A plane choice: coordinating timing and orientation of cell division during plant development

An incoherent feed-forward loop involving bHLH transcription factors, Auxin and CYCLIN-Ds regulates style radial symmetry establishment in Arabidopsis

The bilateral-to-radial symmetry transition occurring during the development of the Arabidopsis thaliana female reproductive organ (gynoecium) is a crucial biological process linked to plant fertilization and seed production. Despite its significance, the cellular mechanisms governing the establishment and breaking of radial symmetry at the gynoecium apex (style) remain unknown. To fill this gap, we employed quantitative confocal imaging coupled with MorphoGraphX analysis, in vivo and in vitro transcriptional experiments, and genetic analysis encompassing mutants in two bHLH transcription factors necessary and sufficient to promote transition to radial symmetry, SPATULA (SPT) and INDEHISCENT (IND). Here, we show that defects in style morphogenesis correlate with defects in cell-division orientation and rate. We showed that the SPT-mediated accumulation of auxin in the medial-apical cells undergoing symmetry transition is required to maintain cell-division-oriented perpendicular to the direction of organ growth (anticlinal, transversal cell division). In addition, SPT and IND promote the expression of specific core cell-cycle regulators, CYCLIN-D1;1 (CYC-D1;1) and CYC-D3;3, to support progression through the G1 phase of the cell cycle. This transcriptional regulation is repressed by auxin, thus forming an incoherent feed-forward loop mechanism. We propose that this mechanism fine-tunes cell division rate and orientation with the morphogenic signal provided by auxin, during patterning of radial symmetry at the style.

The microtubular preprophase band recruits Myosin XI to the cortical division site to guide phragmoplast expansion during plant cytokinesis

In plant vegetative tissues, cell division employs a mitotic microtubule array called the preprophase band (PPB) that marks the cortical division site. This transient cytoskeletal array imprints the spatial information to be read by the cytokinetic phragmoplast at later stages of mitotic cell division. In Arabidopsis thaliana, we discovered that the PPB recruited the Myosin XI motor MYA1/Myo11F to the cortical division site, where it joined microtubule-associated proteins and motors to form a ring of prominent cytoskeletal assemblies that received the expanding phragmoplast. Such a myosin localization pattern at the cortical division site was dependent on the POK1/2 Kinesin-12 motors. This regulatory function of MYA1/Myo11F in phragmoplast guidance was dependent on intact actin filaments. The discovery of these cytoskeletal motor assemblies pinpoints a mechanism underlying how two dynamic cytoskeletal networks work in concert to govern PPB-dependent division plane orientation in flowering plants.

Opposing polarity domains provide direction and play a role in cell division in plant growth

Polar localization of proteins is important for plant growth and development. Identifying the interactors of polarized proteins provides spatial information and cell-type functions. In this issue of Developmental Cell, Wallner et al. (2024) utilize opposing polarity domain proteins to identify interactors and their functions during cell division in Arabidopsis stomata.

SHR and SCR coordinate root patterning and growth early in the cell cycle

Precise control of cell division is essential for proper patterning and growth during the development of multicellular organisms. Coordination of formative divisions that generate new tissue patterns with proliferative divisions that promote growth is poorly understood. SHORTROOT (SHR) and SCARECROW (SCR) are transcription factors that are required for formative divisions in the stem cell niche of Arabidopsis roots1,2. Here we show that levels of SHR and SCR early in the cell cycle determine the orientation of the division plane, resulting in either formative or proliferative cell division. We used 4D quantitative, long-term and frequent (every 15 min for up to 48 h) light sheet and confocal microscopy to probe the dynamics of SHR and SCR in tandem within single cells of living roots. Directly controlling their dynamics with an SHR induction system enabled us to challenge an existing bistable model3 of the SHR-SCR gene-regulatory network and to identify key features that are essential for rescue of formative divisions in shr mutants. SHR and SCR kinetics do not align with the expected behaviour of a bistable system, and only low transient levels, present early in the cell cycle, are required for formative divisions. These results reveal an uncharacterized mechanism by which developmental regulators directly coordinate patterning and growth.

