Biomedical subjects

Ruiqin Zhong

Publications and source records attributed to Ruiqin Zhong.

15 recordsLinked to original sources

PubMed2006

SND1, a NAC domain transcription factor, is a key regulator of secondary wall synthesis in fibers of Arabidopsis.

Secondary walls in fibers and tracheary elements constitute the most abundant biomass produced by plants. Although a number of genes involved in the biosynthesis of secondary wall components have been characterized, little is known about the molecular mechanisms underlying the coordinated expression of these genes. Here, we demonstrate that the Arabidopsis thaliana NAC (for NAM, ATAF1/2, and CUC2) domain transcription factor, SND1 (for secondary wall-associated NAC domain protein), is a key transcriptional switch regulating secondary wall synthesis in fibers. We show that SND1 is expressed specifically in interfascicular fibers and xylary fibers in stems and that dominant repression of SND1 causes a drastic reduction in the secondary wall thickening of fibers. Ectopic overexpression of SND1 results in activation of the expression of secondary wall biosynthetic genes, leading to massive deposition of secondary walls in cells that are normally nonsclerenchymatous. In addition, we have found that SND1 upregulates the expression of several transcription factors that are highly expressed in fibers during secondary wall synthesis. Together, our results reveal that SND1 is a key transcriptional activator involved in secondary wall biosynthesis in fibers.

Arabidopsis↗

PubMed2006

The poplar glycosyltransferase GT47C is functionally conserved with Arabidopsis Fragile fiber8.

Xylan is the major hemicellulose in dicot wood. Unraveling genes involved in the biosynthesis of xylan will be of importance in understanding the process of wood formation. In this report, we investigated the possible role of poplar GT47C, a glycosyltransferase belonging to family GT47, in the biosynthesis of xylan. PoGT47C from the hybrid poplar Populus alba x tremula exhibits 84% sequence similarity to Fragile fiber8 (FRA8), which is involved in the biosynthesis of glucuronoxylan in Arabidopsis. Phylogenetic analysis of glycosyltransferase family GT47 in the Populus trichocarpa genome revealed that GT47C is the only close homolog of FRA8. In situ hybridization showed that the PoGT47C gene was expressed in developing primary xylem, secondary xylem and phloem fibers of stems, and in developing secondary xylem of roots. Sequence analysis suggests that PoGT47C is a type II membrane protein, and study of the subcellular localization demonstrated that fluorescent protein-tagged PoGT47C was located in the Golgi. Immunolocalization with a xylan monoclonal antibody LM10 revealed a nearly complete loss of xylan signals in the secondary walls of fibers and vessels in the Arabidopsis fra8 mutant. Expression of PoGT47C in the fra8 mutant restored the secondary wall thickness and xylan content to the wild-type level. Together, these results suggest that PoGT47C is functionally conserved with FRA8 and it is probably involved in xylan synthesis during wood formation.

Amino Acid Sequence↗

PubMed2005

Arabidopsis fragile fiber8, which encodes a putative glucuronyltransferase, is essential for normal secondary wall synthesis.

Secondary walls in vessels and fibers of dicotyledonous plants are mainly composed of cellulose, xylan, and lignin. Although genes involved in biosynthesis of cellulose and lignin have been intensively studied, little is known about genes participating in xylan synthesis. We found that Arabidopsis thaliana fragile fiber8 (fra8) is defective in xylan synthesis. The fra8 mutation caused a dramatic reduction in fiber wall thickness and a decrease in stem strength. FRA8 was found to encode a member of glycosyltransferase family 47 and exhibits high sequence similarity to tobacco (Nicotiana plumbaginifolia) pectin glucuronyltransferase. FRA8 is expressed specifically in developing vessels and fiber cells, and FRA8 is targeted to Golgi. Comparative analyses of cell wall polysaccharide fractions from fra8 and wild-type stems showed that the xylan and cellulose contents are drastically reduced in fra8, whereas xyloglucan and pectin are elevated. Further structural analysis of cell walls revealed that although wild-type xylans contain both glucuronic acid and 4-O-methylglucuronic acid residues, xylans from fra8 retain only 4-O-methylglucuronic acid, indicating that the fra8 mutation results in a specific defect in the addition of glucuronic acid residues onto xylans. These findings suggest that FRA8 is a glucuronyltransferase involved in the biosynthesis of glucuronoxylan during secondary wall formation.

