Transient and sustained increases in inositol 1,4,5-trisphosphate precede the differential growth response in gravistimulated maize pulvini

The Vacuolar Inositol Transporter BvINT1;1 Contributes to Raffinose Biosynthesis and Reactive Oxygen Species Scavenging During Cold Stress in Sugar Beet

Despite a high sucrose accumulation in its taproot vacuoles, sugar beet (Beta vulgaris subsp. vulgaris) is sensitive to freezing. Earlier, a taproot-specific accumulation of raffinose was shown to have beneficial effects on the freezing tolerance of the plant. However, synthesis of raffinose and other oligosaccharides of the raffinose family depends on the availability of myo-inositol. Since inositol and inositol-metabolising enzymes reside in different organelles, functional inositol metabolism and raffinose synthesis depend on inositol transporters. We identified five homologues of putative inositol transporters in the sugar beet genome, two of which, BvINT1;1 and BvINT1;2, are localised at the tonoplast. Among these, only the transcript of BvINT1;1 is highly upregulated in sugar beet taproots under cold. BvINT1;1 exhibits a high transport specificity for inositol and sugar beet mutants lacking functional BvINT1;1 contain increased inositol levels, likely accumulating in the vacuole, and decreased raffinose contents under cold treatment. Due to the quenching capacity of raffinose for Reactive Oxygen Species (ROS), which accumulate under cold stress, bvint1;1 sugar beet plants show increased expression of both, ROS marker genes and detoxifying enzymes. Based on these findings, we conclude that the vacuolar inositol transporter BvINT1;1 is contributing to ROS-homoeostasis in the cold metabolism of sugar beet.

Getting attached to membranes-How plant signaling networks employ PtdIns(4,5)P2

In eukaryotes, a small subset of membrane lipids, the phosphoinositides (PIs), exert regulatory effects on membrane-associated processes with profound impact on the organism, and PIs are relevant also for the physiology and development of plants. The PI, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) has emerged as an important regulatory player in plants, and in recent years this lipid has received substantial attention. This Update Review focuses on our current understanding of how PtdIns(4,5)P2 exerts its regulatory functions, how biosynthesis and degradation of this important regulatory lipid are controlled, and how PtdIns(4,5)P2 is linked to upstream and downstream elements within plant signalling networks.

Phosphoinositides in plant-pathogen interaction: trends and perspectives

Phosphoinositides are important regulatory membrane lipids, with a role in plant development and cellular function. Emerging evidence indicates that phosphoinositides play crucial roles in plant defence and are also utilized by pathogens for infection. In this review, we highlight the role of phosphoinositides in plant-pathogen interaction and the implication of this remarkable convergence in the battle against plant diseases.

Regulation of inositol 1,2,4,5,6-pentakisphosphate and inositol hexakisphosphate levels in Gossypium hirsutum by IPK1

The IPK1 genes, which code for 2-kinases that can synthesize Ins(1,2,4,5,6)P₅ from Ins(1,4,5,6)P₄, are expressed throughout cotton plants, resulting in the highest Ins(1,2,4,5,6)P₅ concentrations in young leaves and flower buds. Cotton leaves contain large amounts of Ins(1,2,4,5,6)P₅ and InsP₆ compared to plants not in the Malvaceae family. The inositol polyphosphate pathway has been linked to stress tolerance in numerous plant species. Accordingly, we sought to determine why cotton and other Malvaceae have such high levels of these inositol phosphates. We have quantified the levels of InsP₅ and InsP₆ in different tissues of cotton plants and determined the expression of IPK1 (inositol 1,3,4,5,6-pentakisphosphate 2-kinase gene) in vegetative and reproductive tissues. Gossypium hirsutum was found to contain four IPK1 genes that were grouped into two pair (AB, CD) where each pair consists of very similar sequences that were measured together. More IPK1AB is expressed in leaves than in roots, whereas more IPK1CD is expressed in roots than in leaves. Leaves and flower buds have more InsP₅ and InsP₆ than stems and roots. Leaves and roots contain more InsP₅ than InsP₆, whereas flower buds and stems contain more InsP₆ than InsP₅. Dark-grown seedlings contain more InsP₅ and InsP₆ than those grown under lights, and the ratio of InsP₅ to InsP₆ is greater in the light-grown seedlings. During 35 days of the life cycle of the third true leaf, InsP₅ and InsP₆ gradually decreased by more than 50%. Silencing IPK1AB and IPK1CD with Cotton Leaf Crumple Virus-induced gene silencing (VIGS) resulted in plants with an intense viral phenotype, reduced IPK1AB expression and lowered amounts of InsP₅. The results are consistent with Ins(1,2,4,5,6)P₅ synthesis from Ins(1,4,5,6)P₄ by IPK1. This study detailed the central role of IPK1 in cotton inositol polyphosphate metabolism, which has potential to be harnessed to improve the resistance of plants to different kinds of stress.

