9.3 Growth in Plants

Meristems

Regions of undifferentiated cells capable of indeterminate growth

Apical: Root and stem tips, lengthening, taller, new leaves and flowers

Lateral: Between xylem and phloem, occurs at the cambium, widening, bark, plant widens

Hormones

Molecules produced to affect physiological activity, auxins main hormone responsible for controlling plant growth

Apical Dominance

Plants grow up before out, lateral buds inhibited by auxins

Auxin Efflux Pumps and Phototropism

• Light from a certain direction is detected by phototropins

• Phototropins undergo a conformational change and bind to receptors within cells

• Receptors control the expression of genes that make PIN3 membrane proteins that transport auxins from cell to cell

• PIN3 proteins pump auxins away from cells at the shoot tip to the shaded side of the tip

• Presence of auxins on the shaded side promotes growth by increasing cell elongation

• Auxin stimulates the movement of H+ ions to the cell wall, decreasing pH and and causing cross-links between cellulose microfibrils to break, loosening the walls

• Auxin also upregulates expression of expansins which further increase elasticity of the cell walls

• Cell walls are more flexible, the cells can take up more water by osmosis and elongate

• Cells on shaded side elongate more than light side, causing the shoot to bend towards the light

• Reverse occurs at root tips

Micropropagation

• Specific plant tissue (typically the undifferentiated shoot apex) is selected from a stock plant and sterilised

• The tissue sample (called the explant) is grown on a sterile nutrient agar gel

• The explant is treated with growth hormones (e.g. auxins) to stimulate shoot and root development

• The growing shoots can be continuously divided and separated to form new samples (multiplication phase)

• Once the root and shoot are developed, the cloned plant can be transferred to soil