Physiological response of Rhizobium leguminosarum during bacteroid development

Hood, Graham (2013) Physiological response of Rhizobium leguminosarum during bacteroid development. Doctoral thesis, University of East Anglia.

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Abstract

legume-rhizobia symbioses, free-living rhizobia colonise root nodules and
develop into N2 fixing specialists known as bacteroids. During bacteroid
development, rhizobia must adapt to the nodule environment, consisting of reactive
oxygen species, low oxygen, antimicrobial secondary metabolites, low pH and in
some nodules, antimicrobial peptides. This study offers a holistic insight into the
processes required by R. leguminosarum during bacteroid development in nodules
formed on four legumes: Pisum sativum, Vicia faba, Vicia hirsuta and Phaseolus
vulgaris.
Initially, a high-throughput mutagenesis strategy was used to target genes
upregulated during bacteroid development. Screening forty-two mutants on P.
sativum identified some moderate phenotypes but more importantly, highlighted
functional redundancy between certain gene products. A clear example of functional
redundancy was seen between the Mn2+ transporters SitABCD and MntH. Single
mutations in sitA or mntH did not cause a symbiotic phenotype whereas the double
mutant could not form bacteroids on P. sativum, V. faba or V. hirsuta. Intriguingly,
no symbiotic phenotype for the double mutant was observed on P. vulgaris. In
addition to Mn2+ transporters, a Mg2+ channel, MgtE, that is essential for growth in
Mg2+-limited medium at low pH was identified. As with the Mn2+ transporters, the
requirement of MgtE during symbiosis depended upon the species of the hostlegume.
Reasons for host-dependent requirement of SitABCD, MntH and MgtE are
discussed.
The requirement of three O2-responsive regulators that govern regulatory pathways
essential to N2 fixation was also investigated. FnrN appears to be the major O2-
responsive regulator required for symbiosis but in addition to fnrN, two genes, fixL
and fixLc, need to be mutated to prohibit N2 fixation. Other findings include a
putative toxin-antitoxin system that hinders N2 fixation when disturbed.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Mia Reeves
Date Deposited: 11 Jun 2014 11:49
Last Modified: 11 Jun 2014 11:49
URI: https://ueaeprints.uea.ac.uk/id/eprint/48693
DOI:

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