Root Systems
Root Development
Tap Root
Fibrous Root
1. Root morphology and variation
2. Regions of root development
Variations in apical organization
Stem cell tiers at the apical meristem
Xylem and phloem development
Organization of the primary root
3. Lateral root development
4. Secondary growth in roots
Vetiver grass in China
Specialized Roots
Root Morphology
1. Primary root
2. Lateral/secondary roots
Contractile
3. Root hairs
4. Root cap
Taxodium pneumatophores
Aerial
Adventitious
Prop
1
Variations in Root Apical Organization
Closed Apical Meristem
Onion
Open Meristem
Corn - Closed Meristem
Fern
Single Apical Cell
Root Apical Meristem – Quiescent Center
Root Development
Cell types and initials in the Arabidopsis Root Apex
1. Autoradiography with Tritiated (H3)Thymidine Æ No labeling of nuclei.
2. Initials/stem cells at quiescent center
1. Tier 1 Initials
Columella
Epidermis + Lateral Root Cap
2. Tier 2 Initials
Quiescent Center
Cortex & Endodermis
3. Tier 3 Initials
Stele – Vascular cylinder
2
Arabidopsis Root Apical Meristem
Tiers of Cells at the RAM
Tier 3: Stele Initials
Tier 1: Lateral Columella Initials &
Epidermal initials
Root
Development
Tier 1: Root Cap Columella Initials
Divide periclinally
Tier 2: Quiescent Center
Tier 2 (Ring): Cortex Initials
Root
Anatomy
Root Vascular
Cylinder
Epidermis
Endodermis
Cortex
Casparian Strip
Stele
Xylem
Phloem
Procambium
Pericycle
3
Root Primary Growth
Root Development - Regions
Region of Cell Division
Developmental Regions
1. Cell Division
2. Elongation
Procambium
3. Maturation
Procambium
Ground Meristem
Protoderm
“Tier 3”
“Tier 2”
“Tier 1”
Root Development - Regions
Region of Elongation
Basal End
Root Development
Region of Elongation - Early
Developmental Regions
1. Cell Division
2. Elongation
Developmental Regions
3. Maturation
1. Cell Division
Phloem
2. Elongation
Xylem
3. Maturation
Endodermis
Cortex
Longitudinal
Epidermis
Root Cap
Apical End
Root Apical Meristem
4
Root Development
Region of Elongation
Root Development - Region of Maturation
1o Xylem
phloem
procambium
xylem
Summary – Root Development
Primary
Tissues
mature
Maturation
1. Dermal Tissue System
Æ epidermal cells mature
Æ root hairs develop
Lateral Root Development
2. Ground Meristem
Æ cortex cells mature
Æ endodermis may lignify
3. Procambium
Æ primary phloem matures
Æ primary xylem matures
1. Protoderm
Æ cells immature
Elongation
Primary
meristems
differentiate
Cell division
Apical
Meristem
3
2
1
2. Ground Meristem
Æ cortex cells enlarging
Æ endodermis forms
3. Procambium
Æ mature protophloem
Æ immature protoxylem ∼ 2o walls
Tier 1 ÆRoot cap initials
Æ Protoderm
Tier 2 Æ Quiescent Center
Æ Ring of cortical initials
or ground meristem
Tier 3 Æ Procambium
5
Lateral Root Development
How do cells organize and proliferate into organs?
(1) Dedifferentiation of pericycle cells.
(2) Ordered cell divisions and cell differentiation to generate
a highly organized Lateral Root Primordium… which may
include cells that function as an apical meristem.
(3) Lateral Root emergence via cell expansion.
(4) Formation of a lateral root apical meristem to allow
continued growth of the organized lateral root.
Lateral Root Development - Assignment
Go to paper by Malamy, J.E. & P.N. Benfey. Organization and cell
differentiation in lateral roots of Arabidopsis thaliana.
