The Arabidopsis stem cell factor POLTERGEIST is membrane localized and phospholipid stimulated

Transcriptomic and lipidomic profiles of glycerolipids during Arabidopsis flower development

Flower glycerolipids are the yet-to-be discovered frontier of the lipidome. Although ample evidence suggests important roles for glycerolipids in flower development, stage-specific lipid profiling in tiny Arabidopsis flowers is challenging. Here, we utilized a transgenic system to synchronize flower development in Arabidopsis. The transgenic plant PAP1::AP1-GR ap1-1 cal-5 showed synchronized flower development upon dexamethasone treatment, which enabled massive harvesting of floral samples of homogenous developmental stages for glycerolipid profiling. Glycerolipid profiling revealed a decrease in concentrations of phospholipids involved in signaling during the early development stages, such as phosphatidic acid and phosphatidylinositol, and a marked increase in concentrations of nonphosphorous galactolipids during the late stage. Moreover, in the midstage, phosphatidylinositol 4,5-bisphosphate concentration was increased transiently, which suggests the stimulation of the phosphoinositide metabolism. Accompanying transcriptomic profiling of relevant glycerolipid metabolic genes revealed simultaneous induction of multiple phosphoinositide biosynthetic genes associated with the increased phosphatidylinositol 4,5-bisphosphate concentration, with a high degree of differential expression patterns for genes encoding other glycerolipid-metabolic genes. The phosphatidic acid phosphatase mutant pah1 pah2 showed flower developmental defect, suggesting a role for phosphatidic acid in flower development. Our concurrent profiling of glycerolipids and relevant metabolic gene expression revealed distinct metabolic pathways stimulated at different stages of flower development in Arabidopsis.

Arabidopsis SABRE and CLASP interact to stabilize cell division plane orientation and planar polarity

The orientation of cell division and the coordination of cell polarity within the plane of the tissue layer (planar polarity) contribute to shape diverse multicellular organisms. The root of Arabidopsis thaliana displays regularly oriented cell divisions, cell elongation and planar polarity providing a plant model system to study these processes. Here we report that the SABRE protein, which shares similarity with proteins of unknown function throughout eukaryotes, has important roles in orienting cell division and planar polarity. SABRE localizes at the plasma membrane, endomembranes, mitotic spindle and cell plate. SABRE stabilizes the orientation of CLASP-labelled preprophase band microtubules predicting the cell division plane, and of cortical microtubules driving cell elongation. During planar polarity establishment, sabre is epistatic to clasp at directing polar membrane domains of Rho-of-plant GTPases. Our findings mechanistically link SABRE to CLASP-dependent microtubule organization, shedding new light on the function of SABRE-related proteins in eukaryotes.

Bipolar Plasma Membrane Distribution of Phosphoinositides and Their Requirement for Auxin-Mediated Cell Polarity and Patterning in Arabidopsis

Cell polarity manifested by asymmetric distribution of cargoes, such as receptors and transporters, within the plasma membrane (PM) is crucial for essential functions in multicellular organisms. In plants, cell polarity (re)establishment is intimately linked to patterning processes. Despite the importance of cell polarity, its underlying mechanisms are still largely unknown, including the definition and distinctiveness of the polar domains within the PM. Here, we show in Arabidopsis thaliana that the signaling membrane components, the phosphoinositides phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] as well as PtdIns4P 5-kinases mediating their interconversion, are specifically enriched at apical and basal polar plasma membrane domains. The PtdIns4P 5-kinases PIP5K1 and PIP5K2 are redundantly required for polar localization of specifically apical and basal cargoes, such as PIN-FORMED transporters for the plant hormone auxin. As a consequence of the polarity defects, instructive auxin gradients as well as embryonic and postembryonic patterning are severely compromised. Furthermore, auxin itself regulates PIP5K transcription and PtdIns4P and PtdIns(4,5)P₂ levels, in particular their association with polar PM domains. Our results provide insight into the polar domain-delineating mechanisms in plant cells that depend on apical and basal distribution of membrane lipids and are essential for embryonic and postembryonic patterning.

