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Abstract
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 transcription 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.
Highlights
In plants, animals and microbes, many aspects of physiology, metabolism and development exhibit 24-hour rhythmicity controlled by a circadian clock
We suggest that circadian regulation ip of Nodule-specific Cysteine Rich peptides (NCRs) gene expression in nodules may play a role to ensure temporal coordination of bacterial r activity with the rhythms of the plant host
We found an alternative version of the EE motif, which we c call Evening Element Related (EER) (Fig 4A), to be 3.6 fold and significantly enriched (P = 5.19e-147), s with 57.6% of rhythmic NCRs containing an EER motif compared to an abundance of 16.2% of all NCR u promoters
Summary
Animals and microbes, many aspects of physiology, metabolism and development exhibit 24-hour rhythmicity controlled by a circadian clock. We investigate potential mechanisms through analysis of the rhythmic transcriptome in nodules and reveal circadian control t of a subset of NCR genes through EE motifs in their promoters.
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