The Arabidopsis TRM1-TON1 interaction reveals a recruitment network common to plant cortical microtubule arrays and eukaryotic centrosomes

An overview of plant division-plane orientation

Contents Summary 505 I. Introduction 505 II. Models of plant cell division 505 III. Establishing the division plane 506 IV. Maintaining the division plane during mitosis and cytokinesis 509 Acknowledgements 510 References 510 SUMMARY: Plants, a significant source of planet-wide biomass, have an unique type of cell division in which a new cell wall is constructed de novo inside the cell and guided towards the cell edge to complete division. The elegant control over positioning this new cell wall is essential for proper patterning and development. Plant cells, lacking migration, tightly coordinate division orientation and directed expansion to generate organized shapes. Several emerging lines of evidence suggest that the proteins required for division-plane establishment are distinct from those required for division-plane maintenance. We discuss recent shape-based computational models and mutant analyses that raise questions about, and identify unexpected connections between, the roles of well-known proteins and structures during division-plane orientation.

Why plants make puzzle cells, and how their shape emerges

The shape and function of plant cells are often highly interdependent. The puzzle-shaped cells that appear in the epidermis of many plants are a striking example of a complex cell shape, however their functional benefit has remained elusive. We propose that these intricate forms provide an effective strategy to reduce mechanical stress in the cell wall of the epidermis. When tissue-level growth is isotropic, we hypothesize that lobes emerge at the cellular level to prevent formation of large isodiametric cells that would bulge under the stress produced by turgor pressure. Data from various plant organs and species support the relationship between lobes and growth isotropy, which we test with mutants where growth direction is perturbed. Using simulation models we show that a mechanism actively regulating cellular stress plausibly reproduces the development of epidermal cell shape. Together, our results suggest that mechanical stress is a key driver of cell-shape morphogenesis.

Emerging roles of cortical microtubule-membrane interactions

Plant cortical microtubules have crucial roles in cell wall development. Cortical microtubules are tightly anchored to the plasma membrane in a highly ordered array, which directs the deposition of cellulose microfibrils by guiding the movement of the cellulose synthase complex. Cortical microtubules also interact with several endomembrane systems to regulate cell wall development and other cellular events. Recent studies have identified new factors that mediate interactions between cortical microtubules and endomembrane systems including the plasma membrane, endosome, exocytic vesicles, and endoplasmic reticulum. These studies revealed that cortical microtubule-membrane interactions are highly dynamic, with specialized roles in developmental and environmental signaling pathways. A recent reconstructive study identified a novel function of the cortical microtubule-plasma membrane interaction, which acts as a lateral fence that defines plasma membrane domains. This review summarizes recent advances in our understanding of the mechanisms and functions of cortical microtubule-membrane interactions.

The preprophase band of microtubules controls the robustness of division orientation in plants

Controlling cell division plane orientation is essential for morphogenesis in multicellular organisms. In plant cells, the future cortical division plane is marked before mitotic entry by the preprophase band (PPB). Here, we characterized an Arabidopsis trm (TON1 Recruiting Motif) mutant that impairs PPB formation but does not affect interphase microtubules. Unexpectedly, PPB disruption neither abolished the capacity of root cells to define a cortical division zone nor induced aberrant cell division patterns but rather caused a loss of precision in cell division orientation. Our results advocate for a reassessment of PPB function and division plane determination in plants and show that a main output of this microtubule array is to limit spindle rotations in order to increase the robustness of cell division.

Suppressive effect of microRNA319 expression on rice plant height

KEY MESSAGE: miR319 was identified as a dwarf-inducing gene from Shiokari and its dwarf near isogenic line, and its transgenic rice showed a reduced plant height. This finding reveals the potential application of miR319 in future molecular breeding. It is well known that microRNAs (miRNAs) play important roles in plant physiology, especially in development and stress responses. However, little is known about the role of miRNAs in plant height. In this study, the rice cultivar Shiokari and its dwarf near isogenic line Shiokari-d6 were analysed to identify and characterize plant height-associated miRNAs. This anatomic and morphological investigation revealed that the major cause of the shorter height of Shiokari-d6 is the significantly dis-elongated internodes, particularly the second internode and those underneath it. The results of miRNA microarray profiling and real-time RT-PCR indicated that miR319 is expressed at a significantly higher level in Shiokari-d6 than in Shiokari. Transgenic rice overexpressing miR319 in Oryza sativa L. cv. Tainung 67 generated through Agrobacterium-mediated transformation had a stable dwarf phenotype regardless of whether the plants were from the T1 or T2 generation. We also found that the internodes of miR319-overexpressing rice are shortened, particularly the third internode and those underneath it. Furthermore, we identified three putative miR319 target genes that were previously uncharacterized with expression levels that were negatively correlated with the expression of miR319. In conclusion, miR319 is the first miRNA proposed to be involved in plant height regulation, and its function may influence the elongation of internodes, which leads to decreased plant height.

