Topic 9.3 – Plant Growth

This movement is regulated by hormones called auxins. Plant hormones control growth in the shoot apex. Can either be in the direction of the stimulus (positive) or away from the stimulus (negative) This movement is regulated by hormones called auxins. Auxins promote growth by lengthening the cells in which they are introduced Auxins are produced by the coleoptile, which is a protective sheath that comes around the emerging plant Normally auxins are produced evenly throughout the plant, which causes the plant to grow normally If the plant’s photoreceptors detect light, auxin will move in the opposite direction of the light, causing cells on that side of the plant to lengthen themselves. This leads to the plant growing unevenly, but growing in the direction of the light. Mitosis and cell division in the shoot apex provide the cells needed for extension of the stem and development of leaves.

Practice: Print out this slide and draw on each of the following trees where auxin would have been in higher concentration during growth:

30 Minute Inquiry – Plant Tropisms You and your group must find one specific example of a plant that is going through each of the types of tropisms (can be three different plants or different parts of same plant): Gravitropism/Geotropism Phototropism Thigmotropism In addition to this, you must determine the where auxin is being produced in the plant, and whether it is a positive or a negative tropism Example: Plantus originalus The stem of P. originalus is seen growing in this image towards the light. This is coming from equal auxin expression in the tip of the shoot, implying a positive phototropism.

Auxin efflux pumps can set up concentration gradients of auxin in plant tissue. All plant cells contain auxin receptors When auxin binds to receptors, transcription of specific genes is promoted. A concentration gradient of auxin influences cell growth rates by changing the pattern of gene expression. The expression of these genes causes secretion of H+ ions into the cell wall, which loosen the connections between the cellulose fibers that make them, and allow for cellular expansion This all occurs in the shoot apex Auxin affects gene expression in shoots

Undifferentiated cells in the meristems of plants allow for indeterminate growth. Plants only grow where they have undifferentiated cells – meristems. These meristems act like pockets of stem cells do in humans. These can either be apical (meaning at the root apex or shoot apex) or lateral (thickening layers of the cambium) Shoot Apex Lateral Meristem

Apical Meristems These areas of growth are found at the terminal bud of the plant. This portion will continue to grow indiscriminately lengthwise and size wise for the plant Each time the plant adds an additional section of growth, it leaves behind an axillary bud. This bud is temporarily inactive until it needs to be used. A hormone inhibitor prevents growth before the plant needs it

Lateral Meristems As plants grow up, they also need to grow wider to support their height. Lateral Meristem growth results in secondary growth (xylem growth) in a ring inside the cambium along with secondary phloem growth If you have ever wondered where the rings in a tree come from, this is where! The outermost layer of this is called cork, which is produced by cork cambium. This is the bark of a tree!

Micropropogation of plants can be done using tissues from the shoot apex, nutrient agar gels and growth hormones. Micropropogation is the method scientists use to produce many clonal offspring from one plant. It requires the use of plant growth hormones (auxin), nutrient agar, and a sample of the plant tissue containing at least one meristem. Steps: 1. Grow mother plant with desired genetics. 2. Cut mother plant into pieces containing at least one meristem/bud 3. Place pieces of plant into nutrient agar containing hormones. 4. Allow many baby plants to be formed. In what way will these plants be different than if you simply bred a rose with one with the same phenotype?

Application: Use of micropropagation for rapid bulking up of new varieties, production of virus-free strains of existing varieties and propagation of orchids and other rare species. Orchids are the most commonly micropropogated species, due to the high values of them, and extreme focus on genetics. Varieties of virus-free orchids and specific cross breeds can only be created by micropropogation. Orchids – Many varieties can not reproduce on their own, given a reduced reproductive structure from selective breeding. Micropropogation allows for creative cross breeds to be formed as well.