AUX/LAX genes encode a family of auxin influx transporters that perform distinct functions during Arabidopsis development

Auxin transport, which is mediated by specialized influx and efflux carriers, plays a major role in many aspects of plant growth and development. AUXIN1 (AUX1) has been demonstrated to encode a high-affinity auxin influx carrier. In Arabidopsis thaliana, AUX1 belongs to a small multigene family comprising four highly conserved genes (i.e., AUX1 and LIKE AUX1 [LAX] genes LAX1, LAX2, and LAX3). We report that all four members of this AUX/LAX family display auxin uptake functions. Despite the conservation of their biochemical function, AUX1, LAX1, and LAX3 have been described to regulate distinct auxin-dependent developmental processes. Here, we report that LAX2 regulates vascular patterning in cotyledons. We also describe how regulatory and coding sequences of AUX/LAX genes have undergone subfunctionalization based on their distinct patterns of spatial expression and the inability of LAX sequences to rescue aux1 mutant phenotypes, respectively. Despite their high sequence similarity at the protein level, transgenic studies reveal that LAX proteins are not correctly targeted in the AUX1 expression domain. Domain swapping studies suggest that the N-terminal half of AUX1 is essential for correct LAX localization. We conclude that Arabidopsis AUX/LAX genes encode a family of auxin influx transporters that perform distinct developmental functions and have evolved distinct regulatory mechanisms.

Unique contribution of IRF-5-Ikaros axis to the B-cell IgG2a response

IRF-5 is a transcription factor activated by toll like receptor (TLR)7 and TLR9 during innate immune responses. IRF-5 activates not only Type I IFN, but also inflammatory cytokines. Most importantly, a genetic variation in the IRF-5 gene shows a strong association with autoimmune diseases such as Lupus. Here, we report that IRF5-deficient mice have attenuated IgG2a/c responses to T-cell-dependent and -independent antigens and to polyoma virus infection. This defect is due to the intrinsic deletion of IRF-5 in B cells, as SCID mice reconstituted with Irf5-/- B cells show a decrease in IgG2a/c expression after viral infection compared with mice that received wild-type B cells. Irf5-/-B cells in vitro have diminished TLR and cytokine-induced class switching to IgG2a/c. Addressing the molecular mechanism, we show that IRF-5 regulates IgG2a/c expression by decreasing Ikaros expression; reconstitution of IRF-5 in Irf5-/- B cells downregulates Ikaros levels and increases switching to IgG2a/c. The IRF site in ikzf1 promoter binds IRF-5, IRF-4 and IRF-8. We show that IRF-8 but not IRF-4 activates the ikzf1 promoter, and IRF-5 inhibits the transcriptional activity of IRF-8. Collectively, these results identify the IRF-5-Ikaros axis as a critical modulator of IgG2a/c class switching.

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.

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.

A defective ABC transporter of the MRP family, responsible for the bean lpa1 mutation, affects the regulation of the phytic acid pathway, reduces seed myo-inositol and alters ABA sensitivity

• We previously identified the lpa1 (low phytic acid) 280-10 line that carries a mutation conferring a 90% reduction in phytic acid (InsP(6) ) content. In contrast to other lpa mutants, lpa1(280-10) does not display negative pleiotropic effects. In the present paper, we have identified the mutated gene and analysed its impact on the phytic acid pathway. • Here, we mapped the lpa1(280-10) mutation by bulk analysis on a segregating F(2) population, an then, by comparison with the soybean genome, we identified and sequenced a candidate gene. The InsP(6) pathway was analysed by gene expression and quantification of metabolites. • The mutated Pvmrp1(280-10) cosegregates with the lpa1(280-10) mutation, and the expression level of several genes of the InsP(6) pathway are reduced in the lpa1(280-10) mutant as well as the inositol and raffinosaccharide content. PvMrp2, a very similar paralogue of PvMrp1 was also mapped and sequenced. • The lpa1 mutation in beans is likely the result of a defective Mrp1 gene (orthologous to the lpa genes AtMRP5 and ZmMRP4), while its Mrp2 paralog is not able to complement the mutant phenotype in the seed. This mutation appears to down-regulate the InsP(6) pathway at the transcriptional level, as well as altering inositol-related metabolism and affecting ABA sensitivity.