Amino Acid Sequence↗

PubMed2005

Mutation of SAC1, an Arabidopsis SAC domain phosphoinositide phosphatase, causes alterations in cell morphogenesis, cell wall synthesis, and actin organization.

SAC (for suppressor of actin) domain proteins in yeast and animals have been shown to modulate the levels of phosphoinositides, thereby regulating several cellular activities such as signal transduction, actin cytoskeleton organization, and vesicle trafficking. Nine genes encoding SAC domain-containing proteins are present in the Arabidopsis thaliana genome, but their roles in plant cellular functions and plant growth and development have not been characterized. In this report, we demonstrate the essential roles of one of the Arabidopsis SAC domain proteins, AtSAC1, in plant cellular functions. Mutation of the AtSAC1 gene in the fragile fiber7 (fra7) mutant caused a dramatic decrease in the wall thickness of fiber cells and vessel elements, thus resulting in a weak stem phenotype. The fra7 mutation also led to reduced length and aberrant shapes in fiber cells, pith cells, and trichomes and to an alteration in overall plant architecture. The AtSAC1 gene was found to be expressed in all tissues in elongating organs; however, it showed predominant expression in vascular tissues and fibers in nonelongating parts of stems. In vitro activity assay demonstrated that AtSAC1 exhibited phosphatase activity toward phosphatidylinositol 3,5-biphosphate. Subcellular localization studies showed that AtSAC1 was colocalized with a Golgi marker. Truncation of the C terminus by the fra7 mutation resulted in its localization in the cytoplasm but had no effect on phosphatase activity. Furthermore, examination of the cytoskeleton organization revealed that the fra7 mutation caused the formation of aberrant actin cables in elongating cells but had no effect on the organization of cortical microtubules. Together, these results provide genetic evidence that AtSAC1, a SAC domain phosphoinositide phosphatase, is required for normal cell morphogenesis, cell wall synthesis, and actin organization.

Actins↗

PubMed2004

FRAGILE FIBER3, an Arabidopsis gene encoding a type II inositol polyphosphate 5-phosphatase, is required for secondary wall synthesis and actin organization in fiber cells.

Type II inositol polyphosphate 5-phosphatases (5PTases) in yeast and animals have been known to regulate the level of phosphoinositides and thereby influence various cellular activities, such as vesicle trafficking and actin organization. In plants, little is known about the phosphatases involved in hydrolysis of phosphoinositides, and roles of type II 5PTases in plant cellular functions have not yet been characterized. In this study, we demonstrate that the FRAGILE FIBER3 (FRA3) gene of Arabidopsis thaliana, which encodes a type II 5PTase, plays an essential role in the secondary wall synthesis in fiber cells and xylem vessels. The fra3 mutations caused a dramatic reduction in secondary wall thickness and a concomitant decrease in stem strength. These phenotypes were associated with an alteration in actin organization in fiber cells. Consistent with the defective fiber and vessel phenotypes, the FRA3 gene was found to be highly expressed in fiber cells and vascular tissues in stems. The FRA3 protein is composed of two domains, an N-terminal localized WD-repeat domain and a C-terminal localized 5PTase catalytic domain. In vitro activity assay demonstrated that recombinant FRA3 exhibited phosphatase activity toward PtdIns(4,5)P2, PtdIns(3,4,5)P3, and Ins(1,4,5)P3, with the highest substrate affinity toward PtdIns(4,5)P2. The fra3 missense mutation, which caused an amino acid substitution in the conserved motif II of the 5PTase catalytic domain, completely abolished the FRA3 phosphatase activity. Moreover, the endogenous levels of PtdIns(4,5)2 and Ins(1,4,5)P3 were found to be elevated in fra3 stems. Together, our findings suggest that the FRA3 type II 5PTase is involved in phosphoinositide metabolism and influences secondary wall synthesis and actin organization.

Actin Cytoskeleton↗

PubMed2004

Amphivasal vascular bundle 1, a gain-of-function mutation of the IFL1/REV gene, is associated with alterations in the polarity of leaves, stems and carpels.

