Auxin induces rapid changes in phosphatidylinositol metabolites

Phosphatidylinositol-specific phospholipase C2 functions in auxin-modulated root development

Nine phosphatidylinositol-specific phospholipases C (PLCs) have been identified in the Arabidopsis genome; among the importance of PLC2 in reproductive development is significant. However, the role of PLC2 in vegetative development such as in root growth is elusive. Here, we report that plc2 mutants displayed multiple auxin-defective phenotypes in root development, including short primary root, impaired root gravitropism, and inhibited root hair growth. The DR5:GUS expression and the endogenous indole-3-acetic acid (IAA) content, as well as the responses of a set of auxin-related genes to exogenous IAA treatment, were all decreased in plc2 seedlings, suggesting the influence of PLC2 on auxin accumulation and signalling. The root elongation of plc2 mutants was less sensitive to the high concentration of exogenous auxins, and the application of 1-naphthaleneacetic acid or the auxin transport inhibitor N-1-naphthylphthalamic acid could rescue the root hair growth of plc2 mutants. In addition, the PIN2 polarity and cycling in plc2 root epidermis cells were altered. These results demonstrate a critical role of PLC2 in auxin-mediated root development in Arabidopsis, in which PLC2 influences the polar distribution of PIN2.

Auxin and chloroplast movements

Auxin is involved in a wide spectrum of physiological processes in plants, including responses controlled by the blue light photoreceptors phototropins: phototropic bending and stomatal movement. However, the role of auxin in phototropin-mediated chloroplast movements has never been studied. To address this question we searched for potential interactions between auxin and the chloroplast movement signaling pathway using different experimental approaches and two model plants, Arabidopsis thaliana and Nicotiana tabacum. We observed that the disturbance of auxin homeostasis by shoot decapitation caused a decrease in chloroplast movement parameters, which could be rescued by exogenous auxin application. In several cases, the impairment of polar auxin transport, by chemical inhibitors or in auxin carrier mutants, had a similar negative effect on chloroplast movements. This inhibition was not correlated with changes in auxin levels. Chloroplast relocations were also affected by the antiauxin p-chlorophenoxyisobutyric acid and mutations in genes encoding some of the elements of the SCF(TIR1)-Aux/IAA auxin receptor complex. The observed changes in chloroplast movement parameters are not prominent, which points to a modulatory role of auxin in this process. Taken together, the obtained results suggest that auxin acts indirectly to regulate chloroplast movements, presumably by regulating gene expression via the SCF(TIR1)-Aux/IAA-ARF pathway. Auxin does not seem to be involved in controlling the expression of phototropins.

Transcriptome analysis identifies genes involved in adventitious branches formation of Gracilaria lichenoides in vitro

Tissue culture could solve the problems associated with Gracilaria cultivation, including the consistent supply of high-quality seed stock, strain improvement, and efficient mass culture of high-yielding commercial strains. However, STC lags behind that of higher plants because of the paucity of genomic information. Transcriptome analysis and the identification of potential unigenes involved in the formation and regeneration of callus or direct induction of ABs are essential. Herein, the CK, EWAB and NPA G. lichenoides transcriptomes were analyzed using the Illumina sequencing platform in first time. A total of 17,922,453,300 nucleotide clean bases were generated and assembled into 21,294 unigenes, providing a total gene space of 400,912,038 nucleotides with an average length of 1,883 and N 50 of 5,055 nucleotides and a G + C content of 52.02%. BLAST analysis resulted in the assignment of 13,724 (97.5%), 3,740 (26.6%), 9,934 (70.6%), 10,611 (75.4%), 9,490 (67.4%), and 7,773 (55.2%) unigenes were annotated to the NR, NT, Swiss-Prot, KEGG, COG, and GO databases, respectively, and the total of annotated unigenes was 14,070. A total of 17,099 transcripts were predicted to possess open reading frames, including 3,238 predicted and 13,861 blasted based on protein databases. In addition, 3,287 SSRs were detected in G.lichenoides, providing further support for genetic variation and marker-assisted selection in the future. Our results suggest that auxin polar transport, auxin signal transduction, crosstalk with other endogenous plant hormones and antioxidant systems, play important roles for ABs formation in G. lichenoides explants in vitro. The present findings will facilitate further studies on gene discovery and on the molecular mechanisms underlying the tissue culture of seaweed.

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.

