Legumes engage in a symbiotic relationship with nitrogen-fixing bacteria that infect the plant’s roots, forming nodules. However, plants control the number of nodules through a process called autoregulation of nodulation (AON, also abbreviated AUT) that involves a root-generated signal and a shoot-generated signal. Previous work in Lotus japonicus had suggested that members of the CLE family of signaling peptides, LjCLE-RS1 and LjCLE-RS2, were involved in regulating nodule formation. Mortier et al. have identified in Medicago truncatula a pair of CLE proteins, encoded by MtCLE12 and MtCLE13, that are homologous to the L. japonicus LjCLE-RS1 and LjCLE-RS2. Quantitative reverse transcription and promoter reporter analysis indicated that MtCLE12 and MtCLE13 were differentially expressed in the sites of root nodule formation and in stems and cotyledons. After root infection, MtCLE13 showed increased expression in a decreasing gradient from the inner cortical region (where cell proliferation occurs) to the outer cortical cells and pericycle, whereas MtCLE12 showed low expression at the earliest stages of nodule development and increased expression once round nodules had formed. Expression of MtCLE12 and MtCLE13 was lost in plants carrying mutations that rendered them nodulation-incompetent. The balance of cytokinin and auxin signaling regulates nodulation, and root application of the synthetic cytokinin 6-benzylaminopurine (BAP), but not auxin, increased the abundance of MtCLE13 transcripts. Ectopic expression of either gene completely suppressed nodule formation, suggesting that these two CLE members, like the related L. japonicus peptides, serve as negative regulators of nodulation. Composite plants containing both wild-type roots and roots overexpressing either or both MtCLE12 and MtCLE13 lost the ability to form nodules both in the wild-type roots and in the transgenic roots, suggesting that MtCLE13 and MtCLE12 served as long-distance signals to suppress nodulation. Plants with transgenic roots overexpressing either or both MtCLE13 and MtCLE12 exhibited longer petioles than wild-type plants. This effect on the leaf stem was lost in plants deficient for the receptor SUNN (sunn-1), and the inhibitory effect of ectopic MtCLE12 and MtCLE13 was partially blocked in sunn-1 plants. Thus, two members of the CLE family appear to mediate local and long-distance signals that regulate petiole growth and nodulation.
V. Mortier, G. Den Herder, R. Whitford, W. Van de Velde, S. Rombauts, K. D’haeseleer, M. Holsters, S. Goormachtig, CLE peptides control Medicago truncatula nodulation locally and systemically. Plant Physiol. 153, 222–237 (2010). [Online Journal]