Biased gene expression reveals the contribution of subgenome to altitude adaptation in allopolyploid Isoetes sinensis

Allopolyploids are believed to inherit the genetic characteristics of its progenitors and exhibit stronger adaptability and vigor. The allotetraploid Isoetes sinensis was formed by the natural hybridization and polyploidization of two diploid progenitors, Isoetes taiwanensis and Isoetes yunguiensis, and was believed to have the potential to adapt to plateau environments. To explore the expression pattern of homoeologous genes and their contributions to altitude adaptation, we transplanted natural allotetraploid I. sinensis (TnTnYnYn) along the altitude gradient for a long-term, and harvested them in summer and winter, respectively. One year after transplanting, it still lived well, even in the extreme environment of the Qinghai-Tibet Plateau. Then, we performed high-throughput RNA sequencing to measure their gene expression level. A total of 7801 homoeologous genes were expressed, among which 5786 were identified as shared expression in different altitudes and seasons. We further found that altitude variations could change the subgenome bias trend of I. sinensis, but season could not. Moreover, the functions of uniquely expressed genes indicated that temperature might be an important restrictive factor during the adaptation process. Through the analysis of DEGs and uniquely expressed genes, we found that Y subgenome provided more contributions to high altitude adaptation than T subgenome. These adaptive traits to high altitude may be inherited from its plateau progenitor I. yunguiensis. Through weighted gene co-expression network analysis, pentatricopeptide repeats gene family and glycerophospholipid metabolism pathway were considered to play important roles in high-altitude adaptation. Totally, this study will enrich our understanding of allopolyploid in environmental adaptation.

Exogenous melatonin enhances the growth and production of bioactive metabolites in Lemna aequinoctialis culture by modulating metabolic and lipidomic profiles

BACKGROUND

Lemna species are cosmopolitan floating plants that have great application potential in the food/feed, pharmaceutical, phytoremediation, biofuel, and bioplastic industries. In this study, the effects of exogenous melatonin (0.1, 1, and 10 µM) on the growth and production of various bioactive metabolites and intact lipid species were investigated in Lemna aequinoctialis culture.

RESULTS

Melatonin treatment significantly enhanced the growth (total dry weight) of the Lemna aequinoctialis culture. Melatonin treatment also increased cellular production of metabolites including β-alanine, ascorbic acid, aspartic acid, citric acid, chlorophyll, glutamic acid, phytosterols, serotonin, and sucrose, and intact lipid species; digalactosyldiacylglycerols, monogalactosyldiacylglycerols, phosphatidylinositols, and sulfoquinovosyldiacylglycerols. Among those metabolites, the productivity of campesterol (1.79 mg/L) and stigmasterol (10.94 mg/L) were the highest at day 28, when 10 µM melatonin was treated at day 7.

CONCLUSION

These results suggest that melatonin treatment could be employed for enhanced production of biomass or various bioactive metabolites and intact lipid species in large-scale L. aequinoctialis cultivation as a resource for food, feed, and pharmaceutical industries.

The functions of phospholipases and their hydrolysis products in plant growth, development and stress responses

Cell membranes are the initial site of stimulus perception from environment and phospholipids are the basic and important components of cell membranes. Phospholipases hydrolyze membrane lipids to generate various cellular mediators. These phospholipase-derived products, such as diacylglycerol, phosphatidic acid, inositol phosphates, lysophopsholipids, and free fatty acids, act as second messengers, playing vital roles in signal transduction during plant growth, development, and stress responses. This review focuses on the structure, substrate specificities, reaction requirements, and acting mechanism of several phospholipase families. It will discuss their functional significance in plant growth, development, and stress responses. In addition, it will highlight some critical knowledge gaps in the action mechanism, metabolic and signaling roles of these phospholipases and their products in the context of plant growth, development and stress responses.

Plant Gravitropism: From Mechanistic Insights into Plant Function on Earth to Plants Colonizing Other Worlds

Gravitropism, the growth of roots and shoots toward or away from the direction of gravity, has been studied for centuries. Such studies have not only led to a better understanding of the gravitropic process itself, but also paved new paths leading to deeper mechanistic insights into a wide range of research areas. These include hormone biology, cell signal transduction, regulation of gene expression, plant evolution, and plant interactions with a variety of environmental stimuli. In addition to contributions to basic knowledge about how plants function, there is accumulating evidence that gravitropism confers adaptive advantages to crops, particularly under marginal agricultural soils. Therefore, gravitropism is emerging as a breeding target for enhancing agricultural productivity. Moreover, research on gravitropism has spawned several studies on plant growth in microgravity that have enabled researchers to uncouple the effects of gravity from other tropisms. Although rapid progress on understanding gravitropism witnessed during the past decade continues to be driven by traditional molecular, physiological, and cell biological tools, these tools have been enriched by technological innovations in next-generation omics platforms and microgravity analog facilities. In this chapter, we review the field of gravitropism by highlighting recent landmark studies that have provided unique insights into this classic research topic while also discussing potential contributions to agriculture on Earth and beyond.

The AtCRK5 Protein Kinase Is Required to Maintain the ROS NO Balance Affecting the PIN2-Mediated Root Gravitropic Response in Arabidopsis

The Arabidopsis AtCRK5 protein kinase is involved in the establishment of the proper auxin gradient in many developmental processes. Among others, the Atcrk5-1 mutant was reported to exhibit a delayed gravitropic response via compromised PIN2-mediated auxin transport at the root tip. Here, we report that this phenotype correlates with lower superoxide anion (O2•-) and hydrogen peroxide (H2O2) levels but a higher nitric oxide (NO) content in the mutant root tips in comparison to the wild type (AtCol-0). The oxidative stress inducer paraquat (PQ) triggering formation of O2•- (and consequently, H2O2) was able to rescue the gravitropic response of Atcrk5-1 roots. The direct application of H2O2 had the same effect. Under gravistimulation, correct auxin distribution was restored (at least partially) by PQ or H2O2 treatment in the mutant root tips. In agreement, the redistribution of the PIN2 auxin efflux carrier was similar in the gravistimulated PQ-treated mutant and untreated wild type roots. It was also found that PQ-treatment decreased the endogenous NO level at the root tip to normal levels. Furthermore, the mutant phenotype could be reverted by direct manipulation of the endogenous NO level using an NO scavenger (cPTIO). The potential involvement of AtCRK5 protein kinase in the control of auxin-ROS-NO-PIN2-auxin regulatory loop is discussed.