The low phytic acid1-241 (lpa1-241) maize mutation alters the accumulation of anthocyanin pigment in the kernel

The lpa1 mutations in maize are caused by lesions in the ZmMRP4 (multidrug resistance-associated proteins 4) gene. In previous studies (Raboy et al. in Plant Physiol 124:355-368, 2000; Pilu et al. in Theor Appl Genet 107:980-987, 2003a; Shi et al. Nat Biotechnol 25:930-937, 2007), several mutations have been isolated in this locus causing a reduction of phytic acid (myo-inositol-1,2,3,4,5,6-hexakisphosphate, or InsP(6)) content and an equivalent increasing of free phosphate. In particular, the lpa1-241 mutation causes a reduction of up to 90% of phytic acid, associated with strong pleiotropic effects on the whole plant. In this work, we show, for the first time to our knowledge, an interaction between the accumulation of anthocyanin pigments in the kernel and the lpa mutations. In fact the lpa1-241 mutant accumulates a higher level of anthocyanins as compared to wild type either in the embryo (about 3.8-fold) or in the aleurone layer (about 0.3-fold) in a genotype able to accumulate anthocyanin. Furthermore, we demonstrate that these pigments are mislocalised in the cytoplasm, conferring a blue pigmentation of the scutellum, because of the neutral/basic pH of this cellular compartment. As a matter of fact, the propionate treatment, causing a specific acidification of the cytoplasm, restored the red pigmentation of the scutellum in the mutant and expression analysis showed a reduction of ZmMRP3 anthocyanins' transporter gene expression. On the whole, these data strongly suggest a possible interaction between the lpa mutation and anthocyanin accumulation and compartmentalisation in the kernel.

A tip-localized RhoGAP controls cell polarity by globally inhibiting Rho GTPase at the cell apex

BACKGROUND

Highly elongated eukaryotic cells (e.g., neuronal axons, fungal hyphae, and pollen tubes) are generated through continuous apically restricted growth (tip growth), which universally requires tip-localized Rho GTPases. We used the oscillating pollen tube as a model system to determine the function and regulation of Rho GTPases in tip growth. Our previous work showed that the spatiotemporal dynamics of the apical cap of the activated Rho-like GTPase from Plant 1 (ROP1) are critical for tip growth in pollen tubes. However, the underlying mechanism for the generation and maintenance of this dynamic apical cap is poorly understood.

RESULTS

A screen for mutations that enhance ROP1-overexpression-induced depolarization of pollen-tube growth identified REN1 (ROP1 enhancer 1) in Arabidopsis, whose null mutations turn elongated pollen tubes into bulbous cells. REN1 encodes a novel Rho GTPase-activating protein (RhoGAP) required for restricting the ROP1 activity to the pollen-tube tip. REN1 was localized to exocytic vesicles accumulated in the pollen-tube apex, as well as to the apical plasma membrane at the site of ROP1 activation. The apical localization of REN1 and its function in controlling growth polarity was compromised by disruption of ROP1-dependent F-actin and vesicular trafficking, which indicates that REN1 targeting and function is regulated by ROP1 downstream signaling.

CONCLUSIONS

Our findings suggest that the REN1 RhoGAP controls a negative-feedback-based global inhibition of ROP1. This function provides a critical self-organizing mechanism, by which ROP signaling is spatially limited to the growth site and temporally oscillates during continuous tip growth. Similar spatiotemporal control of Rho GTPase signaling may also play an important role in cell-polarity control in other systems, including tip growth in fungi and cell movement in animals.

Isolation and characterisation of an lpa (low phytic acid) mutant in common bean (Phaseolus vulgaris L.)

Phytic acid is considered as one of the major antinutritional compounds in cereal and legume seeds. The development of lpa (low phytic acid) grains, resulting in increased mineral cation availability, is considered a major goal in the improvement of the nutritional quality of seed crops, especially those largely consumed in developing countries. From a mutagenized population of common bean we isolated a homozygous lpa mutant line (lpa-280-10) showing, compared to wild type, a 90% reduction of phytic acid, a 25% reduction of raffinosaccharides and a much higher amount of free or weakly bound iron cations in the seed. Genetic analysis showed that the lpa character is due to a recessive mutation that segregates in a monogenic, Mendelian fashion. Germination tests performed using varying ageing or stress conditions, clearly showed that the bean line lpa-280-10 has a better germination response than the wild type. These data, together with those obtained from 2 years of agronomic trials showing that the mutant seed yield is close to that of its parents and other evidence, indicate that the new lpa-280-10 mutation might be the first devoid of visible macroscopic negative effects in plants, pods and seeds.

