Abstract
Peptides are signaling molecules regulating various aspects of plant development, including the balance between cell division and differentiation in different meristems. Among those, CLAVATA3/Embryo Surrounding Region-related (CLE-ESR) peptide activity depends on leucine-rich-repeat receptor-like-kinases (LRR-RLK) belonging to the subclass XI. In legume plants, such as the Medicago truncatula model, specific CLE peptides were shown to regulate root symbiotic nodulation depending on the LRR-RLK SUNN (Super Numeric Nodules). Amongst the ten M. truncatula LRR-RLK most closely related to SUNN, only one showed a nodule-induced expression, and was so-called MtNRLK1 (Nodule-induced Receptor-Like Kinase 1). MtNRLK1 expression is associated to root and nodule vasculature as well as to the proximal meristem and rhizobial infection zone in the nodule apex. Except for the root vasculature, the MtNRLK1 symbiotic expression pattern is different than the one of MtSUNN. Functional analyses either based on RNA interference, insertional mutagenesis, and overexpression of MtNRLK1 however failed to identify a significant nodulation phenotype, either regarding the number, size, organization or nitrogen fixation capacity of the symbiotic organs formed.
Highlights
Plant growth relies on stem cells located in meristems from which most tissues and organs are post-embryonically derived e.g. the Shoot and Root Apical Meristems (SAM and RAM)[1,2,3]
In different legumes including M. truncatula, the Autoregulation of Nodulation” (AON) pathway was shown to rely on a CLE/leucine-rich-repeat receptor-likekinases (LRR-RLK) regulatory module, involving the production in rhizobia inoculated roots of peptides such as MtCLE12/MtCLE13 or LjCLE-RS1/LjCLE-RS2 (CLE-Root Signal) that are proposed to move to shoots through the xylem vasculature and to act depending on the MtSUNN (Super Numeric Nodules)/LjHAR1 (Hypernodulation and Aberrant Root)/GmNARK (Nodule Autoregulation Receptor Kinase) subclass XI LRR-RLK receptor closely related to Arabidopsis CLV116–25
MtNRLK1 is most closely related to the A. thaliana AtBAM3 receptor while MtSUNN is the closest M. truncatula homolog of AtCLV121. As both AtBAM3 and AtCLV1 function in SAM homeostasis by regulating stem cell identity[40,41,42], we have made the hypothesis that MtNRLK1 might be involved in controlling the balance between cell proliferation and differentiation within the nodule potentially in relation with MtCLE12 and MtCLE13 peptides previously shown to be expressed in differentiated nodules[16]
Summary
Plant growth relies on stem cells located in meristems from which most tissues and organs are post-embryonically derived e.g. the Shoot and Root Apical Meristems (SAM and RAM)[1,2,3]. In different legumes including M. truncatula, the AON pathway was shown to rely on a CLE/LRR-RLK regulatory module, involving the production in rhizobia inoculated roots of peptides such as MtCLE12/MtCLE13 or LjCLE-RS1/LjCLE-RS2 (CLE-Root Signal) that are proposed to move to shoots through the xylem vasculature and to act depending on the MtSUNN (Super Numeric Nodules)/LjHAR1 (Hypernodulation and Aberrant Root)/GmNARK (Nodule Autoregulation Receptor Kinase) subclass XI LRR-RLK receptor closely related to Arabidopsis CLV116–25. In contrast to the nark/sunn/har[1] AtCLV1-related mutants, MtCLV2- and MtRPK2-related mutants show SAM defects, indicating that partially overlapping receptor complexes controls SAM activity and nodule number in legumes[27,29] Another family of secreted peptides, referred to as CEPs (C-terminal Encoding Peptides), mostly produced under nitrogen-starvation, was shown to positively regulate nodulation and to inhibit lateral root formation[30]. In M. truncatula, the CEP1 peptide requires CRA2 to regulate nodule and lateral root development[35]
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