Nov. 8th; Plant Development and Hormones

Male Gametophytes:  Pollen

Female Gametophytes: Embryo Sac

Generative Cell will divid to become the two sperm. The tube cell makes up the pollen tube into the pistil of the plant.

One sperm fertilizes the egg, the other will fuse with the polar nucei. The fertilized egg will become the embryo. The two polar nuclei and sperm will be the endosperm that provides nutrients to the embryo. This is known as Double Fertilization.

Embryo Development

The last step of seed development is that they become desiccated, they only have 5-20% water. The seeds coat will harden.

Inside of the seed will be the seed coat, the radicle (embryonic root), Cotyledon, Epicotyl (above cotyledon), Hypocotyl (below cotyledon).

Cotyledon are seed leaves that are packed with nutrients. In monocots, the cotyledon will wrap to become the Coleoptile.

Plant Hormones

Five Main Classes of Plant Hormones

The five hormones: Auxin, Cytokinins, Gibberellins, Ethylene, Abscisic Acid

Other Plant Growth Requlators: Salicylic Acid, Jasmonic Acid, Systemin

Working with Coleoptiles Charles Darwin and son Francis studies phototrophism. They found that cutting the tip prevents it from bending. They put an opaque cap on the tip and it did not bend. If covered with a clear cap it did. If they covered the bottom with a cap it still bent. This means that there was a signal from the tip of the plant that must have been responsible for this phototrophism. Peter Boysen Jensen took it a step further and studied the chemical signal. Fritz went isolated the chemical responsible for the signal. This chemical was an AUXIN.

Auxin

This is a the first plant hormone discovered. It is the Greek word for increases. Natural auxins consist of things like indole acetic acid (IAA) and relate to molecultes. They are produced in the apical meristems of shoots. They are usually transported from the top down, basipetal transport. This gradient become really important.

The process in which auxin phototrophism occurs is the ACID GROWTH HYPOTHESIS. Auxin increases proton pump activity. The cell wall becomes more acidic, and unzips the hydrogen bonds. This makes the wall soft allowing turgor pressure to push cell wall outward.

Auxin is also involved in gravitrophism. The starch-statolith hypothesis is an explaination of gravitrophism. The amyloplasts, leucoplasts that store starch, are used to figure out what is up and what is down. Whe the weight of the amyloplasts hit it triggers auxin production. Auxin is produces in the shoots.

Auxin is really important for apical dominance, tissue differentiation, lateral roots, fruit development, leaf abscission, and herbicide. In tissue differentaitaion, if you damage the vascular tissues, auxin initiates formation of vascular tissue to heal the wound. Auxin stimulates pericycl cells to divide.  The auxin enhances fruit growth, developing seeds are the source of IAA. The absicssion, or dropping of leaves and other plant parts, are a cause of a drop in auxin that drops the part. Synthestic auxins are used as weed killers.

Cytokinins

Johannes van Overbeek discovered that coconut milk possessed factors that accelerated the growth of plant embryo and isolted tissues and cells in test tubes. 

They are found in actively dividing tissues, cytokinesis. The are shown to cause plant cells to grow in an undifferentiated mass of cells (callus). The combination of auxin from above and cytokinin from the root tell the tissue what to differentiate into. Cytokinen is produced from the roots. They can delay and reverse senescence (the process of ageing) in plants. They release buds from apical dominance.

Gibberellins

E. Kurosawa was working on the disease of rice that produced fast growing, pale-colored and sickly plants. This was caused by a fungus isolated from the rice and later bean plants. The bean plants showed gibberellins were being released from the plant itself instead of the fungus.

Gibberellins are all about stem elongation. Dwarf plants are a varity of mutations that cause a lack of gibberellins or the inability to process gibberellins. Bolting, or the sudden verticle growth associated with flowering and death, is mediated by gibberellins. There is a spike in the gibberellins concentration. They are also really important for germination.  Gibberellins will induce genes to make enzymes that break down starch in the remaining endosperm. The biggest current use of gibberellins today is in Thompson’s seedless grapes.

Abscisic Acid

Abscisic acid causes stomatal closure, and prevents premature germination of seeds. It is very important for water regulation.

Ethylene

Ethylene is a gas that was discovered by accident by citrus growers. They noticed that if they kept them in a warm shed they could ripen the fruit faster. Ethylene gas hastens the ripening of fruits. It is importatn in seed germination, fruit ripening, and absission.

 

 

 

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