The Rab GTPase RabA4d regulates pollen tube tip growth in Arabidopsis thaliana

During reproduction in flowering plants, pollen grains form a tube that grows in a polarized fashion through the female tissues to eventually fertilize the egg cell. These highly polarized pollen tubes have a rapid rate of growth that is supported by a tip-focused delivery of membrane and cell wall components. To gain a better understanding of how this growth is regulated, we investigated the function RABA4D, a member of the Arabidopsis thaliana RabA4 subfamily of Rab GTPase proteins. Here, we show that RABA4D was expressed in a pollen-specific manner and that enhanced yellow fluorescent protein (EYFP)-RabA4d-labeled membrane compartments localized to the tips of growing pollen tubes. Mutant pollen in which the RABA4D gene was disrupted displayed bulged pollen tubes with a reduced rate of growth in vitro and displayed altered deposition of some cell wall components. Expression of EYFP-RabA4d restored wild-type phenotypes to the raba4d mutant pollen tubes, while expression of EYFP-RabA4b did not rescue the raba4d phenotype. In vivo, disruption of RABA4D resulted in a male-specific transmission defect with mutant raba4d pollen tubes displaying aberrant growth in the ovary and reduced guidance at the micropyle. We propose that RabA4d plays an important role in the regulation of pollen tube tip growth.

Imaging phosphatidylinositol 4-phosphate dynamics in living plant cells

Polyphosphoinositides represent a minor group of phospholipids, accounting for less than 1% of the total. Despite their low abundance, these molecules have been implicated in various signalling and membrane trafficking events. Phosphatidylinositol 4-phosphate (PtdIns4P) is the most abundant polyphosphoinositide. (32)Pi-labelling studies have shown that the turnover of PtdIns4P is rapid, but little is known about where in the cell or plant this occurs. Here, we describe the use of a lipid biosensor that monitors PtdIns4P dynamics in living plant cells. The biosensor consists of a fusion between a fluorescent protein and a lipid-binding domain that specifically binds PtdIns4P, i.e. the pleckstrin homology domain of the human protein phosphatidylinositol-4-phosphate adaptor protein-1 (FAPP1). YFP-PH(FAPP1) was expressed in four plant systems: transiently in cowpea protoplasts, and stably in tobacco BY-2 cells, Medicago truncatula roots and Arabidopsis thaliana seedlings. All systems allowed YFP-PH(FAPP1) expression without detrimental effects. Two distinct fluorescence patterns were observed: labelling of motile punctate structures and the plasma membrane. Co-expression studies with organelle markers revealed strong co-labelling with the Golgi marker STtmd-CFP, but not with the endocytic/pre-vacuolar marker GFP-AtRABF2b. Co-expression with the Ptdins3P biosensor YFP-2 x FYVE revealed totally different localization patterns. During cell division, YFP-PH(FAPP1) showed strong labelling of the cell plate, but PtdIns3P was completely absent from the newly formed cell membrane. In root hairs of M. truncatula and A. thaliana, a clear PtdIns4P gradient was apparent in the plasma membrane, with the highest concentration in the tip. This only occurred in growing root hairs, indicating a role for PtdIns4P in tip growth.

Phytic acid prevents oxidative stress in seeds: evidence from a maize (Zea mays L.) low phytic acid mutant

A maize mutant defective in the synthesis of phytic acid during seed maturation was used as a tool to study the consequences of the lack of this important reserve substance on seed survival. Data on germinability, free iron level, free radical relative abundance, protein carbonylation level, damage to DNA, degree of lipid peroxidation, alpha- and gamma-tocopherol amount and antioxidant capacity were recorded on seeds of maize B73 and of an isogenic low phytic acid mutant (lpa1-241), either unaged or incubated for 7 d in accelerated ageing conditions (46 degrees C and 100% relative humidity). The lpa1-241 mutant, compared to wild type (wt), showed a lower germination capacity, which decreased further after accelerated ageing. Whole lpa1-241 mutant kernels contained about 50% more free or weakly bound iron than wt ones and showed a higher content of free radicals, mainly concentrated in embryos; in addition, upon accelerated ageing, lpa1-241 seed proteins were more carbonylated and DNA was more damaged, whereas lipids did not appear to be more peroxidated, but the gamma-tocopherol content was decreased by about 50%. These findings can be interpreted in terms of previously reported but never proven antioxidant activity of phytic acid through iron complexation. Therefore, a novel role in plant seed physiology can be assigned to phytic acid, that is, protection against oxidative stress during the seed's life span. As in maize kernels the greater part of phytic acid (and thus of metal ions) is concentrated in the embryo, its antioxidant action may be of particular relevance in this crop.

