Molecular effect of phytohormone - Auxin in regulation of cell extension is shown in this presentation. Auxin is an important plant hormone which helps in elongation but how the process of elonagation is done?? this presentation is made for you to understand the molecular, chemical effect of auxin on plants.

1. Plant Biochemistry
Molecular effect of Auxin in regulation of cell extension
VANSHIKA VARSHNEY
MSc BIOCHEMISTRY
1ST YEAR
CCS UNIVERSITY, UP

2. ● Auxins are a group of naturally occurring and artificially synthesised plant
hormones. They play an important role in the regulation of plant growth. Auxins
were initially isolated from human urine.
● Auxin means to “enlarge” or “increase”. They induce cell division, differentiation
and elongation.
● Charles Darwin detected phototropism movement (bending of plants towards
light) in the coleoptile of canary grass. He observed that there was some
influencer at the tip of the coleoptile, which was responsible for the bending
towards the light.
● Later, Frits Went isolated and named the substance as “Auxin”, which was
responsible for phototropic movement in oat coleoptile.
● Kenneth Thimann purified and elucidated the structure of primary auxins, e.g.
IAA (Indoleacetic acid).
● Naturally Occurring Auxins: Indole-3-acetic acid (IAA), Indole butyric acid (IBA)
● Artificially synthesized Auxins: 2,4-dichlorophenoxyacetic acid (2,4-D),
Naphthalene acetic acid (NAA)

5. Biosynthesis of Auxin
● Indole-3-acetic acid (IAA), the most important natural auxin in plants, is mainly synthesized from the amino acid
tryptophan (Trp).
● Auxin biosynthesis occurs in both aerial portions of the plant and in roots; thus, the auxin required for root
development could come from either source, or both.
● Biosynthesis of Auxin has two different pathways: 1) Trp Dependent Biosynthesis
2) Trp Independent Biosynthesis
● In Trp-dependent IAA biosynthesis, four pathways have been postulated in plants: (i) the indole-3-acetamide (IAM)
pathway; (ii) the indole-3-pyruvic acid (IPA) pathway; (iii) the tryptamine (TAM) pathway; and (iv) the indole-3-
acetaldoxime (IAOX) pathway.

7. Tryptophan
aminotransfer
ase of
Arabidopsis 1
Indole 3
acetaldehyde
oxidase

8. ● Plant cells are surrounded by cell walls, which are dynamic structures displaying a strictly
regulated balance between rigidity and flexibility.
● Walls are fairly rigid to provide support and protection, but also extensible, to allow cell growth,
which is triggered by a high intracellular turgor pressure.
● Wall properties regulate the differential growth of the cell, resulting in a diversity of cell sizes and
shapes.
● The plant hormone auxin is well known to stimulate cell elongation via increasing wall
extensibility.
● Auxin participates in the regulation of cell wall properties by inducing wall loosening.
● Plasticity refers to the ability of a plant cell wall to stretch irreversibly.
● Auxin increases wall plasticity, and thereby enables shoot elongation and such phenomena as
phototropism, the shoot's tendency to grow in the direction of a light source.
● For the plant to grow, its cells must expand, and for this to happen the rigid structural elements
of the cell wall must relax.
● The cell wall can stretch when its cellulose fibers are allowed to slide past one another.

9. ● Auxin loosens the meshwork of cellulose fibers and matrix polysaccharides by stimulating the
transport of H+ ions into the cell wall.
● The resulting decrease in pH activates cell wall proteins called expansins, which alter the
hydrogen bonding between the matrix polysaccharides that link the cellulose fibers together.
● Scientists still don't know exactly how expansins loosen the meshwork, but one suggestion is that
expansins allow the cellulose fibers and matrix polysaccharides to slide relative to one another, so
that the wall stretches and the cell expands.
● Plants grow when their cells expand. Because a rigid cell wall encloses the plant cell, the cell wall
ultimately controls the rate and direction of growth. The cell wall contains cellulose fibers, which
line up parallel to each other in bundles linked by matrix polysaccharides.
● The driving force for cell growth is turgor pressure. For the cell to expand, these cellulose
bundles, called microfibrils, must be allowed to slide past one another, permitting the cell wall to
stretch. Once the cell wall is stretched, new cellulose microfibrils are laid down to maintain the

10. ● In his experiment with oat coleoptiles, Frits Went demonstrated that a hormone, named auxin,
stimulates plant growth. Subsequently, other researchers have wondered how auxin might act on
the molecular level to enable the plant to grow. Let's examine some of these experiments.
● When segments of oat coleoptiles are stretched, they do not recover completely. Attaching a
weight to the coleoptile segment causes it to bend, stretching it. After the weight is removed, the
shoot returns only part of the way to its original position. Thus, some of the stretching was
reversible, but some was not.
● Irreversible stretching or bending is called plasticity, and reversible stretching is called elasticity.
● Considering what you know about auxin's effect on plant growth, what do you think might be
the effect of treating the shoots with auxin before stretching? Would auxin increase the segments'
plasticity or their elasticity or perhaps neither?
● An auxin-treated segment will bend farther than an untreated segment when a weight is added.
After the weight is removed, the auxin-treated segment shows less recovery, and the net
displacement is therefore greater than for an untreated segment. The auxin significantly increases

11. ● Scientists wanted to know how auxin loosened the cell wall. Studies in the 1970s showed that acidifying
the medium in which coleoptile segments were growing—that is, adding hydrogen ions—caused the
segments to grow just as rapidly as shoots treated with auxin.
● In addition, treating coleoptile segments with auxin (A) increased the hydrogen ion concentration in the
growth medium. These results suggested that auxin might loosen the cell wall by causing the release of
hydrogen ions, thereby decreasing the pH. How might this "acid growth" process occur in nature?
● Auxin increases the activity of proteins in the plasma membrane that pump hydrogen ions into the cell
wall. Biologists hypothesized that these ions activate one or more cell wall proteins, and they began their
search for such proteins.
● In the 1990s, proteins called expansins were isolated and purified from cell walls. When expansins are
added to isolated plant cell walls, the ability of the walls to be stretched increases. Expansins, activated by
H+ ions, alter the pattern of hydrogen bonding between the polysaccharides in the cell wall, possibly
allowing these macromolecules to slip past each other so that the cell wall stretches.

13. Mode of action