Gene expression during development is regulated both transcriptionally and translationally; however, relatively few examples of translational regulation are known. On , Imai and colleagues describe for the first time how an upstream open reading frame (uORF) mediates translational control during plant development. Loss-of-function mutants of Arabidopsis ACAULIS 5(ACL5), which encodes spermine synthase, have a dwarf phenotype because of a defect in stem elongation. To find out how ACL5regulates stem elongation, the researchers isolated a dominant suppressor mutant of the acl5 phenotype - sac51-d. They show that sac51-d disrupts a short uORF of SAC51, which encodes a bHLH transcription factor. Other experiments indicate that this disruption might increase the translation of SAC51. Thus, the researchers suggest that the uORF-encoded protein normally prevents the initiation of SAC51translation, and that ACL5 acts directly or indirectly (possibly through spermine's effects on protein synthesis) to activate SAC51translation and subsequent stem elongation.
