Polar targeting of proteins - a green perspective.

Cell polarity - the asymmetric distribution of molecules and cell structures within the cell - is a feature that almost all cells possess. Even though the cytoskeleton and other intracellular organelles can have a direction and guide protein distribution, the plasma membrane is, in many cases, essential for the asymmetric localization of proteins because it helps to concentrate proteins and restrict their localization. Indeed, many proteins that exhibit asymmetric or polarized localization are either embedded in the PM or located close to it in the cellular cortex. Such proteins, which we refer to here as 'polar proteins', use various mechanisms of membrane targeting, including vesicle trafficking, direct phospholipid binding, or membrane anchoring mediated by post-translational modifications or binding to other proteins. These mechanisms are often shared with non-polar proteins, yet the unique combinations of several mechanisms or protein-specific factors assure the asymmetric distribution of polar proteins. Although there is a relatively detailed understanding of polar protein membrane targeting mechanisms in animal and yeast models, knowledge in plants is more fragmented and focused on a limited number of known polar proteins in different contexts. In this Review, we combine the current knowledge of membrane targeting mechanisms and factors for known plant transmembrane and cortical proteins and compare these with the mechanisms elucidated in non-plant systems. We classify the known factors as general or polarity specific, and we highlight areas where more knowledge is needed to construct an understanding of general polar targeting mechanisms in plants or to resolve controversies.

Pole position: How plant cells polarize along the axes.

Having a sense of direction is a fundamental cellular trait that can determine cell shape, division orientation, or function, and ultimately the formation of a functional, multicellular body. Cells acquire and integrate directional information by establishing discrete subcellular domains along an axis with distinct molecular profiles, a process known as cell polarization. Insight into the principles and mechanisms underlying cell polarity has been propelled by decades of extensive research mostly in yeast and animal models. Our understanding of cell polarity establishment in plants, which lack most of the regulatory molecules identified in other eukaryotes, is more limited, but significant progress has been made in recent years. In this review, we explore how plant cells coordinately establish stable polarity axes aligned with the organ axes, highlighting similarities in the molecular logic used to polarize both plant and animal cells. We propose a classification system for plant cell polarity events and nomenclature guidelines. Finally, we provide a deep phylogenetic analysis of polar proteins and discuss the evolution of polarity machineries in plants.

Mechanical conflict caused by a cell-wall-loosening enzyme activates de novo shoot regeneration.

Cellular heterogeneity is a hallmark of multicellular organisms. During shoot regeneration from undifferentiated callus, only a select few cells, called progenitors, develop into shoot. How these cells are selected and what governs their subsequent progression to a patterned organ system is unknown. Using Arabidopsis thaliana, we show that it is not just the abundance of stem cell regulators but rather the localization pattern of polarity proteins that predicts the progenitor's fate. A shoot-promoting factor, CUC2, activated the expression of the cell-wall-loosening enzyme, XTH9, solely in a shell of cells surrounding the progenitor, causing different mechanical stresses in these cells. This mechanical conflict then activates cell polarity in progenitors to promote meristem formation. Interestingly, genetic or physical perturbations to cells surrounding the progenitor impaired the progenitor and vice versa. These suggest a feedback loop between progenitors and their neighbors for shoot regeneration in the absence of tissue-patterning cues.

Auxin-dependent control of cytoskeleton and cell shape regulates division orientation in the Arabidopsis embryo.

Premitotic control of cell division orientation is critical for plant development, as cell walls prevent extensive cell remodeling or migration. While many divisions are proliferative and add cells to existing tissues, some divisions are formative and generate new tissue layers or growth axes. Such formative divisions are often asymmetric in nature, producing daughters with different fates. We have previously shown that, in the Arabidopsis thaliana embryo, developmental asymmetry is correlated with geometric asymmetry, creating daughter cells of unequal volume. Such divisions are generated by division planes that deviate from a default "minimal surface area" rule. Inhibition of auxin response leads to reversal to this default, yet the mechanisms underlying division plane choice in the embryo have been unclear. Here, we show that auxin-dependent division plane control involves alterations in cell geometry, but not in cell polarity axis or nuclear position. Through transcriptome profiling, we find that auxin regulates genes controlling cell wall and cytoskeleton properties. We confirm the involvement of microtubule (MT)-binding proteins in embryo division control. Organization of both MT and actin cytoskeleton depends on auxin response, and genetically controlled MT or actin depolymerization in embryos leads to disruption of asymmetric divisions, including reversion to the default. Our work shows how auxin-dependent control of MT and actin cytoskeleton properties interacts with cell geometry to generate asymmetric divisions during the earliest steps in plant development.

The C. elegans Crumbs family contains a CRB3 homolog and is not essential for viability.

Crumbs proteins are important regulators of epithelial polarity. In C. elegans, no essential role for the two described Crumbs homologs has been uncovered. Here, we identify and characterize an additional Crumbs family member in C. elegans, which we termed CRB-3 based on its similarity in size and sequence to mammalian CRB3. We visualized CRB-3 subcellular localization by expressing a translational GFP fusion. CRB-3::GFP was expressed in several polarized tissues in the embryo and larval stages, and showed apical localization in the intestine and pharynx. To identify the function of the Crumbs family in C. elegans development, we generated a triple Crumbs deletion mutant by sequentially removing the entire coding sequence for each crumbs homolog using a CRISPR/Cas9-based approach. Remarkably, animals lacking all three Crumbs homologs are viable and show normal epithelial polarity. Thus, the three C. elegans Crumbs family members do not appear to play an essential role in epithelial polarity establishment.