ROOT HAIRS
Cloe de Luxán Hernández
PCDU Seminar
DEFINITION
 Long tubular-shaped outgrowths from
root epidermal cells
 Increase root surface area and diameter
Grierson, C; Nielsen, E; Ketelaar, T and Schiefelbein, J. The Arabidopsis Book – Root hairs. American Society of Plant Biologists (2014)
PATTERN OF EPIDERMAL CELLS IN THE ROOT
 Root hair cells and non-hair cells are
interspersed.
H position: intercelular space between
two cortical cells
N position: over a single cortical cell
 Cell- cell comunication events are critical for the establishment of cell indentity in
the root epidermis.
Grierson, C; Nielsen, E; Ketelaar, T and Schiefelbein, J. The Arabidopsis Book – Root hairs. American Society of Plant Biologists (2014)
NATURE OF THE CELL PATTERNING
INFORMATION
 Position-dependent epidermal cell pattern is determined at early stages of
epidermal development.
 Trichoblasts and Atrichoblasts are distinguished before hair outgrowth.
Trichoblasts vs Atrichoblasts
Division
High
Low
Length
Small
Big
Cytoplasmic density
High
Low
Rate of vacuolation
Low
High
NATURE OF THE CELL PATTERNING
INFORMATION
 GLABRA 2 (specifical expression in non-hair cells) fused to GUS reporter gene
H position
N position
Grierson, C; Nielsen, E; Ketelaar, T and Schiefelbein, J. The Arabidopsis Book – Root hairs. American Society of Plant Biologists (2014)
NATURE OF THE CELL PATTERNING
INFORMATION
 GLABRA 2 (specifical expression in non-hair
cells) fused to GFP
 Epidermial cell pattern initiated during
embryogenesis
External application of auxin and ethylene
induces root hair cell differentiation in cells in
both H and N position.
Grierson, C; Nielsen, E; Ketelaar, T and Schiefelbein, J. The Arabidopsis Book – Root hairs. American Society of Plant Biologists (2014)
ROOT HAIR FORMATION

Two main steps:
 Root hair initiation
 Root hair tip growth
Small area of the
cell wall loosens to
form a swelling
Hair grows by
targeted secretion
Grierson, C; Nielsen, E; Ketelaar, T and Schiefelbein, J. The Arabidopsis Book – Root hairs. American Society of Plant Biologists (2014)
ROOT HAIR INITIATION
 Rop accumulation at the apical plasma
membrane domains in the tip of root hairs
assisted by receptor-like kinase FERONIA.
 pH drop that leads to the activation of
expansins.
 ER condensation and actin accumulation.
Grierson, C; Nielsen, E; Ketelaar, T and Schiefelbein, J. The Arabidopsis Book – Root hairs. American Society of Plant Biologists (2014)
ROOT HAIR TIP GROWTH
 Extremely polarized type of cell expansion that results in the formation of a tubular cell.
 Sustained by exocytosis of vesicles in the root hair apex.
 Vesicles produced by smooth and rough ER and Golgi complexes.
Cell wall materials
accumulation
ROPs
cell wall polysaccharides and cell wall (glyco)proteins
ROOT HAIR TIP GROWTH
 High polarization of the citoplasm.
 Tip growth unit: all celular constituents that support tip-growth
 Delivery of new vesicles is assisted by cytoplasmic streaming
Ketelaar, T. The actin cytoskeleton in root hairs: all is fine at the tip. Current opinion in Plant Biology (2013) 16: 1-8
CALCIUM IN TIP GROWTH
 Root hairs of 5-10 μm long experiment an increase of Ca2+ concentration from about 200
nM to at least 1μM .
 Calcium gradient at the tip of the hair controls direction of growth (Facilitates fusion of
exocytotic vesicles with PM)
 ROP  RHD2 (NADPHox)  ROS  activation of calcium channels
Tip focused Ca2+ gradient remains throughout tip growth
 Related to ABP
ACTIN IN TIP GROWTH
 Functions as a backbone for cytoplasmic streaming.
 Organizes the cytoplasm of the subapical region
Actin
inhibitors
Latrunculin B
G-actin
G-actin
DEPOLYMERIZATION
POLYMERIZATION
F-actin
Bulge formation
Tip growth
ACTIN IN TIP GROWTH
 Actin organization is highly polarized.
Supports cytoplasmic
streaming in the cortical
cytoplasm
Ketelaar, T. The actin cytoskeleton in root hairs: all is fine at the tip. Current opinion in Plant Biology (2013) 16: 1-8
Supports accumulation
of cytoplasm in the
(sub)apical part
ACTIN IN TIP GROWTH
Actin plays a role in determining root hair width
[Actin inhibitors]
[Actin inhibitors]
ACTIN BINDING PROTEINS (ABPS)
 Play an important role in determining the higher-order organization of the actin
cytoskeleton.
