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

Abstract

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 bryophyte Physcomitrium patens has a single copy of SABRE, 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 of SABRE, 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 in Δsabre cells, 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.

COPII Sec23 proteins form isoform-specific ER exit sites with differential effects on polarized growth

“…In fact, tip growth in Δsec23d resembles the recently identified P. patens Δsabre mutant plants (Cheng & Bezanilla, 2021). SABRE associates with regions of the ER and its absence has no effect on the cytoskeleton but rather impacts ER morphology by accumulating aggregates that label with an ER lumenal marker (Cheng & Bezanilla, 2021). Taken together these data suggest that abrogated ER function impacts polarized growth without altering cytoskeletal organization and dynamics.…”

“…While ∆sec23d protonemal cells grow significantly slower than wild type leading to smaller plants, we were unable to identify any defects in either the actin or microtubules cytoskeletons, suggesting that Sec23D contributes to polarized cell expansion independent of the cytoskeleton. In fact, tip growth in ∆sec23d resembles the recently identified P. patens ∆sabre mutant plants (Cheng & Bezanilla, 2021). SABRE associates with regions of the ER and its absence has no effect on the cytoskeleton but rather impacts ER morphology by accumulating aggregates that label with an ER lumenal marker (Cheng & Bezanilla, 2021).…”

The Hob proteins are novel and conserved lipid binding proteins at ER-PM contact sites

“…Mutation of A. thaliana KIP, a putative paralog of SABRE, causes defects in pollen tube growth (Procissi et al, 2003), as does mutation of Z. mays APT1 (Xu and Dooner, 2006). Finally, mutation of the SABRE ortholog in the moss P. patens causes stunted growth, defects in polarized growth, and failures in cell division (Cheng and Bezanilla, 2020). Although genetic screens in plants and insects have revealed phenotypic insights, the molecular function of hobbit and its orthologs has remained elusive.…”