The intracellular and intercellular cross-talk during subsidiary cell formation in Zea mays: existing and novel components orchestrating cell polarization and asymmetric division

Stomatal development and orientation: a phylogenetic and ecophysiological perspective

BACKGROUND

Oriented patterning of epidermal cells is achieved primarily by transverse protodermal cell divisions perpendicular to the organ axis, followed by axial cell elongation. In linear leaves with parallel venation, most stomata are regularly aligned with the veins. This longitudinal patterning operates under a strong developmental constraint and has demonstrable physiological benefits, especially in grasses. However, transversely oriented stomata characterize a few groups, among both living angiosperms and extinct Mesozoic seed plants.

SCOPE

This review examines comparative and developmental data on stomatal patterning in a broad phylogenetic context, focusing on the evolutionary and ecophysiological significance of guard-cell orientation. It draws from a diverse range of literature to explore the pivotal roles of the plant growth hormone auxin in establishing polarity and chemical gradients that enable cellular differentiation.

CONCLUSIONS

Transverse stomata evolved iteratively in a few seed-plant groups during the Mesozoic era, especially among parasitic or xerophytic taxa, such as the hemiparasitic mistletoe genus Viscum and the xerophytic shrub Casuarina, indicating a possible link with ecological factors such as the Cretaceous CO2 decline and changing water availability. The discovery of this feature in some extinct seed-plant taxa known only from fossils could represent a useful phylogenetic marker.

The OPAQUE1/DISCORDIA2 myosin XI is required for phragmoplast guidance during asymmetric cell division in maize

Formative asymmetric divisions produce cells with different fates and are critical for development. We show the maize (Zea mays) myosin XI protein, OPAQUE1 (O1), is necessary for asymmetric divisions during maize stomatal development. We analyzed stomatal precursor cells before and during asymmetric division to determine why o1 mutants have abnormal division planes. Cell polarization and nuclear positioning occur normally in the o1 mutant, and the future site of division is correctly specified. The defect in o1 becomes apparent during late cytokinesis, when the phragmoplast forms the nascent cell plate. Initial phragmoplast guidance in o1 is normal; however, as phragmoplast expansion continues o1 phragmoplasts become misguided. To understand how O1 contributes to phragmoplast guidance, we identified O1-interacting proteins. Maize kinesins related to the Arabidopsis thaliana division site markers PHRAGMOPLAST ORIENTING KINESINs (POKs), which are also required for correct phragmoplast guidance, physically interact with O1. We propose that different myosins are important at multiple steps of phragmoplast expansion, and the O1 actin motor and POK-like microtubule motors work together to ensure correct late-stage phragmoplast guidance.

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.

Division site determination during asymmetric cell division in plants

During development, both animals and plants exploit asymmetric cell division (ACD) to increase tissue complexity, a process that usually generates cells dissimilar in size, morphology, and fate. Plants lack the key regulators that control ACD in animals. Instead, plants have evolved two unique cytoskeletal structures to tackle this problem: the preprophase band (PPB) and phragmoplast. The assembly of the PPB and phragmoplast and their contributions to division plane orientation have been extensively studied. However, how the division plane is positioned off the cell center during asymmetric division is poorly understood. Over the past 20 years, emerging evidence points to a critical role for polarly localized membrane proteins in this process. Although many of these proteins are species- or cell type specific, and the molecular mechanism underlying division asymmetry is not fully understood, common features such as morphological changes in cells, cytoskeletal dynamics, and nuclear positioning have been observed. In this review, we provide updates on polarity establishment and nuclear positioning during ACD in plants. Together with previous findings about symmetrically dividing cells and the emerging roles of developmental cues, we aim to offer evolutionary insight into a common framework for asymmetric division-site determination and highlight directions for future work.

The maize single-nucleus transcriptome comprehensively describes signaling networks governing movement and development of grass stomata

The unique morphology of grass stomata enables rapid responses to environmental changes. Deciphering the basis for these responses is critical for improving food security. We have developed a planta platform of single-nucleus RNA-sequencing by combined fluorescence-activated nuclei flow sorting, and used it to identify cell types in mature and developing stomata from 33,098 nuclei of the maize epidermis-enriched tissues. Guard cells (GCs) and subsidiary cells (SCs) displayed differential expression of genes, besides those encoding transporters, involved in the abscisic acid, CO2, Ca2+, starch metabolism, and blue light signaling pathways, implicating coordinated signal integration in speedy stomatal responses, and of genes affecting cell wall plasticity, implying a more sophisticated relationship between GCs and SCs in stomatal development and dumbbell-shaped guard cell formation. The trajectory of stomatal development identified in young tissues, and by comparison to the bulk RNA-seq data of the MUTE defective mutant in stomatal development, confirmed known features, and shed light on key participants in stomatal development. Our study provides a valuable, comprehensive, and fundamental foundation for further insights into grass stomatal function.

