Plant-Microbe Symbioses BIOL 456

Mutualistic

Nutrient acquisition

Water acquisition

Pathogen defense

Parasitic

Various leaf and

root pathogens

decomposition

In the nitrogen cycle, soil microbes mediate plant nitrogen (N) acquisition through, _______ and N fixation pathways.

soil nutrient recycling

Microbial extracellular enzymes drive ________ through decomposition

The rhizosphere priming effect

__________:

Plants exude some of their photosynthates into soil which can exert control over the decomposition of soil organic matter.

Evidence suggests that plants can dynamically moderate this process depending on their nutrient needs.

leguminous plants

The mutualistic relationship between ________ and nitrogen-fixing bacteria is one of the most important symbioses known

Legumes

_______- family of flowering plants that often bear their seeds in pods

○ e.g., soybeans, clover, alfalfa, beans, and peas

• Responsible for 25% of the N2 fixed globally each year

rhizobia

The Legume–Root Nodule Symbiosis:

Bacterial symbiont termed ______- best known in genus rhizobium

root nodules

Legume Symbiosis: Infection of legume roots by nitrogen-fixing bacteria leads to the formation of ________ that fix nitrogen

• grow well in areas where other plants would not.

nitrogenase

•N2 fixation reaction is catalyzed by ________ enzyme

• Sensitive to the presence of oxygen

inactivated

•Nitrogen-fixing bacteria need to perform aerobic respiration with O2 to generate energy for N2

fixation, but nitrogenases are ______ by O2

leghemoglobin

•In the nodule, O2 levels are controlled by the O2 -binding protein _______

Nod factors

Legume Symbiosis:

• Bacteria signal the plant with ______

• Induce root hair curling

• Trigger plant cell division

• Signal legumes to develop root nodules

nodABC gene

Legume Symbiosis:

_______ encodes proteins that produce lipochitin oligosaccharides

Free Living Nitrogen Fixation

a natural process where heterotrophic bacteria, living independently in the soil or water, convert atmospheric nitrogen into usable forms for plants

Mycorrhizal fungi

• From the fungal phylum Glomeromycota

• Colonize the root systems of plants, providing increased water and nutrient absorption (N, P, etc.) capabilities.

• Plant provides the fungus with carbohydrates formed from photosynthesis.

Arbuscular mycorrhizae

__________ – penetrate and live within root cells

Ectomycorrhizae

_________– surround but do not penetrate root cells

extension

Mycorrhizal fungi improve water and nutrient absorption

• Fungal mycelium acts as an “_______” of the root, increasing surface area exposed to soil

obligate

Most mycorrhizae are ______- rarely found in nature not in association with roots

Arbuscular Mycorrhizae

• Most common mycorrhizal association

• Found in >80% of terrestrial plant species

•Ancient symbiosis – established

~400 million years ago – allowed evolution of land plants

•Obligately biotrophic

oligosaccharide

Arbuscular Mycorrhizae:

• Fungal mycelium becomes deeply embedded within the root tissue

•Use _______ signaling factors (Myc factors)

Ectomycorrhizae

• Fungal cells form an extensive sheath around the outside of the root

• Found primarily in forest trees, particularly boreal and temperate forests

• Associate with specific tree species (pine, firs, spruce, hemlock, oak, hickory, alder, and beech) whereas ash and sugar maples are AMF associated

• More recent association than AM in terms of evolutionary time

saprotroph

Some ECM can also exist as free-living ________ (have maintained ability to break down soil organic matter)

hyphae

Tripartite plant-mycorrhizae-bacteria symbioses:

Increasing evidence suggests that mycorrhizae host bacterial communities inside and outside of ______ and spores

hyphosphere

Tripartite plant-mycorrhizae-bacteria symbioses:

Hyphae have been shown to deliver plant photosynthates to its own “_______”

sanction

Mycorrhizae and the parasite-mutualist continuum:

Plants can sense and “_______” AMF that are taking more than they give.

Molecular Koch’s Postulates

• The gene or mutation must be found in all or most cases of the disease.

• Inactivation or deletion of the gene or mutation should reduce or eliminate the disease symptoms.

  • Reintroduction of the gene or mutation into a disease-free host should restore the disease symptoms.

Koch’s Postulates

• The microbe must be present in all cases of the disease. 

• The microbe must be isolated and grown in pure culture from the diseased host. 

• The pure culture must cause the disease when inoculated into a healthy host. 

