Role of the cell wall in cell shape acquisition

The growth and development of an organism depend on the coordinated expansion and shape acquisition of individual cells. The epidermis, primarily controls morphogenesis as well as acts as an essential component at the interface with the environment. In plants, the cell wall, a polysaccharide network located outside the plasma membrane, ensures tight junctions between cells and determines the expansion rate and direction of each neighbouring cell, thereby determining cell shape and tissue morphology. Interestingly, plant cells are characterized by a great diversity of shapes, which vary from simple isodiametric forms to more complex structures such as in the puzzle-shaped pavement cells (PCs), displaying alternating lobes and necks, which are observed in the leaf epidermis. In our studies, we investigated the role of wall composition and mechanical properties in cell shape acquisition. We found that in Arabidopsis thaliana, cell wall integrity is essential for proper PC shape formation and that the mechanical properties of the cell wall between two mature PCs are heterogeneous. Further detailed examinations revealed the existence of a stiffness gradient across the curved cell wall at the lobes. We then showed that locally softer regions display an increased accumulation of specific pectic components such as galactans and arabinans, demonstrating their role in the regulation of wall mechanical properties. Furthermore, the appearance of these local heterogeneities precedes the cell morphological changes, indicating that the wall modifications are needed to initiate the lobing process. The cell wall composition was also studied in another species, Cinnamomum camphora (camphor tree), revealing a polarization of some cell wall components in PCs, and, uniquely, the presence of wall lignification in both epidermal and mesophyll cells. We also demonstrated that PC division pattern and development are correlated with an auxin gradient generated by directional transport, making a direct link with what is known on auxin stimulated acid growth and transcriptional response of genes controlling cell wall biosynthesis and remodelling. Altogether, our results support a major role for plant cell walls in cell shape acquisition. Our data reveal a striking dynamicity of PC cell walls, displaying the polarly distributed mechano-chemical properties required for lobing, which change according to the cell developmental stage. Furthermore, our work tightly links the master growth regulator auxin to the regulation of cell shape via a complex and dynamic control of cell wall remodelling

Fluctuating auxin response gradients determine pavement cell-shape acquisition

Puzzle-shaped pavement cells provide a powerful model system to investigate the cellular and subcellular processes underlying complex cell-shape determination in plants. To better understand pavement cell-shape acquisition and the role of auxin in this process, we focused on the spirals of young stomatal lineage ground cells of Arabidopsis leaf epidermis. The predictability of lobe formation in these cells allowed us to demonstrate that the auxin response gradient forms within the cells of the spiral and fluctuates based on the particular stage of lobe development. We revealed that specific localization of auxin transporters at the different membranes of these young cells changes during the course of lobe formation, suggesting that these fluctuating auxin response gradients are orchestrated via auxin transport to control lobe formation and determine pavement cell shape

MHC Architecture in Amphibians—Ancestral Reconstruction, Gene Rearrangements, and Duplication Patterns

The hypervariable major histocompatibility complex (MHC) is a crucial component of vertebrate adaptive immunity, but largescale studies on MHC macroevolution in nonmodel vertebrates have long been constrained by methodological limitations. Here, we used rapidly accumulating genomic data to reconstruct macroevolution of the MHC region in amphibians. We retrieved contigs containing the MHC region from genome assemblies of 32 amphibian species and examined major structural rearrangements, duplication patterns, and gene structure across the amphibian phylogeny. Based on the few available caecilian and urodele genomes, we showed that the structure of ancestral MHC region in amphibians was probably relatively simple and compact, with a close physical linkage between MHC-I and MHC-II regions. This ancestral MHC architecture was generally conserved in anurans, although the evolution of class I subregion proceeded toward more extensive duplication and rapid expansion of gene copy number, providing evidence for dynamic evolutionary trajectories. Although, in anurans, we recorded tandems of duplicated MHC-I genes outside the core subregion, our phylogenetic analyses of MHC-I sequences provided little support for an expansion of nonclassical MHC-Ib genes across amphibian families. Finally, we found that intronic regions of amphibian classical MHC genes were much longer when compared with other tetrapod lineages (birds and mammals), which could partly be driven by the expansion of genome size. Our study reveals novel evolutionary patterns of the MHC region in amphibians and provides a comprehensive framework for further studies on the MHC macroevolution across vertebrates

