Chemical control in plants

​​In a nutshell

Plant growth is controlled by different chemicals which are responsible for regulating processes in the plant.  Chemicals are produced in different areas of the plant and respond to light, stimulate growth and initiate development.

Plant growth chemicals

The majority of responses in plants are induced by growth patterns and growth rates. Plant chemicals are usually produced in the root or stem of a plant in unspecialised cells. Plant growth chemicals are not always transported and they usually induce responses at the site where they are produced. 

Auxins

Auxins are the main plant growth chemical and they exert their effect on plant cell walls. The meristem is an area of dividing cells found at the root and shoot tips of a plant. Auxins allow plant cell walls to be more flexible so plant cells can expand further through tropisms. Tropism is a response carried out by plant cells where they respond to the direction of gravity and light. 

Indoleacetic acid (IAA) is an important auxin and is produced in the tips of stems. IAA controls tropisms by moving through the plant via diffusion, active transport and the phloem. IAA promotes plant cell elongation. 

Cytokinins

Cytokinins are a plant growth hormone produced in the roots and lower shoots of a plant. They are responsible for stimulating root growth and the growth of side shoots. Cytokinins move up the plant and auxins move down the plant through the phloem. 

Cytokinins interact with receptors on cell surface membranes triggering formation of transcription factors and eventually cell replication. 

Gibberellins

Gibberellins are a plant growth hormone formed during germination. They are important chemicals involved in plant germination and seed development. Gibberellins are also responsible for controlling the internode length. The internode is the distance between branches along the plant stem.

Gibberellins interact with intracellular proteins called DELLA proteins. Following this interaction DELLA proteins undergo a conformational change which allows the transcription of genes initiating germination. 

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Apical dominance

Definition

Apical dominance is a feature utilised in plants to keep the plant growing upwards towards brighter sunlight compared to promoting the growth of side shoots. A feature of apical dominance is the antagonistic effect of auxins and cytokinins. 

Auxins promote changes which prevent the activation of genes that lead to cytokinin production. Therefore, the lower sections of the plant grow the most outshoots, whereas, the upper sections of the plant grow upwards. 

When cytokinins promote shoot growth,  a new growing point will be generated and this will produce auxins. This encourages the side shoot to grow upwards rather than outwards. This restores apical dominance.  

Phytochrome

Phytochrome is a blue-green pigment found in green plants in small concentrations. Phytochrome has a large protein structure which drastically changes after interaction with different wavelengths of light. 

P_R and P​_FR

Phytochrome presents itself in two different forms P_R and P_FR. P_R is a blue pigment that absorbs red light at a wavelength of $$660\space nm$$. P_FR is a blue-green pigment that absorbs far-red light at a wavelength of $$730\space nm$$. Essentially, this means that when exposed to light, phytochrome changes its structure to P_FR ​and when phytochrome is in the dark it changes its structure to P_R.

Phytochrome control

Phytochrome can monitor the length of day and night. When phytochrome switches from P_R to P_FR this signals the beginning of the day. When phytochrome switches back this signals the end of the day. 

When the length of day reaches the right time period, then phytochrome will activate genes in the plant which stimulate the growth of flowers. Therefore, phytochrome allows plants to flower at the same particular time in the year. 

Phytochrome also stimulates chlorophyll when light is at its strongest. This influences leaf shape, so light absorbance is optimal. These processes are known as photomorphogenesis. Photomorphogenic processes are plant mechanisms that allow plant growth and development to be influenced by light.