Novel receptor kinases involved in growth regulation

The growth of plant cells involves a constant adjustment of synthesis and rearrangement of cell wall polymers. Recently, three plasma membrane-bound receptor kinases related to CrRLK1 have been shown to be involved in the negative control of cell growth in different contexts. THESEUS1 is activated in mutants deficient for cellulose and may act as a cell wall integrity sensor inhibiting cell elongation. FERONIA is polarly localized in synergid cells of the female gametophyte and is required for growth cessation of compatible pollen tubes and subsequent delivery of sperm cells. AmRLK is involved in the control of the polar conical outgrowth of epidermal cells of Antirrhinum petals. The conservation of both extracellular and kinase domains suggests that the three receptors bind to related ligands and have similar cellular outputs, which may involve the production of reactive oxygen species.     

Growth Control: A Saga of Cell Walls,ROS, and Peptide Receptors

Despite an increasingly detailed understanding of endogenous and environmental growth-controlling signals and their signaling networks, little is known on how these networks are integrated with the cell expansion machinery. Members of the CrRLK1L family control cell wall properties and cell expansion in a variety of developmental and environmental contexts. Two recent reports provide exciting new insights into the mode of action of these RLKs. One study shows that one family member, FERONIA (FER), is required for the production of hydroxyl radicals in the female gametophyte, which causes pollen tube rupture and sperm cell release during fertilization. Another study shows that FER is a receptor for a signaling peptide (Rapid Alkalinization Factor 1 [RALF1]) that triggers cell wall alkalinization and growth arrest, possibly through the inhibition of plasma membrane H⁺-ATPase activity. RALF1 belongs to a large gene family, with a wide range of expression patterns. Other CrRLK1L family members therefore may also be receptors for RALF-like peptides. These findings have important implications for our understanding of the control of cell wall integrity during growth and raise new intriguing questions.     

植物类受体激酶CrRLK1-L亚家族及其生物学功能

植物CrRLK1-L亚家族类受体激酶的胞外域具有新颖结构基序,但功能大都未知.该家族成员广泛存在于被子植物中,但在动物和微生物中不存在其同源物.CrRLK1-L家族成员相对较少,但组织表达非常广泛.它们定位于细胞质膜上,并且部分成员的定位还具有极性,这与其参与雌雄配子体的识别和受精作用密切相关.该家族成员普遍具有激酶活性,该活性对其功能的发挥至关重要.目前仅报道在拟南芥中参与助细胞与花粉的识别和调控营养组织的细胞伸长,但参与这些生物学过程的作用机制似乎独立于已知的信号通路之外,可能有自身独特的信号传导机制.所以对这一类具特有结构基序的类受体激酶基因的功能研究,将有助于解析植物特有生物学过程的分子作用机制,特别是在植物有性生殖过程中,合理利用这些分子开展育种实践对未来农业生产具有潜在的应用价值.     

Regulation and function of connective tissue growth factor/CCN2 in tissue repair, scarring and fibrosis

Connective tissue growth factor (CTGF, CCN2) is a secreted protein with major roles in angiogenesis, chondrogenesis, osteogenesis, tissue repair, cancer and fibrosis. It is a member of the CCN family of immediate-early gene products which are characterised by four discrete protein modules in which reside growth factor binding domains, functional motifs for integrin recognition, heparin and proteoglycan binding, and dimerization motifs. A primary function of CTGF is to modulate and coordinate signaling responses involving cell surface proteoglycans, key components of the extracellular matrix, and growth factors. Integration of these molecular cues regulates growth factor and receptor interactions, cell motility and mesenchymal cell activation and differentiation in tissue remodelling. Abnormal amplification of CTGF dependent signals results in a failure to terminate tissue repair, leading pathological scarring in conditions such as fibrosis and cancer.     

Crystal structures of the extracellular domains of the CrRLK1L receptor‐like kinases ANXUR1 and ANXUR2

Catharanthus roseus Receptor‐Like Kinase 1‐like (CrRLK1L) proteins contain two tandem malectin‐like modules in their extracellular domains (ECDs) and function in diverse signaling pathways in plants. Malectin is a carbohydrate‐binding protein in animals and recognizes a number of diglucosides; however, it remains unclear how the two malectin‐like domains in the CrRLK1L proteins sense the ligand molecule. In this study, we reveal the crystal structures of the ECDs of ANXUR1 and ANXUR2, two CrRLK1L members in Arabidopsis thaliana that have critical functions in controlling pollen tube rupture during the fertilization process. We show that the two malectin‐like domains in these proteins pack together to form a rigid architecture. Unlike animal malectin, these malectin‐like domains lack residues involved in binding to the diglucosides, suggesting that they have a distinct ligand‐binding mechanism. A cleft is observed between the two malectin‐like domains, which might function as a potential ligand‐binding pocket.     