The IQD Family of Calmodulin-Binding Proteins Links Calcium Signaling to Microtubules, Membrane Subdomains, and the Nucleus

Calcium (Ca2+) signaling and dynamic reorganization of the cytoskeleton are essential processes for the coordination and control of plant cell shape and cell growth. Calmodulin (CaM) and closely related calmodulin-like (CML) polypeptides are principal sensors of Ca2+ signals. CaM/CMLs decode and relay information encrypted by the second messenger via differential interactions with a wide spectrum of targets to modulate their diverse biochemical activities. The plant-specific IQ67 DOMAIN (IQD) family emerged as possibly the largest class of CaM-interacting proteins with undefined molecular functions and biological roles. Here, we show that the 33 members of the IQD family in Arabidopsis (Arabidopsis thaliana) differentially localize, using green fluorescent protein (GFP)-tagged proteins, to multiple and distinct subcellular sites, including microtubule (MT) arrays, plasma membrane subdomains, and nuclear compartments. Intriguingly, the various IQD-specific localization patterns coincide with the subcellular patterns of IQD-dependent recruitment of CaM, suggesting that the diverse IQD members sequester Ca2+-CaM signaling modules to specific subcellular sites for precise regulation of Ca2+-dependent processes. Because MT localization is a hallmark of most IQD family members, we quantitatively analyzed GFP-labeled MT arrays in Nicotiana benthamiana cells transiently expressing GFP-IQD fusions and observed IQD-specific MT patterns, which point to a role of IQDs in MT organization and dynamics. Indeed, stable overexpression of select IQD proteins in Arabidopsis altered cellular MT orientation, cell shape, and organ morphology. Because IQDs share biochemical properties with scaffold proteins, we propose that IQD families provide an assortment of platform proteins for integrating CaM-dependent Ca2+ signaling at multiple cellular sites to regulate cell function, shape, and growth.

PIF4 Promotes Expression of LNG1 and LNG2 to Induce Thermomorphogenic Growth in Arabidopsis

Arabidopsis plants adapt to high ambient temperature by a suite of morphological changes including elongation of hypocotyls and petioles and leaf hyponastic growth. These morphological changes are collectively called thermomorphogenesis and are believed to increase leaf cooling capacity by enhancing transpiration efficiency, thereby increasing tolerance to heat stress. The bHLH transcription factor PHYTOCHROME INTERACTING FACTOR4 (PIF4) has been identified as a major regulator of thermomorphogenic growth. Here, we show that PIF4 promotes the expression of two homologous genes LONGIFOLIA1 (LNG1) and LONGIFOLIA2 (LNG2) that have been reported to regulate leaf morphology. ChIP-Seq analyses and ChIP assays showed that PIF4 directly binds to the promoters of both LNG1 and LNG2. The expression of LNG1 and LNG2 is induced by high temperature in wild type plants. However, the high temperature activation of LNG1 and LNG2 is compromised in the pif4 mutant, indicating that PIF4 directly regulates LNG1 and LNG2 expression in response to high ambient temperatures. We further show that the activities of LNGs support thermomorphogenic growth. The expression of auxin biosynthetic and responsive genes is decreased in the lng quadruple mutant, implying that LNGs promote thermomorphogenic growth by activating the auxin pathway. Together, our results demonstrate that LNG1 and LNG2 are directly regulated by PIF4 and are new components for the regulation of thermomorphogenesis.

Asymmetric zygote division: A mystery starting point of embryogenesis

In angiosperm, asymmetric zygote division is critical for embryogenesis. The molecular mechanism underlying this process has gained a great attention recently. Some players involve in the control of both accurate position and correct orientation of zygote division plane have been found, which provide useful clues for the extensive investigations. It is getting clear that both internal and external factors are involved in this complex regulatory mechanism and the asymmetric zygote division seems with great impact in cell fate determination and embryo pattern formation.

Tissue and Organ Initiation in the Plant Embryo: A First Time for Everything

Land plants can grow to tremendous body sizes, yet even the most complex architectures are the result of iterations of the same developmental processes: organ initiation, growth, and pattern formation. A central question in plant biology is how these processes are regulated and coordinated to allow for the formation of ordered, 3D structures. All these elementary processes first occur in early embryogenesis, during which, from a fertilized egg cell, precursors for all major tissues and stem cells are initiated, followed by tissue growth and patterning. Here we discuss recent progress in our understanding of this phase of plant life. We consider the cellular basis for multicellular development in 3D and focus on the genetic regulatory mechanisms that direct specific steps during early embryogenesis.

