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growth in plants
Essential idea: Plants adapt their growth to environmental conditions.
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 has led to advances in the understanding of plant hormones and their effect on gene expression.
Understandings:
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 has led to advances in the understanding of plant hormones and their effect on gene expression.
Understandings:
- Undifferentiated cells in the meristems of plants allow indeterminate growth.
- Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves.
- Plant hormones control growth in the shoot apex.
- Plant shoots respond to the environment by tropisms.
- Auxin efflux pumps can set up concentration gradients of auxin in plant tissue.
- Auxin influences cell growth rates by changing the pattern of gene expression.
- Application: Micropropagation of plants using tissue from the shoot apex, nutrient agar gels and growth hormones.
- 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.
- Auxin is the only named hormone that is expected.
growth in plants
Undifferentiated cells in the meristems of plants allow indeterminate growth.
The growth of a plant is an everyday phenomenon, but it is nonetheless remarkable. Most animals and some plants organs undergo determinate growth which is there is either a defined juvenile or embryonic period or growth stops when a certain size is reached or a structure is fully formed. Growth can also be indeterminable when cells continue to divide indefinitely. Plants in general have indeterminate growth.
Meristems: Growth in plants is confined to regions known as meristems. They are composed of undifferentiated cells that are undergoing active cell division. Primary meristems are found at the tips of the stems and roots. They are called apical meristems. The root apical meristems are responsible for the growth of the root. The shoot apical meristem is at the tip of the stem. Many dicotyledenous plants also develop lateral meristems.
The growth of a plant is an everyday phenomenon, but it is nonetheless remarkable. Most animals and some plants organs undergo determinate growth which is there is either a defined juvenile or embryonic period or growth stops when a certain size is reached or a structure is fully formed. Growth can also be indeterminable when cells continue to divide indefinitely. Plants in general have indeterminate growth.
Meristems: Growth in plants is confined to regions known as meristems. They are composed of undifferentiated cells that are undergoing active cell division. Primary meristems are found at the tips of the stems and roots. They are called apical meristems. The root apical meristems are responsible for the growth of the root. The shoot apical meristem is at the tip of the stem. Many dicotyledenous plants also develop lateral meristems.
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.
Cells in meristems are small and go through the cell cycle repeatedly to produce more cells, by mitosis an 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, once cell remains in the meristem while the other increases in size and differentiates as it si pushed away from the meristem region.
Each apical meristem can give rise to additional meristems including protoderm, procambium, and ground meristem, in general these give rise to different tissues.
Cells in meristems are small and go through the cell cycle repeatedly to produce more cells, by mitosis an 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, once cell remains in the meristem while the other increases in size and differentiates as it si pushed away from the meristem region.
Each apical meristem can give rise to additional meristems including protoderm, procambium, and ground meristem, in general these give rise to different tissues.
plant hormones affect shoot growth
Plant hormones control growth in the shoot apex.
A hormone is a 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 roots, influencing the development of fruits and regulating lead development. The most abundant auxin is indole-3-acetic acid (IAA) which has a role in the control of growth in the shoot apex. Among other effects, the IAA promotes the elongation of cells in stems. IAA 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 eh junction or node of the stem, as the shoot apical meristem grows and forms leaves regions of meristem are left behind at the node. Growth at these nodes is inhibited by auxin produced by the shoot apical meristem. The further 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 by auxin. In addition, cytokinins hormones produced in the roots promote axillary bud growth, just as the gibberellins.
A hormone is a 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 roots, influencing the development of fruits and regulating lead development. The most abundant auxin is indole-3-acetic acid (IAA) which has a role in the control of growth in the shoot apex. Among other effects, the IAA promotes the elongation of cells in stems. IAA 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 eh junction or node of the stem, as the shoot apical meristem grows and forms leaves regions of meristem are left behind at the node. Growth at these nodes is inhibited by auxin produced by the shoot apical meristem. The further 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 by auxin. In addition, cytokinins hormones produced in the roots promote axillary bud growth, just as the gibberellins.
plant tropism
Plants respond to the environment by tropisms.
Plants use hormones to control the growth of stems and roots. Both the rate and the direction of growth are controlled. 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 to gravity. These directional growth responses to external stimuli are called tropisms. Growth towards the light is called phototropism and growth in response to gravitational force is called gravitropism.
Plants use hormones to control the growth of stems and roots. Both the rate and the direction of growth are controlled. 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 to gravity. These directional growth responses to external stimuli are called tropisms. Growth towards the light is called phototropism and growth in response to gravitational force is called gravitropism.
auxin influences gene expression
Auxin influences cell growth rates by changing the pattern of gene expression.
The first stage in phototropism is the absorption of light by photoreceptors. Proteins called phototropins have this role. When they absorb light of an appropriate wavelength, their conformation changes. They can then bind to receptors within the cell, which control the transcription of specific genes. Although much research is still needed in this field, it seems likely that the genes involved are those coding for a group of glycoproteins located in the plasma membrane of cells in the stem that transport the plant hormone auxin from cell to cell, called PIN3 proteins.
The first stage in phototropism is the absorption of light by photoreceptors. Proteins called phototropins have this role. When they absorb light of an appropriate wavelength, their conformation changes. They can then bind to receptors within the cell, which control the transcription of specific genes. Although much research is still needed in this field, it seems likely that the genes involved are those coding for a group of glycoproteins located in the plasma membrane of cells in the stem that transport the plant hormone auxin from cell to cell, called PIN3 proteins.
intracellular pumps
Auxin efflux pumps can set up concentration gradients of auxin in plant tissue.
The position and type of PIN3 proteins can be varied to transport auxin to where growth is needed. If phototropins in the tips 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 shader side of the stem cause greater growth on this side , so the stem grows in a curve towards the source of the brighter light. The leaves attached to the stem will therefore receive more light and be able to photosynthesize at a greater rate.
The position and type of PIN3 proteins can be varied to transport auxin to where growth is needed. If phototropins in the tips 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 shader side of the stem cause greater growth on this side , so the stem grows in a curve towards the source of the brighter light. The leaves attached to the stem will therefore receive more light and be able to photosynthesize at a greater rate.
Gravitropism is also auxin dependent. The upward growth of shoots and the downwards growth of roots occurs in response to gravity. If a root is placed on its side, gravity causes cellular organelles called statoliths to accumulate on the lower side of cells. This leads to the distribution of PIN3 transporter proteins that direct auxin transport to the bottom of the cells. High concentrations of auxin inhibit root cel elongation so the top cells elongate at a higher rate than the bottom cells causing the root to bend downwards.
Theory of knowledge:
- Plants communicate chemically both internally and externally. To what extent can plants be said to have language?