Physical forces regulate plant development and morphogenesis

"Shape of Cell"-An Auxin and Cell Wall Duet

Understanding the mechanisms underlying cell shape acquisition is of fundamental importance in plant science, as this process ultimately defines the structure and function of plant organs. Plants produce cells of diverse shapes and sizes, including pavement cells and stomata of leaves, elongated epidermal cells of the hypocotyl, and cells with outgrowths such as root hairs, and so forth. Plant cells experience mechanical forces of variable magnitude during their development and interaction with neighboring cells and the surrounding environment. From the time of cytokinesis, they are encaged in a complex cell wall matrix, which offers mechanical support and enables directional growth and a differential rate of expansion towards adjacent cells via its mechanochemical heterogeneity. The phytohormone auxin is well characterized for its role in cell expansion and cell elasticity. The interaction between dynamic auxin redistribution and the mechanical properties of the cell wall within tissues drives the development of specific cell shapes. Here, we focus on the regulatory feedback loop involving auxin activity, its influence on cell wall chemistry and mechanical properties, and the coordination of cell shape formation. Integrating insights from molecular and cell biology, biophysics, and computational modeling, we explore the mechanistic link between auxin signaling and cell wall dynamics in shaping plant cells.

Genome-Wide Identification and Expression Analysis of TONNEAU1 Recruited Motif (TRM) Gene Family in Tomato

The TONNEAU1 Recruited Motif (TRM) gene family is integral to the growth and development of various plants, playing a particularly critical role in regulating the shape of plant organs. While the functions of the TRM gene family in other plant species have been documented, knowledge regarding the members of the tomato (Solanum lycopersicum). SlTRM gene family remains limited, and their specific roles are not yet well understood. In this study, we identified and analyzed 28 members of the SlTRM gene family in tomato using bioinformatics approaches based on the latest whole genome data. Our analysis included the examination of protein structures, physicochemical properties, collinearity analysis, gene structures, conserved motifs, and promoter cis-acting elements of the SlTRM gene family members. The phylogenetic analysis indicated that both tomato and Arabidopsis thaliana are categorized into five distinct subfamilies. Furthermore, we conducted a three-dimensional structure prediction of 28 SlTRM genes for the first time, utilizing AlphaFold3, a deep learning architecture developed by DeepMind. Subsequently, we analyzed public transcriptome data to assess the tissue specificity of these 28 genes. Additionally, we examined the expression of SlTRM genes using RNA-seq and qRT-PCR techniques. Our analysis revealed that SlTRM25 was significantly differentially expressed, leading us to hypothesize that it may be involved in the development of lateral branches in tomatoes. Finally, we predicted the regulatory interaction network of SlTRM25 and identified that it interacts with genes such as SlFAF3/4b, SlCSR-like1, SlCSR-like2, and SlTRM19. This study serves as a reference for the investigation of the tomato TRM gene family members and introduces a novel perspective on the role of this gene family in the formation of lateral branches in tomatoes, offering both theoretical and practical significance.

The mutual influence of microtubules and the cortical ER on their coordinated organisation

The endoplasmic reticulum (ER) is the largest organelle in terms of membrane content, occupying the entire cytoplasmic volume. It is tethered to the cell cortex through ER-plasma membrane contact sites (EPCS). Previous studies have shown that EPCSs labelled by VAP27 align with cortical microtubules, and that ER tubules elongate along microtubules. Here, we addressed the question whether this relationship is bidirectional, with EPCSs influencing microtubule organisation. Using TIRF microscopy to track EPCSs and microtubule dynamics simultaneously, we demonstrate that while EPCSs remain stable, microtubules are highly dynamic and can adjust their positioning based on nearby EPCS in Arabidopsis cotyledon epidermis. In lobes of epidermal cells enclosed by two indentations, where microtubules bundle together, EPCSs flank the bundles and exhibit a distinctive arrangement, forming symmetric arcs in relation to the lobe axis. In guard cells, transversely oriented ER tubules co-align with microtubules. Disrupting microtubules with the drug oryzalin leads to transient guard cells-ER remodelling, followed by its reorganisation into transverse tubules before microtubule recovery. Taken together our observations suggest, that the positioning of EPCSs and cortical microtubules, can affect each other and the organisation of cortical ER.

Actin-mediated avoidance of tricellular junction influences global topology at the Arabidopsis shoot apical meristem

Division plane orientation contributes to cell shape and topological organization, playing a key role in morphogenesis, but the precise physical and molecular mechanism influencing these processes remains largely obscure in plants. In particular, it is less clear how the placement of the new walls occurs in relation to the walls of neighboring cells. Here, we show that genetic perturbation of the actin cytoskeleton results in more rectangular cell shapes and higher incidences of four-way junctions, perturbing the global topology of cells in the shoot apical meristem of Arabidopsis thaliana. Actin mutants also exhibit changes in the expansion rate of the new versus the maternal cell wall after division, affecting the evolution of internal angles at tricellular junctions. Further, the increased width of the preprophase band in the actin mutant contributes to inaccuracy in the placement of the new cell wall. Computational simulation further substantiates this hypothesis and reproduces the observed cell shape defects.

Nature's Load-Bearing Design Principles and Their Application in Engineering: A Review

Biological structures optimized through natural selection provide valuable insights for engineering load-bearing components. This paper reviews six key strategies evolved in nature for efficient mechanical load handling: hierarchically structured composites, cellular structures, functional gradients, hard shell-soft core architectures, form follows function, and robust geometric shapes. The paper also discusses recent research that applies these strategies to engineering design, demonstrating their effectiveness in advancing technical solutions. The challenges of translating nature's designs into engineering applications are addressed, with a focus on how advancements in computational methods, particularly artificial intelligence, are accelerating this process. The need for further development in innovative material characterization techniques, efficient modeling approaches for heterogeneous media, multi-criteria structural optimization methods, and advanced manufacturing techniques capable of achieving enhanced control across multiple scales is underscored. By highlighting nature's holistic approach to designing functional components, this paper advocates for adopting a similarly comprehensive methodology in engineering practices to shape the next generation of load-bearing technical components.

Mechanical forces exerted on floral primordia with a novel experimental system modify floral development in Arabidopsis thaliana

Mechanical forces play a crucial role in plant development, including floral development. We previously reported that the phyllotactic variation in the staminate flowers of Ceratophyllum demersum may be caused by mechanical forces on the adaxial side of floral primordia, which may be a common mechanism in angiosperms. On the basis of this result, we developed a novel experimental system for analysis of the effects of mechanical forces on the floral meristem of Arabidopsis thaliana, aiming to induce morphological changes in flowers. In this experimental system, a micromanipulator equipped with a micro device, which is shaped to conform with the contour of the abaxial side of the young floral primordium, is used to exert contact pressure on a floral primordium. In the present study, we conducted contact experiments using this system and successfully induced diverse morphological changes during floral primordial development. In several primordia, the tip of the abaxial sepal primordium was incised with two or three lobes. A different floral primordium developed an additional sepal on the abaxial side (i.e., two abaxial sepals). Additionally, we observed the fusion of sepals in some floral primordia. These results suggest that mechanical forces have multiple effects on floral development, and changes in the tensile stress pattern in the cells of floral primordia are induced by the mechanical forces exerted with the micro device. These effects, in turn, lead to morphological changes in the floral primordia.