Great expectations for PIP: phosphoinositides as regulators of signaling during development and disease

Phosphoinositides function as signaling precursors as well as regulators and scaffolds of signaling molecules required for important cellular processes such as membrane trafficking. Although a picture of the biochemical and cell biological functions of phosphoinositides is emerging, less is known about how these functions impact signaling on a broader scale during development. This review summarizes recent work on the role of phosphoinositides in developmental signaling and in a number of diseases and developmental disorders.

Nitric oxide triggers phosphatidic acid accumulation via phospholipase D during auxin-induced adventitious root formation in cucumber

Auxin and nitric oxide (NO) play fundamental roles throughout plant life. NO is a second messenger in auxin signal transduction leading to root developmental processes. The mechanisms triggered by auxin and NO that direct adventitious root (AR) formation are beginning to be unraveled. The goal of this work was to study phospholipid (PL) signaling during the auxin- and NO-induced AR formation in cucumber (Cucumis sativus) explants. Explants were labeled with 32P-inorganic phosphate and treated with the auxins indole-3-acetic acid or 1-naphthylacetic acid, or the NO donor S-nitroso N-acetyl penicillamine, in the presence or absence of the specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. PLs were separated by thin-layer chromatography and quantified. We report that the signaling PLs phosphatidic acid (PA), phosphatidylinositol phosphate, and phosphatidylinositol bisphosphate accumulated within 1 min after auxin or NO treatment. Both auxin and NO evoked similar and transient time course responses, since signaling PLs returned to control levels after 20 or 30 min of treatment. The results indicate that auxin relies on NO in inducing PA, phosphatidylinositol phosphate, and phosphatidylinositol bisphosphate accumulation. Furthermore, we demonstrate that auxin and NO trigger PA formation via phospholipase D (PLD) activity. Explants treated for 10 min with auxin or NO displayed a 200% increase in AR number compared with control explants. In addition, PLD activity was required for the auxin- and NO-induced AR formation. Finally, exogenously applied PA increased up to 300% the number of ARs. Altogether, our data support the idea that PLD-derived PA is an early signaling event during AR formation induced by auxin and NO in cucumber explants.

Phytohormone collaboration: zooming in on auxin-brassinosteroid interactions

Similar to animal hormones, classic plant hormones are small organic molecules that regulate physiological and developmental processes. In development, this often involves the regulation of growth through the control of cell size or division. The plant hormones auxin and brassinosteroid modulate both cell expansion and proliferation and are known for their overlapping activities in physiological assays. Recent molecular genetic analyses in the model plant Arabidopsis suggest that this reflects interdependent and often synergistic action of the two hormone pathways. Such pathway interactions probably occur through the combinatorial regulation of common target genes by auxin- and brassinosteroid-controlled transcription factors. Moreover, auxin and brassinosteroid signaling and biosynthesis and auxin transport might be linked by an emerging upstream connection involving calcium-calmodulin and phosphoinositide signaling.

Preparation of polyclonal antibody specific for AtPLC4, an Arabidopsis phosphatidylinositol-specific phospholipase C in rabbits

Phosphoinositide-specific phospholipase Cs (PI-PLCs) are important enzymes in eukaryotes, which catalyze the hydrolysis of phosphatidylinositol 4,5-bisphosphate into the two second messengers inositol 1,4,5-trisphosphate and diacylglycerol. The Arabidopsis genome contains nine putative PI-PLC genes. AtPLC4, an abiotic stress induced gene, has been reported to encode an active PI-PLC isoform. However, the exact roles of putative AtPLC4 in plant remain to be elicited. The first 108 amino acid residues of the N-terminal of AtPLC4, referred to as AtPLC4 N, was expressed as a recombinant protein in Escherichia coli and used as antigen in generating antibody. Purified recombinant proteins including AtPLC1 to AtPLC5, AtPLC8, AtPLC9 and AtPLC4 N were transferred onto the same blot to test specificity of the prepared antibody. Western blot result shows that only AtPLC4 and AtPLC4 N can be recognized by the antibody. The antibody recognized a protein of approximately 68kDa in the plasma membrane fraction and cytosolic fractions prepared from Arabidopsis thaliana plants. This corresponds very well with the calculated molecular weight of AtPLC4. The results suggest that AtPLC4 may encode a plasma membrane-associated protein.

