External Mechanical Cues Reveal a Katanin-Independent Mechanism behind Auxin-Mediated Tissue Bending in Plants

A 3-component module maintains sepal flatness in Arabidopsis

As in origami, morphogenesis in living systems heavily relies on tissue curving and folding through the interplay between biochemical and biomechanical cues. By contrast, certain organs maintain their flat posture over several days. Here, we identified a pathway that is required for the maintenance of organ flatness, taking the sepal, the outermost floral organ, in Arabidopsis as a model system. Through genetic, cellular, and mechanical approaches, our results demonstrate that the global gene expression regulator VERNALIZATION INDEPENDENCE 4 (VIP4) fine-tunes the mechanical properties of sepal cell walls and maintains balanced growth on both sides of the sepals, mainly by orchestrating the distribution pattern of AUXIN RESPONSE FACTOR 3 (ARF3). vip4 mutation results in softer cell walls and faster cell growth on the adaxial sepal side, which eventually cause sepals to bend outward. Downstream of VIP4, ARF3 works through modulating auxin to downregulate pectin methylesterase VANGUARD1, resulting in decreased cell wall stiffness. Thus, our work unravels a 3-component module that relates hormonal patterns to organ curvature and actively maintains sepal flatness during its growth.

Arabidopsis floral buds are locked through stress-induced sepal tip curving

In most plant species, sepals-the outermost floral organs-provide a protective shield for reproductive organs. How the floral bud becomes sealed is unknown. In Arabidopsis, we identified a small region at the sepal tip that is markedly curved inward early on and remains curved even after anthesis. Through modelling and quantitative growth analysis, we find that this hook emerges from growth arrest at the tip at a stage when cortical microtubules align with growth-derived tensile stress. Depolymerizing microtubules specifically at young sepal tips hindered hook formation and resulted in open floral buds. Mutants with defective growth pattern at the tip failed to curve inwards, whereas mutants with enhanced alignment of cortical microtubules at the tip exhibited a stronger hook. We propose that floral buds are locked due to a stress-derived growth arrest event curving the sepal tip and forming a rigid hook early on during flower development.

New insights into the functions and regulations of MAP215/MOR1 and katanin, two conserved microtubule-associated proteins in Arabidopsis

Plant microtubules (MTs) form highly dynamic and distinct arrays throughout the cell cycle and are essential for cell and organ morphogenesis. A plethora of microtubule associated-proteins (MAPs), both conserved and plant-specific, ensure the dynamic response of MTs to internal and external cues. The MAP215 family MT polymerase/nucleation factor and the MT severing enzyme katanin are among the most conserved MAPs in eukaryotes. Recent studies have revealed unexpected functional and physical interactions between MICROTUBULE ORGANIZATION 1 (MOR1), the Arabidopsis homolog of MAP215, and KATANIN 1 (KTN1), the catalytic subunit of katanin. In this minireview, we provide a short overview on current understanding of the functions and regulations of MOR1 and katanin in cell morphogenesis and plant growth and development.

A 3-component module maintains sepal flatness in Arabidopsis

As in origami, morphogenesis in living systems heavily relies on tissue curving and folding, through the interplay between biochemical and biomechanical cues. In contrast, certain organs maintain their flat posture over several days. Here we identified a pathway, which is required for the maintenance of organ flatness, taking the sepal, the outermost floral organ, in Arabidopsis as a model system. Through genetic, cellular and mechanical approaches, our results demonstrate that global gene expression regulator VERNALIZATION INDEPENDENCE 4 (VIP4) fine-tunes the mechanical properties of sepal cell walls and maintains balanced growth on both sides of the sepals, mainly by orchestrating the distribution pattern of AUXIN RESPONSE FACTOR 3 (ARF3). vip4 mutation results in softer cell walls and faster cell growth on the adaxial sepal side, which eventually cause sepals to bend outward. Downstream of VIP4, ARF3 works through modulating auxin signaling to down-regulate pectin methylesterase VANGUARD1, resulting in decreased cell wall stiffness. Our work unravels a 3-component module, which relates hormonal patterns to organ curvature, and actively maintains sepal flatness during its growth.

