Cortical microtubules optimize cell-wall crystallinity to drive unidirectional growth in Arabidopsis

The heat shock factor 20-HSF4-cellulose synthase A2 module regulates heat stress tolerance in maize

Temperature shapes the geographical distribution and behavior of plants. Understanding the regulatory mechanisms underlying the plant heat stress response is important for developing climate-resilient crops, including maize (Zea mays). To identify transcription factors (TFs) that may contribute to the maize heat stress response, we generated a dataset of short- and long-term transcriptome changes following a heat treatment time course in the inbred line B73. Co-expression network analysis highlighted several TFs, including the class B2a heat shock factor (HSF) ZmHSF20. Zmhsf20 mutant seedlings exhibited enhanced tolerance to heat stress. Furthermore, DNA affinity purification sequencing and Cleavage Under Targets and Tagmentation assays demonstrated that ZmHSF20 binds to the promoters of Cellulose synthase A2 (ZmCesA2) and three class A Hsf genes, including ZmHsf4, repressing their transcription. We showed that ZmCesA2 and ZmHSF4 promote the heat stress response, with ZmHSF4 directly activating ZmCesA2 transcription. In agreement with the transcriptome analysis, ZmHSF20 inhibited cellulose accumulation and repressed the expression of cell wall-related genes. Importantly, the Zmhsf20 Zmhsf4 double mutant exhibited decreased thermotolerance, placing ZmHsf4 downstream of ZmHsf20. We proposed an expanded model of the heat stress response in maize, whereby ZmHSF20 lowers seedling heat tolerance by repressing ZmHsf4 and ZmCesA2, thus balancing seedling growth and defense.

Heterogeneous identity, stiffness and growth characterise the shoot apex of Arabidopsis stem cell mutants

Stem cell homeostasis in the shoot apical meristem involves a core regulatory feedback loop between the signalling peptide CLAVATA3 (CLV3), produced in stem cells, and the transcription factor WUSCHEL, expressed in the underlying organising centre. clv3 mutant meristems display massive overgrowth, which is thought to be caused by stem cell overproliferation, although it is unknown how uncontrolled stem cell divisions lead to this altered morphology. Here, we reveal local buckling defects in mutant meristems, and use analytical models to show how mechanical properties and growth rates may contribute to the phenotype. Indeed, clv3 mutant meristems are mechanically more heterogeneous than the wild type, and also display regional growth heterogeneities. Furthermore, stereotypical wild-type meristem organisation, in which cells simultaneously express distinct fate markers, is lost in mutants. Finally, cells in mutant meristems are auxin responsive, suggesting that they are functionally distinguishable from wild-type stem cells. Thus, all benchmarks show that clv3 mutant meristem cells are different from wild-type stem cells, suggesting that overgrowth is caused by the disruption of a more complex regulatory framework that maintains distinct genetic and functional domains in the meristem.

Growth and tension in explosive fruit

Exploding seed pods of the common weed Cardamine hirsuta have the remarkable ability to launch seeds far from the plant. The energy for this explosion comes from tension that builds up in the fruit valves. Above a critical threshold, the fruit fractures along its dehiscence zone and the two valves coil explosively, ejecting the seeds. A common mechanism to generate tension is drying, causing tissues to shrink. However, this does not happen in C. hirsuta fruit. Instead, tension is produced by active contraction of growing exocarp cells in the outer layer of the fruit valves. Exactly how growth causes the exocarp tissue to contract and generate pulling force is unknown. Here we show that the reorientation of microtubules in the exocarp cell cortex changes the orientation of cellulose microfibrils in the cell wall and the consequent cellular growth pattern. We used mechanical modeling to show how tension emerges through growth due to the highly anisotropic orientation of load-bearing cellulose microfibrils and their effect on cell shape. By explicitly defining the cell wall as multi-layered in our model, we discovered that a cross-lamellate pattern of cellulose microfibrils further enhances the developing tension in growing cells. Therefore, the interplay of cell wall properties with turgor-driven growth enables the fruit exocarp to generate sufficient tension to power explosive seed dispersal.

Open questions in plant cell wall synthesis

Plant cells are surrounded by strong yet flexible polysaccharide-based cell walls that support the cell while also allowing growth by cell expansion. Plant cell wall research has advanced tremendously in recent years. Sequenced genomes of many model and crop plants have facilitated cataloging and characterization of many enzymes involved in cell wall synthesis. Structural information has been generated for several important cell wall synthesizing enzymes. Important tools have been developed including antibodies raised against a variety of cell wall polysaccharides and glycoproteins, collections of enzyme clones and synthetic glycan arrays for characterizing enzymes, herbicides that specifically affect cell wall synthesis, live-cell imaging probes to track cell wall synthesis, and an inducible secondary cell wall synthesis system. Despite these advances, and often because of the new information they provide, many open questions about plant cell wall polysaccharide synthesis persist. This article highlights some of the key questions that remain open, reviews the data supporting different hypotheses that address these questions, and discusses technological developments that may answer these questions in the future.

DEFECTIVE KERNEL1 regulates cellulose synthesis and affects primary cell wall mechanics

The cell wall is one of the defining features of plants, controlling cell shape, regulating growth dynamics and hydraulic conductivity, as well as mediating plants interactions with both the external and internal environments. Here we report that a putative mechanosensitive Cys-protease DEFECTIVE KERNEL1 (DEK1) influences the mechanical properties of primary cell walls and regulation of cellulose synthesis. Our results indicate that DEK1 is an important regulator of cellulose synthesis in epidermal tissue of Arabidopsis thaliana cotyledons during early post-embryonic development. DEK1 is involved in regulation of cellulose synthase complexes (CSCs) by modifying their biosynthetic properties, possibly through interactions with various cellulose synthase regulatory proteins. Mechanical properties of the primary cell wall are altered in DEK1 modulated lines with DEK1 affecting both cell wall stiffness and the thickness of the cellulose microfibril bundles in epidermal cell walls of cotyledons.

Dinoflagellate Amphiesmal Dynamics: Cell Wall Deposition with Ecdysis and Cellular Growth

Dinoflagellates are a major aquatic protist group with amphiesma, multiple cortical membranous "cell wall" layers that contain large circum-cortical alveolar sacs (AVs). AVs undergo extensive remodeling during cell- and life-cycle transitions, including ecdysal cysts (ECs) and resting cysts that are important in some harmful algal bloom initiation-termination. AVs are large cortical vesicular compartments, within which are elaborate cellulosic thecal plates (CTPs), in thecate species, and the pellicular layer (PL). AV-CTPs provide cellular mechanical protection and are targets of vesicular transport that are replaced during EC-swarmer cell transition, or with increased deposition during the cellular growth cycle. AV-PL exhibits dynamical-replacement with vesicular trafficking that are orchestrated with amphiesmal chlortetracycline-labeled Ca²⁺ stores signaling, integrating cellular growth with different modes of cell division cycle/progression. We reviewed the dynamics of amphiesma during different cell division cycle modes and life cycle stages, and its multifaceted regulations, focusing on the regulatory and functional readouts, including the coral-zooxanthellae interactions.

