Growth Factors in Plants

Plant Hormones - Plant growth factors

Plant growth factors are chemical messengers made in small quantities by various tissues spread throughout the plant body and not specialised cells within glands like animal hormones.

Growth factors are transported through the phloem or xylem sap or can be diffused between cells directly to target nearby cells. This results in a growth-related response such as cell elongation affecting the direction of plant growth. There are essentially two types of plant growth factors: auxins, of which indoleacetic acid (IAA) is the most important and gibberellins.

Gibberellins

Gibberellins are produced throughout the plant body in high concentrations in young leaves and seeds. These factors are essentially involved in regulating seed germination and controlling stem elongation.

Gibberellins are produced in the early stages of germination by the nascent plant embryo and stimulate cells to synthesise amylase, which is crucial to obtaining energy necessary for embryo growth.

Gibberellins are also present in plant stems, where they’re essential in determining their growth and how tall the plant is by stimulating cell division and cell elongation within the stem.

Indoleacetic acid

Indoleacetic acid (IAA) is the most important auxin made by plants. This growth factor is produced in the dividing tips or meristems of roots and shoots and controls their growth by influencing plant cell elongation.

IAA action results in plant cell elongation because it increases young plant cells' cell wall plasticity, allowing their expansion. The acid growth hypothesis presents a possible explanation for this mechanism. According to this hypothesis, IAA increases plant cell wall plasticity by stimulating targeted cells to increase the active transport of hydrogen ions from the cell cytoplasm into the cell wall. This acidifies the wall and loosens bonds in its structure, allowing it to stretch and become longer.

Tropisms

A tropism is a type of differential plant growth response to stimuli that allows plants to maximize their environmental conditions.

There are several types of tropisms depending on the nature of the originating stimulus, but all tropisms cause directional growth of a part of the body of a plant, like its shoots or roots. Types of tropisms include:

• Phototropism (light stimuli)
• Geotropism/gravitropism (gravity stimuli)
• Chemotropism (chemical stimuli)
• Thigmotropism (touch stimuli)

This growth can be directed towards the stimuli (positive response) or away from the stimuli (negative response), meaning the direction of the response (growth) is dependent on the direction of the stimulus. Below are two examples of the most relevant tropism responses in plants: phototropism and gravitropism.

Phototropism

Phototropism is the directional growth of plants’ body parts like shoots or roots dependent on the direction of light stimuli.

When plants grow in all-around light, whether bright or low light and normal gravity conditions, they tend to grow straight upwards; in low light conditions, plants tend to grow upwards more and faster than in bright conditions. However, phototropism happens when plants are exposed to unilateral light or light brighter on one side than another. When plants are exposed to these lighting conditions, phototropism causes shoots to grow towards the light source (or the brighter side) and roots to grow away. Since shoots grow in the direction of the stimuli, they have a positive phototropism, while roots that grow away have negative phototropism. These plant responses help ensure survival by making sure shoots receive as much light as possible to perform photosynthesis, and roots stay in the ground buried by moving away from the light should they emerge from the soil.

Negative and positive phototropism is the result of the plant body part being exposed to light and bending either towards or away from the light source. This bending response happens because of uneven IAA distributions throughout the plant body caused, in this case, by light stimuli.

As mentioned before, IAA is an auxin produced in the growing tips of shoots and roots and is evenly distributed throughout the plant body part in a normal illumination scenario. However, uneven light distribution stimuli cause accumulation of IAA on the more shaded side of the exposed plant body part. In shoots, cells on the shaded side with more IAA will start to elongate more and bend the entire structure towards the light. In roots, however, the opposite happens because higher concentrations of IAA will inhibit cell elongation, which means that the light side of roots will elongate faster than the shaded side, causing the root to bend away into the soil.

Gravitropism

Besides light, plants are also susceptible to gravity, which influences their growth direction through gravitropism. Like everything else on earth, plants receive the same unidirectional gravitation stimulus that pulls us down towards the core of the earth.

Responses to gravitational stimuli are important to control plant growth. For example, shoots that grow away from the gravitational pull into the light are negatively gravitropic, while the plant’s roots grow towards the gravitational pull into the soil and are therefore positively gravitropic.

Gravitropism effects are perhaps more evident in horizontally growing plants. In horizontally-growing plants, roots still display positive gravitropism and the shoots, negative gravitropism. Similar to phototropism, gravity influences IAA distribution in plants. When growing laterally, IAA accumulates on the lower side of the body part (root or shoot), meaning the side closer to the ground. In shoots, IAA accumulation promotes cell elongation, which means the lower side cells elongate, and the structure bends and grows upwards, much like it would if the plant were vertical. While in the roots, the IAA accumulation inhibits cell elongation, which means cells on the opposite side will elongate more, and the structure will bend and grow downwards.