The interplay between cell wall integrity and cell cycle progression in plants

Plant cell walls are dynamic structures that play crucial roles in growth, development, and stress responses. Despite our growing understanding of cell wall biology, the connections between cell wall integrity (CWI) and cell cycle progression in plants remain poorly understood. This review aims to explore the intricate relationship between CWI and cell cycle progression in plants, drawing insights from studies in yeast and mammals. We provide an overview of the plant cell cycle, highlight the role of endoreplication in cell wall composition, and discuss recent findings on the molecular mechanisms linking CWI perception to cell wall biosynthesis and gene expression regulation. Furthermore, we address future perspectives and unanswered questions in the field, such as the identification of specific CWI sensing mechanisms and the role of CWI maintenance in the growth-defense trade-off. Elucidating these connections could have significant implications for crop improvement and sustainable agriculture.

Mutant IAA21 genes from Dendrocalamus sinicus Chia et J. L. Sun inhibit stem and root growth in transgenic tobacco by interacting with ARF5

Woody bamboos are important resource of industrial fibres. Auxin signaling plays a key role in multiple plant developmental processes, as yet the role of auxin/indole acetic acid (Aux/IAA) in culm development of woody bamboos has not been previously characterized. Dendrocalamus sinicus Chia et J. L. Sun is the largest woody bamboo documented in the world. Here, we identified two alleles of DsIAA21 gene (sIAA21 and bIAA21) from the straight- and bent-culm variants of D. sinicus, respectively, and studied how the domains I, i, and II of DsIAA21 affect the gene transcriptional repression. The results showed that bIAA21 expression was rapidly induced by exogenous auxin in D. sinicus. In transgenic tobacco, sIAA21 and bIAA21 mutated in domains i, and II significantly regulated plant architecture and root development. Stem cross sections revealed that parenchyma cells were smaller in transgenic plants than that in wild type plants. Domain i mutation changed the leucine and proline at position 45 to proline and leucine (siaa21L45P and biaa21P45L) strongly repressed cell expansion and root elongation by reducing the gravitropic response. Substitution of isoleucine with valine in domain II of the full length DsIAA21 resulted in dwarf stature in transgenic tobacco plants. Furthermore, the DsIAA21 interacted with auxin response factor 5 (ARF5) in transgenic tobacco plants, suggesting that DsIAA21 might inhibit stem and root elongation via interacting with ARF5. Taken together, our data indicated that DsIAA21 was a negative regulator of plant development and suggested that amino acid differences in domain i of sIAA21 versus bIAA21 affected their response to auxin, and might play a key role in the formation of the bent culm variant in D. sinicus. Our results not only shed a light on the morphogenetic mechanism in D. sinicus, but also provided new insights into versatile function of Aux/IAAs in plants.

Endoreplication controls cell size via mechanochemical signaling

During hypocotyl development, an asymmetric auxin gradient causes differential cell elongation, leading to tissue bending and apical hook formation. Recently, Ma et al. identified a molecular pathway that links auxin with endoreplication and cell size through cell wall integrity sensing, cell wall remodeling, and regulation of cell wall stiffness.

Discretizing the cellular bases of plant morphogenesis: Emerging properties from subcellular and noisy patterning

A fundamental question in biology is how multicellular organisms robustly shape their organs. In the past decade, much progress has been made not just in identifying biochemical and biophysical factors underpinning morphogenesis, but also in analyzing their spatio-temporal dynamics. A remarkable outcome of such analyses is that morphogenesis involves high levels of heterogeneity and fluctuations at local scales. Although this could be considered as white noise to be averaged over time, there is increasing evidence that these heterogeneities and fluctuations are instructive cues for development. In this review, we highlight some of the new questions that such heterogeneities raise for plant morphogenesis. We also investigate their effects across scales, focusing on how subcellular heterogeneities contribute to organ shape robustness and evolvability.

Noninvasive Long-Term Imaging of the Cytoskeleton in Arabidopsis Seedlings

The preparation of biological samples, especially for live-cell microscopy, remains a major experimental challenge in the lab despite technological advances. In addition, high-resolution microscopy techniques require higher sample quality and uniformity, which is difficult to ensure during manual preparation while maintaining "ideal" growth conditions. In this protocol, we provide a way out by growing Arabidopsis thaliana seedlings directly in an imaging chamber, which eliminates invasive sample preparation directly before imaging. This method hinges on the precise placement of seeds into imaging chambers, which can be grown in conventional climate chambers. We detail three methods to grow hypocotyls, cotyledons, leaves, and roots for high-resolution and long-term imaging of the plant cytoskeleton. Furthermore, we show that the growth and development of seedlings inside the chambers can be externally manipulated by the addition of chemicals.

