Legume - Rhizobia symbiosis Flashcards
what family and class is Rhizobium a part of?
Rhizobium is part of the Rhizobiaceae family, within the alpha-protobacteria class
What is legume-rhizobium symbiosis based on?
Legume-rhizobium symbiosis is based on a molecular dialogue, involving flavonoids and Nod-factors
What are the steps undergone within the root when establishing symbiosis?
Nod-factors induce root deformation, creating an infection pocket. The nucleus moves closer to the infection pocket, creating an infection thread, established via invagination of the plasma membrane.
The Rhizobium proliferates into the infection thread.
Why are root nodules produced in legumes?
Rhizobium infiltrates root cortex and inhabits intercellular space. The plant responds by producing more cortical cells
What happens on a cellular level, when establishing legume-Rhizobium symbiosis?
Bacteria are released into cells via endocytosis and are enclosed in symbiosomes (a vacuole like structure), where around half of them differentiate into bacterioids
what flavonoids and Nod-factors specific?
true
not all species of legume are compatible with every species of rhizobium, and vice versa.
there are receptors in legumes that have a high affinity to perceive specific Nod factors
tell me about Nod factors
Nod factors, an LCO (lipochitooligosacchiride), are needed in nanomolar concentrations to create change in root hairs.
Synthesis of Nod-factors is conducted by Nod-genes.
Transcription of Nod-genes depends on interaction between flavonoid and transcription factor protein (Nod-D). presence of Nod-D activates transcription
tell me about the structure of Nod-factors and how its encoded
Each rhizobium produces a different chemical decoration of Nod-factor. The structure is a chain of 3 - 6 N-acetylgucosamine groups and a fatty acid, encoded by 5 genes:
NOD A, B and C do the core structure:
- C forms short N-acetylglucosamine chains
- B deacetylates the first group
- A attached fatty acid
NOD E and F synthesise fatty acid
tell me about the Ca^2+ signalling pathway
NFs are perceived by a plasma membrane receptor (Nod-factor receptors or LysM receptor like kinase). receptor like kinase transports a Ca^2+ signalling message to the nucleus, where the spike in Ca activates (via phosphorylation) transcription factors transcribing symbiosis genes
what are the two types of Nodules?
Interdeterminate: where differentiated and un-differentiated cells are seperated.
Determinate: they are held together
What is the cost of symbiosis?
the plant receives as NH4+ fixated by the bacteria, in response to providing malate and succinate to rhizobium, which through the TCA cycle (and pentose-phosphate pathway) yields ATP and reducing power. lots of reducing power is required.
tell me about legume-rhizobium senescence
symbiosis ends when the legume requires more sugars and proteins, and cannot afford to provide for the bacteria. the nodule stops growing rhizobium returns to soil. However only undifferentiated bacteria can continue as free living organism.
why does rhizobium turn green during senescence?
It loses its heme group from leghaemoglobin
