barberry from infection by basidiospores Life Cycle of Puccinia graminis IV I O II III

13 Puccinia graminis f.sp. tritici life cycle host ploidy Overwintering and transport of urediospores

14 Stage IV Basidia bearing basidiospores (n) in the spring teliospore germinates a promycelium diploid nucleus migrates into the promycelium and undergoes meiosis four haploid nuclei migrate into developing sterigmata & are incorporated into basidiospores basidiospores reinfect alternate host

15 teliospore germinates, gives rise to a short germ tube of determinate growth known as the promycelium. Promycelium: site of meiosis & formation of sterigmata and basidiospores

16 Stage 0: Spermogonia bearing spermatia (n) and receptive hyphae (n) fertilization of the receptive hyphae by spermatia initiates the dikaryon and the formation of aecia Stage 0 and I produced on “alternate” host helios.bto.ed.ac.uk/bto/microbes/biotroph.htm

17 Puccinia plasmogamy

18 Stage II: Uredinia bearing urediniospores (n+n) reinfect primary host amplifies disease within primary host uredinia can eventually develop into telia helios.bto.ed.ac.uk/bto/microbes/biotroph.htm

19 Teliospore: site of karyogamy technically part of the basidium Stage III: Telia bearing teliospores (n+n  2n) final stage on primary host overwinters as dikaryon

20 Facultative parasitism: Magnaporthe grisea infection

21 Magnaporthe grisea / Oryza sativa Some non pathogenic M. grisea strains can grow in host plants if wound inoculated

22 Host resistance and basic compatibility Most plants are not attacked by the vast majority of potential pathogens Most plants are not attacked by the vast majority of potential pathogens Preformed defenses Preformed defenses Potential pathogens secrete chemicals during growth that can be detected Potential pathogens secrete chemicals during growth that can be detected

23 What is a pathogenicity gene? A gene whose product contributes to successful fungal establishment in the host A gene whose product contributes to successful fungal establishment in the host Examples Examples Hydrolytic enzymes (especially for necrotrophs) Hydrolytic enzymes (especially for necrotrophs) Compatibility determinants (especially for biotrophs) Compatibility determinants (especially for biotrophs) Defense avoidance/detoxification Defense avoidance/detoxification

24 What is a resistance gene? A gene whose product enables the host to detect a pathogen and/or mount a defense A gene whose product enables the host to detect a pathogen and/or mount a defense The fungal product that is detected The fungal product that is detected does not have to be directly involved in pathogenesis does not have to be directly involved in pathogenesis is defined as being produced by an avirulence gene is defined as being produced by an avirulence gene

25 Heath – host pathogen interactions Ann Bot 80, 713 Ann Bot 80, 713

26 Fungal pathogenicity on plants Fungal pathogens of plants include opportunists, necrotrophs and biotrophs Fungal pathogens of plants include opportunists, necrotrophs and biotrophs Resistance is seen at several levels Resistance is seen at several levels Non-host resistance – Non-host resistance – Widespread, early onset, effective Widespread, early onset, effective Passive – attachment/germination Passive – attachment/germination Active – initial colonization, e. g. wall apposition Active – initial colonization, e. g. wall apposition Hypersensitivity Hypersensitivity Durable Durable

27 Varietal resistance Superimposed on basic compatibility Superimposed on basic compatibility Often based on a single resistance gene Often based on a single resistance gene Typically not durable Typically not durable

28 Gene for gene interactions HostRr HostRr PathogenAresistsusc PathogenAresistsusc asuscsusc asuscsusc basic compatibility overcomes nonhost defense basic compatibility overcomes nonhost defense pressure on host to detect pathogen leads to (temporary) resistance pressure on host to detect pathogen leads to (temporary) resistance pressure on pathogen to overcome/evade resistance pressure on pathogen to overcome/evade resistance

29 Breeding for resistance Identify likely targets, disrupt, look for attenuation of pathogenicity Identify likely targets, disrupt, look for attenuation of pathogenicity Cross commercial susceptible strains to wild relatives, backcross to retain yield and desireable characters in resistant strain Cross commercial susceptible strains to wild relatives, backcross to retain yield and desireable characters in resistant strain 8-10 years; resistance may last 3-5 years 8-10 years; resistance may last 3-5 years Pyramid strategies Pyramid strategies Horizontal resistance Horizontal resistance


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