3,4-Dehydro-L-proline Induces Programmed Cell Death in the Roots of Brachypodium distachyon

As cell wall proteins, the hydroxyproline-rich glycoproteins (HRGPs) take part in plant growth and various developmental processes. To fulfil their functions, HRGPs, extensins (EXTs) in particular, undergo the hydroxylation of proline by the prolyl-4-hydroxylases. The activity of these enzymes can be inhibited with 3,4-dehydro-L-proline (3,4-DHP), which enables its application to reveal the functions of the HRGPs. Thus, to study the involvement of HRGPs in the development of root hairs and roots, we treated seedlings of Brachypodium distachyon with 250 µM, 500 µM, and 750 µM of 3,4-DHP. The histological observations showed that the root epidermis cells and the cortex cells beneath them ruptured. The immunostaining experiments using the JIM20 antibody, which recognizes the EXT epitopes, demonstrated the higher abundance of this epitope in the control compared to the treated samples. The transmission electron microscopy analyses revealed morphological and ultrastructural features that are typical for the vacuolar-type of cell death. Using the TUNEL test (terminal deoxynucleotidyl transferase dUTP nick end labelling), we showed an increase in the number of nuclei with damaged DNA in the roots that had been treated with 3,4-DHP compared to the control. Finally, an analysis of two metacaspases' gene activity revealed an increase in their expression in the treated roots. Altogether, our results show that inhibiting the prolyl-4-hydroxylases with 3,4-DHP results in a vacuolar-type of cell death in roots, thereby highlighting the important role of HRGPs in root hair development and root growth.

Synthesis and regulation of auxin and abscisic acid in maize

Indole-3-acetic acid (IAA), the primary auxin in higher plants, and abscisic acid (ABA) play crucial roles in the ability of maize (Zea mays L.) to acclimatize to various environments by mediating growth, development, defense and nutrient allocation. Although understanding the biochemical reactions for IAA and ABA biosynthesis and signal transduction has progressed, the mechanisms by which auxin and ABA are synthesized and transduced in maize have not been fully elucidated to date. The synthesis and signal transduction pathway of IAA and ABA in maize can be analyzed using an existing model. This article focuses on the research progress toward understanding the synthesis and signaling pathways of IAA and ABA, as well as IAA and ABA regulation of maize growth, providing insight for future development and the significance of IAA and ABA for maize improvement.

The Role of Grass MUTE Orthologues During Stomatal Development

Gas exchange between the plant and the atmosphere takes place through stomatal pores formed by paired guard cells. Grasses develop a unique stomatal structure that consists of two dumbbell-shaped guard cells flanked by lateral subsidiary cells. These structures confer a very efficient gas exchange capacity, which may have contributed to the evolutionary success of grasses. Recent works have identified orthologues of Arabidopsis MUTE in three grass species: BdMUTE in Brachypodium distachyon, BZU2/ZmMUTE in maize, and OsMUTE in rice. These genes induce the recruitment of subsidiary cells, and it appears to rely upon the ability of intercellular movement, from the guard mother cell to subsidiary mother cells, of the proteins encoded by them. Unexpectedly, this function of these grass MUTE genes contrasts with that of Arabidopsis MUTE, which promotes guard mother cell identity. These MUTE orthologues also appear to control guard mother cell fate progression, with the action of BdMUTE being less severe than those of BZU2/ZmMUTE and OsMUTE. The emerging picture unravels that grass MUTE genes have not only diverged, due to neo-functionalization, from Arabidopsis MUTE, but also among them.

Flanking Support: How Subsidiary Cells Contribute to Stomatal Form and Function

Few evolutionary adaptations in plants were so critical as the stomatal complex. This structure allows transpiration and efficient gas exchange with the atmosphere. Plants have evolved numerous distinct stomatal architectures to facilitate gas exchange, while balancing water loss and protection from pathogens that can egress via the stomatal pore. Some plants have simple stomata composed of two kidney-shaped guard cells; however, the stomatal apparatus of many plants includes subsidiary cells. Guard cells and subsidiary cells may originate from a single cell lineage, or subsidiary cells may be recruited from cells adjacent to the guard mother cell. The number and morphology of subsidiary cells varies dramatically, and subsidiary cell function is also varied. Subsidiary cells may support guard cell function by offering a mechanical advantage that facilitates guard cell movements, and/or by acting as a reservoir for water and ions. In other cases, subsidiary cells introduce or enhance certain morphologies (such as sunken stomata) that affect gas exchange. Here we review the diversity of stomatal morphology with an emphasis on multi-cellular stomata that include subsidiary cells. We will discuss how subsidiary cells arise and the divisions that produce them; and provide examples of anatomical, mechanical and biochemical consequences of subsidiary cells on stomatal function.