Inositol 1,4,5-trisphosphate and Ran expression during simulated and real microgravity

In order to gain further insight into the signal transduction pathway concerning gravitropism, we studied the expression profiles of mRNA in etiolated sunflower (Helianthus annuus L.) seedlings. Differential-display reverse transcriptase PCR product assayed by capillary electrophoresis revealed the small GTPase Ran, regulating nuclear import and export of proteins. Parallel analysis of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) release by a highly advanced system of metal-dye detection combined with high-performance liquid chromatography provided evidence that the second messenger Ins(1,4,5)P3 is modulated by changes of the gravity vector. Investigations by fast clinorotation and sounding rockets established a positive correlation between the Ins(1,4,5)P3 level and the expression rate of Ran mRNA during simulated and real microgravity. Since an asymmetric distribution of auxin during graviresponse is suggested to induce differential cell elongation, additional information on the perception and transduction pathways was achieved by auxin stimulation experiments. While we were able to demonstrate an auxin-dependent production of Ins(1,4,5)P3, the expression of Ran mRNA was not affected by auxin. Finally, besides the phosphoinositide system as one element of the signal transduction chain linking graviperception to graviresponse, a Ran-mediated interaction model of extracellular microgravity signal perception and intercellular transduction pathway is proposed.

Biosynthesis of storage lipids in plant cell and embryo cultures

The biosynthesis of storage lipids in plant cell and embryo cultures is discussed in the light of their significance in the breeding of agriculturally important oil seed crops. After a short introduction to the biosynthesis of storage lipids, i.e. triacylglycerols and wax esters, this review covers the occurrence and biosynthesis of storage lipids in plant cell and embryo cultures. Plant cells in culture generally contain low levels of both unusual fatty acids and triacylglycerols indicating that these cells are quite different from cells of oil storage tissues. There are a few exceptions to this rule which demonstrate that induction of genes involved in the expression of fatty acid modification and triacylglycerol assembly is possible in plant cell cultures. Such biosynthetically active plant cells may be of particular interest in future studies of storage lipid assembly. Both somatic and gametophytic embryos of oil plants exhibit high capacities for storage lipid biosynthesis and accumulation in vitro compared to cultured plant cells. Above all, the microspore-derived embryo system is recommended to both plant breeders and plant biochemists for the selection and multiplication of plants of superior quality.

Auxin induces an increase of Ca2+ concentration in the cytosol of wheat leaf protoplasts

Auxin addition to protoplasts isolated from leaves of 6-day-old wheat seedlings (Triticum aestivum L. cv. Kadett) induced a rapid increase in the cytosolic calcium concentration [Ca2+]cyt. The shifts in [Ca2+]cyt were detected by use of fluorescence microscopy in single protoplasts loaded with the calcium binding tetra[acetoxymethyl]ester of the fluorescent dye, Fura 2. Addition of the synthetic auxin naphthyl acetic acid, 1-NAA, induced an increase in [Ca2+]cyt within 5-10s, while the physiologically non-active analogue, 2-NAA, did not. The amplitude of calcium increase depended on the concentration of 1-NAA. Since the process was affected by different concentrations of Ca2+ in the external medium, and since the calcium channel blockers (nifedipine and verapamil) postponed and inhibited the reaction, it is suggested that auxin primarily activates Ca2+-permeable channels in the plasma membrane. In the presence of low external calcium concentration (0.1 mM), 5 mM LiCl almost totally blocked the increase in [Ca2+]cyt, indicating a possible involvement of tonoplast Ca2+-channels in the auxin-induced [Ca2+]cyt shift. Thus, calcium signalling induced by auxin involves both external and internal Ca2+ pools.

The G-protein-coupled receptor GCR1 regulates DNA synthesis through activation of phosphatidylinositol-specific phospholipase C

Different lines of evidence suggest that specific events during the cell cycle may be mediated by a heterotrimeric G-protein activated by a cognate G-protein coupled receptor. However, coupling between the only known Galpha-subunit of the heterotrimeric G-protein (GPA1) and the only putative G-protein coupled receptor (GCR1) of plants has never been shown. Using a variety of approaches, we show here that GCR1-enhanced thymidine incorporation into DNA depends on an increase in phosphatidylinositol-specific phospholipase C activity and an elevation of inositol 1,4,5-trisphosphate levels in the cells. Tobacco (Nicotiana tabacum) cells that overexpress either Arabidopsis GCR1 or GPA1 display this phenomenon. We suggest on the basis of these results that GCR1-controlled events during the cell cycle involve phosphatidylinositol-specific phospholipase C as an effector of GCR1 and inositol 1,4,5-trisphosphate as a second messenger, and that GCR1 and GPA1 are both involved in this particular signaling pathway.

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.

