Disruption of a dna topoisomerase I gene affects morphogenesis in arabidopsis

The genesis of phyllotaxis, which often is associated with the Fibonacci series of numbers, is an old unsolved puzzle in plant morphogenesis. Here, we show that disruption of an Arabidopsis topoisomerase (topo) I gene named TOP1α affects phyllotaxis and plant architecture. The divergence angles and internode lengths between two successive flowers were more random in the top1α mutant than in the wild type. The top1α plants sporadically produced multiple flowers from one node, and the number of floral organ primordia often was different. The mutation also caused the twisting of inflorescences and individual flowers and the serration of leaf margins. These morphological abnormalities indicate that TOP1α may play a critical role in the maintenance of a regular pattern of organ initiation. The top1α mutant transformed with the RNA interference construct for TOP1β, another topo I gene arrayed tandemly with TOP1α, was found to be lethal at young seedling stages, suggesting that topo I activity is essential in plants.