Formation mechanism and regulation analysis of trumpet leaf in Ginkgo biloba L

Introduction

The research on plant leaf morphology is of great significance for understanding the development and evolution of plant organ morphology. As a relict plant, the G. biloba leaf morphology typically exhibits bifoliate and peltate forms. However, throughout its long evolutionary history, Ginkgo leaves have undergone diverse changes.

Methods

This study focuses on the distinct "trumpet" leaves and normal fan-shaped leaves of G. biloba for analysis of their phenotypes, photosynthetic activity, anatomical observations, as well as transcriptomic and metabolomic analyses.

Results

The results showed that trumpet-shaped G. biloba leaves have fewer cells, significant morphological differences between dorsal and abaxial epidermal cells, leading to a significantly lower net photosynthetic rate. Additionally, this study found that endogenous plant hormones such as GA, auxin, and JA as well as metabolites such as flavonoids and phenolic acids play roles in the formation of trumpet-shaped G. biloba leaves. Moreover, the experiments revealed the regulatory mechanisms of various key biological processes and gene expressions in the trumpet-shaped leaves of G. biloba.

Discussion

Differences in the dorsal and abdominal cells of G. biloba leaves can cause the leaf to curl, thus reducing the overall photosynthetic efficiency of the leaves. However, the morphology of plant leaves is determined during the primordia leaf stage. In the early stages of leaf development, the shoot apical meristem (SAM) determines the developmental morphology of dicotyledonous plant leaves. This process involves the activity of multiple gene families and small RNAs. The establishment of leaf morphology is complexly regulated by various endogenous hormones, including the effect of auxin on cell walls. Additionally, changes in intracellular ion concentrations, such as fluctuations in Ca2+ concentration, also affect cell wall rigidity, thereby influencing leaf growth morphology.

Structure and growth of plant cell walls

Plant cells build nanofibrillar walls that are central to plant growth, morphogenesis and mechanics. Starting from simple sugars, three groups of polysaccharides, namely, cellulose, hemicelluloses and pectins, with very different physical properties are assembled by the cell to make a strong yet extensible wall. This Review describes the physics of wall growth and its regulation by cellular processes such as cellulose production by cellulose synthase, modulation of wall pH by plasma membrane H+-ATPase, wall loosening by expansin and signalling by plant hormones such as auxin and brassinosteroid. In addition, this Review discusses the nuanced roles, properties and interactions of cellulose, matrix polysaccharides and cell wall proteins and describes how wall stress and wall loosening cooperatively result in cell wall growth.

Transcriptome and functional analyses of phenotypic plasticity in sea grape Caulerpa okamurae

Caulerpa is a marine green macroalga distinguished by a large single cell with multiple nuclei. It also exhibits remarkable morphological intraspecies variations, in response to diverse environmental types. However, the molecular mechanisms underlying this phenotypic plasticity remain poorly understood. In this work, we compare the transcriptomes of Caulerpa okamurae Weber Bosse, 1897 displaying altered phenotypes of cultivation and natural phenotypes and investigate significantly regulated genes and their biological functions using differential expression analyses. We observe light-harvesting complex upregulation and cellular framework stability downregulation in altered phenotypes compared to the natural phenotypes. Intertidal macrophytes reduce light capture to avoid photodamage and regulate their morphology to protect against wave damage. In contrast, the lower light conditions and the cultivation environment augment light capture and increase a morphology prioritizing light trapping. Moreover, the addition of simulated wave-sweeping stimuli induces a return to the natural morphology under high-light conditions, showing how mechanical stress affects morphological organization in C. okamurae. We provide detailed gene expression patterns in C. okamurae under varying light intensities and water conditions, suggesting a distinct influence on its morphological traits.

The nonlinear mechanics of highly extensible plant epidermal cell walls

Plant epidermal cell walls maintain the mechanical integrity of plants and restrict organ growth. Mechanical analyses can give insights into wall structure and are inputs for mechanobiology models of plant growth. To better understand the intrinsic mechanics of epidermal cell walls and how they may accommodate large deformations during growth, we analyzed a geometrically simple material, onion epidermal strips consisting of only the outer (periclinal) cell wall, ~7 μm thick. With uniaxial stretching by >40%, the wall showed complex three-phase stress-strain responses while cyclic stretching revealed reversible and irreversible deformations and elastic hysteresis. Stretching at varying strain rates and temperatures indicated the wall behaved more like a network of flexible cellulose fibers capable of sliding than a viscoelastic composite with pectin viscosity. We developed an analytic framework to quantify nonlinear wall mechanics in terms of stiffness, deformation, and energy dissipation, finding that the wall stretches by combined elastic and plastic deformation without compromising its stiffness. We also analyzed mechanical changes in slightly dehydrated walls. Their extension became stiffer and more irreversible, highlighting the influence of water on cellulose stiffness and sliding. This study offers insights into the structure and deformation modes of primary cell walls and presents a framework that is also applicable to tissues and whole organs.

Stiffness transitions in new walls post-cell division differ between Marchantia polymorpha gemmae and Arabidopsis thaliana leaves

Plant morphogenesis is governed by the mechanics of the cell wall-a stiff and thin polymeric box that encloses the cells. The cell wall is a highly dynamic composite material. New cell walls are added during cell division. As the cells continue to grow, the properties of cell walls are modulated to undergo significant changes in shape and size without breakage. Spatial and temporal variations in cell wall mechanical properties have been observed. However, how they relate to cell division remains an outstanding question. Here, we combine time-lapse imaging with local mechanical measurements via atomic force microscopy to systematically map the cell wall's age and growth, with their stiffness. We make use of two systems, Marchantia polymorpha gemmae, and Arabidopsis thaliana leaves. We first characterize the growth and cell division of M. polymorpha gemmae. We then demonstrate that cell division in M. polymorpha gemmae results in the generation of a temporary stiffer and slower-growing new wall. In contrast, this transient phenomenon is absent in A. thaliana leaves. We provide evidence that this different temporal behavior has a direct impact on the local cell geometry via changes in the junction angle. These results are expected to pave the way for developing more realistic plant morphogenetic models and to advance the study into the impact of cell division on tissue growth.

How metabolism and development are intertwined in space and time

Developmental transitions, occurring throughout the life cycle of plants, require precise regulation of metabolic processes to generate the energy and resources necessary for the committed growth processes. In parallel, the establishment of new cells, tissues, and even organs, alongside their differentiation provoke profound changes in metabolism. It is increasingly being recognized that there is a certain degree of feedback regulation between the components and products of metabolic pathways and developmental regulators. The generation of large-scale metabolomics datasets during developmental transitions, in combination with molecular genetic approaches has helped to further our knowledge on the functional importance of metabolic regulation of development. In this perspective article, we provide insights into studies that elucidate interactions between metabolism and development at the temporal and spatial scales. We additionally discuss how this influences cell growth-related processes. We also highlight how metabolic intermediates function as signaling molecules to direct plant development in response to changing internal and external conditions.