Elongation of wood fibers combines features of diffuse and tip growth

Xylem fibers are highly elongated cells that are key constituents of wood, play major physiological roles in plants, comprise an important terrestrial carbon reservoir, and thus have enormous ecological and economic importance. As they develop, from fusiform initials, their bodies remain the same length while their tips elongate and intrude into intercellular spaces.To elucidate mechanisms of tip elongation, we studied the cell wall along the length of isolated, elongating aspen xylem fibers and used computer simulations to predict the forces driving the intercellular space formation required for their growth.We found pectin matrix epitopes (JIM5, LM7) concentrated at the tips where cellulose microfibrils have transverse orientation, and xyloglucan epitopes (CCRC-M89, CCRC-M58) in fiber bodies where microfibrils are disordered. These features are accompanied by changes in cell wall thickness, indicating that while the cell wall elongates strictly at the tips, it is deposited all over fibers. Computer modeling revealed that the intercellular space formation needed for intrusive growth may only require targeted release of cell adhesion, which allows turgor pressure in neighboring fiber cells to 'round' the cells creating spaces.These characteristics show that xylem fibers' elongation involves a distinct mechanism that combines features of both diffuse and tip growth

Mechanochemical Polarization of Contiguous Cell Walls Shapes Plant Pavement Cells.

The epidermis of aerial plant organs is thought to be limiting for growth, because it acts as a continuous load-bearing layer, resisting tension. Leaf epidermis contains jigsaw puzzle piece-shaped pavement cells whose shape has been proposed to be a result of subcellular variations in expansion rate that induce local buckling events. Paradoxically, such local compressive buckling should not occur given the tensile stresses across the epidermis. Using computational modeling, we show that the simplest scenario to explain pavement cell shapes within an epidermis under tension must involve mechanical wall heterogeneities across and along the anticlinal pavement cell walls between adjacent cells. Combining genetics, atomic force microscopy, and immunolabeling, we demonstrate that contiguous cell walls indeed exhibit hybrid mechanochemical properties. Such biochemical wall heterogeneities precede wall bending. Altogether, this provides a possible mechanism for the generation of complex plant cell shapes

Using positional information to provide context for biological image analysis with MorphoGraphX 2.0

Positional information is a central concept in developmental biology. In developing organs, positional information can be idealized as a local coordinate system that arises from morphogen gradients controlled by organizers at key locations. This offers a plausible mechanism for the integration of the molecular networks operating in individual cells into the spatially-coordinated multicellular responses necessary for the organization of emergent forms. Understanding how positional cues guide morphogenesis requires the quantification of gene expression and growth dynamics in the context of their underlying coordinate systems. Here we present recent advances in the MorphoGraphX software (Barbier de Reuille et al., 2015)⁠ that implement a generalized framework to annotate developing organs with local coordinate systems. These coordinate systems introduce an organ-centric spatial context to microscopy data, allowing gene expression and growth to be quantified and compared in the context of the positional information thought to control them

How Cell Geometry and Cellular Patterning Influence Tissue Stiffness

Cell growth in plants occurs due to relaxation of the cell wall in response to mechanical forces generated by turgor pressure. Growth can be anisotropic, with the principal direction of growth often correlating with the direction of lower stiffness of the cell wall. However, extensometer experiments on onion epidermal peels have shown that the tissue is stiffer in the principal direction of growth. Here, we used a combination of microextensometer experiments on epidermal onion peels and finite element method (FEM) modeling to investigate how cell geometry and cellular patterning affects mechanical measurements made at the tissue level. Simulations with isotropic cell-wall material parameters showed that the orientation of elongated cells influences tissue apparent stiffness, with the tissue appearing much softer in the transverse versus the longitudinal directions. Our simulations suggest that although extensometer experiments show that the onion tissue is stiffer when stretched in the longitudinal direction, the effect of cellular geometry means that the wall is in fact softer in this direction, matching the primary growth direction of the cells

