Nuclear auxin signaling is essential for organogenesis but not for cell survival in the liverwort Marchantia polymorpha

Abstract: Auxin plays pleiotropic roles in plant development via gene regulation upon perception by the receptors TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX (TIR1/AFBs). This nuclear auxin signaling (NAS) originated in the common ancestor of land plants. Although complete loss of TIR1/AFBs causes embryonic lethality in Arabidopsis thaliana, it is unclear whether the requirement for TIR1-mediated auxin perception in cell viability can be generalized. The model liverwort Marchanita polymorpha has a minimal NAS system with only a single TIR1/AFB, MpTIR1. Here we show by genetic, biochemical, and transcriptomic analyses that MpTIR1 functions as an evolutionarily conserved auxin receptor. Null mutants and conditionally knocked-out mutants of MpTIR1 were viable but incapable of forming any organs and grew as cell masses. Principal component analysis performed using transcriptomes at various developmental stages indicated that MpTIR1 is involved in the developmental transition from spores to organized thalli, during which apical notches containing stem cells are established. In Mptir1 cells, stem-cell- and differentiation-related genes were up- and down-regulated, respectively. Our findings suggest that, in M. polymorpha, NAS is dispensable for cell division but essential for three-dimensional body plans by establishing pluripotent stem cells for organogenesis, a derived trait of land plants.