Major components in the KARRIKIN INSENSITIVE2-ligand signaling pathway are conserved in the liverwort, Marchantia polymorpha

KARRIKIN INSENSITIVE2 (KAI2) was first identified in Arabidopsis thaliana as a receptor of karrikin, a smoke-derived germination stimulant. KAI2 is also considered a receptor of an unidentified endogenous molecule called the KAI2-ligand (KL). Upon KAI2 activation, signals are transmitted through degradation of D53/SMXL proteins via ubiquitination by a Skp-Cullin-F-box (SCF) E3 ubiquitin ligase complex. All components in the KL signaling pathway exist in the liverwort Marchantia polymorpha, namely MpKAI2A and MpKAI2B, MpMAX2 encoding the F-box protein, and MpSMXL, indicating that the signaling pathway became functional in the common ancestor of bryophytes and seed plants. Genetic analysis using knock-out mutants of these KL signaling genes, produced using the CRISPR system, indicated that MpKAI2A, MpMAX2 and MpSMXL act in the same genetic pathway and control early gemma growth. Introduction of MpSMXLd53, in which a domain required for degradation is mutated, into wild-type plants caused phenotypes resembling those of the Mpkai2a and Mpmax2 mutants. In addition, Citrine fluorescence was detected in tobacco cells transiently transformed with the 35S:MpSMXL-Citrine gene construct and treated with MG132, a proteasome inhibitor. On the other hand, introduction of 35S:MpSMXLd53-Citrine conferred Citrine fluorescence without MG132 treatment. These findings imply that MpSMXL is subjected to degradation, and that degradation of MpSMXL is crucial for KL signaling in M. polymorpha. We also showed that MpSMXL is negatively regulated by KL signaling. Taken together, this study demonstrates that basic mechanisms in the KL signaling pathway are conserved in M. polymorpha.