Autoregulation of nodulation (AON) serves as a vital negative feedback mechanism that governs the quantity of nodules formed on legume roots. Its significance lies in the fact that nodule formation and nitrogen fixation impose substantial energy costs on the host plant. In the case of the model legume Medicago truncatula, the AON process unfolds as follows: (i) Rhizobial infection triggers the expression of two peptide-coding genes, MtCLE12 and MtCLE13, (ii) The mature CLE peptides migrate to the shoot and bind to their receptor SUNN, (iii) SUNN activation leads to a reduction in the expression of microRNA 2111, which acts as a negative regulator of transcript levels of TMLs (TOO MUCH LOVE), (iv) TMLs encode a Kelch repeat-containing F-box protein in the root that negatively modulates nodule numbers.
However, the downstream components of the CLE/SUNN/miR2111/TML-mediated AON mechanism remain unknown, including their role in controlling nodule formation. To address this, we employed RNA-Seq analysis coupled with reverse genetics to identify and characterise downstream target genes, including transcription factors that relay AON signals, thereby influencing root nodule formation. We have elucidate the potential functions of the newly discovered components within these peptide signalling pathways and explore their interactions with other signalling pathways that regulate nodule formation. By presenting a comprehensive model of the molecular regulation of nodulation, we aim to enhance our understanding of this intricate process, ultimately paving the way for improved sustainable crop productivity and enhanced food security.