secondary growth

plant growth
involving increase in width

apical meristems/primary
meristems

regions of growth at
the tips of plants

cork cambium

meristematic
tissue which produces new cork/
bark cells

mitosis

cell division where one
diploid cell becomes two diploid
cells

shoot apex

apical meristem and
its surrounding developing plant
tissue at the tip of a stem

meristematic tissue

tissue of the
meristem which allows continual
growth

differentiation

series of changes
which transforms unspecialized
cells into specialized cells and
tissues in multicellular organisms

meristematic cell

undifferentiated cell of plant
meristems

protoderm

early shoot apex area
which becomes the mature plant
epidermis

leaf primordia

early shoot apex
area which become the leaf

hormone

chemical messenger
produced in very small amounts
by an endocrine gland

target cell

cells that respond to a
particular hormone

Factors which affect plant development and growth include:

• environmental factors, such as day length and water availability
• receptors, which allow the plant to detect certain environmental factors
• the genetic make-up of the plant
• hormones of several types produced in various plant regions.

auxins

group of plant hormones
affecting plant growth and
development

gibberellin

plant hormone
which promotes seed germination
and stem growth

cytokinin

plant hormone
which promotes root growth and
maintains leaf health

abscisic acid

plant hormone
which closes stomata and
maintains seed dormancy in some
plant species

ethylene

plant hormone
involved in promoting fruit
ripening

micropropagation

process of
culturing cells from the shoot
apex of plants on nutrient gels to
produce desired plants

tropism

plant growth or
movement to directional stimuli
in the immediate environment

positive tropism

plant growth
toward a specifi c environmental
stimulus

negative tropism

plant
growth away from a specifi c
environmental stimulus

gravitropism

growth or
movement in plants due to gravity

positive gravitropism

plant
growth toward gravity such as
roots show

negative gravitropism

plant
growth away from gravity

phototropism

plant growth or
movement towards or away from
light

negatively phototropic

plant
growth away from light

positively phototropic

plant
growth toward light

Common stimuli for plant tropisms are:

• chemicals
• light
• gravity
• touch.

Sequence of events in plant phototropism:

1 Auxin is produced by all cells in the plant shoot region exposed to light.
2 Auxin effl ux pumps move auxin out of cells on the shoot side closest to the light
and into the intercellular space.
3 Auxin is moved to the intercellular space of the shoot cells further away
from the light.
4 The auxin activates proton pumps in the cell membranes of the shoot cells
further from the light.
5 The proton pumps cause a build-up of hydrogen ions in the intercellular space.
6 Increased hydrogen ions in the intercellular space causes a decrease in pH.
7 A decrease in pH breaks hydrogen bonds in the cellulose of the cell walls of cells
away from the light.
8 The result of breaking cellulose hydrogen bonds is elongation of cells on the
darker side and bending toward the light.
The particular auxin involved in this elongation process of phototropism is called
indole-3-acetic acid (IAA).

concentration gradient

difference in chemical
concentrations between two
regions

auxin effl ux pump

PIN3
proteins of plant cell
membranes involved in moving
auxins in plant tissues

PIN3 proteins

proteins making
up auxin effl ux pumps

indole-3-acetic acid

specific
plant auxin involved in elongating
plant stems in response to light

transcriptional repressors

protein which binds to DNA
and prevents transcription of a
particular region or gene

The ways auxins affect gene expression and plant growth are as follows:

1 When auxin comes into contact with receptors on the nuclear membrane of
these repressor-containing cells, an auxin–receptor complex is formed.
2 This complex moves to and binds with the repressor of the DNA that codes for
growth-stimulating genes.
3 The binding with the repressor results in the breakdown of the repressor.
4 Growth-stimulating genes are transcribed when the repressor is no longer present.
5 The transcription of the growth-stimulating genes causes growth.

auxin–receptor

complex combination of an
auxin and a transcriptional
repressor which leads to
breakdown of the repressor and
transcription occurring

Meristems are tissues in a plant consisting of undifferentiated cells capable of indeterminate growth

• They are analagous to totipotent stem cells in animals, except that they have specific regions of growth and development
• Meristematic tissue can allow plants to regrow structures or even form entirely new plants (vegetative propagation)