To curve for survival: Apical hook development

Apical hook is a simple curved structure formed at the upper part of hypocotyls when dicot seeds germinate in darkness. The hook structure is transient but essential for seedlings' survival during soil emergence due to its efficient protection of the delicate shoot apex from mechanical injury. As a superb model system for studying plant differential growth, apical hook has fascinated botanists as early as the Darwin age, and significant advances have been achieved at both the morphological and molecular levels to understand how apical hook development is regulated. Here, we will mainly summarize the research progress at these two levels. We will also briefly compare the growth dynamics between apical hook and hypocotyl gravitropic bending at early seed germination phase, with the aim to deduce a certain consensus on their connections. Finally, we will outline the remaining questions and future research perspectives for apical hook development.

The Course of Mechanical Stress: Types, Perception, and Plant Response

Mechanical stimuli, together with the corresponding plant perception mechanisms and the finely tuned thigmomorphogenetic response, has been of scientific and practical interest since the mid-17th century. As an emerging field, there are many challenges in the research of mechanical stress. Indeed, studies on different plant species (annual/perennial) and plant organs (stem/root) using different approaches (field, wet lab, and in silico/computational) have delivered insufficient findings that frequently impede the practical application of the acquired knowledge. Accordingly, the current work distils existing mechanical stress knowledge by bringing in side-by-side the research conducted on both stem and roots. First, the various types of mechanical stress encountered by plants are defined. Second, plant perception mechanisms are outlined. Finally, the different strategies employed by the plant stem and roots to counteract the perceived mechanical stresses are summarized, depicting the corresponding morphological, phytohormonal, and molecular characteristics. The comprehensive literature on both perennial (woody) and annual plants was reviewed, considering the potential benefits and drawbacks of the two plant types, which allowed us to highlight current gaps in knowledge as areas of interest for future research.

Multiple mechanisms behind plant bending

To survive, plants constantly adapt their body shape to their environment. This often involves remarkably rapid bending of their organs such as stems, leaves and roots. Since plant cells are enclosed by stiff cell walls, they use various strategies for bending their organs, which differ from bending mechanisms of soft animal tissues and involve larger physical forces. Here we attempt to summarize and link different viewpoints on bending mechanisms: genes and signalling, mathematical modelling and biomechanics. We argue that quantifying cell growth and physical forces could open a new level in our understanding of bending and resolve some of its paradoxes.

Transmembrane kinase 1-mediated auxin signal regulates membrane-associated clathrin in Arabidopsis roots

Clathrin-mediated endocytosis (CME) is the major endocytic pathway in eukaryotic cells that directly regulates abundance of plasma membrane proteins. Clathrin triskelia are composed of clathrin heavy chains (CHCs) and light chains (CLCs), and the phytohormone auxin differentially regulates membrane-associated CLCs and CHCs, modulating the endocytosis and therefore the distribution of auxin efflux transporter PIN-FORMED2 (PIN2). However, the molecular mechanisms by which auxin regulates clathrin are still poorly understood. Transmembrane kinase (TMKs) family proteins are considered to contribute to auxin signaling and plant development; it remains unclear whether they are involved in PIN transport by CME. We assessed TMKs involvement in the regulation of clathrin by auxin, using genetic, pharmacological, and cytological approaches including live-cell imaging and immunofluorescence. In tmk1 mutant seedlings, auxin failed to rapidly regulate abundance of both CHC and CLC and to inhibit PIN2 endocytosis, leading to an impaired asymmetric distribution of PIN2 and therefore auxin. Furthermore, TMK3 and TMK4 were shown not to be involved in regulation of clathrin by auxin. In summary, TMK1 is essential for auxin-regulated clathrin recruitment and CME. TMK1 therefore plays a critical role in the establishment of an asymmetric distribution of PIN2 and an auxin gradient during root gravitropism.