Polarity in plant asymmetric cell division: Division orientation and cell fate differentiation

Asymmetric cell division (ACD) is universally required for the development of multicellular organisms. Unlike animal cells, plant cells have a rigid cellulosic extracellular matrix, the cell wall, which provides physical support and forms communication routes. This fundamental difference leads to some unique mechanisms in plants for generating asymmetries during cell division. However, plants also utilize intrinsically polarized proteins to regulate asymmetric signaling and cell division, a strategy similar to the differentiation mechanism found in animals. Current progress suggests that common regulatory modes, i.e. protein spontaneous clustering and cytoskeleton reorganization, underlie protein polarization in both animal and plant cells. Despite these commonalities, it is important to note that intrinsic mechanisms in plants are heavily influenced by extrinsic cues. To control physical asymmetry in cell division, although our understanding is fragmentary thus far, plants might have evolved novel polarization strategies to orientate cell division plane. Recent studies also suggest that the phytohormone auxin, one of the most pivotal small molecules in plant development, regulates ACD in plants.

Cell division plane orientation based on tensile stress in Arabidopsis thaliana

Cell geometry has long been proposed to play a key role in the orientation of symmetric cell division planes. In particular, the recently proposed Besson-Dumais rule generalizes Errera's rule and predicts that cells divide along one of the local minima of plane area. However, this rule has been tested only on tissues with rather local spherical shape and homogeneous growth. Here, we tested the application of the Besson-Dumais rule to the divisions occurring in the Arabidopsis shoot apex, which contains domains with anisotropic curvature and differential growth. We found that the Besson-Dumais rule works well in the central part of the apex, but fails to account for cell division planes in the saddle-shaped boundary region. Because curvature anisotropy and differential growth prescribe directional tensile stress in that region, we tested the putative contribution of anisotropic stress fields to cell division plane orientation at the shoot apex. To do so, we compared two division rules: geometrical (new plane along the shortest path) and mechanical (new plane along maximal tension). The mechanical division rule reproduced the enrichment of long planes observed in the boundary region. Experimental perturbation of mechanical stress pattern further supported a contribution of anisotropic tensile stress in division plane orientation. Importantly, simulations of tissues growing in an isotropic stress field, and dividing along maximal tension, provided division plane distributions comparable to those obtained with the geometrical rule. We thus propose that division plane orientation by tensile stress offers a general rule for symmetric cell division in plants.

Overview of OVATE FAMILY PROTEINS, A Novel Class of Plant-Specific Growth Regulators

OVATE FAMILY PROTEINS (OFPs) are a class of proteins with a conserved OVATE domain. OVATE protein was first identified in tomato as a key regulator of fruit shape. OFPs are plant-specific proteins that are widely distributed in the plant kingdom including mosses and lycophytes. Transcriptional activity analysis of Arabidopsis OFPs (AtOFPs) in protoplasts suggests that they act as transcription repressors. Functional characterization of OFPs from different plant species including Arabidopsis, rice, tomato, pepper, and banana suggests that OFPs regulate multiple aspects of plant growth and development, which is likely achieved by interacting with different types of transcription factors including the KNOX and BELL classes, and/or directly regulating the expression of target genes such as Gibberellin 20 oxidase (GA20ox). Here, we examine how OVATE was originally identified, summarize recent progress in elucidation of the roles of OFPs in regulating plant growth and development, and describe possible mechanisms underpinning this regulation. Finally, we review potential new research directions that could shed additional light on the functional biology of OFPs in plants.

Natural Variations in SLG7 Regulate Grain Shape in Rice

Rice (Oryza sativa) grain shape, which is controlled by quantitative trait loci (QTL), has a strong effect on yield production and quality. However, the molecular basis for grain development remains largely unknown. In this study, we identified a novel QTL, Slender grain on chromosome 7 (SLG7), that is responsible for grain shape, using backcross introgression lines derived from 9311 and Azucena. The SLG7 allele from Azucena produces longer and thinner grains, although it has no influence on grain weight and yield production. SLG7 encodes a protein homologous to LONGIFOLIA 1 and LONGIFOLIA 2, both of which increase organ length in Arabidopsis. SLG7 is constitutively expressed in various tissues in rice, and the SLG7 protein is located in plasma membrane. Morphological and cellular analyses suggested that SLG7 produces slender grains by longitudinally increasing cell length, while transversely decreasing cell width, which is independent from cell division. Our findings show that the functions of SLG7 family members are conserved across monocots and dicots and that the SLG7 allele could be applied in breeding to modify rice grain appearance.

