Symbiosis receptor kinase (SYMRK) is indispensable for activation of root nodule symbiosis (RNS) at both epidermal and cortical levels and is functionally conserved in legumes. Previously, we reported SYMRK to be phosphorylated on "gatekeeper" Tyr both in vitro as well as in planta. Since gatekeeper phosphorylation was not necessary for activity, the significance remained elusive. Herein, we show that substituting gatekeeper with nonphosphorylatable residues like Phe or Ala significantly affected autophosphorylation on selected targets on activation segment/αEF and β3-αC loop of SYMRK. In addition, the same gatekeeper mutants failed to restore proper symbiotic features in a symrk null mutant where rhizobial invasion of the epidermis and nodule organogenesis was unaffected but rhizobia remain restricted to the epidermis in infection threads migrating parallel to the longitudinal axis of the root, resulting in extensive infection patches at the nodule apex. Thus, gatekeeper phosphorylation is critical for synchronizing epidermal/cortical responses in RNS.

Symbiosis Receptor Kinase (SYMRK), a member of the Nod factor signaling pathway, is indispensible for both nodule organogenesis and intracellular colonization of symbionts in rhizobia-legume symbiosis. Here, we show that the intracellular kinase domain of a SYMRK (SYMRK-kd) but not its inactive or full-length version leads to hyperactivation of the nodule organogenic program in Medicago truncatula TR25 (symrk knockout mutant) in the absence of rhizobia. Spontaneous nodulation in TR25/SYMRK-kd was 6-fold higher than rhizobia-induced nodulation in TR25/SYMRK roots. The merged clusters of spontaneous nodules indicated that TR25 roots in the presence of SYMRK-kd have overcome the control over both nodule numbers and their spatial position. In the presence of rhizobia, SYMRK-kd could rescue the epidermal infection processes in TR25, but colonization of symbionts in the nodule interior was significantly compromised. In summary, ligand-independent deregulated activation of SYMRK hyperactivates nodule organogenesis in the absence of rhizobia, but its ectodomain is required for proper symbiont colonization.

Plant receptor‐like kinases (RLKs) share their evolutionary origin with animal interleukin‐1 receptor‐associated kinase (IRAK)/Pelle family of soluble kinases and are distinguished by having tyrosine as ‘gatekeeper’. This position is adjacent to the hinge region and is hidden in a hydrophobic pocket of the catalytic cleft of protein kinases and is therefore least probable to be a target for any modification. This communication illustrates the accessibility of the gatekeeper site (Y670) towards both autophosphorylation and dephosphorylation in the recombinant cytoplasmic domain of symbiosis receptor kinase from Arachis hypogaea ( Ah SYMRK). Autophosphorylation on gatekeeper tyrosine was detected prior to extraction but never under in vitro conditions. We hypothesize gatekeeper phosphorylation to be associated with synthesis/maturation of Ah SYMRK and this phenomenon may be prevalent among RLKs.