IB Bio 9.3 Growth in plants

indeterminate growth

The growth of something is not fully determined. Stem cell tissue in plants can carry out mitosis continuously and have the potential to grow e to an indefinite length. Undifferentiated cells in the meristems of a plant allow for indeterminant growth.

meristems

1. regions where small undifferentiated cells continue to divide and grow

2. are tissues in a plant consisting of undifferentiated cells capable of indeterminante growth

apical meristems

Apical meristems occur at the roots and shoot tips and are responsible for primary growth (lengthening), Produces leaves and flowers, and Apical meristems give rise to new leaves and flowers.

lateral meristems

Occurs at the cambium and are responsible for secondary growth (widening and thickening), Extra xylem and phloem tissues, and Responsible for the production of bark

outline the growth of a shoot

The shoot of the plant is the stem together with the leaves. At the tip of the shoot there is a meristem called the shoot apical meristem. The cells there carry out mitosis and cell division repeatedly, to generate the cells needed for extension of the stem and development of leaves.
1. Some of the undifferentiated cells stay in the meristem and continue through the cell cycle, producing more cells.
2. The continuous production of cells results in the displacement of other cells at the edge of the meristem.
3. Cells at the edge stop dividing and undergo rapid growth and differentiation to become stem or leaf tissue.

structure of a shoot apical meristem

A. leaf primordia
B. procambium
C. apical meristem
E. auxiliary buds

plant hormones and shoot growth

plant hormones control growth in teh shoot apex. A hormone is a chemical messge that is produced and released in one part of an organism that has an affect on another. An exmaple of a plant hormone is auxin. Auxin hormones are the growth hormones.

auxins

Auxins are a group of hormones produced by the tip of a shoot or root (i.e. apical meristems) that regulate plant growth

Auxin efflux pumps can set up concentration gradients within tissues - changing the distribution of auxin within the plant
These pumps can control the direction of plant growth by determining which regions of plant tissue have high auxin levels
Auxin efflux pumps can change position within the membrane (due to fluidity) and be activated by various factors

auxins- growth rates and gene expression

Auxin is a plant hormone and influences cell growth rates by changing the pattern of gene expression with a plant's cells

Auxin's mechanism of action is different in shoots and roots as different gene pathways are activated in each tissue

In shoots, auxin increases the flexibility of the cell wall to promote plant growth via cell elongation

Auxin activates a proton pump in the plasma membrane which causes the secretion of H+ ions into the cell wall
The resultant decrease in pH causes cellulose fibers within the cell wall to loosen (by breaking the bonds between them)
Additional
ly, auxin upregulates expression of expansins, which similarly increases the elasticity of the cell wall
With the cell wall now more flexible, an influx of water (to be stored in the vacuole) causes the cell to increase in size

auxins and phototropism

When light comes in a certain direction, the plant grows and turns toward the direction of the light. Photoreceptors receive the light.The hormone instead, is concentrated in the area that is away from the light. When auxin moves from the cytoplasm to the cell was, different light intensities cause auxin efflux pumps to move and a concentration gradient is set up by the movement of auxin. The auxin (hormone) then affects gene expression of the cells on the side that it moved too. Auxin binds to auxin receptors and promote transcription of certain genes. As the result of certain genes getting expressed more, protons (hydrogen ions) are secreted more, and the more protons the looser the cellulose fibers of the plant. The cellulose can then stretch out and elongate.

Auxin has different mechanism of action in the roots of plants versus the shoots of plants

In the shoots, auxin stimulates cell elongation and thus high concentrations of auxin promote growth (cells become larger)
In the roots, auxin inhibits cell elongation and thus high concentrations of auxin limit growth (cells become relatively smaller)

Abscisic Acid

functions to inhibit plant growth a

Gibberellins

initiates plant growth

Cytokinins

promotes cell division to ensure roots and shoots grow at similar or equal rates

plant tropisms (plants respond to the environmetn by tropisms)

Plant hormones control both the growth of plants (stems and roots). The direction in which stems grow is influenced by two external stimuli: light and gravity.

Phototropism- growth towards the light

Gravitropism (or geotropism)- growth in response to gravitational force

hydrotropism- response to water gradient

thigmotropism- response to tactile stimulus

define tropisms

Directional growth in repsosne to directional external stimuli

micropropagation

a technique that is used to produce genetically identical plants from the apical meristems of parent plants. The apical meristem is cut and placed in a sterile nutrient environment. The developing callus (tissue lumps)is placed in agar gel with auxin or cytokinins or gibberllins (plant hormone) that will stimulate the growth of roots and shoots. The plantlet that grows will be transferred into another environment such as a potted soil to continue growing.

benefits of micropropagation

rapid bulking- parent plant can be cloned in large amounts to produce selectively bred plants. This is beneficial in field such as agriculture to increase the amount of plants (for sale, consumption, purchase etc)

Virus-Free Stains- plant virus can cripple plant populations and affect the ecosystem. Micropropagation can be used to produce virus resistant/virus free plants so that they can survive and reproduce.

Propagation of rare species- to preserve delicate ecosystems rather than taking plants from them, plants can be clones and transported without harming the ecosystem.

Grafting

older less sophisticated ay of growing plants. Plant parts are attached from different plants and attached with a tool such as a clothes pin.

Define the role gibberellin plays in plants

A plant hormone that promotes shoot growth