Unequal signalling and biosynthesis in daughter cells

The stem cells in the root apical meristem divide to give rise to transit-amplifying cells, which eventually exit from the cell cycle for elongation and differentiation. Low levels of brassinosteroids are required for cell cycle progression in the meristem, whereas high levels promote cell cycle exit in the elongation zone. Brassinosteroid signalling in dividing cells is therefore probably dynamically regulated. OCTOPUS (OPS) and OCTOPUS-LIKE (OPL) proteins are brassinosteroid-responsive regulators of phloem development that are polarly localized to the apical plasma membrane, but whether they are involved in dynamic regulation of brassinosteroid signalling in dividing cells was unknown.

The researchers first obtained single-cell RNA-sequencing datasets to generate a cell cycle reference for root tissue. They found that brassinosteroid-responsive genes are enriched in a module that corresponds to the G1 phase. This was confirmed by live-cell imaging, which showed that nuclear accumulation of the brassinosteroid-responsive transcription factor BRASSINAZOLE RESISTANT 1 (BZR1) peaked in the G1 phase and declined during mitosis. The brassinosteroid biosynthesis gene DWARF 4 (DWF4) displays a similar cell cycle expression pattern to that of BZR1, and levels were at their minimum during mitosis. Interestingly, the post-mitotic recovery of BZR1 nuclear accumulation is more pronounced in the upper daughter cell, whereas the recovery of DWF4 initially occurs in the lower daughter cell. The unequal recovery of BZR1 in the daughter cells did not depend on gene expression or protein degradation, but was rather due to phosphorylation-dependent displacement from the nucleus mediated by Arabidopsis thaliana SHAGGY-related protein kinases (ATSKs). Interestingly, OPL2 interacts with ATSK32; overexpression of OPL2 reduces the nuclear localization and total level of ATSK32. As OPS and OPL proteins are passed on to the upper daughter cell owing to their polar localization in the mother cell, this could explain the preferential post-mitotic nuclear accumulation of BZR1 in the upper daughter cell. Conversely, OPS or OPL mediated nuclear BZR1 accumulation is delayed in the lower daughter cell, which allows it to escape the negative feedback loop and express brassinosteroid biosynthesis genes such as DWF4. This mechanism establishes a balance between brassinosteroid signalling and biosynthesis that optimizes root growth, as was also demonstrated by computational modelling.