Auxin stimulates S6 ribosomal protein phosphorylation in maize thereby affecting protein synthesis regulation

Auxin is known to stimulate protein synthesis in many plant tissues, but the mechanisms involved in this process are unknown. The present research inquires whether auxin might regulate selective translation of mRNAs by inducing S6 ribosomal protein phosphorylation on the 40S ribosomal subunit in maize (Zea mays L.). Maize embryonic axes auxin-stimulated by natural (IAA) or synthetic (Dicamba or 1-NAA) auxins, selectively increased ribosomal protein synthesis. This effect was not reproduced by auxin inactive analogue 2-NAA. Enhanced S6 ribosomal protein phosphorylation on the 40S ribosomal subunit was also observed after auxin stimulation, as measured by [32P] incorporation into this protein. This increment did not occur when stimulation was performed with the inactive auxin analogue. Further, increased recruitment into polysomes of two 5'TOP-like mRNAs, encoding for the initiation translation factor eIF-iso4E and the S6 ribosomal protein, was also found after auxin stimulation of maize axes. A positive correlation was established between the levels of S6 ribosomal protein phosphorylation and the S6 ribosomal protein transcript recruitment into polysomes by means of okadaic acid or heat shock application to maize axes. These data indicate that auxin stimulates S6 ribosomal protein phosphorylation on maize ribosomes, concomitant to the recruitment of specific mRNAs (5'TOP-like mRNAs) into polysomes for translation. It is proposed that by this mechanism auxin regulate the synthesis of specific proteins in maize tissues.

Vascular tissue differentiation and pattern formation in plants

Vascular tissues, xylem and phloem, are differentiated from meristematic cells, procambium, and vascular cambium. Auxin and cytokinin have been considered essential for vascular tissue differentiation; this is supported by recent molecular and genetic analyses. Xylogenesis has long been used as a model for study of cell differentiation, and many genes involved in late stages of tracheary element formation have been characterized. A number of mutants affecting vascular differentiation and pattern formation have been isolated in Arabidopsis. Studies of some of these mutants have suggested that vascular tissue organization within the bundles and vascular pattern formation at the organ level are regulated by positional information.

PLANTCELLULAR ANDMOLECULARRESPONSES TOHIGHSALINITY

Plant responses to salinity stress are reviewed with emphasis on molecular mechanisms of signal transduction and on the physiological consequences of altered gene expression that affect biochemical reactions downstream of stress sensing. We make extensive use of comparisons with model organisms, halophytic plants, and yeast, which provide a paradigm for many responses to salinity exhibited by stress-sensitive plants. Among biochemical responses, we emphasize osmolyte biosynthesis and function, water flux control, and membrane transport of ions for maintenance and re-establishment of homeostasis. The advances in understanding the effectiveness of stress responses, and distinctions between pathology and adaptive advantage, are increasingly based on transgenic plant and mutant analyses, in particular the analysis of Arabidopsis mutants defective in elements of stress signal transduction pathways. We summarize evidence for plant stress signaling systems, some of which have components analogous to those that regulate osmotic stress responses of yeast. There is evidence also of signaling cascades that are not known to exist in the unicellular eukaryote, some that presumably function in intercellular coordination or regulation of effector genes in a cell-/tissue-specific context required for tolerance of plants. A complex set of stress-responsive transcription factors is emerging. The imminent availability of genomic DNA sequences and global and cell-specific transcript expression data, combined with determinant identification based on gain- and loss-of-function molecular genetics, will provide the infrastructure for functional physiological dissection of salt tolerance determinants in an organismal context. Furthermore, protein interaction analysis and evaluation of allelism, additivity, and epistasis allow determination of ordered relationships between stress signaling components. Finally, genetic activation and suppression screens will lead inevitably to an understanding of the interrelationships of the multiple signaling systems that control stress-adaptive responses in plants.

Decreased inositol 1,4,5-trisphosphate content in pathogen-challenged soybean cells

Phosphoinositide-specific phospholipase C (PI-PLC) has been shown to be transiently activated when plant cells were treated with elicitors. We thus investigated the activity of PI-PLC when soybean cells were infected with the bacterial pathogen Pseudomonas syringae pv. glycinea, by measuring cellular cytosolic inositol 1,4,5-trisphosphate (IP3) levels. We observed that IP3 content decreased in both compatible and incompatible interactions. In vitro phosphatase activities were similar in both water control and infected cells with slightly lower IP3 degradation observed for infected cells, indicating that the reduced IP3 content in infected cells most likely results from reduced PI-PLC activity. We hypothesize that reduced IP3 content following infection may lead to suppression of various housekeeping activities of the cells, thus diverting the cellular resources either to the synthesis of defense-related compounds against pathogens, and/or to the growth of pathogens.

