Plant-Microbe Interactions: Nitrogen Fixation, Mycorrhizae, and Mutualisms in Ecology

What visual indicator shows an imminent phosphate deficiency in the research plant Arabidopsis?

The plant develops a blue color.

In 'smart plants' designed to detect phosphorus deficiency, what type of DNA sequence binds RNA polymerase more readily when phosphorus levels decline?

A promoter.

The promoter in phosphorus-detecting 'smart plants' is linked to a 'reporter' gene that leads to the production of a _____ in the leaf cells.

light blue pigment

What is the term for relationships between different species where each species provides a substance or service that benefits the other?

Mutualism.

What type of organism synthesizes the nerve toxin tetrodotoxin found in puffer fish?

Mutualistic bacteria (Vibrio species).

In the mutualism between the floating fern Azolla and a cyanobacterium, what does the fern provide to the bacterium?

The fern provides carbohydrates.

What benefit does the floating fern Azolla receive from the nitrogen-fixing cyanobacterium that lives in its leaves?

The fern receives fixed nitrogen from the cyanobacterium.

In the mutualism between leaf-cutter ants and fungi, what do the ants cultivate the fungus on?

The ants cultivate the fungus on harvested leaves.

What are the mutualistic associations between plant roots and fungi called?

Mycorrhizae.

In mycorrhizal associations, what is the first benefit that the fungus's mycelium provide to the plant roots?

It increases the surface area for the uptake of water and minerals.

What do some Acacia plants provide to the ants that aggressively defend them?

The plants provide nourishment in the form of protein-rich structures and nectar.

What is the term for bacteria that live in close association with plant roots or in the soil closely surrounding them?

Rhizobacteria.

What is the rhizosphere?

The soil closely surrounding plant roots.

Up to what percentage of a plant's photosynthetic production may be used to fuel rhizobacterial communities?

Up to 20%.

Besides stimulating growth, what are two benefits plants can receive from rhizobacteria?

Protection from disease via antibiotics and making nutrients more available.

What is the term for rhizobacteria that live between cells within the plant?

Endophytes.

What are the two forms of nitrogen that plants can absorb and use?

Nitrate ($NO_3^-$) and ammonium ($NH_4^+$).

The two-step process of oxidizing ammonia ($NH_3$) to nitrite ($NO_2^-$) and then to nitrate ($NO_3^-$) is called _____.

nitrification

In the nitrogen cycle, the conversion of organic nitrogen from dead material into $NH_3$ by decomposers is called _____.

ammonification

What is the function of denitrifying bacteria in the nitrogen cycle?

They convert nitrate ($NO_3^-$) to gaseous nitrogen ($N_2$), which returns to the atmosphere.

Why can't plants use the abundant gaseous nitrogen ($N_2$) in the atmosphere?

There is a triple bond between the two nitrogen atoms, making the molecule almost inert.

What is the process of converting atmospheric nitrogen (N2) into ammonia (NH3)?

Nitrogen fixation.

What is the name of the enzyme complex that drives the reaction of nitrogen fixation?

Nitrogenase.

How many ATP molecules are required for a nitrogen-fixing bacterium to synthesize one molecule of ammonia (NH3)?

Eight ATP molecules.

Members of the genus _____ form intimate nitrogen-fixing associations with the roots of legumes.

Rhizobium

What are the swellings along a legume's roots that are composed of plant cells infected by Rhizobium bacteria called?

Nodules.

Inside root nodules, Rhizobium bacteria assume a form called what? which are contained within vesicles in the root cells.

theyre called bacteroids

What is the first step in the development of a soybean root nodule?

Roots emit chemical signals that attract Rhizobium, which then emit signals stimulating root hairs to elongate.

How do Rhizobium bacteria enter a legume's root hair during nodule formation?

They enter through an 'infection thread' formed by an invagination of the plasma membrane.

After penetrating the root cortex, vesicles containing bacteria bud off from the infection thread and the bacteria develop into nitrogen-fixing _____.

bacteroids

What is leghemoglobin?

An iron-containing protein in root nodules that binds to oxygen.

What is the primary function of leghemoglobin in root nodules?

It acts as an oxygen buffer, maintaining an anaerobic environment for nitrogen fixation.

In the mutualistic relationship between legumes and Rhizobium, what does the plant provide to the bacteria?

The plant provides carbohydrates and other organic compounds.

What agricultural practice involves planting a non-legume one year and a legume the next to restore fixed nitrogen in the soil?

Crop rotation.

When a legume crop is ploughed under to decompose and enrich the soil, it is referred to as _____.

green manure

Rice farmers often culture the aquatic fern Azolla, which has a mutualistic relationship with _____ that fix nitrogen.

cyanobacteria

In the mycorrhizal relationship, what does the host plant provide to the fungus?

The host plant provides the fungus with a steady supply of sugar.

what is the second benefit mycorrhizal fungi provide to plants?

They secrete growth factors or antibiotics that protect the plant from pathogens.

The mutualistic symbiosis of _____ is thought to be one of the evolutionary adaptations that helped plants first colonize land.

mycorrhizae

Which type of mycorrhizae forms a dense sheath, or mantle, of mycelia over the surface of the root?

Ectomycorrhizae.

In ectomycorrhizae, the fungal hyphae grow into the root cortex but do not penetrate the root cells, instead forming a network in the _____.

apoplast (extracellular space)

Which type of mycorrhizae is characterized by hyphae that penetrate the root cortex cell walls but not the plasma membrane?

Arbuscular mycorrhizae.

What are the densely branched structures formed by arbuscular mycorrhizae inside root cells, which are important sites of nutrient transfer?

Arbuscules.

Which type of mycorrhizae is more common, found in over 85% of plant species, including most crops?

Arbuscular mycorrhizae.

In an experiment on petroleum-contaminated soil, adding a mixture of _____ and mycorrhizal fungi helped grasses produce more biomass.

plant growth promoting rhizobacteria (PGPR)

What is the term for the use of plants and associated microbes to clean up contaminated environments?

Phytoremediation.

How do salmon contribute marine-derived nutrients to temperate rainforest ecosystems?

They migrate to freshwater streams, die after spawning, and are carried into the forest by predators like bears, where they decompose.

The rarer, heavier stable isotope of nitrogen, _____, is more abundant in marine ecosystems and is used to trace nutrient flow from salmon to forests.

$^{15}N$