Phosphoinositides in plants: novel functions in membrane trafficking

Tight regulation of membrane trafficking is crucial to the proper maintenance of the endomembrane trafficking system of eukaryotic cells. Distinct organelles must maintain their identities while at the same time continuously accepting, sorting, and exchanging membrane and luminal cargo constituents. Additionally, many of these organelles differentiate specialized subdomains containing distinct sets of lipids and proteins and restrict certain aspects of membrane trafficking to these regions of the organelle. Phosphoinositides (PIs) are a class of membrane lipids that have emerged as key components in some of these membrane trafficking events. The ability of these lipids to be rapidly produced, modified, and hydrolyzed by distinct classes of phosphatidylinositol (PtdIns) kinases, phosphatases, and phospholipases, allows for their use as finely tuned spatial and temporal landmarks for organelle and sub-organelle domains. In this review we will attempt to highlight some of the recent studies of the roles of this class of lipids in plant membrane trafficking, particularly on their important roles in polarized membrane trafficking in plants.

The auxin influx carrier LAX3 promotes lateral root emergence

Lateral roots originate deep within the parental root from a small number of founder cells at the periphery of vascular tissues and must emerge through intervening layers of tissues. We describe how the hormone auxin, which originates from the developing lateral root, acts as a local inductive signal which re-programmes adjacent cells. Auxin induces the expression of a previously uncharacterized auxin influx carrier LAX3 in cortical and epidermal cells directly overlaying new primordia. Increased LAX3 activity reinforces the auxin-dependent induction of a selection of cell-wall-remodelling enzymes, which are likely to promote cell separation in advance of developing lateral root primordia.

Rho-GTPase-dependent filamentous actin dynamics coordinate vesicle targeting and exocytosis during tip growth

The dynamic activity of tip-localized filamentous actin (F-actin) in pollen tubes is controlled by counteracting RIC4 and RIC3 pathways downstream of the ROP1 guanosine triphosphatase promoting actin assembly and disassembly, respectively. We show here that ROP1 activation is required for both the polar accumulation and the exocytosis of vesicles at the plasma membrane apex. The apical accumulation of exocytic vesicles oscillated in phase with, but slightly behind, apical actin assembly and was enhanced by overexpression of RIC4. However, RIC4 overexpression inhibited exocytosis, and this inhibition could be suppressed by latrunculin B treatment or RIC3 overexpression. We conclude that RIC4-dependent actin assembly is required for polar vesicle accumulation, whereas RIC3-mediated actin disassembly is required for exocytosis. Thus ROP1-dependent F-actin dynamics control tip growth through spatiotemporal coordination of vesicle targeting and exocytosis.

Root hair defective4 encodes a phosphatidylinositol-4-phosphate phosphatase required for proper root hair development in Arabidopsis thaliana

Polarized expansion of root hair cells in Arabidopsis thaliana is improperly controlled in root hair-defective rhd4-1 mutant plants, resulting in root hairs that are shorter and randomly form bulges along their length. Using time-lapse fluorescence microscopy in rhd4-1 root hairs, we analyzed membrane dynamics after labeling with RabA4b, a marker for polarized membrane trafficking in root hairs. This revealed stochastic loss and recovery of the RabA4b compartment in the tips of growing root hairs, consistent with a role for the RHD4 protein in regulation of polarized membrane trafficking in these cells. The wild-type RHD4 gene was identified by map-based cloning and was found to encode a Sac1p-like phosphoinositide phosphatase. RHD4 displayed a preference for phosphatidylinositol-4-phosphate [PI(4)P] in vitro, and rhd4-1 roots accumulated higher levels of PI(4)P in vivo. In wild-type root hairs, PI(4)P accumulated primarily in a tip-localized plasma membrane domain, but in rhd4-1 mutants, significant levels of PI(4)P were detected associated with internal membranes. A fluorescent RHD4 fusion protein localized to membranes at the tips of growing root hairs. We propose that RHD4 is selectively recruited to RabA4b-labeled membranes that are involved in polarized expansion of root hair cells and that, in conjunction with the phosphoinositide kinase PI-4Kbeta1, RHD4 regulates the accumulation of PI(4)P on membrane compartments at the tips of growing root hairs.