• The microbe must be re-isolated from the experimentally infected host. 

Ascomycota and Basidiomycota

Phytopathogenic fungi

• Mostly from the fungal phyla ________

• Plant pathogens vary in their life strategy

Necrotrophs

Phytopathogenic fungi:

______- kill plants and feed on dead tissue

Biotrophs

Phytopathogenic fungi:

_______- colonize and feed on living plant tissue (need the plant to be alive).

• Many mutualists are biotrophs! Pathogens/parasites are freeloading biotrophs

Hemibiotrophs

Phytopathogenic fungi:

________- exhibit biotrophy initially, then are necrotrophs

Necrotrophic phytopathogenic fungi

• Opportunistic pathogens that are able to take on free-living or symbiotic lifestyles.

• Take advantage of wounds or natural openings in plant tissues

• Broad range of hosts, easy to culture in the lab

• Cause rapid, uncontrolled death of plant cells and ultimately death of the plant

• Examples: Botrytis cinerea (grey mold), Fusarium oxysporum (vascular wilt)

Biotrophic phytopathogenic fungi

• Feed on living plant tissue (e.g., consuming sugars inside of cells)

• Generally obligate pathogens with a narrow host range

• Difficult or impossible to cultivate outside of host

• Plant defense mechanisms may cause programmed cell death to compartmentalize diseased tissue

• Examples: Ustilago maydis (maize smut) and Phytophthora infestans (potato late blight)

Hemibiotrophic phytopathogenic fungi

• At first, behave as biotrophs but enter necrotrophic phase ultimately killing plants.

• Rice blast caused by Magnaporthe oryzae is an especially prolific example

• This disease has been estimated to destroy enough rice to feed 60 million people yearly

Root knot nematodes

plant-parasitic from the genus Meloidogyne. They exist in soil in areas with hot climates or short winters. About 2000 species of plants worldwide are susceptible to infection from this and they cause approximately 5% of global crop loss

Agrobacterium tumefaciens

Plant pathogenic bacteria:

• Soil-borne pathogen that is the causative agent of crown gall disease (abnormal cell growth resulting in tumor formation)

• Especially consequential for fruit and nut trees, leading to major economic impacts to apple, pear, and grape industries

A. tumefaciens cells

Agrobacterium tumefaciens:

To initiate tumor formation, ________ must attach to the wound site on the plant

Ti plasmid

Agrobacterium tumefaciens:

• Attached cells synthesize cellulose microfibrils and transfer a portion of the _______ to plant cells

• DNA transfer is mediated by vir-encoded proteins

Tobacco Mosaic Virus (TMV)

Plant Virus - _________

• First viral pathogen to be discovered (1930)

• Is simply a strand of RNA with a protein coat

• RNA hijacks plant cellular machinery to reproduce the virus

• Must have living cells to multiply, but can survive in dormant state in dead plant tissue for years

cell-to-cell contact

Tobacco Mosaic Virus:

Spreads by _________, resulting in areas of dead tissue and stunted growth, but generally doesn’t kill plants

Chemical communication

Signaling between plants and microbes

_________: through volatile and dissolved molecules

pathogen-associated molecular patterns (PAMPs)

The Plant Immune System:

Plant cell surfaces possess pattern recognition receptors (PRRs) that recognize

____________

• e.g., molecular patterns associated with pathogen flagellar proteins

These receptors can cause expression of genes to synthesize hormones, antimicrobial compounds, and reactive oxygen species to fend off pathogens

PAMP signaling pathways

The Plant Immune System:

If a pathogen is able to contact the cell and inject effector molecules, intracellular receptors called nucleotide-binding domain, leucine-rich repeat receptors (NLRs) may

• secondarily trigger _________

• undergo programmed cell death

elicit induced systemic resistance

Microbial mutualists in the rhizosphere can __________ in plants

microbial-associated molecular patterns (MAMPs)

Microbially induced systemic acquired resistance (SAR)

Some microbes in the rhizosphere recognize pathogens and release chemical signals (e.g., VOCs, small peptides) called __________

defense-related genes

Microbially induced systemic acquired resistance (SAR):

Root cell receptors recognize MAMPs and trigger hormone pathways (e.g., jasmonic acid synthesis) which upregulate _________ to, for example:

• Synthesize defensive secondary metabolites (e.g., antimicrobials)

• Change root morphology to fend off pathogens (e.g., lignify root tissue).