Metabolomics by UHPLC-HRMS reveals the impact of heat stress on pathogen-elicited immunity in maize

INTRODUCTION

Studies investigating crop resistance to abiotic and biotic stress have largely focused on plant responses to singular forms of stress and individual biochemical pathways that only partially represent stress responses. Thus, combined abiotic and biotic stress treatments and the global assessment of their elicited metabolic expression remains largely unexplored. In this study, we employed targeted and untargeted metabolomics to investigate the molecular responses of maize (Zea mays) to abiotic, biotic, and combinatorial stress.

OBJECTIVE

We compared the inducible metabolomes of heat-stressed (abiotic) and C. heterostrophus-infected (biotic) maize and examined the effects of heat stress on the ability of maize to defend itself against C. heterostrophus.

METHODS

Ultra-high-performance liquid chromatography-high-resolution mass spectrometry was performed on plants grown under control conditions (28 °C), heat stress (38 °C), Cochliobolus heterostrophus infection, or combinatorial stress [heat (38 °C) + C. heterostrophus infection].

RESULTS

Multivariate analyses revealed differential metabolite expression between heat stress, C. heterostrophus infection, and their respective controls. In combinatorial experiments, treatment with heat stress prior to fungal inoculation negatively impacted maize disease resistance against C. heterostrophus, and distinct metabolome separation between combinatorial stressed plants and the non-heat-stressed infected controls was observed. Targeted analysis revealed inducible primary and secondary metabolite responses to abiotic/biotic stress, and combinatorial experiments indicated that deficiency in the hydroxycinnamic acid, p-coumaric acid, may contribute to the heat-induced susceptibility of maize to C. heterostrophus.

CONCLUSION

These findings demonstrate that abiotic stress can predispose crops to more severe disease symptoms, underlining the increasing need to investigate defense chemistry in plants under combinatorial stress.

A putative tomato inositol polyphosphate 5-phosphatase, Le5PT1, is involved in plant growth and abiotic stress responses

Based on sequence similarity to Arabidopsis inositol polyphosphate 5-phosphatases (5PTases) involved in abiotic stress responses and development, four tomato cDNAs (Le5PT1-4) encoding putative 5PTase proteins were identified. The predicted protein sequences of the Le5PTs include conserved catalytic domains required for 5PTase enzyme activity. Le5PT1, 2, and 3 showed high amino acid sequence identity with At5PTase2, At5PTase1 and At5PTase3, and At5PTase5 and At5PTase6, respectively. The expression of Le5PT1 was downregulated soon after initiation of dehydration and salt stress as well as exposure to polyethylene glycol (PEG) and NaCl, but not by exogenous ABA treatment. On the other hand, the expression of Le5PT2 gradually increased with time in all treatments. Transgenic tobacco plants overexpressing Le5PT1 exhibited reduced growth in height, leaf area, and dry weight compared to wild type plants. Transgenic plants also had lower water use efficiency (WUE) than wild type and the downregulation of the drought-responsive gene, NtERD10B. Together these results suggest that Le5PT1 may have a negative role in response to water deficit through the repression of drought-inducible genes that in turn affects plant growth and development.

A PAMP-triggered MAPK cascade inhibits phosphatidylinositol 4,5-bisphosphate production by PIP5K6 in Arabidopsis thaliana

The phosphoinositide kinase PIP5K6 has recently been identified as a target for the mitogen-activated protein kinase (MAPK) MPK6. Phosphorylation of PIP5K6 inhibited the production of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂ ), impacting membrane trafficking and cell expansion in pollen tubes. Here, we analyzed whether MPK6 regulated PIP5K6 in vegetative Arabidopsis cells in response to the pathogen-associated molecular pattern (PAMP) flg22. Promoter-β-glucuronidase analyses and quantitative real-time reverse transcription polymerase chain reaction data show PIP5K6 expressed throughout Arabidopsis tissues. Upon flg22 treatment of transgenic protoplasts, the PIP5K6 protein was phosphorylated, and this modification was reduced for a PIP5K6 variant lacking MPK6-targeted residues, or in protoplasts from mpk6 mutants. Upon flg22 treatment of Arabidopsis plants, phosphoinositide levels mildly decreased and a fluorescent reporter for PtdIns(4,5)P₂ displayed reduced plasma membrane association, contrasting with phosphoinositide increases reported for abiotic stress responses. Flg22 treatment and chemical induction of the upstream MAPK kinase, MKK5, decreased phosphatidylinositol 4-phosphate 5-kinase activity in mesophyll protoplasts, indicating that the flg22-activated MAPK cascade limited PtdIns(4,5)P₂ production. PIP5K6 expression or PIP5K6 protein abundance changed only marginally upon flg22 treatment, consistent with post-translational control of PIP5K6 activity. PtdIns(4,5)P₂ -dependent endocytosis of FM 4-64, PIN2 and the NADPH-oxidase RbohD were reduced upon flg22 treatment or MKK5 induction. Reduced RbohD-endocytosis was correlated with enhanced ROS production. We conclude that MPK6-mediated phosphorylation of PIP5K6 limits the production of a functional PtdIns(4,5)P₂ pool upon PAMP perception.

