KinG Is a Plant-Specific Kinesin That Regulates Both Intra- and Intercellular Movement of SHORT-ROOT

Distributing Plant Developmental Regulatory Proteins via Plasmodesmata

During plant development, mobile proteins, including transcription factors, abundantly serve as messengers between cells to activate transcriptional signaling cascades in distal tissues. These proteins travel from cell to cell via nanoscopic tunnels in the cell wall known as plasmodesmata. Cellular control over this intercellular movement can occur at two likely interdependent levels. It involves regulation at the level of plasmodesmata density and structure as well as at the level of the cargo proteins that traverse these tunnels. In this review, we cover the dynamics of plasmodesmata formation and structure in a developmental context together with recent insights into the mechanisms that may control these aspects. Furthermore, we explore the processes involved in cargo-specific mechanisms that control the transport of proteins via plasmodesmata. Instead of a one-fits-all mechanism, a pluriform repertoire of mechanisms is encountered that controls the intercellular transport of proteins via plasmodesmata to control plant development.

Traveling with purpose: cell-to-cell transport of plant mRNAs

Messenger RNAs (mRNAs) in multicellular organisms can act as signals transported cell-to-cell and over long distances. In plants, mRNAs traffic cell-to-cell via plasmodesmata (PDs) and over long distances via the phloem vascular system to control diverse biological processes - such as cell fate and tissue patterning - in destination organs. Research on long-distance transport of mRNAs in plants has made remarkable progress, including the cataloguing of many mobile mRNAs, characterization of mRNA features important for transport, identification of mRNA-binding proteins involved in their transport, and understanding of the physiological roles of mRNA transport. However, information on short-range mRNA cell-to-cell transport is still limited. This review discusses the regulatory mechanisms and physiological functions of mRNA transport at the cellular and whole plant levels.

Calponin homology domain containing kinesin, KIS1, regulates chloroplast stromule formation and immunity

Chloroplast morphology changes during immunity, giving rise to tubule-like structures known as stromules. Stromules extend along microtubules and anchor to actin filaments along nuclei to promote perinuclear chloroplast clustering. This facilitates the transport of defense molecules/proteins from chloroplasts to the nucleus. Evidence for a direct role for stromules in immunity is lacking since, currently, there are no known genes that regulate stromule biogenesis. We show that a calponin homology (CH) domain containing kinesin, KIS1 (kinesin required for inducing stromules 1), is required for stromule formation during TNL [TIR (Toll/Interleukin-1 receptor)-type nucleotide-binding leucine-rich repeat]-immune receptor-mediated immunity. Furthermore, KIS1 is required for TNL-mediated immunity to bacterial and viral pathogens. The microtubule-binding motor domain of KIS1 is required for stromule formation while the actin-binding, CH domain is required for perinuclear chloroplast clustering. We show that KIS1 functions through early immune signaling components, EDS1 and PAD4, with salicylic acid-induced stromules requiring KIS1. Thus, KIS1 represents a player in stromule biogenesis.

Microtubule Regulation in Plants: From Morphological Development to Stress Adaptation

Microtubules (MTs) are essential elements of the eukaryotic cytoskeleton and are critical for various cell functions. During cell division, plant MTs form highly ordered structures, and cortical MTs guide the cell wall cellulose patterns and thus control cell size and shape. Both are important for morphological development and for adjusting plant growth and plasticity under environmental challenges for stress adaptation. Various MT regulators control the dynamics and organization of MTs in diverse cellular processes and response to developmental and environmental cues. This article summarizes the recent progress in plant MT studies from morphological development to stress responses, discusses the latest techniques applied, and encourages more research into plant MT regulation.

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.