Drought stress delays endosperm development and misregulates genes associated with cytoskeleton organization and grain quality proteins in developing wheat seeds

Drought stress is a major yield-limiting factor for wheat. Wheat yields are particularly sensitive to drought stress during reproductive development. Early seed development stage is an important determinant of seed size, one of the yield components. We specifically examined the impact of drought stress imposed during postzygotic early seed development in wheat. We imposed a short-term drought stress on plants with day-old seeds and observed that even a short-duration drought stress significantly reduced the size of developing seeds as well as mature seeds. Drought stress delayed the developmental transition from syncytial to cellularized stage of endosperm. Coincident with reduced seed size and delayed endosperm development, a subset of genes associated with cytoskeleton organization was misregulated in developing seeds under drought-stressed. Several genes linked to hormone pathways were also differentially regulated in response to drought stress in early seeds. Notably, drought stress strongly repressed the expression of wheat storage protein genes such as gliadins, glutenins and avenins as early as 3 days after pollination. Our results provide new insights on how some of the early seed developmental events are impacted by water stress, and the underlying molecular pathways that can possibly impact both grain size and quality in wheat.

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.

Plant cytokinesis-No ring, no constriction but centrifugal construction of the partitioning membrane

Plants have evolved a unique way of partitioning the cytoplasm of dividing cells: Instead of forming a contractile ring that constricts the plasma membrane, plant cells target membrane vesicles to the plane of division where the vesicles fuse with one another to form the partitioning membrane. Plant cytokinesis starts in the centre and progresses towards the periphery, culminating in the fusion of the partitioning membrane with the parental plasma membrane. This membrane dynamics is orchestrated by a specific cytoskeletal array named phragmoplast that originates from interzone spindle remnants. Here we review the properties of the process as well as molecules that play specific roles in that process.

Natural Variations in SLG7 Regulate Grain Shape in Rice

Rice (Oryza sativa) grain shape, which is controlled by quantitative trait loci (QTL), has a strong effect on yield production and quality. However, the molecular basis for grain development remains largely unknown. In this study, we identified a novel QTL, Slender grain on chromosome 7 (SLG7), that is responsible for grain shape, using backcross introgression lines derived from 9311 and Azucena. The SLG7 allele from Azucena produces longer and thinner grains, although it has no influence on grain weight and yield production. SLG7 encodes a protein homologous to LONGIFOLIA 1 and LONGIFOLIA 2, both of which increase organ length in Arabidopsis. SLG7 is constitutively expressed in various tissues in rice, and the SLG7 protein is located in plasma membrane. Morphological and cellular analyses suggested that SLG7 produces slender grains by longitudinally increasing cell length, while transversely decreasing cell width, which is independent from cell division. Our findings show that the functions of SLG7 family members are conserved across monocots and dicots and that the SLG7 allele could be applied in breeding to modify rice grain appearance.

Copy number variation at the GL7 locus contributes to grain size diversity in rice

Copy number variants (CNVs) are associated with changes in gene expression levels and contribute to various adaptive traits. Here we show that a CNV at the Grain Length on Chromosome 7 (GL7) locus contributes to grain size diversity in rice (Oryza sativa L.). GL7 encodes a protein homologous to Arabidopsis thaliana LONGIFOLIA proteins, which regulate longitudinal cell elongation. Tandem duplication of a 17.1-kb segment at the GL7 locus leads to upregulation of GL7 and downregulation of its nearby negative regulator, resulting in an increase in grain length and improvement of grain appearance quality. Sequence analysis indicates that allelic variants of GL7 and its negative regulator are associated with grain size diversity and that the CNV at the GL7 locus was selected for and used in breeding. Our work suggests that pyramiding beneficial alleles of GL7 and other yield- and quality-related genes may improve the breeding of elite rice varieties.

Mechanisms of plant cell division

Plant cells are confined by a network of cellulosic walls that imposes rigid control over the selection of division plane orientations, crucial for morphogenesis and genetically regulated. While in animal cells and yeast, the actin cytoskeleton is instrumental in the execution of cytokinesis, in plant cells the microtubule cytoskeleton is taking the lead in spatially controlling and executing cytokinesis by the formation of two unique, plant-specific arrays, the preprophase band (PPB) and the phragmoplast. The formation of microtubule arrays in plant cells is contingent on acentrosomal microtubule nucleation. At the onset of mitosis, the PPB defines the plane of cell division where the partitioning cell wall is later constructed by the cytokinetic phragmoplast, imposing a spatio-temporal relationship between the two processes. Current research progress in the field of plant cell division focuses on identifying and tying the links between early and late events in spatial control of cytokinesis and how microtubule array formation is regulated in plant cells.

