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Root Hair Formation

Root hairs are specialized projections from epidermal cells in plant roots that enhance water absorption, with their formation occurring in three stages: specification, initiation, and outgrowth. Key genes such as expansins and XETs are crucial for cell wall loosening during hair formation, while various mutants in Arabidopsis demonstrate the genetic regulation of root hair development. Additionally, factors like the TTG gene and treatments with hormones can influence root hair density and formation.

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Renu Jangid
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33 views14 pages

Root Hair Formation

Root hairs are specialized projections from epidermal cells in plant roots that enhance water absorption, with their formation occurring in three stages: specification, initiation, and outgrowth. Key genes such as expansins and XETs are crucial for cell wall loosening during hair formation, while various mutants in Arabidopsis demonstrate the genetic regulation of root hair development. Additionally, factors like the TTG gene and treatments with hormones can influence root hair density and formation.

Uploaded by

Renu Jangid
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Root Hair Formation

Dr. Renu Jangid


Introduction
• Root hairs are projections of the epidermal cells of the root
in the zone of differentiation and extend out tortuously into
the soil, greatly enhancing water absorption by the root.

• In most dicotyledons and some monocotyledons, these


outgrowths develop more or less at random from the cells of
the epidermis, yielding a pattern of normal epidermal cells
interspersed with root hair cells.
• Use of a fluorescent marker has revealed that in
Arabidopsis seedling roots, the small size of the cell and
the presence of a dense, intensely stained cytoplasm stand
out as obvious features of the differentiating root hair
cell. Differences between the hair-forming and hairless
cells, such as in the presence of surface deposits, timing
of vacuolation, and the extent of elongation, begin to
appear during later stages of differentiation.
• In wild-type Arabidopsis, root hairs are formed by
epidermal cells termed trichoblasts which overlie the
boundary between two cortical cells.
Root hair formation
• The formation of a root hair can be divided into three distinct
stages, namely specification, initiation, and outgrowth.

1. Specification: Epidermal cells differentiate into root hair cells


(trichoblasts) or non-hair cells based on positional cues.

2. Initiation: A localized bulge forms at the basal end of


trichoblasts, marked by the localization of Rop GTPase.

(Rho GTPase are small signaling proteins that play a crucial role
in regulating cell polarity and development in plants).

3. Outgrowth: The bulge elongates into a tubular structure


through tip growth driven by cell wall loosening and cytoskeletal
• The Arabidopsis expansin genes (AtEXP7 and AtEXP18) and
xyloglucan endotransglycosylase (XET) play crucial role in
root hair formation. They are contribute a role in the loosening
of the cell wall to promote hair initiation and outgrowth.

• Expansin (EXP) are expressed in trichoblast but not


atrichoblast cells.

• XETs act by breaking and reforming the glycosidic bonds of


xyloglucan which cross-links cellulose microfibrils whereas
the expansins mediate cell wall loosening without undergoing
breakage of the major structural components of the cell wall.
Several mutants have been described in Arabidopsis which
exhibit abnormal cell expansion and several of these are affected
in cellulose biosynthesis. For example, root epidermal
bulger/root hair defective (reb1/rhd1-6), root hair less (rhl),
cow1 and ttg, gl mutants.
• RHD6 is a positive regulator of root hair initiation. It
promotes the expression of expansin genes
(AtEXP7 and AtEXP18), which are essential for root hair
bulge formation. The reb1/rhd6 mutant exhibit significantly
reduced root hair numbers.
• The procuste 1 (prc1) gene also plays a crucial role in
the early stages of root hair formation, specially in cell
wall loosening, which are essential steps of hair
initiation and outgrowth.
• The prc1 mutant affect cellulose synthase and reduced
cellulose content in cell wall, leading to bulging of root
hair forming cells and show altered hair positioning due
to weakened cell walls.
• Mutation in both rhd and prc genes can distrupt root hair
formation process and lead to defects in root hair
morphology.
Root hair development
in rhd6-1 is partially
restored in the prc1-
1 mutant background.
Primary root tissues of
wild-type (A),
rhd6-1 (B),
prc1-1 (C)
rhd6-1/prc1-1(D)
seedlings which have
been grown on MS agar
for 5 days.
• Two other mutations that induce hair formation on non-hair-
forming cells of the root epidermis of Arabidopsis are ttg and
gI2.
• TTG (TRANSPARENT TESTA GLABRA) acts as a negative
regulator in the early stages of root hair development, but can
also promote root hair outgrowth later in the process.
• Mutations in TTG1 can lead to an increase in root hair density,
suggesting it normally helps to restrain root hair formation.
• Arabidopsis is a MYB-like protein encoded by the
CAPRICE (CPC) gene. Mutant cpc seedlings have fewer
root hairs than wild-type plants, whereas transgenic plants
overexpressing the CPC gene generate more root hairs
than the wild type.
• Treatment of wild-type seedlings of Arabidopsis with the
ethylene, cytokinin and IAA also induces root hair cell
formation.
• RHL1 genes are involved in the early stages of cell
specification and hair initiation.
• COW 1 involved in root hair tip growth and elongation.

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