Analysis of formin functions during cytokinesis using specific inhibitor SMIFH2

Regulation of cytokinetic machinery in plants

Plant cells divide by constructing a two-dimensional membrane compartment filled with oligosaccharides known as the cell plate. The cell plate is produced by the phragmoplast, a plant-specific structure composed of cytoskeletal polymers, membranes, and associated proteins. Initially, the phragmoplast forms as a disk between daughter nuclei at the end of anaphase, then continues to expand outward until the cell plate connects to the parental cell wall. Phragmoplast expansion encompasses dramatic reorganization of microtubules. At the start, microtubules form short antiparallel overlaps that initiate cell plate biogenesis by recruiting membrane material in the form of cytokinetic vesicles. Subsequent membrane expansion and remodeling processes are accompanied by dissolution of the antiparallel overlaps and attachment of microtubules to the cell plate biogenesis machinery. Deposition of oligosaccharides into the lumen confers mechanical rigidity to the cell plate that triggers depolymerization of microtubules. Precise coordination of microtubule organization with vesicle trafficking, membrane remodeling, and the deposition of oligosaccharides plays a critical role for cell plate production. This review summarizes current understanding of key signaling pathways that couple diverse processes in the phragmoplast.

A RhoGAP controls apical actin polymerization by inhibiting formin in Arabidopsis pollen tubes

Formin is an important player in promoting apical actin polymerization in pollen tubes, but the mechanism regulating its activity remains unknown. We here identify REN1, a Rho GTPase-activating protein, as a negative regulator of formins in Arabidopsis pollen tubes. Specifically, we found that depletion of REN1 promotes apical actin polymerization and increases the amount of filamentous actin in pollen tubes. Interestingly, the effect of REN1 loss of function phenocopies the effect of formin gain of function, as it causes the formation of supernumerary membrane-derived actin bundles, which leads to tube swelling and membrane deformation. Importantly, inhibition of formins suppresses the phenotypic defects in ren1 mutant pollen tubes. We further demonstrate that REN1 physically interacts with the Arabidopsis formin protein AtFH5, predominantly with the C terminus, and inhibits the ability of AtFH5 to nucleate and assemble actin in vitro. Depletion of AtFH5 partially suppresses the phenotype in ren1 mutant pollen tubes, demonstrating that REN1 regulates apical actin polymerization at least partially through inhibiting AtFH5. We thus uncover a novel mechanism regulating formins and actin polymerization in plants.

Endomembrane trafficking driven by microtubule growth regulates stomatal movement in Arabidopsis

Microtubule-based vesicle trafficking usually relies upon kinesin and dynein motors and few reports describe microtubule polymerisation driving directional vesicle trafficking. Here we show that Arabidopsis END BINDING1b (EB1b), a microtubule plus-end binding protein, directly interacts with SYP121, a SNARE protein that mediates the trafficking of the K+ channel KAT1 and its distribution to the plasma membrane (PM) in Arabidopsis guard cells. Knockout of AtEB1b and its homologous proteins results in a modest but significant change in the distribution of KAT1 and SYP121 in guard cells and consequently delays light-induced stomatal opening. Live-cell imaging reveals that a portion of SYP121-associated endomembrane compartments co-localise with AtEB1b at the growing ends of microtubules, trafficking along with the growth of microtubules for targeting to the PM. Our study reveals a mechanism of vesicle trafficking driven by microtubule growth, which is involved in the redistribution of PM proteins to modulate guard cell movement.

Transmembrane formins as active cargoes of membrane trafficking

Formins are a large, evolutionarily old family of cytoskeletal regulators whose roles include actin capping and nucleation, as well as modulation of microtubule dynamics. The plant class I formin clade is characterized by a unique domain organization, as most of its members are transmembrane proteins with possible cell wall-binding motifs exposed to the extracytoplasmic space-a structure that appears to be a synapomorphy of the plant kingdom. While such transmembrane formins are traditionally considered mainly as plasmalemma-localized proteins contributing to the organization of the cell cortex, we review, from a cell biology perspective, the growing evidence that they can also, at least temporarily, reside (and in some cases also function) in endomembranes including secretory and endocytotic pathway compartments, the endoplasmic reticulum, the nuclear envelope, and the tonoplast. Based on this evidence, we propose that class I formins may thus serve as 'active cargoes' of membrane trafficking-membrane-embedded proteins that modulate the fate of endo- or exocytotic compartments while being transported by them.