Alpha-catenin-dependent recruitment of the centrosomal protein CAP350 to adherens junctions allows epithelial cells to acquire a columnar shape

Epithelial morphogenesis involves a dramatic reorganisation of the microtubule cytoskeleton. How this complex process is controlled at the molecular level is still largely unknown. Here, we report that the centrosomal microtubule (MT)-binding protein CAP350 localises at adherens junctions in epithelial cells. By two-hybrid screening, we identified a direct interaction of CAP350 with the adhesion protein α-catenin that was further confirmed by co-immunoprecipitation experiments. Block of epithelial cadherin (E-cadherin)-mediated cell-cell adhesion or α-catenin depletion prevented CAP350 localisation at cell-cell junctions. Knocking down junction-located CAP350 inhibited the establishment of an apico-basal array of microtubules and impaired the acquisition of columnar shape in Madin-Darby canine kidney II (MDCKII) cells grown as polarised epithelia. Furthermore, MDCKII cystogenesis was also defective in junctional CAP350-depleted cells. CAP350-depleted MDCKII cysts were smaller and contained either multiple lumens or no lumen. Membrane polarity was not affected, but cortical microtubule bundles did not properly form. Our results indicate that CAP350 may act as an adaptor between adherens junctions and microtubules, thus regulating epithelial differentiation and contributing to the definition of cell architecture. We also uncover a central role of α-catenin in global cytoskeleton remodelling, in which it acts not only on actin but also on MT reorganisation during epithelial morphogenesis.

In epithelial cells, the normally centrosomal protein CAP350 binds to α-catenin at adherens junctions and helps to establish the cells' parallel apico-basal microtubule array and columnar shape.

Epithelia cover all the surfaces of and the cavities throughout the body and serve as barriers between the organism and its external environment. Epithelial differentiation requires the coordination in space and time of several mechanisms that ultimately lead to the acquisition of distinctive epithelial features, including apical-basal polarity, specialised cell-cell junctions, and columnar shape. Epithelial differentiation also induces the reorganisation of three cytoskeletal networks: actin filaments, intermediate filaments, and microtubules. In simple epithelia, cadherins and their cytoplasmic binding partners catenins play a crucial role in connecting cell-cell junctions to the actin cytoskeleton. The cadherin extracellular domain forms adhesive contacts between adjacent cells, and their cytoplasmic tail indirectly binds the actin-binding protein α-catenin, thus linking cell-cell junctions to the underlying actin cytoskeleton. We report here an additional role of α-catenin in remodelling microtubules during epithelial differentiation. In most epithelial cells, microtubules are organised as parallel bundles aligned along the apico-basal axis and as apical and basal plasma membrane-associated networks. We demonstrate that the microtubule-binding protein CAP350, which is only localised at the centrosome in most cells, is also recruited at cell–cell junctions in epithelial cells through its binding to α-catenin. In the absence of junctional CAP350, microtubules are unable to reorganise in bundles, and cells do not acquire columnar shape. Our results suggest that recruitment of centrosomal proteins to cell-cell junctions could be a general mechanism to control microtubule reorganisation in neighbour cells during epithelial differentiation.

Control of oriented cell division in the Arabidopsis embryo

Multicellular plant development requires strict control of cell division orientation. A key unanswered question is how developmental regulators interact with the generic cell division machinery to trigger oriented divisions. We discuss the Arabidopsis embryo as a model for addressing this question. Recent progress in 3D imaging and computation now allows sketching of a framework for the developmental control of division orientation in which the signaling molecule auxin controls oriented division by preventing a geometrically defined default plane. We expect that the identification of auxin effectors, together with the identification of novel regulators of cell division will help to link developmental regulators to the division machinery.

Evolutionary developmental genetics of fruit morphological variation within the Solanaceae

Morphological variations of fruits such as shape and size, and color are a result of adaptive evolution. The evolution of morphological novelties is particularly intriguing. An understanding of these evolutionary processes calls for the elucidation of the developmental and genetic mechanisms that result in particular fruit morphological characteristics, which determine seed dispersal. The genetic and developmental basis for fruit morphological variation was established at a microevolutionary time scale. Here, we summarize the progress on the evolutionary developmental genetics of fruit size, shape and color in the Solanaceae. Studies suggest that the recruitment of a pre-existing gene and subsequent modification of its interaction and regulatory networks are frequently involved in the evolution of morphological diversity. The basic mechanisms underlying changes in plant morphology are alterations in gene expression and/or gene function. We also deliberate on the future direction in evolutionary developmental genetics of fruit morphological variation such as fruit type. These studies will provide insights into plant developmental processes and will help to improve the productivity and fruit quality of crops.

The functions of the cytoskeleton and associated proteins during mitosis and cytokinesis in plant cells

In higher plants, microtubule (MT)-based, and actin filament (AF)-based structures play important roles in mitosis and cytokinesis. Besides the mitotic spindle, the evolution of a band comprising cortical MTs and AFs, namely, the preprophase band (PPB), is evident in plant cells. This band forecasts a specific division plane before the initiation of mitosis. During cytokinesis, another plant-specific cytoskeletal structure called the phragmoplast guides vesicles in the creation of a new cell wall. In addition, a number of cytoskeleton-associated proteins are reportedly involved in the formation and function of the PPB, mitotic spindle, and phragmoplast. This review summarizes current knowledge on the cytoskeleton-associated proteins that mediate the cytoskeletal arrays during mitosis and cytokinesis in plant cells and discusses the interaction between MTs and AFs involved in mitosis and cytokinesis.