Single-cell transcriptome sequencing revealing the difference in photosynthesis and carbohydrate metabolism between epidermal cells and non-epidermal cells of Gracilariopsis lemaneiformis (Rhodophyta)

The allocation of photoassimilates is considered as a key factor for determining plant productivity. The difference in photosynthesis and carbohydrate metabolism between source and sink cells provide the driven force for photoassimilates' allocation. However, photosynthesis and carbohydrate metabolism of different cells and the carbon allocation between these cells have not been elucidated in Gracilariopsis lemaneiformis. In the present study, transcriptome analysis of epidermal cells (EC) and non-epidermal cells (NEC) of G. lemaneiformis under normal light conditions was carried out. There were 3436 differentially expressed genes (DEGs) identified, and most of these DEGs were related to photosynthesis and metabolism. Based on a comprehensive analysis both at physiological and transcriptional level, the activity of photosynthesis and carbohydrate metabolism of EC and NEC were revealed. Photosynthesis activity and the synthesis activity of many low molecular weight carbohydrates (floridoside, sucrose, and others) in EC were significantly higher than those in NEC. However, the main carbon sink, floridean starch and agar, had higher levels in NEC. Moreover, the DEGs related to transportation of photoassimilates were found in this study. These results suggested that photoassimilates of EC could be transported to NEC. This study will contribute to our understanding of the source and sink relationship between the cells in G. lemaneiformis.

CDC48B facilitates the intercellular trafficking of SHORT-ROOT during radial patterning in roots

CELL DIVISION CONTROL PROTEIN48 (CDC48) is essential for membrane fusion, protein degradation, and other cellular processes. Here, we revealed the crucial role of CDC48B in regulating periclinal cell division in roots by analyzing the recessive gen1 mutant. We identified the GEN1 gene through map-based cloning and verified that GEN1 encodes CDC48B. gen1 showed severely inhibited root growth, increased periclinal cell division in the endodermis, defective middle cortex (MC) formation, and altered ground tissue patterning in roots. Consistent with these phenotypes, CYCLIND 6;1(CYCD6;1), a periclinal cell division marker, was upregulated in gen1 compared to Col-0. The ratio of SHR_pro :SHR-GFP fluorescence in pre-dividing nuclei versus the adjacent stele decreased by 33% in gen1, indicating that the trafficking of SHORT-ROOT (SHR) decreased in gen1 when endodermal cells started to divide. These findings suggest that the loss of function of CDC48B inhibits the intercellular trafficking of SHR from the stele to the endodermis, thereby decreasing SHR accumulation in the endodermis. These findings shed light on the crucial role of CDC48B in regulating periclinal cell division in roots.

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.

The functions of kinesin and kinesin-related proteins in eukaryotes

Kinesins constitute a superfamily of ATP-driven microtubule motor enzymes that convert the chemical energy of ATP hydrolysis into mechanical work along microtubule tracks. Kinesins are found in all eukaryotic organisms and are essential to all eukaryotic cells, involved in diverse cellular functions such as microtubule dynamics and morphogenesis, chromosome segregation, spindle formation and elongation and transport of organelles. In this review, we explore recently reported functions of kinesins in eukaryotes and compare their specific cargoes in both plant and animal kingdoms to understand the possible roles of uncharacterized motors in a kingdom based on their reported functions in other kingdoms.

Maintaining the structural and functional homeostasis of the plant endoplasmic reticulum

The endoplasmic reticulum (ER) is a ubiquitous organelle that is vital to the life of eukaryotic cells. It synthesizes essential lipids and proteins and initiates the glycosylation of intracellular and surface proteins. As such, the ER is necessary for cell growth and communication with the external environment. The ER is also a highly dynamic organelle, whose structure is continuously remodeled through an interaction with the cytoskeleton and the action of specialized ER shapers. Recent and significant advances in ER studies have brought to light conserved and unique features underlying the structure and function of this organelle in plant cells. In this review, exciting developments in the understanding of the mechanisms for plant ER structural and functional homeostasis, particularly those that underpin ER network architecture and ER degradation, are presented and discussed.

