Division site determination during asymmetric cell division in plants

RHO OF PLANTS signalling and the activating ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS: specificity in cellular signal transduction in plants

Every cell constantly receives signals from its neighbours or the environment. In plants, most signals are perceived by RECEPTOR-LIKE KINASEs (RLKs) and then transmitted into the cell. The molecular switches RHO OF PLANTS (ROP) are critical proteins for polar signal transduction and regulate multiple cell polarity processes downstream of RLKs. Many ROP-regulating proteins and scaffold proteins of the ROP complex are known. However, the spatiotemporal ROP signalling complex composition is not yet understood. Moreover, how specificity is achieved in different ROP signalling pathways within one cell still needs to be determined. This review gives an overview of recent advances in ROP signalling and how specificity by downstream scaffold proteins can be achieved. The composition of the ROP signalling complexes is discussed, focusing on the possibility of the simultaneous presence of ROP activators and inactivators within the same complex to balance ROP activity. Furthermore, this review highlights the function of plant-specific ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS polarizing ROP signalling and defining the specificity of the initiated ROP signalling pathway.

Dehydration-responsive cytoskeleton proteome of rice reveals reprograming of key molecular pathways to mediate metabolic adaptation and cell survival

The plant cytoskeletal proteins play a key role that control cytoskeleton dynamics, contributing to crucial biological processes such as cell wall morphogenesis, stomatal conductance and abscisic acid accumulation in repercussion to water-deficit stress or dehydration. Yet, it is still completely unknown which specific biochemical processes and regulatory mechanisms the cytoskeleton uses to drive dehydration tolerance. To better understand the role of cytoskeleton, we developed the dehydration-responsive cytoskeletal proteome map of a resilient rice cultivar. Initially, four-week-old rice plants were exposed to progressive dehydration, and the magnitude of dehydration-induced compensatory physiological responses was monitored in terms of physicochemical indices. The organelle fractionation in conjunction with label-free quantitative proteome analysis led to the identification of 955 dehydration-responsive cytoskeletal proteins (DRCPs). To our knowledge, this is the first report of a stress-responsive plant cytoskeletal proteome, representing the largest inventory of cytoskeleton and cytoskeleton-associated proteins. The DRCPs were apparently involved in a wide array of intra-cellular molecules transportation, organelles positioning, cytoskeleton organization followed by different metabolic processes including amino acid metabolism. These findings presented open a unique view on global regulation of plant cytoskeletal proteome is intimately linked to cellular metabolic rewiring of adaptive responses, and potentially confer dehydration tolerance, especially in rice, and other crop species, in general.

Comprehensive and quantitative analysis of intracellular structure polarization at the apical-basal axis in elongating Arabidopsis zygotes

A comprehensive and quantitative evaluation of multiple intracellular structures or proteins is a promising approach to provide a deeper understanding of and new insights into cellular polarity. In this study, we developed an image analysis pipeline to obtain intensity profiles of fluorescent probes along the apical-basal axis in elongating Arabidopsis thaliana zygotes based on two-photon live-cell imaging data. This technique showed the intracellular distribution of actin filaments, mitochondria, microtubules, and vacuolar membranes along the apical-basal axis in elongating zygotes from the onset of cell elongation to just before asymmetric cell division. Hierarchical cluster analysis of the quantitative data on intracellular distribution revealed that the zygote may be compartmentalized into two parts, with a boundary located 43.6% from the cell tip, immediately after fertilization. To explore the biological significance of this compartmentalization, we examined the positions of the asymmetric cell divisions from the dataset used in this distribution analysis. We found that the cell division plane was reproducibly inserted 20.5% from the cell tip. This position corresponded well with the midpoint of the compartmentalized apical region, suggesting a potential relationship between the zygote compartmentalization, which begins with cell elongation, and the position of the asymmetric cell division.

Membrane microdomains: Structural and signaling platforms for establishing membrane polarity

Cell polarity results from the asymmetric distribution of cellular structures, molecules, and functions. Polarity is a fundamental cellular trait that can determine the orientation of cell division, the formation of particular cell shapes, and ultimately the development of a multicellular body. To maintain the distinct asymmetric distribution of proteins and lipids in cellular membranes, plant cells have developed complex trafficking and regulatory mechanisms. Major advances have been made in our understanding of how membrane microdomains influence the asymmetric distribution of proteins and lipids. In this review, we first give an overview of cell polarity. Next, we discuss current knowledge concerning membrane microdomains and their roles as structural and signaling platforms to establish and maintain membrane polarity, with a special focus on the asymmetric distribution of proteins and lipids, and advanced microscopy techniques to observe and characterize membrane microdomains. Finally, we review recent advances regarding membrane trafficking in cell polarity establishment and how the balance between exocytosis and endocytosis affects membrane polarity.

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.

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.

Rab GTPases, tethers, and SNAREs work together to regulate Arabidopsis cell plate formation

Cell plates are transient structures formed by the fusion of vesicles at the center of the dividing plane; furthermore, these are precursors to new cell walls and are essential for cytokinesis. Cell plate formation requires a highly coordinated process of cytoskeletal rearrangement, vesicle accumulation and fusion, and membrane maturation. Tethering factors have been shown to interact with the Ras superfamily of small GTP binding proteins (Rab GTPases) and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which are essential for cell plate formation during cytokinesis and are fundamental for maintaining normal plant growth and development. In Arabidopsis thaliana, members of the Rab GTPases, tethers, and SNAREs are localized in cell plates, and mutations in the genes encoding these proteins result in typical cytokinesis-defective phenotypes, such as the formation of abnormal cell plates, multinucleated cells, and incomplete cell walls. This review highlights recent findings on vesicle trafficking during cell plate formation mediated by Rab GTPases, tethers, and SNAREs.

Asymmetric cell division in plant development

Asymmetric cell division (ACD) is a fundamental process that generates new cell types during development in eukaryotic species. In plant development, post-embryonic organogenesis driven by ACD is universal and more important than in animals, in which organ pattern is preset during embryogenesis. Thus, plant development provides a powerful system to study molecular mechanisms underlying ACD. During the past decade, tremendous progress has been made in our understanding of the key components and mechanisms involved in this important process in plants. Here, we present an overview of how ACD is determined and regulated in multiple biological processes in plant development and compare their conservation and specificity among different model cell systems. We also summarize the molecular roles and mechanisms of the phytohormones in the regulation of plant ACD. Finally, we conclude with the overarching paradigms and principles that govern plant ACD and consider how new technologies can be exploited to fill the knowledge gaps and make new advances in the field.

Plant cytokinesis and the construction of new cell wall

Cytokinesis in plants is fundamentally different from that in animals and fungi. In plant cells, a cell plate forms through the fusion of cytokinetic vesicles and then develops into the new cell wall, partitioning the cytoplasm of the dividing cell. The formation of the cell plate entails multiple stages that involve highly orchestrated vesicle accumulation, fusion, and membrane maturation, which occur concurrently with the timely deposition of polysaccharides such as callose, cellulose, and cross-linking glycans. This review summarizes the major stages in cytokinesis, endomembrane components involved in cell plate assembly and its transition to a new cell wall. An animation that can be widely used for educational purposes further summarizes the process.