Endoreplication mediates cell size control via mechanochemical signaling from cell wall

Endoreplication is an evolutionarily conserved mechanism for increasing nuclear DNA content (ploidy). Ploidy frequently scales with final cell and organ size, suggesting a key role for endoreplication in these processes. However, exceptions exist, and, consequently, the endoreplication-size nexus remains enigmatic. Here, we show that prolonged tissue folding at the apical hook in Arabidopsis requires endoreplication asymmetry under the control of an auxin gradient. We identify a molecular pathway linking endoreplication levels to cell size through cell wall remodeling and stiffness modulation. We find that endoreplication is not only permissive for growth: Endoreplication reduction enhances wall stiffening, actively reducing cell size. The cell wall integrity kinase THESEUS plays a key role in this feedback loop. Our data thus explain the nonlinearity between ploidy levels and size while also providing a molecular mechanism linking mechanochemical signaling with endoreplication-mediated dynamic control of cell growth.

New Wine in an Old Bottle: Utilizing Chemical Genetics to Dissect Apical Hook Development

The apical hook is formed by dicot seedlings to protect the tender shoot apical meristem during soil emergence. Regulated by many phytohormones, the apical hook has been taken as a model to study the crosstalk between individual signaling pathways. Over recent decades, the roles of different phytohormones and environmental signals in apical hook development have been illustrated. However, key regulators downstream of canonical hormone signaling have rarely been identified via classical genetics screening, possibly due to genetic redundancy and/or lethal mutation. Chemical genetics that utilize small molecules to perturb and elucidate biological processes could provide a complementary strategy to overcome the limitations in classical genetics. In this review, we summarize current progress in hormonal regulation of the apical hook, and previously reported chemical tools that could assist the understanding of this complex developmental process. We also provide insight into novel strategies for chemical screening and target identification, which could possibly lead to discoveries of new regulatory components in apical hook development, or unidentified signaling crosstalk that is overlooked by classical genetics screening.

Fourteen Stations of Auxin

Auxin has always been at the forefront of research in plant physiology and development. Since the earliest contemplations by Julius von Sachs and Charles Darwin, more than a century-long struggle has been waged to understand its function. This largely reflects the failures, successes, and inevitable progress in the entire field of plant signaling and development. Here I present 14 stations on our long and sometimes mystical journey to understand auxin. These highlights were selected to give a flavor of the field and to show the scope and limits of our current knowledge. A special focus is put on features that make auxin unique among phytohormones, such as its dynamic, directional transport network, which integrates external and internal signals, including self-organizing feedback. Accented are persistent mysteries and controversies. The unexpected discoveries related to rapid auxin responses and growth regulation recently disturbed our contentment regarding understanding of the auxin signaling mechanism. These new revelations, along with advances in technology, usher us into a new, exciting era in auxin research.

Plant cell walls as mechanical signaling hubs for morphogenesis

The instructive role of mechanical cues during morphogenesis is increasingly being recognized in all kingdoms. Patterns of mechanical stress depend on shape, growth and external factors. In plants, the cell wall integrates these three parameters to function as a hub for mechanical feedback. Plant cells are interconnected by cell walls that provide structural integrity and yet are flexible enough to act as both targets and transducers of mechanical cues. Such cues may act locally at the subcellular level or across entire tissues, requiring tight control of both cell-wall composition and cell-cell adhesion. Here we focus on how changes in cell-wall chemistry and mechanics act in communicating diverse cues to direct growth asymmetries required for plant morphogenesis. We explore the role of cellulose microfibrils, microtubule arrays and pectin methylesterification in the transduction of mechanical cues during morphogenesis. Plant hormones can affect the mechanochemical composition of the cell wall and, in turn, the cell wall can modulate hormone signaling pathways, as well as the tissue-level distribution of these hormones. This also leads us to revisit the position of biochemical growth factors, such as plant hormones, acting both upstream and downstream of mechanical signaling. Finally, while the structure of the cell wall is being elucidated with increasing precision, existing data clearly show that the integration of genetic, biochemical and theoretical studies will be essential for a better understanding of the role of the cell wall as a hub for the mechanical control of plant morphogenesis.