Morphogenesis of leaves: from initiation to the production of diverse shapes

The manner by which plant organs gain their shape is a longstanding question in developmental biology. Leaves, as typical lateral organs, are initiated from the shoot apical meristem that harbors stem cells. Leaf morphogenesis is accompanied by cell proliferation and specification to form the specific 3D shapes, with flattened lamina being the most common. Here, we briefly review the mechanisms controlling leaf initiation and morphogenesis, from periodic initiation in the shoot apex to the formation of conserved thin-blade and divergent leaf shapes. We introduce both regulatory gene patterning and biomechanical regulation involved in leaf morphogenesis. How phenotype is determined by genotype remains largely unanswered. Together, these new insights into leaf morphogenesis resolve molecular chains of events to better aid our understanding.

IQ67 DOMAIN protein 21 is critical for indentation formation in pavement cell morphogenesis

In plants, cortical microtubules anchor to the plasma membrane in arrays and play important roles in cell shape. However, the molecular mechanism of microtubule binding proteins, which connect the plasma membrane and cortical microtubules in cell morphology remains largely unknown. Here, we report that a plasma membrane and microtubule dual-localized IQ67 domain protein, IQD21, is critical for cotyledon pavement cell (PC) morphogenesis in Arabidopsis. iqd21 mutation caused increased indentation width, decreased lobe length, and similar lobe number of PCs, whereas IQD21 overexpression had a different effect on cotyledon PC shape. Weak overexpression led to increased lobe number, decreased indentation width, and similar lobe length, while moderate or great overexpression resulted in decreased lobe number, indentation width, and lobe length of PCs. Live-cell observations revealed that IQD21 accumulation at indentation regions correlates with lobe initiation and outgrowth during PC development. Cell biological and genetic approaches revealed that IQD21 promotes transfacial microtubules anchoring to the plasma membrane via its polybasic sites and bundling at the indentation regions in both periclinal and anticlinal walls. IQD21 controls cortical microtubule organization mainly through promoting Katanin 1-mediated microtubule severing during PC interdigitation. These findings provide the genetic evidence that transfacial microtubule arrays play a determinant role in lobe formation, and the insight into the molecular mechanism of IQD21 in transfacial microtubule organization at indentations and puzzle-shaped PC development.

Localized growth and remodelling drives spongy mesophyll morphogenesis

The spongy mesophyll is a complex, porous tissue found in plant leaves that enables carbon capture and provides mechanical stability. Unlike many other biological tissues, which remain confluent throughout development, the spongy mesophyll must develop from an initially confluent tissue into a tortuous network of cells with a large proportion of intercellular airspace. How the airspace in the spongy mesophyll develops while the tissue remains mechanically stable is unknown. Here, we use computer simulations of deformable polygons to develop a purely mechanical model for the development of the spongy mesophyll tissue. By stipulating that cell wall growth and remodelling occurs only near void space, our computational model is able to recapitulate spongy mesophyll development observed in Arabidopsis thaliana leaves. We find that robust generation of pore space in the spongy mesophyll requires a balance of cell growth, adhesion, stiffness and tissue pressure to ensure cell networks become porous yet maintain mechanical stability. The success of this mechanical model of morphogenesis suggests that simple physical principles can coordinate and drive the development of complex plant tissues like the spongy mesophyll.

Differential growth dynamics control aerial organ geometry

How gene activities and biomechanics together direct organ shapes is poorly understood. Plant leaf and floral organs develop from highly similar initial structures and share similar gene expression patterns, yet they gain drastically different shapes later-flat and bilateral leaf primordia and radially symmetric floral primordia, respectively. We analyzed cellular growth patterns and gene expression in young leaves and flowers of Arabidopsis thaliana and found significant differences in cell growth rates, which correlate with convergence sites of phytohormone auxin that require polar auxin transport. In leaf primordia, the PRESSED-FLOWER-expressing middle domain grows faster than adjacent adaxial domain and coincides with auxin convergence. In contrast, in floral primordia, the LEAFY-expressing domain shows accelerated growth rates and pronounced auxin convergence. This distinct cell growth dynamics between leaf and flower requires changes in levels of cell-wall pectin de-methyl-esterification and mechanical properties of the cell wall. Data-driven computer model simulations at organ and cellular levels demonstrate that growth differences are central to obtaining distinct organ shape, corroborating in planta observations. Together, our study provides a mechanistic basis for the establishment of early aerial organ symmetries through local modulation of differential growth patterns with auxin and biomechanics.

Candida albicans CHK1 gene regulates its cross-kingdom interactions with Streptococcus mutans to promote caries

The cross-kingdom interactions between Candida albicans and Streptococcus mutans have played important roles in early childhood caries (ECC). However, the key pathways of C. albicans promoting the cariogenicity of S. mutans are still unclear. Here, we found that C. albicans CHK1 gene was highly upregulated in their dual-species biofilms. C. albicans chk1Δ/Δ significantly reduced the synergistical growth promotion, biofilm formation, and exopolysaccharides (EPS) production of S. mutans, the key cariogenic agent, compared to C. albicans wild type (WT) and CHK1 complementary strains. C. albicans WT upregulated the expressions of S. mutans EPS biosynthesis genes gtfB, gtfC, and gtfD, and their regulatory genes vicR and vicK, but chk1Δ/Δ had no effects. Both C. albicans WT and chk1Δ/Δ failed to promote the biofilm formation and EPS production of S. mutans ΔvicK and antisense-vicR strains, indicating that C. albicans CHK1 upregulated S. mutans vicR and vicK to increase the EPS biosynthesis gene expression, then enhanced the EPS production and biofilm formation to promote the cariogenicity. In rat caries model, the coinfection with chk1Δ/Δ and S. mutans decreased the colonization of S. mutans and developed less caries especially the severe caries compared to that from the combinations of S. mutans with C. albicans WT, indicating the essential role of C. albicans CHK1 gene in the development of dental caries. Our study for the first time demonstrated the key roles of C. albicans CHK1 gene in dental caries and suggested that it may be a practical target to reduce or treat ECC. KEY POINTS: • C. albicans CHK1 gene is important for its interaction with S. mutans. • CHK1 regulates S. mutans two-component system to promote its cariogenicity. • CHK1 gene regulates the cariogenicity of S. mutans in rat dental caries.