Meristematic tissue can be divided into apical meristems and lateral meristems:

• Apical meristems occur at shoot and root tips and are responsible for primary growth (i.e. plant lengthening)
• Lateral meristems occur at the cambium and are responsible for secondary growth (i.e. plant widening / thickening)
• Apical meristems give rise to new leaves and flowers, while lateral meristems are responsible for the production of bark

The apical meristems give rise to primary growth (lengthening) and occurs at the tips of the roots and shoots

• Growth at these regions is due to a combination of cell enlargement and repeated cell division (mitosis and cytokinesis)
• Differentiation of the dividing meristem gives rise to a variety of stem tissues and structures – including leaves and flowers

In the stem, growth occurs in sections called nodes – with the remaining meristem tissue forming an inactive axillary bud

• These axillary (lateral) buds have the potential to form new branching shoots, complete with leaves and flowers

The growth of the stem and the formation of new nodes is controlled by plant hormones released from the shoot apex

• One of the main groups of plant hormones involved in shoot and root growth are auxins (e.g. indole-3-acetic acid / IAA)

When auxins are produced by the shoot apical meristem, it promotes growth in the shoot apex via cell elongation and division

• The production of auxins additionally prevents growth in lateral (axillary) buds, a condition known as apical dominance
• Apical dominance ensures that a plant will use its energy to grow up towards the light in order to outcompete other plants
• As the distance between the terminal bud and axillary bud increases, the inhibition of the axillary bud by auxin diminishes
• Different species of plants will show different levels of apical dominance

Auxins are a group of hormones produced by the tip of a shoot or root (i.e. apical meristems) that regulate plant growth

• Auxin efflux pumps can set up concentration gradients within tissues – changing the distribution of auxin within the plant
• These pumps can control the direction of plant growth by determining which regions of plant tissue have high auxin levels
• Auxin efflux pumps can change position within the membrane (due to fluidity) and be activated by various factors

Auxin has different mechanism of action in the roots of plants versus the shoots of plants:

• In the shoots, auxin stimulates cell elongation and thus high concentrations of auxin promote growth (cells become larger)
• In the roots, auxin inhibits cell elongation and thus high concentrations of auxin limit growth (cells become relatively smaller)

Auxin is a plant hormone and influences cell growth rates by changing the pattern of gene expression with a plant’s cells

• Auxin’s mechanism of action is different in shoots and roots as different gene pathways are activated in each tissue

In shoots, auxin increases the flexibility of the cell wall to promote plant growth via cell elongation

• Auxin activates a proton pump in the plasma membrane which causes the secretion of H⁺ ions into the cell wall
• The resultant decrease in pH causes cellulose fibres within the cell wall to loosen (by breaking the bonds between them)
• Additionally, auxin upregulates expression of expansins, which similarly increases the elasticity of the cell wall
• With the cell wall now more flexible, an influx of water (to be stored in the vacuole) causes the cell to increase in size

Tropisms describe the growth or turning movement of an plant in response to a directional external stimulus

• Phototropism is a growth movement in response to a unidirectional light source
• Geotropism (or gravitropism) is a growth movement in response to gravitational forces
• Other tropisms include hydrotropism (responding to a water gradient) and thigmotropism (responding to a tactile stimulus)

Both phototropism and geotropism are controlled by the distribution of auxin within the plant cells:

• In geotropism, auxin will accumulate on the lower side of the plant in response to the force of gravity
• In phototropism, light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant

In shoots, high auxin concentrations promote cell elongation, meaning that:

• The dark side of the shoot elongates and shoots grow towards the light (positive phototropism)
• The lower side of the shoot elongates and roots grow away from the ground

In roots, high auxin concentrations inhibit cell elongation, meaning that:

• The dark side of the root becomes shorter and the roots grow away from the light (negative phototropism)
• The lower side of the root becomes shorter and the roots turn downwards into the earth

Micropropagation is a technique used to produce large numbers of identical plants (clones) from a selected stock plant

• Plants can reproduce asexually from meristems because they are undifferentiated cells capable of indeterminate growth
• When a plant cutting is used to reproduce asexually in the native environment it is called vegetative propagation
• When plant tissues are cultured in the laboratory (in vitro) in order to reproduce asexually it is called micropropagation

