The role of auxin-induced peaks of α-expansin expression during lateral root primordium formation in Pinus taeda

Lateral root primordium (LRP) formation in the four vascular poles of 7- to 10-day-old loblolly pine (Pinus taeda L.) seedlings was promoted by the auxin alpha-naphthalene acetic acid (NAA) and occurred closer to the root tip than in the controls. These observations support a role of auxin located within the vascular cylinder in the development of LRP. Adjacent LRP almost never occurred in the same vascular pole, but NAA increased the probability that this would happen by two- to six-fold. Expression of genes for alpha-expansin was induced by auxin in hypocotyls but occurred spontaneously in primary roots. In situ localization revealed that expansin was expressed in the vascular parenchyma where LRP formed spontaneously in roots, and where adventitious root meristems formed in auxin-treated hypocotyls. Expansin expression was not uniform in the LRP-forming zone of the primary root. The number of cells exhibiting expansin expression longitudinally occurred in distinct peaks, which were more frequent after auxin treatment. These peaks may reflect non-uniform distribution of auxin in the stele or localization of cells with increased sensitivity to auxin. However, LRP were spaced about 10-fold further apart than the peaks of expansin expression. Therefore, localized peak expansin expression did not always predict the location of LRP. We speculate that other factors must interact with locally high auxin concentrations to specify the location of LRP.