Molecular and biochemical evidence for the involvement of calcium/calmodulin in auxin action

The use of (35)S-labeled calmodulin (CaM) to screen a corn root cDNA expression library has led to the isolation of a CaM-binding protein, encoded by a cDNA with sequence similarity to small auxin up RNAs (SAURs), a class of early auxin-responsive genes. The cDNA designated as ZmSAUR1 (Zea mays SAURs) was expressed in Escherichia coli, and the recombinant protein was purified by CaM affinity chromatography. The CaM binding assay revealed that the recombinant protein binds to CaM in a calcium-dependent manner. Deletion analysis revealed that the CaM binding site was located at the NH(2)-terminal domain. A synthetic peptide of amino acids 20-45, corresponding to the potential CaM binding region, was used for calcium-dependent mobility shift assays. The synthetic peptide formed a stable complex with CaM only in the presence of calcium. The CaM affinity assay indicated that ZmSAUR1 binds to CaM with high affinity (K(d) approximately 15 nM) in a calcium-dependent manner. Comparison of the NH(2)-terminal portions of all of the characterized SAURs revealed that they all contain a stretch of the basic alpha-amphiphilic helix similar to the CaM binding region of ZmSAUR1. CaM binds to the two synthetic peptides from the NH(2)-terminal regions of Arabidopsis SAUR-AC1 and soybean 10A5, suggesting that this is a general phenomenon for all SAURs. Northern analysis was carried out using the total RNA isolated from auxin-treated corn coleoptile segments. ZmSAUR1 gene expression began within 10 min, increased rapidly between 10 and 60 min, and peaked around 60 min after 10 microM alpha-naphthaleneacetic acid treatment. These results indicate that ZmSAUR1 is an early auxin-responsive gene. The CaM antagonist N-(6-aminohexyl)5-chloro-1-naphthalenesulfonamide hydrochloride inhibited the auxin-induced cell elongation but not the auxin-induced expression of ZmSAUR1. This suggests that calcium/CaM do not regulate ZmSAUR1 at the transcriptional level. CaM binding to ZmSAUR1 in a calcium-dependent manner suggests that calcium/CaM regulate ZmSAUR1 at the post-translational level. Our data provide the first direct evidence for the involvement of calcium/CaM-mediated signaling in auxin-mediated signal transduction.

Abscisic acid induces oscillations in guard-cell cytosolic free calcium that involve phosphoinositide-specific phospholipase C

Oscillations in cytosolic free Ca2+ concentration ([Ca2+]cyt) are an important component of Ca2+-based signal transduction pathways. This fact has led us to investigate whether oscillations in [Ca2+]cyt are involved in the response of stomatal guard cells to the plant hormone abscisic acid (ABA). We show that ABA induces oscillations in guard-cell [Ca2+]cyt. The pattern of the oscillations depended on the ABA concentration and correlated with the final stomatal aperture. We examined the mechanism by which ABA generates oscillations in guard-cell [Ca2+]cyt by using 1-(6-[17beta-3-methoxyestra-1,3, 5(10)-trien-17-yl]aminohexyl)-1H-pyrrole-2,5-dione (U-73122), an inhibitor of phosphoinositide-specific phospholipase C (PI-PLC)-dependent processes in animals. U-73122 inhibited the hydrolysis of phosphatidylinositol 4,5-bisphosphate by a recombinant PI-PLC, isolated from a guard-cell-enriched cDNA library, in a dose-dependent manner. This result confirms that U-73122 is an inhibitor of plant PI-PLC activity. U-73122 inhibited both ABA-induced oscillations in [Ca2+]cyt and stomatal closure. In contrast, U-73122 did not inhibit external Ca2+-induced oscillations in guard-cell [Ca2+]cyt and stomatal closure. Furthermore, there was no effect of the inactive analogue 1-(6-[17beta-3-methoxyestra-1,3, 5(10)-trien-17-yl]aminohexyl)-2,5-pyrrolidinedione on recombinant PI-PLC activity or ABA-induced and external Ca2+-induced oscillations in [Ca2+]cyt and stomatal closure. This lack of effect suggests that the effects of U-73122 in guard cells are the result of inhibition of PI-PLC and not a consequence of nonspecific effects. Taken together, our data suggest a role for PI-PLC in the generation of ABA-induced oscillations in [Ca2+]cyt and point toward the involvement of oscillations in [Ca2+]cyt in the maintenance of stomatal aperture by ABA.

