Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essential for plant body development. The Dutch biologist Frits Warmolt Went first described auxins and their role in plant growth in the 1920s.
Kenneth V. Thimann became the first to isolate one of these phytohormones and to determine its chemical structure as indole-3-acetic acid (IAA). Went and Thimann co-authored a book on plant hormones, Phytohormones, in 1937.
The auxin signal cascade: In the absence of auxin, Aux/IAA bind to and suppress the transcriptional activity of ARFs. When auxin is present it forms a 'molecular glue' between TIR1 and Aux/IAAs, which leads to the degradation of these repressors. ARFs are then free to bind to DNA and to cause changes in transcription.
Auxin diffuses along the shaded side of the plant, and causes cellulose in the cell wall to break, allowing turgor (water pressure) to expand the cell.
A healthy Arabidopsis thaliana plant (left) next to an auxin signal-transduction mutant with a repressed response to auxin.
Crown galls are caused by Agrobacterium tumefaciens bacteria; they produce and secrete auxin and cytokinin, which interfere with normal cell division and cause tumors.
Plant hormones are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis, the regulation of organ size, pathogen defense, stress tolerance and reproductive development. Unlike in animals each plant cell is capable of producing hormones. Went and Thimann coined the term "phytohormone" and used it in the title of their 1937 book.
Lack of the plant hormone auxin can cause abnormal growth (right)
Phyllody on a purple coneflower (Echinacea purpurea), a plant development abnormality where leaf-like structures replace flower organs. It can be caused by hormonal imbalance, among other reasons.
