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 Mp KAI2A and Mp KAI2B , Mp MAX2 encoding the F-box protein, and Mp SMXL , 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 Mp KAI2A , Mp MAX2 and Mp SMXL act in the same genetic pathway and control early gemma growth. Introduction of Mp SMXL ^d53 , in which a domain required for degradation is mutated, into wild-type plants caused phenotypes resembling those of the Mp kai2a and Mp max2 mutants. In addition, Citrine fluorescence was detected in tobacco cells transiently transformed with the 35S :Mp SMXL - Citrine gene construct and treated with MG132, a proteasome inhibitor. On the other hand, introduction of 35S :Mp SMXL ^d53 - 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 .