CLAVATA3 dodecapeptide modified CdTe nanoparticles: a biocompatible quantum dot probe for in vivo labeling of plant stem cells

CLAVATA3 (CLV3) dodecapeptides function in plant stem cell maintenance, but CLV3 function in cell-cell communication remains less clear. Here, we coupled CLV3 dodecapeptides to synthesized CdTe nanoparticles to track their bioactivity on stem cells in the root apical meristem. To achieve this, we first synthesized CdTe quantum dots (QDs) using a one-pot method, and then evaluated the cytotoxicity of the QDs in BY-2 cells. The results showed that QDs in plant cells must be used at low concentrations and for short treatment time. To make biocompatible probes to track stem cell fate, we conjugated CLV3 dodecapeptides to the QDs by the zero-coupling method; this modification greatly reduced the cytotoxicity of the QDs. Furthermore, we detected CLV3-QDs localized on the cell membrane, consistent with the known localization of CLV3. Our results indicate that using surface-modified QDs at low concentrations and for short time treatment can improve their utility for plant cell imaging.

Molecular control of cell specification and cell differentiation during procambial development

Land plants develop vascular tissues that enable the long-distance transport of water and nutrients in xylem and phloem, provide mechanical support for their vertical growth, and produce cells in radial growth. Vascular tissues are produced in many parts of the plant and during different developmental stages. Early vascular development is focused in procambial meristems, and in some species it continues during the secondary phase of plant development in cambial meristems. In this review, we highlight recent progress in understanding procambial development. This involves the analysis of stem cell-like properties of procambial tissues, specification of xylem and phloem, and differentiation of the conductive tissues. Several major plant hormones, small-RNA species, and transcriptional networks play a role in vascular development. We describe current approaches to integrating these networks as well as their potential role in explaining the diversity and evolution of plant vascular systems.

The role of lipid post-translational modification in plant developmental processes

Most eukaryotic proteins are post-translationally modified, and modification has profound effects on protein function. One key modification is the attachment of a lipid group to certain amino acids; this typically facilitates subcellular targeting (association with a membrane) and protein-protein interactions (by virtue of the large hydrophobic moiety). Most widely recognized are lipid modifications of proteins involved in developmental signaling, but proteins with structural roles are also lipid-modified. The three known types of intracellular protein lipid modifications are S-acylation, N-myristoylation, and prenylation. In plants, genetic analysis of the enzymes involved, along with molecular analysis of select target proteins, has recently shed light on the roles of lipid modification in key developmental processes, such as meristem function, flower development, polar cell elongation, cell differentiation, and hormone responses. In addition, while lipid post-translational mechanisms are generally conserved among eukaryotes, plants differ in the nature and function of target proteins, the effects of lipid modification on target proteins, and the roles of lipid modification in developmental processes.

A functional genomic analysis of Arabidopsis thaliana PP2C clade D

In the reference dicot plant Arabidopsis thaliana, the PP2C family of P-protein phosphatases includes the products of 80 genes that have been separated into ten multi-protein clades plus six singletons. Clade D includes the products of nine genes distributed among three chromosomes (APD1, At3g12620; APD2, At3g17090; APD3, At3g51370; APD4, At3g55050; APD5, At4g33920; APD6, At4g38520; APD7, At5g02760; APD8, At5g06750; and APD9, At5g66080). As part of a functional genomics analysis of protein phosphorylation, we retrieved expression data from public databases and determined the subcellular protein localization of the members of clade D. While the nine proteins have been grouped together based upon primary sequence alignments, we observed no obvious common patterns in expression or localization. We found chimera with the GFP associated with the nucleus, plasma membrane, the endomembrane system, and mitochondria in transgenic plants.

