We discuss manuscripts, preprints, and peer-reviewed papers in a journal club format each week. Discussion hosts can choose to post their notes here.
2024-10-23
Hermann .. Torii 2024, bioRxiv, “Chemical genetics reveals cross-activation of plant developmental signaling by the immune peptide-receptor pathway” (link)
Herrmann et al 2024 report a novel inhibitor of MAPK3/6 activation, a small molecule termed kC9. kC9 treatment leads to increased stomatal density when applied at micromolar concentrations. Genetic experiments show that kC9 acts downstream of ERECTA-inhibited stomatal development but upstream of MUTE and FAMA. Biochemical experiments showed the kC9 binds MAPK6 with micromolar affinity, and models of the interaction predict binding at the kinase’s ATP binding pocket.
The new kC9 reagent allowed dissection of flg22 vs EPF siganling. Flg22 normally does not suppress stomatal development, but interestingly either induction of EPF or exogenous flg22 treatment can counteract the effects of kC9-induced stomatal formation. This suggests that kC9 effects on MAPK activation can be bypassed by strong receptor signaling from either the ER or FLS2 pathway. A timecourse of kC9 and flg22 treatment post-germination identified a potential timepoint ~4 dpg at which the two signals are converging to determine cell fate, especially affecting the presence of enigmatic “small cells” which have stomatal lineage appearance but lack typical marker gene expression.
We discussed the “model” for specificity that they were trying to test. Although both stomatal development and flagellin recognition use similar MAPK pathways, they are thought to be distinct due to the MAPKKK having either stomatal or immune phenotypes. However, there is evidence for overlapping roles from MAPKKK double mutants (Wang 2022) and from reports of immune phenotypes for yda and erecta (Sopena-Torres 2018, Jorda 2016). Since the paper now finds a chemically-induced condition where flg22 could affect stomatal development, this gives further support for the shared pathway model.
Other points of discussion:
- We reviewed the ER signaling pathways compared to other LRR-family receptors and thought about potential upstream sources of specificity, such as TMM or SOBIR1 for immunity-related RLPs.
- We looked at the original yoda phenotype from Lukowitz 2004 Cell (Somerville lab) – originally identified in a screen for defective suspensor development in embryos!! Wow!
- We appreciated the chemical genetic screen to identify kC9 and testing of related compound kC9-3 to test specificity.
- We were not experts in stomatal mutant phenotypes so we hoped to see WT plants in panel 1F. We assumed kC9 would not lead to stomata on WT hypocotyls, in contrast to its strong elicitation on tmm plants. We also thought the panels in Fig 1D might be reversed top vs bottom.
- We found the biochemical data convincing that kC9 binds MAPK6, but wanted to see its affinity relative to ATP in competition experiments.
- We liked the 3x3 self-organized map analysis of RNAseq data – very intuitive clusters! (edited)