The dynamics and biophysics of shape formation: Common themes in plant and animal morphogenesis

The emergence of tissue form in multicellular organisms results from the complex interplay between genetics and physics. In both plants and animals, cells must act in concert to pattern their behaviors. Our understanding of the factors sculpting multicellular form has increased dramatically in the past few decades. From this work, common themes have emerged that connect plant and animal morphogenesis-an exciting connection that solidifies our understanding of the developmental basis of multicellular life. In this review, we will discuss the themes and the underlying principles that connect plant and animal morphogenesis, including the coordination of gene expression, signaling, growth, contraction, and mechanical and geometric feedback.

Update: on selected ROP cell polarity mechanisms in plant cell morphogenesis

The unequal (asymmetric) distribution of cell structures and proteins within a cell is designated as cell polarity. Cell polarity is a crucial prerequisite for morphogenetic processes such as oriented cell division and directed cell expansion. Rho-related GTPase from plants (ROPs) are required for cellular morphogenesis through the reorganization of the cytoskeleton and vesicle transport in various tissues. Here, I review recent advances in ROP-dependent tip growth, vesicle transport, and tip architecture. I report on the regulatory mechanisms of ROP upstream regulators found in different cell types. It appears that these regulators assemble in nanodomains with specific lipid compositions and recruit ROPs for activation in a stimulus-dependent manner. Current models link mechanosensing/mechanotransduction to ROP polarity signaling involved in feedback mechanisms via the cytoskeleton. Finally, I discuss ROP signaling components that are upregulated by tissue-specific transcription factors and exhibit specific localization patterns during cell division, clearly suggesting ROP signaling in division plane alignment.

A polarized nuclear position specifies the correct division plane during maize stomatal development

Asymmetric cell division generates different cell types and is a feature of development in multicellular organisms. Prior to asymmetric cell division, cell polarity is established. Maize (Zea mays) stomatal development serves as an excellent plant model system for asymmetric cell division, especially the asymmetric division of the subsidiary mother cell (SMC). In SMCs, the nucleus migrates to a polar location after the accumulation of polarly localized proteins but before the appearance of the preprophase band. We examined a mutant of an outer nuclear membrane protein that is part of the LINC (linker of nucleoskeleton and cytoskeleton) complex that localizes to the nuclear envelope in interphase cells. Previously, maize linc kash sine-like2 (mlks2) was observed to have abnormal stomata. We confirmed and identified the precise defects that lead to abnormal asymmetric divisions. Proteins that are polarly localized in SMCs prior to division polarized normally in mlks2. However, polar localization of the nucleus was sometimes impaired, even in cells that have otherwise normal polarity. This led to a misplaced preprophase band and atypical division planes. MLKS2 localized to mitotic structures; however, the structure of the preprophase band, spindle, and phragmoplast appeared normal in mlks2. Time-lapse imaging revealed that mlks2 has defects in premitotic nuclear migration toward the polarized site and unstable position at the division site after formation of the preprophase band. Overall, our results show that nuclear envelope proteins promote premitotic nuclear migration and stable nuclear position and that the position of the nucleus influences division plane establishment in asymmetrically dividing cells.