Phosphatidylserine synthase 1 is required for inflorescence meristem and organ development in Arabidopsis

Phosphatidylserine (PS), a quantitatively minor membrane phospholipid, is involved in many biological processes besides its role in membrane structure. One PS synthesis gene, PHOSPHATIDYLSERINE SYNTHASE1 (PSS1), has been discovered to be required for microspore development in Arabidopsis thaliana L. but how PSS1 affects postembryonic development is still largely unknown. Here, we show that PSS1 is also required for inflorescence meristem and organ development in Arabidopsis. Disruption of PSS1 causes severe dwarfism, smaller lateral organs and reduced size of inflorescence meristem. Morphological and molecular studies suggest that both cell division and cell elongation are affected in the pss1-1 mutant. RNA in situ hybridization and promoter GUS analysis show that expression of both WUSCHEL (WUS) and CLAVATA3 (CLV3) depend on PSS1. Moreover, the defect in meristem maintenance is recovered and the expression of WUS and CLV3 are restored in the pss1-1 clv1-1 double mutant. Both SHOOTSTEMLESS (STM) and BREVIPEDICELLUS (BP) are upregulated, and auxin distribution is disrupted in rosette leaves of pss1-1. However, expression of BP, which is also a regulator of internode development, is lost in the pss1-1 inflorescence stem. Our data suggest that PSS1 plays essential roles in inflorescence meristem maintenance through the WUS-CLV pathway, and in leaf and internode development by differentially regulating the class I KNOX genes.

WUSCHEL-responsive At5g65480 interacts with CLAVATA components in vitro and in transient expression

The CLAVATA (CLV) signaling pathway is essential for shoot meristem homeostasis in Arabidopsis. CLV acts to limit the expression domain of the stem cell-promoting gene WUSCHEL (WUS). The closely related receptor-kinases CLV1 and BAM1 are key components in this pathway; however, the downstream factors that link the receptors to WUS regulation are poorly understood. The Arabidopsis gene At5g65480 was recently identified as a direct transcriptional target up-regulated by WUS. We have independently identified this gene which we term CCI1 as a CLV1 and BAM1 interacting protein in vitro and in transient expression. CCI1 has phosphatidylinositide-binding activity in vitro and localizes to the plasma membrane in transient expression. Furthermore, CLV signaling components and CCI1 both partition to detergent-resistant membrane microdomains characterized as lipid rafts.

Warm temperatures induce transgenerational epigenetic release of RNA silencing by inhibiting siRNA biogenesis in Arabidopsis

Owing to their sessile nature, plants have evolved sophisticated genetic and epigenetic regulatory systems to respond quickly and reversibly to daily and seasonal temperature changes. However, our knowledge of how plants sense and respond to warming ambient temperatures is rather limited. Here we show that an increase in growth temperature from 22 °C to 30 °C effectively inhibited transgene-induced posttranscriptional gene silencing (PTGS) in Arabidopsis. Interestingly, warmth-induced PTGS release exhibited transgenerational epigenetic inheritance. We discovered that the warmth-induced PTGS release occurred during a critical step that leads to the formation of double-stranded RNA (dsRNA) for producing small interfering RNAs (siRNAs). Deep sequencing of small RNAs and RNA blot analysis indicated that the 22-30 °C increase resulted in a significant reduction in the abundance of many trans-acting siRNAs that require dsRNA for biogenesis. We discovered that the temperature increase reduced the protein abundance of SUPPRESSOR OF GENE SILENCING 3, as a consequence, attenuating the formation of stable dsRNAs required for siRNA biogenesis. Importantly, SUPPRESSOR OF GENE SILENCING 3 overexpression released the warmth-triggered inhibition of siRNA biogenesis and reduced the transgenerational epigenetic memory. Thus, our study reveals a previously undescribed association between warming temperatures, an epigenetic system, and siRNA biogenesis.

