AtCSLD2 is an integral Golgi membrane protein with its N-terminus facing the cytosol

Characterization of Cellulose synthase-like D (CSLD) family revealed the involvement of PtrCslD5 in root hair formation in Populus trichocarpa

Cellulose synthase-like D (CSLD) family was characterized for their expression and functions in Populus trichocarpa. Ten members, PtrCslD1-10, were identified in the P. trichocarpa genome, and they belong to 4 clades by phylogenetic tree analysis. qRT-PCR and promoter:GUS assays in Arabidopsis and P. trichocarpa displayed divergent expression patterns of these 10 PtrCSLD genes in root hairs, root tips, leaves, vascular tissues, xylem and flowers. Among PtrCslD2, PtrCslD4, PtrCslD5, PtrCslD6, and PtrCslD8 that all exhibited expression in root hairs, only PtrCslD5 could restore the root hairless phenotype of the atcsld3 mutant, demonstrating that PtrCslD5 is the functional ortholog of AtCslD3 for root hair formation. Our results suggest more possible functions for other PtrCslD genes in poplar.

Cellulose synthase-like D1 controls organ size in maize

BACKGROUND

Plant architecture is a critical factor that affects planting density and, consequently, grain yield in maize. The genes or loci that determine organ size are the key regulators of plant architecture. Thus, understanding the genetic and molecular mechanisms of organ size will inform the use of a molecular manipulation approach to improve maize plant architecture and grain yield.

RESULTS

A total of 18 unique quantitative trait loci (QTLs) were identified for 11 agronomic traits in the F₂ and F_2:3 segregating populations derived from a cross between a double haploid line with a small plant architecture (MT03-1) and an inbred line with a large plant architecture (LEE-12). Subsequently, we showed that one QTL, qLW10, for multiple agronomic traits that relate to plant organ size reflects allelic variation in ZmCSLD1, which encodes a cellulose synthase-like D protein. ZmCSLD1 was localized to the trans-Golgi and was highly expressed in the rapidly growing regions. The loss of ZmCSLD1 function decreased cell division, which resulted in smaller organs with fewer cell numbers and, in turn, pleiotropic effects on multiple agronomic traits. In addition, intragenic complementation was investigated for two Zmcsld1 alleles with nonsynonymous SNPs in different functional domains, and the mechanism of this complementation was determined to be through homodimeric interactions.

CONCLUSIONS

Through positional cloning by using two populations and allelism tests, qLW10 for organ size was resolved to be a cellulose synthase-like D family gene, ZmCSLD1. ZmCSLD1 has pleiotropic effects on multiple agronomic traits that alter plant organ size by changing the process of cell division. These findings provide new insight into the regulatory mechanism that underlies plant organ development.

Cellular localization and biochemical characterization of a chimeric fluorescent protein fusion of Arabidopsis cellulose synthase-like A2 inserted into Golgi membrane

Cellulose synthase-like (Csl) genes are believed to encode enzymes for the synthesis of cell wall matrix polysaccharides. The subfamily of CslA is putatively involved in the biosynthesis of β -mannans. Here we report a study on the cellular localization and the enzyme activity of an Arabidopsis CslA family member, AtCslA2. We show that the fluorescent protein fusion AtCslA2-GFP, transiently expressed in tobacco leaf protoplasts, is synthesized in the ER and it accumulates in the Golgi stacks. The chimera is inserted in the Golgi membrane and is functional since membrane preparations obtained by transformed protoplasts carry out the in vitro synthesis of a 14C-mannan starting from GDP-D-[U-14C]mannose as substrate. The enzyme specific activity is increased by approximately 38% in the transformed protoplasts with respect to wild-type. Preliminary tests with proteinase K, biochemical data, and TM domain predictions suggest that the catalytic site of AtCslA2 faces the Golgi lumen.

Golgi-localized enzyme complexes for plant cell wall biosynthesis

The plant cell wall mostly comprises complex glycans, which are synthesized by numerous enzymes located in the Golgi apparatus and plasma membrane. Protein-protein interactions have been shown to constitute an important organizing principle for glycan biosynthetic enzymes in mammals and yeast. Recent genetic and biochemical data also indicate that such interactions could be common in plant cell wall biosynthesis. In this review, we examine the new findings in protein-protein interactions among plant cell wall biosynthetic enzymes and discuss the possibilities for enzyme complexes in the Golgi apparatus. These new insights in the field may contribute to novel strategies for molecular engineering of the cell wall.