Cellulose synthesis in land plants

All plant cells are surrounded by a cell wall that provides cohesion, protection, and a means of directional growth to plants. Cellulose microfibrils contribute the main biomechanical scaffold for most of these walls. The biosynthesis of cellulose, which typically is the most prominent constituent of the cell wall and therefore Earth's most abundant biopolymer, is finely attuned to developmental and environmental cues. Our understanding of the machinery that catalyzes and regulates cellulose biosynthesis has substantially improved due to recent technological advances in, for example, structural biology and microscopy. Here, we provide a comprehensive overview of the structure, function, and regulation of the cellulose synthesis machinery and its regulatory interactors. We aim to highlight important knowledge gaps in the field, and outline emerging approaches that promise a means to close those gaps.

Tools for studying the cytoskeleton during plant cell division

The plant cytoskeleton regulates fundamental biological processes, including cell division. How to experimentally perturb the cytoskeleton is a key question if one wants to understand the role of both actin filaments (AFs) and microtubules (MTs) in a given biological process. While a myriad of mutants are available, knock-out in cytoskeleton regulators, when nonlethal, often produce little or no phenotypic perturbation because such regulators are often part of a large family, leading to functional redundancy. In this review, alternative techniques to modify the plant cytoskeleton during plant cell division are outlined. The different pharmacological and genetic approaches already developed in cell culture, transient assays, or in whole organisms are presented. Perspectives on the use of optogenetics to perturb the plant cytoskeleton are also discussed.

Regulation of cellulose synthesis via exocytosis and endocytosis

Cellulose is a critical component of plant cell walls. Cellulose is made at the plasma membrane by cellulose synthase (CESA) enzymes organized into large, multi-subunit cellulose synthase complexes (CSCs). Although CESAs are only active at the plasma membrane, fluorescently-tagged CESAs also substantially label the Golgi apparatus and other intracellular compartments, even when cellulose synthesis rates are high. These data imply that CESA activity is regulated by trafficking to the plasma membrane (exocytosis) and removal from the plasma membrane (endocytosis), as well as recycling of endocytosed CESAs back to the plasma membrane. Key molecular components and events of CESA exocytosis and endocytosis have recently been defined, primarily using mutant analysis and live-cell imaging in Arabidopsis thaliana. Here, we integrate these data into a working model of CESA regulation by exocytosis and endocytosis and highlight key outstanding questions. We present the hypothesis that cycling of CESAs between the plasma membrane and the endomembrane system is important for regulating cellulose synthesis and for maintaining a robust population of active CSCs in the plasma membrane.

The Combined Effect of Heat and Osmotic Stress on Suberization of Arabidopsis Roots

The simultaneous occurrence of heat stress and drought is becoming more regular as a consequence of climate change, causing extensive agricultural losses. The application of either heat or osmotic stress increase cell-wall suberization in different tissues, which may play a role in improving plant resilience. In this work, we studied how the suberization process is affected by the combination of drought and heat stress by following the expression of suberin biosynthesis genes, cell-wall suberization and the chemical composition in Arabidopsis roots. The Arabidopsis plants used in this study were at the onset of secondary root development. At this point, one can observe a developmental gradient in the main root, with primary development closer to the root tip and secondary development, confirmed by the suberized phellem, closer to the shoot. Remarkably, we found a differential response depending on the root zone. The combination of drought and heat stress increased cell wall suberization in main root segments undergoing secondary development and in lateral roots (LRs), while the main root zone, at primary development stage, was not particularly affected. We also found differences in the overall chemical composition of the cell walls in both root zones in response to combined stress. The data gathered showed that, under combined drought and heat stress, Arabidopsis roots undergo differential cell wall remodeling depending on developmental stage, with modifications in the biosynthesis and/or assembly of major cell wall components.

Building an extensible cell wall

This article recounts, from my perspective of four decades in this field, evolving paradigms of primary cell wall structure and the mechanism of surface enlargement of growing cell walls. Updates of the structures, physical interactions, and roles of cellulose, xyloglucan, and pectins are presented. This leads to an example of how a conceptual depiction of wall structure can be translated into an explicit quantitative model based on molecular dynamics methods. Comparison of the model's mechanical behavior with experimental results provides insights into the molecular basis of complex mechanical behaviors of primary cell wall and uncovers the dominant role of cellulose-cellulose interactions in forming a strong yet extensible network.

Arabinogalactan Proteins: Focus on the Role in Cellulose Synthesis and Deposition during Plant Cell Wall Biogenesis

Arabinogalactan proteins (AGPs) belong to a family of glycoproteins that are widely present in plants. AGPs are mostly composed of a protein backbone decorated with complex carbohydrate side chains and are usually anchored to the plasma membrane or secreted extracellularly. A trickle of compelling biochemical and genetic evidence has demonstrated that AGPs make exciting candidates for a multitude of vital activities related to plant growth and development. However, because of the diversity of AGPs, functional redundancy of AGP family members, and blunt-force research tools, the precise functions of AGPs and their mechanisms of action remain elusive. In this review, we put together the current knowledge about the characteristics, classification, and identification of AGPs and make a summary of the biological functions of AGPs in multiple phases of plant reproduction and developmental processes. In addition, we especially discuss deeply the potential mechanisms for AGP action in different biological processes via their impacts on cellulose synthesis and deposition based on previous studies. Particularly, five hypothetical models that may explain the AGP involvement in cellulose synthesis and deposition during plant cell wall biogenesis are proposed. AGPs open a new avenue for understanding cellulose synthesis and deposition in plants.

