Download Root Structure and Function - Oxford Academic

Root Structure and Function
The following papers resulted from a symposium held at the XVII International Botanical Congress,
Vienna, Austria, July 2005
Organized by
T. L. ROST (University of California, Davis)
A. J. BLOOM (University of California, Davis)
Annals of Botany 97: 837–838, 2006
doi:10.1093/aob/mcl042, available online at www.aob.oxfordjournals.org
Root Structure and Function
PREFACE
This highlight section of the Annals of Botany contains
a collection of articles that focus on the structure and function of roots of flowering plants. Most of the papers presented are based on talks given at a symposium on root
structure and function at the XVII International Botanical
Congress, Vienna, Austria, July 2005. Many of the articles,
briefly capsulated below, examine the interactions between
plants and the rhizosphere—the below-ground environment
that immediately surrounds the root. The rhizosphere is far
more complex physically, chemically and biologically than
the above-ground environment of the shoot because (1)
rapid, long-distance movement of energy or matter is
limited through the mix of solid, liquid and gas phases
below ground, (2) extreme changes in chemical composition occur below ground during the course of minutes and in
the space of millimeters, and (3) the density as well as the
diversity of organisms is orders of magnitude greater below
ground.
Plant roots serve as the major conduit of energy to the
rhizosphere and of nutrients and water from the rhizosphere;
therefore, plant roots dominate the dynamic habitat below
ground. The articles presented here literally begin to scratch
the surface and explore the depths of our understanding of
roots. What follows are brief statements about the contents
of each paper, the first four discuss root–soil interactions
and mineral nutrition, and the last four deal with aspects of
root structure–function related to hormonal and developmental regulation of root processes, lateral root initiation
and root cap function.
The article by Watt et al. highlights the paucity of reliable
data on root–organism interactions in the field. The authors
present an analysis that sets forth an appropriate perspective
for observing root and rhizosphere processes in the field. It
may help define the probability of different rhizosphere
processes on roots in different soil conditions.
In seedlings grown on a slope, Scippa et al. have found
that changes in root system morphology, pulling strength
and chemical lignin content were related to seedling age.
cDNA–AFLP analysis revealed that the expression of
several genes in root systems varied with slope. BLAST
analysis showed that some differentially expressed genes
are homologues to genes induced by environmental stresses
in other plant species, and/or are involved in the production
of strengthening materials.
Bloom et al. show that in a nutrient solution, root elongation of a maize seedling—even one with ample nitrogen
reserves—depended most strongly on exogenous inorganic
nitrogen, and less so, if at all, on either the pH of the bulk
nutrient solution or the mechanical properties of cell walls.
The presence of ammonium or nitrate in the medium stimulated elongation by 29 % or 14 %, respectively. Acidifying
the root medium from pH 65 to 56 nearly doubled the
elasticity of the seminal root, but slightly decreased its
elongation. Addition of an osmoticum to the medium had no
effect on root elongation in the absence of inorganic nitrogen, but diminished the stimulation of elongation in the
presence of ammonium and nitrate. This indicates that
these ions or their by-products serve partially as osmolytes.
The paper by Walch-Liu et al. reviews the different ways
in which the N supply can modify root branching, summarizing the current understanding of the mechanism of nitrate
stimulation of lateral root growth and the role of the ANR1
gene. In addition, the authors discuss the possible role of
auxin in regulating the systemic inhibition of early lateral
root development by high rates of nitrate supply. Finally,
they examine recent evidence that an amino acid, L-glutamate, can act as an external signal to elicit complex changes
in root growth and development. It is clear that plants have
evolved sophisticated pathways for sensing and responding
to changes in different components of the external N supply
as well as their own internal N status.
Cytokinin (CK) synthesized in the root cap promotes
cytokinesis, vascular cambium sensitivity, vascular differentiation and root apical dominance. Auxin (indole-3-acetic
acid, IAA) produced in young shoot organs promotes root
development, induces vascular differentiation while auxin,
together with CK, regulates root gravitropism. The article
by Aloni et al. analyses the hormonal mechanisms that
induce the root’s primary vascular system, explains how
differentiating protoxylem vessels promote lateral root
initiation, proposes the concept of CK-dependent root apical
dominance, and visualizes the CK and IAA regulation of
root gravitropism.
