Plant Pathogens and Fungal Endophytes Flashcards
How much of global crop losses is due to pests?
40%, fungi account for 2/3 of that
How much rice is lost per year due to fungal infection?
50 million tonnes
What percent of wheat is susceptible to emerging fungal pathogens?
90%
What are biotrophs?
Fungi that derive nutrients from the living cells of a host
They tend to be obligate and have specialized structures for nutrient absorption
What are hemibiotrophs?
Fungi that are pathogens and establish as biotrophs and later switch to being nectrotrophs
What are necrotrophs?
Fungi that kill the cells of a living host and utilize their remains as food
What nutritional lifestyle are NOT included in plant pathogens?
Saprotrophs which are decomposers of already dead organic matter
What is the obligacy and host range of biotrophs?
Specialized, obligate
Narrow range
What is the obligacy and host range of necrotrophs?
Unspecialized, facultative
Broad or narrow range
What is the state of the host on entry and axenic culturability of biotrophs?
All ages; in possession of host defense
Not easy and sometimes not yet possible
What is the state of the host on entry and axenic culturability of necrotrophs?
Often immature, overmature, or damaged
Easy
What is the entry into the host, production of appressoria and haustoria, and damage to host tissues of biotrophs?
Specialized, like direct penetration of the cell walls
Appressoria or appressoria-like structures are evident; haustoria is generally present
Little damage in the compatible host
What is the entry into the host, production of appressoria and haustoria, and damage to host tissues of necrotrophs?
Unspecialized; via wounds or natural openings
Not usually any appressoria or haustoria
Rapid cell death of the host tissues
What is the production of lytic enzymes, production of toxins, survival following host death, control by resistance/susceptibility genes, and host defense pathways like in biotrophs?
Lytic enzymes are localized to hyphae and limited in quantity
Toxins are not usually produced
They have little saprotrophic ability, they often survive as dormant spores following host death
PRR proteins; R gene products
Host defense pathways are NPR1; salicylic acid
What is the production of lytic enzymes, production of toxins, survival following host death, control by resistance/susceptibility genes, and host defense pathways like in necrotrophs?
Production of lytic enzymes is dependent on the mode of killing, often copious doing massive damage
Toxins produced dependent on the mode of killing and are often produced acting relatively locally or spread extensively in the xylem; some produce host-specific toxins
Can grow saprotrophically following host death
PRR proteins; host-specific toxin binding susceptibility gene products
Host defense pathways are COI1/EIN2; jasmonic acid/ethylene
What is the disease life cycle?
Arrival
Attachment
Entry
Host recognition
establishment and symptom development
Exit and Survival
How do non-motile spores undergo arrival?
They must wait for the plant to reach them vis roots and shoots
How do motile spores undergo arrival?
They are attracted to their hosts
How do some fungi arrive?
By growing towards their hosts as mycelia or being delivered by other infected plant parts
Which fungi are incapable of spreading through the soil?
Biotrophs, hemibiotrophs, and most necrotrophs
How do fungi attach and enter their host?
They enter through wounds, natural openings, or intact surfaces
Penetration through intact surfaces involves appressoria
What does appressoria involve?
High turgor pressure at the penetrating tip
How do necrotrophs establish via damping off?
The fungi secrete toxins and lytic enzymes
Pectolytic enzymes attack cell walls and cause plants to lose/ooze cytoplasm
Watery plant surfaces involved in damping-off diseases
How do necrotrophs establish via wilts?
Other types of necrotrophs enter vascular systems and inject toxins into the vasculature
Low-weight toxins are transported to the leaves and shut down the stomata and transpiration
These mechanisms lead to wilt diseases
How do biotrophs establish?
Their enzymatic secretion is more localized
Hyphae spread through the plant and may take up nutrients either on the surface or through haustoria
They are nutrient sinks for the plant
What biotrophic mechanism leads to the green island phenomenon?
Producing plant cytokinin hormones that increase chlorophyll retention and protein synthesis in the vicinity of the infection
What is Green Island caused by?
A powdery mildew
What is the exit and survival strategy of fungi?
Many fungi begin to produce reproductive structures in response to nutrient depletion
Many rely on long-distance dispersal
What is the first stage of a plant fungal infection?
The conidium forms an appressorium
How is the conidium emptied?
Glycerol accumulates
Water is drawn to glycerol
Glycerol cannot escape due to melanin
Pressure increases
What are the 3 prerequisites to appressorium formation?
Recognition and glycerol + MAPK pathway upregulation
3-4-1 nucleus sequence
Conidium autophagy
What are the two pathways that govern appressorium formation?