The Arabidopsis apyrase AtAPY1 is localized in the Golgi instead of the extracellular space

BACKGROUND

The two highly similar Arabidopsis apyrases AtAPY1 and AtAPY2 were previously shown to be involved in plant growth and development, evidently by regulating extracellular ATP signals. The subcellular localization of AtAPY1 was investigated to corroborate an extracellular function.

RESULTS

Transgenic Arabidopsis lines expressing AtAPY1 fused to the SNAP-(O(6)-alkylguanine-DNA alkyltransferase)-tag were used for indirect immunofluorescence and AtAPY1 was detected in punctate structures within the cell. The same signal pattern was found in seedlings stably overexpressing AtAPY1-GFP by indirect immunofluorescence and live imaging. In order to identify the nature of the AtAPY1-positive structures, AtAPY1-GFP expressing seedlings were treated with the endocytic marker stain FM4-64 (N-(3-triethylammoniumpropyl)-4-(p-diethylaminophenyl-hexatrienyl)-pyridinium dibromide) and crossed with a transgenic line expressing the trans-Golgi marker Rab E1d. Neither FM4-64 nor Rab E1d co-localized with AtAPY1. However, live imaging of transgenic Arabidopsis lines expressing AtAPY1-GFP and either the fluorescent protein-tagged Golgi marker Membrin 12, Syntaxin of plants 32 or Golgi transport 1 protein homolog showed co-localization. The Golgi localization was confirmed by immunogold labeling of AtAPY1-GFP. There was no indication of extracellular AtAPY1 by indirect immunofluorescence using antibodies against SNAP and GFP, live imaging of AtAPY1-GFP and immunogold labeling of AtAPY1-GFP. Activity assays with AtAPY1-GFP revealed GDP, UDP and IDP as substrates, but neither ATP nor ADP. To determine if AtAPY1 is a soluble or membrane protein, microsomal membranes were isolated and treated with various solubilizing agents. Only SDS and urea (not alkaline or high salt conditions) were able to release the AtAPY1 protein from microsomal membranes.

CONCLUSIONS

AtAPY1 is an integral Golgi protein with the substrate specificity typical for Golgi apyrases. It is therefore not likely to regulate extracellular nucleotide signals as previously thought. We propose instead that AtAPY1 exerts its growth and developmental effects by possibly regulating glycosylation reactions in the Golgi.

GO-PROMTO illuminates protein membrane topologies of glycan biosynthetic enzymes in the Golgi apparatus of living tissues

The Golgi apparatus is the main site of glycan biosynthesis in eukaryotes. Better understanding of the membrane topology of the proteins and enzymes involved can impart new mechanistic insights into these processes. Publically available bioinformatic tools provide highly variable predictions of membrane topologies for given proteins. Therefore we devised a non-invasive experimental method by which the membrane topologies of Golgi-resident proteins can be determined in the Golgi apparatus in living tissues. A Golgi marker was used to construct a series of reporters based on the principle of bimolecular fluorescence complementation. The reporters and proteins of interest were recombinantly fused to split halves of yellow fluorescent protein (YFP) and transiently co-expressed with the reporters in the Nicotiana benthamiana leaf tissue. Output signals were binary, showing either the presence or absence of fluorescence with signal morphologies characteristic of the Golgi apparatus and endoplasmic reticulum (ER). The method allows prompt and robust determinations of membrane topologies of Golgi-resident proteins and is termed GO-PROMTO (for GOlgi PROtein Membrane TOpology). We applied GO-PROMTO to examine the topologies of proteins involved in the biosynthesis of plant cell wall polysaccharides including xyloglucan and arabinan. The results suggest the existence of novel biosynthetic mechanisms involving transports of intermediates across Golgi membranes.

Biosynthesis of non-cellulosic polysaccharides of plant cell walls

Enzymes that make the polymer backbones of plant cell wall polysaccharides have proven to be recalcitrant to biochemical purification. Availability of mutational genetics and genomic tools paved the way for rapid progress in identifying genes encoding various cell wall glycan synthases. Mutational genetics, the primary tool used in unraveling cellulose biosynthesis, was ineffective in assigning function to any of the hemicellulosic, polymerizing glycan synthases. A combination of comparative genomics and functional expression in a heterologous system allowed identification of various cellulose synthase-like (Csl) sequences as being involved in the formation of β-1,4-mannan, β-1,4-glucan, and mixed-linked glucan. A number of xylose-deficient mutants have led to a variety of genes, none of which thus far possesses the motifs known to be conserved among polymerizing β-glycan synthases. Except for xylan synthase, which appears to be an agglomerate of proteins just like cellulose synthase, Golgi glycan synthases already identified suggest that the catalytic polypeptide by itself is sufficient for enzyme activity, most likely as a homodimer. Several of the Csl genes remain to be assigned a function. The possibility of the involvement of various Csl genes in making more than one product remains.