Sequence analysis of a 40-kb Arabidopsis thaliana genomic region located at the top of chromosome 1

As a contribution to the European Scientists Sequencing Arabidopsis (BIOTECH ESSA) project, a contig of almost 40kb has been sequenced at the extreme top of chromosome 1, around the Arabidopsis thaliana gene coding for a member of the 1-aminocyclopropane-1-carboxylate synthesis gene family. The region contains, besides the ACS1 gene itself, 10 putative genes, all new for Arabidopsis. Among these are three genes encoding kinases, a late embryogenesis-abundant protein, a MADS box-containing protein, a dehydrogenase, and a Myb-related transcription factor. In addition, six cDNAs have been sequenced that correspond to this region.

Disruption of a gene encoding phosphatidic acid phosphatase causes abnormal phenotypes in cell growth and abnormal cytokinesis in Saccharomyces cerevisiae

Phosphatidic acid phosphatase (PAP) is an enzyme involved in lipid metabolism. Diacylglycerol (DG) and phosphatidic acid (PA) are a substrate and a product of PAP, respectively, and function as second messengers in several signal transduction pathways in animals. To investigate the function of PAP in Saccharomyces cerevisiae, we analyzed changes in cellular phenotypes of a mutant that has a disrupted PAP gene. Two putative genes for PAP (ScPAP1 and ScPAP2) are in the S. cerevisiae genome. We generated a ScPAP1-null mutant and observed its cellular phenotypic changes. The ScPAP1-null mutant cells aggregated in liquid culture, and microscopical analyses showed that these mutant cells have an abnormal cell shape and abnormal cytokinesis during cell division. The ScPAP1 is possibly involved in cell growth and cytokinesis in S. cerevisiae. Yeast phosphatidic acid phosphatase is possibly involved in cell growth and cytokinesis.

Molecular cloning of a cDNA encoding diacylglycerol kinase (DGK) in Arabidopsis thaliana

Diacylglycerol kinase (DGK) synthesizes phosphatidic acid from diacylglycerol, an activator of protein kinase C (PKC), to resynthesize phosphatidylinositols. The structure of DGK has not been characterized in plants. We report the cloning of a cDNA, cATDGK1, encoding DGK from Arabidopsis thaliana. The cATDGK1 CDNA contains an open reading frame of 2184 bp, and encodes a putative protein of 728 amino acids with a predicted molecular mass of 79.4 kDa. The deduced ATDGK1 amino acid sequence exhibits significant similarity to that of rat, pig, and Drosophila DGKs. The ATDGK1 mRNA was detected in roots, shoots, and leaves. Southern blot analysis suggests that the ATDGK1 gene is a single-copy gene. The existence of DGK as well as phospholipase C suggests the existence of PKC in plants.

A gene encoding a phosphatidylinositol-specific phospholipase C is induced by dehydration and salt stress in Arabidopsis thaliana

A cDNA corresponding to a putative phosphatidylinositol-specific phospholipase C (PI-PLC) in the higher plant Arabidopsis thaliana was cloned by use of the polymerase chain reaction. The cDNA, designated cAtPLC1, encodes a putative polypeptide of 561 aa with a calculated molecular mass of 64 kDa. The putative product includes so-called X and Y domains found in all PI-PLCs identified to date. In mammalian cells, there are three types of PI-PLC, PLC-beta, -gamma, and -delta. The overall structure of the putative AtPLC1 protein is most similar to that of PLC-delta, although the AtPLC1 protein is much smaller than PLCs from other organisms. The recombinant AtPLC1 protein synthesized in Escherichia coli was able to hydrolyze phosphatidylinositol 4,5-bisphosphate and this activity was completely dependent on Ca2+, as observed also for mammalian PI-PLCs. These results suggest that the AtPLC1 gene encodes a genuine PI-PLC of a higher plant. Northern blot analysis showed that the AtPLC1 gene is expressed at very low levels in the plant under normal conditions but is induced to a significant extent under various environmental stresses, such as dehydration, salinity, and low temperature. These observations suggest that AtPLC1 might be involved in the signal-transduction pathways of environmental stresses and that an increase in the level of AtPLC1 might amplify the signal, in a manner that contributes to the adaptation of the plant to these stresses.

