A SOSEKI-based coordinate system interprets global polarity cues in Arabidopsis

Polar expedition: mechanisms for protein polar localization

Most cells show asymmetry in their shape or in the organization of their components that results in poles with different properties. This is a fundamental feature that participates in modulating the development of an organism and its responses to external stimuli. In plants, a number of proteins that are important for developmental and physiological processes have been shown to display polar localization. However, how these polarities are established, maintained, or dynamically modulated is still largely unclear for most of these proteins. In this review we report recent updates on the mechanisms of polar protein localization, focusing on a subset of these proteins that are the focus of current research efforts.

Square one: zygote polarity and early embryogenesis in flowering plants

In the last two decades, work on auxin signaling has helped to understand many aspects of the fundamental process underlying the specification of tissue types in the plant embryo. However, the immediate steps after fertilization including the polarization of the zygote and the initial body axis formation remained poorly understood. Valuable insight into these enigmatic processes has been gained by studying fertilization in grasses. Recent technical advances in transcriptomics of developing embryos with high spatial and temporal resolution give an emerging picture of the rapid changes of the zygotic developmental program. Together with the use of live imaging of novel fluorescent marker lines, these data are now the basis of unraveling the very first steps of the embryonic patterning process.

DIX Domain Polymerization Drives Assembly of Plant Cell Polarity Complexes

Cell polarity is fundamental for tissue morphogenesis in multicellular organisms. Plants and animals evolved multicellularity independently, and it is unknown whether their polarity systems are derived from a single-celled ancestor. Planar polarity in animals is conferred by Wnt signaling, an ancient signaling pathway transduced by Dishevelled, which assembles signalosomes by dynamic head-to-tail DIX domain polymerization. In contrast, polarity-determining pathways in plants are elusive. We recently discovered Arabidopsis SOSEKI proteins, which exhibit polar localization throughout development. Here, we identify SOSEKI as ancient polar proteins across land plants. Concentration-dependent polymerization via a bona fide DIX domain allows these to recruit ANGUSTIFOLIA to polar sites, similar to the polymerization-dependent recruitment of signaling effectors by Dishevelled. Cross-kingdom domain swaps reveal functional equivalence of animal and plant DIX domains. We trace DIX domains to unicellular eukaryotes and thus show that DIX-dependent polymerization is an ancient mechanism conserved between kingdoms and central to polarity proteins.

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SOSEKI proteins are deeply conserved polar proteins in land plants

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A DIX domain mediates polymerization and polarization of SOSEKI proteins

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SOSEKI polymerization allows polar recruitment of an effector protein

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DIX-dependent polymerization is shared between animal and plant polarity proteins

Coordination of tissue cell polarity by auxin transport and signaling

Plants coordinate the polarity of hundreds of cells during vein formation, but how they do so is unclear. The prevailing hypothesis proposes that GNOM, a regulator of membrane trafficking, positions PIN-FORMED auxin transporters to the correct side of the plasma membrane; the resulting cell-to-cell, polar transport of auxin would coordinate tissue cell polarity and induce vein formation. Contrary to predictions of the hypothesis, we find that vein formation occurs in the absence of PIN-FORMED or any other intercellular auxin-transporter; that the residual auxin-transport-independent vein-patterning activity relies on auxin signaling; and that a GNOM-dependent signal acts upstream of both auxin transport and signaling to coordinate tissue cell polarity and induce vein formation. Our results reveal synergism between auxin transport and signaling, and their unsuspected control by GNOM in the coordination of tissue cell polarity during vein patterning, one of the most informative expressions of tissue cell polarization in plants.

Plants, animals and other living things grow and develop over their lifetimes: for example, oak trees come from acorns and chickens begin their lives as eggs. To achieve these transformations, the cells in those living things must grow, divide and change their shape and other features.

Plants and animals specify the directions in which their cells will grow and develop by gathering specific proteins to one side of the cells. This makes one side different from all the other sides, which the cells use as an internal compass that points in one direction. To align their internal compasses, animal cells touch one another and often move around inside the body. Plant cells, on the other hand, are surrounded by a wall that keeps them apart and prevents them from moving around. So how do plant cells align their internal compasses?