Photosynthesis and yield response to elevated CO2, C4 plant foxtail millet behaves similarly to C3 species

Foxtail millet (Setaria italica) is a nutrient-rich food source traditionally grown in arid and semi-arid areas, as it is well adapted to drought climate. Yet there is limited information as how the crop responses to the changing climate. In order to investigate the response of foxtail millet to elevated [CO₂] and the underlying mechanism, the crop was grown at ambient [CO₂] (400 μmol mol^-1) and elevated [CO₂] (600 μmol mol^-1) in an open-top chamber (OTC) experimental facility in North China. The changes in leaf photosynthesis, chlorophyll fluorescence, biomass, yield and global gene expression in response to elevated [CO₂] were determined. Despite foxtail millet being a C₄ photosynthetic crop, photosynthetic rates (P_N) and intrinsic water-use efficiency (WUEi), were increased under elevated [CO₂]. Similarly, grain yield and above-ground biomass also significantly increased (P <  0.05) for the two years of experimentation under elevated [CO₂]. Increases in seeds and tiller number, spike and stem weight were the main contributors to the increased grain yield and biomass. Using transcriptomic analyses, this study further identified some genes which play a role in cell wall reinforcement, shoot initiation, stomatal conductance, carbon fixation, glycolysis / gluconeogenesis responsive to elevated [CO₂]. Changes in these genes reduced plant height, increased stem diameters, and promote CO₂ fixation. Higher photosynthetic rates at elevated [CO₂] demonstrated that foxtail millet was not photosynthetically saturated at elevated [CO₂] and its photosynthesis response to elevated [CO₂] were analogous to C₃ plants.

The Tonoplastic Inositol Transporter INT1 From Arabidopsis thaliana Impacts Cell Elongation in a Sucrose-Dependent Way

The tonoplastic inositol transporter INT1 is the only known transport protein in Arabidopsis that facilitates myo-inositol import from the vacuole into the cytoplasm. Impairment of the release of vacuolar inositol by knockout of INT1 results in a severe inhibition of cell elongation in roots as well as in etiolated hypocotyls. Importantly, a more strongly reduced cell elongation was observed when sucrose was supplied in the growth medium, and this sucrose-dependent effect can be complemented by the addition of exogenous myo-inositol. Comparing int1 mutants (defective in transport) with mutants defective in myo-inositol biosynthesis (mips1 mutants) revealed that the sucrose-induced inhibition in cell elongation does not just depend on inositol depletion. Secondary effects as observed for altered availability of inositol in biosynthesis mutants, as disturbed membrane turnover, alterations in PIN protein localization or alterations in inositol-derived signaling molecules could be ruled out to be responsible for impairing the cell elongation in int1 mutants. Although the molecular mechanism remains to be elucidated, our data implicate a crucial role of INT1-transported myo-inositol in regulating cell elongation in a sucrose-dependent manner and underline recent reports of regulatory roles for sucrose and other carbohydrate intermediates as metabolic semaphores.

Knock-Down of Arabidopsis PLC5 Reduces Primary Root Growth and Secondary Root Formation While Overexpression Improves Drought Tolerance and Causes Stunted Root Hair Growth

Phospholipase C (PLC) is a well-known signaling enzyme in metazoans that hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to produce inositol 1,4,5-trisphosphate and diacylglycerol as second messengers involved in mutiple processes. Plants contain PLC too, but relatively little is known about its function there. The model system Arabidopsis thaliana contains nine PLC genes. Reversed genetics have implicated several roles for PLCs in plant development and stress signaling. Here, PLC5 is functionally addressed. Promoter-β-glucuronidase (GUS) analyses revealed expression in roots, leaves and flowers, predominantly in vascular tissue, most probably phloem companion cells, but also in guard cells, trichomes and root apical meristem. Only one plc5-1 knock-down mutant was obtained, which developed normally but grew more slowly and exhibited reduced primary root growth and decreased lateral root numbers. These phenotypes could be complemented by expressing the wild-type gene behind its own promoter. Overexpression of PLC5 (PLC5-OE) using the UBQ10 promoter resulted in reduced primary and secondary root growth, stunted root hairs, decreased stomatal aperture and improved drought tolerance. PLC5-OE lines exhibited strongly reduced phosphatidylinositol 4-monophosphate (PIP) and PIP2 levels and increased amounts of phosphatidic acid, indicating enhanced PLC activity in vivo. Reduced PIP2 levels and stunted root hair growth of PLC5-OE seedlings could be recovered by inducible overexpression of a root hair-specific PIP 5-kinase, PIP5K3. Our results show that PLC5 is involved in primary and secondary root growth and that its overexpression improves drought tolerance. Independently, we provide new evidence that PIP2 is essential for the polar tip growth of root hairs.

Arabidopsis Phospholipase C3 is Involved in Lateral Root Initiation and ABA Responses in Seed Germination and Stomatal Closure

Phospholipase C (PLC) is well known for its role in animal signaling, where it generates the second messengers, inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), by hydrolyzing the minor phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP2), upon receptor stimulation. In plants, PLC's role is still unclear, especially because the primary targets of both second messengers are lacking, i.e. the ligand-gated Ca2+ channel and protein kinase C, and because PIP2 levels are extremely low. Nonetheless, the Arabidopsis genome encodes nine PLCs. We used a reversed-genetic approach to explore PLC's function in Arabidopsis, and report here that PLC3 is required for proper root development, seed germination and stomatal opening. Two independent knock-down mutants, plc3-2 and plc3-3, were found to exhibit reduced lateral root densities by 10-20%. Mutant seeds germinated more slowly but were less sensitive to ABA to prevent germination. Guard cells of plc3 were also compromised in ABA-dependent stomatal closure. Promoter-β-glucuronidase (GUS) analyses confirmed PLC3 expression in guard cells and germinating seeds, and revealed that the majority is expressed in vascular tissue, most probably phloem companion cells, in roots, leaves and flowers. In vivo 32Pi labeling revealed that ABA stimulated the formation of PIP2 in germinating seeds and guard cell-enriched leaf peels, which was significantly reduced in plc3 mutants. Overexpression of PLC3 had no effect on root system architecture or seed germination, but increased the plant's tolerance to drought. Our results provide genetic evidence for PLC's involvement in plant development and ABA signaling, and confirm earlier observations that overexpression increases drought tolerance. Potential molecular mechanisms for the above observations are discussed.