The IPGA1-ANGUSTIFOLIA module regulates microtubule organisation and pavement cell shape in Arabidopsis

Plant cells continuously experience mechanical stress resulting from the cell wall that bears internal turgor pressure. Cortical microtubules align with the predicted maximal tensile stress direction to guide cellulose biosynthesis and therefore results in cell wall reinforcement. We have previously identified Increased Petal Growth Anisotropy (IPGA1) as a putative microtubule-associated protein in Arabidopsis, but the function of IPGA1 remains unclear. Here, using the Arabidopsis cotyledon pavement cell as a model, we demonstrated that IPGA1 forms protein granules and interacts with ANGUSTIFOLIA (AN) to cooperatively regulate microtubule organisation in response to stress. Application of mechanical perturbations, such as cell ablation, led to microtubule reorganisation into aligned arrays in wild-type cells. This microtubule response to stress was enhanced in the IPGA1 loss-of-function mutant. Mechanical perturbations promoted the formation of IPGA1 granules on microtubules. We further showed that IPGA1 physically interacted with AN both in vitro and on microtubules. The ipga1 mutant alleles exhibited reduced interdigitated growth of pavement cells, with smooth shape. IPGA1 and AN had a genetic interaction in regulating pavement cell shape. Furthermore, IPGA1 genetically and physically interacted with the microtubule-severing enzyme KATANIN. We propose that the IPGA1-AN module regulates microtubule organisation and pavement cell shape.

Cotton (Gossypium hirsutum) VIRMA as an N6-Methyladenosine RNA Methylation Regulator Participates in Controlling Chloroplast-Dependent and Independent Leaf Development

N6-methyladenosine (m⁶A) is one of the most abundant internal modifications of mRNA, which plays important roles in gene expression regulation, and plant growth and development. Vir-like m⁶A methyltransferase associated (VIRMA) serves as a scaffold for bridging the catalytic core components of the m⁶A methyltransferase complex. The role of VIRMA in regulating leaf development and its related mechanisms have not been reported. Here, we identified and characterized two upland cotton (Gossypium hirsutum) VIRMA genes, named as GhVIR-A and GhVIR-D, which share 98.5% identity with each other. GhVIR-A and GhVIR-D were ubiquitously expressed in different tissues and relatively higher expressed in leaves and main stem apexes (MSA). Knocking down the expression of GhVIR genes by the virus-induced gene silencing (VIGS) system influences leaf cell size, cell shape, and total cell numbers, thereby determining cotton leaf morphogenesis. The dot-blot assay and colorimetric experiment showed the ratio of m⁶A to A in mRNA is lower in leaves of GhVIR-VIGS plants compared with control plants. Messenger RNA (mRNA) high-throughput sequencing (RNA-seq) and a qRT-PCR experiment showed that GhVIRs regulate leaf development through influencing expression of some transcription factor genes, tubulin genes, and chloroplast genes including photosystem, carbon fixation, and ribosome assembly. Chloroplast structure, chlorophyll content, and photosynthetic efficiency were changed and unsuitable for leaf growth and development in GhVIR-VIGS plants compared with control plants. Taken together, our results demonstrate GhVIRs function in cotton leaf development by chloroplast dependent and independent pathways.

Leaf morphogenesis: The multifaceted roles of mechanics

Plants produce a rich diversity of biological forms, and the diversity of leaves is especially notable. Mechanisms of leaf morphogenesis have been studied in the past two decades, with a growing focus on the interactive roles of mechanics in recent years. Growth of plant organs involves feedback by mechanical stress: growth induces stress, and stress affects growth and morphogenesis. Although much attention has been given to potential stress-sensing mechanisms and cellular responses, the mechanical principles guiding morphogenesis have not been well understood. Here we synthesize the overarching roles of mechanics and mechanical stress in multilevel and multiple stages of leaf morphogenesis, encompassing leaf primordium initiation, phyllotaxis and venation patterning, and the establishment of complex mature leaf shapes. Moreover, the roles of mechanics at multiscale levels, from subcellular cytoskeletal molecules to single cells to tissues at the organ scale, are articulated. By highlighting the role of mechanical buckling in the formation of three-dimensional leaf shapes, this review integrates the perspectives of mechanics and biology to provide broader insights into the mechanobiology of leaf morphogenesis.

Expansin-mediated developmental and adaptive responses: A matter of cell wall biomechanics?

Biomechanical properties of the cell wall (CW) are important for many developmental and adaptive responses in plants. Expansins were shown to mediate pH-dependent CW enlargement via a process called CW loosening. Here, we provide a brief overview of expansin occurrence in plant and non-plant species, their structure and mode of action including the role of hormone-regulated CW acidification in the control of expansin activity. We depict the historical as well as recent CW models, discuss the role of expansins in the CW biomechanics and address the developmental importance of expansin-regulated CW loosening in cell elongation and new primordia formation. We summarise the data published so far on the role of expansins in the abiotic stress response as well as the rather scarce evidence and hypotheses on the possible mechanisms underlying expansin-mediated abiotic stress resistance. Finally, we wrap it up by highlighting possible future directions in expansin research.

MDP25 mediates the fine-tuning of microtubule organization in response to salt stress

Microtubules are dynamic cytoskeleton structures playing fundamental roles in plant responses to salt stress. The precise mechanisms by which microtubule organization is regulated under salt stress are largely unknown. Here, we report that Arabidopsis thaliana MICROTUBULE-DESTABILIZING PROTEIN 25 (MDP25; also known as PLASMA MEMBRANE-ASSOCIATED CATION-BINDING PROTEIN 1 (PCaP1)) helps regulate microtubule organization. Under salt treatment, elevated cytosolic Ca²⁺ concentration caused MDP25 to partially dissociate from the plasma membrane, promoting microtubule depolymerization. When Ca²⁺ signaling was blocked by BAPTA-AM or LaCl₃ , microtubule depolymerization in wild-type and MDP25-overexpressing cells was slower, while there was no obvious change in mdp25 cells. Knockout of MDP25 improved microtubule reassembly and was conducive to microtubule integrity under long-term salt treatment and microtubule recovery after salt stress. Moreover, mdp25 seedlings exhibited a higher survival rate under salt stress. The presence microtubule-disrupting reagent oryzalin or microtubule-stabilizing reagent paclitaxel differentially affected the survival rates of different genotypes under salt stress. MDP25 promoted microtubule instability by affecting the catastrophe and rescue frequencies, shrinkage rate and time in pause phase at the microtubule plus-end and the depolymerization rate at the microtubule minus-end. These findings reveal a role for MDP25 in regulating microtubule organization under salt treatment by affecting microtubule dynamics.

Mutagenic Effect of 60Co γ-Irradiation on Rosa multiflora ‘Libellula’ and the Mechanism Underlying the Associated Leaf Changes

Gamma (γ)-irradiation can induce changes in plant morphology, cellular physiological activities, and genetic material. To date, there has been limited research on the molecular basis of leaf morphological abnormalities and physiological changes in irradiated rose plants. In this study, Rosa multiflora ‘Libellula’ plants were treated with ⁶⁰Co γ-rays. The irradiation resulted in the distortion of blade morphology. Additionally, the leaf chlorophyll content decreased, whereas the accumulation of reactive oxygen species increased. The differentially expressed genes between the control and 2–3 plants irradiated with 50 Gy were analyzed by RNA-seq technology, which revealed genes related to chlorophyll metabolism were differentially expressed. The expression levels of genes related to the regulation of antioxidant enzyme synthesis were downregulated. An RNA-seq analysis also identified the differentially expressed regulatory genes involved in leaf morphology development. Four genes (RcYABBY1, RcARF18, RcARF9, and RcWOX8) were selected, and their expression patterns in different leaf development stages and in various plant organs were analyzed. Furthermore, virus-induced gene silencing technology was used to verify that RcYABBY1 is involved in the morphogenesis of R. multiflora ‘Libellula’ leaves. The results of this study are useful for clarifying the molecular, physiological, and morphological changes in irradiated rose plants.