Histological examination by Gladish et al. showed that
some cells in the elongation zone of the primary root tips of
pea have morphological abnormalities including misshapen
and fragmented nuclei and cytoplasmic shrinking and
fragmentation. TEM revealed lobing, invagination and
chromatin aggregation in nuclei. The affected cells were
found to be positive for chromatin fragmentation, indicating
that apoptosis-like programmed cell death is involved in
these processes. Programmed root tip death may rapidly
reduce oxygen demand and sink strength allowing more
rapid diversion of resources to lateral roots growing in
more permissive conditions.
Dubrovsky et al. show that in Arabidopsis thaliana lateral
root primordia (LRP) initiation occurs strictly acropetally:
de novo initiation events being absent between already
developed lateral roots (LR) or LRPs. Thus, there is a narrow developmental window for LR initiation. No specific
cell-count or distance-measuring mechanisms have been
found that determine the site of successive initiation events.
Nevertheless, the branching density and lateral organ density (the density of LRs and LRPs) are accession-specific,
and the average distance between successive LRs can be
predicted based on the lateral density.
Ó The Author 2006. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved.
For Permissions, please email: [email protected]
838
Rost and Bloom — Root Structure and Function
Hamamoto et al. report, for the first time, a correlation
between root apical meristem (RAM) organization type and
the production and release of root cap border cells. Root cap
border cells are released into the rhizosphere where
they survive for a period and interact with beneficial
micro-organisms in the soil. This interaction could provide
a favourable benefit to the plant. Species exhibiting open
RAM organization produced significantly more border cells
than species exhibiting closed apical organization.
In conclusion, this diverse collection of papers should
dispel the image of roots as simple structures quietly sipping
nutrients and water from a relatively static medium. Roots
themselves rapidly change shape in response to environmental cues, and these changes dramatically alter both
their abiotic and biotic environment. The authors, through
their constant digging, have begun to expose the under
side of plants that for so long was out of sight and out of
mind.
Bloom AJ, Frensch J, Taylor AR. 2006. Influence of inorganic nitrogen
and pH on the elongation of maize seminal roots. Annals of Botany 97:
867–873.
Dubrovsky JG, Gambetta GA, Hernández-Barrera A, Shishkova S,
Gonzaléz I. 2006. Lateral root initiation in Arabidopsis: developmental window, spatial patterning, density, and predictability. Annals
of Botany 97: 903–915.
Gladish DK, Xu J, Niki T. 2006. Apoptosis-like programmed cell
death occurs in procambium and ground meristem of pea (Pisum
sativum) root tips exposed to sudden flooding. Annals of Botany 97:
895–902.
Hamamoto L, Hawes MC, Rost TL. 2006. The production and release of
living root cap border cells is a function of root apical meristem type
in dicotyledonous angiosperm plants. Annals of Botany 97:
917–923.
Scippa GS, Di Michele M, Di Iorio A, Costa A, Lasserre B, Chiatante D.
2006. The response of Spartium junceum roots to slope: anchorage and
gene factors. Annals of Botany 97: 857–866.
Walch-Liu P, Ivanov II, Filleur S, Gan Y, Remans T, Forde BG.
2006. Nitrogen regulation of root branching. Annals of Botany 97:
875–881.
Watt M, Silk WK, Passioura JB. 2006. Rates of root and organism growth,
soil conditions, and temporal and spatial development of the rhizosphere. Annals of Botany 97: 839–855.
LITERATURE CITED
Aloni R, Aloni E, Langhans M, Ullrich CI. 2006. Role of cytokinin
and auxin in shaping root architecture: regulating vascular
differentiation, lateral root initiation, root apical dominance and root
gravitropism. Annals of Botany 97: 883–893.
T. L. Rost and A. J. Bloom
E-mail [email protected]