Functional characterization of a cellulose synthase, CtCESA1, from the marine red alga Calliarthron tuberculosum (Corallinales)

In land plants and algae, cellulose is important for strengthening cell walls and preventing breakage due to physical forces. Though our understanding of cellulose production by cellulose synthases (CESAs) has seen significant advances for several land plant and bacterial species, functional characterization of this fundamental protein is absent in red algae. Here we identify CESA gene candidates in the calcifying red alga Calliarthron tuberculosum using sequence similarity-based approaches, and elucidate their phylogenetic relationship with other CESAs from diverse taxa. One gene candidate, CtCESA1, was closely related to other putative red algal CESA genes. To test if CtCESA1 encoded a true cellulose synthase, CtCESA1 protein was expressed and purified from insect and yeast expression systems. CtCESA1 showed glucan synthase activity in glucose tracer assays. CtCESA1 activity was relatively low when compared with plant and bacterial CESA activity. In an in vitro assay, a predicted N-terminal starch-binding domain from CtCESA1 bound red algal floridean starch extracts, representing a unique domain in red algal CESAs not present in CESAs from other lineages. When the CtCESA1 gene was introduced into Arabidopsis thaliana cesa mutants, the red algal CtCESA1 partially rescued the growth defects of the primary cell wall cesa6 mutant, but not cesa3 or secondary cell wall cesa7 mutants. A fluorescently tagged CtCESA1 localized to the plasma membrane in the Arabidopsis cesa6 mutant background. This study presents functional evidence validating the sequence annotation of red algal CESAs. The relatively low activity of CtCESA1, partial complementation in Arabidopsis, and presence of unique protein domains suggest that there are probably functional differences between the algal and land plant CESAs.

Cell biology of primary cell wall synthesis in plants

Building a complex structure such as the cell wall, with many individual parts that need to be assembled correctly from distinct sources within the cell, is a well-orchestrated process. Additional complexity is required to mediate dynamic responses to environmental and developmental cues. Enzymes, sugars, and other cell wall components are constantly and actively transported to and from the plasma membrane during diffuse growth. Cell wall components are transported in vesicles on cytoskeletal tracks composed of microtubules and actin filaments. Many of these components, and additional proteins, vesicles, and lipids are trafficked to and from the cell plate during cytokinesis. In this review, we first discuss how the cytoskeleton is initially organized to add new cell wall material or to build a new cell wall, focusing on similarities during these processes. Next, we discuss how polysaccharides and enzymes that build the cell wall are trafficked to the correct location by motor proteins and through other interactions with the cytoskeleton. Finally, we discuss some of the special features of newly formed cell walls generated during cytokinesis.

Analysis of cellulose synthase activity in Arabidopsis using spinning disk microscopy

We describe sample preparation and visualization of fluorescently tagged cellulose synthases in cellulose synthase complexes at the plasma membrane of Arabidopsis hypocotyl epidermal cells using live-cell imaging via spinning disk microscopy. We present a technique for sample mounting that may be suitable for imaging other samples. Additionally, we offer free, open-source solutions for image analysis and provide extensive troubleshooting suggestions. For complete information on the use and execution of this protocol, please refer to McFarlane et al., 2021.

Brassinosteroids promote parenchyma cell and secondary xylem development in sugar beet (Beta vulgaris L.) root

Increasing crop yield has always been an important goal in agriculture. Brassinosteroids (BRs) are growth-promoting steroid hormones with vital roles in many root developmental processes. Sugar beet (Beta vulgaris L.) is a root crop with a tertiary root structure. The differentiation of vascular bundles and the division of cambial cells increase root diameter. However, little is known about how BRs regulate the transverse growth of beetroot. Therefore, sugar beet with eight leaves was grown in medium containing epibrassinolide or brassinazole, an inhibitor of BR biosynthesis. BRs increased the spacing between the cambial rings by increasing the size of parenchyma cells between the rings and ultimately increasing root diameter. BRs also promoted secondary xylem differentiation. Moreover, the gene expression analysis of BvXTH33, BvSHV3, BvCESA6, BvPARVUS, and BvCEL1, which were related to the cell wall biosynthesis, indicated that BR could promote the growth of cell wall. These findings showed that BRs function in transverse development in beetroot.

Tailoring renewable materials via plant biotechnology

Plants inherently display a rich diversity in cell wall chemistry, as they synthesize an array of polysaccharides along with lignin, a polyphenolic that can vary dramatically in subunit composition and interunit linkage complexity. These same cell wall chemical constituents play essential roles in our society, having been isolated by a variety of evolving industrial processes and employed in the production of an array of commodity products to which humans are reliant. However, these polymers are inherently synthesized and intricately packaged into complex structures that facilitate plant survival and adaptation to local biogeoclimatic regions and stresses, not for ease of deconstruction and commercial product development. Herein, we describe evolving techniques and strategies for altering the metabolic pathways related to plant cell wall biosynthesis, and highlight the resulting impact on chemistry, architecture, and polymer interactions. Furthermore, this review illustrates how these unique targeted cell wall modifications could significantly extend the number, diversity, and value of products generated in existing and emerging biorefineries. These modifications can further target the ability for processing of engineered wood into advanced high performance materials. In doing so, we attempt to illuminate the complex connection on how polymer chemistry and structure can be tailored to advance renewable material applications, using all the chemical constituents of plant-derived biopolymers, including pectins, hemicelluloses, cellulose, and lignins.

The thickening and modification of the galactan-enriched layer during primary phloem fibre development in Cannabis sativa

Primary phloem fibres (PPFs) have higher fibre quality and are economically more important for the textile sector than secondary phloem fibres. Both the chemical composition and mechanical structure of the secondary cell wall mainly influence the quality of bast fibres. We investigated the thickening of the galactan-enriched (Gn) layer and its modification process into a gelatinous (G)-layer, which is the largest portion of the secondary cell wall, during the development of the PPF in Cannabis sativa. Stem segments of hemp collected at 17, 29, 52 and 62 days after sowing were comparatively examined using light microscopy, scanning electron microscopy and transmission electron microscopy. The initial cells of PPF started the proliferation and differentiation at 17 days, but the secondary cell wall thickening had already commenced before the 29 days. Both the G- and Gn-layer were rapidly added onto the S-layer of PPFs; thus, the secondary cell wall thickness increased approximately 2-fold at 52 days (from the 29-day mark), and 8-fold at 62 days. The cortical microtubule arrays appeared adjacent to the plasma membrane of PPF cells related to the cellulose synthesis. Additionally, cross-sectioned microfibrils were observed on Gn-layer as the cluster of tiny spots. At 62 days, the specific stratification structure consisting of several lamellae occurred on the G-layer of the secondary cell wall. The secondary cell wall thickened remarkably at 52 days through 62 days so that the mature secondary cell wall consisted of three distinctive layers, the S-, G- and Gn-layer. Cortical microtubule arrays frequently appeared adjacent to the plasma membrane together with cellulose microfibrils on secondary cell wall. The G-layer of PPF at 62 days exhibited the characteristic stratification structure, which demonstrates the modification of the Gn-layer into the G-layer.

The functions of kinesin and kinesin-related proteins in eukaryotes

Kinesins constitute a superfamily of ATP-driven microtubule motor enzymes that convert the chemical energy of ATP hydrolysis into mechanical work along microtubule tracks. Kinesins are found in all eukaryotic organisms and are essential to all eukaryotic cells, involved in diverse cellular functions such as microtubule dynamics and morphogenesis, chromosome segregation, spindle formation and elongation and transport of organelles. In this review, we explore recently reported functions of kinesins in eukaryotes and compare their specific cargoes in both plant and animal kingdoms to understand the possible roles of uncharacterized motors in a kingdom based on their reported functions in other kingdoms.