Photomorphogenesis of Diverse Autonomous Traveling Waves in a Monolithic Soft Artificial Muscle

Biological organisms (e.g., batoid fish, etc.) possess the remarkable ability to morph their soft, sheet-like tissues into wavy morphologies and self-oscillate to make traveling waves, enabling myriad functionalities in propulsion, locomotion, and transportation. In contrast, current manmade soft robotic systems cannot adaptively make wavy morphologies and concurrently achieve wave propagation because the controllable actuation of desired 3D morphologies in entirely soft materials is a formidable challenge due to their continuously deformable bodies that own a large number of actuable degrees of freedom. Here, we report a bioinspired robotic system that not only allows photomorphogenesis of on-demand 3D wavy morphologies but also enables autonomous wave propagation in a monolithic soft artificial muscle (MSAM). This system employs a conceptually different design strategy based on a combination of two principles derived from plant morphogenesis and the undulatory motion of ray fish. The former offers a shaping principle based on differential growth that enables morphing MSAM into target wavy configurations, while the latter inspires a driving principle that induces autonomous propagation of shaped waves by rhythmic motor patterns. This waving system can be used as adaptive "soft engines/motors" that enable directional locomotion, intelligent transportation of cargo, and autonomous propulsion. It even produces programmable, complex artificial peristaltic waves. Our design allows controllable formation of 3D wavy morphologies and autonomous wave behaviors in the soft robotic system that would be useful for broad applications in adaptive, self-regulated mechanical systems for advanced robotics, soft machines, and energy harvest.

Cell biology of the leaf epidermis: Fate specification, morphogenesis, and coordination

As the outermost layer of plants, the epidermis serves as a critical interface between plants and the environment. During leaf development, the differentiation of specialized epidermal cell types, including stomatal guard cells, pavement cells, and trichomes, occurs simultaneously, each providing unique and pivotal functions for plant growth and survival. Decades of molecular-genetic and physiological studies have unraveled key players and hormone signaling specifying epidermal differentiation. However, most studies focus on only one cell type at a time, and how these distinct cell types coordinate as a unit is far from well-comprehended. Here we provide a review on the current knowledge of regulatory mechanisms underpinning the fate specification, differentiation, morphogenesis, and positioning of these specialized cell types. Emphasis is given to their shared developmental origins, fate flexibility, as well as cell cycle and hormonal controls. Furthermore, we discuss computational modeling approaches to integrate how mechanical properties of individual epidermal cell types and entire tissue/organ properties mutually influence each other. We hope to illuminate the underlying mechanisms coordinating the cell differentiation that ultimately generate a functional leaf epidermis.

Stem Cell Basis for Fractal Patterns: Axillary Meristem Initiation

Whereas stem cell lineages are of enormous importance in animal development, their roles in plant development have only been appreciated in recent years. Several specialized lineages of stem cells have been identified in plants, such as meristemoid mother cells and vascular cambium, as well as those located in the apical meristems. The initiation of axillary meristems (AMs) has recently gained intensive attention. AMs derive from existing stem cell lineages that exit from SAMs and define new growth axes. AMs are in fact additional rounds of SAMs, and display the same expression patterns and functions as the embryonic SAM, creating a fractal branching pattern. Their formation takes place in leaf-meristem boundaries and mainly comprises two key stages. The first stage is the maintenance of the meristematic cell lineage in an undifferentiated state. The second stage is the activation, proliferation, and re-specification to form new stem cell niches in AMs, which become the new postembryonic "fountain of youth" for organogenesis. Both stages are tightly regulated by spatially and temporally interwound signaling networks. In this mini-review, I will summarize the most up-to-date understanding of AM establishment and mainly focus on how the leaf axil meristematic cell lineage is actively maintained and further activated to become CLV3-expressed stem cells, which involves phytohormonal cascades, transcriptional regulations, epigenetic modifications, as well as mechanical signals.

Plant cell polarity as the nexus of tissue mechanics and morphogenesis

How reproducible body patterns emerge from the collective activity of individual cells is a key question in developmental biology. Plant cells are encaged in their walls and unable to migrate. Morphogenesis thus relies on directional cell division, by precise positioning of division planes, and anisotropic cellular growth, mediated by regulated mechanical inhomogeneity of the walls. Both processes require the prior establishment of cell polarity, marked by the formation of polar domains at the plasma membrane, in a number of developmental contexts. The establishment of cell polarity involves biochemical cues, but increasing evidence suggests that mechanical forces also play a prominent instructive role. While evidence for mutual regulation between cell polarity and tissue mechanics is emerging, the nature of this bidirectional feedback remains unclear. Here we review the role of cell polarity at the interface of tissue mechanics and morphogenesis. We also aim to integrate biochemistry-centred insights with concepts derived from physics and physical chemistry. Lastly, we propose a set of questions that will help address the fundamental nature of cell polarization and its mechanistic basis.

From genes to networks: The genetic control of leaf development

Substantial diversity exists for both the size and shape of the leaf, the main photosynthetic organ of flowering plants. The two major forms of leaf are simple leaves, in which the leaf blade is undivided, and compound leaves, which comprise several leaflets. Leaves form at the shoot apical meristem from a group of undifferentiated cells, which first establish polarity, then grow and differentiate. Each of these processes is controlled by a combination of transcriptional regulators, microRNAs and phytohormones. The present review documents recent advances in our understanding of how these various factors modulate the development of both simple leaves (focusing mainly on the model plant Arabidopsis thaliana) and compound leaves (focusing mainly on the model legume species Medicago truncatula).

Solving the Puzzle of Shape Regulation in Plant Epidermal Pavement Cells

The plant epidermis serves many essential functions, including interactions with the environment, protection, mechanical strength, and regulation of tissue and organ growth. To achieve these functions, specialized epidermal cells develop into particular shapes. These include the intriguing interdigitated jigsaw puzzle shape of cotyledon and leaf pavement cells seen in many species, the precise functions of which remain rather obscure. Although pavement cell shape regulation is complex and still a long way from being fully understood, the roles of the cell wall, mechanical stresses, cytoskeleton, cytoskeletal regulatory proteins, and phytohormones are becoming clearer. Here, we provide a review of this current knowledge of pavement cell morphogenesis, generated from a wealth of experimental evidence and assisted by computational modeling approaches. We also discuss the evolution and potential functions of pavement cell interdigitation. Throughout the review, we highlight some of the thought-provoking controversies and creative theories surrounding the formation of the curious puzzle shape of these cells.