Axes and polarities in leaf vein formation

For multicellular organisms to develop, cells must grow, divide, and differentiate along preferential or exclusive orientations or directions. Moreover, those orientations, or axes, and directions, or polarities, must be coordinated between cells within and between tissues. Therefore, how axes and polarities are coordinated between cells is a key question in biology. In animals, such coordination mainly depends on cell migration and direct interaction between proteins protruding from the plasma membrane. Both cell movements and direct cell-cell interactions are prevented in plants by cell walls that surround plant cells and keep them apart and in place. Therefore, plants have evolved unique mechanisms to coordinate their cell axes and polarities. Here I will discuss evidence suggesting that understanding how leaf veins form may uncover those unique mechanisms. Indeed, unlike previously thought, the cell-to-cell polar transport of the plant hormone auxin along developing veins cannot account for many features of vein patterning. Instead, those features can be accounted for by models of vein patterning that combine polar auxin transport with auxin diffusion through plasmodesmata along the axis of developing veins. Though it remains unclear whether such a combination of polar transport and axial diffusion of auxin can account for the formation of the variety of vein patterns found in plant leaves, evidence suggests that such a combined mechanism may control plant developmental processes beyond vein patterning.

Divided by fate: The interplay between division orientation and cell shape underlying lateral root initiation in Arabidopsis

The development of lateral roots starts with a round of anticlinal, asymmetric cell divisions in lateral root founder cells in the pericycle, deep within the root. The reorientation of the cell division plane occurs in parallel with changes in cell shape and needs to be coordinated with its direct neighbor, the endodermis. This accommodation response requires the integration of biochemical and mechanical signals in both cell types. Recently, it was reported that dynamic changes in the cytoskeleton and possibly the cell wall are part of the molecular mechanism required to correctly orient and position the cell division plane. Here we discuss the latest progress made towards our understanding of the regulation of cell shape and division plane orientation underlying lateral root initiation in Arabidopsis.

D, Noriega A, Drakakaki G, Sylvester AW, Rasmussen CG. Subcellular positioning during cell division and cell plate formation in maize

Introduction

During proliferative plant cell division, the new cell wall, called the cell plate, is first built in the middle of the cell and then expands outward to complete cytokinesis. This dynamic process requires coordinated movement and arrangement of the cytoskeleton and organelles.

Methods

Here we use live-cell markers to track the dynamic reorganization of microtubules, nuclei, endoplasmic reticulum, and endomembrane compartments during division and the formation of the cell plate in maize leaf epidermal cells.

Results

The microtubule plus-end localized protein END BINDING1 (EB1) highlighted increasing microtubule dynamicity during mitosis to support rapid changes in microtubule structures. The localization of the cell-plate specific syntaxin KNOLLE, several RAB-GTPases, as well as two plasma membrane localized proteins was assessed after treatment with the cytokinesis-specific callose-deposition inhibitor Endosidin7 (ES7) and the microtubule-disrupting herbicide chlorpropham (CIPC). While ES7 caused cell plate defects in Arabidopsis thaliana, it did not alter callose accumulation, or disrupt cell plate formation in maize. In contrast, CIPC treatment of maize epidermal cells occasionally produced irregular cell plates that split or fragmented, but did not otherwise disrupt the accumulation of cell-plate localized proteins.

Discussion

Together, these markers provide a robust suite of tools to examine subcellular trafficking and organellar organization during mitosis and cell plate formation in maize.

The making of a ring: Assembly and regulation of microtubule-associated proteins during preprophase band formation and division plane set-up

The preprophase band (PPB) is a transient cytokinetic structure that marks the future division plane at the onset of mitosis. The PPB forms a dense cortical ring of mainly microtubules, actin filaments, endoplasmic reticulum, and associated proteins that encircles the nucleus of mitotic cells. After PPB disassembly, the positional information is preserved by the cortical division zone (CDZ). The formation of the PPB and its contribution to timely CDZ set-up involves activities of functionally distinct microtubule-associated proteins (MAPs) that interact physically and genetically to support robust division plane orientation in plants. Recent studies identified two types of plant-specific MAPs as key regulators of PPB formation, the TON1 RECRUITMENT MOTIF (TRM) and IQ67 DOMAIN (IQD) families. Both families share hallmarks of disordered scaffold proteins. Interactions of IQDs and TRMs with multiple binding partners, including the microtubule severing KATANIN1, may provide a molecular framework to coordinate PPB formation, maturation, and disassembly.