Metabolism of abscisic acid in two contrasting rice genotypes submitted to recurrent water deficit

Drought is the main constrain for crops worldwide, however, the effects of recurrent water deficit remain still hidden. We analysed two rice genotypes, 'BRS-Querência' (lowlands) and 'AN-Cambará' (uplands), after 7 days of recurrent drought followed by 24 h of rehydration, hypothesising that genotypes grown in regions with different water availabilities respond differently to water deficits, and that a previous exposure to stress could alter abscisic acid (ABA) metabolism. The results showed that both genotypes reduced stomatal conductance and increased ABA concentration. After rehydration, the ABA levels decreased, mainly in the plants of BRS-Querência subjected to recurrent stress. However, the levels of ABA were higher in plants in recurrent water deficit compared to non-recurrent stress plants in both genotypes. Remarkably in the lowland genotype, the ABA glucosyl-ester (ABA-GE) concentration increased after recovery in the plants under recurrent stress. Regarding of gene expression, the genes associated in ABA biosynthesis with the highest expression levels were NCED2, NCED3, NCED4 and AAO2. However, 'AN-Cambará' showed less transcriptional activation. Taking into account the genes involved in ABA catabolism, ABAH1 appears to play an important role related to the recurrent stress in upland plants. These results indicate that one of the factors that can promote greater tolerance for the upland genotype is the tradeoff between ABA and ABA-GE when plants are subjected to water deficits. In addition, they indicate that abscisic acid metabolism is altered due to the genotype (upland or lowland) and pre-exposure to stress can also modify adaptive responses in rice varieties (recurrent stress).

Structure of the Bacterial Cellulose Ribbon and Its Assembly-Guiding Cytoskeleton by Electron Cryotomography

This work’s relevance for the microbiology community is twofold. It delivers for the first time high-resolution near-native snapshots of Gluconacetobacter spp. (previously Komagataeibacter spp.) in the process of cellulose ribbon synthesis, in their native biofilm environment.

Cellulose is a widespread component of bacterial biofilms, where its properties of exceptional water retention, high tensile strength, and stiffness prevent dehydration and mechanical disruption of the biofilm. Bacteria in the genus Gluconacetobacter secrete crystalline cellulose, with a structure very similar to that found in plant cell walls. How this higher-order structure is produced is poorly understood. We used cryo-electron tomography and focused-ion-beam milling of native bacterial biofilms to image cellulose-synthesizing Gluconacetobacter hansenii and Gluconacetobacter xylinus bacteria in a frozen-hydrated, near-native state. We confirm previous results suggesting that cellulose crystallization occurs serially following its secretion along one side of the cell, leading to a cellulose ribbon that can reach several micrometers in length and combine with ribbons from other cells to form a robust biofilm matrix. We were able to take direct measurements in a near-native state of the cellulose sheets. Our results also reveal a novel cytoskeletal structure, which we have named the cortical belt, adjacent to the inner membrane and underlying the sites where cellulose is seen emerging from the cell. We found that this structure is not present in other cellulose-synthesizing bacterial species, Agrobacterium tumefaciens and Escherichia coli 1094, which do not produce organized cellulose ribbons. We therefore propose that the cortical belt holds the cellulose synthase complexes in a line to form higher-order cellulose structures, such as sheets and ribbons.

IMPORTANCE This work’s relevance for the microbiology community is twofold. It delivers for the first time high-resolution near-native snapshots of Gluconacetobacter spp. (previously Komagataeibacter spp.) in the process of cellulose ribbon synthesis, in their native biofilm environment. It puts forward a noncharacterized cytoskeleton element associated with the side of the cell where the cellulose synthesis occurs. This represents a step forward in the understanding of the cell-guided process of crystalline cellulose synthesis, studied specifically in the Gluconacetobacter genus and still not fully understood. Additionally, our successful attempt to use cryo-focused-ion-beam milling through biofilms to image the cells in their native environment will drive the community to use this tool for the morphological characterization of other studied biofilms.

Cell Wall Biology: Dual Control of Cellulose Synthase Guidance

Cortical microtubules can direct the orientation of newly synthesized cellulose fibres in plant cell walls. However, cell wall-mediated steering mechanisms have also been anticipated. New research reveals that cellulose synthesis may be directed by pre-existing cellulose fibres in the walls.

Long-term live-cell imaging techniques for visualizing pavement cell morphogenesis

Recent advancements in microscopy and biological technologies have allowed scientists to study dynamic plant developmental processes with high temporal and spatial resolution. Pavement cells, epidermal cells found on leaf tissue, form complex shapes with alternating regions of indentations and outgrowths that are postulated to be driven by the microtubule cytoskeleton. Given their complex shapes, pavement cells and the microtubule contribution towards morphogenesis have been of great interest in the field of developmental biology. Here, we focus on two live-cell imaging methods that allow for early and long-term imaging of the cotyledon (embryonic leaf-like tissue) and leaf epidermis with minimal invasiveness in order to study microtubules throughout pavement cell morphogenesis. The methods described in this chapter can be applied to studying other developmental processes associated with cotyledon and leaf tissue.

Mutation of rice bc1 gene affects internode elongation and induces delayed cell wall deposition in developing internodes

A rice COBRA-like gene, BRITTLE CULM1 (BC1) has been shown to be involved in assembling cell wall components and cellulose crystallinity, which determines mechanical strength in above ground organs. However, the detailed roles of BC1 in rice development are poorly understood. In this study, we found that, unlike the known brittle culm mutants, the internode length of the bc1 mutant was ~1.27 times longer than that of wild type in rice. In order to analyze the effects of bc1 mutation on internode development, we compared the deposition of cell wall components among each developmental stage of the elongating second internodes from wild type, Kinmaze, and the bc1 mutant. In wild type, histochemical observations of lignin revealed that lignin deposition was gradually increased after the cell elongation stage of the internodes. Cellulose and p-coumaric acid (pCA) content also gradually increased along with the progress of the developmental stage. The ferulic acid (FA) content rapidly increased in the cell elongation stage and decreased at the late secondary cell wall formation stage. In the bc1 mutant, the contents of cell wall components were lower than those of wild type from the cell elongation stage, in which the BC1 started to express at this stage in wild type. In the bc1 mutant, the deposition patterns of cell wall components, especially phenolic components including lignin, pCA, and FA, were delayed compared with those of wild type. These results suggest that the BC1 gene plays a role in synthesizing appropriate cell walls at each stage in the developing internode.