Development 124, 33-44 (1997)
Online at: http://dev.biologists.org/cgi/reprint/124/1/33
1. Develop a diagram for Early Developmental Stages of Root
Formation. i.e. formation of Lateral Root Primordium (LRP)
2. What is GUS, what is a root cell type-specific expression line and
what is the application of GUS expression used with root cell typespecific expression lines?
3. Diagram the level of lateral root development as the lateral root
a) breaks through the epidermis
b) breaks through cortical parenchyma cells
c) breaks through the parent root epidermis
Lateral Root Development
I.
II.
III.
Lateral Root Development
IV.
V.
Periclinal ÷ IL
endodermis
Anticlinal ÷ OL1 & OL2
Anticlinal ÷ OL1 & 2
Periclinal ÷ IL1
1st
2nd
Anticlinal divisions
in pericycle cells &
radial enlargement
Periclinal divisions
in pericycle cells
create outer &
inner cell layers &
radial expansion
3rd
Periclinal divisions in
outer layer creates 2 outer
& 1 inner cell layer
IL2
IL1 OL2
÷
÷
÷
OL1
Midway
thru
cortex
6
VII & VIII
VI.
Simultaneous cell ÷
– LR Primordium Æ LR Root Meristem
Emergence Through Epidermis
Elongating cells
at core of LRP
Outer
epidermis
Cell Expansion in basal cells
Epidermis
Cortex
Endodermis
Root cap
Stele
Increased Cell # at apex
Meristem initials contribute to growth
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Comments
1. Limitations of histology:
(a) Cell position suggestive of cell fate, but not definitive.
(b) Cell types cannot be distinguished at early stages
How (DNA)Enhancers Work
Binding of Activator (protein) to Enhancer (DNA) allows activator to interact
with transcription factors (proteins) associated with RNA polymerase. This
process stimulates transcription.
2. Use of root cell type-specific marker lines
Cell-type specific markers confirm cell developmental
patterns based on histology.
3. Methodology
(a) Develop Enhancer Trapping Cassette
GUS + CaMV promoter
(b) Insert into root cells
Agrobacterium + Ti plasmid-mediated root
transformation
(c) Use GUS expression for cell-specific marker
Enhancers
Coding
region
Activator
Activators
General
factors
Enhancer
Coactivator
Activator
B
TAFs
F
E
TFIID
RNA
polymerase II
These regulatory proteins bind to DNA at
distant sites known as enhancers. When
DNA folds so that the enhancer is
brought into proximity with the initiation
complex, the activator proteins interact
with the complex to increase the rate of
transcription.
Coactivators
These transcription factors transmit
signals from activator proteins to the
general factors.
H
Core
promoter
General Factors
These transcription factors position RNA
polymerase at the start of a proteincoding sequence and then release the
polymerase to initiate transcription.
7
GUS Insertion in Plant Genome
GUS Reporter Gene Expression
1. GUS produces β-glucuronidase.
2. β-glucuronidase produces a blue reaction product with a chromogenic
substrate
Observations of Marker Studies
Marker Lines in Arabidopsis roots with GUS expression
Stele &
Pericycle
Line: Ste05
Endodermis
Line:
End195
Columella Root
Cap
Line: CRC219
Lateral Root
Cap
Line: LRC244
LRP
Stage
Marker Observed & Location
VI
Epidermal Marker in OL1
VII
Cortex Marker in OL2
V
Endodermal Marker in OL2
Emergent Root
Stele & Root Cap Markers
8
GUS Expression in Lateral Root Primordium
GUS Expression in Lateral Root Primordium
Stage
Mature Root
Epidermis
Endodermis
Cortex
Lateral
root base
Endodermis
Summary - Lateral Root Development
epidermis
cortex
vasculature
Quiescent center
Root cap
pericycle
endodermis
Lateral root formation can be divided into the following major stages:
(1) Stimulation and dedifferentiation of pericycle cells.
(2) Ordered cell divisions and cell differentiation to generate
a highly organized lateral root primordium, which may include a group of
cells that function as an apical meristem.
(3) Emergence via cell expansion.
(4) Activation of the lateral root meristem to allow continued growth of
the organized lateral root.
9
END Roots
10