 Its activity is modulated by the tip-focused Ca2+ gradient
Several groups:
 Actin nucleating proteins
 Actin depolymerizers
 Actin bundling proteins
 Motor proteins
ACTIN NUCLEATING PROTEINS
 Arp2/3 complex:
 In some mutants: wavy, swollen and branching root hairs with reduced
length
 No important role in actin organization during root hair growth
Arp2/3 complex
ACTIN NUCLEATING PROTEINS
 Formins: 2 classes
Accumulation of FH8 causes an accumulation of F-actin in growing root hair tips and
defects in actin organization and growth defects such as growth reduction, tip swelling,
wavyness and branching
Highly related to profilins.
Overexpression of PFN1 causes the formation of root hairs that are twice as long as root
hairs in WT.
Actin trimer
Formin dimer
ACTIN DEPOLYMERIZERS
 Actin depolymerizing factor (ADF) : increases actin turnover by severing actin filaments and
increasing the dissociation rate from their minus ends.
 Its activity is enhanced by high [Ca2+]
Enhancement of ADF activity plays a role in fine F-actin formation.
 Actin Interacting Protein 1 (AIP1): enhances ADF activity
 Reduced expression: growth velocity decreases
 Increased expression: root hair tip swelling and inhibition of root hair growth
ACTIN BUNDLING PROTEINS
 Villins: 3 activities:
 Ca2+/calmodulin-activated actin severing and capping
 Actin filament bundling
 VILLIN4 gene mutation: finer actin organization, lower elongation rate, and decreased
cytoplasmic streaming rate. (Villins implicated in actin bundling in root hairs)
 Ca2+ dependent actin severing and capping activities of villin are involved in the regulation
of actin dynamics in the apical area of pollen tubes. (Likely same role in root hairs)
MOTOR PROTEINS
 Myosin XI: function in actin-dependent organelle trafficking and actin-mediated
cytoplasmic restructuring.
 Mutations in Myosin XI genes: root hair growth reduction and altered actin organization
 Ca2+ dependent inhibition (reduced cytoplasmic streaming velocities)
POLARIZED MEMBRANE TRAFFICKING IN ROOT
HAIRS
 Plant Rab GTPases control this mechanism
Secretory vesicles
TGN membranes
Prisca Campanoni, P. and Blatt M.R. Membrane trafficking and polar growth in root hairs and pollen tubes. Journal of Experimental Botany (2007) Vol. 58, No. 1, pp. 65–74
POLARIZED MEMBRANE TRAFFICKING IN ROOT
HAIRS
 Caffeine treatment: modulates tip-focused calcium gradient
 Yariv reagent treatment: precipitation of arabinogalactan
proteins
Inhibit
elongations of
pollen tubes
 Secretion and growth may be uncoupled
 More membrane is added to the plasma membrane than is required for growth. (Excess
membrane being retrieved by endocytosis)
POLARIZED MEMBRANE TRAFFICKING IN ROOT
HAIRS
Clathrin-coated vesicle
MVB
Uncoated vesicle
Recycling pathway
Secretory vesicle
Vacuole
- Complete fusión with the
PM
- Kiss and run mechanism
Degradation pathway
Exocytotic pathway
Recycling pathway
CESSATION OF TIP GROWTH
 Tip Growth is sustained by auxin provided by neighbouring non-hair cells
Auxin
transport
Root tip
growing
 End of growth:




Production of a symmetrical dome-shaped tip
Dispersal of the citoplasm at the tip
Enlargement of the vacuole into the dome
Lost of Rop protein, calcium gradient and calcium cannel activity at the tip
ROOT HAIRS AND NUTRIENT
ACQUISITION
 Root hairs facilitate the uptake of nutrients from the soil.
 Root hair development is strongly regulated by nutrient concentration (P, Fe, Mn and Zn)
[P]
1μM
1000μM
Root hair density
High
Low
REFERENCES
 Grierson, C; Nielsen, E; Ketelaar, T and Schiefelbein, J. The Arabidopsis Book – Root hairs. American
Society of Plant Biologists (2014)
 Prisca Campanoni, P. and Blatt M.R. Membrane trafficking and polar growth in root hairs and pollen tubes.
Journal of Experimental Botany (2007) Vol. 58, No. 1, pp. 65–74
 Ketelaar, T. The actin cytoskeleton in root hairs: all is fine at the tip. Current opinion in Plant Biology (2013)
16: 1-8
 Schneider, S.W. Kiss and run mechanism in exocytosis. The Journal of Membrane Biology (2001) 181: 67-76
 Ovecka, M; Lang, I; Baluska, F; Ismail, A; Illes, P; Lichtscheidl, I.K. Endocytosis and vesicle trafficking during
tip growth of root hairs. Protoplasma (2005) 226: 39-54.