Protein S-ACYL Transferase10 is critical for development and salt tolerance in Arabidopsis

Protein S-acylation, commonly known as palmitoylation, is a reversible posttranslational modification that catalyzes the addition of a saturated lipid group, often palmitate, to the sulfhydryl group of a Cys. Palmitoylation regulates enzyme activity, protein stability, subcellular localization, and intracellular sorting. Many plant proteins are palmitoylated. However, little is known about protein S-acyl transferases (PATs), which catalyze palmitoylation. Here, we report that the tonoplast-localized PAT10 is critical for development and salt tolerance in Arabidopsis thaliana. PAT10 loss of function resulted in pleiotropic growth defects, including smaller leaves, dwarfism, and sterility. In addition, pat10 mutants are hypersensitive to salt stresses. We further show that PAT10 regulates the tonoplast localization of several calcineurin B-like proteins (CBLs), including CBL2, CBL3, and CBL6, whose membrane association also depends on palmitoylation. Introducing a C192S mutation within the highly conserved catalytic motif of PAT10 failed to complement pat10 mutants, indicating that PAT10 functions through protein palmitoylation. We propose that PAT10-mediated palmitoylation is critical for vacuolar function by regulating membrane association or the activities of tonoplast proteins.

Core pathways controlling shoot meristem maintenance

Essential to the function of shoot meristems in plants to act as sites of continuous organ and tissue formation is the ability of cells within the meristem to remain undifferentiated and proliferate indefinitely. These are characteristics of the stem cells within meristems that are critical for their growth properties. Stem cells are found in tight association with the stem cell niche-those cells that signal to maintain stem cells. Shoot meristems are unique among stem cell systems in that the stem cell niche is a constantly changing population of recent daughter stem cells. Recent progress from Arabidopsis and other systems have uncovered a large number of genes with defined roles in meristem structure and maintenance. This review will focus on well-studied pathways that represent signaling between the stem cells and the niche, that prevent ectopic differentiation of stem cells, that regulate the chromatin status of stem cell factors, and that reveal intersection of hormone signaling and meristem maintenance.

Arabidopsis SABRE and CLASP interact to stabilize cell division plane orientation and planar polarity

The orientation of cell division and the coordination of cell polarity within the plane of the tissue layer (planar polarity) contribute to shape diverse multicellular organisms. The root of Arabidopsis thaliana displays regularly oriented cell divisions, cell elongation and planar polarity providing a plant model system to study these processes. Here we report that the SABRE protein, which shares similarity with proteins of unknown function throughout eukaryotes, has important roles in orienting cell division and planar polarity. SABRE localizes at the plasma membrane, endomembranes, mitotic spindle and cell plate. SABRE stabilizes the orientation of CLASP-labelled preprophase band microtubules predicting the cell division plane, and of cortical microtubules driving cell elongation. During planar polarity establishment, sabre is epistatic to clasp at directing polar membrane domains of Rho-of-plant GTPases. Our findings mechanistically link SABRE to CLASP-dependent microtubule organization, shedding new light on the function of SABRE-related proteins in eukaryotes.

CLE peptides in plants: proteolytic processing, structure-activity relationship, and ligand-receptor interaction

Ligand-receptor signaling initiated by the CLAVATA3/ ENDOSPERM SURROUNDING REGION (CLE) family peptides is critical in regulating cell division and differentiation in meristematic tissues in plants. Biologically active CLE peptides are released from precursor proteins via proteolytic processing. The mature form of CLE ligands consists of 12-13 amino acids with several post-translational modifications. This review summarizes recent progress toward understanding the proteolytic activities that cleave precursor proteins to release CLE peptides, the molecular structure and function of mature CLE ligands, and interactions between CLE ligands and corresponding leucine-rich repeat (LRR) receptor-like kinases (RLKs).

