Crop-Rhizobial Symbiosis Flashcards
(14 cards)
What is an example of a mycorrhiza?
Glomeromycotan
Can form symbiosis with most plant species
Exchanges nutrients and nitrogen for carbon
What is an arbuscule?
A highly branched intracellular hyphal structure surrounded by a membrane
In the cortical layer
How long has mycorrhizal and rhizobial symbiosis been around for?
Mycorrhizal: >400 million yrs
Rhizobial: ~60 million yrs
What is shared between mycorrhizal and rhizobial symbiosis?
Much of the signalling downstream of the LysM-RLK receptor is shared between rhizobial and mycorrhizal symbiosis
But, then they diverge downstream of CCaMK, suggesting CCaMK plays a role in differentiating the responses
What is not required for mycorrhizal association but is for rhizobial?
LysM-RLK
What are the Myc factors?
Lipochito-oligosaccharides
Found a mixture of sulphated and non-sulphated LCOs
Found they increase mycorrhizal associations in a number of plants
What was found about Myc factors in Medicago?
Tested non-sulphated Myc factors
Found it induced root branching
What did NOD mutants do to Myc factors in Medicago?
NOD mutants blocked the response of Myc factors
Specifically:
- The LysM-RLK NFP mutant blocked this response (plays a role in both rhizobial and mycorrhizal associations)
- But LYK3 mutants did not (only plays a role in mycorrhizal associations)
What is Parasponia?
The only non-legume plant known to form rhizobial symbiosis
Ideal to study as they recently evolved this trait independently
How was Parasponia used to test the relationship between rhizobial and mycorrhizal associations?
What was found?
- Parasponia NFP homologue was identified (PaNFP)
- Reduced expression of PaNFP, using RNAi
- Observed the effects on mycorrhizal and rhizobial symbiosis
- Found that RNAi lines with reduced PaNFP impaired the plants ability to form both mycorrhizal and rhizobial associations
- Shows PaNFP recognises both Myc and NOD factors
- Therefore, a Myc factor sensing LysM-RLK has been recruited to sense rhizobial NOD factors to allow for rhizobial interactions
How could rhizobial symbiosis have come from existing mycorrhizal pathways?
Legumes could have undergone whole genome duplication
They have 2 copies of NFP / NFR5 receptors:
- One copy likely evolved to recognise NOD factors
- The other retained its ancestral role of recognising Myc factors
Seems likely that rhizobia gained the ability to produce lipochito-oligosaccharides, so they could mimic mycorrhizal Myc factors
How could rhizobia have gained the ability to produce lipochito-oligosaccharides?
- Horizontal gene transfer from fungi, supported by evidence of rhizobia bacteria inside glomeromycota
- Independent evolution (phenocopied)
Is it possible to engineer rhizobial symbiosis into crops?
Too complex to just transfer a few genes to non-legumes and expect them to nodulate
However, most crops form mycorrhizal symbiosis, so core symbiotic machinery is already there
Rhizobial specific components could be introduced to the existing pathway
What is N-Fix?
An endophytic nitrogen fixer: Gluconacetobacter diazotrophicus
- Fixes nitrogen using denitrogenase
- Lives inside plant tissues, first found in sugarcane
- Can colonise tomato, wheat, maize, rice
- Secretes extracellular mucus to provide a suitable environment for N2 fixation
- Increases respiration to decrease O2 levels
- Produces nitrogenase protecting enzymes under high O2 conditions
- Currently being commercialised as N-Fix, a non-GMO seed coating
- Claims to provide ~50% of the plants nitrogen needs
