Communication and Communication and control in flowering control in flowering

plantsplants

ALBIO9700/2006JK

• In some species of plants (In some species of plants (MimosaMimosa and and Venus fly-trap) electrical signals rather Venus fly-trap) electrical signals rather like the action potentials can be detectedlike the action potentials can be detected

• Most communication within plants Most communication within plants depends on chemicals (depends on chemicals (plant hormonesplant hormones or or plant growth regulatorsplant growth regulators))

• Plant growth regulators are not produced Plant growth regulators are not produced in endocrine glands but in a variety of in endocrine glands but in a variety of tissues in small quantitiestissues in small quantities

• They move in plant either directly from They move in plant either directly from cell to cell (diffusion or active transport) cell to cell (diffusion or active transport) or carried in the phloem sap or xylem or carried in the phloem sap or xylem vessels; some don’t move at all vessels; some don’t move at all

ALBIO9700/2006JK

ALBIO9700/2006JK

Auxins and apical Auxins and apical dominancedominance

• AuxinAuxin ( (IAAIAA: indole 3-acetic acid) is synthesised in : indole 3-acetic acid) is synthesised in the growing tips of roots and shoots, where the the growing tips of roots and shoots, where the cells are dividingcells are dividing

• Involved in determining whether a plant grows Involved in determining whether a plant grows upwards or whether it branches sidewaysupwards or whether it branches sideways

• When a plant has an active growing point at its When a plant has an active growing point at its apex, this tends to stop apex, this tends to stop lateral budslateral buds from from growing growing

• However, if the However, if the apical budapical bud is cut off, then the is cut off, then the lateral buds start to growlateral buds start to grow

• Clearly the presence of the apical bud is stopping Clearly the presence of the apical bud is stopping the lateral bud from growing (the lateral bud from growing (apical dominanceapical dominance))

• Auxin in the apical bud is transported down the Auxin in the apical bud is transported down the stem to the lateral budsstem to the lateral buds

• It seems like that other plant growth substances It seems like that other plant growth substances ((cytokininscytokinins and and abscisic acidabscisic acid) are also involved) are also involved

ALBIO9700/2006JK

ALBIO9700/2006JK

Gibberellins and stem Gibberellins and stem elongationelongation

• Gibberellins are synthesised in most parts of Gibberellins are synthesised in most parts of plants (young leaves and seeds, stems: plants (young leaves and seeds, stems: determining growth)determining growth)

• The height of some plants is partly The height of some plants is partly controlled by their genescontrolled by their genes

• The dominant allele of this gene regulates The dominant allele of this gene regulates the synthesis of an enzyme that catalyses the synthesis of an enzyme that catalyses the synthesis of an active form of the synthesis of an active form of gibberellin, GAgibberellin, GA11

• If only recessive allele is present, plant If only recessive allele is present, plant contains only inactive forms of gibberellincontains only inactive forms of gibberellin

• Active gibberellin stimulates cell division Active gibberellin stimulates cell division and cell elongation in the stem (causing and cell elongation in the stem (causing plant to grow tall)plant to grow tall)

ALBIO9700/2006JK

Gibberellins and seed Gibberellins and seed germinationgermination

• In some seeds, gibberellins are involved in the In some seeds, gibberellins are involved in the control of germinationcontrol of germination

• When the seed absorbs water, this stimulates the When the seed absorbs water, this stimulates the production of gibberellin by the production of gibberellin by the embryoembryo and the and the gibberellin stimulates the synthesis of gibberellin stimulates the synthesis of amylaseamylase by the cells in the by the cells in the aleurone layeraleurone layer

• Amylase hydrolyses starch molecules in Amylase hydrolyses starch molecules in endospermendosperm – maltose molecules – glucose – – maltose molecules – glucose – transported to the embryo – respiration to provide transported to the embryo – respiration to provide energy as embryo begins to growenergy as embryo begins to grow