Four-dimensional quantitative analysis of cell plate development in Arabidopsis using lattice light sheet microscopy identifies robust transition points between growth phases

Cell plate formation during cytokinesis entails multiple stages occurring concurrently and requiring orchestrated vesicle delivery, membrane remodelling, and timely deposition of polysaccharides, such as callose. Understanding such a dynamic process requires dissection in time and space; this has been a major hurdle in studying cytokinesis. Using lattice light sheet microscopy (LLSM), we studied cell plate development in four dimensions, through the behavior of yellow fluorescent protein (YFP)-tagged cytokinesis-specific GTPase RABA2a vesicles. We monitored the entire duration of cell plate development, from its first emergence, with the aid of YFP-RABA2a, in both the presence and absence of cytokinetic callose. By developing a robust cytokinetic vesicle volume analysis pipeline, we identified distinct behavioral patterns, allowing the identification of three easily trackable cell plate developmental phases. Notably, the phase transition between phase I and phase II is striking, indicating a switch from membrane accumulation to the recycling of excess membrane material. We interrogated the role of callose using pharmacological inhibition with LLSM and electron microscopy. Loss of callose inhibited the phase transitions, establishing the critical role and timing of the polysaccharide deposition in cell plate expansion and maturation. This study exemplifies the power of combining LLSM with quantitative analysis to decode and untangle such a complex process.

The microtubule-nucleating factor MACERATOR tethers AUGMIN7 to microtubules and governs phragmoplast architecture

The plant cytokinetic microtubule array, called the phragmoplast, exhibits higher microtubule dynamics in its center (midzone) than at the periphery (distal zone). This behavior is known as the axial asymmetry. Despite being a major characteristic of the phragmoplast, little is known about regulators of this phenomenon. Here we address the role of microtubule nucleation in axial asymmetry by characterizing MACERATOR (MACET) proteins in Arabidopsis thaliana and Nicotiana benthamiana with a combination of genetic, biochemical, and live-cell imaging assays, using photo-convertible microtubule probes, and modeling. MACET paralogs accumulate at the shrinking microtubule ends and decrease the tubulin OFF rate. Loss of MACET4 and MACET5 function abrogates axial asymmetry by suppressing microtubule dynamicity in the midzone. MACET4 also narrows the microtubule nucleation angle at the phragmoplast leading edge and functions as a microtubule tethering factor for AUGMIN COMPLEX SUBUNIT 7 (AUG7). The macet4 macet5 double mutant shows diminished clustering of AUG7 in the phragmoplast distal zone. Knockout of AUG7 does not affect MACET4 localization, axial asymmetry, or microtubule nucleation angle, but increases phragmoplast length and slows down phragmoplast expansion. The mce4-1 mce5 aug7-1 triple knockout is not viable. Experimental data and modeling demonstrate that microtubule nucleation factors regulate phragmoplast architecture and axial asymmetry directly by generating new microtubules and indirectly by modulating the abundance of free tubulin.

EB1C forms dimer and interacts with protein phosphatase 2A (PP2A) to regulate fiber elongation in upland cotton (Gossypium hirsutum)

Cotton is the most economically important natural fiber crop grown in more than sixty-five countries of the world. Fiber length is the main factor affecting fiber quality, but the existing main varieties are short in length and cannot suit the higher demands of the textile industry. It is necessary to discover functional genes that enable fiber length improvement in cotton through molecular breeding. In this study, overexpression of GhEB1C in Arabidopsis thaliana significantly promotes trichomes, tap roots, and root hairs elongation. The molecular regulation of GhEB1C involves its interactions with itself and GhB'ETA, and the function of GhEB1C regulation mainly depends on the two cysteine residues located at the C-terminal. In particular, the function activity of GhEB1C protein triggered with the regulation of protein phosphatase 2A, while silencing of GhEB1C in cotton significantly influenced the fiber protrusions and elongation mechanisms., Further, influenced the expression of MYB-bHLH-WD40 complex, brassinosteroids, and jasmonic acid-related genes, which showed that transcriptional regulation of GhEB1C is indispensable for cotton fiber formation and elongation processes. Our study analyzed the brief molecular mechanism of GhEB1C regulation. Further elucidated that GhEB1C can be a potential target gene to improve cotton fiber length through transgenic breeding.

Tools for studying the cytoskeleton during plant cell division

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

Reforming the Barrier: The Role of Formins in Wound Repair

The restoration of an intact epidermal barrier after wound injury is the culmination of a highly complex and exquisitely regulated physiological process involving multiple cells and tissues, overlapping dynamic events and protein synthesis and regulation. Central to this process is the cytoskeleton, a system of intracellular proteins that are instrumental in regulating important processes involved in wound repair including chemotaxis, cytokinesis, proliferation, migration, and phagocytosis. One highly conserved family of cytoskeletal proteins that are emerging as major regulators of actin and microtubule nucleation, polymerization, and stabilization are the formins. The formin family includes 15 different proteins categorized into seven subfamilies based on three formin homology domains (FH1, FH2, and FH3). The formins themselves are regulated in different ways including autoinhibition, activation, and localization by a range of proteins, including Rho GTPases. Herein, we describe the roles and effects of the formin family of cytoskeletal proteins on the fundamental process of wound healing and highlight recent advances relating to their important functions, mechanisms, and regulation at the molecular and cellular levels.

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.

Cell biology of primary cell wall synthesis in plants

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