Comparative phytohormone profiles, lipid kinase and lipid phosphatase activities in barley aleurone, coleoptile, and root tissues

We analyzed lipid kinase and lipid phosphatase activities and determined endogenous phytohormone levels by liquid chromatography-tandem mass spectrometry in root and coleoptile tissues following germination of barley (Hordeum vulgare) seeds. The enzymes showing highest activity in aleurone cells were diacylglycerol kinase (DAG-k, EC 2.7.1.107) and phosphatidate kinase (PA-k). The ratio of gibberellins (GAs) to abscisic acid (ABA) was 2-fold higher in aleurone than in coleoptile or root tissues. In coleoptiles, phosphatidylinositol 4-kinase (PI4-k, EC 2.7.1.67) showed the highest enzyme activity, and jasmonic acid (JA) level was higher than in aleurone. In roots, activities of PI4-k, DAG-k, and PA-k were similar, and salicylic acid (SA) showed the highest concentration. In the assays to evaluate the hydrolysis of DGPP (diacylglycerol pyrophosphate) and PA (phosphatidic acid) we observed that PA hydrolysis by LPPs (lipid phosphate phosphatases) was not modified; however, the diacylglycerol pyrophosphate phosphatase (DGPPase) was strikingly higher in coleoptile and root tissues than to aleurone. Relevance of these findings in terms of signaling responses and seedling growth is discussed.

A role for phosphoinositides in regulating plant nuclear functions

Nuclear localized inositol phospholipids and inositol phosphates are important for regulating many essential processes in animal and yeast cells such as DNA replication, recombination, RNA processing, mRNA export and cell cycle progression. An overview of the current literature indicates the presence of a plant nuclear phosphoinositide (PI) pathway. Inositol phospholipids, inositol phosphates, and enzymes of the PI pathway have been identified in plant nuclei and are implicated in DNA replication, chromatin remodeling, stress responses and hormone signaling. In this review, the potential functions of the nuclear PI pathway in plants are discussed within the context of the animal and yeast literature. It is anticipated that future research will help shed light on the functional significance of the nuclear PI pathway in plants.

AGD1, a class 1 ARF-GAP, acts in common signaling pathways with phosphoinositide metabolism and the actin cytoskeleton in controlling Arabidopsis root hair polarity

The Arabidopsis thaliana AGD1 gene encodes a class 1 adenosine diphosphate ribosylation factor-gtpase-activating protein (ARF-GAP). Previously, we found that agd1 mutants have root hairs that exhibit wavy growth and have two tips that originate from a single initiation point. To gain new insights into how AGD1 modulates root hair polarity we analyzed double mutants of agd1 and other loci involved in root hair development, and evaluated dynamics of various components of root hair tip growth in agd1 by live cell microscopy. Because AGD1 contains a phosphoinositide (PI) binding pleckstrin homology (PH) domain, we focused on genetic interactions between agd1 and root hair mutants altered in PI metabolism. Rhd4, which is knocked-out in a gene encoding a phosphatidylinositol-4-phosphate (PI-4P) phosphatase, was epistatic to agd1. In contrast, mutations to PIP5K3 and COW1, which encode a type B phosphatidylinositol-4-phosphate 5-kinase 3 and a phosphatidylinositol transfer protein, respectively, enhanced the root hair defects of agd1. Enhanced root hair defects were also observed in double mutants to AGD1 and ACT2, a root hair-expressed vegetative actin isoform. Consistent with our double-mutant studies, targeting of tip growth components involved in PI signaling (PI-4P), secretion (RABA4b) and actin regulation (ROP2), were altered in agd1 root hairs. Furthermore, tip cytosolic calcium ([Ca²⁺](cyt) ) oscillations were disrupted in root hairs of agd1. Taken together, our results indicate that AGD1 links PI signaling to cytoskeletal-, [Ca²⁺](cyt-) , ROP2-, and RABA4b-mediated root hair development.