Redundant mechanisms in division plane positioning

Redundancies in plant cell division contribute to the maintenance of proper division plane orientation. Here we highlight three types of redundancy: 1) Temporal redundancy, or correction of earlier defects that results in proper final positioning, 2) Genetic redundancy, or functional compensation by homologous genes, and 3) Synthetic redundancy, or redundancy within or between pathways that contribute to proper division plane orientation. Understanding the types of redundant mechanisms involved provides insight into current models of division plane orientation and opens up new avenues for exploration.

Polarly localized WPR proteins interact with PAN receptors and the actin cytoskeleton during maize stomatal development

Polarization of cells prior to asymmetric cell division is crucial for correct cell divisions, cell fate, and tissue patterning. In maize (Zea mays) stomatal development, the polarization of subsidiary mother cells (SMCs) prior to asymmetric division is controlled by the BRICK (BRK)-PANGLOSS (PAN)-RHO FAMILY GTPASE (ROP) pathway. Two catalytically inactive receptor-like kinases, PAN2 and PAN1, are required for correct division plane positioning. Proteins in the BRK-PAN-ROP pathway are polarized in SMCs, with the polarization of each protein dependent on the previous one. As most of the known proteins in this pathway do not physically interact, possible interactors that might participate in the pathway are yet to be described. We identified WEAK CHLOROPLAST MOVEMENT UNDER BLUE LIGHT 1 (WEB1)/PLASTID MOVEMENT IMPAIRED 2 (PMI2)-RELATED (WPR) proteins as players during SMC polarization in maize. WPRs physically interact with PAN receptors and polarly accumulate in SMCs. The polarized localization of WPR proteins depends on PAN2 but not PAN1. CRISPR-Cas9-induced mutations result in division plane defects in SMCs, and ectopic expression of WPR-RFP results in stomatal defects and alterations to the actin cytoskeleton. We show that certain WPR proteins directly interact with F-actin through their N-terminus. Our data implicate WPR proteins as potentially regulating actin filaments, providing insight into their molecular function. These results demonstrate that WPR proteins are important for cell polarization.

The localization of PHRAGMOPLAST ORIENTING KINESIN1 at the division site depends on the microtubule-binding proteins TANGLED1 and AUXIN-INDUCED IN ROOT CULTURES9 in Arabidopsis

Proper plant growth and development require spatial coordination of cell divisions. Two unrelated microtubule-binding proteins, TANGLED1 (TAN1) and AUXIN-INDUCED IN ROOT CULTURES9 (AIR9), are together required for normal growth and division plane orientation in Arabidopsis (Arabidopsis thaliana). The tan1 air9 double mutant has synthetic growth and division plane orientation defects, while single mutants lack obvious defects. Here we show that the division site-localized protein, PHRAGMOPLAST ORIENTING KINESIN1 (POK1), was aberrantly lost from the division site during metaphase and telophase in the tan1 air9 mutant. Since TAN1 and POK1 interact via the first 132 amino acids of TAN1 (TAN11-132), we assessed the localization and function of TAN11-132 in the tan1 air9 double mutant. TAN11-132 rescued tan1 air9 mutant phenotypes and localized to the division site during telophase. However, replacing six amino-acid residues within TAN11-132, which disrupted the POK1-TAN1 interaction in the yeast-two-hybrid system, caused loss of both rescue and division site localization of TAN11-132 in the tan1 air9 mutant. Full-length TAN1 with the same alanine substitutions had defects in phragmoplast guidance and reduced TAN1 and POK1 localization at the division site but rescued most tan1 air9 mutant phenotypes. Together, these data suggest that TAN1 and AIR9 are required for POK1 localization, and yet unknown proteins may stabilize TAN1-POK1 interactions.