In Arabidopsis stems, the vascular bundles in the stele are arranged in a ring-like pattern and the vascular tissues in each bundle are organized in a collateral pattern. We have shown previously that the semidominant amphivasal vascular bundle 1 (avb1) mutation transforms the collateral vascular bundles into amphivasal bundles and disrupts the ring-like arrangement of vascular bundles in the stele. In this study, we show that the avb1 mutation occurred in the putative microRNA 165 target sequence in the IFL1/REV gene and caused an amino acid substitution in the putative sterol/lipid-binding START domain. We present direct evidence that the wild-type IFL1/REV mRNA was cleaved within the microRNA 165 target sequence and the avb1 mutation resulted in an inhibition of cleavage and a higher level accumulation of full-length mRNA, suggesting a role of microRNA 165 in the regulation of IFL1/REV gene expression. In addition to an alteration in vascular patterning, the avb1 mutation also caused dramatic changes in fiber cell wall thickening and organ polarity, including aberrant formation and proliferation of cauline leaves and branches, production of trumpet-shaped leaves with reversed adaxial-abaxial identity, ectopic growth of carpel-like structures on the outer surface of carpels, and fasciation of inflorescence. Ectopic overexpression of the avb1 mutant cDNA not only phenocopied most of the avb1 mutant phenotypes but also led to additional novel phenotypes such as formation of leaves with extremely narrow blades and ectopic production of branches in the axil of siliques. Taken together, these results suggest that the avb1 gain-of-function mutation of the IFL1/REV gene alters the positional information that determines vascular patterning and organ polarity.

Amino Acid Sequence↗

PubMed2004

Molecular and biochemical characterization of three WD-repeat-domain-containing inositol polyphosphate 5-phosphatases in Arabidopsis thaliana.

Type II inositol polyphosphate 5-phosphatases (5PTases) in animals and yeast have been known to be important for regulating inositol and phospholipid signaling by hydrolyzing phosphate from both inositol polyphosphates and phosphoinositides. However, the molecular and biochemical properties of type II 5PTases in plants have not yet been studied. In this report, we show that three Arabidopsis genes, At5PTase12, At5PTase13 and At5PTase14, encode proteins with a 5PTase domain and a WD-repeat domain, a novel combination present only in plant 5PTases. We demonstrate that these genes are differentially expressed in Arabidopsis organs and At5PTase13 is induced in response to ABA and wounding treatments. Our biochemical studies reveal that although both At5PTase12 and At5PTase13 exhibit phosphatase activity toward only Ins(1,4,5)P3, At5PTase14 hydrolyzes phosphate from PI(4,5)P2, PI(3,4,5)P3 and Ins(1,4,5)P3 with the highest substrate affinity toward PI(4,5)P2. All three At5PTases require Mg2+ for their phosphatase activities. Our molecular and biochemical characterization of three WD-repeat-domain-containing At5PTases provides a foundation for further elucidation of their cellular functions in Arabidopsis.

Amino Acid Sequence↗

PubMed2003

The Arabidopsis RHD3 gene is required for cell wall biosynthesis and actin organization.

The Arabidopsis thaliana (L.) Heynh. ROOT HAIR DEFECTIVE3 (RHD3) gene has previously been shown to be essential for normal cell expansion [H. Wang et al. (1997) Genes Dev 11:799-811]. In this report, we demonstrated that mutation of the RHD3 gene in the Arabidopsis fragile fiber 4 (fra4) mutant caused a dramatic reduction in the wall thickness of fibers, vessels, and pith cells in the inflorescence stems and, concomitantly, a decrease in the mechanical strength of stems. The reduced wall thickness in the fra4 mutant was accompanied by an alteration in cell wall composition. Consistent with the defective fiber and vessel wall phenotypes, the RHD3 gene exhibited a strong expression in developing fiber and xylem cells. We showed that the Arabidopsis genome contains two additional RHD3-like genes, one of which was expressed specifically in flowers. In addition, we found that mutation of the RHD3 gene caused an alteration in the organization of the actin cytoskeleton but no effects on cortical microtubules. Our findings suggest an essential role of RHD3 in cell wall biosynthesis and actin organization, both of which are known to be important for cell expansion.