CLASP: a microtubule-based integrator of the hormone-mediated transitions from cell division to elongation

Plants use robust mechanisms to optimize organ size to prevailing conditions. Modulating the transition from cell division to elongation dramatically affects morphology and size. Although it is well established that auxin, cytokinin and brassinosteroid mediate these transitions, recent works show that the cytoskeleton, which is normally thought to act downstream of these hormones, plays a key role in this regulatory process. In particular, the microtubule-associated protein CLASP has a dual role in meristem maintenance. CLASP modulates levels of the auxin efflux carrier PIN2 by tethering SNX1 endosomes to cortical microtubules, which in turn fine tunes auxin maxima in the root apical meristem. CLASP is also required for transfacial microtubule bundle formation at the sharp cell edges, a feature strongly associated with maintaining the capacity for further cell division.

Cell wall-related proteins of unknown function: missing links in plant cell wall development

Lignocellulosic biomass is an important feedstock for the pulp and paper industry as well as emerging biofuel and biomaterial industries. However, the recalcitrance of the secondary cell wall to chemical or enzymatic degradation remains a major hurdle for efficient extraction of economically important biopolymers such as cellulose. It has been estimated that approximately 10-15% of about 27,000 protein-coding genes in the Arabidopsis genome are dedicated to cell wall development; however, only about 130 Arabidopsis genes thus far have experimental evidence validating cell wall function. While many genes have been implicated through co-expression analysis with known genes, a large number are broadly classified as proteins of unknown function (PUFs). Recently the functionality of some of these unknown proteins in cell wall development has been revealed using reverse genetic approaches. Given the large number of cell wall-related PUFs, how do we approach and subsequently prioritize the investigation of such unknown genes that may be essential to or influence plant cell wall development and structure? Here, we address the aforementioned question in two parts; we first identify the different kinds of PUFs based on known and predicted features such as protein domains. Knowledge of inherent features of PUFs may allow for functional inference and a concomitant link to biological context. Secondly, we discuss omics-based technologies and approaches that are helping identify and prioritize cell wall-related PUFs by functional association. In this way, hypothesis-driven experiments can be designed for functional elucidation of many proteins that remain missing links in our understanding of plant cell wall biosynthesis.

RoKSN, a floral repressor, forms protein complexes with RoFD and RoFT to regulate vegetative and reproductive development in rose

FT/TFL1 family members have been known to be involved in the development and flowering in plants. In rose, RoKSN, a TFL1 homologue, is a key regulator of flowering, whose absence causes continuous flowering. Our objectives are to functionally validate RoKSN and to explore its mode of action in rose. We complemented Arabidopsis tfl1 mutants and ectopically expressed RoKSN in a continuous-flowering (CF) rose. Using different protein interaction techniques, we studied RoKSN interactions with RoFD and RoFT and possible competition. In Arabidopsis, RoKSN complemented the tfl1 mutant by rescuing late flowering and indeterminate growth. In CF roses, the ectopic expression of RoKSN led to the absence of flowering. Different branching patterns were observed and some transgenic plants had an increased number of leaflets per leaf. In these transgenic roses, floral activator transcripts decreased. Furthermore, RoKSN was able to interact both with RoFD and the floral activator, RoFT. Protein interaction experiments revealed that RoKSN and RoFT could compete with RoFD for repression and activation of blooming, respectively. We conclude that RoKSN is a floral repressor and is also involved in the vegetative development of rose. RoKSN forms a complex with RoFD and could compete with RoFT for repression of flowering.

MAPs: cellular navigators for microtubule array orientations in Arabidopsis

Microtubules are subcellular nanotubes composed of α- and β-tubulin that arise from microtubule nucleation sites, mainly composed of γ-tubulin complexes [corrected]. Cell wall encased plant cells have evolved four distinct microtubule arrays that regulate cell division and expansion. Microtubule-associated proteins, the so called MAPs, construct, destruct and reorganize microtubule arrays thus regulating their spatiotemporal transitions during the cell cycle. By physically binding to microtubules and/or modulating their functions, MAPs control microtubule dynamic instability and/or interfilament cross talk. We survey the recent analyses of Arabidopsis MAPs such as MAP65, MOR1, CLASP, katanin, TON1, FASS, TRM, TAN1 and kinesins in terms of their effects on microtubule array organizations and plant development.

