Nodule Specific CLE Peptides Mediate Crosstalk Between Local Nodule Development and Systemic Auto-Regulation of Nodulation Pathways in Medicago truncatula

Root nodule development is controlled by the local Nod Factor and cytokinin-dependent pathways and systemically by the autoregulation of nodulation (AON) pathway (1-3). In Medicago truncatula, the AON pathway controls nodule number by suppressing nodulation competency in yet-to-be-infected roots via the shoot-expressing SUNN receptor kinase (2). Recently, MtCLE12 and MtCLE13, which show nodule-specific expression and encode CLE (CLAVATA3/ESR) peptide ligands, have been shown to regulate AON (3). The ectopic expression of MtCLE12 or MtCLE13 (but not MtCLE26 which does not have nodule specific expression) abolishes nodulation in wild type roots but not in the super nodulating null mutant sunn-4 (defective in SUNN). This suggests that ectopic over-expression of MtCLE12 or 13 in roots triggers constitutive AON which suppresses nodulation competency in a SUNN-dependent manner. Using qRT-PCR we showed that three local nodulation pathway genes: NODULE INCEPTION (NIN, a common regulator of the Nod Factor and cytokinin pathways), MtEFD (ethylene response factor and negative regulator of nodule differentiation) and MtRR8 (a type-A response regulator involved in negative regulation of cytokinin signaling) were each regulated specifically by MtCLE12 in a SUNN-dependent manner. This suggests that MtCLE12 expression in the first formed nodules influences shoot SUNN to produce the shoot AON signal. We propose that this results in up regulated MtEFD and MtRR8 in roots which reduces nodulation competency by down regulating NIN which is at the crossroads of Nod Factor and cytokinin signaling. This proposed model links for the first time the pathways for Nod factor signaling, cytokinin perception and AON and provides a means for the plant to regulate nodule numbers in tune with its physiological status.

1. I.M.L. Saur, et al. New Phytologist 190, 865-874, (2011).
2. D. E. Reid, et al. Mol Plant Microbe Interact. 24, 606 (2011).
3. V. Mortier et al., Plant Physiology 153, 222 (2010).