SABRE populates ER domains essential for cell plate maturation and cell expansion influencing cell and tissue patterning

The SABRE protein, originally identified in plants, is found throughout eukaryotes. In plants, SABRE has been implicated in cell expansion, division plane orientation and planar polarity. However, how SABRE mediates these processes remains an open question. Here, we have taken advantage of the fact that the bryophytePhyscomitrium patenshas a single copy ofSABRE, is an excellent model for cell biology and is readily amenable to precise genetic alterations to investigate SABRE’s mechanism of action. We discovered that SABRE null mutants were stunted in both polarized growing and diffusely growing tissues, similar to reported phenotypes in seed plants. However, in polarized growing cells, we observed significant delays in cell plate formation and sometimes catastrophic failures in cell division. We generated a functional SABRE fluorescent fusion protein and determined that it forms dynamic puncta on regions of the endoplasmic reticulum (ER) both in the cytoplasm during interphase and at the new cell plate during division. In the absence ofSABRE, ER morphology was severely compromised with large aggregates accumulating in the cytoplasm and abnormal buckling along the developing cell plate late in cytokinesis. In fact, SABRE and the ER maximally accumulated on the developing plate specifically during cell plate maturation, coincident with the timing of the onset of failures in cell plate formation in cells lacking SABRE. Further we discovered that callose deposition is delayed inΔsabrecells, and in cells that failed to divide, abnormal callose accumulations formed at the cell plate. Our findings demonstrated that SABRE functions by influencing the ER and callose deposition, revealing a surprising and essential role for the ER in cell plate maturation. Given that SABRE is conserved, understanding how SABRE influences cell and tissue patterning has profound significance across eukaryotes.