Arabidopsis WD repeat domain55 Interacts with DNA damaged binding protein1 and is required for apical patterning in the embryo

CUL4-RING ubiquitin E3 ligases (CRL4s) were recently shown to exert their specificity through the binding of various substrate receptors, which bind the CUL4 interactor DNA damaged binding protein1 (DDB1) through a WDxR motif. In a segregation-based mutagenesis screen, we identified a WDxR motif-containing protein (WDR55) required for male and female gametogenesis and seed development. We demonstrate that WDR55 physically interacts with Arabidopsis thaliana DDB1A in planta, suggesting that WDR55 may be a novel substrate recruiter of CRL4 complexes. Examination of mutants revealed a failure in the fusion of the polar cells in embryo sac development, in addition to embryo and endosperm developmental arrest at various stages ranging from the zygote stage to the globular stage. wdr55-2 embryos suggest a defect in the transition to bilateral symmetry in the apical embryo domain, further supported by aberrant apical embryo localization of DORNROESCHEN, a direct target of the auxin response factor protein monopteros. Moreover, the auxin response pattern, as determined using the synthetic auxin-responsive reporter ProDR5:green fluorescent protein, was shifted in the basal embryo and suspensor but does not support a strong direct link to auxin response. Interestingly, the observed embryo and endosperm phenotype is reminiscent of CUL4 or DDB1A/B loss of function and thus may support a regulatory role of a putative CRL4(WDR55) E3 ligase complex.

Phosphoinositide signaling

"All things flow and change…even in the stillest matter there is unseen flux and movement." Attributed to Heraclitus (530-470 BC), from The Story of Philosophy by Will Durant. Heraclitus, a Greek philosopher, was thinking on a much larger scale than molecular signaling; however, his visionary comments are an important reminder for those studying signaling today. Even in unstimulated cells, signaling pathways are in constant metabolic flux and provide basal signals that travel throughout the organism. In addition, negatively charged phospholipids, such as the polyphosphorylated inositol phospholipids, provide a circuit board of on/off switches for attracting or repelling proteins that define the membranes of the cell. This template of charged phospholipids is sensitive to discrete changes and metabolic fluxes-e.g., in pH and cations-which contribute to the oscillating signals in the cell. The inherent complexities of a constantly fluctuating system make understanding how plants integrate and process signals challenging. In this review we discuss one aspect of lipid signaling: the inositol family of negatively charged phospholipids and their functions as molecular sensors and regulators of metabolic flux in plants.

Green light for polyphosphoinositide signals in plants

Plant genomes lack homologues of the inositol 1,4,5-trisphosphate receptor and protein kinase C, which are important components of the canonical phospholipase C signalling system in animals. Instead, plants seem to utilize alternative downstream signalling molecules, that is, InsP(6) and phosphatidic acid. Inositol lipids may also function as second messengers themselves. By reversible phosphorylation of the inositol headgroup, five biologically active plant polyphosphoinositides can be detected. Protein targets interact with specific polyphosphoinositide isomers via selective lipid-binding domains, thereby altering their intracellular localization and/or enzymatic activity. Such lipid-binding domains have also been used to create GFP based-lipid biosensors to visualize PPIs dynamics in vivo. Here, we highlight some recent advances and ideas on PPIs' role in plant signalling.

The Function of the CLE Peptides in Plant Development and Plant-Microbe Interactions

