development. This indicates that NIN acts
downstream of CCaMK and the GRAS tran-
scription factors to regulate symbiotic root hair
responses (Marsh et al. 2007 ).Ninis activated by
cytokinin through a L. japonicus cytokinin
receptor, LHK1 (Tirichine et al. 2007 ). Gain-of-
function LHK1 spontaneously induces nodules
without rhizobial infection (Tirichine et al.
2007 ). Ectopic expression ofNinalso induces
cortical cell divisions in the absence of rhizobia
(Soyano et al. 2013 ). NIN regulates cortical cell
divisions downstream of the cytokinin signalling.
LjNF-YA1andLjNF-YB1have been identified
as direct targets of NIN (Soyano et al. 2013 ).
They are involved in stimulation of cell division.
They encode different subunits of a CCAAT-box
binding heterotrimeric complex. Knock-down of
LjNF-YA1 prevents the nodule formation. Co-
overexpression of the two Lotus NF-Y genes
stimulates cells division in lateral root primordia
as well as cortical cell division. NF-Y regulates
expression of its target genes by influencing his-
tone modification and requires an additional
transcriptional activator to efficiently activate
transcription. NIN may also regulate other tran-
scription factors that act together with the NF-Y
to induce cortical cell division.ArabidopsisNIN-
like proteins (NLPs) play a central role in the
transcriptional regulation of nitrate-responsive
genes and target nitrate-responsive elements
(Konishi and Yanagisawa 2013 ), which are
almost identical to NIN-binding nucleotide
sequences. Nitrate is known as an inhibitor of
nodulation. There may be a linkage between
nodulation control and nitrate-response pathways.
Acknowledgments We thank Sylvia Singh for carefully
reading the manuscript.
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