Cellulose Synthase-Like D1 is integral to normal cell division, expansion, and leaf development in maize

The Cellulose Synthase-Like D (CslD) genes have important, although still poorly defined, roles in cell wall formation. Here, we show an unexpected involvement of CslD1 from maize (Zea mays) in cell division. Both division and expansion were altered in the narrow-organ and warty phenotypes of the csld1 mutants. Leaf width was reduced by 35%, due mainly to a 47% drop in the number of cell files across the blade. Width of other organs was also proportionally reduced. In leaf epidermis, the deficiency in lateral divisions was only partially compensated by a modest, uniform increase in cell width. Localized clusters of misdivided epidermal cells also led to the formation of warty lesions, with cell clusters bulging from the epidermal layer, and some cells expanding to volumes 75-fold greater than normal. The decreased cell divisions and localized epidermal expansions were not associated with detectable changes in the cell wall composition of csld1 leaf blades or epidermal peels, yet a greater abundance of thin, dense walls was indicated by high-resolution x-ray tomography of stems. Cell-level defects leading to wart formation were traced to sites of active cell division and expansion at the bases of leaf blades, where cytokinesis and cross-wall formation were disrupted. Flow cytometry confirmed a greater frequency of polyploid cells in basal zones of leaf blades, consistent with the disruption of cytokinesis and/or the cell cycle in csld1 mutants. Collectively, these data indicate a previously unrecognized role for CSLD activity in plant cell division, especially during early phases of cross-wall formation.

A role for CSLD3 during cell-wall synthesis in apical plasma membranes of tip-growing root-hair cells

In plants, cell shape is defined by the cell wall, and changes in cell shape and size are dictated by modification of existing cell walls and deposition of newly synthesized cell-wall material. In root hairs, expansion occurs by a process called tip growth, which is shared by root hairs, pollen tubes and fungal hyphae. We show that cellulose-like polysaccharides are present in root-hair tips, and de novo synthesis of these polysaccharides is required for tip growth. We also find that eYFP-CSLD3 proteins, but not CESA cellulose synthases, localize to a polarized plasma-membrane domain in root hairs. Using biochemical methods and genetic complementation of a csld3 mutant with a chimaeric CSLD3 protein containing a CESA6 catalytic domain, we provide evidence that CSLD3 represents a distinct (1→4)-β-glucan synthase activity in apical plasma membranes during tip growth in root-hair cells.

Root hair-specific disruption of cellulose and xyloglucan in AtCSLD3 mutants, and factors affecting the post-rupture resumption of mutant root hair growth

The glycosyl transferase encoded by the cellulose synthase-like gene CSLD3/KJK/RHD7 (At3g03050) is required for cell wall integrity during root hair formation in Arabidopsis thaliana but it remains unclear whether it contributes to the synthesis of cellulose or hemicellulose. We identified two new alleles, root hair-defective (rhd) 7-1 and rhd7-4, which affect the C-terminal end of the encoded protein. Like root hairs in the previously characterized kjk-2 putative null mutant, rhd7-1 and rhd7-4 hairs rupture before tip growth but, depending on the growth medium and temperature, hairs are able to survive rupture and initiate tip growth, indicating that these alleles retain some function. At 21°C, the rhd7 tip-growing root hairs continued to rupture but at 5ºC, rupture was inhibited, resulting in long, wild type-like root hairs. At both temperatures, the expression of another root hair-specific CSLD gene, CSLD2, was increased in the rhd7-4 mutant but reduced in the kjk-2 mutant, suggesting that CSLD2 expression is CSLD3-dependent, and that CSLD2 could partially compensate for CSLD3 defects to prevent rupture at 5°C. Using a fluorescent brightener (FB 28) to detect cell wall (1 → 4)-β-glucans (primarily cellulose) and CCRC-M1 antibody to detect fucosylated xyloglucans revealed a patchy distribution of both in the mutant root hair cell walls. Cell wall thickness varied, and immunogold electron microscopy indicated that xyloglucan distribution was altered throughout the root hair cell walls. These cell wall defects indicate that CSLD3 is required for the normal organization of both cellulose and xyloglucan in root hair cell walls.