Distribution of Endogenous NO Regulates Early Gravitropic Response and PIN2 Localization in Arabidopsis Roots

High-resolution and automated image analysis of individual roots demonstrated that endogenous nitric oxide (NO) contribute significantly to gravitropism of Arabidopsis roots. Lowering of endogenous NO concentrations strongly reduced and even reversed gravitropism, resulting in upward bending, without affecting root growth rate. Notably, the asymmetric accumulation of NO along the upper and lower sides of roots correlated with a positive gravitropic response. Detection of NO by the specific DAF-FM DA fluorescent probe revealed that NO was higher at the lower side of horizontally-oriented roots returning to initial values 2 h after the onset of gravistimulation. We demonstrate that NO promotes plasma membrane re-localization of PIN2 in epidermal cells, which is required during the early root gravitropic response. The dynamic and asymmetric localization of both auxin and NO is critical to regulate auxin polar transport during gravitropism. Our results collectively suggest that, although auxin and NO crosstalk occurs at different levels of regulation, they converge in the regulation of PIN2 membrane trafficking in gravistimulated roots, supporting the notion that a temporally and spatially coordinated network of signal molecules could participate in the early phases of auxin polar transport during gravitropism.

Phosphoinositide signaling in plant development

The membranes of eukaryotic cells create hydrophobic barriers that control substance and information exchange between the inside and outside of cells and between cellular compartments. Besides their roles as membrane building blocks, some membrane lipids, such as phosphoinositides (PIs), also exert regulatory effects. Indeed, emerging evidence indicates that PIs play crucial roles in controlling polarity and growth in plants. Here, I highlight the key roles of PIs as important regulatory membrane lipids in plant development and function.

Guilt by Association: A Phenotype-Based View of the Plant Phosphoinositide Network

Eukaryotic membranes contain small amounts of phospholipids that have regulatory effects on the physiological functions of cells, tissues, and organs. Phosphoinositides (PIs)-the phosphorylated derivatives of phosphatidylinositol-are one example of such regulatory lipids. Although PIs were described in plants decades ago, their contribution to the regulation of physiological processes in plants is not well understood. In the past few years, evidence has emerged that PIs are essential for plant function and development. Recently reported phenotypes associated with the perturbation of different PIs suggest that some subgroups of PIs influence specific processes. Although the molecular targets of PI-dependent regulation in plants are largely unknown, the effects of perturbed PI metabolism can be used to propose regulatory modules that involve particular downstream targets of PI regulation. This review summarizes phenotypes associated with the perturbation of the plant PI network to categorize functions and suggest possible downstream targets of plant PI regulation.

Male functions and malfunctions: the impact of phosphoinositides on pollen development and pollen tube growth

Phosphoinositides in pollen. In angiosperms, sexual reproduction is a series of complex biological events that facilitate the distribution of male generative cells for double fertilization. Angiosperms have no motile gametes, and the distribution units of generative cells are pollen grains, passively mobile desiccated structures, capable of delivering genetic material to compatible flowers over long distances and in an adverse environment. The development of pollen (male gametogenesis) and the formation of a pollen tube after a pollen grain has reached a compatible flower (pollen tube growth) are important aspects of plant developmental biology. In recent years, a wealth of information has been gathered about the molecular control of cell polarity, membrane trafficking and cytoskeletal dynamics underlying these developmental processes. In particular, it has been found that regulatory membrane phospholipids, such as phosphoinositides (PIs), are critical regulatory players, controlling key steps of trafficking and polarization. Characteristic features of PIs are the inositol phosphate headgroups of the lipids, which protrude from the cytosolic surfaces of membranes, enabling specific binding and recruitment of numerous protein partners containing specific PI-binding domains. Such recruitment is globally an early event in polarization processes of eukaryotic cells and also of key importance to pollen development and tube growth. Additionally, PIs serve as precursors of other signaling factors with importance to male gametogenesis. This review highlights the recent advances about the roles of PIs in pollen development and pollen function.

Brevis plant1, a putative inositol polyphosphate 5-phosphatase, is required for internode elongation in maize

In maize (Zea mays L.), as in other grass species, stem elongation occurs during growth and most noticeably upon the transition to flowering. Genes that reduce stem elongation have been important to reduce stem breakage, or lodging. Stem elongation has been mediated by dwarf and brachytic/brevis plant mutants that affect giberellic acid and auxin pathways, respectively. Maize brevis plant1 (bv1) mutants, first identified over 80 years ago, strongly resemble brachytic2 mutants that have shortened internodes, short internode cells, and are deficient in auxin transport. Here, we characterized two novel bv1 maize mutants. We found that an inositol polyphosphate 5-phosphatase orthologue of the rice gene dwarf50 was the molecular basis for the bv1 phenotype, implicating auxin-mediated inositol polyphosphate and/or phosphoinositide signalling in stem elongation. We suggest that auxin-mediated internode elongation involves processes that also contribute to stem gravitropism. Genes misregulated in bv1 mutants included genes important for cell wall synthesis, transmembrane transport, and cytoskeletal function. Mutant and wild-type plants were indistinguishable early in development, responded similarly to changes in light quality, had unaltered flowering times, and had normal flower development. These attributes suggest that breeding could utilize bv1 alleles to increase crop grain yields.

