ACAULIS5, an Arabidopsis gene required for stem elongation, encodes a spermine synthase

Polyamines have been implicated in a wide range of biological processes, in cluding growth and development in bacteria and animals, but their function in higher plants is unclear. Here we show that the Arabidopsis ACAULIS5 (AC L5) gene, whose inactivation causes a defect in the elongation of stem inte rnodes by reducing cell expansion, encodes a protein that shares sequence s imilarity with the polyamine biosynthetic enzymes spermidine synthase and s permine synthase, Expression of the recombinant ACL5 protein in Escherichia coli showed that ACL5 possesses spermine synthase activity. Restoration of the acl5 mutant phenotype by somatic reversion of a transposon-induced all ele suggests a non-cell-autonomous function for the ACL5 gene product, We a lso found that expression of the ACL5 cDNA under the control of a heat shoc k gene promoter in acl5 mutant plants restores the phenotype in a heat shoc k-dependent manner. The results of the experiments showed that polyamines p lay an essential role in promotion of internode elongation through cell exp ansion in Arabidopsis, We discuss the relationships to plant growth regulat ors such as auxin and gibberellins that have related functions.