Sangamon and Pre-Sangamon Interglaciations in the Hudson Bay Lowlands of Manitoba

The manuscript documents voluminous new and published stratigraphie and paléontologie data from a region unique in North America, and possibly the world, in that it has abundant natural exposures recording a long and consistent succession of Quaternary tills and fossiliferous intertill sediments. Two major non-glacial intervals have been recognized under tills. Various non-glacial sediments have been correlated using their stratigraphie position, pollen analysis, and beetle assemblages, and are found to be equivalent to the Missinaibi Formation in Ontario. Non-finite high pressure radiocarbon dates, and aspartic acid ratios on wood suggest that the uppermost and most prevalent interval is of Sangamonian age. The Sangamonian record begins and ends with cool intervals, separated by a warmer period when climates were similar to or slightly warmer than present, and when the northern treeline was near its present position. The interval began with a high glacioisostatic sea, which regressed to levels below present Hudson Bay datum. The Sundance paleosol and silt beds below the Amery till relate to an earlier interglaciation characterized by cool tundra conditions. As an analogue for predicting future global change, the record preserved in the Hudson Bay Lowlands is second to none: additional intensive research is strongly recommended.

Fluvial Sedimentology and Paleoecology of Holocene Alluvial Deposits, Red River, Manitoba

Stratigraphie and paleoecological analyses at five sections, together with age determinations based on 19 previously published and 21 new radiocarbon dates, provide a detailed late Holocene history of the Red River, Manitoba. Ecological information, such as age frequency analysis, relative abundance, diversity and association of species was drawn from 19 mollusc species. These data indicate that the Red and Assiniboine rivers cut the valleys they occupy today within a thousand years of the regression of Lake Agassiz. In the south, up to 14 m of alluvium has accumulated during the last 7000 years. A decrease in the sedimentation rate at 1400 BP is coincident with the shift in the position of the Assiniboine from the valley of the La Salle River to its present position. Overbank sedimentation did not start in the northern part of the area until ca. 5200 BP. Initial rapid sedimentation rates in this area are attributed to increased precipitation and a brief eastward excursion of the Assiniboine River into the Red. In spite of increased precipitation, flood frequencies remained low in the north until 1400 BP. Increased overbank sedimentation after 1400 BP is attributed to the northward shift in the position ot the Assiniboine.

Developing a new interdisciplinary lab course for undergraduate and graduate students: Plant cells and proteins

Studies of protein function increasingly use multifaceted approaches that span disciplines including recombinant DNA technology, cell biology, and analytical biochemistry. These studies rely on sophisticated equipment and methodologies including confocal fluorescence microscopy, mass spectrometry, and X-ray crystallography that are beyond the scope of traditional laboratory courses. To equip the advanced undergraduate and beginning graduate students with an enabling base of knowledge and initial experience with advanced protein research methodologies, a laboratory course entitled Plant Cells and Proteins was developed in a partnership between Washington University and the Donald Danforth Plant Science Center in St. Louis. In this one semester course, 10-12 students obtain hands-on experience with plant tissue culture, gene transformation, subcellular localization of fluorescent recombinant proteins using confocal microscopy, purification of affinity-tagged recombinant proteins, isolation of total protein extracts, enzymatic assays, one- and two-dimensional gel electrophoresis, MALDI-TOF and ESI-Q-TOF mass spectrometry, protein crystallization, and X-ray diffraction. The course is taught as a series of modules, each led by an expert researcher. Students are evaluated based on a series of graded written reports and tests of their mastery of key concepts, interpretations, and the limitations of the experimental methods.