Functional analysis of related CrRLK1L receptor‐like kinases in pollen tube reception

The Catharanthus roseus Receptor‐Like Kinase 1‐like (CrRLK1L) family of 17 receptor‐like kinases (RLKs) has been implicated in a variety of signaling pathways in Arabidopsis, ranging from pollen tube (PT) reception and tip growth to hormonal responses. The extracellular domains of these RLKs have malectin‐like domains predicted to bind carbohydrate moieties. Domain swap analysis showed that the extracellular domains of the three members analyzed (FER, ANX1, HERK1) are not interchangeable, suggesting distinct upstream components, such as ligands and/or co‐factors. In contrast, their intercellular domains are functionally equivalent for PT reception, indicating that they have common downstream targets in their signaling pathways. The kinase domain is necessary for FER function, but kinase activity itself is not, indicating that other kinases may be involved in signal transduction during PT reception.     

植物类LORELEI糖基磷脂酰肌醇锚定蛋白研究进展

类LORELEI糖基磷脂酰肌醇锚定蛋白(LLG)定位于细胞质膜外表面, 作为CrRLK1L家族类受体激酶的分子伴侣, 参与其转运和胞外信号转导, 从而调控植物生殖发育以及免疫与逆境应答等过程。LLG2/3与ANX和BUPS互作, 调控花粉管顶端生长与爆裂。LLG1与FER (FERONIA)互作, 调控下游的NADPH氧化酶产生活性氧(ROS), 促进根部细胞伸长和根毛生长。此外, LLG1作为FER的共受体, 与快速碱化因子(RALFs)互作, 调节G蛋白β亚基(AGB1)和质膜H ⁺-ATPase功能、胞内ROS稳态以及Ca ²⁺瞬变, 引起根部和气孔的盐应答反应。LLG1与FLS2和EFR互作激活下游RbohD, 调节ROS产生, 调控植物免疫应答。该文综述了植物LLG的相关研究进展, 可为深入理解LLG的生物学功能提供重要信息。     

New insights into the functional roles of CrRLKs in the control of plant cell growth and development

Receptor-like kinases (RLKs) are a family of transmembrane proteins with a variable ligand-binding extracellular domain and a cytoplasmic kinase domain. In Arabidopsis, there are ∼600 RLKs believed to have diverse functions during plant growth, development and interactions with the environment. Based on the variable extracellular domain, RLKs can be classified into different subfamilies. The CrRLK subfamily contains 17 members in Arabidopsis and characterization of some of its members suggests a role for these proteins in the regulation of growth and reproduction. This review focuses on the roles of CrRLKs in the regulation of polarized growth with emphasis on the newly identified signal transduction pathways activated downstream of CrRLKs. A picture is emerging where CrRLKs are part of a conserved signal transduction cascade important for growth maintenance in different cell types.Key words: CrRLKs, FERONIA, RAC/ROP, ROS, polar growthThe ability of plants to perceive and process environmental and internal information into coordinated responses is crucial to their adaptability and survival in constantly changing environments. Most of signal perception occurs at the plasma membrane of cells where membrane-associated receptors receive signals to activate downstream signaling cascades that regulate growth and development. In plants and animals alike, receptor-like kinases (RLKs) mediate many of the signaling events at the cell surface and in the model plant Arabidopsis they comprise a monophyletic family with more than 600 members.¹ RLKs are transmembrane proteins with a variable N-terminal extracellular domain and a Ser/Thr intracellular kinase domain. The diversity of their extracellular domains suggests involvement in the transduction of a wide range of signals and allows them to be classified into different sub-families.² The CrRLK1L subfamily (from here on referred to as CrRLK) is named after the first member characterized in Catharanthus roseus cell cultures³ and contains 17 members in Arabidopsis.⁴ Several members of this family have now been implicated in growth regulatory processes.THESEUS1 (THE1) was identified through a suppressor screen of a cellulose-deficient mutant (prc1-1) which has a short hypocotyl phenotype.⁵ Loss of THE1 function resulted in reduced growth inhibition in the prc1-1 the1 double mutant. Interestingly, the the1 mutation itself has no effect in wild type background, thus leading to the suggestion that THE1 functions as a sensor of cell wall integrity in situations where the cell wall is weakened and organ elongation would be detrimental for the plant.^4,5A second CrRLK, FERONIA (FER), was first implicated in the regulation of female control of fertility. In the female gametophyte FER is involved in sensing pollen tube arrival and promoting its rupture which is necessary for double fertilization to occur.^6,7 FER is in fact involved in several processes depending on the tissue where it is expressed. In hypocotyls, FER is involved in the integration of ethylene and brassinosteroid (BR) signals to regulate hypocotyl elongation in the dark.⁸ Moreover, FER, THE1 and the closely related HERCULES1 (HERK1), were found to regulate cell elongation by interacting with BR signaling.⁹ More recently, roles for FER in the regulation of root hair development and fungal invasion have been established.^10,11 The pollen-specific ANXUR1 (ANX1) and ANXUR2 (ANX2) are closely related to FER and act redundantly to maintain pollen tube growth integrity during its journey through the style and ensure against precocious pollen tube rupture before reaching the ovule.^12,13Apparently with different biological roles, all the CrRLK members analyzed thus far have an effect on the growth of plant cells. The present review focuses on their role during cell growth with emphasis on polar cell growth and the downstream pathways activated by CrRLKs.     