- Recognition of waxy leaf surface signals Pth11 to upregulate glycerol formation - runs through cAMP
- Similar surface recognition signals upregulation of phosphorylation cascade through a series of MAP kinases - provides energy for lots of other stuff
What does the deletion of a single MAP kinase do to a fungus?
This makes it unable to infect the plant
What is the 3-4-1 nucleus sequence?
?
What does the interruption of cell death and autophagy of the conidium inhibit?
Appressorium formation
What are some things that the appressorium does?
Develops a penetration peg
Rearranges its cytoskeleton
Assembles a septin ring
What does the septin ring in the appressorium do?
Septins scaffold the actin filaments and hold them in place
What happens if we knock out any septin?
It leaves a tangled mess of actin and infection will fail
What do septins require?
Specific very long chain fatty acids, which can be targeted by fungicides
How does Magnaporthe move from cell to cell?
Uses the plant’s plasmodesmata
What do hyphae form at the plasmodesmata?
Hyphae form transpressoria with actomyosin
What does the plant develop at the surface of the invading hyphae?
A biotrophic interfacial complex
What does the biotrophic interfacial complex become a conduit for?
Effects
The fungus uses the BIC to transmit effectors into plant cells, overwhelming immune defenses
What are the points at which a plant fungal pathogen is highly susceptible to interruption?
Any of the three steps in appressorium formation
PMK1-dependent formation of transpressoria
What are endophytes?
Symptomless fungi that live within plant tissues
Their function is unclear
Some may be beneficial, others injurious
From all fungi, but especially ascomycetes
How are endophytes assessed?
Surface sterilization of plant tissue
Culturing on a variety of media
Culture-free methods (PCR)
Both culture and PCR
What is the transmission type of Class I (Clavicipitaceous) endophytes?
Primarily vertical - maternal plants pass fungi on to offspring vis seed infections
What are some characteristics of Class I endophytes?
Frequently increase plant biomass
Confer drought tolerance
Produce chemicals that are toxic to animals and decrease herbivory such as alkaloids
Where are Class I endophytes thought to derive from?
Insect-parasitic ancestors
What is some evidence that C-endophytes may have evolved from insect pathogens?
Reduction of enzymatic capabilities
Increased dependence on the host plant for nutrients
An apparent increase in the production of particular secondary metabolites beneficial in the symbiosis
What is the function of Class I endophytes in the plant?
Enhance resistance of hosts to insect-feeding
Some endophytes deter feeding by mammalian herbivores
What is sleepy grass?
A plant that harbors a C-endophyte that produces lysergic acid amide
What are the characteristics of non-C endophytes?
Not deriving from Clavicipitaceae
Typically broad host ranges
Include classes II-IV
What are Class II endophytes characterized by?
Extensive infections: they colonize roots, stems, and leaves
Are transmitted vertically or horizontally via seed coats and/or rhizomes
Have low abundance in the rhizosphere
Confer habitat-adapted fitness benefits in addition to non-habitat-adapted benefits
Typically have high infection frequencies in plants growing in high-stress habitats
What do class II endophytes enhance?
Habitat adaptation
What is Curvularia protuberata?
Colonizes all non-embryonic tissues of the geothermal plant Dichanthelium lanuginosum
Grows in 55-degree soils in nature
When grown alone, neither the plant nor the fungus can tolerate temperates above 38 degrees
What is Fusarium culmorum?
Colonizes all non-embryonic tissues of coastal dune grass
If grown alone, the host plant does not survive and the endophyte’s growth is retarded when exposed to levels of salinity experienced in their native habitat
Both partners tolerate seawater levels of salinity when grown symbiotically
What effects do class II endophytes have on plant fitness?
Most increase shoot and/or root biomass, possibly as a result of the induction of plant hormones by the host or biosynthesis of plant hormones by the fungi
How are class III endophytes distinguished?
Their occurrence primarily or exclusively in above-ground tissues, especially leaves
Horizontal transmission
The formation of highly localized infections
The potential to confer benefits or costs on hosts that are not necessarily habitat-specific
Extremely high biodiversity
Low biomass, mosaic-like infections of leaves
What is the major difference between class II and class III endophytes?
Seedlings raised under sterile conditions do not contain culturable class III endophytes due to horizontal transmission
Class II is transmitted vertically
Why are class III endophytes considered a wastebasket category?
Because it is for endophytes that don’t fit in with the other 3 classes
What are the characteristics of class IV fungi?
Originally called pseudomycorrhizal fungi
Dark septate endophytes
Mostly associated with roots
Mostly ascomycetes
Often broad host range
What are the characteristics of pseudomycorrhizal fungi?
Occur worldwide
No apparent pathogenic effects
