Norspermine substitutes for thermospermine in the control of stem elongation in Arabidopsis thaliana.

Thermospermine is a structural isomer of spermine and is required for stem elongation in Arabidopsis thaliana. We noted the C3C3 arrangement of carbon chains in thermospermine (C3C3C4), which is not present in spermine (C3C4C3), and examined if it is functionally replaced with norspermine (C3C3C3) or not. Exogenous application of norspermine to acl5, a mutant defective in the synthesis of thermospermine, partially suppressed its dwarf phenotype, and down-regulated the level of the acl5 transcript which is much higher than that of the ACL5 transcript in the wild type. Furthermore, in the Zinnia culture, differentiation of mesophyll cells into tracheary elements was blocked by thermospermine and norspermine but not by spermine. Our results indicate that norspermine can functionally substitute for thermospermine.