L13 Fungal Mutualists
Advantages of mutualism for fungi
"sweets for my sweet" -plants are an abundant source of sugars for the fungus
Arbuscular mycorrhizas: formation
*an infection process; however, extensive communication between the fungi and the root facilitates mutualism 1. spores germinate in response to root signaling molecules 2. hyphae penetrate the root tissues 3. hyphae grow between the cells in the root cortex and form appressoria that penetrate the plant cells 4. the host plasmalemma invaginates and proliferates around the fungus 5. repeated dichotomous branching of the fungal hyphae produces the arbuscule inside the cortical cell
Nutrient recycling: endophytes
initiate the biological degradation of dead or dying host-plant
lolines
powerful against invertebrates
Lichen naming
Fungal component: "mycobiont" Algal/cyanobacterial component: "photobiont" Scientific name is the same as that of the mycobiont
Photobiont
-90% green algae, 10% cyanobacterium -many are obligate symbionts -habitat may impact selection of photobiont by mycobiont
Mycobiont
-98% Ascomycota (40% are lichen formers), rest are Basidiomycota -1/5 of all fungi are obligate lichen formers -Communicate with photobiont using unique SMs Complex reproduction: -sexual mycobiont: fungal spores must find compatible photobiont upon germination and resynthesize the lichen symbiosis (horizontal transmission) -asexual mycobiont: photobiont is transmitted with the mycobiont through vegetative propegules or a fragmented thallus (vertical transmission)
Class 1 Endophytes
-Clavicipitaceous endophytes -fungal genera: Epichloe and Neotyphodium -resistricted to grasses -can grow in both above and below ground tissue
Arbuscule
-Highly branched bunch of hyphae within the plant root -Site of nutrient transfer between the plant and the fungus -4-15 day lifespan; upon death, they break down and the plant cell returns to normal
Endophytes: risks/benefits to the plant host
-Increase plant fitness by conferring stress tolerance -Decrease plant fitness by altering resource allocation
How do mycorrhizal fungi benefit plants?
-Increase the root SA (because they are smaller), allowing for better uptake of water and P, K, N (fungi can turn inorganic nitrogen into usable forms; NOT NITROGEN FIXERS THO) -Improve stress resistance -Protect against nematodes and disease -Protect against contaminant absorption -Enhance coil structure for increased water capacity and nutrient trapping -Promote faster rehabilitation of degraded sites -Improve biocontrol of pathogens
Arbuscular mycorrhizas: AM signals to host plants
-Lipo-chitooligosaccharides (LCO's) and chitooligosaccharides (CO's) -Chitin oligomers are present in all fungi and induce plant defense; however, when present in short chains (4-5) they perceive them as symbiotic signals and do not elicit a defense response
Endophytes: definition
-Microorganisms that internally infect living plant tissues without causing any visible manifestation of disease, and live in mutualistic association with plants for at least part of their life cycle
Mycorrhizal networks
-Most do not have host specificity -Most plant roots are colonized by multiple fungi -Plants are usually connected by mycorrhizal mycelial networks: "wood-wide-webs" -These networks facilitate transfer of carbon and nitrogen from one plant to another -form the worlds largest organism; 3.5 miles and 3 feet into the ground; 8,000 years old
Mycorrhizae
-Mutually beneficial relationship between plant roots and fungi -Mycorrhizae filaments in the soil function has extensions of root systems and are more effective at water and nutrient absorption than the roots themselves -Almost ALL terrestrial plant species carry and depend on mycorrhizae to some extent
immunosuppressive drugs
-Mycophenolic acid -used to prevent rejection in organ transplantation that was isolated from Penicillium brevicompactum
Arbuscular mycorrhizas: host plant signals to AM fungi
-Strigolactones are plant derived hormones that stimulate the branching and metabolism of presymbiotic hyphae in AM fungi -AM fungi perceive the strigolactones via a calcium-mediated pathway and activate multiple responses involving fungal cell wall metabolism -Cutin monomers: derived from the cuticle, make up the waxy coating of the leaf --> fungi perceive them as signals for producing "infections" of the roots
Lichens: benefit to fungus
-algae/cyanobacteria photosynthesizes to provide food for the fungus -provide benefit of nitrogen fixation
Lichens: important for nature and humans
-beautiful to look at -converse CO2 into O2, making them an important cleaner-upper of pollutants
biotic stress tolerance: endophytes
-by clavicipitaceous endophytes -enhane resistance to insect feeding and protect plants from herbivory through SMs
Non-clavicipitaceous (NC-endophytes; Non-Balansiaceous) endophytes
-can associate with a large array of different plant species -classes 2, 3, and 4
Classification of endophytes
-consist of mainly Ascomyctoa -two groups: 1. Clavicipitaceous (C-endophytes; Balansiaceous) 2. non-clavicipitaceous (NC-endophytes; Non-Balansiaceous)
abiotic stress tolerance: endophytes
-drought, metal contamination via chelation
Ectomycorrhiza (ECM) structural features
-form an extensive network of hyphae in the root cortex that makes the Hartig net -forms sheath of tissue around the outside of the root that extends into the soil: the fungal mantle -forms roots that are shorter and wider than uninfected roots