Xyloglucan endotransglucosylase-hydrolase30 negatively affects salt tolerance in Arabidopsis

Plants have evolved various strategies to sense and respond to saline environments, which severely reduce plant growth and limit agricultural productivity. Alteration to the cell wall is one strategy that helps plants adapt to salt stress. However, the physiological mechanism of how the cell wall components respond to salt stress is not fully understood. Here, we show that expression of XTH30, encoding xyloglucan endotransglucosylase-hydrolase30, is strongly up-regulated in response to salt stress in Arabidopsis. Loss-of-function of XTH30 leads to increased salt tolerance and overexpression of XTH30 results in salt hypersensitivity. XTH30 is located in the plasma membrane and is highly expressed in the root, flower, stem, and etiolated hypocotyl. The NaCl-induced increase in xyloglucan (XyG)-derived oligosaccharide (XLFG) of the wild type is partly blocked in xth30 mutants. Loss-of-function of XTH30 slows down the decrease of crystalline cellulose content and the depolymerization of microtubules caused by salt stress. Moreover, lower Na+ accumulation in shoot and lower H2O2 content are found in xth30 mutants in response to salt stress. Taken together, these results indicate that XTH30 modulates XyG side chains, altered abundance of XLFG, cellulose synthesis, and cortical microtubule stability, and negatively affecting salt tolerance.

Connected through the force: mechanical signals in plant development

As multicellular organisms, plants acquire characteristic shapes through a complex set of biological processes known as morphogenesis. Biochemical signalling underlies much of development, as it allows cells to acquire specific identities based on their position within tissues and organs. However, as growing physical structures, plants, and their constituent cells, also experience internal and external physical forces that can be perceived and can influence key processes such as growth, polarity, and gene expression. This process, which adds another layer of control to growth and development, has important implications for plant morphogenesis. This review provides an overview of recent research into the role of mechanical signals in plant development and aims to show how mechanical signalling can be used, in concert with biochemical signals, as a cue allowing cells and tissues to coordinate their behaviour and to add robustness to developmental processes.

Primary wall cellulose synthase regulates shoot apical meristem mechanics and growth

How organisms attain their specific shapes and modify their growth patterns in response to environmental and chemical signals has been the subject of many investigations. Plant cells are at high turgor pressure and are surrounded by a rigid yet flexible cell wall, which is the primary determinant of plant growth and morphogenesis. Cellulose microfibrils, synthesized by plasma membrane-localized cellulose synthase complexes, are major tension-bearing components of the cell wall that mediate directional growth. Despite advances in understanding the genetic and biophysical regulation of morphogenesis, direct studies of cellulose biosynthesis and its impact on morphogenesis of different cell and tissue types are largely lacking. In this study, we took advantage of mutants of three primary cellulose synthase (CESA) genes that are involved in primary wall cellulose synthesis. Using field emission scanning electron microscopy, live cell imaging and biophysical measurements, we aimed to understand how the primary wall CESA complex acts during shoot apical meristem development. Our results indicate that cellulose biosynthesis impacts the mechanics and growth of the shoot apical meristem.

Anisotropic growth is achieved through the additive mechanical effect of material anisotropy and elastic asymmetry

Fast directional growth is a necessity for the young seedling; after germination, it needs to quickly penetrate the soil to begin its autotrophic life. In most dicot plants, this rapid escape is due to the anisotropic elongation of the hypocotyl, the columnar organ between the root and the shoot meristems. Anisotropic growth is common in plant organs and is canonically attributed to cell wall anisotropy produced by oriented cellulose fibers. Recently, a mechanism based on asymmetric pectin-based cell wall elasticity has been proposed. Here we present a harmonizing model for anisotropic growth control in the dark-grown Arabidopsis thaliana hypocotyl: basic anisotropic information is provided by cellulose orientation) and additive anisotropic information is provided by pectin-based elastic asymmetry in the epidermis. We quantitatively show that hypocotyl elongation is anisotropic starting at germination. We present experimental evidence for pectin biochemical differences and wall mechanics providing important growth regulation in the hypocotyl. Lastly, our in silico modelling experiments indicate an additive collaboration between pectin biochemistry and cellulose orientation in promoting anisotropic growth.

The effects of Bisphenol A on the seagrass Cymodocea nodosa: Leaf elongation impairment and cytoskeleton disturbance

Bisphenol A (BPA) is an emerging pollutant of environmental concern, classified as "moderately toxic" and "toxic", causing adverse effects on aquatic biota. Although information about BPA toxicity on aquatic fauna is available, the data about BPA effects on aquatic flora remain scarce, missing for marine macrophytes. The effects of environmentally relevant BPA concentrations (ranging from 0.03 to 3 μg L^-1) on juvenile leaf elongation and the cytoskeleton (microtubules, MTs and actin filaments, AFs) were studied in the seagrass Cymodocea nodosa for 1-10 days. The suitability of cytoskeleton disturbance and leaf elongation impairment as "biomarkers" for BPA stress were tested. The highest BPA concentrations (0.3, 0.5, 1 and 3 μg L^-1) affected significantly leaf elongation from the onset of the experiment, while defects of the cytoskeleton were observed even at lower concentrations. In particular, MTs were initially disrupted (i.e. "lowest observed effect concentrations", LOECs) at 0.1 μg L^-1, while AFs were damaged even at 0.03 μg L^-1. AFs appeared thus to be more sensitive to lower BPA concentrations, while there was a correlation between leaf elongation impairment and MT defects. Thus, AF damages, MT disruption and leaf elongation impairment in C. nodosa, in this particular order, appear to be sensitive "biomarkers" of BPA stress, at the above environmentally relevant BPA concentrations.

EB1 contributes to microtubule bundling and organization, along with root growth, in Arabidopsis thaliana

Microtubules are involved in plant development and adaptation to their environment, but the sustaining molecular mechanisms remain elusive. Microtubule-end-binding 1 (EB1) proteins participate in directional root growth in Arabidopsis thaliana. However, a connection to the underlying microtubule array has not been established yet. We show here that EB1 proteins contribute to the organization of cortical microtubules in growing epidermal plant cells, without significant modulation of microtubule dynamics. Using super-resolution stimulated emission depletion (STED) microscopy and an original quantification approach, we also demonstrate a significant reduction of apparent microtubule bundling in cytoplasmic-EB1-deficient plants, suggesting a function for EB1 in the interaction between adjacent microtubules. Furthermore, we observed root growth defects in EB1-deficient plants, which are not related to cell division impairment. Altogether, our results support a role for EB1 proteins in root development, in part by maintaining the organization of cortical microtubules.

This article has an associated First Person interview with the first author of the paper.

Summary: EB1 proteins affect cortical-microtubule bundling and organization in Arabidopsis thaliana, without significant modulation of microtubule dynamics. They also participate in root growth, further linking microtubules to plant development.

