UNIT ___: Plant Response and Growth Name: ________________________
Essential Idea(s):
Structure and function are correlated in the phloem in plants.
IB Assessment Statements and Class Objectives
9.3.U1: Undifferentiated cells in the meristems of plants allow indeterminate growth
- Define indeterminate growth and totipotent.
- State that most plants have indeterminate growth and have totipotent cells.
- Define meristem.
- Compare apical and lateral meristems.
9.3.U2: Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves
- Outline role of mitosis in the growth of stem and leaves while maintaining a meristem.
9.3.U3: Plant hormones control growth in the shot apex
- State the generic function of plant hormones.
- Outline how auxin concentrations regulate plant growth in the root and stem.
- Outline the role of auxin in apical dominance.
9.3.NOS: Developments in scientific research follow improvements in analysis and education-improvements in analytical techniques allowing the detection of trace amounts of substances has led to advances in the understanding of plant hormones and their effect on gene expression.
- Outline role of microarrays in understanding role of plant hormones.
9.3.U4: Plant shoots response to the environment by tropisms
- State two external factors that control the growth of roots and stems.
- Define tropism, phototropism and gravitropism.
9.3.U5: Auxin efflux pumps can set up concentration gradients of auxin in plants tissue
- Outline how PIN-transport proteins can direct direction of auxin flow.
- Explain how auxin concentrations allow for phototropism in the stem.
- Explain how auxin concentrations allow for gravitropism in the root.
9.3.U6: Auxin influences of cell growth rates by changing the pattern of gene expression
- State that auxin influences cell growth rates by changing gene expression.
3.5.U6: Many plants species and some animal species have natural methods of cloning
- Define clone.
- Outline two examples of natural cloning in plants.
3.5.S1: Design of an experiment to assess one factor affecting the rooting of stem-cuttings
- Outline preparation of a plant for rooting of a stem cutting.
- List manipulated, responding and controlled variables in an experiment of rooting stem-cuttings.
9.3.A1: Micropropagation of plants using tissue from the shoot apex nutrient agar gels and growth hormones
- Define micropropagation.
- Outline how changing auxin and cytokinin ratios can lead to development of roots or shoots from the same explant tissue.
9.3.A2: 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
- Outline three roles of micropropagation of plant species.
Plant Growth at Meristems
- Unlike animals, plants have indeterminate growth.
- Regions from which a plant can grow are called “meristems.”
- Meristems have cells that are totipotent.
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- There are two types of meristems:
- Growth at meristems is caused by two factors:
- Cell enlargement
- Cell division (mitosis and cytokinesis)
- When mitosis occurs in the meristem, each daughter cell has a different fate
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Differentiation to give rise to specialized structures (leaves, flowers, roots)
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- In the stem, growth occurs at meristem tissue located at nodes (junctions) with the remaining meristem tissue forming an inactive “axillary bud.”
- Axillary buds have the potential to form branching shoots.
Plant Hormones: Auxin
- Auxins are a type of plant hormone that influences cell growth rates by changing gene expression.
- Auxin has different effects with different concentrations.
- Auxin production in the shoot apical meristem
- The distribution of auxin can be changed within the plant by altering the location of auxin transport proteins in the plant cell membrane.
- “Efflux” proteins transport molecules OUT of a cell.
- PIN proteins are integral membrane proteins that are involved in the transport of auxin out of the cell.
- PIN proteins can be located along specific sides of the plant cell, thereby directing the direction of auxin movement (and in turn determining which region of the plant will grow).
- PIN proteins can change position within the membrane.
- Auxin’s mechanism of action is different in shoots and roots as different gene pathways are activated in each tissue
- 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)
Tropisms
- Tropism definition:
- 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. Remember, in shoots auxin stimulates cell growth. So, the dark side of the shoot elongates and shoots grow towards the light.
Micropropagation
- Key steps:
- Meristem plant tissue is selected from a stock plant.
- The meristem cells are totipotent, so…
- The removed tissue is called the “explant”
- The explant tissue is grown on sterile nutrient agar and treated with growth hormones to stimulate growth
- More cytokinin (a different plant hormone)…
- Uses of micropropagation:
Rapid Bulking - Desirable stock plants can be cloned to maintain a selected characteristic
- More reliable that selective breeding because new plants are genetically identical to the parent “stock” plant
- 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 Species - Used to increase numbers of rare or endangered plant species
- Used to increase numbers of species that are difficult to breed sexually (e.g. orchids)
- Used to increase numbers of plant species that are commercially in demand
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- Plant propagation from stem cutting
- How to prepare a plant for rooting of a stem cutting:
- List potential manipulated, responding and controlled variables in an experiment of rooting stem-cuttings.
MANIPULATED
| RESPONDING
| CONTROLLED
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Natural Cloning in Plants
- In plants, clones form through vegetative propagation:
- Virtually all types of roots and shoots are capable of vegetative propagation
- Garlic and onion bulbs are modified plant leaves – all the bulbs in a group are genetically identical
- Underground stems (e.g. potato tubers) can form new plants which are genetically identical to the parent plant
- Certain plants can form horizontal stems called runners (or stolons) that grow roots and develop into clones