Thirteen a-expansin genes were isolated from Rumex palustris, adding to the six already documented forthis species. Five a-expansin genes were selected for expression studies in various organs/tissues ofR. palustris, with a focus on roots exposed to aerated or O2-deficient conditions, using real-time RT-PCR.Several cases of differential expression of a-expansin genes in the various root types of R. palustris weredocumented, and the identity of the dominant transcript differed between root types (i.e., tap root vs.lateral roots vs. adventitious roots). Several genes were expressed differentially in response to low O2. In situhybridizations showed expansin mRNA expression in the oldest region of the tap root was localized to cellsnear the vascular cambium; this being the first report of expansin expression associated with secondarygrowth in roots. In situ hybridization also showed abundant expression of expansin mRNA in the mostapical 1 mm of adventitious roots. Such early expression of expansin mRNA in cells soon after beingproduced by the root apex presumably enables cell wall loosening in the elongation zone of roots. Inaddition, expression of some expansin mRNAs increased in ‘mature zones’ of roots; these expansins mightbe involved in root hair formation or in formation of lateral root primordia. The present findings supportthe notion that large gene families of a-expansins enable flexibility in expression for various organs andtissues as a normal part of plant development, as well as in response to abiotic stress.