The Arabidopsis AAA ATPase SKD1 is involved in multivesicular endosome function and interacts with its positive regulator LYST-INTERACTING PROTEIN5

In yeast and mammals, the AAA ATPase Vps4p/SKD1 (for Vacuolar protein sorting 4/SUPPRESSOR OF K(+) TRANSPORT GROWTH DEFECT1) is required for the endosomal sorting of secretory and endocytic cargo. We identified a VPS4/SKD1 homolog in Arabidopsis thaliana, which localizes to the cytoplasm and to multivesicular endosomes. In addition, green fluorescent protein-SKD1 colocalizes on multivesicular bodies with fluorescent fusion protein endosomal Rab GTPases, such as ARA6/RabF1, RHA1/RabF2a, and ARA7/RabF2b, and with the endocytic marker FM4-64. The expression of SKD1(E232Q), an ATPase-deficient version of SKD1, induces alterations in the endosomal system of tobacco (Nicotiana tabacum) Bright Yellow 2 cells and ultimately leads to cell death. The inducible expression of SKD1(E232Q) in Arabidopsis resulted in enlarged endosomes with a reduced number of internal vesicles. In a yeast two-hybrid screen using Arabidopsis SKD1 as bait, we isolated a putative homolog of mammalian LYST-INTERACTING PROTEIN5 (LIP5)/SKD1 BINDING PROTEIN1 and yeast Vta1p (for Vps twenty associated 1 protein). Arabidopsis LIP5 acts as a positive regulator of SKD1 by increasing fourfold to fivefold its in vitro ATPase activity. We isolated a knockout homozygous Arabidopsis mutant line with a T-DNA insertion in LIP5. lip5 plants are viable and show no phenotypic alterations under normal growth conditions, suggesting that basal SKD1 ATPase activity is sufficient for plant development and growth.

AtCAT6, a sink-tissue-localized transporter for essential amino acids in Arabidopsis

Amino acids represent the major form of reduced nitrogen that is transported in plants. Amino acid transporters in plants often show tissue-specific expression patterns and are used by plants to transport these metabolites from source to sink during development and under changing environmental conditions. We identified one amino acid transporter, AtCAT6, which is expressed in sink tissues such as lateral root primordia, flowers and seeds. Additionally AtCAT6 was induced during infestation of roots by the plant-parasitic root-knot nematode, Meloidogyne incognita. Quantitative reverse-transcriptase PCR revealed nematode inducibility throughout the duration of nematode infestation and in nematode-induced feeding sites. Promoter analyses confirmed expression in endogenous sink tissues and nematode-induced feeding sites. In Xenopus oocytes, AtCAT6 mediated electrogenic transport of proteinogenic as well as non-proteinogenic amino acids with moderate affinity. AtCAT6 transported large, neutral and cationic amino acids in preference to other amino acids. Knockout mutants of this transporter failed to grow on medium containing l-glutamine as the sole nitrogen source. Our data suggest that AtCAT6 plays a role in supplying amino acids to sink tissues of plants and nematode-induced feeding structures.

A role for the RabA4b effector protein PI-4Kbeta1 in polarized expansion of root hair cells in Arabidopsis thaliana

The RabA4b GTPase labels a novel, trans-Golgi network compartment displaying a developmentally regulated polar distribution in growing Arabidopsis thaliana root hair cells. GTP bound RabA4b selectively recruits the plant phosphatidylinositol 4-OH kinase, PI-4Kβ1, but not members of other PI-4K families. PI-4Kβ1 colocalizes with RabA4b on tip-localized membranes in growing root hairs, and mutant plants in which both the PI-4Kβ1 and -4Kβ2 genes are disrupted display aberrant root hair morphologies. PI-4Kβ1 interacts with RabA4b through a novel homology domain, specific to eukaryotic type IIIβ PI-4Ks, and PI-4Kβ1 also interacts with a Ca²⁺ sensor, AtCBL1, through its NH₂ terminus. We propose that RabA4b recruitment of PI-4Kβ1 results in Ca²⁺-dependent generation of PI-4P on this compartment, providing a link between Ca²⁺ and PI-4,5P₂–dependent signals during the polarized secretion of cell wall components in tip-growing root hair cells.

