Iron homeostasis in root nodules

Lead Research Organisation: John Innes Centre
Department Name: Contracts Office


Janneke Balk leads a research group studying the biogenesis of metal proteins in plants, algae and fungi. She has previously identified several proteins that are involved in the assembly of iron-sulfur cofactors. As one of her core projects, Dr Balk will investigate metal homeostasis during nodule development, in both the plant and bacterial partners of the symbiotic interaction. Several key enzymes required for biological nitrogen fixation require large amounts of iron and molybdenum, but little is know how these metals are acquired and correctly distributed.

1. Confirm that the host plant provides the metals, which has, surprisingly, not been demonstrated thus far. Or, in other words, we need to exclude that the bacteria bring a hoard of iron and molybdenum with them when invading the plant roots. We will measure the metal content of free-living Rhizobia and Rhizobia at different stages of the colonization process (exposure to root exudates, entering the root hair cells, migration to the cortex, early nodules and N2-fixing nodules), using a number of techniques such as mass spectrometry, colorimetric assays and in-situ Fe staining.
2. Identify plant and bacterial genes that are up-regulated in nodules AND are putatively involved in metal homeostasis. Transcriptomics data sets of pea nodules, before and after inoculation, are available in collaboration with Phil Poole.
3. Functional characterization of selected genes identified in (2), using (i) a genetic strategy and (ii) recombinant protein techniques. Rhizobium genes will be deleted and the strains tested for their ability to infect the host plant and/or fix nitrogen. The plant genes will be studied using gene silencing or available insertion mutants. The same bacterial and plant genes will be expressed in E. coli, purified, and their biochemical function studied in-vitro.
Description Legumes and a few other plant species are able to form an intimate symbiosis with Rhizobium bacteria to obtain nitrogen in the form of ammonium in exchange for carbohydrates derived from photosynthesis. The bacteria have a high demand for iron, for example to provide sufficient iron cofactors for the enzyme nitrogenase that turns dinitrogen into ammonium. We have identified two iron transporter genes in the legume Medicago truncatula that are required for iron delivery to the bacteria. One of those is expressed early in the infection process, and the other is expressed in the late stage of nodule development. We demonstrated that iron delivery from the host plant to the bacteria is impaired in mutants in these transporter genes, particularly in the second case, by developing a luminescent reporter expressed in the bacteria. The manuscript was published in the journal "New Phytologist" on 10 June 2020.
Exploitation Route The transporter can be transferred to crop species, together with several other genes, to engineer the nitrogen-fixing pathway in species other than legumes. This will decrease fertilizer use in a sustainable fashion, but is a long-term goal.
Sectors Agriculture, Food and Drink,Environment

Description Sainsbury PhD Studentship
Amount £100,000 (GBP)
Organisation Gatsby Charitable Foundation 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2013 
End 09/2017
Description SEN1 mutant line (Kalo) 
Organisation National Agricultural Research and Innovation Center
Country Hungary 
Sector Public 
PI Contribution A PhD student in my lab, Jenny Walton, spent 2 weeks at the Agricultural Biotechnology Institute to investigate nodule development. After the visit, Ms Walton did further analysis on a Medicago line given to us by Dr Kalo.
Collaborator Contribution Dr Peter Kalo at the Agricultural Biotechnology Institute, Hungary, hosted a PhD student, Jenny Walton, from my laboratory. She used the microscopy facilities to study iron transport in nodulation. Peter Kalo also gave us seeds of a Medicago line that is of interest to our research.
Impact A manuscript is being prepared
Start Year 2015