Fungal relationships with plants Obligate and facultative parasitism/pathogenicity

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

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

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

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

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

Symptom types – hormone induced Hypertrophy auxin Taphrina deformans Synchytrium endobioticum http://www.maff.gov.uk/planth/pestnote/pwd.htm

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

Symptom types – abscission Hemileia vastatrix rust.lbl.gov

Symptom types – sterilization Ustilago maydis Claviceps purpurea – ergot

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

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

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

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

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

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

Puccinia plasmogamy www.apsnet.org/education/LessonsPlantPath/StemRustWheat

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

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

Facultative parasitism: Magnaporthe grisea infection

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

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

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

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

Heath – host pathogen interactions Ann Bot 80, 713

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

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

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

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