Stochasticity and the limits of molecular signaling in plant development

Understanding plant development is in part a theoretical endeavor that can only succeed if it is based upon a correctly contrived axiomatic framework. Here I revisit some of the basic assumptions that frame our understanding of plant development and suggest that we consider an alternative informational ecosystem that more faithfully reflects the physical and architectural realities of plant tissue and organ growth. I discuss molecular signaling as a stochastic process and propose that the iterative and architectural nature of plant growth is more usefully represented by deterministic models based upon structural, surficial, and stress-mechanical information networks that come into play at the trans-cellular level.

Plant cell division from the perspective of polarity

The orientation of cell division is a major determinant of plant morphogenesis. In spite of considerable efforts over the past decades, the precise mechanism of division plane selection remains elusive. The majority of studies on the topic have addressed division orientation from either a predominantly developmental or a cell biological perspective. Thus, mechanistic insights into the links between developmental and cellular factors affecting division orientation are particularly lacking. Here, I review recent progress in the understanding of cell division orientation in the embryo and primary root meristem of Arabidopsis from both developmental and cell biological standpoints. I offer a view of multilevel polarity as a central aspect of cell division: on the one hand, the division plane is a readout of tissue- and organism-wide polarities; on the other hand, the cortical division zone can be seen as a transient polar subcellular plasma membrane domain. Finally, I argue that a polarity-focused conceptual framework and the integration of developmental and cell biological approaches hold great promise to unravel the mechanistic basis of plant cell division orientation in the near future.

Defects in division plane positioning in the root meristematic zone affect cell organization in the differentiation zone

Cell division plane orientation is critical for plant and animal development and growth. TANGLED1 (TAN1) and AUXIN-INDUCED-IN-ROOT-CULTURES9 (AIR9) are division-site localized microtubule-binding proteins required for division plane positioning. tan1 and air9 Arabidopsis thaliana single mutants have minor or no noticeable phenotypes but the tan1 air9 double mutant has synthetic phenotypes including stunted growth, misoriented divisions, and aberrant cell-file rotation in the root differentiation zone. These data suggest that TAN1 plays a role in nondividing cells. To determine whether TAN1 is required in elongating and differentiating cells in the tan1 air9 double mutant, we limited its expression to actively dividing cells using the G2/M-specific promoter of the syntaxin KNOLLE (pKN:TAN1-YFP). Unexpectedly, in addition to rescuing division plane defects, pKN:TAN1-YFP rescued root growth and the root differentiation zone cell file rotation defects in the tan1 air9 double mutant. This suggests that defects that occur in the meristematic zone later affect the organization of elongating and differentiating cells.

Combined computational modeling and experimental analysis integrating chemical and mechanical signals suggests possible mechanism of shoot meristem maintenance

Stem cell maintenance in multilayered shoot apical meristems (SAMs) of plants requires strict regulation of cell growth and division. Exactly how the complex milieu of chemical and mechanical signals interact in the central region of the SAM to regulate cell division plane orientation is not well understood. In this paper, simulations using a newly developed multiscale computational model are combined with experimental studies to suggest and test three hypothesized mechanisms for the regulation of cell division plane orientation and the direction of anisotropic cell expansion in the corpus. Simulations predict that in the Apical corpus, WUSCHEL and cytokinin regulate the direction of anisotropic cell expansion, and cells divide according to tensile stress on the cell wall. In the Basal corpus, model simulations suggest dual roles for WUSCHEL and cytokinin in regulating both the direction of anisotropic cell expansion and cell division plane orientation. Simulation results are followed by a detailed analysis of changes in cell characteristics upon manipulation of WUSCHEL and cytokinin in experiments that support model predictions. Moreover, simulations predict that this layer-specific mechanism maintains both the experimentally observed shape and structure of the SAM as well as the distribution of WUSCHEL in the tissue. This provides an additional link between the roles of WUSCHEL, cytokinin, and mechanical stress in regulating SAM growth and proper stem cell maintenance in the SAM.

