R2D2: Auxin marker from Star Wars

Recently "Star Wars: The Last Jedi" is released. Definitely, a great news for Star Wars' fan. I'm going to exploit this opportunity from a plant biologist point and can't resist that. Although I'm a huge fan of BB-8, who can move in any direction with enormous flexibility, this post is about R2D2. From Star Wars series, R2D2 is the robot character and appeared in most of the episodes so far. It appears for a shorter time in The Last Jedi also. It's a very good friend of Luke Skywalker. 



R2D2 from plant biology perspective in an auxin-responsive marker. It stands for the Ratiometric version of 2 D2’s. Th simplest way to explain auxin signaling pathway is that ARF (Auxin Responsive Factor) is usually bound to repressor AUX/IAA protein in absence of auxin. Upon the arrival and binding of auxin, ARF becomes free to execute the down-stream event and AUX/IAA is subjected to degradation via ubiquitination. AUX/IAA has 5 domains and here D2 indicates the domain II of AUX/IAA. So, the take-home message is we can simply observe DII breakdown to get an understanding of auxin signaling or response. 

This is the last and most sensitive auxin marker reported and gained popularity for semi-quantitative and rapid response characteristics. This reporter has combined degradable DII and mutated undegradable DII domain. Degradable one is tagged with Venus and mutated undegradable one fluoresces red. As a result, in an usual case, there will be both green and red fluorescence which shows yellow color. After the application of exogenous auxin, degradable DII will degrade and green color will go down. So, there will be a shift from yellow to red. From that colorimetric ratio (yellow/red), we can get the idea about auxin response in quantitative and spontaneous fashion. In addition, they use RPS5A promoter rather than constitutive CaMV 35S, for driving both DII domains, which is active in dividing cells. 

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