Arabidopsis mannan synthase CSLA9 and glucan synthase CSLC4 have opposite orientations in the Golgi membrane

Several proteins encoded by the cellulose synthase-like (CSL) gene family are known to be processive glycan synthases involved in the synthesis of cell-wall polysaccharides. These include CSLA proteins, which synthesize β-(1→4)-linked mannans found in the walls of many plant species, and CSLC proteins, which are thought to synthesize the β-(1→4)-linked glucan backbone of xyloglucan, an abundant polysaccharide in the primary walls of many plants. CSLA and CSLC proteins are predicted to have multiple membrane spans, and their products (mannan and xyloglucan) accumulate in the Golgi lumen. Knowing where these proteins are located in the cell and how they are orientated in the membrane is important for understanding many aspects of mannan and xyloglucan biosynthesis. In this study, we investigate the subcellular localization and membrane protein topology of CSLA9 and CSLC4, the members of these two families that are most highly expressed in Arabidopsis. CSLA9 and CSLC4 are found predominantly in Golgi membranes, based on co-localization with the known ER/Golgi marker ERD2-YFP. The topology of epitope-tagged proteins was examined using protease protection experiments. Experiments were designed to determine the positions of both the protein termini and the active loop of the CSL proteins investigated. The topology of CSLA9 is characterized by an odd number of transmembrane domains (probably five) and an active site that faces the Golgi lumen. In contrast, CSLC4 has an even number of transmembrane domains (probably six) and an active site that faces the cytosol. The implications of these topologies on various aspects of hemicellulose biosynthesis are discussed.

A glucurono(arabino)xylan synthase complex from wheat contains members of the GT43, GT47, and GT75 families and functions cooperatively

Glucuronoarabinoxylans (GAXs) are the major hemicelluloses in grass cell walls, but the proteins that synthesize them have previously been uncharacterized. The biosynthesis of GAXs would require at least three glycosyltransferases (GTs): xylosyltransferase (XylT), arabinosyltransferase (AraT), and glucuronosyltransferase (GlcAT). A combination of proteomics and transcriptomics analyses revealed three wheat (Triticum aestivum) glycosyltransferase (TaGT) proteins from the GT43, GT47, and GT75 families as promising candidates involved in GAX synthesis in wheat, namely TaGT43-4, TaGT47-13, and TaGT75-4. Coimmunoprecipitation experiments using specific antibodies produced against TaGT43-4 allowed the immunopurification of a complex containing these three GT proteins. The affinity-purified complex also showed GAX-XylT, GAX-AraT, and GAX-GlcAT activities that work in a cooperative manner. UDP Xyl strongly enhanced both AraT and GlcAT activities. However, while UDP arabinopyranose stimulated the XylT activity, it had only limited effect on GlcAT activity. Similarly, UDP GlcUA stimulated the XylT activity but had only limited effect on AraT activity. The [(14)C]GAX polymer synthesized by the affinity-purified complex contained Xyl, Ara, and GlcUA in a ratio of 45:12:1, respectively. When this product was digested with purified endoxylanase III and analyzed by high-pH anion-exchange chromatography, only two oligosaccharides were obtained, suggesting a regular structure. One of the two oligosaccharides has six Xyls and two Aras, and the second oligosaccharide contains Xyl, Ara, and GlcUA in a ratio of 40:8:1, respectively. Our results provide a direct link of the involvement of TaGT43-4, TaGT47-13, and TaGT75-4 proteins (as a core complex) in the synthesis of GAX polymer in wheat.

Identification and characterization of NARROW AND ROLLED LEAF 1, a novel gene regulating leaf morphology and plant architecture in rice

Leaf morphology is an important agronomic trait in rice breeding. We isolated three allelic mutants of NARROW AND ROLLED LEAF 1 (nrl1) which showed phenotypes of reduced leaf width and semi-rolled leaves and different degrees of dwarfism. Microscopic analysis indicated that the nrl1-1 mutant had fewer longitudinal veins and smaller adaxial bulliform cells compared with the wild-type. The NRL1 gene was mapped to the chromosome 12 and encodes the cellulose synthase-like protein D4 (OsCslD4). Sequence analyses revealed single base substitutions in the three allelic mutants. Genetic complementation and over-expression of the OsCslD4 gene confirmed the identity of NRL1. The gene was expressed in all tested organs of rice at the heading stage and expression level was higher in vigorously growing organs, such as roots, sheaths and panicles than in elsewhere. In the mutant leaves, however, the expression level was lower than that in the wild-type. We conclude that OsCslD4 encoded by NRL1 plays a critical role in leaf morphogenesis and vegetative development in rice.

