Control and Coordination in Plants PDF

## 15.13 Control and coordination in plants **On these pages you will learn to:** - Describe the rapid response of the Venus fly trap to stimulation of hairs on the lobes of modified leaves and explain how the closure of the trap is achieved - Explain the role of auxin in elongation growth by stimu...

## 15.13 Control and coordination in plants **On these pages you will learn to:** - Describe the rapid response of the Venus fly trap to stimulation of hairs on the lobes of modified leaves and explain how the closure of the trap is achieved - Explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls - Describe the role of gibberellin in the germination of wheat or barley - Explain the role of gibberellin in stem elongation including the role of the dominant allele, Le, that codes for a functioning enzyme in the gibberellin synthesis pathway, and the recessive allele, le, that codes for a non-functional enzyme **Figure 1** Venus flytrap Plants can respond to a stimulus very rapidly: for example the sudden closure of leaves on a Venus flytrap when capturing an insect. For the most part, plant responses are slower than those of animals but they are still complex processes. Different concentrations of a plant hormone can have very different effects. ### Role of auxins in elongation growth Auxins are a group of chemical substances of which indoleacetic acid (IAA) is the most common. The transport of auxin is in one direction, away from the tip of shoots and roots where it is produced. Auxin has a number of effects on plant cells, including altering the state of the cell wall so that permanent elongation of the cells can occur. This is only effective on young cell walls before they have developed greater rigidity through secondary thickening. The proposed explanation of how auxin has a role in this elongation growth is called the **acid growth hypothesis** and is outlined below. - Auxin binds to receptors, which causes transport proteins (ATPases) in the cell surface membrane to actively transport protons (hydrogen ions) from the cytoplasm into spaces in the cell wall. - The protons cause the fluid-filled spaces in the cell wall to become more acidic. - The decrease in pH creates the optimum conditions for proteins known as expansins to weaken the cell wall, temporarily disrupting the hydrogen bonding between cellulose microfibrils and between these and other cell wall polysaccharides. - This causes a loosening of the cell wall. - Water entering the cell by osmosis will cause an increase in turgor pressure and the loosening of the cell wall means that during active cell growth its protoplast can expand, causing elongation of the cell. The hypothesis is supported by experiments in which buffer solutions that neutralise the acidity of cell walls are shown to prevent cell elongation. Conversely, agents that acidify cell walls cause cell elongation. It has also been observed that protons are released from cells in response to auxin. The elongation of cells on one side only of a stem or root can lead to the stems or roots bending. This is the means by which plants respond relatively quickly to environmental stimuli such as light, water and gravity. These responses can be explained in terms of the stimuli causing uneven distribution of auxin as it moves away from the tip of the stem or root. ### Response of a Venus flytrap The Venus flytrap lives in a boggy habitat with few nutrients. In order to obtain nutrients such as nitrogen, needed for growth, it has to capture and digest small insects. When an insect lands on the specialised leaf of the Venus flytrap, the two lobes of the leaf that are usually convex suddenly become concave, snapping shut and so trapping the insect in the cavity that is formed. It is not clear how this rapid response is brought about, but one explanation is as follows. - The mechanical stimulus of the insect touching hairs on the lobes of the leaves generates an action potential that passes to the lower cells of the midrib, known as hinge cells. It requires at least two hairs, which act as receptors, to be triggered in quick succession cause a response. - A proton ($H^+$) pump moves protons out of the hinge cells and into the cell wall spaces between the cells in response to action potentials from the trigger hairs. This may be in response to auxin, the concentration of which has been observed to increase in the hinge cells. By the acid growth hypothesis described earlier, the increased acidity dissolves the calcium pectate that holds the cell walls together. - The loss of protons from the hinge cells makes them more negative, and so positively charged ions such as calcium ions are attracted into the cells, decreasing their water potential. - There is now a water potential gradient that causes water to enter the cells by osmosis. As the cell walls are much less rigid and more flexible, the cells rapidly expand, causing the lobes of the leaf to become concave, closing together and trapping the insect. Alternative theories involve loss of turgor in other cells, causing the response. As there is some experimental evidence for each, it may be that both mechanisms are involved. ### Role of gibberellins in stem elongation Gibberellins are a group of over 90 plant growth regulators found not only in flowering plants but also in fungi, algae and some bacteria. They are thought to be made in developing seeds and apical portions of stems and roots. One gibberellin increases the length of stems and so increases the height of plants. If the gibberellin is added to certain genetic dwarf varieties of plant, the plants grow to normal size. This is probably because height is controlled by a gene with two alleles: - a dominant allele (Le) that controls the production of an enzyme needed in the synthesis pathway of gibberellin; plants with at least one dominant allele therefore grow to normal height - a recessive allele (le) that codes for a non-functioning enzyme involved in the synthesis pathway of gibberellin; plants with both alleles of the recessive form therefore cannot synthesise gibberellin and so develop into dwarf varieties. ### Role of gibberellins in the germination of barley seeds Once plant seeds are formed they often remain dormant for some time before they germinate. This allows them to overcome adverse conditions like the cold temperatures of winter and allows time for them to be dispersed to new regions by wind or animals. This dormancy is, in part, due to the very low water content - between 5 and 10% - of most seeds. What then breaks dormancy and starts the process of germination? To answer this question, we need first to look at the structure of a typical endospermous seed such as wheat or barley. These seeds are made up of: **Figure 2** Structure of a barley seed and the role of gibberellin in its germination The barley seed structure contains the annotations: a-amylase, starch, Pericarp and testa, Aleurone layer, Endosperm, maltose, glucose, gibberellin, Scutellum, water and embryo. - **pericarp and testa** - an outer tough, protective layer made up of the testa (seed coat) and pericarp (fruit coat) fused together - **aleurone layer** - a protein-rich layer just beneath the testa - **endosperm** - a large region of stored starch that provides an energy source for the growing embryo - **scutellum** - a modified form of the single cotyledon (seed leaf) of the seed - **embryo** - the result of mitotic division of the zygote, this will develop into the new plant. The process of germination typically requires the presence of water, oxygen and a favourable temperature. Proteins called DELLA proteins normally inhibit germination (Topic 16.15), as follows (the numbers refer to those on the summary of the process in Figure 1): 1. Water softens the pericarp and testa covering and enters the rest of the seed. 2. The water stimulates the embryo to produce gibberellin. 3. The gibberellin diffuses into the cells of the aleurone layer, where it causes the breakdown of DELLA proteins and so induces transcription of the genes producing α-amylase and other enzymes that are manufactured from its protein store in the aleurone layer. $f(x) = -4(x + 3)­­­­2 + 2$ 4. The a-amylase and other enzymes diffuse into the endosperm. 5. α-amylase hydrolyses the starch in the endosperm into maltose, which in turn is hydrolysed by maltase to glucose. 6. The glucose diffuses, via the scutellum, into the embryo where it is used to provide the ATP (from respiratory breakdown) and raw material (e.g. cellulose) needed for germination and growth.