The process of micropropagation involves a number of key steps:

• Specific plant tissue (typically the undifferentiated shoot apex) is selected from a stock plant and sterilised
• The tissue sample (called the explant) is grown on a sterile nutrient agar gel
• The explant is treated with growth hormones (e.g. auxins) to stimulate shoot and root development
• The growing shoots can be continuously divided and separated to form new samples (multiplication phase)
• Once the root and shoot are developed, the cloned plant can be transferred to soil

Micropropagation is used to rapidly produce large numbers of cloned plants under controlled conditions:

Rapid Bulking

• Desirable stock plants can be cloned via micropropagation to conserve the fidelity of the selected characteristic
• This process is more reliable that selective breeding because new plants are genetically identical to the stock plant
• This technique is also used to rapidly produce large quantities of plants created via genetic modification

Virus-Free Strains

• Plant viruses have the potential to decimate crops, crippling economies and leading to famine
• Viruses typically spread through infected plants via the vascular tissue – which meristems do not contain
• Propagating plants from the non-infected meristems allows for the rapid reproduction of virus-free plant strains

Propagation of Rare Species

• Micropropagation is commonly used to increase numbers of rare or endangered plant species
• It is also used to increase numbers of species that are difficult to breed sexually (e.g. orchids)
• It may also be used to increase numbers of plant species that are commercially in demand

Plant growth is initiated at regions called meristems – undifferentiated cells capable of indeterminate divisions

• Meristems are equivalent to embyronic stem cells in animals, but are retained throughout the adult life of the plant
• This allows plants to regrow structures and even reproduce asexually (vegetative propagation)

All the differentiated tissues in a plant are derived from meristems – either apical or lateral meristems

• Apical meristems give rise to the primary tissues needed to increase a plant’s length and grow new leaves and fruits
• Lateral meristems give rise to the secondary tissues needed to support an increase in the plant’s width (e.g. bark)

Secondary (Lateral) Growth

The thickening of a plant’s stem (secondary growth) is controlled by the cambium (where lateral meristems are found)

• Vascular cambium cells give rise to secondary xylem and phloem, which facilitate water and nutrient transport in the plant
• Relative to the cambium, secondary xylem cells are formed internally and secondary phloem cells are formed externally
• Cork cambium cells produce a progressively thickening layer of cork, which contributes to the bark of a plant
• These cells add girth to the plant stem, resulting in the thickening of the trunk (lateral growth)

Growth Rings

The rate of secondary growth in a plant will change throughout the year according to the seasons

• Growth rates will slow in winter when there is less light available for photosynthesis

This results in discrete growth rings occurring within the plant stem, which are visible when the trunk is cut in cross-section

• Each ring typically marks the passage of one year in the life of the tree
• Growth rings can be counted to estimate the age of the plant (dendrochronology)

Plant growth and development are controlled by plant hormones (phytohormones)

• There are 5 main plant hormones that coordinate plant growth and development

Auxins:

• Promote primary growth (lengthening) by promoting cell elongation and increasing the rate of cell division
• Promote apical dominance – whereby the apex / tip of a plant grows while the lateral buds remain undeveloped
• Auxin concentrations may change in response to directional stimuli (i.e. play a key role in tropisms)

Cytokinins:

• Promote cell division (cytokinesis) and ensure roots and shoots grow at equal rates
• Promotes secondary growth (thickening) and help to control the rate of branching by a plant
• Cytokinins are also involved in stimulating the growth of fruit

Gibberellins:

• Triggers germination in dormant seeds (initiates plant growth)
• Gibberellin also causes stem elongation by promoting cell elongation and cell division

Ethylene:

• A gas which acts as a plant hormone and stimulates maturation and ageing (senescence)
• It is responsible for the ripening of certain fruit (auxins and gibberellins promote fruit growth but inhibit ripening)
• It also contributes to the loss of leaves (abscission) and the death of flowers

Abscisic Acid:

• Abscisic acid (ABA) principally functions to inhibit plant growth and development
• It promotes the death of leaves (abscission) and is responsible for seed dormancy
• It generally initiates stress responses in plants (like winter dormancy in deciduous plants)
• Abscisic acid controls the closing of stomata and hence regulates water loss in plants