Thread by @MichaelRaissig

Over the moon to share @danzhanghappy et al’s #preprint “Opposite polarity programs regulate asymmetric subsidiary cell divisions in grasses” - we identified a novel, distal polarity domain formed by the POLAR homolog in #Brachypodium @biorxiv_plants biorxiv.org/content/10.110…🧵/1

We transcriptionally profiled the unique bdmute phenotype–viable but no stomatal subsidiary cells (SCs)–to identify genes associated with SC formation -> 4th most downregulated gene = BdPOLAR!! /2

CRISPRed bdpolar mutants showed defective SC divisions, much like classical polarity mutants that affect subsidiary mother cell (SMC) polarity factors like PANGLOSS1 homologs. Indeed, also bdpan1 mutants show this defect and double (polar;pan1) has 80% (!) defective SCs /3

The HUGE surprise was that BdPOLAR forms a novel distal polarity domain that is reciprocal to the classical, proximal BdPAN1 domain in SMCs 🤯 - all so far identified polarity players with a role in SC division go to the proximal, PAN1-dominated domain /4

Also very cool is that BdPOLAR expression and polarization but not BdPAN1 expression and polarization depends on BdMUTE establishing SC identity. In bdmute mutants BdPOLAR is simply not expressed in SMCs! /5

Next, we found a one-way inhibitory relationship between the two polarity domains, where BdPAN1 seems to actively exclude BdPOLAR from its domain. In bdpan1 mutant SMCs, BdPOLAR-mVenus was able to invade the BdPAN1 domain! Yet, BdPAN1 is unaffected in bdpolar /6

The proximal BdPAN1 domain seems–like in maize–to be responsible for “attracting” the nucleus before division. Interestingly, nuclear migration defects in bdpan1 (~40% defective SCs) are the same as in bdpolar;bdpan1 doubles (~80% defective SCs) -> distinct role for BdPOLAR? /7

Indeed, the cortical division site marker BdTANGLED1 was much more often mislocalized in bdpolar than bdpan1 and sometimes even formed three (!) sites - we thus propose that BdPOLAR affects cell division orientation rather than nuclear migration during SC formation /8

Finally, we confirm that defective SCs affect stomatal opening and closing speed and gas exchange capacity in #Brachypodium - interestingly, however, to a lesser degree than in bdmute that lacks SC identity-> residual SC identity in improperly divided “SCs” in bdpolar;bdpan1? /9

Apparently, the extreme asymmetric and curved SC division requires not just one but TWO polarity domains - a primary, cell fate independent, proximal domain (PAN1) and a cell fate dependent, distal domain (POLAR) that are linked (1-way inhibition) yet functionally distinct /10

This story is a wonderful collaboration between @stanfordstomata’s lab (Emily Abrash, @plantaximena) and @HeikeLindner & my lab (@danzhanghappy @TiagoDanielGN @Ins29432437 Barbara Jesenofsky) - funded by @HHMINEWS & @dfg_public /11

Huge shout-out to 4 amazing #WomenInSTEM - @danzhanghappy for doing most of the work, Emily Abrash for setting up #Brachypodium as a stomatal model and her work on PAN1, @stanfordstomata for being an amazing mentor and, of course, @HeikeLindner for being always on my side /end