Plant lecture 14 - Symbiotic N fixation Flashcards

1
Q

Nitrogen fixation

Nitrogenase

A
  • Dinitrogenase reductase = Fe protein, supplies reducing power, also MoFe protein that reduces N2
  • Symbiotic N fixing = when N fixing bacteria associate with a plant
  • E.g. rhizobia + legumes forming root nodules
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2
Q

Nodules development in legumes 1

A
  • Flavonoids released by roots of host plant → change in rhizobia gene expression (nod genes)
  • Nod genes allow synthesis of nod factors (lipochitin oligosaccharides) that initiate change in host gene expression (ENOD genes encoding for early nodulins)
  • In addition to change gene expression, Nod factors also trigger influxes, membrane depolarisation
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3
Q

Nod factors

A
  • Typically lipochitin oligosaccharide w/ NAG backbone
  • Chemical modifications on terminal residue are important for interaction with host plant + encodes specificity of interaction
  • Plant releases compounds into rhizosphere, some detected by Rhizobia which produces a particular Nod factor that if recognised by plant starts formation of nodule
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4
Q

Nod factor signalling

A
  • DS signalling component identified by genetic analysis of Lotus japonius
  • Signal received by receptor kinase. Causes Ca2+ influx which impacts TF NSP1/2
  • NSP1/2 can also be activated by DMI1/2 (ligand gated cation channel/ receptor kinase) which triggers ca2+ spiking around nucleus + activates DMI3 (calcium/calmodulin-dependent protein kinase)
  • If deregulate DM13 by removing autoinhibition domain, causes nodulation w/o rhizobia (rhizobia critical in cascade)

Ubiquination

  • LYK3 receptor kinase = nod factor protein that assists binding of nod factor to kinase
  • W/o nod factor, PUBI = E3 ubiquitin ligase that is active. Ubiquinates a protein needed for infection + infection x
  • When nod factor binds LYK3, LYK3 phosphorylates PUB1 + inactivates

Bacterial exopolysaccharide (EPS)

  • Important for host invasion by symbiotic bacteria
  • In Lotus japonicus, nod factor signal transduction induces transcription of the host gene Epr3
  • This encodes a receptor protein similar to NRF1 family which can recognise EPS
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5
Q

Nodule development in legumes 2

During infection

A
  1. Bacteria attach to root harms promote growth + curling
  2. Modification of cell wall followed by invagination of the plasma membrane → infection thread
  3. Invagination propagates into root cortex + nodule primordial is formed in cortex
  4. LOF + GOF mutations in cytokinin receptor show activation of LHK1 is sufficient to trigger nodule formation
  • Signalling coordinates activity at surface vs centre of root cortex (cortex is ready for invading bacteria)
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6
Q

Role of nodule inception protein in nodulation (NIN)

A
  • NIN - bifunctional TF suppressing ENOD11 expression in epidermis + promoting transcription of cytokinin receptor CRE1 in root cortex
  • Nod factors arrive at root hair + trigger expression of NIN
  • NIN activates the cortical program leading to organogenesis
  • NIN activates NPL that breaks down cell wall + allows invag. of plasma membrane to create infection thread
  • Some signal triggers cytokinin production that → expression of NIN in cortex
  • Sets up +ve feedback loop: production of NIN ↑ expression of CRE1, which ↑ zone within root sensitive to cytokinin. Eventually NIN inhibited
  • NIN controls a diversity of functions inc. cell wall modification via NPL, GA biosynthesis by genes like CPS1, nutrient uptake + DNA synthesis
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7
Q

Nodule development in legumes - 3

Nodule formation

A
  1. The infection thread extends into the nodule primordium, allowing rhizobia to enter the plant.
  2. Bacteria are released into the cytoplasm, forming symbiosomes that occupy up to 80% of the cell volume and contain up to 20 bacteria surrounded by the plant-derived peribacteroid membrane.
  3. The bacteria differentiate into endosymbiotic bacteroids + the nodule primordium develops into a mature nodule.
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8
Q

Symbiotic bacterial gene expression

A
  • Nod, nol and noe genes are required for the synthesis of the nod factors that initiate nodule development:
    1. The nodD gene product activates the expression of the other nod genes after forming a complex with secondary metabolites in the root exudate
    2. NodA-C gene products make lipochitin oligosaccharide backbone
    3. Other nod gene products determine specificity by controlling the chemical modification of terminal residues
    4. Nif genes = both symbiotic bacteria + N2 fixing. Gene products Include Fe protein + MoFe, regulatory proteins for bif gene expression, NifA/L to name a few
    5. Fix genes - only occur in symbiotic N2 fixing bacteria. Gene products include components of O2 sensing system (FixJ,L,K) + structural proteins of high affinity bacterial terminal oxidase. Protect nitrogenase from denaturation by O2
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9
Q

Maintaining an anerobic environment

A

3 factors help ↓ O2 conc.

  1. Restricted diffusion, based on a variable permeability barrier controlling O2 exchange at the nodule periphery and the reduction of intercellular air spaces.
  2. Binding to leghaemoglobin reduces the free O2 concentration in the host cytosol to 10-25 nM. RNAi-induced abolition of leghaemoglobin synthesis prevents symbiotic N fixation.
    - Bacterial respiration, aided by the ↑ affinity terminal oxidase (Km ~ 7 nM), acts as a major oxygen sink.
    - Low O2 limits nodule respiration + nitrogenase. Effect greater in bacteroid than host cytosol as host mit. localise at cell surface near intracellular air spaces
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10
Q

Nitrogen sensing

A
  • Free living diazotrophs use 2 component N control system to regulate Nif gene expression
  • NtrB/C. Less common in symbiotic diazotrophs, reflects role of symbiotic diazotroph as source of fixed N for host + importance of O2 sensing
  • NtrC-P activates NifA/L
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11
Q

Symbiotic plant gene expression

A
  1. Early nodulins
    - Tissue specific expression = essential for successful infection + nodule development
    - Epidermal Nod factor-induced ENOD expression occurs in the vicinity of actively growing root hairs
    - Pectate lyase induced in the nodule primordium degrades the plant cell wall around the infection thread, facilitating the formation of symbiosomes
  2. Late nodulins
    - Establishes metabolic conditions for N2 fixation in the host cytosol:
    - leghaemoglobin for controlling oxygen availability
    - glutamine synthetase for ammonium assimilation
    - sucrose synthase for sucrose breakdown
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