Opposite polarity programs regulate asymmetric subsidiary cell divisions in grasses

Grass stomata recruit lateral subsidiary cells (SCs), which are key to the unique stomatal morphology and the efficient plant-atmosphere gas exchange in grasses. Subsidiary mother cells (SMCs) strongly polarise before an asymmetric division forms a SC. Yet apart from a proximal polarity module that includes PANGLOSS1 (PAN1) and guides nuclear migration, little is known regarding the developmental processes that form SCs. Here, we used comparative transcriptomics of developing wild-type and SC-less bdmute leaves in the genetic model grass Brachypodium distachyon to identify novel factors involved in SC formation. This approach revealed BdPOLAR, which forms a novel, distal polarity domain in SMCs that is opposite to the proximal PAN1 domain. Both polarity domains are required for the formative SC division yet exhibit various roles in guiding pre-mitotic nuclear migration and SMC division plane orientation, respectively. Nonetheless, the domains are linked as the proximal domain controls polarisation of the distal domain. In summary, we identified two opposing polarity domains that coordinate the SC division, a process crucial for grass stomatal physiology.

Brassinosteroids regulate petal spur length in Aquilegia by controlling cell elongation

BACKGROUND AND AIMS

Aquilegia produce elongated, three-dimensional petal spurs that fill with nectar to attract pollinators. Previous studies have shown that the diversity of spur length across the Aquilegia genus is a key innovation that is tightly linked with its recent and rapid diversification into new ranges, and that evolution of increased spur lengths is achieved via anisotropic cell elongation. Previous work identified a brassinosteroid response transcription factor as being enriched in the early developing spur cup. Brassinosteroids are known to be important for cell elongation, suggesting that brassinosteroid-mediated response may be an important regulator of spur elongation and potentially a driver of spur length diversity in Aquilegia. In this study, we investigated the role of brassinosteroids in the development of the Aquilegia coerulea petal spur.

METHODS

We exogenously applied the biologically active brassinosteroid brassinolide to developing petal spurs to investigate spur growth under high hormone conditions. We used virus-induced gene silencing and gene expression experiments to understand the function of brassinosteroid-related transcription factors in A. coerulea petal spurs.

KEY RESULTS

We identified a total of three Aquilegia homologues of the BES1/BZR1 protein family and found that these genes are ubiquitously expressed in all floral tissues during development, yet, consistent with the previous RNAseq study, we found that two of these paralogues are enriched in early developing petals. Exogenously applied brassinosteroid increased petal spur length due to increased anisotropic cell elongation as well as cell division. We found that targeting of the AqBEH genes with virus-induced gene silencing resulted in shortened petals, a phenotype caused in part by a loss of cell anisotropy.

CONCLUSIONS

Collectively, our results support a role for brassinosteroids in anisotropic cell expansion in Aquilegia petal spurs and highlight the brassinosteroid pathway as a potential player in the diversification of petal spur length in Aquilegia.

Confocal Analysis of Arabidopsis Root Cell Divisions in 3D: A Focus on the Endodermis

The development of multicellular organisms requires coordinated cell divisions for the production of diverse cell types and body plan elaboration and growth. There are two main types of cell divisions: proliferative or symmetric divisions, which produce more cells of a given type, and formative or asymmetric divisions, which produce cells of different types. Because plant cells are surrounded by cell walls, the orientation of plant cell divisions is particularly important in cell fate specification and tissue or organ morphology. The cellular organization of the Arabidopsis thaliana root makes an excellent tool to study how oriented cell division contributes to tissue patterning during organ development. To understand how division plane orientation in a specific genotype or growth condition may impact organ or tissue development, a detailed characterization of cell division orientation is required. Here we describe a confocal microscopy-based, live imaging method for Arabidopsis root tips to examine the 3D orientations of cell division planes and quantify formative, proliferative, and atypical endodermal cell divisions.