Intercellular trafficking via plasmodesmata: molecular layers of complexity

Plasmodesmata are intercellular pores connecting together most plant cells. These structures consist of a central constricted form of the endoplasmic reticulum, encircled by some cytoplasmic space, in turn delimited by the plasma membrane, itself ultimately surrounded by the cell wall. The presence and structure of plasmodesmata create multiple routes for intercellular trafficking of a large spectrum of molecules (encompassing RNAs, proteins, hormones and metabolites) and also enable local signalling events. Movement across plasmodesmata is finely controlled in order to balance processes requiring communication with those necessitating symplastic isolation. Here, we describe the identities and roles of the molecular components (specific sets of lipids, proteins and wall polysaccharides) that shape and define plasmodesmata structural and functional domains. We highlight the extensive and dynamic interactions that exist between the plasma/endoplasmic reticulum membranes, cytoplasm and cell wall domains, binding them together to effectively define plasmodesmata shapes and purposes.

Why are ATP-driven microtubule minus-end directed motors critical to plants? An overview of plant multifunctional kinesins

In plants, microtubule and actin cytoskeletons are involved in key processes including cell division, cell expansion, growth and development, biotic and abiotic stress, tropisms, hormonal signalling as well as cytoplasmic streaming in growing pollen tubes. Kinesin enzymes have a highly conserved motor domain for binding microtubule cytoskeleton assisting these motors to organise their own tracks, the microtubules by using chemical energy of ATP hydrolysis. In addition to this conserved binding site, kinesins possess non-conserved variable domains mediating structural and functional interaction of microtubules with other cell structures to perform various cellular jobs such as chromosome segregation, spindle formation and elongation, transport of organelles as well as microtubules-actins cross linking and microtubules sliding. Therefore, how the non-motor variable regions specify the kinesin function is of fundamental importance for all eukaryotic cells. Kinesins are classified into ~17 known families and some ungrouped orphans, of which ~13 families have been recognised in plants. Kinesin-14 family consisted of plant specific microtubules minus end-directed motors, are much diverse and unique to plants in the sense that they substitute the functions of animal dynein. In this review, we explore the functions of plant kinesins, especially from non-motor domains viewpoint, focussing mainly on recent work on the origin and functional diversity of motors that drive microtubule minus-end trafficking events.

Actin Filament Disruption Alters Phragmoplast Microtubule Dynamics during the Initial Phase of Plant Cytokinesis

Plant growth and development relies on the accurate positioning of the cell plate between dividing cells during cytokinesis. The cell plate is synthetized by a specialized structure called the phragmoplast, which contains bipolar microtubules that polymerize to form a framework with the plus ends at or near the division site. This allows the transport of Golgi-derived vesicles toward the plus ends to form and expand the cell plate. Actin filaments play important roles in cell plate expansion and guidance in plant cytokinesis at the late phase, but whether they are involved at the early phase is unknown. To investigate this further, we disrupted the actin filaments in cell cycle-synchronized tobacco BY-2 cells with latrunculin B (LatB), an actin polymerization inhibitor. We observed the cells under a transmission electron microscope or a spinning-disk confocal laser scanning microscope. We found that disruption of actin filaments by LatB caused the membrane vesicles at the equatorial plane of the cell plate to be dispersed rather than form clusters as they did in the untreated cells. The midzone constriction of phragmoplast microtubules also was perturbed in LatB-treated cells. The live cell imaging and kymograph analysis showed that disruption of actin filaments also changed the accumulation timing of NACK1 kinesin, which plays a crucial role in cell plate expansion. This suggests that there are two functionally different types of microtubules in the phragmoplast. Together, our results show that actin filaments regulate phragmoplast microtubules at the initial phase of plant cytokinesis.

From plasmodesma geometry to effective symplasmic permeability through biophysical modelling

Regulation of molecular transport via intercellular channels called plasmodesmata (PDs) is important for both coordinating developmental and environmental responses among neighbouring cells, and isolating (groups of) cells to execute distinct programs. Cell-to-cell mobility of fluorescent molecules and PD dimensions (measured from electron micrographs) are both used as methods to predict PD transport capacity (i.e., effective symplasmic permeability), but often yield very different values. Here, we build a theoretical bridge between both experimental approaches by calculating the effective symplasmic permeability from a geometrical description of individual PDs and considering the flow towards them. We find that a dilated central region has the strongest impact in thick cell walls and that clustering of PDs into pit fields strongly reduces predicted permeabilities. Moreover, our open source multi-level model allows to predict PD dimensions matching measured permeabilities and add a functional interpretation to structural differences observed between PDs in different cell walls.

