Auxin

Most commonly found as… … Indole-3-acetic acid or IAA. What is it? IAA is a weak acid similar to tryptophan. Its existence was first suggested by the Darwin brothers as a compound responsible for 'the power of movement in plants', but it was not named and chemically defined until much later. What does it do? Everything! What, everything? Yes, auxin apparently regulates almost everything in plants including rates of cell expansion, rates of cell division and establishment and maintenance of pattern during growth and development. How does it manage that? This is still a mystery, but it appears that the hub of auxin signalling is targeted protein degradation. Components of the ubiquitinylation system, including a SCF-type ubiquitin–protein ligase, SCF TIR1 , and the ubiquitin-related protein, RUB1, are essential for normal auxin signalling. Targets for SCF TIR1 include a large family of transcriptional regulators called the Aux/IAA family. Aux/IAAs are characterised by four highly conserved domains, one of which, domain II, acts as a destabilisation signal and is both necessary and sufficient for the interaction of Aux/IAAs with SCF TIR1. Auxin promotes this interaction and reduces the stability of the Aux/IAAs below an already low level. What do Aux/IAAs do? The Aux/IAAs appear to regulate transcription through their interaction with Auxin Response Factors (ARFs). Aux/IAAs and ARFs can form homo and heterodimers through domains III and IV, which are found in both protein families. ARFs bind to the auxin response elements of auxin-regulated genes and act most efficiently as dimers. Aux/IAAs appear to compete for dimerisation and hence block ARF function. Auxin destabilises Aux/IAAs and hence activates ARFs. Aux/IAA genes are themselves regulated by this system, forming a negative feedback loop. The complexities of auxin signalling may reflect the varied dynamics of induction, activation and degradation of these gene families. A major missing link in the model described above is a lack of knowledge about how auxin regulates Aux/IAA stability. By analogy with similar systems in other species it is likely that auxin results in a modification of the Aux/IAAs, for example phosphorylation, allowing recognition of the Aux/IAA by SCF TIR1. Is there a receptor? I bet you think that's an innocent little question. Well you are sadly mistaken. There is a well-characterised protein that binds auxin called the Auxin Binding Protein 1 or ABP1. This protein is essential for embryogenesis and manipulation of its levels in transgenic tobacco plants results …