I am a complete sell out. I have worked for a plant biotech company and getting funding from Gatsby (a charity with a goal, alone with others, is to get scientists interesting in plants). These people have been very nice by helping me pay rent, buy DVDs and science magazines as well as sending me to Mexico for a conference. Therefore, I thought I should give something back by educating my friends in a bit of plant biology. Don’t worry faint hearted, I will keep it nice and molecular as that is the way I like it.
After returning from my Mexico trip I realised a few things, English seems to be spoken by everyone, don’t eat food from Mexico City airport and auxin is a fascinating plant hormone and virtually everyone in the plant community agrees. Most talks there were on auxin biosynthesis, receptor function and structure, its control of shoot branching and transport around the plant. However, I do not recall all of it. Or hardy any of it as I was jet lagged with little prior knowledge of plant hormones (plus it has been over a year since the conference).
Auxin was the most interesting of all the plant signalling molecules. It has been studied for over 100 years and only today we are starting to understand how it controls plant development and cell biology. Auxin is a small molecule made from an amino acid and its most biologically active for is called IAA. Auxin controls cell elongation and division, and can promote these or stop them depending on the tissue type auxin enters. Auxin causes lateral root growth and patterns the vascular system of plants. Auxin starts shaping the plant in the embryo. What is interesting is how it regulates many of these things.
It controls gene expression by activating Auxin Response Factors (ARFs). This is done by destabilizing proteins called Aux/IAAs. Genes activated by the addition of auxin contain Auxin Response Elements (ARE) which an ARF binds to. When no auxin is present, an ARF is bound to an ARE but it is diamerized with an Aux/IAA, which represses transcription of this gene. When auxin enters the cell, it gets ride of this Aux/IAA and the ARF diamerizes with another ARF. This causes transcription of the auxin regulated gene. How does auxin cause breakdown of Aux/IAA? (I thought I would sound more intelligent if I asked a lot of questions) Auxin doesn’t have a nice receptor at the cell membrane that activates a second message or a phosphorylation pathway, no that would be to simple. In 2005 Ottoline Leyser and Stefan Kepinski (then post-doc, now lecturer at Leeds) published in Nature that TIR1 was an auxin receptor. tir1 mutants had been known to be deficient in auxin signalling for a long time but not thought to be a mutation in the receptor. TIR1 is an F-box protein, which are not famous for being receptors (until now). Normally they simply act as an E3 ubiqutin (Ub) ligase, meaning they take Ub from one protein and add it to another creating a poly-ubiquitin tag that sends a protein for degradation by the 26S proteasome. Ubiqutination and targeting to the proteasome is found animals and fungi and plays an important role in signalling, including regulation of the cell cycle! TIR1 adds Ub to the Aux/IAA when auxin enters the cell. It was assumed that auxin was perceived by some other protein in the cell and caused some modification of the Aux/IAA or TIR1 to cause this to happen, probably by phosphorylation. Now we know that auxin binds toTIR1 and acts as a molecular glue, bridging the gap between TIR1 and Aux/IAA so it can be broken down. I think this is a brilliant method. I am glad to see plants are being original and creating cool new signalling pathways instead of being just like boring old animals and fungi that love there MAPK so much they should just go along and marry it. Please see my simplified diagram of the auxin signalling pathway, it is not perfect, but who is......except maybe Colombo. But then how does auxin have very different effects on different cell types. This appears to be because different tissues expresses different ARFs and Aux/IAAs and these turn on different genes but little is known right now.
One final thing, is TIR1 the only receptor? A good question indeed. Like I said earlier, I like asking questions to make me sound smart. Other F-box proteins appear to do the same job. The same only story of redundancy. However, this style of signalling cannot explain all the effects auxin has on plants. Some auxin responses occur very quickly after auxin addition. These happen so quickly, it is unlikely proteolysis followed by transcription and translation of effectors can account for them. Things like guard cell outward K+ current up, increased cytoplasm calcium, cell wall acidification starts (to help increase cell wall expansion) and elongation growth. How can we explain auxin’s influence over these physiological changes? There is debate and investigation over what causes these things. Some are caused by changes in membrane potential and perhaps Auxin Binding Protein 1 (ABP1) has some control over some changes but I am not convinced and other features are unrelated to this. After auxin addition MAP kinase activity increase but this is unrelated to both TIR1 and ABP1. There are still a lot of unanswered questions.
Here is a short, concise review by Ottoline’s old post-doc (now at Leeds), which I found yesterday and does a far better job of introducing people to this that I have!
The anatomy of auxin perception
Another one talking a more detailed look at auxin receptors and the debate around them. I have not read it fully, sadly.
Receptors for auxin: will it all end in TIRs?
I have not referenced here for two reasons. First, I can’t be bothered and secondly, a lot of this is from conferences or meetings I have been to. I hope you enjoyed this little rant from the plant person.