Class 1 endophytes: Epichloe endophytes
-form associations almost exclusively with grasses from the subfamily of Pooideae -host specific -grow intracellularly and systematically, infecting both vegetative and reproductive organs of their host plants including developing seeds -inherited vertically -provide protection against herbivores, enhance plant growth, photosynthetic rate, and stress tolerance -fungus receives nutrition, protection, and dispersion via host seeds -have successfully been developed and commercialized for agricultural use in the USA, Australia and New Zealand (example: MaxQ); increase vigor and growth
Lichens: benefit to algae
-fungi form protective cortex that protect the algae/cyanobacteria from UV rays -enables them to survive environments that could not otherwise
ergot alkaloids and indole diterpenes (lolitrem B)
-highly toxic to vertebrates -perennial ryegrass toxicosis from Neotyphodium endophytes
Ectomycorrhiza (ECM)
-hyphae grow between plant root cells -colonize only 10% of plant families, mostly woody plants -not host specific -most have above ground fruiting bodies but some have underground fruiting bodies, like a truffle -feed via hartig cells
Arbuscular mycorrhizas
-hyphae penetrate into the root cells -obligate biotrophs -only Glomeromycota -most common type of mycorrhizal association (80-90% of land plants) -Not very host specific -feed via arbuscule
Clavicipitaceous (C-endophytes; Balansiaceous) endophytes
-infect grasses -most common class 1 Epichloe + Neotyphodium
Class 1 endophytes: B. Neotyphodium endophytes
-mutualists with grasses, including important forage species -vertical transmission by growing into the forming seed and establishing resting mycelium in the embryo -produce alkaloid SMs: peramine, lolines, ergot alkaloids, and indole diterpenes
Photobiont switching
-mycobiont can associate with a new algal/cyanobacterial parter -allows for rapid adaption to the environment -allow lichen to acclimate
peramine
-no biotoxic effects -alkaloids -deters some invertebrates from feeding -produced by Neotyphodium
Why are we interested in endophytes?
-rich source of SMs -of high pharmaceutical, industrial, and agricultural importance Anticancer drugs, anti-fungal, anti-HIV, and other antimicrobial drugs, anti-cholesterol drugs, immunosuppressive drugs
Perennial ryegrass toxicosis aka "staggers" disease
-seasonal mycotoxicosis of grazing livestock -characterized by tremors, loss of coordination, convulsions, heat stress -death due to drowning
Ericoid mycorrhizae
-specialized to infect in the order of Ericales family or Ericaceae -form only endomycorrhizal associations -mostly colonize mature epidermal root hairs, penetrating them and filling them with coiled hyphae -It is through these coils that nutrient exchange occurs -80% of root volume can be fungal tissue -Ascomycota
Orchid mycorrhizae
-specialized to infect orchids -form mostly endomycorrhizal associations -most orchid seeds will not germinate unless they are infected -fungus may be the sole source of nutrition, particularly carbohydrates, needed for early development -Basidiomycota: "Rhizoctonia" (also known as plant pathogens)
Lichens
-symbiosis between fungi and green algae (85%), cyanobacteria (10%) or both (5%) -fungus is the dominant partner, determining shape and characteristics -green algae and cyanobacteria are photosynthetic
Endophytes: main characteristics
-ubiquitous in plants and have a profound impact on plant ecology, fitness, and evolution (like mycorrhizae) -reside entirely within the plant tissues and may grow within roots, stems, and/or leaves (unlike mycorrhizae) -can be transmitted vertically, horizontally, or both
Major types of Mycorrhizae
1. Endomycorrhizae or Arbuscular mycorrhizas (AM) -grow intracellularly -Glomeromycota -associated with 90%of plants 2. Ectomycorrhizae (ECM) -grow intercellularly -can be part of different categories -associate with 10% of plants
Endophytes vs. Mycorrhizas
1. Never see endophytes 2. Endophytes Can be above or below ground, mycorrhizae only associate with roots
Lolium perenne and Neotyphodium lolii
Lolium perenne: -"perennial ryegrass" -important grazing grass Neotyphodium lolii: -endophyte of perennial ryegrass -production of bioactive ergot alkaloids: ergovaline and lolitrem B (known to have toxic effects on mammals)
anti-fungal, anti-HIV, and other antimicrobial drugs
Micafungin: echinocandin antifungal isolated from Coleophoma empetri Altertoxin: a potent anti-HIV drug isolated from Alternaria tenuissima
Neotyphodium lolii mycotoxins
Neotyphodium endophyte Tremorgenic neurotoxins -cause incoordination by interference with neuronal transmission -"mimic" neurotransmitters like serotonin -induces heat stress and depresses prolactin levels, reducing milk yield Disease: perennial ryegrass staggers
Anticancer drugs
Paclitaxel: the world's first billion-dollar anticancer drug -isolated from Taxus brevifolia and Taxus wallinchiana, which harbor endophytic fungi Other anticancer drugs isolated from endophytic fungi including podophyllotoxin, kaempferol, torreyanic acid, and cajanol
anti-cholesterol drugs
Rosuvastatin, a statin isolated from Penicillium citrinum and P. brevicompactum
What happens when a mycorrhizae from one plant meets the mycorrhize from another plant?
a mushroom!
phytostimulation: endophytes
stimulate longer root hairs which makes more efficient absorption of essential nutrients