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Topic 9 Plant
Biology
9.3 Growth in Plants
Nature of Science
Developments in scientific research follow
improvements in analysis and deduction:
improvements in analytical techniques allowing
the detection of trace amounts of substances
have led to advances in the understanding of
plant hormones and their effect on gene
expression.
Growth in Plants

Undifferentiated cells in the meristems of plants
allow indeterminate growth.
o
Growth can be indeterminate when cells
continue to divide indefinitely. Plants, in
general, have indeterminate growth.
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Many plant cells, including some fully
differentiated types, have the capacity to
generate whole plants: i.e. the cells are
totipotent (this phenomenon is what sets plant
cells apart from most animals.
o
Growth in plants happens in regions called
meristems.
o
Apical meristems – primary meristems found at
the tips of stems and roots.
o
Many dicot plants also develop lateral
meristems (which allow horizontal growth).
Role of mitosis in stem extension and leaf
development.
• Mitosis and cell division in the shoot apex provide cells needed for
extension of the stem and development of leaves.
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Cells in the meristem are small and go through the cell cycle repeatedly to
produce more cells, by mitosis and cytokinesis. These new cells absorb nutrients
and water and so increase in volume and mass.
The root apical meristem is responsible for the growth of the root.
The shoot apical meristem is more complex – it throws off the cells that are needed
for the growth of the stem and also produces the groups of cells that grow and
develop into leaves and flowers. With each division one cell remains in the
meristem while the other increases in size and differentiates as it is pushed away
from the meristem region.
Each apical meristem can give rise to additional meristems including the
protoderm, procambium, and ground meristem. In general these give rise to
different tissue.
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Protoderm – gives rise to the epidermis.
Procambium – gives rise to vascular tissue.
Ground meristem – gives rise to pith.
Plant hormones affect shoot growth
•
Plant hormones control growth in the shoot apex.
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Hormone – chemical message that is produced and released in one part of an organism to
have an effect in another part of the organism.
Auxins are hormones that have a broad range of functions including initiating the growth of
the roots, influencing the development of fruits and regulating leaf development.
 Most abundant auxin is indole-3-acetic acid (IAA) – has a role in the control of growth in
the shoot apex, promotes the elongation of cells in stems.
 IAA is is synthesized in the apical meristem of the shoot and is transported down the
stem to stimulate growth. At very high concentrations, it can inhibit growth.
Axillary buds are shoots that form at the junction, or node, of the stem and the base of a
leaf. As the shoot apical meristem grows and forms leaves, regions of meristems are left
behind at the node. Growth at these nodes is inhibited by auxin produced by the shoot
apical meristems. This is called apical dominance. The further the distant a node is from
the shoot apical meristem, the lower the concentration of auxin and the less likely that
growth in the axillary bud will be inhibited.
Cytokinins – hormones produced in the root, promote axillary bud growth. The relative ratio
of cytokinins and auxins determine whether the axillary bud will develop.
Giberellins are another category of hormones that contribute to stem elongation.
Plant Tropisms
• Plants respond to the environment by
tropisms.
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Tropism – plant responses to external
stimuli.
The direction in which stems grow can be
influenced by two external stimuli: light
and gravity.
Stems grow towards the source of the
brightest light or in the absence of light
they grow upwards, in the opposite
direction of gravity.
Phototropism = growth towards light.
Gravitropism = growth in response to
gravitational force.
Auxin influences gene expression
 Auxin influences cell growth rates by changing the pattern
of gene expression.
• In a phototropism, the first step is the absorption of light by photoreceptors
(phototropins). When they absorb light of an appropriate wavelength, their
conformation changes. They can bind to receptors within the cell, which
control the transcription of specific genes. It seems likely that the genes
involved are those coding for a group of glycoproteins located in the plasma
membrane of the cells in the stem that transport the plant hormone auxin
from cell to cell.
Intracellular Pumps
• Auxin efflux pumps can set up concentration gradients of auxin in
plant tissues.
o
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The position and type of PIN3 proteins can be varied to transport auxin to
where growth is needed.
If phototropins in the tip detect a greater intensity of light on one side of the
stem than the other, auxin is transported laterally from the side with brighter
light to the more shaded side. Higher concentrations of auxin on the
shadier side cause greater growth (cell elongation) on this side, so the
stems curves towards the source brighter light. The leaves attached to the
stem will therefore receive more light and be able to photosynthesize at a
greater rate.
Application
• Micropropagation of plants using tissue from the
shoot apex, nutrient agar gels and growth
hormones.
• Micropropagation is an in vitro procedure that
produces large numbers of identical plants.
Application
• Use of micropropagation for rapid bulking up of
new varieties, production of virus-free strains of
existing varieties and propagation of orchids
and other rare species.
• Read pg 427 and explain how this works.