Auxin action and auxin-binding proteins

The plant growth regulator auxin mediates an enormous range of developmental and growth responses, some of which are manifest rapidly and others manifest only after considerable lag periods. The protein that perceives auxin, the auxin receptor, has been sought by many laboratories and the search has identified a good number of candidates. However, a receptor must not only bind auxin, but also transduce the auxin stimulus into the responses we recognize. Finding evidence for this second condition has always proved very demanding. A key requisite is a convenient assay for auxin activity and preferably one involving a rapid response because this is likely to be linked directly to the perception event. For one auxin-binding protein (ABP1) there is growing evidence that it is a functional auxin receptor. The assays used in this work have been rapid auxin-induced changes in protoplast electrophysiology. There are many other responses induced rapidly by auxin for which a link to ABP1 has yet to be established. We have reviewed the whole range of rapid auxin-mediated responses and by doing so we hope to have provided a comprehensive picture of the many events to which a receptor (or receptors) must connect. Against this framework we match the known properties of all putative receptors, including ABP1. Not only have we tried to identify auxin-binding proteins unlikely to be receptors, but we also highlight the remaining gaps in our understanding of the more likely receptor candidates. Contents Summary 167 I. Introduction 168 II. Gene activation 168 III. Mutants 179 IV. Auxin-induced elongation growth 179 V. Other auxin-binding proteins 191 VI. Auxins and signal transduction 192 VII. Overview 194 Acknowledgements 195 References 195.

5-Hydroxytryptamine affects turnover of polyphosphoinositides in maize and stimulates nitrate reductase in the absence of light

Incubation of etiolated maize leaves for 5 min in 5-hydroxytryptamine increased phosphatidylinositol-4,5-bisphosphate levels but on longer incubation its level decreased and a corresponding increase in inositol-trisphosphate was observed. The increase in phosphatidylinositol-4,5-bisphosphate by 5-hydroxytryptamine was similar to that obtained after short irradiation of leaves with red light. Nitrate-inducible and phytochrome-stimulated enzyme nitrate reductase could be stimulated in darkness if the leaves were incubated in the presence of nitrate and 5-hydroxytryptamine. These results indicate that one of the initial events in phytochrome-mediated enzyme stimulation could be through the generation of 'signals' from the turnover of the phosphoinositide cycle.

Physiological, biochemical and molecular processes associated with gravitropism in roots of maize

This research aims to characterize regulation of the principal cytosolic protein kinases in maize, cultivar 'Merit' root tips, since much evidence indicates that stimuli which modulate the gravitropic response in this system act through regulation of activity of these enzymes. To this end, we have cloned a maize protein kinase belonging to a group of plant protein kinases with a catalytic domain similar in primary structure to the second messenger-regulated protein kinases known in animal and fungal systems. However, both the unique structural features conserved among plant protein kinases in this group, and lack of evidence for cyclic nucleotide signalling in plants point to operation of a novel protein kinase regulatory mechanism in plants. In order to test effects of possible regulators on protein kinase activity, we developed a sensitive method for detecting regulation of autophosphoryl labelling of protein kinases in unfractionated maize protein extracts. Regulation of protein kinase autophosphorylation in these extracts was different from that known in animals and fungi, further suggesting operation of unique protein kinase regulatory mechanisms in plants. Previous research has shown that light, or factors modulated by light, regulate plant protein kinase activity. We found that protein kinase activity was co-immunoprecipitated with the plant photoreceptor phytochrome, and was associated with phytochrome by high-affinity chemical interactions. Far-red reversibility of red-light regulation of phytochrome phosphorylation by the associated protein kinase indicates that it may modulate or transduce the light signals which lead to gravitropic sensitivity in 'Merit' maize.

Modulation of K+ channels in Vicia stomatal guard cells by peptide homologs to the auxin-binding protein C terminus

Transduction of the auxin stimulus in plants is thought to entail binding of the hormone to a soluble auxin-binding protein (ABP) outside the cell and subsequent interaction between this auxin-protein complex and an integral membrane receptor ("docking") protein that couples the signal across the plasma membrane. To explore the structural requirements for ABP function, synthetic peptides were prepared to the amino acid sequences of the predicted surface domains of ABPzm1, the dominant ABP from Zea. Biological function was assayed under voltage clamp, monitoring the ability of the peptides to evoke auxin-related modulations in inward- (IK,in) and outward-rectifying (IK,out) K+ channel activities of Vicia guard cells in the absence of added auxin. Only the peptide corresponding to the C-terminal domain of ABPzm1 was active. The dominant response was an inactivation of IK,in, although the peptide also evoked an activation of IK,out. Inactivation of IK,in was complete within 20-30 s and was fully reversible, was marked by a slowing of voltage-dependent activation and deactivation, and was dependent on peptide concentration (K1/2, 16 +/- 6 microM). Buffering cytoplasmic-free [Ca2+] with EGTA had no effect on IK,in response to the peptide. However, virtually complete and reversible block of the response was achieved when cytoplasmic pH (pHi) was brought under experimental control using the weak acid butyrate. Parallel measurements of pHi using the fluorescent dye 2',7'-bis(2-carboxyethyl-5(6)-carboxyfluorescein (BCECF) and dual-wavelength laser-scanning confocal microscopy demonstrated that the C-terminal peptide evoked rapid and reversible cytoplasmic alkalinizations of 0.4 +/- 0.1 pHi unit and confirmed the antagonism of the pHi response in the presence of butyrate. These, and comparable results with the auxins indole acetic acid and 1-naphthyleneacetic acid, implicate the C-terminal domain of ABPzm1 in auxin-ABP coupling to pHi and an associated intracellular signaling cascade.