Identification and characterization of non-classical Major Histocompatibility Complex (MHC) class I genes in genomes of frogs

Geny głównego kompleksu zgodności tkankowej (MHC) kodują białka pełniące wiodącą rolę w rozpoznawaniu patogenów atakujących organizm. Badania przeprowadzone na rodzaju Xenopus wskazują, że kluczową rolę w odporności przeciwko ranawirusą i bakteriom z rodziny Mycobacteriaceae, które są jednymi z głównych patogenów atakujących ten rodzaj płazów bezogonowych, mogą pełnić nieklasyczne białka MHC klasy I (MHC-Ib) i ściśle z nimi powiązane niekonwencjonalne limfoctyty T. Do tej pory region MHC został najlepiej poznany u rodzaju Xenopus. Wiadomo, że posiadają jedną kopię genu klasycznego MHC klasy I (MHC-Ia) i wiele kopii genu MHC-Ib. Głównym celem pracy było sprawdzenie czy płazy bezogonowe inne niż Xenopus posiadają wiele kopii genów MHC-Ib. Praca skupiła się również na analizie liczby genów MHC-Ia, jak i porównaniu architektury genomowej regionów MHC pomiędzy gatunkami. Analiza opierała się na przeszukaniu genomowych anotacji białek pochodzących z wysokiej jakości genomów płazów bezogonowych pod kątem białek MHC I na podstawie sekwencji aminokwasowej genów MHC-Ia i MHC-Ib pochodzących od X. laevis oraz sekwencji białka HLA-A człowieka. Uzyskane sekwencje białek zostały użyte do konstrukcji drzewa filogenetycznego, którego założeniem jest prześledzenie ewolucji tych białek. Dodatkowo określono stopień zakonserwowania sekwencji białkowych, co ma na celu pomóc rozróżnić geny między MHC-Ia, a MHC-Ib. Było to możliwe dzięki badaniom, które określiły pozycje 9 aminokwasów silnie zakonserwowanych w toku ewolucji w białkach MHC I u innych organizmów. Co więcej, w celu lepszego poznania architektury genomowej regionu MHC, geny zostały zmapowane na chromosomy.Analiza ujawniła, iż wielość genów MHC-Ib jest charakterystyczna jedynie dla rodzaju Xenopus, jednocześnie wszystkie z badanych płazów bezogonowych posiadają zarówno geny MHC-Ib, jak i MHC-Ia. Możliwym jest, że u pozostałych gatunków działają inne mechanizmy broniące płazy bezogonowe przed groźnymi patogenami środowiskowymi.Major Histocompatibility Complex (MHC) genes encode the proteins that play a crucial role in pathogens recognition. Studies on the genus Xenopus indicate for adaptive immunity the key role might be played by the non-classic MHC class I genes and closely related T lymphocytes. Until now, the MHC region was investigated in Xenopus family. It is well known that it possess one classical MHC class I (MHC-Ia) locus and plenty of non- classical MHC class I (MHC-Ib) genes.The main aim of the study was to check whether the frogs other than Xenopus have multiple MHC-Ib genes. Paper focuses also on count of MHC-Ia genes and the differences in genomic architecture of MHC region among frogs. The analysis was based on screening the genomic annotation of proteins from high quality genomes of frogs for MHC I proteins based on the amino acid sequence of the MHC-Ia and MHC-Ib genes derived from X. laevis and the sequence of the human HLA-A protein. In order that get familiar with the evolution of those proteins, the obtained sequences were used to construct a phylogenetic tree. In addition, the degree of conservation of the protein sequences was determined, with the purpose to distinguish genes between MHC-Ia and MHC-Ib. This was possible thanks to studies that have identified the positions of 9 amino acids highly conserved during evolution in MHC I proteins in other organisms. Moreover, in order that better understand the genomic architecture of the MHC region, genes were mapped to chromosomes.The analysis revealed that the multiplicity of MHC-Ib genes is characteristic only for the genus Xenopus, However, all of the tested frogs have both the MHC-Ib and MHC-Ia genes. It is possible that other species have other mechanisms to protect frogs against dangerous environmental pathogens