Differential expression of two barley XET-related genes during coleoptile growth

Plant cell elongation depends on the physical properties of the primary cell wall. Because xyloglucan endotransglycosylases (XETs) are enzymes that mediate cleavage and rejoining of the beta(1-4)-XG backbone of primary cell wall, they are potentially involved in cell elongation. In this paper, the growth of the barley coleoptile was related to the expression patterns of two genes from this family (hvEXT, hvXEB) in experiments where coleoptile elongation varied according to light/dark treatments in order to assess the potential role of these genes in cell elongation. In dark-grown and light-grown coleoptiles, growth rate variations were associated with altered levels of expression of hvEXT and hvXEB: they were higher in dark-grown than in light-grown seedlings, and decreased after 5 d in darkness, and after 4 d in continuous light. In 4-d-old seedlings, coleoptile elongation decreased significantly 4 h after the onset of a continuous white- light irradiation, and hvXEB and hvEXT mRNA levels decreased, respectively, 2 h and 4 h after the onset of white-light irradiation. Moreover, the distribution of hvXEB and hvEXT along the coleoptiles of 4-d-old dark-grown seedlings were different. Altogether, these results suggest a complex pattern of temporal and positional expression for the different genes of the XET-related family.     

The SUN41 and SUN42 genes are essential for cell separation in Candida albicans

Completion of the yeast cell cycle involves extensive remodelling of the cell wall upon separation of mother and daughter cells. We have studied two members of the ascomycete-specific SUN gene family in Candida albicans. Inactivation of SUN41 yields defects in cell separation and hyphal elongation while inactivation of SUN42 results in minor phenotypic alterations. Simultaneous inactivation of SUN41 and SUN42 is synthetically lethal due to lysis of mother cells after septation. Electronic microscopy reveals cell wall defects mainly localized in the region surrounding the septa. This phenotype is osmoremediable and the conditional double mutants show increased sensitivity to cell wall or cell membrane perturbing agents. The essential function shared by Sun41p and Sun42p is conserved among yeasts because UTH1, a Saccharomyces cerevisiae SUN gene, suppresses the lethality of SUN41 and SUN42 conditional mutants. Investigation of functional genomic data obtained in S. cerevisiae reveals links between members of the SUN gene family and the RAM pathway regulating cell wall-degrading enzymes specifically involved during cell separation. Thus, the main function of ascomycetous Sun proteins appears linked to cell wall remodelling, with a probable role in counter-balancing cell wall degradation to avoid cell lysis upon cell separation.     