Plant phosphoinositide signaling - dynamics on demand

Eukaryotic membranes contain small amounts of lipids with regulatory roles. An important class of such regulatory lipids are phosphoinositides (PIs). Within membranes, PIs serve as recruitment signals, as regulators of membrane protein function or as precursors for second messenger production, thereby influencing a multitude of cellular processes with key importance for plant function and development. Plant PIs occur locally and transiently within membrane microdomains, and their abundance is strictly controlled. To understand the functions of the plant PI-network it is important to understand not only downstream PI-effects, but also to identify and characterize factors contributing to dynamic PI formation. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Immunohistochemistry relative to gravity: a simple method to retain information about gravity for immunolocalization and histochemistry

We describe a simple method to preserve information about a plant organ's orientation relative to the direction of the gravity vector during sample processing for immunolocalization or histochemical analysis of cell biological processes. This approach has been used in gravity stimulated roots of Arabidopsis thaliana and Zea mays to study PIN3 relocalization, study the asymmetrical remodeling of the actin network and the cortical microtubule array, and to reveal the asymmetrical expression of the auxin signaling reporter DR5::GUS. This method enables the rapid analysis of a large number of samples from a variety of genotypes, as well as from tissue that may be too thick for microscopy in live plants.

A Bird's-Eye View of Molecular Changes in Plant Gravitropism Using Omics Techniques

During evolution, plants have developed mechanisms to adapt to a variety of environmental stresses, including drought, high salinity, changes in carbon dioxide levels and pathogens. Central signaling hubs and pathways that are regulated in response to these stimuli have been identified. In contrast to these well studied environmental stimuli, changes in transcript, protein and metabolite levels in response to a gravitational stimulus are less well understood. Amyloplasts, localized in statocytes of the root tip, in mesophyll cells of coleoptiles and in the elongation zone of the growing internodes comprise statoliths in higher plants. Deviations of the statocytes with respect to the earthly gravity vector lead to a displacement of statoliths relative to the cell due to their inertia and thus to gravity perception. Downstream signaling events, including the conversion from the biophysical signal of sedimentation of distinct heavy mass to a biochemical signal, however, remain elusive. More recently, technical advances, including clinostats, drop towers, parabolic flights, satellites, and the International Space Station, allowed researchers to study the effect of altered gravity conditions - real and simulated micro- as well as hypergravity on plants. This allows for a unique opportunity to study plant responses to a purely anthropogenic stress for which no evolutionary program exists. Furthermore, the requirement for plants as food and oxygen sources during prolonged manned space explorations led to an increased interest in the identi-fication of genes involved in the adaptation of plants to microgravity. Transcriptomic, proteomic, phosphoproteomic, and metabolomic profiling strategies provide a sensitive high-throughput approach to identify biochemical alterations in response to changes with respect to the influence of the gravitational vector and thus the acting gravitational force on the transcript, protein and metabolite level. This review aims at summarizing recent experimental approaches and discusses major observations.

Strategies of seedlings to overcome their sessile nature: auxin in mobility control

Plants are sessile organisms that are permanently restricted to their site of germination. To compensate for their lack of mobility, plants evolved unique mechanisms enabling them to rapidly react to ever changing environmental conditions and flexibly adapt their postembryonic developmental program. A prominent demonstration of this developmental plasticity is their ability to bend organs in order to reach the position most optimal for growth and utilization of light, nutrients, and other resources. Shortly after germination, dicotyledonous seedlings form a bended structure, the so-called apical hook, to protect the delicate shoot meristem and cotyledons from damage when penetrating through the soil. Upon perception of a light stimulus, the apical hook rapidly opens and the photomorphogenic developmental program is activated. After germination, plant organs are able to align their growth with the light source and adopt the most favorable orientation through bending, in a process named phototropism. On the other hand, when roots and shoots are diverted from their upright orientation, they immediately detect a change in the gravity vector and bend to maintain a vertical growth direction. Noteworthy, despite the diversity of external stimuli perceived by different plant organs, all plant tropic movements share a common mechanistic basis: differential cell growth. In our review, we will discuss the molecular principles underlying various tropic responses with the focus on mechanisms mediating the perception of external signals, transduction cascades and downstream responses that regulate differential cell growth and consequently, organ bending. In particular, we highlight common and specific features of regulatory pathways in control of the bending of organs and a role for the plant hormone auxin as a key regulatory component.

New insights into root gravitropic signalling

An important feature of plants is the ability to adapt their growth towards or away from external stimuli such as light, water, temperature, and gravity. These responsive plant growth movements are called tropisms and they contribute to the plant's survival and reproduction. Roots modulate their growth towards gravity to exploit the soil for water and nutrient uptake, and to provide anchorage. The physiological process of root gravitropism comprises gravity perception, signal transmission, growth response, and the re-establishment of normal growth. Gravity perception is best explained by the starch-statolith hypothesis that states that dense starch-filled amyloplasts or statoliths within columella cells sediment in the direction of gravity, resulting in the generation of a signal that causes asymmetric growth. Though little is known about the gravity receptor(s), the role of auxin linking gravity sensing to the response is well established. Auxin influx and efflux carriers facilitate creation of a differential auxin gradient between the upper and lower side of gravistimulated roots. This asymmetric auxin gradient causes differential growth responses in the graviresponding tissue of the elongation zone, leading to root curvature. Cell biological and mathematical modelling approaches suggest that the root gravitropic response begins within minutes of a gravity stimulus, triggering genomic and non-genomic responses. This review discusses recent advances in our understanding of root gravitropism in Arabidopsis thaliana and identifies current challenges and future perspectives.