The CLAVATA3 (CLV3)/ENDOSPERM SURROUNDING REGION (ESR) (CLE) peptides consist of 12 or 13 amino acids, including hydroxylated proline residues that may or may not contain sugar modifications, and function in a non-cell-autonomous fashion. The CLE gene was first reported in Zea mays (maize) as an endosperm-specific gene, ESR, in 1997 (Opsahl-Ferstad et al., 1997). CLE genes encode secreted peptides that function in the extracellular space as intercellular signaling molecules and bind to cellular surface receptor-like proteins to transmit a signal. CLE peptides regulate various physiological and developmental processes and its signaling pathway are conserved in diverse land plants. Recent CLE functional studies have pointed to their significance in regulating meristematic activity in plant meristems, through the CLE-receptor kinase-WOX signaling node. CLV3 and CLE40 are responsible for maintenance of shoot apical meristem (SAM) and root apical meristem (RAM) function, regulating homeodomain transcription factors, WUSCHEL (WUS) and WUSCHEL-related homeobox 5 (WOX5), respectively. CLE and WOX form an interconnected and self-correcting feedback loop to provide robustness to stem cell homeostasis. CLE peptides are required for certain plant-microbe interactions, such as those that occur during legume symbiosis and phytopathogenic nematode infection. Understanding the molecular properties of CLE peptides may provide insight into plant cell-cell communication, and therefore also into plant-microbe interactions.

The role of multifunctional M1 metallopeptidases in cell cycle progression

BACKGROUND

Metallopeptidases of the M1 family are found in all phyla (except viruses) and are important in the cell cycle and normal growth and development. M1s often have spatiotemporal expression patterns which allow for strict regulation of activity. Mutations in the genes encoding M1s result in disease and are often lethal. This family of zinc metallopeptidases all share the catalytic region containing a signature amino acid exopeptidase (GXMXN) and a zinc binding (HEXXH[18X]E) motif. In addition, M1 aminopeptidases often also contain additional membrane association and/or protein interaction motifs. These protein interaction domains may function independently of M1 enzymatic activity and can contribute to multifunctionality of the proteins.

SCOPE

A brief review of M1 metalloproteases in plants and animals and their roles in the cell cycle is presented. In animals, human puromycin-sensitive aminopeptidase (PSA) acts during mitosis and perhaps meiosis, while the insect homologue puromycin-sensitive aminopeptidase (PAM-1) is required for meiotic and mitotic exit; the remaining human M1 family members appear to play a direct or indirect role in mitosis/cell proliferation. In plants, meiotic prophase aminopeptidase 1 (MPA1) is essential for the first steps in meiosis, and aminopeptidase M1 (APM1) appears to be important in mitosis and cell division.

CONCLUSIONS

M1 metalloprotease activity in the cell cycle is conserved across phyla. The activities of the multifunctional M1s, processing small peptides and peptide hormones and contributing to protein trafficking and signal transduction processes, either directly or indirectly impact on the cell cycle. Identification of peptide substrates and interacting protein partners is required to understand M1 function in fertility and normal growth and development in plants.

The RLK/Pelle family of kinases

The RLK/Pelle class of proteins kinases is composed of over 600 members in Arabidopsis. Many of the proteins in this family are receptor-like kinases (RLK), while others have lost their extracellular domains and are found as cytoplasmic kinases. Proteins in this family that are RLKs have a variety of extracellular domains that drive function in a large number of processes, from cell wall interactions to disease resistance to developmental control. This review will briefly cover the major subclasses of RLK/Pelle proteins and their roles. In addition, two specific groups on RLKs will be discussed in detail, relating recent findings in Arabidopsis and how well these conclusions have been able to be translated to agronomically important species. Finally, some details on kinase activity and signal transduction will be addressed, along with the mystery of RLK/Pelle members lacking kinase enzymatic activity.

Determination of symmetric and asymmetric division planes in plant cells

The cellular organization of plant tissues is determined by patterns of cell division and growth coupled with cellular differentiation. Cells proliferate mainly via symmetric division, whereas asymmetric divisions are associated with initiation of new developmental patterns and cell types. Division planes in both symmetrically and asymmetrically dividing cells are established through the action of a cortical preprophase band (PPB) of cytoskeletal filaments, which is disassembled upon transition to metaphase, leaving behind a cortical division site (CDS) to which the cytokinetic phragmoplast is later guided to position the cell plate. Recent progress has been made in understanding PPB formation and function as well as the nature and function of the CDS. In asymmetrically dividing cells, division plane establishment is governed by cell polarity. Recent work is beginning to shed light on polarization mechanisms in asymmetrically dividing cells, with receptor-like proteins and potential downstream effectors emerging as important players in this process.