A crosstalk between auxin and brassinosteroid regulates leaf shape by modulating growth anisotropy

Leaf shape is highly variable within and among plant species, ranging from slender to oval shaped. This is largely determined by the proximodistal axis of growth. However, little is known about how proximal-distal growth is controlled to determine leaf shape. Here, we show that Arabidopsis leaf and sepal proximodistal growth is tuned by two phytohormones. Two class A AUXIN RESPONSE FACTORs (ARFs), ARF6 and ARF8, activate the transcription of DWARF4, which encodes a key brassinosteroid (BR) biosynthetic enzyme. At the cellular level, the phytohormones promote more directional cell expansion along the proximodistal axis, as well as final cell sizes. BRs promote the demethyl-esterification of cell wall pectins, leading to isotropic in-plane cell wall loosening. Notably, numerical simulation showed that isotropic cell wall loosening could lead to directional cell and organ growth along the proximodistal axis. Taken together, we show that auxin acts through biosynthesis of BRs to determine cell wall mechanics and directional cell growth to generate leaves of variable roundness.

The Arabidopsis embryo as a quantifiable model for studying pattern formation

Phenotypic diversity of flowering plants stems from common basic features of the plant body pattern with well-defined body axes, organs and tissue organisation. Cell division and cell specification are the two processes that underlie the formation of a body pattern. As plant cells are encased into their cellulosic walls, directional cell division through precise positioning of division plane is crucial for shaping plant morphology. Since many plant cells are pluripotent, their fate establishment is influenced by their cellular environment through cell-to-cell signaling. Recent studies show that apart from biochemical regulation, these two processes are also influenced by cell and tissue morphology and operate under mechanical control. Finding a proper model system that allows dissecting the relationship between these aspects is the key to our understanding of pattern establishment. In this review, we present the Arabidopsis embryo as a simple, yet comprehensive model of pattern formation compatible with high-throughput quantitative assays.

Candida albicans CHK1 gene from two-component system is essential for its pathogenicity in oral candidiasis

The roles of Candida albicans CHK1, a key gene from two-component system, in oral mucosal infection are not clear. This study evaluated the key roles of CHK1 gene in vitro and in vivo. The expression of CHK1 and its regulated virulence factors were tested during the oral epithelial cell infection. The production of lactate dehydrogenase, ROS, and IL-1α combined with the confocal and scanning electron microscope observation was employed to identify the capability of CHK1 in damaging the epithelial cells. Both immunocompetent and immunodeficient mice oropharyngeal infection models were involved to confirm the roles of CHK1 gene in vivo. The expression of CHK1 gene was significantly increased during the oral epithelial cell infection. The chk1Δ/Δ mutant failed to damage the epithelial cells or induce IL-α and ROS production. Interestingly, chk1Δ/Δ can also form the similar hyphae with WT and complementary strains. Accordingly, chk1Δ/Δ did not affect the adhesion and invasion rates of C. albicans to oral epithelial cells. However, chk1Δ/Δ significantly decreased the expression levels of the virulence factors, including ALS2, SAP6, and YWP1. The chk1Δ/Δ also failed to cause oral candidiasis in both immunocompetent and immunodeficient mice indicating that CHK1 gene from the two-component system is essential for the pathogenicity of C. albicans. KEY POINTS: • CHK1gene is essential for C. albicans in oral candidiasis • C. albicans without CHK1 gene can form "non-pathogenic" hyphae. • CHK1 gene regulates the virulence of C. albicans.

Auxin biosynthesis and cellular efflux act together to regulate leaf vein patterning

Our current understanding of vein development in leaves is based on canalization of the plant hormone auxin into self-reinforcing streams which determine the sites of vascular cell differentiation. By comparison, how auxin biosynthesis affects leaf vein patterning is less well understood. Here, after observing that inhibiting polar auxin transport rescues the sparse leaf vein phenotype in auxin biosynthesis mutants, we propose that the processes of auxin biosynthesis and cellular auxin efflux work in concert during vein development. By using computational modeling, we show that localized auxin maxima are able to interact with mechanical forces generated by the morphological constraints which are imposed during early primordium development. This interaction is able to explain four fundamental characteristics of midvein morphology in a growing leaf: (i) distal cell division; (ii) coordinated cell elongation; (iii) a midvein positioned in the center of the primordium; and (iv) a midvein which is distally branched. Domains of auxin biosynthetic enzyme expression are not positioned by auxin canalization, as they are observed before auxin efflux proteins polarize. This suggests that the site-specific accumulation of auxin, as regulated by the balanced action of cellular auxin efflux and local auxin biosynthesis, is crucial for leaf vein formation.

An integral insight into pollen wall development: involvement of physical processes in exine ontogeny in Calycanthus floridus L., with an experimental approach

We aimed to understand the underlying mechanisms of development in the sporopollenin-containing part of the pollen wall, the exine, one of the most complex cell walls in plants. Our hypothesis is that distinct physical processes, phase separation and micellar self-assembly, underpinexine development by taking the molecular building blocks, determined and synthesised by the genome, through several phase transitions. To test this hypothesis, we traced each stage of microspore development in Calycanthus floridus with transmission electron microscopy and then generated in vitro experimental simulations corresponding to every developmental stage. The sequence of structures observed within the periplasmic space around developing microspores starts with spherical units, which are rearranged into columns to then form rod-like units (the young columellae) and, finally, white line centred endexine lamellae. Phase separation precedes each developmental stage. The set of experimental simulations, obtained as self-assembled micellar mesophases formed at the interface between lipid and water compartments, was the same: spherical micelles; columns of spherical micelles; cylindrical micelles; and laminate micelles, separated by gaps, resembling white-lined lamellae. Thus, patterns simulating structures observed at the main stages of exine development in C. floridus were obtained from in vitro experiments, and hence purely physicochemical processes can construct exine-like patterns. This highlights the important part played by physical processes that are not under direct genomic control and share influence on the emerging ultrastructure with the genome during exine development. These findings suggest that a new approach to ontogenetic studies, including a consideration of physical factors, is required for a better understanding of developmental processes.

Photosynthesis-assisted remodeling of three-dimensional printed structures

The mechanical properties of engineering structures continuously weaken during service life because of material fatigue or degradation. By contrast, living organisms are able to strengthen their mechanical properties by regenerating parts of their structures. For example, plants strengthen their cell structures by transforming photosynthesis-produced glucose into stiff polysaccharides. In this work, we realize hybrid materials that use photosynthesis of embedded chloroplasts to remodel their microstructures. These materials can be used to three-dimensionally (3D)-print functional structures, which are endowed with matrix-strengthening and crack healing when exposed to white light. The mechanism relies on a 3D-printable polymer that allows for an additional cross-linking reaction with photosynthesis-produced glucose in the material bulk or on the interface. The remodeling behavior can be suspended by freezing chloroplasts, regulated by mechanical preloads, and reversed by environmental cues. This work opens the door for the design of hybrid synthetic-living materials, for applications such as smart composites, lightweight structures, and soft robotics.