G-protein from Medicago sativa: functional association to photoreceptors

G-protein subunits were characterized from Medicago sativa (alfalfa) seedlings. Crude membranes and GTP-Sepharose-purified fractions were electrophoresed on SDS/polyacrylamide gels and analysed by Western blotting with 9193 (anti-alpha common) and AS/7 (anti-alpha t, anti-alpha i1 and anti-alpha i2) polyclonal antibodies. These procedures led to the identification of a specific polypeptide band of about 43 kDa. Another polypeptide reacting with the SW/1 (anti-beta) antibody, of about 37 kDa, was also detected. The 43 kDa polypeptide bound specifically [alpha-32P]GTP by a photoaffinity reaction and was ADP-ribosylated by activated cholera toxin, but not by pertussis toxin. Irradiation of etiolated Medicago sativa protoplast preparations at 660 nm for 1 min produced a maximal increase in the guanosine 5'-[gamma-thio]triphosphate (GTP[35S])-binding rate. After this period of irradiation, the binding rate tended to decrease. The effect of a red-light (660 nm) pulse on the binding rate was reversed when it was immediately followed by a period of far-red (> 730 nm) illumination. These results may suggest that activation of GTP[S]-binding rate was a consequence of conversion of phytochrome Pr into the Ptr form.

Phospholipase A2 affects the activity of fusicoccin receptors

Biochemical properties of fusicoccin receptors are strongly influenced by the phospholipid environment. In this report we have studied the effect of different exogenous phospholipases on fusicoccin binding ability of both plasma membrane and solubilised receptors. Among the phospholipases tested only phospholipase A2 showed an inhibitory effect on fusicoccin binding. In particular, the influence of this enzyme on the time course and reversibility of the fusicoccin binding reaction was studied. The inhibitory effect of phospholipase A2 was the consequence of fatty acid release. The usual fatty acids of plasma membrane phospholipids were active in inhibiting the interaction of fusicoccin with its receptors. It is concluded that a phospholipid associated to the fusicoccin receptor might play a significant role in the modulation of binding.

The plant phosphoinositide system

Images

3- and 4-phosphorylated phosphatidylinositols in the aquatic plant Spirodela polyrhiza L

Labelling of Spirodela polyrhiza L. plants with [3H]inositol and [32P]Pi yielded a series of phosphoinositides which were identified as PtdIns, PtdIns4P and PtdIns(4,5)P2. In addition, systematic degradation of a phospholipid extract identified PtdIns3P. Analysis of the distribution of 32P label between the monoester and diester phosphate groups of PtdIns3P and PtdIns4P revealed differences in the labelling of the monoester phosphate, suggesting that the two PtdInsP species are not synthesized or metabolized in a co-ordinate manner.

Regulatory functions of soluble auxin-binding proteins

Since the effects of auxin on plant tissues are complex, the mode of action of auxin at the molecular level may not depend on a single mechanism. There may be a mechanism by which the interaction of auxin with receptors localized in the cytoplasmic membranes activates certain enzymes which are necessary to generate the putative second messengers. On the other hand, soluble auxin-binding proteins have been isolated from a variety of plant tissues. Some of these proteins have a high affinity for auxins and the binding is auxin specific, reversible, and saturable, characteristics which suggest that these proteins may be auxin receptors. Although these criteria are often used to distinguish real receptors from nonfunctional binding proteins, it is necessary to clarify the biological function of the binding proteins to classify them as putative receptors. The reported results on the function of soluble auxin-binding proteins demonstrate that, in the transcription system composed of isolated nuclei, auxin interacts with soluble auxin-binding proteins and stimulates the expression of specific genes. Thus, one of the mechanisms of action of auxin may involve a direct interaction with a soluble receptor protein, such that the resultant auxin-receptor complex, possibly together with other protein factors, can subsequently recognize the promoter region of specific gene(s) and interact with RNA polymerase II to cause a transcription of the gene(s).