Plant Hormone Mechanisms: Auxins, Amylases, Ethylene

Posted on Mar 12, 2025 in Biology

Rhizobium-Legume Symbiosis: Infection Process

  1. Union of *Rhizobium* to a radical emergent hair by chemotaxis.
  2. The hair grows curved, and bacteria grow inside.
  3. Degradation of the cell wall allows infection.
  4. The infection cord reaches the base of the hair.
  5. *Rhizobium* is freed in the apoplast and initiates the formation of a new cord.
  6. The infection branches and releases vesicles into the cytosol. Nodule differentiation occurs.

Auxin Action Mechanism: Cell Elongation and Growth

Auxins induce cell growth and cause cells to increase in size successively. This is known as the acid growth mechanism.

Indole-3-acetic acid (IAA), the main auxin, reaches the cell and binds to its receptor, initiating signal transduction via secondary messengers. This can act at two levels:

  1. A secondary messenger of IAA causes activation of the ATPase pump, increasing its activity and the concentration of hydrogen ions (H+).
  2. The secondary messenger of IAA acts directly at the nucleus, activating the expression of genes encoding H+-ATPases in the membrane.

Both mechanisms increase the number of H+. This acidifies the cell wall, activating hydrolytic enzymes that degrade components of the wall, reducing its tensile stiffness and allowing cell expansion.

Amylase Synthesis: Gibberellin-Powered Mechanism

When the hormone gibberellin (GA) binds to its receptor, it activates a G protein, acting at two levels:

  1. Via Ca2+-dependent pathway: Active secretion of vesicles containing GA occurs.
  2. Via a Ca2+-independent pathway: A signal cascade reaches the nucleus and interacts with a repressor protein that prevents the transcription of GA-MYB (an amylase synthesis promoter). The signal inhibits the repressor, inactivating it and allowing transcription.

The resulting mRNA leaves the nucleus to the cytosol, leading to the synthesis of the GA-MYB protein. This protein is secreted into vesicles from the endoplasmic reticulum and activated in the previous step. These vesicles migrate to the membrane and release amylases that degrade starch, mobilizing reserves and enabling seed germination.

Ethylene Biosynthesis

Ethylene biosynthesis occurs in three steps, with an optional fourth conjugation step:

  1. Transfer of the amino acid methionine to S-adenosylmethionine (SAM), catalyzed by SAM synthase with one ATP molecule. This is not specific to ethylene biosynthesis; SAM is a precursor in other metabolic pathways.
  2. Transfer from SAM to 1-aminocyclopropane-1-carboxylic acid (ACC), catalyzed by the key regulatory enzyme ACC synthase.
  3. Formation of ethylene from ACC, catalyzed by ACC oxidase.
  4. Complementary action: ACC can be irreversibly conjugated to form malonyl-ACC, considered a storage form of the ethylene precursor.

This entire process is called the methionine or Yang cycle, which allows for the regeneration of methionine. If methionine is used frequently, ethylene formation may cease due to methionine depletion.

Ethylene Action Mechanism

When there is no ethylene, the ETR1 receptor is inactive, and CTR1 (a molecule that acts as an inhibitor of ethylene action) is active. This means there is no response to ethylene. However, when ethylene binds to ETR1, it causes the inactivation of CTR1, resulting in a downstream response. The transmembrane protein EIN2 is activated, allowing the influx of ions, and transcription factors (EIN3) are activated, leading to the expression of ERF1. These act on genes that respond to ethylene, resulting in the ethylene response.