Microtubule organization and microtubule-associated proteins in plant cells

Plants have unique microtubule (MT) arrays, cortical MTs, preprophase band, mitotic spindle, and phragmoplast, in the processes of evolution. These MT arrays control the directions of cell division and expansion especially in plants and are essential for plant morphogenesis and developments. Organizations and functions of these MT arrays are accomplished by diverse MT-associated proteins (MAPs). This review introduces 10 of conserved MAPs in eukaryote such as γ-TuC, augmin, katanin, kinesin, EB1, CLASP, MOR1/MAP215, MAP65, TPX2, formin, and several plant-specific MAPs such as CSI1, SPR2, MAP70, WVD2/WDL, RIP/MIDD, SPR1, MAP18/PCaP, EDE1, and MAP190. Most of the studies cited in this review have been analyzed in the particular model plant, Arabidopsis thaliana. The significant knowledge of A. thaliana is the important established base to understand MT organizations and functions in plants.

What lies beyond the eye: the molecular mechanisms regulating tomato fruit weight and shape

Domestication of fruit and vegetables resulted in a huge diversity of shapes and sizes of the produce. Selections that took place over thousands of years of alleles that increased fruit weight and altered shape for specific culinary uses provide a wealth of resources to study the molecular bases of this diversity. Tomato (Solanum lycopersicum) evolved from a wild ancestor (S. pimpinellifolium) bearing small and round edible fruit. Molecular genetic studies led to the identification of two genes selected for fruit weight: FW2.2 encoding a member of the Cell Number Regulator family; and FW3.2 encoding a P450 enzyme and the ortholog of KLUH. Four genes were identified that were selected for fruit shape: SUN encoding a member of the IQD family of calmodulin-binding proteins leading to fruit elongation; OVATE encoding a member of the OVATE family proteins involved in transcriptional repression leading to fruit elongation; LC encoding most likely the ortholog of WUSCHEL controlling meristem size and locule number; FAS encoding a member in the YABBY family controlling locule number leading to flat or oxheart shape. For this article, we will provide an overview of the putative function of the known genes, when during floral and fruit development they are hypothesized to act and their potential importance in regulating morphological diversity in other fruit and vegetable crops.

Reorganization of the plant cortical microtubule array

The interphase microtubule arrays in flowering plant cells assemble at the cell cortex into patterns that affect cellular morphogenesis. A decade of live cell imaging studies has provided significant information about the in vivo properties of the microtubule polymers. Efforts to extrapolate individual properties to larger roles in organizing or patterning the microtubule array have produced models focused on self-organization and local levels of biological control. Recent studies looking at cortical microtubule arrays as they transition from an existing pattern to a new pattern have re-emerged as a testbed for examining these models and the molecular hypotheses underpinning them. The evidence suggests that microtubule patterning is locally controlled on the scale of a cell face, using or circumventing self-organizating properties as necessary.

The mechanics behind cell division

It is now well established that the orientation of the plane of cell division highly depends on cell geometry in plants. However, the related molecular mechanism remains largely unknown. Recent data in animal systems highlight the role of the cytoskeleton response to mechanical stress in this process. Interestingly, these results are consistent with some data obtained in parallel in plants. Here we review and confront these studies, across kingdoms, and we explore the possibility that the intrinsic mechanical properties of the cytoskeleton play a key role in the nexus between cell division and mechanical stress. This opens many avenues for future research that are also discussed in this review.

Interaction of the microtubule-associated host protein HIP2 with viral helper component proteinase is important in infection with potato virus A

Microtubules (MT) outline and maintain the overall shape of cells and can reorganize cellular membranes to serve as sites of RNA virus replication. Here, we provide data on involvement of an MT-associated protein in infection of plants with a potyvirus, Potato virus A (PVA), representing the largest family of plant-infecting RNA viruses. Our results showed that helper-component proteinase (HCpro)-interacting protein 2 (HIP2) of potato (Solanum tuberosum) is an MT-associated protein similar to Arabidopsis SPR2. Virus-induced silencing of HIP2 in Nicotiana benthamiana resulted in a spiral-like growth phenotype, similar to the Arabidopsis spr2 mutant, and the spr2 phenotype in Arabidopsis was complemented with potato HIP2. HCpro of PVA interacted with HIP2 of potato and tobacco (Nicotiana tabacum). The interaction was detected by bimolecular fluorescence complementation in PVA-infected leaves on MT and MT intersections at the cell cortex. HIP2-HCpro interaction was determined by the C-proximal α-helix-rich domain of HIP2, whereas the N-proximal putative TOG domain and the central coiled-coil domain of HIP2 controlled HIP2 dimerization and binding to MT. Accumulation of PVA was significantly reduced in the HIP2-silenced leaves of N. benthamiana, which indicates that HIP2-HCpro interactions are important for virus infection.

