9.3. Growth in plants

Miltiadis-Spyridon Kitsos Platon IB Diploma

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The official IB Diploma Biology guideEssential idea: Plants adapt their growth to environmental conditions.

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Growth in plants Undifferentiated cells in the meristems of the plants allow indeterminate growth

In plants growth is indeterminate that is, the cells have the potential to divide indefinitely.

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Whole plants may develop from differentiated cells. Side stems known as runners, tubers and bulbs are very good examples.

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Growth in plants Undifferentiated cells in the meristems of the plants allow indeterminate growthPlant growth is taking place at the meristems. Meristems are areas where undifferentiated cells undergo division.

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Undifferentiated cells in the meristems of the plants allow indeterminate growth

Mitosis and plant growthMitosis and cell division in the shoot apex provide cell needed for extension for the stem and development of the leaves

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Different meristematic tissues result from the apical meristem which in turn give rise to different tissues. Chemical signals play a role in differentiation.

Hormones and plant growth Plant hormones control growth in the shoot ape

A hormone is a chemical compound produced and released somewhere in the plant, transported via the sap and having an effect on another part of the plant.

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Specific functions of auxin are:• root and shoot growth• Flowering• fruit development• leaf development• wound response

9.3.U4 Plant shoots respond to the environment by tropisms.

http://www.kscience.co.uk/animations/auxin.htm

Gravitropism (aka Geotropism) is the response to gravity. This response can be both positive and negative.

Phototropism interactive animation

Gravitropsimtutorials & animations

Edited from: http://www.slideshare.net/gurustip/plant-structure-and-growth-ahl

http://leavingbio.net/plant%20responses.htm

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Plant tropismsPlant respond to environment by tropisms

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In this image you can see different experimental setups to investigate the effect of light on plant grown. Can you deduce the respective results?

Phototropims= Photo (light) + Tropos (movement)

Gravitropism = Gravity + Tropos (movement)Growth following the gravitational force

Plant tropismsPlant respond to environment by tropisms

https://youtu.be/4-2DZo2ppAY

http://www.kscience.co.uk/animations/auxin.swf

http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp18/animations/ch18/demonstrating_phototropism.swf

http://www.kscience.co.uk/animations/auxin.htm

Phototropism interactive animation

Auxin affects gene expression in shoots:• Cells contain an auxin receptor.• When auxin binds to receptors, transcription of specific genes is promoted.• The expression of these genes causes secretion of hydrogen ions into cell walls.• hydrogen ions loosen connections between cellulose fibres, allowing cell

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Auxin and gene expressionAuxin influences cell growth rates by changing the pattern of gene expression

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9.3.U5 Auxin efflux pumps can set up concentration gradients of auxin in plant tissue. 9.3.U6 Auxin influences cell growth rates by changing the pattern of gene expression.

Different factors can affect transporter proteins and hence the direction in which auxin can move:• The location of transporter proteins can be

changed as the plasma membrane is fluid, e.g. efflux transporters can congregate at the top of cells in roots to move auxin upwards

• Transporter proteins can be activated and/or inhibited by stimuli such as light

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Concentration gradients of auxin are necessary to control the direction of plant growth.

Auxin can enter the cell by:• Diffusion• influx transporter proteins

Auxin can move out of the cell by through efflux transporters (called PIN proteins)

This requires that auxin is unevenly transported amongst plant tissues.

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Auxin and gene expressionAuxin efflux pumps can set up concentration gradients of auxin in plant tissue.

Phototropins at the apex receive the light

They undergo conformational changes and bind to receptors within the cell

This binding promotes the expression of specific genes which encode for plasma proteins called PIN3 glycoproteins

Intracellular movement of auxinAuxin efflux pumps can set up concentration gradients of auxin in plant tissue

PIN3 proteins may determine the direction of auxin transport. See these examples

When the sun moves sideways PIN3 channels enable the later movement of auxin.

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http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp18/animations/ch18/auxin_mode_of_action.swf

Intracellular movement of auxinAuxin efflux pumps can set up concentration gradients of auxin in plant tissue

Amyloplast statoliths pulled by gravity will change the distribution of PIN3 proteins which accumulate at the bottom of the cells.

Unlike in the stems, high auxin concentration in the roots inhibits cell elongation.

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Micropropagation of plants Micropropagation is an in vitro technique of developing numerous identical plants on an artificial culture medium under aseptic and controlled conditions. Replaces traditional propagation techniques Lead to the development of virus-free plants – selective

breeding Saves time and labor

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Micropropagation of plants Explants that is sterilized pieces of plants are cut into pieces and placed in a growth medium along with plant hormones. Sterilise media to prevent growth of fungi and other microorganisms

Formation of an undifferentiated mass called the callus (totipotent cells)

Based on the relative proportion of hormones development of stems or roots

Hardening process

Transfer to soil

Cytokinins cons > auxin cons development of shoot

Cytokinins cons < auxin cons development of roots

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Micropropagation of plants Micropropagation of plants is used for:

• A cost-effective production of a significant number of plants.• Conservation of species close to extinction. • Isolation of diseases from plants. • Development of genetically modified plants.

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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. (1.8)

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The interactions between different plant hormones and multiple plant genes are very complex and not fully understood. Our understanding of this area of science is steadily growing due to our ability to detect trace (very small) amounts of molecules.

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