PREreview of Cell wall damage impairs root hair cell patterning and tissue morphogenesis mediated by the Arabidopsis receptor kinase STRUBBELIG

Cell wall damage impairs root hair cell patterning and tissue morphogenesis mediated by the Arabidopsis receptor kinase STRUBBELIG

Rajneesh Singhal and Deepak D. Bhandari

Department of Plant Biology, Michigan State University, East Lansing, USA.

This study by Chaudhary and colleagues show that the reduction of cellulose in the cell wall is accompanied by the downregulation of the atypical receptor kinase STRUBBELIG (SUB). Using a combination of genetic mutants, chemical treatments, and microscopy the authors attempt to connect SUB regulation with root hair patterning and cell shape. The authors show that treatment with the cellulose biosynthesis inhibitor, isoxaben (isx), leads to a reduction in SUB transcript levels and SUB-complex at the plasma membrane. The authors suggest that this is indicative of differing assembly of SUB-complexes depending on cell wall damage signals, no direct experiments addressing this have been performed. Though cell wall associated damage signals have been well-studied, a unifying model is unlikely given the diverse elicitors, hence it is plausible that SUB complexes differ upon cell wall damage status, and further experiments addressing this would be interesting. Further, the authors show that isoxaben treatment leads to alteration in the root hair patterning and floral development in WT seedlings, similar alteration happens in sub-9 mutant allele without any isoxaben treatment implying that the alterations are a result of attenuation in SUB signaling. The presented results largely support the hypothesis of SUB being involved in cell wall morphogenesis and paves way for further research aimed at integrating SUB function/s in cell wall development and damage-associated signaling.

The study is well-planned and executed, however, few questions remain

1. The authors mention that the reduction in cellulose levels in response to isx treatment leads to a “compensatory” cell wall damage response. It would be useful if they could elaborate on what these responses are? And whether or not SUB is connected to them.
2. The authors have earlier shown that the treatment with isx (which results in reduced SUB levels) leads to the production of ROS in WT seedlings, however, sub null alleles also have reduced ROS levels. The reason behind this reduction is not explained, could it be possible that ROS induction and SUB downregulation are not connected?
3. The two mutants used in the study in Fig 1 and Fig 1 (s), ixr2-1 and prc1-1 both have mutations in CESA 6 gene. However, in ixr2-1 the SUB is expressed at normal levels with or without isx treatment (Fig S1A) but prc1 has a reduced level of SUB (Fig 1H). It would be useful to know why is this observed?
4. In figure 1, the measurement of SUB transcripts seems to be arbitrary and not comparable to the images shown in Fig. 1A. It would be useful if the authors could elaborate on the rationale behind the time points tested.
5. SUB forms a complex with a transmembrane protein QUIRKY (QKY). qky mutants have reduced SUB levels and the treatment of qky mutants with isoxaben further reduces the levels of SUB. Does isoxaben also influence the levels of QKY in WT seedlings? And do sub null alleles also influence QKY?
6. Could the authors clarify what is the difference between sub-1 and sub-9, are their growth phenotypes comparable, or are they in different accessions?
7. Fig (3A-G). How were hair and non-hair cells determined, if the authors could elaborate on this (beyond GL2) and possibly mark it on the figure it would be easy for the readers. Also, we would recommend that this figure be re-arranged and the figure elaborated to ensure that the figure is easy to interpret for readers who are not intimately associated with the field. 
8. Why do roots treated with 150mM NaCl (Fig 3R) show altered GL2 expression? This is not clear from the text.
9. The authors have treated the seedlings with sub-lethal concentrations of isx at 1, 2, and 3 nM. What is the effect of these concentrations on the cellulose levels in the seedlings?
10. It is still not clear what is the link between the SUB levels and the Cell wall mediated response? In other words, is it possible that the structural damage to the cell wall due to reduced cellulose might be leading to the alteration in the root hair patterning and other defects and the response due to the SUB signaling might be a secondary response? So, how could the authors differentiate between the primary vs the secondary role played by the different receptor kinase/s present at the plasma membrane?
11. This perhaps is not a direct inquiry into the work presented here, but we believe readers would be very curious to know - How sub-1 or sub-9 roots behave under isx treatments and if this is dose-dependent?