Actins↗

PubMed2003

Unraveling the functions of glycosyltransferase family 47 in plants.

The function of glycosyltransferases (GTs) from family GT47 was first identified in animal exostosins as beta-glucuronyltransferase involved in the synthesis of heparan sulfate. Two recent papers report the functions of two plant members in this family as a pectin beta-glucuronyltransferase and a xyloglucan beta-galactosyltransferase. These findings greatly extend our understanding of the biological functions of family GT47 and also represent an important leap toward the molecular dissection of cell wall biosynthesis.

Amino Acid Sequence↗

PubMed2003

Expression of a mutant form of cellulose synthase AtCesA7 causes dominant negative effect on cellulose biosynthesis.

Cellulose synthase catalytic subunits (CesAs) have been implicated in catalyzing the biosynthesis of cellulose, the major component of plant cell walls. Interactions between CesA subunits are thought to be required for normal cellulose synthesis, which suggests that incorporation of defective CesA subunits into cellulose synthase complex could potentially cause a dominant effect on cellulose synthesis. However, all CesA mutants so far reported have been shown to be recessive in terms of cellulose synthesis. In the course of studying the molecular mechanisms regulating secondary wall formation in fibers, we have found that a mutant allele of AtCesA7 gene in the fra5 (fragile fiber 5) mutant causes a semidominant phenotype in the reduction of fiber cell wall thickness and cellulose content. The fra5 missense mutation occurred in a conserved amino acid located in the second cytoplasmic domain of AtCesA7. Overexpression of the fra5 mutant cDNA in wild-type plants not only reduced secondary wall thickness and cellulose content but also decreased primary wall thickness and cell elongation. In contrast, overexpression of the fra6 mutant form of AtCesA8 did not cause any reduction in cell wall thickness and cellulose content. These results suggest that the fra5 mutant protein may interfere with the function of endogenous wild-type CesA proteins, thus resulting in a dominant negative effect on cellulose biosynthesis.

Arabidopsis↗

PubMed2003

The SAC domain-containing protein gene family in Arabidopsis.

The SAC domain was first identified in the yeast (Saccharomyces cerevisiae) Sac1p phosphoinositide phosphatase protein and subsequently found in a number of proteins from yeast and animals. The SAC domain is approximately 400 amino acids in length and is characterized by seven conserved motifs. The SAC domains of several proteins have been recently demonstrated to possess phosphoinositide phosphatase activities. Sac1p has been shown to regulate the levels of various phosphoinositides in the phosphoinositide pool and affect diverse cellular functions such as actin cytoskeleton organization, Golgi function, and maintenance of vacuole morphology. The Arabidopsis genome contains a total of nine genes encoding SAC domain-containing proteins (AtSACs). The SAC domains of the AtSACs possess the conserved amino acid motifs that are believed to be important for the phosphoinositide phosphatase activities of yeast and animal SAC domain proteins. AtSACs can be divided into three subgroups based on their sequence similarities, hydropathy profiles, and phylogenetic relationship. Gene expression analysis demonstrated that the AtSAC genes exhibited differential expression patterns in different organs and, in particular, the AtSAC6 gene was predominantly expressed in flowers. Moreover, the expression of the AtSAC6 gene was highly induced by salinity. These results provide a foundation for future studies on the elucidation of the cellular functions of SAC domain-containing proteins in Arabidopsis.

Amino Acid Sequence↗

PubMed2002

Vascular development in Arabidopsis.

Vascular tissues, xylem and phloem, form a continuous network throughout the plant body for transport of water, minerals, and food. Characterization of Arabidopsis mutants defective in various aspects of vascular formation has demonstrated that Arabidopsis is an ideal system for investigating the molecular mechanisms controlling vascular development. The processes affected in these mutants include initiation or division of procambium or vascular cambium, formation of continuous vascular cell files, differentiation of procambium or vascular cambium into vascular tissues, cell elongation, patterned secondary wall thickening, and biosynthesis of secondary walls. Identification of the genes affected by some of these mutations has revealed essential roles in vascular development for a cytokinin receptor and several factors mediating auxin transport or signaling. Mutational studies have also identified a number of Arabidopsis mutants defective in leaf venation pattern or vascular tissue organization in stems. Genetic evidence suggests that the vascular tissue organization is regulated by the same positional information that determines organ polarity.

