BIOL 266 MT2 - Intro To Hormones Auxin

Growth and Development are controlled by

Hormones, Environment, and Hormones x environment

Hormone Definition

Naturally occuring organic compounds that at low concentration exert a physiological effect.

Two important commonalities between plant and animals:

1. For a cell to respond to a hormone it must have a receptor for that hormone. The receptor is always a specific protein.

2. There is a signal transduction pathway that begins with the hormone binding to the receptor (which leads to the physiological effect.)

Hormone being detected:

Must be detected, has a specific receptor, light is detectd by photoreceptors.

Hormone after being detected:

Goes into a signal transduction pathway, causes a physiological effect.

Hormone Pathway

Binding → Change in the receptor → Signal Transduction pathway → Cellular Effect

Light Pathway

Absorption → Change in the photoreceptor → Signal Transduction Pathway → Cellular Effect

Plants and Hormones

tissues can produce a lot of different hormones but do not have any specific hormone-producing organs, often transported, and most hormone effects are due to interactions of 2 different hormones (“crosstalk”), younger tissues produce more hormones than older tissues.

Five classes of plant hormones

Auxins, Gibberellins, Cytokinins, Ethylene, Abscisic Acid.

Auxin(s) Introduction

Exhibit phototropism (growing towards the light). Example of auxin in plants are in coleoptiles and the tip being light-sensitive.

Auxin Introduction 2

There is differential growth (coleoptiles detect light but something lower than the tip was the one in charge of phototropism). It was postulated that after the light was detected in the tip, the signal would move down the coleoptile and cause growth.

Auxin Introduction 3

Gelatin blocks was used to test this hypothesis. Coleoptile tips were chopped off and gelatin was put on top of the tip. Gelatin had different levels of light which proved differential growth. The compound responsible for the seedling bending from one side is indoleactic acid.

Auxin and Elongation Growth

Because the dark side gets more auxin (redistributed in the tip, and auxin promotes growth), the dark side grew longer while the light side grew less faster. This caused a difference in elongation growth and the effect of the coleoptile leaning towards the light.

Indoleactid Acid

Major Auxin in all Plants, regulates growth and elongation.

Auxin Other Roles

Aside from Elongation Growth it also helps with root formation. Auxin is translocated (transport) from the shoot (tip of the plant) to its roots. This movement is called basipetal movement. Auxin also promotes adventitous root formation, and when used commercially is a good rooting powder.

Apical Dominance

Is the supression of axillary buds (the buds on top of the stem with a leaf, see image). Because of this, plants grow tall instead of wide. For example, it prevents the plant from growing sideways.

Apical Dominance and Auxin #2

Auxin inhibits axillary bud growth (plants grow sideways and branch out). Exogenous cytokinin works oppositely to auxin and promotes axillary bud growth.

Bioassay

A method used back in the day to quantify/measure how much auxin a stem produced or had. Done by cutting a stem segment and dipping it into a petri dish with water.

Acid Growth Model

Model detailing auxin stimulating elongation growth in stems. One hypothesis it had is that it reduced the cell wall pH, which caused it to loosen, and caused elongation growth. The acid growth model was tested by adding acid to promote elongation, then auxin (with buffed water, allowing it to resist changes) with no promotive growth, then auxin with promotive growth.

The result showed that Auxin does stimulate H+ ATP activity (meaning the pH of the wall was lowered) and the wall loosened.

Expansins

Cell wall proteins that loosen the bonds between cellulose and hemicellulose.

Polar/Basipetal Auxin Transport

Basipetal (transcription of auxin from shoot to tip) transport is considered polar (meaning can be attracted and repelled because it has partial charges). Basipetal also means moving downwards. Auxin is also transported via. vascular parenchyma cells.

The outside of a cell is acidic. A special chemical called IAAH enters the inside of the cell and changes into IAA- due to the inside of the cell being neutral. The 2H+/IAA symporters are special gates that allow IAA into the cell. After IAA- are transported out of the cell via efflux, auxin can diffuse into any direction, allowing for elongation and weakening of cell wall pH.

Gravitropism

The growth of roots downwards and shoots (of plants) upwards.