The Lotus japonicus Genome

5

Genome Sequence and Gene Functions

inMesorhizobium loti and Relatives

Kazuhiko Saeki and Clive W. Ronson

Abstract

Mesorhizobium lotiis a collective name for mesorhizobial species that

establish nitrogen-fixing symbiosis with Lotus species. Accumulating

genetic and genomic data indicate that diverse strains ofM. lotihave been

generated through lateral integration of symbiosis islands into core

chromosomes of a range of bacteria. The M. loti symbiosis islands

probably derived from a common ancestral island and are evolving by

acquiring accessory genetic elements while maintaining gene sets

essential for nodulation and nitrogenfixation together with genes for

some supportive processes. This view was supported by preliminary

mappings of next-generation sequencing data of three strains, R7A,

NZP2037, and NZP2213, on the whole-genome sequence of the strain

MAFF303099. Common properties ofM. lotigenes involved in symbiosis

and their regulation are also described along with genetic resources to

studyM. loti.

5.1 Introduction

Rhizobia are a group of soil bacteria that estab-
lish mutualistic relationships with leguminous
plants. With a compatible legume host, rhizo-
bium cells are capable of inducing root nodules,
within which they reside intracellularly andfix

atmospheric nitrogen to ammonium for nutri-

tional exchange (Perret et al. 2000 ).

This chapter deals with genomic and genetic

analysis of rhizobia that establish symbiosis with

Lotus japonicus. Mesorhizobium loti strains

MAFF303099 and R7A are described mainly,

since they had been used in landmark studies

such as thefirst determination of the complete

genome sequence of a nitrogen-fixing bacterium

(Kaneko et al. 2000 ) and the discovery of a

symbiosis island as an integrative and conjuga-

tive element (ICE) that harbors most genes

involved in root nodule formation and nitrogen

fixation (Sullivan and Ronson 1998 ). However,

taxonomy of rhizobia including those associated

with L. japonicus is still in a state of flux

(Sawada et al. 2003 ). Hence, wefirstly describe

K. Saeki (&)
Department of Biological Science, Faculty of
Science, Nara Women’s University, Nara, Japan
e-mail: [email protected]

C.W. Ronson
Department of Microbiology and Immunology,
University of Otago, PO Box 56, Dunedin
New Zealand

S. Tabata and J. Stougaard (eds.),The Lotus japonicus Genome, Compendium of Plant Genomes,
DOI 10.1007/978-3-662-44270-8_5,©Springer-Verlag Berlin Heidelberg 2014

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