by PhilipJ on 11 February 2009
Plants, like animals, have hormones that deliver chemical messages between distant cells. Charles Darwin and his son discovered this over a century ago—they noticed that if they shined a light on the tips of grass shoots, the stems bend to bring the entire shoot towards the light. Somehow, a message was being sent from the tip down to the stem. You might also have observed the action of hormonal signals in plants: when you prune a tree to make it more bushy, you are modifying the traffic of plant hormones. Both of these effects are caused by the phytohormone auxin.
Auxin is made by cells in the tips of plants, and then it is transported throughout the rest of the plant. It is essential for life of the plant, and the plant quickly dies if deprived of it. Auxin controls many functions, including proper patterning of leaves on stems and response to light and gravity. For instance, auxin controls the branching of plants. When you cut off the tip of a growing plant, you remove the major source of auxin, and the resulting lower levels of auxin cause the remaining parts to branch out instead of growing straight up.
The ultimate effect of auxin is to regulate a collection of genes involved in cell division, elongation and differentiation. However, auxin does not appear to act directly on repressors or activators of these genes—it uses a more circuitous mechanism. Instead, auxin binds to a class of ubiquitin ligases, such as the TIR1 ubiquitin ligase shown here from PDB entry 2p1p. These enzymes assist with the destruction of proteins by the ubiquitin/proteasome system. Auxin binds to the ligase and promotes the ubiquitination of a series of regulatory proteins, termed Aux/IAA proteins. When these are destroyed, they allow a series of auxin response factors (ARF) to interact directly with the genes.
For more on this molecule, read the rest from David Goodsell at the RCSB PDB.