Plant phosphoinositides-complex networks controlling growth and adaptation

Plants differ in many ways from mammals or yeast. However, plants employ phosphoinositides for the regulation of essential cellular functions as do all other eukaryotes. In recent years the plant phosphoinositide system has been linked to the control of cell polarity. Phosphoinositides are also implicated in plant adaptive responses to changing environmental conditions. The current understanding is that plant phosphoinositides control membrane trafficking, ion channels and the cytoskeleton in similar ways as in other eukaryotic systems, but adapted to meet plant cellular requirements and with some plant-specific features. In addition, the formation of soluble inositol polyphosphates from phosphoinositides is important for the perception of important phytohormones, as the relevant receptor proteins contain such molecules as structural cofactors. Overall, the essential nature of phosphoinositides in plants has been established. Still, the complexity of the phosphoinositide networks in plant cells is only emerging and invites further study of its molecular details. This article is part of a special issue entitled Phosphoinositides.

Phosphatidylinositol 4,5-bisphosphate influences PIN polarization by controlling clathrin-mediated membrane trafficking in Arabidopsis

The functions of the minor phospholipid phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] during vegetative plant growth remain obscure. Here, we targeted two related phosphatidylinositol 4-phosphate 5-kinases (PI4P 5-kinases) PIP5K1 and PIP5K2, which are expressed ubiquitously in Arabidopsis thaliana. A pip5k1 pip5k2 double mutant with reduced PtdIns(4,5)P2 levels showed dwarf stature and phenotypes suggesting defects in auxin distribution. The roots of the pip5k1 pip5k2 double mutant had normal auxin levels but reduced auxin transport and altered distribution. Fluorescence-tagged auxin efflux carriers PIN-FORMED (PIN1)-green fluorescent protein (GFP) and PIN2-GFP displayed abnormal, partially apolar distribution. Furthermore, fewer brefeldin A-induced endosomal bodies decorated by PIN1-GFP or PIN2-GFP formed in pip5k1 pip5k2 mutants. Inducible overexpressor lines for PIP5K1 or PIP5K2 also exhibited phenotypes indicating misregulation of auxin-dependent processes, and immunolocalization showed reduced membrane association of PIN1 and PIN2. PIN cycling and polarization require clathrin-mediated endocytosis and labeled clathrin light chain also displayed altered localization patterns in the pip5k1 pip5k2 double mutant, consistent with a role for PtdIns(4,5)P2 in the regulation of clathrin-mediated endocytosis. Further biochemical tests on subcellular fractions enriched for clathrin-coated vesicles (CCVs) indicated that pip5k1 and pip5k2 mutants have reduced CCV-associated PI4P 5-kinase activity. Together, the data indicate an important role for PtdIns(4,5)P2 in the control of clathrin dynamics and in auxin distribution in Arabidopsis.

A type III ACC synthase, ACS7, is involved in root gravitropism in Arabidopsis thaliana

Ethylene is an important plant hormone that regulates developmental processes in plants. The ethylene biosynthesis pathway is a highly regulated process at both the transcriptional and post-translational level. The transcriptional regulation of these ethylene biosynthesis genes is well known. However, post-translational modifications of the key ethylene biosynthesis enzyme 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) are little understood. In vitro kinase assays were conducted on the type III ACS, AtACS7, fusion protein and peptides to determine whether the AtACS7 protein can be phosphorylated by calcium-dependent protein kinase (CDPK). AtACS7 was phosphorylated at Ser216, Thr296, and Ser299 by AtCDPK16 in vitro. To investigate further the function of the ACS7 gene in Arabidopsis, an acs7-1 loss-of-function mutant was isolated. The acs7-1 mutant exhibited less sensitivity to the inhibition of root gravitropism by treatment with the calcium chelator ethylene glycol tetraacetic acid (EGTA). Seedlings were treated with gradient concentrations of ACC. The results showed that a certain concentration of ethylene enhanced the gravity response. Moreover, the acs7-1 mutant was less sensitive to inhibition of the gravity response by treatment with the auxin polar transport inhibitor 1-naphthylphthalamic acid, but exogenous ACC application recovered root gravitropism. Altogether, the results indicate that AtACS7 is involved in root gravitropism in a calcium-dependent manner in Arabidopsis.

Arranged marriage in lipid signalling? The limited choices of PtdIns(4,5)P2 in finding the right partner

Inositol-containing phospholipids (phosphoinositides, PIs) control numerous cellular processes in eukaryotic cells. For plants, a key involvement of PIs has been demonstrated in the regulation of membrane trafficking, cytoskeletal dynamics and in processes mediating the adaptation to changing environmental conditions. Phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2)) mediates its cellular functions via binding to various alternative target proteins. Such downstream targets of PtdIns(4,5)P(2) are characterised by the possession of specific lipid-binding domains, and binding of the PtdIns(4,5)P(2) ligand exerts effects on their activity or localisation. The large number of potential alternative binding partners - and associated cellular processes - raises the question how alternative or even contrapuntal effects of PtdIns(4,5)P(2) are orchestrated to enable cellular function. This article aims to provide an overview of recent insights and new views on how distinct functional pools of PtdIns(4,5)P(2) are generated and maintained. The emerging picture suggests that PtdIns(4,5)P(2) species containing different fatty acids influence the lateral mobility of the lipids in the membrane, possibly enabling specific interactions of PtdIns(4,5)P(2) pools with certain downstream targets. PtdIns(4,5)P(2) pools with certain functions might also be defined by protein-protein interactions of PI4P 5-kinases, which pass PtdIns(4,5)P(2) only to certain downstream partners. Individually or in combination, PtdIns(4,5)P(2) species and specific protein-protein interactions of PI4P 5-kinases might contribute to the channelling of PtdIns(4,5)P(2) signals towards specific functional effects. The dynamic nature of PI-dependent signalling complexes with specific functions is an added challenge for future studies of plant PI signalling.