Exploring Microtubule-Dependent Cellulose-Synthase-Complex Movement with High Precision Particle Tracking

Cellulose synthesis at the plasma membrane is a critical process in plant growth and development. The displacement of cellulose synthase complexes (CSCs) by the rigid cellulose polymers they produce is a measure of enzyme activity. Connections between cortical microtubules and CSCs have been identified but it remains unclear how these affect CSC displacement speed. In this study, we applied a high throughput automated particle tracking method using near-total internal reflection fluorescence microscopy to measure the speed of CSCs. We found CSC speeds did not vary according to their proximity to microtubules, and that inhibiting microtubule polymerization could have opposite effects on CSC speed, depending on the nature of inhibition. While CSC speed increased in the temperature-sensitive mor1-1 mutant, it decreased after treatment with the drug oryzalin. Moreover, introducing the mor1-1 mutation into the CesA1 mutant any1 increased CSC speed, suggesting that microtubule dynamics affect CSC speed by a mechanism other than Cellulose Synthase A (CesA) catalytic activity. CSC speed varied widely in a range of mutants with reduced growth anisotropy, indicating that the relationship between CSC speed and anisotropy is complex. We conclude that microtubules affect CSC speed by finely tuned mechanisms that are independent of their physical association with CSCs.

Cellulose synthase complexes display distinct dynamic behaviors during xylem transdifferentiation

In plants, plasma membrane-embedded CELLULOSE SYNTHASE (CESA) enzyme complexes deposit cellulose polymers into the developing cell wall. Cellulose synthesis requires two different sets of CESA complexes that are active during cell expansion and secondary cell wall thickening, respectively. Hence, developing xylem cells, which first undergo cell expansion and subsequently deposit thick secondary walls, need to completely reorganize their CESA complexes from primary wall- to secondary wall-specific CESAs. Using live-cell imaging, we analyzed the principles underlying this remodeling. At the onset of secondary wall synthesis, the primary wall CESAs ceased to be delivered to the plasma membrane and were gradually removed from both the plasma membrane and the Golgi. For a brief transition period, both primary wall- and secondary wall-specific CESAs coexisted in banded domains of the plasma membrane where secondary wall synthesis is concentrated. During this transition, primary and secondary wall CESAs displayed discrete dynamic behaviors and sensitivities to the inhibitor isoxaben. As secondary wall-specific CESAs were delivered and inserted into the plasma membrane, the primary wall CESAs became concentrated in prevacuolar compartments and lytic vacuoles. This adjustment in localization between the two CESAs was accompanied by concurrent decreased primary wall CESA and increased secondary wall CESA protein abundance. Our data reveal distinct and dynamic subcellular trafficking patterns that underpin the remodeling of the cellulose biosynthetic machinery, resulting in the removal and degradation of the primary wall CESA complex with concurrent production and recycling of the secondary wall CESAs.

CLASP promotes stable tethering of endoplasmic microtubules to the cell cortex to maintain cytoplasmic stability in Arabidopsis meristematic cells

Following cytokinesis in plants, Endoplasmic MTs (EMTs) assemble on the nuclear surface, forming a radial network that extends out to the cell cortex, where they attach and incorporate into the cortical microtubule (CMT) array. We found that in these post-cytokinetic cells, the MT-associated protein CLASP is enriched at sites of EMT-cortex attachment, and is required for stable EMT tethering and growth into the cell cortex. Loss of EMT-cortex anchoring in clasp-1 mutants results in destabilized EMT arrays, and is accompanied by enhanced mobility of the cytoplasm, premature vacuolation, and precocious entry into cell elongation phase. Thus, EMTs appear to maintain cells in a meristematic state by providing a structural scaffold that stabilizes the cytoplasm to counteract actomyosin-based cytoplasmic streaming forces, thereby preventing premature establishment of a central vacuole and rapid cell elongation.

The cell biology of secondary cell wall biosynthesis

BACKGROUND

Secondary cell walls (SCWs) form the architecture of terrestrial plant biomass. They reinforce tracheary elements and strengthen fibres to permit upright growth and the formation of forest canopies. The cells that synthesize a strong, thick SCW around their protoplast must undergo a dramatic commitment to cellulose, hemicellulose and lignin production.

SCOPE

This review puts SCW biosynthesis in a cellular context, with the aim of integrating molecular biology and biochemistry with plant cell biology. While SCWs are deposited in diverse tissue and cellular contexts including in sclerenchyma (fibres and sclereids), phloem (fibres) and xylem (tracheids, fibres and vessels), the focus of this review reflects the fact that protoxylem tracheary elements have proven to be the most amenable experimental system in which to study the cell biology of SCWs.

CONCLUSIONS

SCW biosynthesis requires the co-ordination of plasma membrane cellulose synthases, hemicellulose production in the Golgi and lignin polymer deposition in the apoplast. At the plasma membrane where the SCW is deposited under the guidance of cortical microtubules, there is a high density of SCW cellulose synthase complexes producing cellulose microfibrils consisting of 18-24 glucan chains. These microfibrils are extruded into a cell wall matrix rich in SCW-specific hemicelluloses, typically xylan and mannan. The biosynthesis of eudicot SCW glucuronoxylan is taken as an example to illustrate the emerging importance of protein-protein complexes in the Golgi. From the trans-Golgi, trafficking of vesicles carrying hemicelluloses, cellulose synthases and oxidative enzymes is crucial for exocytosis of SCW components at the microtubule-rich cell membrane domains, producing characteristic SCW patterns. The final step of SCW biosynthesis is lignification, with monolignols secreted by the lignifying cell and, in some cases, by neighbouring cells as well. Oxidative enzymes such as laccases and peroxidases, embedded in the polysaccharide cell wall matrix, determine where lignin is deposited.

Cellulose synthase complex organization and cellulose microfibril structure

Cellulose consists of linear chains of β-1,4-linked glucose units, which are synthesized by the cellulose synthase complex (CSC). In plants, these chains associate in an ordered manner to form the cellulose microfibrils. Both the CSC and the local environment in which the individual chains coalesce to form the cellulose microfibril determine the structure and the unique physical properties of the microfibril. There are several recent reviews that cover many aspects of cellulose biosynthesis, which include trafficking of the complex to the plasma membrane and the relationship between the movement of the CSC and the underlying cortical microtubules (Bringmann et al. 2012 Trends Plant Sci.17, 666-674 (doi:10.1016/j.tplants.2012.06.003); Kumar & Turner 2015 Phytochemistry112, 91-99 (doi:10.1016/j.phytochem.2014.07.009); Schneider et al. 2016 Curr. Opin. Plant Biol.34, 9-16 (doi:10.1016/j.pbi.2016.07.007)). In this review, we will focus on recent advances in cellulose biosynthesis in plants, with an emphasis on our current understanding of the structure of individual catalytic subunits together with the local membrane environment where cellulose synthesis occurs. We will attempt to relate this information to our current knowledge of the structure of the cellulose microfibril and propose a model in which variations in the structure of the CSC have important implications for the structure of the cellulose microfibril produced.This article is part of a discussion meeting issue 'New horizons for cellulose nanotechnology'.

