Plant circadian clock control of Medicago truncatula nodulation involving regulation of Nodule Cysteine-Rich genes

Legumes house nitrogen-fixing endosymbiotic rhizobia in specialized
polyploid cells within root nodules, which undergo tightly regulated
metabolic activity. By carrying out expression analysis of transcripts
over time in Medicago truncatula nodules we found that the circadian clock
enables coordinated control of metabolic and regulatory processes linked
to nitrogen fixation. This involves the circadian clock-associated
transcriptional factor LATE ELONGATED HYPOCOTYL (LHY), with lhy mutants
being affected in nodulation. Rhythmic transcripts in root nodules include
a subset of Nodule-specific Cysteine Rich peptides (NCRs) that have the
LHY-bound conserved Evening Element in their promoters. Until now, studies
have suggested that NCRs act to regulate bacteroid differentiation and
keep the rhizobial population in check. However, these conclusions came
from the study of a few members of this very large gene family that has
complex diversified spatio-temporal expression. We suggest that rhythmic
expression of NCRs may be important for temporal coordination of bacterial
activity with the rhythms of the plant host, in order to ensure optimal
symbiosis.