TANGLED1 mediates microtubule interactions that may promote division plane positioning in maize

The microtubule cytoskeleton serves as a dynamic structural framework for mitosis in eukaryotic cells. TANGLED1 (TAN1) is a microtubule-binding protein that localizes to the division site and mitotic microtubules and plays a critical role in division plane orientation in plants. Here, in vitro experiments demonstrate that TAN1 directly binds microtubules, mediating microtubule zippering or end-on microtubule interactions, depending on their contact angle. Maize tan1 mutant cells improperly position the preprophase band (PPB), which predicts the future division site. However, cell shape–based modeling indicates that PPB positioning defects are likely a consequence of abnormal cell shapes and not due to TAN1 absence. In telophase, colocalization of growing microtubules ends from the phragmoplast with TAN1 at the division site suggests that TAN1 interacts with microtubule tips end-on. Together, our results suggest that TAN1 contributes to microtubule organization to ensure proper division plane orientation.

Differential gene expression profiling of one- and two-dimensional apogamous gametophytes of the fern Dryopteris affinis ssp. affinis

Apomixis was originally defined as the replacement of sexual reproduction by an asexual process that does not involve fertilization but, in angiosperms, it is often used in the more restricted sense of asexual reproduction through seeds. In ferns, apomixis combines the production of unreduced spores (diplospory) and the formation of sporophytes from somatic cells of the prothallium (apogamy). The genes that control the onset of apogamy in ferns are largely unknown. In this study, we describe the gametophyte transcriptome of the apogamous fern Dryopteris affinis ssp. affinis using an RNA-Seq approach to compare the gene expression profiles of one- and two-dimensional gametophytes, the latter containing apogamic centers. After collapsing highly similar de novo transcripts, we obtained 166,191 unigenes, of which 30% could be annotated using public databases. Multiple quality metrics indicate a good quality of the de novo transcriptome with a low level of fragmentation. Our data show a total of 10,679 genes (6% of all genes) to be differentially expressed between gametophytes of filamentous (one-dimensional) and prothallial (two-dimensional) architecture. 6,110 genes were up-regulated in two-dimensional relative to one-dimensional gametophytes, some of which are implicated in the regulation of meristem growth, auxin signaling, reproduction, and sucrose metabolism. 4,570 genes were down-regulated in two-dimensional versus one-dimensional gametophytes, which are enriched in stimulus and defense genes, as well as genes involved in epigenetic gene regulation and ubiquitin degradation. Our results provide insights into free-living gametophyte development, focusing on the filamentous-to-prothallus growth transition, and provide a useful resource for further investigations of asexual reproduction.

Cellular perspectives for improving mesophyll conductance

After entering the leaf, CO2 faces an intricate pathway to the site of photosynthetic fixation embedded within the chloroplasts. The efficiency of CO2 flux is hindered by a number of structural and biochemical barriers which, together, define the ease of flow of the gas within the leaf, termed mesophyll conductance. Previous authors have identified the key elements of this pathway, raising the prospect of engineering the system to improve CO2 flux and, thus, to increase leaf photosynthetic efficiency. In this review, we provide a perspective on the potential for improving the individual elements that contribute to this complex parameter. We lay particular emphasis on generation of the cellular architecture of the leaf which sets the initial boundaries of a number of mesophyll conductance parameters, incorporating an overview of the molecular transport processes which have been proposed as major facilitators of CO2 flux across structural boundaries along the pathway. The review highlights the research areas where future effort might be invested to increase our fundamental understanding of mesophyll conductance and leaf function and, consequently, to enable translation of these findings to improve the efficiency of crop photosynthesis.

