9.3.2 Plant Hormones

• Plant hormones are responsible for most communication within plants
  • Plant hormones are not the same as hormones in animals but they are chemical messengers so the name hormone is often used
  • Plant hormones are sometimes referred to as plant growth regulators
• Auxins are a group of plant hormones that influence many aspects of plant growth
  • A common auxin is known as IAA (indole-3-acetic acid)
• In shoots, auxin is produced in cells at the growing tip before moving away into the surrounding tissues
• Auxin has an important role in regulating shoot growth
  • In shoots, auxin causes cells to elongate, leading to stem growth
    • Note that in roots, auxin inhibits cell growth; the opposite effect to that in shoot cells
    • Note that at very high concentrations, auxin can also inhibit shoot growth
  • Auxin released from the shoot apical meristem inhibits the growth of axillary buds
    • This is known as apical dominance
    • The strength of the inhibitory effect on axillary buds depends on the concentration of auxin; concentrations are lower further away from the shoot apex so axillary buds are more likely to grow lower down the plant
    • Note that cytokinins, another plant hormone, promotes the growth of axillary buds, so the relative concentrations of auxin and cytokinins is also important
    • Cutting off the shoot apex causes a decrease in auxin concentration and therefore promotes shoot growth; this is why pruning (cutting back plant stems) can often help to grow bushier plants!
• Gibberellins are another group of plant hormones involved in regulating stem growth

• It is thought that auxin brings about plant responses such as phototropism by altering the expression of genes inside plant cells
  • A gene is expressed if it is successfully transcribed and translated into a polypeptide
• When light falls on a plant, light energy is absorbed by photoreceptor proteins known as phototropins, causing a change in their 3D shape
• The altered shape of the phototropins allows them to bind to receptors inside the cell, affecting the expression of certain genes
• It is thought that the affected genes could code for glycoproteins called PIN3 proteins
• PIN3 proteins are thought to be involved with the lateral transport of auxin across stems exposed to sunlight, or across stems and roots affected by gravity

• The PIN3 proteins that are thought to be involved with the lateral transport of auxin in stems and roots are also known as efflux pumps
  • The term 'efflux' refers to an outward flow of a substance; in this case auxin is moved out of one cell and into another
• These efflux pumps are important in establishing an auxin gradient across a stem or root in response to a stimulus such as light or gravity
  • E.g. Light is thought to affect the expression of genes that code for the PIN3 protein efflux pumps; light shining on one side of a stem more than the other can therefore lead to an uneven distribution of efflux pumps, creating an auxin gradient
• The cells in a stem or root with a lateral auxin gradient will grow differently depending on the concentration of auxin to which they are exposed

Responding to light

• Light affects the growth of plant shoots in a response known as phototropism
• The concentration of auxin determines the rate of cell elongation within the stem
  • A higher concentration of auxin causes an increase in the rate of cell elongation 
  • If the concentration of auxin is not uniform across the stem then uneven cell growth can occur
• When light shines on a stem from one side, auxin is transported, by PIN3 proteins, from the illuminated side of a shoot to the shaded side
• An auxin gradient is established, with more auxin on the shaded side and less on the illuminated side
• The higher concentration of auxin on the shaded side of the shoot causes a faster rate of cell elongation, and the shoot bends towards the source of light

Higher concentrations of auxin on the shaded side of a stem increases the rate of cell elongation so that the shaded side grows faster than the illuminated side

Responding to gravity

• Roots respond to gravity in a response known as gravitropism
  • Gravitropism is sometimes known as geotropism
• In roots, auxin concentration also affects cell elongation, but higher concentrations of auxin result in a lower rate of cell elongation
  • Note that this is the opposite effect to that of auxin on shoot cells
• Plant cells use organelles called statoliths to detect the direction of gravity
  • Statoliths are starch-filled grains
  • They accumulate on the lower side of plant cells due to gravity
• PIN3 proteins transport auxin towards the lower side of the root cells
• The resulting high concentration of auxin at the lower side of the root inhibits cell elongation
• As a result, the lower side grows at a slower rate than the upper side of the root, causing the root to bend downwards

Auxin inhibits cell elongation in the lower side of roots, causing the cells in the lower side of the root to elongate at a slower rate than the upper side

NOS: Developments in scientific research follow improvements in analysis and deduction; improvements in analytical techniques allowing the detection of trace amounts of substances have led to advances in the understanding of plant hormones and their effect on gene expression

• Early research on plant hormones was carried out by scientists such as Charles Darwin (yes, the Charles Darwin) in the 1800s, and Nikolai Cholodny and Frits Went in the early 1900s
  • Darwin was able to demonstrate the importance of an unknown 'influence' in the bending of coleoptiles (young plant seedlings) towards light
  • Went was able to isolate Darwin's 'influence' and identify it as auxin
  • Cholodny and Went independently proposed that an auxin gradient was responsible for plant tropisms

Studies on plant tropisms carried out by Charles Darwin and Frits Went.  Darwin demonstrated that an ‘influence’ was produced in the tips of plant shoots, while Went showed that an auxin gradient was enough to cause shoot bending

• This early research on tropisms, while essential in progressing scientific understanding at the time, was unable to provide detail on the mechanisms underlying these plant responses beyond the suggestion that an auxin gradient was involved
• Modern analytical techniques mean that scientists today can find out about processes taking place at a cellular level in ways that earlier researchers could not
• An example of this is the science of genomics, which has enabled scientists to make discoveries about the influence of gene expression on plant responses
  • Genomics is the study of genes, including their structure and function
  • One method used in genomics is an analytical technique known as a microarray
    • A series of DNA probes are attached to a surface
    • Tissue samples are added to the microarray, and if a gene is being expressed in the cells, the probes emit fluorescence
    • This shows which genes are being expressed in particular tissue types at particular times
• Microarray research has shown that certain genes are expressed more often in cells on the underside of a plant root and on the shaded side of a plant stem
• Auxin has been shown to affect the expression of certain genes
• Research of this nature often involves organisms that are already well studied; scientists can make use of existing knowledge of their genetics and cell biology
  • A famous example of such a species in plants is Arabidopsis thaliana (thale cress)
  • A. thaliana is known as a model organism due to its frequent use in scientific research