Auxin transport promotes Arabidopsis lateral root initiation

Lateral root development in Arabidopsis provides a model for the study of h ormonal signals that regulate postembryonic organogenesis in higher plants. Lateral roots originate from pairs of pericycle cells, in several cell fil es positioned opposite the xylem pole, that initiate a series of asymmetric , transverse divisions. The auxin transport inhibitor N-1-naphthylphthalami c acid (NPA) arrests lateral root development by blocking the first transve rse division(s). We investigated the basis of NPA action by using a cell-sp ecific reporter to demonstrate that xylem pole pericycle cells retain their identity in the presence of the auxin transport inhibitor. However, NPA ca uses indoleacetic acid (IAA) to accumulate in the root apex while reducing levels in basal tissues critical for lateral root initiation. This pattern of IAA redistribution is consistent with NPA blocking basipetal IAA movemen t from the root tip. Characterization of lateral root development in the sh oot meristemless1 mutant demonstrates that root basipetal and leaf acropeta l auxin transport activities are required during the initiation and emergen ce phases, respectively, of lateral root development.