Abstract
Polyamines are ubiquitous low-molecular-mass polycations involved in many cellular processes, including chromatin condensation, maintenance of DNA structure, RNA processing, regulation of translation, modulation of enzyme activities, and stabilization of membranes. In a previous paper, we reported that loss-of-function mutants of the ACAULIS5 (ACL5) gene in Arabidopsis show a severe dwarf phenotype and it encodes spermine synthase. Only recently, however, ACL5 has been suggested to encode an enzyme synthesizing thermospermine, a structural isomer of spermine.
We examined the effect of exogenous application of polyamines on the growth of acl5 plants. Thermospermine partially rescued the acl5 dwarf phenotype, while spermine had no effects on the growth of acl5 plants. Real-time RT-PCR experiments revealed that acl5 transcript levels in acl5 seedlings, which are up-regulated compared to those in the wild type, were reduced in response to application of thermospermine, suggesting a negative feedback control of thermospermine synthesis.
