Pericycle

Abstract: Lateral root formation in plants involves the stimulation of mature pericycle cells to proliferate and redifferentiate to create a new organ. The simple organization of the root of Arabidopsis thaliana allows the development of lateral root primordia to be characterized histologically. We have divided the process of lateral root development into 8 stages defined by specific anatomical characteristics and cell divisions. To identify the cell types in the developing primordium we have generated a collection of marker lines that express beta-glucuronidase in a tissue- or cell type-specific manner in the root. Using these tools we have constructed a model describing the lineage of each cell type in the lateral root. These studies show that organization and cell differentiation in the lateral root primordia precede the appearance of a lateral root meristem, with differential gene expression apparent after the first set of divisions of the pericycle.

Abstract: 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.

Abstract: uptake of water. On the contrary, at low rates of transpiration such as during the night or during stress conOn the basis of recent results with young primary ditions (drought, high salinity, nutrient deprivation), maize roots, a model is proposed for the movement of the apoplastic path will be less used and the hydraulic water across roots. It is shown how the complex, ‘com- resistance will be high. The role of water channels posite anatomical structure’ of roots results in a ‘com- (aquaporins) in the transcellular path is in the fine posite transport’ of both water and solutes. Parallel adjustment of water flow or in the regulation of uptake apoplastic, symplastic and transcellular pathways play in older, suberized parts of plant roots lacking a suban important role during the passage of water across stantial apoplastic component. The composite transthe different tissues. These are arranged in series port model explains how plants are designed to within the root cylinder (epidermis, exodermis, central optimize water uptake according to demands from the cortex, endodermis, pericycle stelar parenchyma, and shoot and how external factors may influence water tracheary elements). The contribution of these struc- passage across roots. tures to the root’s overall radial hydraulic resistance is examined. It is shown that as soon as early metaxy- Key words: Composite transport model, endodermis, exolem vessels mature, the axial (longitudinal) hydraulic dermis, hydraulic conductivity, reflection coefficient, root, resistance within the xylem is usually not rate-limiting. water, water channels. According to the model, there is a rapid exchange of water between parallel radial pathways because, in contrast to solutes such as nutrient ions, water per- Introduction meates cell membranes readily. The roles of apoplastic One of the essential functions of roots is to supply the barriers (Casparian bands and suberin lamellae) in the shoot with water from the soil. The process of water root’s endo- and exodermis are discussed. The model

Abstract: Lateral root development is a post-embryonic organogenesis event that gives rise to most of the underground parts of higher plants. Auxin promotes lateral root formation, but the molecular mechanisms involved are still unknown. We have isolated a novel Arabidopsis mutant, solitary-root (slr), which has reduced sensitivity to auxin. This dominant slr-1 mutant completely lacks lateral roots, and this phenotype cannot be rescued by the application of exogenous auxin. Analysis with cell-cycle and cell-differentiation markers revealed that the slr-1 mutation blocks cell divisions of pericycle cells in lateral root initiation. The slr-1 mutant is also defective in root hair formation and in the gravitropic responses of its roots and hypocotyls. Map-based positional cloning and isolation of an intragenic suppressor mutant revealed that SLR encodes IAA14, a member of the Aux/IAA protein family. Green fluorescent protein-tagged mutant IAA14 protein was localized in the nucleus, and the gain-of-function slr-1/iaa14 mutation decreased auxin-inducible BA-GUS gene expression in the root, suggesting that SLR/IAA14 acts as a transcriptional repressor. These observations indicate that SLR/IAA14 is a key regulator in auxin-regulated growth and development, particularly in lateral root formation.