High-affinity auxin transport by the AUX1 influx carrier protein

In plants, auxin is a key regulator of development and is unique among plant hormones in that its function requires polarized transport between neighboring cells to form concentration gradients across various plant tissues. Although putative auxin-influx and -efflux transporters have been identified by using molecular genetic approaches, a detailed functional understanding for many of these transporters remains undetermined. Here we present the functional characterization of the auxin-influx carrier AUX1. Upon expression of AUX1 in Xenopus oocytes, saturable, pH-dependent uptake of 3H-IAA was measured. Mutations in AUX1 that abrogate physiological responses to IAA in planta resulted in loss or reduction of 3H-IAA uptake in AUX1-expressing oocytes. AUX1-mediated uptake of 3H-IAA was reduced by the IAA analogs 2,4-D and 1-NOA, but not by other auxin analogs. The measured Km for AUX1-mediated uptake of 3H-IAA was at concentrations at which physiological responses are observed for exogenously added IAA and 2,4-D. This is the first report demonstrating detailed functional characteristics of a plant auxin-influx transporter. This biochemical characterization provides new insights and a novel tool for studying auxin entry into cells and its pivotal roles in plant growth and development.

RNA targeting using peptide nucleic acid

The efforts towards peptide nucleic acid (PNA) drug discovery using cellular RNAs as molecular targets is briefly reviewed, with special emphasis on recent developments. Special attention is given to cellular delivery in vivo bioavailability and the possibilities of using PNA oligomers to (re)direct alternative splicing of pre-messenger (m)RNA.

Observation of three-dimensional long-range order in small ion coulomb crystals in an rf trap

Three-dimensional long-range ordered structures in smaller and near-spherically symmetric Coulomb crystals of (40)Ca(+) ions confined in a linear rf Paul trap have been observed when the number of ions exceeds approximately 1,000 ions. This result is unexpected from ground state molecular dynamics (MD) simulations, but found to be in agreement with MD simulations of metastable ion configurations. Previously, three-dimensional long-range ordered structures have only been reported in Penning traps in systems of approximately 50,000 ions or more.

Nematode-induced changes of transporter gene expression in Arabidopsis roots

Root-knot plant-parasitic nematodes (Meloidogyne spp.) account for much of the damage inflicted to plants by nematodes. The feeding sites of these nematodes consist of "giant" cells, which have characteristics of transfer cells found in other parts of plants. Increased transport activity across the plasma membrane is a hallmark of transfer cells, and giant cells provide nutrition for nematodes; therefore, we initiated a study to identify the transport processes that contribute to the development and function of nematode-induced feeding sites. The study was conducted over a 4-week period, during which time the large changes in the development of giant cells were documented. The Arabidopsis ATH1 GeneChip was used to identify the many transporter genes that were regulated by nematode infestation. Expression of 50 transporter genes from 18 different gene families was significantly changed upon nematode infestation. Sixteen transporter genes were studied in more detail using real-time reverse-transcriptase polymerase chain reaction to determine transcript abundance in nematode-induced galls that contain giant cells and uninfested regions of the root. Certain genes were expressed primarily in galls whereas others were expressed primarily in the uninfested regions of the root, and a third group was expressed evenly throughout the root. Multiple transport processes are regulated and these may play important roles in nematode feeding-site establishment and maintenance.

Radar soundings of the ionosphere of Mars

We report the first radar soundings of the ionosphere of Mars with the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on board the orbiting Mars Express spacecraft. Several types of ionospheric echoes are observed, ranging from vertical echoes caused by specular reflection from the horizontally stratified ionosphere to a wide variety of oblique and diffuse echoes. The oblique echoes are believed to arise mainly from ionospheric structures associated with the complex crustal magnetic fields of Mars. Echoes at the electron plasma frequency and the cyclotron period also provide measurements of the local electron density and magnetic field strength.

Actin-based motility of endosomes is linked to the polar tip growth of root hairs

Plant tip growth has been recognized as an actin-based cellular process requiring targeted exocytosis and compensatory endocytosis to occur at the growth cone. However, the identity of subcellular compartments involved in polarized membrane trafficking pathways remains enigmatic in plants. Here we characterize endosomal compartments in tip-growing root hair cells. We demonstrate their presence at the growing tip and differential distribution upon cessation of tip growth. We also show that both the presence of endosomes as well as their rapid movements within the tip region depends on an intact actin cytoskeleton and involves actin polymerization. In conclusion, actin-propelled endosomal motility is tightly linked to the polar tip growth of root hairs.

