8
Lotus Genes Involved in Nodule
Function and Nitrogen Fixation
Norio Suganuma
Abstract
The existence of host plant genes essential for symbiotic nitrogenfixation
has been suggested by the isolation of legume Fix−mutants, the nodules
of which are normally endocytosed by rhizobia but exhibit little or no
nitrogen-fixing activity. However, it has been difficult to identify the
genes responsible for the Fix−phenotype because of the large genome size
of crop legumes. Genome sequencing of the model legume Lotus
japonicusallowed us to identify the causal genes in Fix−mutants by map-
based cloning. This chapter describes theLotusgenes involved in nodule
function and nitrogenfixation as identified by forward genetics.8.1 Introduction
Inside infected host cells, the tip of infection
threads releases rhizobia within symbiosomes. In
Lotus japonicus nodules, these organelle-like
units usually contain several rhizobia, which
subsequently differentiate into bacteroides and
fix atmospheric nitrogen gas.Mesorhizobium loti
bacteroides, which are compatible withL. japo-
nicus, are slightly larger than free-living rhizobia
(Suganuma et al. 2003 ), despite having similar
morphology and DNA content, and are able to
form colonies on agar plates (Mergaert et al.
2006 ). This is in contrast to bacteroides within
the nodules of galegoid legumes, which are ter-
minally differentiated.
Nitrogenfixation is catalyzed by nitrogenase
complex, the components of which are encoded
in the rhizobial genome. This enzyme is oxygen-
sensitive and has extremely high-energy
requirements. Efficient nitrogenfixation is then
achieved with the help of an oxygen-binding
protein, leghemoglobin, which support the aero-
bic respiration of rhizobia and protects nitroge-
nase against inactivation by free oxygen. Fixed
nitrogen is assimilated in the host cells of nod-
ules and translocated to shoots for optimal plant
growth. In general, nitrogenfixation occurs only
when rhizobia are endocytosed in nodule cells,
with exception ofAzorhizobiumand the photo-
synthetic Bradyrhizobium (Hakoyama et al.
2009 ). Most rhizobia do notfix nitrogen under
free-living conditions. This suggests that nitro-
gen fixation by symbiotic rhizobia is strictly
controlled by the host plants, although it remains
unclear how they regulate the process.N. Suganuma (&)
Department of Life Science, Aichi University of
Education, Kariya, Aichi 448-8542, Japan
e-mail: [email protected]
S. Tabata and J. Stougaard (eds.),The Lotus japonicus Genome, Compendium of Plant Genomes,
DOI 10.1007/978-3-662-44270-8_8,©Springer-Verlag Berlin Heidelberg 2014
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