Longevity in vivo of primary cell wall cellulose synthases

Our work focuses on understanding the lifetime and thus stability of the three main cellulose synthase (CESA) proteins involved in primary cell wall synthesis of Arabidopsis. It had long been thought that a major means of CESA regulation was via their rapid degradation. However, our studies here have uncovered that AtCESA proteins are not rapidly degraded. Rather, they persist for an extended time in the plant cell. Plant cellulose is synthesized by membrane-embedded cellulose synthase complexes (CSCs). The CSC is composed of cellulose synthases (CESAs), of which three distinct isozymes form the primary cell wall CSC and another set of three isozymes form the secondary cell wall CSC. We determined the stability over time of primary cell wall (PCW) CESAs in Arabidopsis thaliana seedlings, using immunoblotting after inhibiting protein synthesis with cycloheximide treatment. Our work reveals very slow turnover for the Arabidopsis PCW CESAs in vivo. Additionally, we show that the stability of all three CESAs within the PCW CSC is altered by mutations in individual CESAs, elevated temperature, and light conditions. Together, these results suggest that CESA proteins are very stable in vivo, but that their lifetimes can be modulated by intrinsic and environmental cues.

How the Depletion in Mineral Major Elements Affects Grapevine (Vitis vinifera L.) Primary Cell Wall

The noteworthy fine remodeling that plant cell walls (CWs) undergo to adapt to developmental, physiological and environmental cues and the observation that its composition and dynamics differ between species represents an opportunity to couple crop species agronomic studies with research on CW modifications. Vitis vinifera is one of the most important crops from an economic point-of-view due to the high value of the fruit, predominantly for winemaking. The availability of some information related to this species' CWs allows researching its responses to imposed conditions that affect the plant's development. Mineral deficiency, in particular nitrogen, phosphorus, potassium and sulfur, strongly affects plant metabolism, reducing both growth and crop yield. Despite the importance of mineral nutrition in development, its influence on CW synthesis and modifications is still insufficiently documented. Addressing this knowledge gap, V. vinifera experimental models were used to study CW responses to imposed mineral depletion in unorganized (callus) and organized (shoots) tissues. The discussion of the obtained results is the main focus of this review. Callus and shoots submitted to mineral restriction are impaired in specific CW components, predominantly cellulose. Reorganization on structure and deposition of several other polymers, in particular the degree and pattern of pectin methyl-esterification and the amount of xyloglucan (XyG), arabinan and extensin, is also observed. In view of recently proposed CW models that consider biomechanical hotspots and direct linkages between pectins and XyG/cellulose, the outcome of these modifications in explaining maintenance of CW integrity through compensatory stiffening can be debated. Nutrient stresses do not affect evenly all tissues with undifferentiated callus tissues showing more pronounced responses, followed by shoot mature internodes, and then newly formed internodes. The impact of nitrogen depletion leads to more noticeable responses, supporting this nutrient's primary role in plant development and metabolism. The consequential compensatory mechanisms highlight the pivotal role of CW in rearranging under environmental stresses.

Formation of wood secondary cell wall may involve two type cellulose synthase complexes in Populus

Cellulose biosynthesis is mediated by cellulose synthases (CesAs), which constitute into rosette-like cellulose synthase complexe (CSC) on the plasma membrane. Two types of CSCs in Arabidopsis are believed to be involved in cellulose synthesis in the primary cell wall and secondary cell walls, respectively. In this work, we found that the two type CSCs participated cellulose biosynthesis in differentiating xylem cells undergoing secondary cell wall thickening in Populus. During the cell wall thickening process, expression of one type CSC genes increased while expression of the other type CSC genes decreased. Suppression of different type CSC genes both affected the wall-thickening and disrupted the multilaminar structure of the secondary cell walls. When CesA7A was suppressed, crystalline cellulose content was reduced, which, however, showed an increase when CesA3D was suppressed. The CesA suppression also affected cellulose digestibility of the wood cell walls. The results suggest that two type CSCs are involved in coordinating the cellulose biosynthesis in formation of the multilaminar structure in Populus wood secondary cell walls.

High Resolution Quantification of Crystalline Cellulose Accumulation in Arabidopsis Roots to Monitor Tissue-specific Cell Wall Modifications

Plant cells are surrounded by a cell wall, the composition of which determines their final size and shape. The cell wall is composed of a complex matrix containing polysaccharides that include cellulose microfibrils that form both crystalline structures and cellulose chains of amorphous organization. The orientation of the cellulose fibers and their concentrations dictate the mechanical properties of the cell. Several methods are used to determine the levels of crystalline cellulose, each bringing both advantages and limitations. Some can distinguish the proportion of crystalline regions within the total cellulose. However, they are limited to whole-organ analyses that are deficient in spatiotemporal information. Others relying on live imaging, are limited by the use of imprecise dyes. Here, we report a sensitive polarized light-based system for specific quantification of relative light retardance, representing crystalline cellulose accumulation in cross sections of Arabidopsis thaliana roots. In this method, the cellular resolution and anatomical data are maintained, enabling direct comparisons between the different tissues composing the growing root. This approach opens a new analytical dimension, shedding light on the link between cell wall composition, cellular behavior and whole-organ growth.

Regulation of cell wall remodeling in grapevine (Vitis vinifera L.) callus under individual mineral stress deficiency

Cell wall (CW) is a dynamic structure that determines the plant form, growth and response to environmental conditions. Vitis vinifera callus grown under nitrogen (-N), phosphorous (-P) and sulfur (-S) deficiency were used as a model system to address the influence of mineral stress in CW remodeling. Callus cells morphology was altered, mostly under -N, resulting in changes in cell length and width compared with the control. CW composition ascertained with specific staining and immuno-detection showed a decrease in cellulose and altered pattern of pectin methylesterification. Under mineral stress genes expression from candidate families disclosed mainly a downregulation of a glycosyl hydrolase family 9C (GH9C), xyloglucan transglycosylase/hydrolases (XTHs) with predicted hydrolytic activity and pectin methylesterases (PMEs). Conversely, upregulation of PMEs inhibitors (PMEIs) was observed. While methylesterification patterns can be associated to PME/PMEI gene expression, the lower cellulose content cannot be attributed to altered cellulose synthase (CesA) gene expression suggesting the involvement of other gene families. Salt extracts from -N and -P callus tissues increased plastic deformation in cucumber hypocotyls while no effect was observed with -S extracts. The lower endo-acting glycosyl hydrolase activity of -N callus extracts pinpoints a more expressive impact of -N on CW-remodeling.

