Auxins are a class of plant hormones (or growth regulators) with certain morphogenic characteristics. Auxins play a central role in coordinating many growth and behavioral processes in the plant life cycle and are essential for plant morphological development. Auxins and their role were first described by Dutch biologist Frits Warmolt Went in the 1920s. Auxins especially stimulate cell elongation in stems and inhibition in roots. They also control abscission and the plant’s responses to light (see phototropism) and gravity (gravitropism).
Auxins accelerate plant growth by stimulating cell division and enlargement, and by interacting with other hormones. Actions include the elongation of cells (by increasing the elasticity of cell walls, allowing the cells to take up more water) in geotropism and phototropism, and fruit drop and leaf fall.
Natural auxins are derivatives of indole. Synthetic auxins are used for crop control, and in rooting powders and weedkillers.
Auxin receptors, signal transduction and physiological responses
Auxin needs specific cellular receptors to which it binds in order to induce a signal transduction that determines biochemical and genetic responses of the target cell. Using radioactively labeled auxin it was noted that most of the radioactivity of cells treated with auxin came from the endoplasmic reticulum, while only a small part came from the plasma membrane and the membrane of the vacuole. This led to the hypothesis that the receptor for this hormone was on the RE. However, by treating plant cells with auxin complexed with the protein albumin in such a way as to form molecules too large to cross the plasma membrane, it was noted that the cell still responded to the presence of auxin. Necessarily then there is a membrane receptor for such hormones. The receptor bound to indol-3-acetic acid was crystallized and named “ABP1” (Auxin Binding Protein 1). In fact, it is not known how the signal transduction of ABP1 proceeds. However, it is known that the binding between receptor and ligand results in the creation of a proton gradient outside the plant cell induced by membrane H+ATPase pumps.
H+ ions activate enzymes called extensins that break part of the hydrogen bonds that anchor hemicelluloses -sugar bridges between cellulose microfibrils- to the microfibrils themselves, resulting in a “loosening” of the cell wall. The H+ATPase pumps of the vacuolar membrane, on the other hand, pump H+ ions inside the vacuole, water follows by osmosis and an increase in the internal pressure of the cell is generated, leading to an increase in its size. This phenomenon is referred to as Acid Growth. It is not known whether IAA-ABP1 binding increases the efficacy of the pumps or induces transcription of the genes that code for them.
Experimental evidence shows that auxin is also able to bind to intracellular receptors and determine the degradation of proteins that function as transcription repressors of genes involved in the signal response. In fact, plants mutant for the TIR and AXR1 genes encoding for protein domains of the E3 and E1 macrostructures, (ubiquitin ligases that determine the degradation of target proteins via the ubiquitin-proteasome pathway) present dwarf phenotype. From the above studies, it was found that the TIR domain of the E3 enzyme, which binds the target protein to be ubiquitinated and sent to the proteasome, is also an auxin receptor. TIR possesses an active site whose apical part binds the target protein, while the deeper part fails to make contact with it. However, the deep cavity of the active site binds auxin. In the absence of auxin the target establishes few interactions with the upper portion of the active site of TIR, whereas with auxin the target establishes interactions with both the upper portion and the deeper bound hormone itself. This results in stable binding.
The target proteins degraded by auxin signaling are those belonging to the AUX/IAA family. These proteins inactivate ARF transcription factors. Once degraded, ARF transcription factors can dimerize and transcribe hormone response genes. Many genes are transcribed such as SAUR, GH, ethylene synthesis genes, etc. The functionality of most of these genes is unknown.
Since the transcriptional profile of auxin signaling is very broad, plant physiological responses are varied:
- Acid growth by cell wall relaxation
- Lateral root growth
- Cell division of the cells of the root apical meristem
- Vessel branching
- Induction of ethylene synthesis