Plant Hormones and the Role of Auxins

• Plants can respond to a stimulus very rapidly, like the Venus flytrap closing its leaves to capture insects, yet plant responses generally are slower than those of animals but remain intricate.
• A plant hormone’s concentration has varying diverse effects.

Role of auxins in elongation growth

• Auxins are a group of chemical substances; indoleacetic acid (IAA) is the the most common one.
• The auxin transport goes in one direction, away from the shoots and roots that produce it.
• Auxin changes the state of the cell wall for permanent cell elongation, effective only on young cell walls before rigidity increases through secondary thickening.
• The acid growth hypothesis explains auxin's role in elongation growth.
• Auxin binds to receptors, triggering transport proteins (ATPases) in the cell surface membrane to actively transport protons from the cytoplasm into cell wall spaces.
• Protons acidify fluid-filled spaces in the cell wall.
• Decreased pH creates the right conditions for expansins to weaken the cell wall by disrupting hydrogen bonds, causing a loosening of the cell wall.
• Osmosis increases turgor pressure, and protoplasts expand during active cell growth, causing elongation.
• Buffer solutions, that neutralise acidity of cell walls, prevent cell elongation.
• Agents that acidify cell walls cause cell elongation, and cells release protons in auxin response.
• Uneven cell elongation on one side of a stem or root causes bending in response to environmental factors (light, water, gravity).
• These responses stem from the stimuli causing uneven auxin distribution as it moves away from the stem or root tip.

Response of a Venus flytrap

• Venus flytraps inhabit nutrient-poor boggy habitats, capturing and digesting insects for nitrogen.
• When an insect lands on specialized leaves, the two lobes become concave, trapping the insect.
• When an insect touches hairs on the lobes, it triggers an action potential to lower midrib cells, or hinge cells in the leaf
• At least two hairs must be stimulated in quick succession to cause it to activate
• A proton pump moves H+ ions from the hinge cells into cell wall spaces after action potentials from trigger hairs.
• Auxin levels increase in hinge cells.
• Increased acidity dissolves pectate holding cell walls together.
• Protons exit hinge cells, making them more negative, attracting positive ions like calcium that decrease water potential.
• Water enters cells by osmosis because of the gradient, and the thinner cell walls allow the cells to rapidly expand.
• This causes the lobes to become concave, trapping the insect.
• Alternative theories suggest turgor loss in other cells causes the response
• There is experimental evidence for both mechanisms causing he insect's response.

Role of gibberellins in stem elongation

• Gibberellins, found in plants, fungi, algae, and bacteria, increase stem length and plant height.
• Adding gibberellin to dwarf plant varieties causes them to grow to normal size.
• Height is controlled by a single gene with two alleles: a dominant allele (Le) and a recessive allele (le).
• Dominant allele (Le) triggers enzyme production for gibberellin synthesis, allow the plant to grow to normal height.
• Recessive allele (le) does not allow gibberellin production, resulting in dwarf varieties.

Role of gibberellins in the germination of barley seeds

• Plant seeds remain dormant after formation to overcome harsh weather conditions and allow for wind or animal dispersion.
• Dormancy is due to low seed water content (5-10%).
• Overcoming dormancy requires understanding endospermous seed structure (wheat or barley).
• Seeds have a pericarp and testa, aleurone layer, endosperm, scutellum, and embryo.
• The pericarp and testa form a protective outer layer.
• The aleurone layer is protein-rich, below the testa.
• The endosperm contains stored starch for energy.
• The scutellum is a modified seed leaf.
• The embryo develops into the new plant.
• Germination requires water, oxygen, and temperature.
• DELLA proteins inhibit germination until broken down by gibberellin.
• Water softens the pericarp and testa, entering the seed.
• Water stimulates gibberellin production in the embryo.
• Gibberellin diffuses into aleurone layer cells, breaking down DELLA proteins and triggering transcription to produce a-amylase and various other enzymes.
• a-amylase and other enzymes diffuse into the endosperm.
• The a-amylase hydrolyzes endosperm starch into maltose, then hydrolyzes into glucose.
• Glucose diffuses into the embryo via the scutellum.
• Glucose provides ATP for germination and cellulose for growth.