in the generation of root-derived signals or per-
ception of shoot-derived signals if they are
responsible for shoot factor-mediated long-dis-
tance control of nodulation. Double-mutant
analyses suggested that TML acts in the same
genetic pathway as HAR1, while PLENTY is
likely to act in a different pathway (Yoro et al.
unpublished data). Inverted-Y grafting experi-
ments suggested that TML is likely to function
downstream of HAR1, possibly as a receptor or a
mediator of the shoot-derived inhibitor (Magori
et al. 2009 ; Magori and Kawaguchi 2009 ). Thus,
it is expected that TML could provide not only a
cue to unveil the as yet unidentified shoot-derived
inhibitor but also a molecular link with compo-
nents of the Nod factor signalling pathway.
Most recently, the map-based cloning and
deep sequencing by a next generation sequencer
identified a candidate of theTMLgene (Takahara
et al. 2013 ). The knock-down of the candidate in
hairy roots ofL. japonicusresulted in a dramatic
increase in nodulation. The putativeTMLgene
encodes a Kelch repeat-containing F-box protein
with two nuclear localizing signals (Takahara
et al. 2013 ). InA. thaliana,there are more than
100 Kelch repeat-containing F-box proteins but
their function remains largely unknown, with the
exception of ZTL, FKF1 and LKP2 that act as
blue light receptors which are critical for light-
controlled plant growth and development (Ito
et al. 2012 ; Schumann et al. 2011 ). FKF1 phys-
ically interacts with a Dof transcription factor
(CDF1) and mediates proteasome degradation of
a CDF1 protein that directly represses CON-
STANSexpression (Imaizumi et al. 2005 ). These
findings led us to speculate that TML functions
as a receptor of shoot-derived inhibitors and
represses nodule development by the degradation
of a transcription factor that constitutes the Nod
factor signalling pathway. The elucidation of the
molecular functions of TML is now required.
7.5 Conclusion
InL. japonicus, HAR1, KLV and CLV2 act in
the shoot, whereas TML acts in the root in the
context of AON (Fig.7.1). Arabinosylated CLE-
RS peptides are expected to link both organs via
long-distance communication. On the other hand,
a second long-distance signal, termed the shoot-
derived inhibitor, is essential for AON. The
shoot-derived inhibitor is synthesized in the
shoots and is translocated to the roots where it
inhibits further nodule development. The per-
ception of the root-derived signal by NARK/
HAR1/SUNN/SYM28, KLAVIER and CLV2 is
then thought to activate the production of the
shoot-derived inhibitor, but the chemical nature
of the long-distance signal of the shoot-derived
inhibitor is currently unknown. To characterize
the shoot-derived inhibitor, Gresshoff and asso-
ciates have developed a novel feeding bioassay
which involves feeding (or introduces) aqueous
leaf extracts directly into the petiole of hyper-
nodulating and supernodulating mutant plants of
Glycine max(soybean) (Lin et al. 2010 ). They
have found that suppression activity is inocula-
tion dependent and Nod factor dependent,
required GmNARK activity, and was heat-,
proteinase K- and ribo-nuclease A-resistant. On
the other hand, Yamaya and Arima ( 2010 ) also
succeeded in detecting nodulation suppression
activity, but their results differ partly. They
showed that the suppressive activity of nodula-
tion is constantly detected irrespective ofB. ja-
ponicum inoculation. Although there is some
information about the shoot-derived inhibitor in
L. japonicus, further studies such as metabolome
and transcriptome analyses will be needed to
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