Plant cell wall matrix polysaccharide biosynthesis

The wall of an expanding plant cell consists primarily of cellulose microfibrils embedded in a matrix of hemicellulosic and pectic polysaccharides along with small amounts of structural and enzymatic proteins. Matrix polysaccharides are synthesized in the Golgi and exported to the cell wall by exocytosis, where they intercalate among cellulose microfibrils, which are made at the plasma membrane and directly deposited into the cell wall. Involvement of Golgi glucan synthesis in auxin-induced cell expansion has long been recognized; however, only recently have the genes corresponding to glucan synthases been identified. Biochemical purification was unsuccessful because of the labile nature and very low abundance of these enzymes. Mutational genetics also proved fruitless. Expression of candidate genes identified through gene expression profiling or comparative genomics in heterologous systems followed by functional characterization has been relatively successful. Several genes from the cellulose synthase-like (Csl) family have been found to be involved in the synthesis of various hemicellulosic glycans. The usefulness of this approach, however, is limited to those enzymes that probably do not form complexes consisting of unrelated proteins. Nonconventional approaches will continue to incrementally unravel the mechanisms of Golgi polysaccharide biosynthesis.

Subcellular localization and functional analysis of the Arabidopsis GTPase RabE

Membrane trafficking plays a fundamental role in eukaryotic cell biology. Of the numerous known or predicted protein components of the plant cell trafficking system, only a relatively small subset have been characterized with respect to their biological roles in plant growth, development, and response to stresses. In this study, we investigated the subcellular localization and function of an Arabidopsis (Arabidopsis thaliana) small GTPase belonging to the RabE family. RabE proteins are phylogenetically related to well-characterized regulators of polarized vesicle transport from the Golgi apparatus to the plasma membrane in animal and yeast cells. The RabE family of GTPases has also been proposed to be a putative host target of AvrPto, an effector protein produced by the plant pathogen Pseudomonas syringae, based on yeast two-hybrid analysis. We generated transgenic Arabidopsis plants that constitutively expressed one of the five RabE proteins (RabE1d) fused to green fluorescent protein (GFP). GFP-RabE1d and endogenous RabE proteins were found to be associated with the Golgi apparatus and the plasma membrane in Arabidopsis leaf cells. RabE down-regulation, due to cosuppression in transgenic plants, resulted in drastically altered leaf morphology and reduced plant size, providing experimental evidence for an important role of RabE GTPases in regulating plant growth. RabE down-regulation did not affect plant susceptibility to pathogenic P. syringae bacteria; conversely, expression of the constitutively active RabE1d-Q74L enhanced plant defenses, conferring resistance to P. syringae infection.

Functional analysis of the cellulose synthase-like genes CSLD1, CSLD2, and CSLD4 in tip-growing Arabidopsis cells

A reverse genetic approach was used to investigate the functions of three members of the cellulose synthase superfamily in Arabidopsis (Arabidopsis thaliana), CELLULOSE SYNTHASE-LIKE D1 (CSLD1), CSLD2, and CSLD4. CSLD2 is required for normal root hair growth but has a different role from that previously described for CSLD3 (KOJAK). CSLD2 is required during a later stage of hair development than CSLD3, and CSLD2 mutants produce root hairs with a range of abnormalities, with many root hairs rupturing late in development. Remarkably, though, it was often the case that in CSLD2 mutants, tip growth would resume after rupturing of root hairs. In silico, semiquantitative reverse transcription-polymerase chain reaction, and promoter-reporter construct analyses indicated that the expression of both CSLD2 and CSLD3 is elevated at reduced temperatures, and the phenotypes of mutants homozygous for insertions in these genes were partially rescued by reduced temperature growth. However, this was not the case for a double mutant homozygous for insertions in both CSLD2 and CSLD3, suggesting that there may be partial redundancy in the functions of these genes. Mutants in CSLD1 and CSLD4 had a defect in male transmission, and plants heterozygous for insertions in CSLD1 or CSLD4 were defective in their ability to produce pollen tubes, although the number and morphology of pollen grains was normal. We propose that the CSLD family of putative glycosyltransferases synthesize a polysaccharide that has a specialized structural role in the cell walls of tip-growing cells.