Stem Elongation and Cell Wall Proteins in Flowering Plants

Abstract: The growth of stems (hypocotyls, epicotyls) and stem-like organs (coleoptiles) in developing seedlings is largely due to the elongation of cells in the sub-apical region of the corresponding organ. According to the organismal concept of plant development, the thick outer epidermal wall, which can be traced back to the peripheral cell wall of the zygote, creates a sturdy organ sheath that determines the rate of stem elongation. The cells of the inner tissues are the products of secondary partitioning of one large protoplast; these turgid, thin-walled cells provide the driving force for organ growth. The structural differences between these types of cell walls are described (outer walls: thick, sturdy, helicoidal cellulose architecture; inner walls: thin, extensible, transversely-oriented cellulose microfibrils). On the basis of these facts, current models of cell wall loosening (and wall stiffening) are discussed with special reference to the expansin, enzymatic polymer remodelling and osmiophilic particle hypothesis. It is concluded that the exact biochemical mechanism(s) responsible for the coordinated yielding of the growth-controlling peripheral organ wall(s) have not yet been identified.     

Genome-Wide Identification and Expression Analysis of the CrRLK1L Gene Family in Apple (Malus domestica)

It has been reported that members of the Catharanthus roseus receptor-like kinase1-like kinase (CrRLK1L) gene family detect cell wall integrity, cell-to-cell communication, and biotic and abiotic stress. We performed a comprehensive study including the genome-wide identification, characterization, and gene expression analysis of CrRLK1Ls in apple (Malus domestica). Sixty-seven M. domestica CrRLK1Ls (MdCrRLK1Ls) were identified based on their domain structure. Molecular weight and pI ranged from 52.36–141 kDa and 5.05–8.9, respectively. They were distributed across 16 of the 18 chromosomes and classified into five phylogenetic branches. Exon-intron structural analysis indicated a wide range of exon numbers. Collinearity analysis showed that both segmental-and tandem-duplication contributed to the expansion of this family. Cis-elements in the MdCrRLK1L promoter region responded mainly to light, circadian rhythm, phytohormones, and biotic or abiotic stress. Many members exhibited tissue-specific expression patterns and differentially expressed under biotic stresses, which may contribute to the different functional roles of MdCrRLK1Ls under physiological stress and/or pathological conditions. This study provides new insights into the CrRLK1Ls in Malus spp.

     

Cell elongation in Arabidopsis hypocotyls involves dynamic changes in cell wall thickness

Field-emission scanning electron microscopy was used to measure wall thicknesses of different cell types in freeze-fractured hypocotyls of Arabidopsis thaliana. Measurements of uronic acid content, wall mass, and wall volume suggest that cell wall biosynthesis in this organ does not always keep pace with, and is not always tightly coupled to, elongation. In light-grown hypocotyls, walls thicken, maintain a constant thickness, or become thinner during elongation, depending upon the cell type and the stage of growth. In light-grown hypocotyls, exogenous gibberellic acid represses the extent of thickening and promotes cell elongation by both wall thinning and increased anisotropy during the early stages of hypocotyl elongation, and by increased wall deposition in the latter stages. Dark-grown hypocotyls, in the 48 h period between cold imbibition and seedling emergence, deposit very thick walls that subsequently thin in a narrow developmental window as the hypocotyl rapidly elongates. The rate of wall deposition is then maintained and keeps pace with cell elongation. The outer epidermal wall is always the thickest ( approximately 1 mum) whereas the thinnest walls, about 50 nm, are found in inner cell layers. It is concluded that control of wall thickness in different cell types is tightly regulated during hypocotyl development, and that wall deposition and cell elongation are not invariably coupled.     

RICE SALT SENSITIVE3 binding to bHLH and JAZ factors mediates control of cell wall plasticity in the root apex

Plasticity of root growth in response to environmental cues and stresses is a fundamental characteristic of plants, in accordance with their sessile lifestyle. This is linked to the balance between plasticity and rigidity of cells in the root apex, and thus is coordinated with the control of cell wall properties. However, mechanisms underlying such harmonization are not well understood, in particular under stressful conditions. We have recently demonstrated that RICE SALT SENSITIVE3 (RSS3), a nuclear factor that mediates restrictive expression of jasmonate-induced genes, plays an important role in root elongation under saline conditions. In this study, we report that loss-of-function of RSS3 results in changes in cell wall properties such as lignin deposition and sensitivity to a cellulose synthase inhibitor, concomitant with altered expression of genes involved in cell wall metabolism. Based on these and previous phenotypic observations of the rss3 mutant, we propose that RSS3 plays a role in the coordinated control of root elongation and cell wall plasticity in the root apex.     

