The Lotus japonicus Genome

(Krusell et al. 2005 ). Thesefindings suggest that
SST1might have been recruited from a preexisting
sulfate transporter gene inL. japonicus.
The SEN1gene encodes an integral mem-
brane protein homologous to nodulin-21 of
Glycine max(Kawaguchi et al. 2002 ; Suganuma
et al. 2003 ; Hakoyama et al.2012a). The location
and function of the SEN1 protein remain to be
demonstrated. However, it is predicted to occur
in the symbiosome membrane and to transport an
essential element required for nitrogenfixation,
because theSEN1gene is expressed exclusively
in infected nodule cells. The nodules of thesen1
mutant completely lack nitrogen-fixation activity,

even though the proteins making up the nitro-

genase complex are expressed (Suganuma et al.

2003 ; Hakoyama et al.2012a); this suggests that

SEN1 is indispensable for the induction of

nitrogen-fixation activity. In addition, symbio-

some and/or bacteroid differentiation are

impaired even at an early stage of nodule

development insen1mutants (Hakoyama et al.

2012a). This suggests that SEN1 is more directly

involved in symbiosome and/or bacteroid differ-

entiation than in nitrogen-fixation activity, in

contrast to SST1. The function of SEN1 needs to

be clarified to elucidate the host regulation of

rhizobial symbiotic nitrogenfixation.

N 2

NH 3

SO 4 2-?

Acetyl-CoA

Homocitrate

CoA-SH

2-Oxoglutarate

?

Symbiosome Nitrogenase

membrane Bacteroid

Vascular strand

Plant cell wall

Plant plasma membrane

Nodule cell

?

Fig. 8.1 Location and function of SST1, SEN1, FEN1,
SYP71, and IGN1 for rhizobial symbiotic nitrogen
fixation inL. japonicusnodules. SST1 located in the
symbiosome membrane transportsSO 42
from the plant
cytosol to bacteroides, as sulfur is required for compo-
nents of the nitrogenase complex. SEN1 is predicted to be
located in the symbiosome membrane and to supply an
essential element for nitrogenase. FEN1 located in the
cytoplasm of infected cells catalyzes the formation of

homocitrate, which is supplied to bacteroides for synthe-

sis of the FeMo-cofactor, one component of the nitroge-

nase complex. SYP71 is expressed in the vascular tissues

and might translocate an unknown substance crucial for

nitrogenfixation. IGN1 localized in the plasma membrane

of infected cells is hypothesized to interact with other

proteins and to function in symbiosome and/or bacteroid

differentiation and maintenance

8 Lotus Genes Involved in Nodule Function and Nitrogen Fixation 81