Physiological response of Rhizobium leguminosarum during bacteroid development

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.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of East Anglia

Year dc:date.issued

2013

Author dc:creator

• Hood, Graham

Language dc:language

en