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Fungal relationships with plants

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Presentation on theme: "Fungal relationships with plants"— Presentation transcript:

1 Fungal relationships with plants
Obligate and facultative parasitism/pathogenicity

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

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

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

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

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

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

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

9 Symptom types – abscission
Hemileia vastatrix

10 Symptom types – sterilization
Ustilago maydis Claviceps purpurea – ergot

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

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 and I produced on “alternate” host
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

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

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 Preformed defenses 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 Examples Hydrolytic enzymes (especially for necrotrophs) Compatibility determinants (especially for biotrophs) 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 The fungal product that is detected does not have to be directly involved in pathogenesis is defined as being produced by an avirulence gene

25 Heath – host pathogen interactions
Ann Bot 80, 713

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

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

28 Gene for gene interactions
Host R r Pathogen A resist susc a susc susc basic compatibility overcomes nonhost defense pressure on host to detect pathogen leads to (temporary) resistance pressure on pathogen to overcome/evade resistance

29 Breeding for resistance
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 8-10 years; resistance may last 3-5 years Pyramid strategies Horizontal resistance

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