Thread by @somssichm

#PlantScienceClassics #10: AuxREs, ARFs & Aux/IAAs. In 1997 the trifecta of Tim Ulmasov (TU), Gretchen Hagen (GH) & Tom Guilfoyle (TG) unleashed their @ScienceMagazine/@ThePlantCell double strike on the plant sciences, making auxin the hot topic of the field for the next decade.

Starting with Charles Darwin & son Francis in 1880, several scientists (J. Sachs, F. Went, N. Cholodny...) had speculated that there must be a mobile substance in plants, acting as messenger to direct growth in response to stimuli such as light or gravity (a ‘growth substance’).

In 1931 Fritz Kögl managed to isolate & describe this substance which he named 'auxin' (greek,‘to grow’). As it resembled animal hormones, he also proposed the name ‘phytohormones’ for this new group of plant substances. How this #Auxin promoted growth was not known yet, however.

In the 1950s Folke Skoog (you know him from his MurashigeSkoog-medium) showed that auxin-treatment increases RNA/DNA content in cells, though at the time this was regarded as nucleic acid metabolism, rather than gene expression, as the latter concept had not yet been established.

Only in the 1980s did GH/TG, Athanasios Theologis & others link the growth effects of auxin to gene regulation by demonstrating that specific mRNAs were upregulated following auxin-treatment. Some of the first auxin-induced genes hence carry the name GH1-3 (GRETCHEN HAGEN1-3).

In the following decade, GH/TG & others tried to identify the Auxin Responsive Elements (AuxREs) in the promoters, using sequencing & alignment of short DNA stretches (no genome data available!) followed by deletions of conserved regions to check for a loss of auxin-induction.

This resulted in the first strike of TU/GH/TG with a 1995 @ThePlantCell paper showing that AuxREs are composite modules with an auxin activated TGTCTC motif, that controls the activity of an overlapping, otherwise constitutively active transcriptional activator site CCTCGTG.

For their 1997 @ScienceMagazine paper TU/GH/TG then use an AuxRE palindrome DNA-stretch in a yeast 1-hybrid screen, pulling out ARF1 as DNA-binding protein. Using DNAse I footprinting & methylation interference, they show that ARF1 indeed binds to the TGTCTC motif (* in fig).

They also find a protein-interaction domain in ARF1, so for their 1997 @ThePlantCell paper they perform a yeast 2-hybrid screen, identifying Aux/IAA proteins as ARF-interactors, and demonstrate that these Aux/IAA proteins appear to be repressors of ARF1-mediated auxin-induction.

However, they also use the palindromic AuxREs they identified to build a synthetic auxin-reporter, the now legendary DR5::GUS line, to visualize auxin-concentrations in planta. Based on their results, TU/GH/TG conclude that binding of Aux/IAAs to ARFs suppresses AuxRE-activity.

In another follow-up in 1999 in @ThePlantJournal, the authors add that it is indeed ARF dimers that occupy the two AuxREs in a palindrome. However, the final piece to obtain a full model of how auxin activates gene expression was added by the @OttolineLeyser & Mark Estelle labs:

In 2001 @Nature William Gray & @StefanKepinski provide evidence that Auxin induces the interaction of the SCF/TIR1 complex with Aux/IAAs, leading to Aux/IAA degradation. This releases ARFs from Aux/IAA-binding/inhibition, enabling them to activate gene expression via AuxREs.

These breakthroughs, together with the concurrent advances in the understanding of polar auxin transport and its involvement in plant morphogenesis (2 key papers in figure), made auxin and phytohormones the hot topic of the plant science field around the turn of the century.

For recent updates following the work presented here, have a look at the nice overview posters from @Dev_journal on Auxin signaling: doi.org/10.1242/dev.13… (by Mark Estelle) and transport: doi.org/10.1242/dev.16… (by @KleineVehnLab).

Sadly, Tom Guilfoyle passed away in 2017. Here’s a nice obituary worth reading by Bruce McClure, published by @Mizzou ipg.missouri.edu/feature-storie…