Katanin-Dependent Microtubule Ordering in Association with ABA Is Important for Root Hydrotropism

Root hydrotropism refers to root directional growth toward soil moisture. Cortical microtubule arrays are essential for determining the growth axis of the elongating cells in plants. However, the role of microtubule reorganization in root hydrotropism remains elusive. Here, we demonstrate that the well-ordered microtubule arrays and the microtubule-severing protein KATANIN (KTN) play important roles in regulating root hydrotropism in Arabidopsis. We found that the root hydrotropic bending of the ktn1 mutant was severely attenuated but not root gravitropism. After hydrostimulation, cortical microtubule arrays in cells of the elongation zone of wild-type (WT) Col-0 roots were reoriented from transverse into an oblique array along the axis of cell elongation, whereas the microtubule arrays in the ktn1 mutant remained in disorder. Moreover, we revealed that abscisic acid (ABA) signaling enhanced the root hydrotropism of WT and partially rescued the oryzalin (a microtubule destabilizer) alterative root hydrotropism of WT but not ktn1 mutants. These results suggest that katanin-dependent microtubule ordering is required for root hydrotropism, which might work downstream of ABA signaling pathways for plant roots to search for water.

Growth asymmetry precedes differential auxin response during apical hook initiation in Arabidopsis

The development of a hook-like structure at the apical part of the soil-emerging organs has fascinated botanists for centuries, but how it is initiated remains unclear. Here, we demonstrate with high-throughput infrared imaging and 2-D clinostat treatment that, when gravity-induced root bending is absent, apical hook formation still takes place. In such scenarios, hook formation begins with a de novo growth asymmetry at the apical part of a straightly elongating hypocotyl. Remarkably, such de novo asymmetric growth, but not the following hook enlargement, precedes the establishment of a detectable auxin response asymmetry, and is largely independent of auxin biosynthesis, transport and signaling. Moreover, we found that functional cortical microtubule array is essential for the following enlargement of hook curvature. When microtubule array was disrupted by oryzalin, the polar localization of PIN proteins and the formation of an auxin maximum became impaired at the to-be-hook region. Taken together, we propose a more comprehensive model for apical hook initiation, in which the microtubule-dependent polar localization of PINs may mediate the instruction of growth asymmetry that is either stochastically taking place, induced by gravitropic response, or both, to generate a significant auxin gradient that drives the full development of the apical hook.

FERONIA and microtubules independently contribute to mechanical integrity in the Arabidopsis shoot

To survive, cells must constantly resist mechanical stress. In plants, this involves the reinforcement of cell walls, notably through microtubule-dependent cellulose deposition. How wall sensing might contribute to this response is unknown. Here, we tested whether the microtubule response to stress acts downstream of known wall sensors. Using a multistep screen with 11 mutant lines, we identify FERONIA (FER) as the primary candidate for the cell’s response to stress in the shoot. However, this does not imply that FER acts upstream of the microtubule response to stress. In fact, when performing mechanical perturbations, we instead show that the expected microtubule response to stress does not require FER. We reveal that the feronia phenotype can be partially rescued by reducing tensile stress levels. Conversely, in the absence of both microtubules and FER, cells appear to swell and burst. Altogether, this shows that the microtubule response to stress acts as an independent pathway to resist stress, in parallel to FER. We propose that both pathways are required to maintain the mechanical integrity of plant cells.

In all living organisms, cells must resist mechanical stress to survive. This study of the model plant Arabidopsis reveals that the candidate cell wall mechanoreceptor FERONIA and microtubules independently contribute to this mechanical feedback.

TMK-based cell-surface auxin signalling activates cell-wall acidification

The phytohormone auxin controls many processes in plants, at least in part through its regulation of cell expansion1. The acid growth hypothesis has been proposed to explain auxin-stimulated cell expansion for five decades, but the mechanism that underlies auxin-induced cell-wall acidification is poorly characterized. Auxin induces the phosphorylation and activation of the plasma membrane H+-ATPase that pumps protons into the apoplast2, yet how auxin activates its phosphorylation remains unclear. Here we show that the transmembrane kinase (TMK) auxin-signalling proteins interact with plasma membrane H+-ATPases, inducing their phosphorylation, and thereby promoting cell-wall acidification and hypocotyl cell elongation in Arabidopsis. Auxin induced interactions between TMKs and H+-ATPases in the plasma membrane within seconds, as well as TMK-dependent phosphorylation of the penultimate threonine residue on the H+-ATPases. Our genetic, biochemical and molecular evidence demonstrates that TMKs directly phosphorylate plasma membrane H+-ATPase and are required for auxin-induced H+-ATPase activation, apoplastic acidification and cell expansion. Thus, our findings reveal a crucial connection between auxin and plasma membrane H+-ATPase activation in regulating apoplastic pH changes and cell expansion through TMK-based cell surface auxin signalling.