De-Esterified Homogalacturonan Enrichment of the Cell Wall Region Adjoining the Preprophase Cortical Cytoplasmic Zone in Some Protodermal Cell Types of Three Land Plants

The distribution of highly de-esterified homogalacturonans (HGs) in dividing protodermal cells of the monocotyledon Zea mays, the dicotyledon Vigna sinensis, and the fern Asplenium nidus was investigated in order to examine whether the cell wall region adjoining the preprophase band (PPB) is locally diversified. Application of immunofluorescence revealed that de-esterified HGs were accumulated selectively in the cell wall adjacent to the PPB in: (a) symmetrically dividing cells of stomatal rows of Z. mays, (b) the asymmetrically dividing protodermal cells of Z. mays, (c) the symmetrically dividing guard cell mother cells (GMCs) of Z. mays and V. sinensis, and (d) the symmetrically dividing protodermal cells of A. nidus. A common feature of the above cell types is that the cell division plane is defined by extrinsic cues. The presented data suggest that the PPB cortical zone-plasmalemma and the adjacent cell wall region function in a coordinated fashion in the determination/accomplishment of the cell division plane, behaving as a continuum. The de-esterified HGs, among other possible functions, might be involved in the perception and the transduction of the extrinsic cues determining cell division plane in the examined cells.

Cell Size Control in Plants

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

An ontogenetic framework for functional studies in the model fern Ceratopteris richardii

As the sister group to seed plants, ferns are a phylogenetically informative lineage. Functional studies in representatives of the fern lineage are helping bridge the knowledge gap in developmental mechanisms between angiosperms and non-vascular plants. The fern life cycle has the advantage of combining a sizable free-living haploid gametophyte, more amenable for developmental studies than the reduced seed plant gametophyte, with an indeterminate and complex diploid sporophyte. Ceratopteris richardii has long been proposed as a model fern and has recently become tractable due to stable transgenesis and increasing genomic resources, allowing researchers to test explicit questions about gene function in a fern for the first time. As with any model system, a detailed understanding of wild-type morphology and a staged ontogeny are indispensable for the characterization of mutant phenotypes resulting from genetic manipulations. Therefore, the goal of this study is to provide a unified reference ontogeny for this emerging model fern as a tool for comparative evolutionary and developmental studies. It complements earlier research by filling gaps in major stages of development of the haploid gametophyte and diploid sporophyte generations, and provides additional descriptions of the shoot apical meristem and early leaf development. This resource is meant to facilitate not only studies of candidate genes within C. richardii, but also broader ontogenetic comparisons to other model plants.

Plant cell division - defining and finding the sweet spot for cell plate insertion

The plant microtubules form unique arrays using acentrosomal microtubule nucleation pathways, yet utilizing evolutionary conserved centrosomal proteins. In cytokinesis, a multi-component cytoskeletal apparatus, the phragmoplast mediates the biosynthesis of the new cell plate by dynamic centrifugal expansion, a process that demands exquisite coordination of microtubule turnover and endomembrane trafficking. At the same time, the phragmoplast is guided to meet with the parental wall at a cortical site that is predefined before mitotic entry and transiently marked by the preprophase band of microtubules. The cortical division zone maintains positional information of the selected division plane for the entire duration of cell division and for the guidance of the phragmoplast during cytokinesis. Its establishment is an essential requirement for normal plant organogenesis, due to the confinement of cells by rigid cell walls.

Cytoskeletal discoveries in the plant lineage using the moss Physcomitrella patens

Advances in cell biology have been largely driven by pioneering work in model systems, the majority of which are from one major eukaryotic lineage, the opisthokonts. However, with the explosion of genomic information in many lineages, it has become clear that eukaryotes have incredible diversity in many cellular systems, including the cytoskeleton. By identifying model systems in diverse lineages, it may be possible to begin to understand the evolutionary origins of the eukaryotic cytoskeleton. Within the plant lineage, cell biological studies in the model moss, Physcomitrella patens, have over the past decade provided key insights into how the cytoskeleton drives cell and tissue morphology. Here, we review P. patens attributes that make it such a rich resource for cytoskeletal cell biological inquiry and highlight recent key findings with regard to intracellular transport, microtubule-actin interactions, and gene discovery that promises for many years to provide new cytoskeletal players.