Abstract
The ACAULIS5 (ACL5) gene, whose inactivation causes a severe defect in the elongation of stem internodes, encodes a polyamine biosynthetic enzyme, spermine synthase. In order to define further the function of polyamines in plant development, we have isolated five suppressor of acl5 (sac) mutants. Although they exhibited various degrees of the recovery in the growth of stem internodes, stem sections and expression analyses of several genes revealed that the recovery of the plant height was resulted from that of the cell length in all five sac mutants. In addition, expression levels of ACL5, which is upregulated in acl5-1 mutants, were restored even before bolting in these sac mutants. Fine mapping located the Sac51 mutation in the 5' leader sequence of the MSJ1.18 gene, which encodes a putative bHLH transcription factor. The possible regulatory mechanism leading to the dominant Sac51 phenotype will be discussed.
