(6 ) and the analogue N ‐(4‐aminobutyl)cyclopentylamine (21 ) promoted the expression level of thermospermine synthase ACAULIS5 (ACL5 ) and enhanced xylemformation. In addition, xylemin (6 ) was found to significantly promote lateral root formation, whereas xyleminanalogues 18 –23 including 21 did not. These results indicate that the analogue 21 has the potential as a novel inhibitor of thermospermine