Callose and homogalacturonan epitope distribution in stomatal complexes of Zea mays and Vigna sinensis

This article deals with the distribution of callose and of the homogalacturonan (HG) epitopes recognized by LM20, JIM5, and 2F4 antibodies in cell walls of differentiating and functioning stomatal complexes of the monocotyledon Zea mays and the dicotyledon Vigna sinensis. The findings revealed that, during stomatal development, in these plant species, callose appears in an accurately spatially and timely controlled manner in cell walls of the guard cells (GCs). In functioning stomata of both plants, callose constitutes a dominant cell wall matrix material of the polar ventral cell wall ends and of the local GC cell wall thickenings. In Zea mays, the LM20, JIM5, or 2F4 antibody-recognized HG epitopes were mainly located in the expanding cell wall regions of the stomatal complexes, while in Vigna sinensis, they were deposited in the local cell wall thickenings of the GCs as well as at the ledges of the stomatal pore. Consideration of the presented data favors the view that in the stomatal complexes of the monocotyledon Z. mays and the dicotyledon V. sinensis, the esterified HGs contribute to the cell wall expansion taking place during GC morphogenesis and the opening of the stomatal pore. Besides, callose and the highly de-esterified HGs allow to GC cell wall regions to withstand the mechanical stresses exerted during stomatal function.

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.

A Subsidiary Cell-Localized Glucose Transporter Promotes Stomatal Conductance and Photosynthesis

It has long been recognized that stomatal movement modulates CO2 availability and as a consequence the photosynthetic rate of plants, and that this process is feedback-regulated by photoassimilates. However, the genetic components and mechanisms underlying this regulatory loop remain poorly understood, especially in monocot crop species. Here, we report the cloning and functional characterization of a maize (Zea mays) mutant named closed stomata1 (cst1). Map-based cloning of cst1 followed by confirmation with the clustered regularly interspaced short palindromic repeats (CRISPR)/ CRISPR associated protein 9 system identified the causal mutation in a Clade I Sugars Will Eventually be Exported Transporters (SWEET) family gene, which leads to the E81K mutation in the CST1 protein. CST1 encodes a functional glucose transporter expressed in subsidiary cells, and the E81K mutation strongly impairs the oligomerization and glucose transporter activity of CST1. Mutation of CST1 results in reduced stomatal opening, carbon starvation, and early senescence in leaves, suggesting that CST1 functions as a positive regulator of stomatal opening. Moreover, CST1 expression is induced by carbon starvation and suppressed by photoassimilate accumulation. Our study thus defines CST1 as a missing link in the feedback-regulation of stomatal movement and photosynthesis by photoassimilates in maize.

The botanical multiverse of Peter Barlow

Dr Peter Barlow, who died in 2017, was one of the most respected botanists and biologists of the latter half of the 20th Century. His interests covered a wide range of plant biological topics, e.g. root growth and development, plant cytoskeleton, effects of gravity, plant intelligence, pattern formation, and evolution of eukaryotic cells. Here we consider Peter's numerous contributions to the: elucidation of plant patterns; understanding of root biology; role of the plant cytoskeleton in growth and development; influence of the Moon on terrestrial vegetation; Cell Body concept; and plant neurobiology. In so doing we attempt not only to provide an overview of Peter's important work in many areas of plant biology, but also to place that work in the context of recent advances in plant and biological sciences.

Cell polarity: compassing cell division and differentiation in plants

Protein polarization underlies directional cell growth, cell morphogenesis, cell division, fate specification and differentiation in plant development. Analysis of in vivo protein dynamics reveals differential mobility of polarized proteins in plant cells, which may arise from lateral diffusion, local protein-protein interactions, and is restricted by protein-membrane-cell wall connections. The asymmetric protein dynamics may provide a mechanism for the regulation of asymmetric cell division and cell differentiation. In light of recent evidence for preprophase band (PPB)-independent mechanisms for orienting division planes, polarity proteins and their dynamics might provide regulation on the PPB at the cell cortex to directly influence phragmoplast positioning or alternatively, impinge on cytoplasmic microtubule-organizing centers (MTOCs) for spindle alignment. Differentiation of specialized cell types is often associated with the spatial regulation of cell wall architecture. Here we discuss the mechanisms of polarized signaling underlying regional cell wall biosynthesis, degradation, and modification during the differentiation of root endodermal cells and leaf epidermal guard cells.

Stomatal Complex Development and F-Actin Organization in Maize Leaf Epidermis Depend on Cellulose Synthesis

Cellulose microfibrils reinforce the cell wall for morphogenesis in plants. Herein, we provide evidence on a series of defects regarding stomatal complex development and F-actin organization in Zea mays leaf epidermis, due to inhibition of cellulose synthesis. Formative cell divisions of stomatal complex ontogenesis were delayed or inhibited, resulting in lack of subsidiary cells and frequently in unicellular stomata, with an atypical stomatal pore. Guard cells failed to acquire a dumbbell shape, becoming rounded, while subsidiary cells, whenever present, exhibited aberrant morphogenesis. F-actin organization was also affected, since the stomatal complex-specific arrays were scarcely observed. At late developmental stages, the overall F-actin network was diminished in all epidermal cells, although thick actin bundles persisted. Taken together, stomatal complex development strongly depends on cell wall mechanical properties. Moreover, F-actin organization exhibits a tight relationship with the cell wall.