1. Communication and
control in flowering plants
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2. • In some species of plants (Mimosa and
Venus fly-trap) electrical signals rather like
the action potentials can be detected
• Most communication within plants
depends on chemicals (plant hormones
or plant growth regulators )
• Plant growth regulators are not produced
in endocrine glands but in a variety of
tissues in small quantities
• They move in plant either directly from cell
to cell (diffusion or active transport) or
carried in the phloem sap or xylem
vessels; some don’t move at all
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4. Auxins and apical dominance
• Auxin (IAA : indole 3-acetic acid) is synthesised in the
growing tips of roots and shoots, where the cells are
dividing
• Involved in determining whether a plant grows upwards
or whether it branches sideways
• When a plant has an active growing point at its apex, this
tends to stop lateral buds from growing
• However, if the apical bud is cut off, then the lateral
buds start to grow
• Clearly the presence of the apical bud is stopping the
lateral bud from growing (apical dominance )
• Auxin in the apical bud is transported down the stem to
the lateral buds
• It seems like that other plant growth substances
(cytokinins and abscisic acid ) are also involved
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6. Gibberellins and stem elongation
• Gibberellins are synthesised in most parts of
plants (young leaves and seeds, stems:
determining growth)
• The height of some plants is partly controlled by
their genes
• The dominant allele of this gene regulates the
synthesis of an enzyme that catalyses the
synthesis of an active form of gibberellin, GA1
• If only recessive allele is present, plant contains
only inactive forms of gibberellin
• Active gibberellin stimulates cell division and cell
elongation in the stem (causing plant to grow
tall)
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7. Gibberellins and seed germination
• In some seeds, gibberellins are involved in the control of
germination
• When the seed absorbs water, this stimulates the
production of gibberellin by the embryo and the
gibberellin stimulates the synthesis of amylase by the
cells in the aleurone layer
• Amylase hydrolyses starch molecules in endosperm –
maltose molecules – glucose – transported to the
embryo – respiration to provide energy as embryo begins
to grow
• Gibberellin causes these effects by regulating genes that
are involved in the synthesis of amylase (in barley seeds,
it has been shown that application of gibberellin causes
an increase in the transcription of mRNA coding for
amylase
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9. Abscisic acid and stomatal closure
• ABA is found in a wide variety of plants including ferns and
mosses as well as flowering plants
• Can be found in every part of the plant and is synthesised in
almost all cells that possess chloroplasts or amyloplasts
(organelles like chloroplasts but contain large starch grains
and no chlorophyll)
• One role of ABA is as a stress hormone
• Plant in drought conditions can have concentrations of ABA in
leaves rise to 40 times the normal (causes stomata to close)
• Not known exactly how ABA achieves the closure of stomata
but the fact that the response is very fast indicates that it is
not done by regulating the expression of genes (stomata
closes within a few minutes of applying ABA to leaf)
• It seems that guard cells have ABA receptors on their plasma
membranes and it is possible that when ABA binds with these
it inhibits the proton pump – stops H+ being pumped out, so K+
and water would not enter and guard cells would become
flaccid and close the stomata
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11. Leaf abscission
• Abscisic acid takes its name from the fact that it was thought to be
closely involved in leaf or fruit fall (abscission )
• The leaves fall because the leaf stalk or petiole breaks off from the stem
• Useful substance withdrawn from leaves and taken back into stem
(breakdown of pigments) – abscission zone forms where petiole
meets stem (2 layers of cell)
• Separation layer (nearest to leaf) – made of small cells with thin walls
• Protective layer (nearest to stem) – made up of cells whose walls
contain suberin
• Enzymes break down the cells walls in the separation layer and the
petiole breaks at this point
• The protective layer remains, forming a ‘scar’ on the stem were leaf
used to be
• Abscisic acid appears to be involved in the senescence of leaves but
not directly in their falling from the plant (auxin is stronger candidate)
• Abscision is usually followed by a drop in auxin concentration in leaf and
can be prevented by applying auxin in early stages
• However, high concentrations of auxin, applied later, can actually
promote fruit drop
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