Bending during seedling emergence: Mechanochemical insight

The apical hook is a crucial structure during seedling development in dicotyledonous plants. It protects the fragile shoot meristem during its journey toward the surface from constraints imposed by the surrounding soil, which safeguards seedling emergence. Emerging evidence sheds light on the regulation of hook development through mechanochemical constraints.

The microtubule-associated protein WDL4 modulates auxin distribution to promote apical hook opening in Arabidopsis

The unique apical hook in dicotyledonous plants protects the shoot apical meristem and cotyledons when seedlings emerge through the soil. Its formation involves differential cell growth under the coordinated control of plant hormones, especially ethylene and auxin. Microtubules are essential players in plant cell growth that are regulated by multiple microtubule-associated proteins (MAPs). However, the role and underlying mechanisms of MAP-microtubule modules in differential cell growth are poorly understood. In this study, we found that the previously uncharacterized Arabidopsis MAP WAVE-DAMPENED2-LIKE4 (WDL4) protein plays a positive role in apical hook opening. WDL4 exhibits a temporal expression pattern during hook development in dark-grown seedlings that is directly regulated by ethylene signaling. WDL4 mutants showed a delayed hook opening phenotype while overexpression of WDL4 resulted in enhanced hook opening. In particular, wdl4-1 mutants exhibited stronger auxin accumulation in the concave side of the apical hook. Furthermore, the regulation of the auxin maxima and trafficking of the auxin efflux carriers PIN-FORMED1 (PIN1) and PIN7 in the hook region is critical for WDL4-mediated hook opening. Together, our study demonstrates that WDL4 positively regulates apical hook opening by modulating auxin distribution, thus unraveling a mechanism for MAP-mediated differential plant cell growth.

GR24, A Synthetic Strigolactone Analog, and Light Affect the Organization of Cortical Microtubules in Arabidopsis Hypocotyl Cells

Strigolactones are plant hormones regulating cytoskeleton-mediated developmental events in roots, such as lateral root formation and elongation of root hairs and hypocotyls. The latter process was addressed herein by the exogenous application of a synthetic strigolactone, GR24, and an inhibitor of strigolactone biosynthesis, TIS108, on hypocotyls of wild-type Arabidopsis and a strigolactone signaling mutant max2-1 (more axillary growth 2-1). Owing to the interdependence between light and strigolactone signaling, the present work was extended to seedlings grown under a standard light/dark regime, or under continuous darkness. Given the essential role of the cortical microtubules in cell elongation, their organization and dynamics were characterized under the conditions of altered strigolactone signaling using fluorescence microscopy methods with different spatiotemporal capacities, such as confocal laser scanning microscopy (CLSM) and structured illumination microscopy (SIM). It was found that GR24-dependent inhibition of hypocotyl elongation correlated with changes in cortical microtubule organization and dynamics, observed in living wild-type and max2-1 seedlings stably expressing genetically encoded fluorescent molecular markers for microtubules. Quantitative assessment of microscopic datasets revealed that chemical and/or genetic manipulation of strigolactone signaling affected microtubule remodeling, especially under light conditions. The application of GR24 in dark conditions partially alleviated cytoskeletal rearrangement, suggesting a new mechanistic connection between cytoskeletal behavior and the light-dependence of strigolactone signaling.

Cell Wall and Hormone Interplay Controls Growth Asymmetry

Plant cell elongation and expansion require the biosynthesis and remodeling of cell wall composition. Recently, Aryal et al. reported how feedback between the cell wall and the auxin response controls differential growth in apical hook development.

Ethylene Signaling Facilitates Plant Adaption to Physical Barriers

The morphological changes are usually observed in the terrestrial plants to respond to physical barriers. The phytohormone ethylene plays an essential role in the morphological development of plants encountering exogenous mechanical impedance, which enables plants to grow optimally in response to physical barriers. Ethylene is shown to regulate these developmental processes directly or in concert with other phytohormones, especially auxin. In this mini review, the involvement of ethylene action in seedling emergence from the soil, root movement within the soil, and parasitic plant invasion of the host plant are described.