Morphological plasticity in the kelp Nereocystis luetkeana (Phaeophyceae) is sensitive to the magnitude, direction, and location of mechanical loading

Nereocystis luetkeana is a canopy-forming kelp that exhibits morphological plasticity across hydrodynamic gradients, producing broad, undulate blades in slow flow and narrow, flattened blades in fast flow, enabling thalli to reduce drag while optimizing photosynthesis. While the functional significance of this phenomenon has been well studied, the developmental and physiological mechanisms that facilitate the plasticity remain poorly understood. In this study, we conducted three experiments to characterize how the (1) magnitude, (2) direction, and (3) location of plasticity-inducing mechanical stimuli affect the morphology of Nereocystis blades. We found that applying a gradient of tensile force caused blades to grow progressively longer, narrower, less ruffled, and heavier in a linear fashion, suggesting that Nereocystis is equally well adapted for all conditions within its hydrodynamic niche. We also found that applying tension transversely across blades caused the growth response to rotate 90°, indicating that there is no substantial separation between the sites of stimulus perception and response and suggesting that a long-distance signaling mechanism, such as a hormone, is unlikely to mediate this phenomenon. Meristoderm cells showed morphological changes that paralleled those of their respective blades in this experiment, implying that tissue-level morphology is influenced by cell growth. Finally, we found that plasticity was only induced when tension was applied directly to the growing tissue, reinforcing that long-distance signaling is probably not involved and possibly indicating that the mechanism on display generally requires an intercalary meristem to facilitate mechanoperception.

Epidermal restriction confers robustness to organ shapes

The shape of comparable tissues and organs is consistent among individuals of a given species, but how this consistency or robustness is achieved remains an open question. The interaction between morphogenetic factors determines organ formation and subsequent shaping, which is ultimately a mechanical process. Using a computational approach, we show that the epidermal layer is essential for the robustness of organ geometry control. Specifically, proper epidermal restriction allows organ asymmetry maintenance, and the tensile epidermal layer is sufficient to suppress local variability in growth, leading to shape robustness. The model explains the enhanced organ shape variations in epidermal mutant plants. In addition, differences in the patterns of epidermal restriction may underlie the initial establishment of organ asymmetry. Our results show that epidermal restriction can answer the longstanding question of how cellular growth noise is averaged to produce precise organ shapes, and the findings also shed light on organ asymmetry establishment.

Touch-induced seedling morphological changes are determined by ethylene-regulated pectin degradation

How mechanical forces regulate plant growth is a fascinating and long-standing question. After germination underground, buried seedlings have to dynamically adjust their growth to respond to mechanical stimulation from soil barriers. Here, we designed a lid touch assay and used atomic force microscopy to investigate the mechanical responses of seedlings during soil emergence. Touching seedlings induced increases in cell wall stiffness and decreases in cell elongation, which were correlated with pectin degradation. We revealed that PGX3, which encodes a polygalacturonase, mediates touch-imposed alterations in the pectin matrix and the mechanics of morphogenesis. Furthermore, we found that ethylene signaling is activated by touch, and the transcription factor EIN3 directly associates with PGX3 promoter and is required for touch-repressed PGX3 expression. By uncovering the link between mechanical forces and cell wall remodeling established via the EIN3-PGX3 module, this work represents a key step in understanding the molecular framework of touch-induced morphological changes.

Nanometer accuracy statistical interferometric technique in monitoring the short-term effects of exogenous plant hormones, auxin and gibberellic acid, on rice plants

Statistical interferometric technique (SIT) is a highly sensitive, high speed non-contact, and non-destructive optical technique developed by our group capable of measuring instantaeoues sub-nanometer displacements. SIT applied to plant leaf elongation revealed nanometric intrinsic fluctuaitons (NIF) that are robust and sensitive to variations in the environment making NIF as a measure of healthiness of the plants. In this study, exogenous plant hormones, auxin (2,4-dichlorophenoxyacetic acid-2,4-D), and gibberellic acid (GA₃), along with an auxin transport inhibitor 2,3,5-triiodobenzoic acid-TIBA, that affect plant growth were used to investigate their effects on NIF. Rice (Oriza sativa) seedlings were used, and their roots were exposed to 1, 2, and 4 µM 2,4-D, and the auxin transport inhibitor, TIBA, of 10, and 20 µM for 22 h and GA₃ solution of different concentrations of 10, 40, and 100 µM for 5 h. Results showed significant increment in NIF for 1 µM and reduction for 4 µM 2,4-D while applicaiton of both 10, and 20 µM TIBA led to reduction in NIF. On the other hand, significant increment in NIF for 40 µM, and a significant reduction at a higher concentration of 100 µM for 5 hours of GA₃ were also observed in comparison to those of control. Our results indicate that NIF as revealed by SIT could show both the positive and negative effects depending on the concentration of exogenous hormones, and transport inhibitors. Results suggest that SIT could be a valuable tool being sensitive enough to speedily assess the effects of plant growth hormones.

Microtubule-Mediated Wall Anisotropy Contributes to Leaf Blade Flattening

Plant organs can adopt a wide range of shapes, resulting from highly directional cell growth and divisions. We focus here on leaves and leaf-like organs in Arabidopsis and tomato, characterized by the formation of thin, flat laminae. Combining experimental approaches with 3D mechanical modeling, we provide evidence that leaf shape depends on cortical microtubule mediated cellulose deposition along the main predicted stress orientations, in particular, along the adaxial-abaxial axis in internal cell walls. This behavior can be explained by a mechanical feedback and has the potential to sustain and even amplify a preexisting degree of flatness, which in turn depends on genes involved in the control of organ polarity and leaf margin formation.

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Microtubules and cellulose microfibrils align along the ad-abaxial direction

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Microtubule-mediated cell growth anisotropy contributes to leaf flattening

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Mechanical feedback accounts for microtubule alignments in the ad-abaxial direction

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Final organ shape depends on the degree of initial asymmetry of primordia

Growing straight through walls

The pollen tube in a flowering plant grows in a direction that is influenced by the mechanical properties of the stigma papillae and the organization of structures called cortical microtubules inside these cells.

Root secondary growth: an unexplored component of soil resource acquisition

BACKGROUND AND AIMS

Despite recent progress in elucidating the molecular basis of secondary growth (cambial growth), the functional implications of this developmental process remain poorly understood. Targeted studies exploring how abiotic and biotic factors affect this process, as well as the relevance of secondary growth to fitness of annual dicotyledonous crop species under stress, are almost entirely absent from the literature. Specifically, the physiological role of secondary growth in roots has been completely neglected yet entails a unique array of implications for plant performance that are distinct from secondary growth in shoot tissue.