The Arabidopsis Rab GTPase RabA4b localizes to the tips of growing root hair cells

Spatial and temporal control of cell wall deposition plays a unique and critical role during growth and development in plants. To characterize membrane trafficking pathways involved in these processes, we have examined the function of a plant Rab GTPase, RabA4b, during polarized expansion in developing root hair cells. Whereas a small fraction of RabA4b cofractionated with Golgi membrane marker proteins, the majority of this protein labeled a unique membrane compartment that did not cofractionate with the previously characterized trans-Golgi network syntaxin proteins SYP41 and SYP51. An enhanced yellow fluorescent protein (EYFP)-RabA4b fusion protein specifically localizes to the tips of growing root hair cells in Arabidopsis thaliana. Tip-localized EYFP-RabA4b disappears in mature root hair cells that have stopped expanding, and polar localization of the EYFP-RabA4b is disrupted by latrunculin B treatment. Loss of tip localization of EYFP-RabA4b was correlated with inhibition of expansion; upon washout of the inhibitor, root hair expansion recovered only after tip localization of the EYFP-RabA4b compartments was reestablished. Furthermore, in mutants with defective root hair morphology, EYFP-RabA4b was improperly localized or was absent from the tips of root hair cells. We propose that RabA4b regulates membrane trafficking through a compartment involved in the polarized secretion of cell wall components in plant cells.

MRI in chronic spinal cord injury patients with and without central pain

BACKGROUND

Central pain following spinal cord injury (SCI) is common and thought to be related to lesion of the spinothalamic pathways.

OBJECTIVE

To examine additional mechanisms of SCI pain.

METHODS

Twenty-three SCI patients with traumatic lesions above T10 (14 with central neuropathic pain and 9 without pain) underwent MRI examination. The authors quantitatively assessed extent of cord lesion on axial T2-weighted images as percentage of 1) gray matter, 2) dorsolateral, 3) anterolateral, and 4) dorsolateral columns based on standardized drawings made by a neuroradiologist blinded to patient history.

RESULTS

At the level of maximal cord injury, 21 patients had lesions involving the entire cord on axial images except for a small border of lower signal intensity, whereas 2 patients had central lesions. Rostral to the main injury, the first image with an incomplete lesion showed significantly more involvement of gray matter in pain than in pain-free patients.

CONCLUSION

Consistent with animal models of SCI, spinothalamic tract lesion together with neuronal hyperexcitability due to lesion of inhibitory interneurons at the rostral end of injury are hypothesized to lead to central pain.

Phenotypic, genetic and molecular characterization of a maize low phytic acid mutant (lpa241)

Phytic acid, myo-inositol 1,2,3,4,5,6-hexakisphosphate, is the major storage compound of phosphorous (P) in plants, predominantly accumulating in seeds (up to 4-5% of dry weight) and pollen. In cereals, phytic acid is deposited in embryo and aleurone grain tissues as a mixed "phytate" salt of potassium and magnesium, although phytates contain other mineral cations such as iron and zinc. During germination, phytates are broken down by the action of phytases, releasing their P, minerals and myo-inositol which become available to the growing seedling. Phytic acid represents an anti-nutritional factor for animals, and isolation of maize low phytic acid ( lpa) mutants provides a novel approach to study its biochemical pathway and to tackle the nutritional problems associated with it. Following chemical mutagenesis of pollen, we have isolated a viable recessive mutant named lpa 241 showing about 90% reduction of phytic acid and about a tenfold increase in seed-free phosphate content. Although germination rate was decreased by about 30% compared to wild-type, developement of mutant plants was apparentely unaffected. The results of the genetic, biochemical and molecular characterization experiments carried out by SSR mapping, MDD-HPLC and RT-PCR are consistent with a mutation affecting the MIPS1S gene, coding for the first enzyme of the phytic acid biosynthetic pathway.

Analysis of the small GTPase gene superfamily of Arabidopsis

Small GTP-binding proteins regulate diverse processes in eukaryotic cells such as signal transduction, cell proliferation, cytoskeletal organization, and intracellular membrane trafficking. These proteins function as molecular switches that cycle between "active" and "inactive" states, and this cycle is linked to the binding and hydrolysis of GTP. The Arabidopsis genome contains 93 genes that encode small GTP-binding protein homologs. Phylogenetic analysis of these genes shows that plants contain Rab, Rho, Arf, and Ran GTPases, but no Ras GTPases. We have assembled complete lists of these small GTPases families, as well as accessory proteins that control their activity, and review what is known of the functions of individual members of these families in Arabidopsis. We also discuss the possible roles of these GTPases in relation to their similarity to orthologs with known functions and localizations in yeast and/or animal systems.