GABI-DUPLO: a collection of double mutants to overcome genetic redundancy in Arabidopsis thaliana

Owing to duplication events in its progenitor, more than 90% of the genes in the Arabidopsis thaliana genome are members of multigene families. A set of 2108 gene families, each consisting of precisely two unlinked paralogous genes, was identified in the nuclear genome of A. thaliana on the basis of sequence similarity. A systematic method for the creation of double knock-out lines for such gene pairs, designated as DUPLO lines, was established and 200 lines are now publicly available. Their initial phenotypic characterisation led to the identification of seven lines with defects that emerge only in the adult stage. A further six lines display seedling lethality and 23 lines were lethal before germination. Another 14 lines are known to show phenotypes under non-standard conditions or at the molecular level. Knock-out of gene pairs with very similar coding sequences or expression profiles is more likely to produce a mutant phenotype than inactivation of gene pairs with dissimilar profiles or sequences. High coding sequence similarity and highly similar expression profiles are only weakly correlated, implying that promoter and coding regions of these gene pairs display different degrees of diversification.

The role of the cytoskeleton and associated proteins in determination of the plant cell division plane

In plants, as in all eukaryotic organisms, microtubule- and actin-filament based structures play fundamental roles during cell division. In addition to the mitotic spindle, plant cells have evolved a unique cytoskeletal structure that designates a specific division plane before the onset of mitosis via formation of a cortical band of microtubules and actin filaments called the preprophase band. During cytokinesis, a second plant-specific microtubule and actin filament structure called the phragmoplast directs vesicles to create the new cell wall. In response to intrinsic and extrinsic cues, many plant cells form a preprophase band in G2 , then the preprophase band recruits specific proteins to populate the cortical division site prior to disassembly of the preprophase band in prometaphase. These proteins are thought to act as a spatial reminder that actively guides the phragmoplast towards the cortical division site during cytokinesis. A number of proteins involved in determination and maintenance of the plane of cell division have been identified. Our current understanding of the molecular interactions of these proteins and their regulation of microtubules is incomplete, but advanced imaging techniques and computer simulations have validated some early concepts of division site determination.

Role of nucleation in cortical microtubule array organization: variations on a theme

The interphase cortical microtubules (CMTs) of plant cells form strikingly ordered arrays in the absence of a dedicated microtubule-organizing center. Considerable research effort has focused on activities such as bundling and severing that occur after CMT nucleation and are thought to be important for generating and maintaining ordered arrays. In this review, we focus on how nucleation affects CMT array organization. The bulk of CMTs are initiated from γ-tubulin-containing nucleation complexes localized to the lateral walls of pre-existing CMTs. These CMTs grow either at an acute angle or parallel to the pre-existing CMT. Although the impact of microtubule-dependent nucleation is not fully understood, recent genetic, live-cell imaging and computer simulation studies have demonstrated that the location, timing and geometry of CMT nucleation have a considerable impact on the organization and orientation of the CMT array. These nucleation properties are defined by the composition, position and dynamics of γ-tubulin-containing nucleation complexes, which represent control points for the cell to regulate CMT array organization.

The emerging function of IQD proteins as scaffolds in cellular signaling and trafficking

Calcium (Ca(2+)) signaling modules are essential for adjusting plant growth and performance to environmental constraints. Differential interactions between sensors of Ca(2+) dynamics and their molecular targets are at the center of the transduction process. Calmodulin (CaM) and CaM-like (CML) proteins are principal Ca(2+)-sensors in plants that govern the activities of numerous downstream proteins with regulatory properties. The families of IQ67-Domain (IQD) proteins are a large class of plant-specific CaM/CML-targets (e.g., 33 members in A. thaliana) which share a unique domain of multiple varied CaM retention motifs in tandem orientation. Genetic studies in Arabidopsis and tomato revealed first roles for IQD proteins related to basal defense response and plant development. Molecular, biochemical and histochemical analysis of Arabidopsis IQD1 demonstrated association with microtubules as well as targeting to the cell nucleus and nucleolus. In vivo binding to CaM and kinesin light chain-related protein-1 (KLCR1) suggests a Ca(2+)-regulated scaffolding function of IQD1 in kinesin motor-dependent transport of multiprotein complexes. Furthermore, because IQD1 interacts in vitro with single-stranded nucleic acids, the prospect arises that IQD1 and other IQD family members facilitate cellular RNA localization as one mechanism to control and fine-tune gene expression and protein sorting.

Strasburger's legacy to mitosis and cytokinesis and its relevance for the Cell Theory

Eduard Strasburger was one of the most prominent biologists contributing to the development of the Cell Theory during the nineteenth century. His major contribution related to the characterization of mitosis and cytokinesis and especially to the discovery of the discrete stages of mitosis, which he termed prophase, metaphase and anaphase. Besides his observations on uninucleate plant and animal cells, he also investigated division processes in multinucleate cells. Here, he emphasised the independent nature of mitosis and cytokinesis. We discuss these issues from the perspective of new discoveries in the field of cell division and conclude that Strasburger's legacy will in the future lead to a reformulation of the Cell Theory and that this will accommodate the independent and primary nature of the nucleus, together with its complement of perinuclear microtubules, for the organisation of the eukaryotic cell.