SCOPE

Since roots are directly responsible for soil resource capture, understanding of the fitness landscape of root phenotypes is important in both basic and applied plant biology. Interactions between root secondary growth, edaphic conditions and soil resource acquisition may have significant effects on plant fitness. Our intention here is not to provide a comprehensive review of a sparse and disparate literature, but rather to highlight knowledge gaps, propose hypotheses and identify opportunities for novel and agriculturally relevant research pertaining to secondary growth of roots. This viewpoint: (1) summarizes evidence from our own studies and other published work; (2) proposes hypotheses regarding the fitness landscape of secondary growth of roots in annual dicotyledonous species for abiotic and biotic stress; and (3) highlights the importance of directing research efforts to this topic within an agricultural context.

CONCLUSIONS

Secondary growth of the roots of annual dicots has functional significance with regards to soil resource acquisition and transport, interactions with soil organisms and carbon sequestration. Research on these topics would contribute significantly toward understanding the agronomic value of secondary growth of roots for crop improvement.

Imaging flowers: a guide to current microscopy and tomography techniques to study flower development

Developmental biology relies heavily on our ability to generate three-dimensional images of live biological specimens through time, and to map gene expression and hormone response in these specimens as they undergo development. The last two decades have seen an explosion of new bioimaging technologies that have pushed the limits of spatial and temporal resolution and provided biologists with invaluable new tools. However, plant tissues are difficult to image, and no single technology fits all purposes; choosing between many bioimaging techniques is not trivial. Here, we review modern light microscopy and computed projection tomography methods, their capabilities and limitations, and we discuss their current and potential applications to the study of flower development and fertilization.

Cytoskeleton Architecture Regulates Glycolysis Coupling Cellular Metabolism to Mechanical Cues

Energy-demanding processes, such as cell growth, migration, and differentiation, are tension modulated, begging the question whether metabolism and mechanical tension are tightly linked. A recent report by Park et al. shows that stiffness in the extracellular matrix (ECM) promotes reorganization of actin, resulting in enhanced glycolysis.

Phyllotaxis Turns Over a New Leaf-A New Hypothesis

Phyllotaxis describes the periodic arrangement of plant organs most conspicuously floral. Oscillators generally underlie periodic phenomena. A hypothetical algorithm generates phyllotaxis regulated by the Hechtian growth oscillator of the stem apical meristem (SAM) protoderm. The oscillator integrates biochemical and mechanical force that regulate morphogenetic gradients of three ionic species, auxin, protons and Ca2+. Hechtian adhesion between cell wall and plasma membrane transduces wall stress that opens Ca2+ channels and reorients auxin efflux "PIN" proteins; they control the auxin-activated proton pump that dissociates Ca2+ bound by periplasmic arabinogalactan proteins (AGP-Ca2+) hence the source of cytosolic Ca2+ waves that activate exocytosis of wall precursors, AGPs and PIN proteins essential for morphogenesis. This novel approach identifies the critical determinants of an algorithm that generates phyllotaxis spiral and Fibonaccian symmetry: these determinants in order of their relative contribution are: (1) size of the apical meristem and the AGP-Ca2+ capacitor; (2) proton pump activity; (3) auxin efflux proteins; (4) Ca2+ channel activity; (5) Hechtian adhesion that mediates the cell wall stress vector. Arguably, AGPs and the AGP-Ca2+ capacitor plays a decisive role in phyllotaxis periodicity and its evolutionary origins.

Calcium and plasma membrane force-gated ion channels behind development

During development, tissues are submitted to high variation of compression and tension forces. The roles of the cell wall, the cytoskeleton, the turgor pressure and the cell geometry during this process have received due attention. In contrast, apart from its role in the establishment of turgor pressure, the involvement of the plasma membrane as a transducer of mechanical forces during development has been under studied. Force-gated (FG) or Mechanosensitive (MS) ion channels embedded in the bilayer represent 'per se' archetypal mechanosensor able to directly and instantaneously transduce membrane forces into electrical and calcium signals. We discuss here how their fine-tuning, combined with their ability to detect micro-curvature and local membrane tension, allows FG channels to transduce mechanical cues into developmental signals.

The Arabidopsis receptor kinase STRUBBELIG regulates the response to cellulose deficiency

Plant cells are encased in a semi-rigid cell wall of complex build. As a consequence, cell wall remodeling is essential for the control of growth and development as well as the regulation of abiotic and biotic stress responses. Plant cells actively sense physico-chemical changes in the cell wall and initiate corresponding cellular responses. However, the underlying cell wall monitoring mechanisms remain poorly understood. In Arabidopsis the atypical receptor kinase STRUBBELIG (SUB) mediates tissue morphogenesis. Here, we show that SUB-mediated signal transduction also regulates the cellular response to a reduction in the biosynthesis of cellulose, a central carbohydrate component of the cell wall. SUB signaling affects early increase of intracellular reactive oxygen species, stress gene induction as well as ectopic lignin and callose accumulation upon exogenous application of the cellulose biosynthesis inhibitor isoxaben. Moreover, our data reveal that SUB signaling is required for maintaining cell size and shape of root epidermal cells and the recovery of root growth after transient exposure to isoxaben. SUB is also required for root growth arrest in mutants with defective cellulose biosynthesis. Genetic data further indicate that SUB controls the isoxaben-induced cell wall stress response independently from other known receptor kinase genes mediating this response, such as THESEUS1 or MIK2. We propose that SUB functions in a least two distinct biological processes: the control of tissue morphogenesis and the response to cell wall damage. Taken together, our results reveal a novel signal transduction pathway that contributes to the molecular framework underlying cell wall integrity signaling.

Plant cells are encapsulated by a semi-rigid and biochemically complex cell wall. This particular feature has consequences for multiple biologically important processes, such as cell and organ growth or various stress responses. For a plant cell to grow the cell wall has to be modified to allow cell expansion, which is driven by outward-directed turgor pressure generated inside the cell. In return, changes in cell wall architecture need to be monitored by individual cells, and to be coordinated across cells in a growing tissue, for an organ to attain its regular size and shape. Cell wall surveillance also comes into play in the reaction against certain stresses, including for example infection by plant pathogens, many of which break through the cell wall during infection, thereby generating wall-derived factors that can induce defense responses. There is only limited knowledge regarding the molecular system that monitors the composition and status of the cell wall. Here we provide further insight into the mechanism. We show that the cell surface receptor STRUBBELIG, previously known to control organ development in Arabidopsis, also promotes the cell’s response to reduced amounts of cellulose, a main component of the cell wall.

Cell Size Control in Plants

The genetic control of the characteristic cell sizes of different species and tissues is a long-standing enigma. Plants are convenient for studying this question in a multicellular context, as their cells do not move and are easily tracked and measured from organ initiation in the meristems to subsequent morphogenesis and differentiation. In this article, we discuss cell size control in plants compared with other organisms. As seen from yeast cells to mammalian cells, size homeostasis is maintained cell autonomously in the shoot meristem. In developing organs, vacuolization contributes to cell size heterogeneity and may resolve conflicts between growth control at the cellular and organ levels. Molecular mechanisms for cell size control have implications for how cell size responds to changes in ploidy, which are particularly important in plant development and evolution. We also discuss comparatively the functional consequences of cell size and their potential repercussions at higher scales, including genome evolution.