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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.
INTRODUCTION
Plants, unlike animals, use postembryonic organogenesis to elaborate their architecture. Lateral branching in root and shoot systems represents a major determinant of plant architecture. Lateral root development in Arabidopsis provides a model for the study of factors that regulate postembryonic organogenesis in higher plants. The Arabidopsis root has a relatively simple anatomy composed of single layers of epidermal, cortical, and endodermal cells surrounding the vascular tissues (Figure 1A; Dolan et al., 1993). Lateral roots in Arabidopsis are derived from pericycle founder cells positioned adjacent to the two protoxylem poles (Figure 1 B; Blakely et al., 1982). Malamy and Benfey (1997) have defined seven developmental stages that precede lateral root emergence. The first periclinal division represents the most common criterion used to define the onset of lateral root formation (Esau, 1977; Lloret et al., 1989). However, in Arabidopsis, the first periclinal divisions occur within groups of eight to 10 short initial cells, indicating that founder cells first must undergo a series of transverse divisions (Malamy and Benfey, 1997). The exact nature and order of the initial transverse divisions have not been characterized in detail in Arabidopsis.
Auxin represents a key regulator of lateral root development (Blakely et al., 1982; Laskowski et al., 1995). Several auxin-related mutants have been...