A role for the endoplasmic reticulum in the cell-to-cell movement of SHORT-ROOT

Plasmodesmata enable the trafficking of various signaling molecules, as well as viruses that exploit these channels for their intercellular movement. Viral movement relies on the endoplasmic reticulum (ER), which serves as a stable platform for the assembly of viral replication complexes and their subsequent shuttling toward plasmodesmata. The role of the ER in the intercellular movement of endogenous proteins is less clear. In the root meristem, the mobile transcription factor SHORT-ROOT (SHR) traffics between cell layers to regulate root radial patterning and differentiation. Movement of SHR is a regulated process that requires several cellular factors including the endomembrane system, intact microtubules and an endosome-associated protein named SHR-interacting-embryonic-lethal (SIEL). Recently, we found that KINESIN G (KinG) interacts with both SIEL and microtubules to support the cell-to-cell movement of SHR. Here, we provide evidence that both SHR-associated endosomes and KinG localize to the endoplasmic reticulum (ER) and that movement of SHR-associated endosomes occurs on the ER. Moreover, we show that compromised ER structure leads to a reduction in the cell-to-cell movement of SHR. Collectively, these results support the hypothesis that the ER plays a role in SHR movement.

Dynamic regulation of plasmodesmatal permeability and its application to horticultural research

Effective cell-to-cell communication allows plants to fine-tune their developmental processes in accordance with the prevailing environmental stimuli. Plasmodesmata (PD) are intercellular channels that span the plant cell wall and serve as cytoplasmic bridges to facilitate efficient exchange of signaling molecules between neighboring cells. The identification of PD-associated proteins and the subsequent elucidation of the regulation of PD structure have provided vital insights into the role of PD architecture in enforcing crucial cellular processes, including callose deposition, ER-Golgi-based secretion, cytoskeleton dynamics, membrane lipid raft organization, chloroplast metabolism, and cell wall formation. In this review, we summarize the emerging discoveries from recent studies that elucidated the regulatory mechanisms involved in PD biogenesis and the dynamics of PD opening-closure. Retrospectively, PD-mediated cell-to-cell communication has been implicated in diverse cellular and physiological processes that are fundamental for the development of horticultural plants. The potential application of PD biotechnological engineering represents a powerful approach for improving agronomic traits in horticultural crops in the future.

The KCH Kinesin Drives Nuclear Transport and Cytoskeletal Coalescence to Promote Tip Cell Growth in Physcomitrella patens

Long-distance transport along microtubules (MTs) is critical for intracellular organization. In animals, antagonistic motor proteins kinesin (plus end directed) and dynein (minus end directed) drive cargo transport. In land plants, however, the identity of motors responsible for transport is poorly understood, as genes encoding cytoplasmic dynein are absent in plant genomes. How other functions of dynein are brought about in plants also remains unknown. Here, we show that a subclass of the kinesin-14 family, KCH (kinesin with calponin homology domain), which can also bind actin, drives MT minus end-directed nuclear transport in the moss Physcomitrella patens When all four KCH genes were deleted, the nucleus was not maintained in the cell center but was translocated to the apical end of protonemal cells. In the knockout (KO) line, apical cell tip growth was also severely suppressed. KCH was localized to MTs, including at the MT focal point near the tip of protonemal cells, where MT plus ends coalesced with actin filaments. MT focus was not stably maintained in KCH KO lines, whereas actin destabilization also disrupted the MT focus in wild-type lines despite KCH remaining on unfocused MTs. KCH had distinct functions in nuclear transport and tip growth, as a truncated KCH construct restored nuclear transport activity, but not tip growth retardation of the KO line. Thus, our study identified KCH as a long-distance retrograde transporter as well as a MT cross-linker, reminiscent of the versatile animal dynein.