Cell polarity in plants: a PARspective on PINs

Plants have acquired the ability for organized multicellular development independent from animals. Because of this, they represent an independent example in nature for the development of coordinated, complex cell polarity from the simple polarity found in unicellular eukaryotes. Plants display a striking array of polarized cell types, with different axes of polarity being defined in one cell. The most investigated and best understood aspect of plant polarity is the apical-basal polarity of the PIN family of auxin efflux facilitators, which are of crucial importance for the organization of the entire plant body. Striking differences exist between the PAR-polarity modules known in animals and the ways PINs polarize plant cells. Nonetheless, a common regulatory logic probably applies to all polarizing eukaryotic cells, which includes self-reinforcing, positive feedback loops, intricate interactions between membrane-attached proteins, lipid signatures, and the targeting of transmembrane proteins to the correct domains of the plasma membrane.     

Cell polarity: fixing cell polarity with Pins

A protein complex is assembled in a step-wise manner at the apical pole of Drosophila neuroblasts. This complex organizes the apical-basal polarity of asymmetrically dividing neuroblasts, and may act via G-protein signalling.     

Cell polarity

Despite extensive genetic analysis of the dynamic multi-phase process that transforms a small population of lateral plate mesoderm into the mature limb skeleton, the mechanisms by which signaling pathways regulate cellular behaviors to generate morphogenetic forces are not known. Recently, a series of papers have offered the intriguing possibility that regulated cell polarity fine-tunes the morphogenetic process via orienting cell axes, division planes and cell movements. Wnt5a-mediated non-canonical signaling, which may include planar cell polarity, has emerged as a common thread in the otherwise distinct signaling networks that regulate morphogenesis in each phase of limb development. These findings position the limb as a key model to elucidate how global tissue patterning pathways direct local differences in cell behavior that, in turn, generate growth and form.     

Cell axiality and polarity in plants — adding pieces to the puzzle

Plant cell polarity is important for cellular function and multicellular development. Classical physiological and cell biological analyses identified cues that orient cell polarity and suggested molecules that translate a cue into intracellular asymmetry. A range of proteins that either mark or are involved in the establishment of a (polar) axis are now available, as are many relevant mutants. These tools are likely to facilitate a dissection of the molecular mechanisms behind cell and organ polarity in the near future.     

Axis formation and polarity in plants.

We have recently gained insight into a number of mechanisms governing the formation of the major axes that define the embryonic and adult plant body plan. Phenotypic analysis and molecular characterization of mutants with aberrant morphogenesis has led to a better understanding of key processes including the generation of the shape of the apical embryo, the establishment and maintenance of the radial pattern of the root, and the placement of lateral organ primordia around the shoot apical meristem.     

Cell polarity in development and cancer

The development of cancer is a multistep process in which the DNA of a single cell accumulates mutations in genes that control essential cellular processes. Loss of cell-cell adhesion and cell polarity is commonly observed in advanced tumours and correlates well with their invasion into adjacent tissues and the formation of metastases. Growing evidence indicates that loss of cell-cell adhesion and cell polarity may also be important in early stages of cancer. The strongest hints in this direction come from studies on tumour suppressor genes in the fruitfly Drosophila melanogaster, which have revealed their importance in the control of apical-basal cell polarity.     

Cell polarity: squaring the circle

Recent studies have found that Drosophila gene products required for zonula adherens formation in the ectoderm are also involved in the asymmetric cell division of the neuroblast. The results illustrate the reiterated use of groups of proteins to dictate cell polarity in epithelial and non-epithelial cells.     

Cell polarity: the PARty expands

Recent work has revealed an evolutionarily conserved trio of proteins that regulate cell polarity in epithelial cells, embryonic blastomeres and neural precursors. This common cell-polarity mechanism is used in cell-specific ways, as highlighted by the recent finding that at least two different types of asymmetric division are observed in Drosophila neural precursors.     

Cell polarity in morphogenesis and metastasis

Most human cancers arise either from epithelial cells or their progenitors. Epithelial cells possess a distinctive apical–basal polarity and loss of polarity is frequently assumed to be a common feature of cancer progression. In particular, cancer cell dissemination to ectopic sites, and metastatic growth at those sites, is often considered to require a mesenchymal transition in which the transformed epithelial cells lose their apical–basal polarity. However, many cancers retain epithelial characteristics, and until recently there has been little conclusive evidence for an involvement of the cell polarity machinery in tumour growth and metastasis. In this article, we discuss evidence that polarity proteins can be potent invasion suppressors but that loss of epithelial character is not essential either for tumour growth and invasion, or metastatic colonization.     

Cell polarity in plants: when two do the same, it is not the same...

In unicellular and multicellular organisms, cell polarity is essential for a wide range of biological processes. An important feature of cell polarity is the asymmetric distribution of proteins in or at the plasma membrane. In plants such polar localized proteins play various specific roles ranging from organizing cell morphogenesis, asymmetric cell division, pathogen defense, nutrient transport and establishment of hormone gradients for developmental patterning. Moreover, flexible respecification of cell polarities enables plants to adjust their physiology and development to environmental changes. Having evolved multicellularity independently and lacking major cell polarity mechanisms of animal cells, plants came up with alternative solutions to generate and respecify cell polarity as well as to regulate polar domains at the plasma membrane.     

Cell polarity in Arabidopsis trichomes

Arabidopsis leaf trichomes are unicellular hairs that display a highly characteristic cell form that has a fixed orientation with respect to the basal-distal leaf axis. The genetic, molecular and cell biological analysis of trichome morphogenesis reveal that various cellular processes need to be coordinated including regulation of the cell cycle, the cell size and the actin and tubulin cytoskeleton. Here we will focus on what is known about the establishment and maintenance of positional information during trichome formation.