Abstract
Loss-of-function mutants of the ACAULIS5 (ACL5) gene have a specific defect in the cell growth in stem internodes. ACL5 encodes a thermospermine synthase. Thermospermine, is an isomer of a common polyamine, spermine. To elucidate the mode of action of thermospermine in the stem elongation, we have isolated suppressor mutants of acl5, named sac, that rescue the dwarf phenotype. The dominant sac51-d allele disrupts a short upstream ORF of SAC51, which encodes a bHLH transcription factor. Genes responsible for sac52 and sac56 encode a ribosomal protein L10A and L4A, respectively. SAC53 was found to encode a receptor for activated C kinase (RACK1A). RACK1 is known to associate with the ribosome and play a key role in translational control in animals and yeasts. We discuss the possibility that thermospermine may be involved in the translational control of the genes required for stem elongation.