Arabidopsis↗

PubMed2002

A kinesin-like protein is essential for oriented deposition of cellulose microfibrils and cell wall strength.

Cortical microtubules have long been hypothesized to regulate the oriented deposition of cellulose microfibrils. However, the molecular mechanisms of how microtubules direct the orientation of cellulose microfibril deposition are not known. We have used fibers in the inflorescence stems of Arabidopsis to study secondary wall deposition and cell wall strength and found a fragile fiber (fra1) mutant with a dramatic reduction in the mechanical strength of fibers. The fra1 mutation did not cause any defects in cell wall composition, secondary wall thickening, or cortical microtubule organization in fiber cells. An apparent alteration was found in the orientation of cellulose microfibrils in fra1 fiber walls, indicating that the reduced mechanical strength of fra1 fibers probably was attributable to altered cellulose microfibril deposition. The FRA1 gene was cloned and found to encode a kinesin-like protein with an N-terminal microtubule binding motor domain. The FRA1 protein was shown to be concentrated around the periphery of the cytoplasm but absent in the nucleus. Based on these findings, we propose that the FRA1 kinesin-like protein is involved in the microtubule control of cellulose microfibril order.

Amino Acid Sequence↗

PubMed2002

Mutation of a chitinase-like gene causes ectopic deposition of lignin, aberrant cell shapes, and overproduction of ethylene.

Chitinase-like proteins have long been proposed to play roles in normal plant growth and development, but no mutations in chitinase-like genes have been obtained previously to support this hypothesis. In this study, we have shown that the gene responsible for the elp1 mutation in Arabidopsis encodes a chitinase-like protein (AtCTL1). Mutation of this chitinase-like gene caused ectopic deposition of lignin and aberrant shapes of cells with incomplete cell walls in the pith of inflorescence stems. The AtCTL1 gene was expressed in all organs during normal plant growth and development, but it was not induced by wounding, salicylic acid, pectin fragments, or ethylene. Consistent with its ubiquitous expression pattern, mutation of the AtCTL1 gene affected many aspects of plant growth and development, including exaggerated hook curvature, reduced length and increased diameter of hypocotyls in dark-grown seedlings, and reduced root length and increased number of root hairs in light-grown seedlings. The mutant phenotypes could be rescued partially by ethylene inhibitors, and ethylene production in the mutant was significantly greater than in the wild type. Together, these results suggest that AtCTL1, a chitinase-like gene, is essential for normal plant growth and development in Arabidopsis.

Amino Acid Sequence↗

PubMed1998

Dual methylation pathways in lignin biosynthesis

Caffeoyl-coenzyme A (CoA) O-methyltransferase (CCoAOMT) has been proposed to be involved in an alternative methylation pathway of lignin biosynthesis. However, no direct evidence has been available to confirm that CCoAOMT is essential for lignin biosynthesis. To understand further the methylation steps in lignin biosynthesis, we used an antisense approach to alter O-methyltransferase (OMT) gene expression and investigated the consequences of this alteration. We generated transgenic tobacco plants with a substantial reduction in CCoAOMT as well as plants with a simultaneous reduction in both CCoAOMT and caffeic acid O-methyltransferase (CAOMT). Lignin analysis showed that the reduction in CCoAOMT alone resulted in a dramatic decrease in lignin content. The reduction in CCoAOMT also led to a dramatic alteration in lignin composition. Both guaiacyl lignin and syringyl lignin were reduced in the transgenic plants. However, guaiacyl lignin was preferentially reduced, which resulted in an increase in the S/G (syringl/guaiacyl) ratio. We have also analyzed lignin content and composition in transgenic plants having a simultaneous reduction in both CCoAOMT and CAOMT. The reduction in both OMTs resulted in a further decrease in total lignin content. This is in sharp contrast to the effect that resulted from the reduction in CAOMT alone, which only decreased the syringl lignin unit without a reduction in overall lignin content. These results unequivocally demonstrate that methylation reactions in lignin biosynthesis are catalyzed by both CCoAOMT and CAOMT.

Journal Article↗