Symbiotic NItrogen Fixation Flashcards
Most common symbiotic nitrogen fixation relationship is between…? and what is this relationship considered?
- Bacteria of the genus rhizobium and the root structures of legumes
This relationship is considered symbiotic
What does symbiotic mean?
Two types of organisms live together as in their mutualistic relationship benefits them from association with one another
how does the bacteria and root structures help each other out in symbiotic nitrogen fixation?
- Bacteria receives a protected environment and nutrients from the plants
- the roots receive fixed nitrogen from the bacteria.
What is Rhizobia?
bacteria that is capable to form symbiotic relationships with legumes and form root nodules
What do rhizobia do?
They enter the root tissues of the plants and form nodules.
what is the purpose of nodules?
they contain and protect the bacteria.
True or false:
all species of the rhizobium genus are capable of either symbiotic relationships or root nodule formation
False, not all of them are
Symbiotic relationships are specific to..?
limits to the rhizobium genus?
- species
- Host
particular species of rhizobium are only able to form nodules on certain legumes.
Proper farming technique since symbiotic relationships are so specific with host and speciies of rhizobium
- farmers inoculate legume seeds with the right rhizobium first before planting
What are legumes?
they are plants that produce seeds in pods and incluide alfalfa, clover , peas
Characteristics of Rhizobium?
- aerobic
- Gram negative rods
- does not produce spores
- motile
First requirement of nodulation? How is this achieved?
- Bring bacterial strain in the soil in close proximity to the root hairs.
- Achieved through the production of a chemical known as flavonoids
What do flavonoids do?
they attract rhizobia, resulting in an accumulation of bacteria on recognition sites on the root hairs.
They activate the transcription of a family of bacterial genes that code for proteins known as the nodulation factors.
What are nodulation factors responsible for?
they are responsible for the multi-step process by which the bacteria are able to infect the plant root hairs and ultimately form nodules.
What genes does the sym plasmid carried by the rhizobium code for
- Nodulation genes
- nitrogenase genes
- host-specificity genes
What happens when a polysaccharide capsule of the rhizobium bind to the cell membrane of the root hair?
the root hair membrane invaginates (turns inside out) forming an infection thread in which the root hair appears to be growing into the root rather than projecting outwards
what is leghemoglobin
it has a high affinity for free oxygen in the environment, keeping oxygen levels low enough to allow nitrogenase function but high enough to allow aerobic respiration in bacteria
Why is a normal alfalfa root nodule pink in color?
sufficient quantities of the reddish-colored leghemoglobin
Why is it important to keep oxygen concentrations low?
to prevent inactivation of the enzyme nitrogenase
Importance of invagination?
it carries surface bound bacteria inside the infection thread resulting in the production of auxin
what is the presence of nitrogenase specific to?
species that are capable of forming nodules
Where in the plant can the ability of fixing nitrogen be utilized?
in the roots.
What do auxins do?
plant growth regulating hormone that stimulate root cells to divide, forming a nodule around the infection thread, which in turn begins to branch further into the nodule
where does rhizobia grow best?
in well-aerated soil
how does the plant solve the issue between needing oxygen for respiration but getting rid of oxygen for nitrogenase function?
they produce a protective compound,
an oxygen carrying protein (therefore it oxygen binds)
called leghemoglobin
