PRE2023-Functional evolution of thermospermine in plants (PID2022-141770NB-I00 B)

Thermospermine (tspm) is a polyamine present in the whole plant kingdom, whose synthesis is catalyzed by the tspm synthase encoded by ACL5. A function for tspm has been proposed in the control of xylem proliferation and differentiation in Arabidopsis thaliana, which is exerted through translational regulation of SACL transcripts. Recent work in our lab has shown that ACL5 and SACL genes are also present in non-vascular plants. However, molecular genetic analysis in the bryophyte Marchantia polymorpha has shown that MpACL5 and MpSACL regulate different processes, and tspm does not regulate MpSACL translation. Plants defective in MpACL5 display accelerated bifurcation of apical meristems (dichotomous branching), while tspm application or MpACL5 overexpression delay bifurcation. Therefore, we propose to address several questions: (1) How does tspm regulate dichotomous branching in M. polymorpha? (2) Does tspm affect dichotomous branching through translational regulation? Of which transcripts? (3) In vascular plants that show dichotomy, such as ferns, does tspm regulate dichotomous branching, xylem proliferation, or both?