Membrane distributions of two ligand-binding receptor complexes in the CLAVATA pathway

Genetic studies have suggested that transmembrane proteins CLAVATA1 (CLV1), CLV2, CORYNE (CRN), BAM1 and BAM2 all play a role in relaying the CLV3 signal and thus regulating stem cell homeostasis at the shoot meristem (SM). The extracellular domain of CLV1 was previously shown to bind the CLE peptide derived from CLV3, providing direct evidence that CLV3-CLV1 function as a ligand-receptor pair. How the other putative receptors function in the CLV pathway, however, remained unclear. We demonstrated in a recent Plant Journal article that the receptor-like protein CLV2 and the receptor-kinases BAM1 and BAM2 also bind to the CLV3 CLE peptide ligand with an affinity similar to that of CLV1. Critically, these ligand binding receptors form two distinct complexes in both transient expression in tobacco and in Arabidopsis meristem cells: a CLV2/CRN multimer and a CLV1/BAM multimer. Here we examine in detail the subcellular membrane partitioning for the receptor proteins in transient expression by two-phase partitioning and co-expression with known subcellular markers. All tested proteins measurably accumulate at the plasma membrane. While CLV1 primarily co-localizes with a plasma membrane marker, CLV2 shows greater co-localization with an endoplasmic reticulum (ER) marker.

Membrane distributions of two ligand-binding receptor complexes in the CLAVATA pathway

Genetic studies have suggested that transmembrane proteins CLAVATA1 (CLV1), CLV2, CORYNE (CRN), BAM1 and BAM2 all play a role in relaying the CLV3 signal and thus regulating stem cell homeostasis at the shoot meristem (SM). The extracellular domain of CLV1 was previously shown to bind the CLE peptide derived from CLV3, providing direct evidence that CLV3-CLV1 function as a ligand-receptor pair. How the other putative receptors function in the CLV pathway, however, remained unclear. We demonstrated in a recent Plant Journal article that the receptor-like protein CLV2 and the receptor-kinases BAM1 and BAM2 also bind to the CLV3 CLE peptide ligand with an affinity similar to that of CLV1. Critically, these ligand binding receptors form two distinct complexes in both transient expression in tobacco and in Arabidopsis meristem cells: a CLV2/CRN multimer and a CLV1/BAM multimer. Here we examine in detail the subcellular membrane partitioning for the receptor proteins in transient expression by two-phase partitioning and co-expression with known subcellular markers. All tested proteins measurably accumulate at the plasma membrane. While CLV1 primarily co-localizes with a plasma membrane marker, CLV2 shows greater co-localization with an endoplasmic reticulum (ER) marker.

The role of the acyl modification, palmitoylation, in Arabidopsis stem cell regulation

Proper control of stem cell populations is key for the development of all multicellular organisms. In Arabidopsis, stem cells are located primarily in the shoot, root and floral meristems where they undergo complex regulation. The Arabidopsis shoot and root meristems are regulated by the related WUS and WOX5 pathways, respectively. Previous studies established that these pathways share the signal transduction components POLTERGEIST (POL) and PLL1. Our latest study in Plant Cell revealed key roles for acyl modifications and lipid binding in the regulation of these two type 2C protein phosphatases. Specifically, POL and PLL1 were shown to localize to the plasma membrane in a myristioylation- and palmitoylation-dependent manner, POL and PLL1 were shown to bind to membrane lipids, and POL activity was found to be stimulated in vitro by the phospholipid PI(4)P. Here, we will discuss what is currently known in Arabidopsis and other organisms about the mechanisms of palmitoylation and provide additional evidence supporting that POL and PLL1 are palmitoylated, including describing the identification of a putative Arabidopsis palmitoyl transferase as a PLL1 interactor.