Brassinosteroids Regulate Root Growth, Development, and Symbiosis

Brassinosteroids (BRs) are natural plant hormones critical for growth and development. BR deficient or signaling mutants show significantly shortened root phenotypes. However, for a long time, it was thought that these phenotypes were solely caused by reduced cell elongation in the mutant roots. Functions of BRs in regulating root development have been largely neglected. Nonetheless, recent detailed analyses, revealed that BRs are not only involved in root cell elongation but are also involved in many aspects of root development, such as maintenance of meristem size, root hair formation, lateral root initiation, gravitropic response, mycorrhiza formation, and nodulation in legume species. In this review, current findings on the functions of BRs in mediating root growth, development, and symbiosis are discussed.

The impact of abiotic factors on cellulose synthesis

As sessile organisms, plants require mechanisms to sense and respond to changes in their environment, including both biotic and abiotic factors. One of the most common plant adaptations to environmental changes is differential regulation of growth, which results in growth either away from adverse conditions or towards more favorable conditions. As cell walls shape plant growth, this differential growth response must be accompanied by alterations to the plant cell wall. Here, we review the impact of four abiotic factors (osmotic conditions, ionic stress, light, and temperature) on the synthesis of cellulose, an important component of the plant cell wall. Understanding how different abiotic factors influence cellulose production and addressing key questions that remain in this field can provide crucial information to cope with the need for increased crop production under the mounting pressures of a growing world population and global climate change.

Microtubules in Plant Cells: Strategies and Methods for Immunofluorescence, Transmission Electron Microscopy, and Live Cell Imaging

Microtubules (MTs) are required throughout plant development for a wide variety of processes, and different strategies have evolved to visualize and analyze them. This chapter provides specific methods that can be used to analyze microtubule organization and dynamic properties in plant systems and summarizes the advantages and limitations for each technique. We outline basic methods for preparing samples for immunofluorescence labeling, including an enzyme-based permeabilization method, and a freeze-shattering method, which generates microfractures in the cell wall to provide antibodies access to cells in cuticle-laden aerial organs such as leaves. We discuss current options for live cell imaging of MTs with fluorescently tagged proteins (FPs), and provide chemical fixation, high-pressure freezing/freeze substitution, and post-fixation staining protocols for preserving MTs for transmission electron microscopy and tomography.

Microtubules and cellulose biosynthesis: the emergence of new players

Microtubules determine the orientation of newly formed cellulose microfibrils in expanding cells. There are many hypotheses regarding how the information is transduced across the plasma membrane from microtubules to cellulose microfibrils. However, the molecular mechanisms underlying the co-alignment between microtubules and cellulose microfibrils were not revealed until the recent discovery of cellulose synthase interacting (CSI) proteins. Characterization of CSIs and additional cellulose synthase-associated proteins will greatly advance the knowledge of how cellulose microfibrils are organized.

Structure, function, and evolution of plant NIMA-related kinases: implication for phosphorylation-dependent microtubule regulation

Microtubules are highly dynamic structures that control the spatiotemporal pattern of cell growth and division. Microtubule dynamics are regulated by reversible protein phosphorylation involving both protein kinases and phosphatases. Never in mitosis A (NIMA)-related kinases (NEKs) are a family of serine/threonine kinases that regulate microtubule-related mitotic events in fungi and animal cells (e.g. centrosome separation and spindle formation). Although plants contain multiple members of the NEK family, their functions remain elusive. Recent studies revealed that NEK6 of Arabidopsis thaliana regulates cell expansion and morphogenesis through β-tubulin phosphorylation and microtubule destabilization. In addition, plant NEK members participate in organ development and stress responses. The present phylogenetic analysis indicates that plant NEK genes are diverged from a single NEK6-like gene, which may share a common ancestor with other kinases involved in the control of microtubule organization. On the contrary, another mitotic kinase, polo-like kinase, might have been lost during the evolution of land plants. We propose that plant NEK members have acquired novel functions to regulate cell growth, microtubule organization, and stress responses.

Multiscale models in the biomechanics of plant growth

Plant growth occurs through the coordinated expansion of tightly adherent cells, driven by regulated softening of cell walls. It is an intrinsically multiscale process, with the integrated properties of multiple cell walls shaping the whole tissue. Multiscale models encode physical relationships to bring new understanding to plant physiology and development.

A Mechanism for Sustained Cellulose Synthesis during Salt Stress

Abiotic stress, such as salinity, drought, and cold, causes detrimental yield losses for all major plant crop species. Understanding mechanisms that improve plants' ability to produce biomass, which largely is constituted by the plant cell wall, is therefore of upmost importance for agricultural activities. Cellulose is a principal component of the cell wall and is synthesized by microtubule-guided cellulose synthase enzymes at the plasma membrane. Here, we identified two components of the cellulose synthase complex, which we call companion of cellulose synthase (CC) proteins. The cytoplasmic tails of these membrane proteins bind to microtubules and promote microtubule dynamics. This activity supports microtubule organization, cellulose synthase localization at the plasma membrane, and renders seedlings less sensitive to stress. Our findings offer a mechanistic model for how two molecular components, the CC proteins, sustain microtubule organization and cellulose synthase localization and thus aid plant biomass production during salt stress. VIDEO ABSTRACT.

Dissecting the contribution of microtubule behaviour in adventitious root induction

Induction of adventitious roots (ARs) in recalcitrant plants often culminates in cell division and callus formation rather than root differentiation. Evidence is provided here to suggest that microtubules (MTs) play a role in the shift from cell division to cell differentiation during AR induction. First, it was found that fewer ARs form in the temperature-sensitive mutant mor1-1, in which the MT-associated protein MOR1 is mutated, and in bot1-1, in which the MT-severing protein katanin is mutated. In the two latter mutants, MT dynamics and form are perturbed. By contrast, the number of ARs increased in RIC1-OX3 plants, in which MT bundling is enhanced and katanin is activated. In addition, any1 plants in which cell walls are perturbed made more ARs than wild-type plants. MT perturbations during AR induction in mor1-1 or in wild-type hypocotyls treated with oryzalin led to the formation of amorphous clusters of cells reminiscent of callus. In these cells a specific pattern of polarized light retardation by the cell walls was lost. PIN1 polarization and auxin maxima were hampered and differentiation of the epidermis was inhibited. It is concluded that a fine-tuned crosstalk between MTs, cell walls, and auxin transport is required for proper AR induction.