Changes in osmotic pressure and cell wall properties during auxin- and ethylene-induced growth of intact coleoptiles of rice

Indole-3-acetic acid and 1-aminocyclopropane-1-carboxylic acid, the precursor of ethylene, stimulated elongation of coleoptiles of seedlings of intact rice ( Oryza sativa L. cv. Sasanishiki) submerged in buffer solution with constant air-bubbling. The osmotic pressure of the cell sap decreased during elongation of coleoptiles. In the presence of 30 μ M aminooxyacetic acid, an inhibitor of ethylene biosynthesis, in-dole-3-acetic acid at 30 μ M accelerated the decrease in the osmotic pressure in the early stage of growth. 1-Aminocyclopropane-1-carboxylic acid at 30 μ M did not influence the decrease in the osmotic pressure.
Both indole-3-acetic acid and 1-aminocyclopropane-1-carboxyIic acid decreased the minimum stress-relaxation time and the relaxation rate of the cell wall, suggesting that both auxin and ethylene induce elongation of rice coleoptiles by stimulating cell wall loosening. These growth regulators caused an increase in the level of glucose in hemicelluloses in the early stage of growth and a decrease in the level in the subsequent last growth phase. Indole-3-acetic acid decreased the hydroxyproline and glucosamine levels per unit dry weight of the cell wall. These changes in the level of cell wall components may be associated with the changes in the mechanical properties of the cell walls caused by auxin and ethylene.     

Building bridges: formin1 of Arabidopsis forms a connection between the cell wall and the actin cytoskeleton

Actin microfilament (MF) organization and remodelling is critical to cell function. The formin family of actin binding proteins are involved in nucleating MFs in Arabidopsis thaliana. They all contain formin homology domains in the intracellular, C‐terminal half of the protein that interacts with MFs. Formins in class I are usually targeted to the plasma membrane and this is true of Formin1 (AtFH1) of A. thaliana. In this study, we have investigated the extracellular domain of AtFH1 and we demonstrate that AtFH1 forms a bridge from the actin cytoskeleton, across the plasma membrane and is anchored within the cell wall. AtFH1 has a large, extracellular domain that is maintained by purifying selection and that contains four conserved regions, one of which is responsible for immobilising the protein. Protein anchoring within the cell wall is reduced in constructs that express truncations of the extracellular domain and in experiments in protoplasts without primary cell walls. The 18 amino acid proline‐rich extracellular domain that is responsible for AtFH1 anchoring has homology with cell‐wall extensins. We also have shown that anchoring of AtFH1 in the cell wall promotes actin bundling within the cell and that overexpression of AtFH1 has an inhibitory effect on organelle actin‐dependant dynamics. Thus, the AtFH1 bridge provides stable anchor points for the actin cytoskeleton and is probably a crucial component of the signalling response and actin‐remodelling mechanisms.     

On the preservation of vessel bifurcations during flow-mediated angiogenic remodelling

During developmental angiogenesis, endothelial cells respond to shear stress by migrating and remodelling the initially hyperbranched plexus, removing certain vessels whilst maintaining others. In this study, we argue that the key regulator of vessel preservation is cell decision behaviour at bifurcations. At flow-convergent bifurcations where migration paths diverge, cells must finely tune migration along both possible paths if the bifurcation is to persist. Experiments have demonstrated that disrupting the cells’ ability to sense shear or the junction forces transmitted between cells impacts the preservation of bifurcations during the remodelling process. However, how these migratory cues integrate during cell decision making remains poorly understood. Therefore, we present the first agent-based model of endothelial cell flow-mediated migration suitable for interrogating the mechanisms behind bifurcation stability. The model simulates flow in a bifurcated vessel network composed of agents representing endothelial cells arranged into a lumen which migrate against flow. Upon approaching a bifurcation where more than one migration path exists, agents refer to a stochastic bifurcation rule which models the decision cells make as a combination of flow-based and collective-based migratory cues. With this rule, cells favour branches with relatively larger shear stress or cell number. We found that cells must integrate both cues nearly equally to maximise bifurcation stability. In simulations with stable bifurcations, we found competitive oscillations between flow and collective cues, and simulations that lost the bifurcation were unable to maintain these oscillations. The competition between these two cues is haemodynamic in origin, and demonstrates that a natural defence against bifurcation loss during remodelling exists: as vessel lumens narrow due to cell efflux, resistance to flow and shear stress increases, attracting new cells to enter and rescue the vessel from regression. Our work provides theoretical insight into the role of junction force transmission has in stabilising vasculature during remodelling and as an emergent mechanism to avoid functional shunting.