Auxin transport promotes Arabidopsis lateral root initiation.

Lateral root development in Arabidopsis provides a model for the study of hormonal signals that regulate postembryonic organogenesis in higher plants. Lateral roots originate from pairs of pericycle cells, in several cell files positioned opposite the xylem pole, that initiate a series of asymmetric, transverse divisions. The auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) arrests lateral root development by blocking the first transverse division(s). We investigated the basis of NPA action by using a cell-specific reporter to demonstrate that xylem pole pericycle cells retain their identity in the presence of the auxin transport inhibitor. However, NPA causes 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 movement from the root tip. Characterization of lateral root development in the shoot meristemless1 mutant demonstrates that root basipetal and leaf acropetal auxin transport activities are required during the initiation and emergence phases, respectively, of lateral root development.

Morphological features of pericycle cells in relation to their topographical location in onion adventitious roots.

Analysis of cell length, cross-sectional area and nuclear length of pericycle cells in adventitious roots of Allium cepa L. has revealed the existence of three cell types whose topographical location is related to the vascular pattern. Cells opposite the xylem are short and narrow with an elongated nucleus. The intervening cells are also short but their cross-sectional area is much greater than that of cells at the xylem pofes, while their nucleus is relatively short. Cells opposite the phloem are long, with an elongated nucleus, and their cross-sectional area is intermediate between that of the other two cell types. Lateral root primordia are always initiated from the population of short cells opposite the xylem. However, the different cell types appear in the pericycle before primordial initiation and not as a result of this event.