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A CEP signaling peptide regulating symbiotic nodules but not roots

The Medicago SymCEP7 hormone increases root nodule number via shoots without compromising lateral root number

 

Legumes acquire soil nutrients both through their nitrogen-fixing root nodules and lateral roots. To balance the costs and benefits of nodulation, legumes negatively control root nodule number by different hormonal pathways. How legumes simultaneously coordinate root nodule and lateral root development to procure nutrients remains however poorly understood. In Medicago (Medicago truncatula), a subset of C-TERMINALLY ENCODED PEPTIDE (CEP) hormones promotes nodule number but all CEP hormones tested to date also negatively regulate lateral root number. In a new collaborative study between the team SILEG at IPS2 and Michael Djordjevic (University of Canberra, Australia), published in Plant Physiology, we report using mass spectrometry that the Medicago CEP7 gene produces a mature peptide, SymCEP7, which promotes from the shoot, in a pM to nM concentration range, nodulation without compromising lateral root number. Fluorescence microscopy and expression analyses demonstrated that SymCEP7 activity requires the COMPACT ROOT ARCHITECTURE 2 (CRA2) receptor and activates the shoot-to-root systemic effector, miR2111. Shoot-applied SymCEP7 also promoted nodule number in White Clover (Trifolium repens) and Lotus (Lotus japonicus), which suggests that this biological function is evolutionarily conserved. We propose that SymCEP7 acts in the Medicago shoot to counterbalance the autoregulation pathways induced rapidly by rhizobia to enable nodulation without compromising lateral root growth, thus promoting the acquisition of nutrients other than nitrogen to support their growth.

 

Identification of SymCEP7 that accumulate in shoot vasculature depending on the CRA2 receptor (A) Predicted CEP7 gene product showing the two CEP peptide domains (orange). (B) High accuracy MS-MS spectrum of the predominant CEP7-derived hormone species identified in vivo in hairy root culture exudates. Highlighted in blue and red, respectively, are the matched y and b ions corresponding to the domain 1 CEP7 peptide hormone. The spectrum is consistent with proline hydroxylation at position 7 and 11 of the identified 15 amino acid product. (C-D) Medicago leaf vascular tissue was isolated from the wild-type A17 (C) and the cra2 mutant (D), incubated with FITC-SymCEP7, washed, formaldehyde cross-linked, and observed using confocal microscopy. FITC fluorescence is green and auto–fluorescence due to cell wall related phenolic compounds is purple.
Identification of SymCEP7 that accumulate in shoot vasculature depending on the CRA2 receptor (A) Predicted CEP7 gene product showing the two CEP peptide domains (orange). (B) High accuracy MS-MS spectrum of the predominant CEP7-derived hormone species identified in vivo in hairy root culture exudates. Highlighted in blue and red, respectively, are the matched y and b ions corresponding to the domain 1 CEP7 peptide hormone. The spectrum is consistent with proline hydroxylation at position 7 and 11 of the identified 15 amino acid product. (C-D) Medicago leaf vascular tissue was isolated from the wild-type A17 (C) and the cra2 mutant (D), incubated with FITC-SymCEP7, washed, formaldehyde cross-linked, and observed using confocal microscopy. FITC fluorescence is green and auto–fluorescence due to cell wall related phenolic compounds is purple.

06/02/2023