2010 Volume 24 Issue 3_4 Pages 153-162
When cucumber (Cucumis sativus L.) seeds germinate in a horizontal position, seedlings develop a specialized protuberance (or peg) on the lower side of the transition zone between the hypocotyl and the root. Gravistimulation induces asymmetric auxin distribution and auxin-inducible gene expression, so that peg formation is suppressed on the upper side of the transition zone. However, we know little of transcriptional regulation of auxin-inducible genes in the transition zone. We demonstrated that Auxin Response Factor 5 (CsARF5) protein functions as a transcriptional repressor of auxin-inducible gene in protoplasts. Our immunoblot analysis suggested that CsARF5 protein is more abundant on the upper side that suppressed peg formation than on the lower side that induced peg formation in the transition zone. When collar-like pegs were induced by exogenous application of auxin to intact seedlings, accumulation of CsARF5 protein decreased substantially. By contrast, exogenous auxin application to segments of the transition zone neither induced collar-like pegs nor decreased CsARF5 proteins. These results suggested that less auxin suppresses peg formation and then indirectly induces accumulation of CsARF5 protein to ensure repression of transcription of auxin-inducible genes.