Scientists have long thought that a protein called GNOM aligns the internal compasses of plant cells. The hypothesis proposes that GNOM gathers another protein, called PIN1, to one side of a cell. PIN1 would then pump a plant hormone known as auxin out of this first cell and, in doing so, would also drain auxin away from the cell on the opposite side. In this second cell, GNOM would then gather PIN1 to the side facing the first cell, and this process would repeat until all the cells' compasses were aligned.

To test this hypothesis, Verna et al. combined microscopy with genetic approaches to study how cells' compasses are aligned in the leaves of a plant called Arabidopsis thaliana. The experiments revealed that auxin needs to move from cell-to-cell to align the cells’ compasses. However, contrary to the above hypothesis, this movement of auxin was not sufficient: the cells also needed to be able to detect and respond to the auxin that entered them. Along with controlling how auxin moved between the cells, GNOM also regulated how the cells responded to the auxin.

These findings reveal how plants specify which directions their cells grow and develop. In the future, this knowledge may eventually aid efforts to improve crop yields by controlling the growth and development of crop plants.

Formation of aperture sites on the pollen surface as a model for development of distinct cellular domains

Pollen grains are covered by the complex extracellular structure, called exine, which in most species is deposited on the pollen surface non-uniformly. Certain surface areas receive fewer exine deposits and develop into regions whose structure and morphology differ significantly from the rest of pollen wall. These regions are known as pollen apertures. Across species, pollen apertures can vary in their numbers, positions, and morphology, generating highly diverse patterns. The process of aperture formation involves establishment of cell polarity, formation of distinct plasma membrane domains, and deposition of extracellular materials at precise positions. Thus, pollen apertures present an excellent model for studying the development of cellular domains and formation of patterns at the single-cell level. Until very recently, the molecular mechanisms underlying the specification and formation of aperture sites were completely unknown. Here, we review recent advances in understanding of the molecular processes involved in pollen aperture formation, focusing on the molecular players identified through genetic approaches in the model plant Arabidopsis. We discuss a potential working model that describes the process of aperture formation, including specification of domains, creation of their defining features, and protection of these regions from exine deposition.

PIN-FORMED and PIN-LIKES auxin transport facilitators

The phytohormone auxin influences virtually all aspects of plant growth and development. Auxin transport across membranes is facilitated by, among other proteins, members of the PIN-FORMED (PIN) and the structurally similar PIN-LIKES (PILS) families, which together govern directional cell-to-cell transport and intracellular accumulation of auxin. Canonical PIN proteins, which exhibit a polar localization in the plasma membrane, determine many patterning and directional growth responses. Conversely, the less-studied non-canonical PINs and PILS proteins, which mostly localize to the endoplasmic reticulum, attenuate cellular auxin responses. Here, and in the accompanying poster, we provide a brief summary of current knowledge of the structure, evolution, function and regulation of these auxin transport facilitators.

Volumetric finite-element modelling of biological growth

Differential growth is the driver of tissue morphogenesis in plants, and also plays a fundamental role in animal development. Although the contributions of growth to shape change have been captured through modelling tissue sheets or isotropic volumes, a framework for modelling both isotropic and anisotropic volumetric growth in three dimensions over large changes in size and shape has been lacking. Here, we describe an approach based on finite-element modelling of continuous volumetric structures, and apply it to a range of forms and growth patterns, providing mathematical validation for examples that admit analytic solution. We show that a major difference between sheet and bulk tissues is that the growth of bulk tissue is more constrained, reducing the possibility of tissue conflict resolution through deformations such as buckling. Tissue sheets or cylinders may be generated from bulk shapes through anisotropic specified growth, oriented by a polarity field. A second polarity field, orthogonal to the first, allows sheets with varying lengths and widths to be generated, as illustrated by the wide range of leaf shapes observed in nature. The framework we describe thus provides a key tool for developing hypotheses for plant morphogenesis and is also applicable to other tissues that deform through differential growth or contraction.