• Gibberellin causes these effects by regulating Gibberellin causes these effects by regulating genes that are involved in the synthesis of genes that are involved in the synthesis of amylase (in barley seeds, it has been shown that amylase (in barley seeds, it has been shown that application of gibberellin causes an increase in application of gibberellin causes an increase in the transcription of mRNA coding for amylasethe transcription of mRNA coding for amylase

ALBIO9700/2006JK

ALBIO9700/2006JK

Abscisic acid and stomatal Abscisic acid and stomatal closureclosure

• ABAABA is found in a wide variety of plants including ferns is found in a wide variety of plants including ferns and mosses as well as flowering plantsand mosses as well as flowering plants

• Can be found in every part of the plant and is synthesised Can be found in every part of the plant and is synthesised in almost all cells that possess chloroplasts or amyloplasts in almost all cells that possess chloroplasts or amyloplasts (organelles like chloroplasts but contain large starch grains (organelles like chloroplasts but contain large starch grains and no chlorophyll)and no chlorophyll)

• One role of ABA is as a One role of ABA is as a stress hormonestress hormone• Plant in drought conditions can have concentrations of ABA Plant in drought conditions can have concentrations of ABA

in leaves rise to 40 times the normal (causes stomata to in leaves rise to 40 times the normal (causes stomata to close)close)

• Not known exactly how ABA achieves the closure of Not known exactly how ABA achieves the closure of stomata but the fact that the response is very fast stomata but the fact that the response is very fast indicates that it is not done by regulating the expression of indicates that it is not done by regulating the expression of genes (stomata closes within a few minutes of applying genes (stomata closes within a few minutes of applying ABA to leaf)ABA to leaf)

• It seems that guard cells have ABA receptors on their It seems that guard cells have ABA receptors on their plasma membranes and it is possible that when ABA binds plasma membranes and it is possible that when ABA binds with these it inhibits the proton pump – stops Hwith these it inhibits the proton pump – stops H++ being being pumped out, so Kpumped out, so K++ and water would not enter and guard and water would not enter and guard cells would become flaccid and close the stomatacells would become flaccid and close the stomata

ALBIO9700/2006JK

ALBIO9700/2006JK

Leaf abscissionLeaf abscission• Abscisic acid takes its name from the fact that it was thought to Abscisic acid takes its name from the fact that it was thought to

be closely involved in leaf or fruit fall (be closely involved in leaf or fruit fall (abscissionabscission))• The leaves fall because the leaf stalk or petiole breaks off from The leaves fall because the leaf stalk or petiole breaks off from

the stemthe stem• Useful substance withdrawn from leaves and taken back into Useful substance withdrawn from leaves and taken back into

stem (breakdown of pigments) – stem (breakdown of pigments) – abscission zoneabscission zone forms where forms where petiole meets stem (2 layers of cell)petiole meets stem (2 layers of cell)

• Separation layerSeparation layer (nearest to leaf) – made of small cells with (nearest to leaf) – made of small cells with thin wallsthin walls

• Protective layerProtective layer (nearest to stem) – made up of cells whose (nearest to stem) – made up of cells whose walls contain walls contain suberinsuberin

• Enzymes break down the cells walls in the separation layer and Enzymes break down the cells walls in the separation layer and the petiole breaks at this pointthe petiole breaks at this point

• The protective layer remains, forming a ‘scar’ on the stem were The protective layer remains, forming a ‘scar’ on the stem were leaf used to beleaf used to be

• Abscisic acid appears to be involved in the senescence of leaves Abscisic acid appears to be involved in the senescence of leaves but not directly in their falling from the plant (auxin is stronger but not directly in their falling from the plant (auxin is stronger candidate)candidate)

• Abscision is usually followed by a drop in auxin concentration in Abscision is usually followed by a drop in auxin concentration in leaf and can be prevented by applying auxin in early stagesleaf and can be prevented by applying auxin in early stages

• However, high concentrations of auxin, applied later, can However, high concentrations of auxin, applied later, can actually actually promotepromote fruit drop fruit drop ALBIO9700/2006JK

ALBIO9700/2006JK