Structure and immunocytochemical localization of photosynthetic enzymes in the lamina joint and sheath pulvinus of the C4 grass Arundinella hirta

The C(4) grass Arundinella hirta exhibits a unique C(4) anatomy, with isolated Kranz cells (distinctive cells) and C(4)-type expression of photosynthetic enzymes in the leaf sheath and stem as well as in the leaf blade. The border zones between these organs are pale green. Those between the leaf blade and sheath and between the sheath and stem are called the lamina joint and sheath pulvinus, respectively, and are involved in gravity sensing. We investigated the structure and localization of C(3) and C(4) photosynthetic enzymes in these tissues. In both zones the epidermis lacked stomata. The inner tissue was composed of parenchyma cells and vascular bundles. The parenchyma cells were densely packed with small intercellular spaces and contained granal chloroplasts with large starch grains. No C(4)-type cellular differentiation was recognized. Western blot analysis showed that the lamina joint and pulvinus accumulated substantial amounts of phosphoenolpyruvate carboxylase (PEPC), pyruvate,Pi dikinase (PPDK), and ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco). Immunogold electron microscopy revealed PEPC in the cytosol and both PPDK and rubisco in the chloroplasts of parenchyma cells, suggesting the occurrence of C(3) and C(4) enzymes within a single type of chlorenchyma cell. These data indicate that the lamina joint and pulvinus have unique expression patterns of C(3) and C(4) enzymes, unlike those in C(4)-type anatomy.

A proteomics approach identifies novel proteins involved in gravitropic signal transduction

PREMISE

Plant organs use gravity as a guide to direct their growth. And although gravitropism has been studied since the time of Darwin, the mechanisms of signal transduction, those that connect the biophysical stimulus perception and the biochemical events of the response, are still not understood.

METHODS

A quantitative proteomics approach was used to identify key proteins during the early events of gravitropism. Plants were subjected to a gravity persistent signal (GPS) treatment, and proteins were extracted from the inflorescence stem at early time points after stimulation. Proteins were labeled with isobaric tags for relative and absolute quantification (iTRAQ) reagents. Proteins were identified and quantified as a single step using tandem mass-spectrometry (MS/MS). For two of the proteins identified, mutants with T-DNA inserts in the corresponding genes were evaluated for gravitropic phenotypes.

KEY RESULTS

A total of 82 proteins showed significant differential quantification between treatment and controls. Proteins were categorized into functional groups based on gene ontology terms and filtered using groups thought to be involved in the signaling events of gravitropism. For two of the proteins selected, GSTF9 and HSP81-2, knockout mutations resulted in defects in root skewing, waving, and curvature as well as in the GPS response of inflorescence stems.

CONCLUSION

Combining a proteomics approach with the GPS response, 82 novel proteins were identified to be involved in the early events of gravitropic signal transduction. As early as 2 and 4 min after a gravistimulation, significant changes occur in protein abundance. The approach was validated through the analysis of mutants exhibiting altered gravitropic responses.

Cereal grass pulvini: agronomically significant models for studying gravitropism signaling and tissue polarity

Cereal grass pulvini have emerged as model systems that are not only valuable for the study of gravitropism, but are also of agricultural and economic significance. The pulvini are regions of tissue that are apical to each node and collectively return a reoriented stem to a more vertical position. They have proven to be useful for the study of gravisensing and response and are also providing clues about the establishment of polarity across tissues. This review will first highlight the agronomic significance of these stem regions and their benefits for use as model systems and provide a brief historical overview. A detailed discussion of the literature focusing on cell signaling and early changes in gene expression will follow, culminating in a temporal framework outlining events in the signaling and early growth phases of gravitropism in this tissue. Changes in cell wall composition and gene expression that occur well into the growth phase will be touched upon briefly. Finally, some ongoing research involving both maize and wheat pulvini will be introduced along with prospects for future investigations.

Brassinosteroid control of shoot gravitropism interacts with ethylene and depends on auxin signaling components

PREMISE OF THE STUDY

To reach favorable conditions for photosynthesis, seedlings grow upward when deprived of light upon underground germination. To direct their growth, they use their negative gravitropic capacity. Negative gravitropism is under tight control of multiple hormones.

METHODS

By counting the number of standing plants in a population or by real time monitoring of the reorientation of gravistimulated seedlings of Arabidopsis thaliana, we evaluated the negative gravitropism of ethylene or brassinosteroid (BR) treated plants. Meta-analysis of transcriptomic data on AUX/IAA genes was gathered, and subsequent mutant analysis was performed.

KEY RESULTS

Ethylene and BR have opposite effects in regulating shoot gravitropism. Lack of BR enhances gravitropic reorientation in 2-d-old seedlings, whereas ethylene does not. Lack of ethylene signaling results in enhanced BR sensitivity. Ethylene and BRs regulate overlapping sets of AUX/IAA genes. BRs regulate a wider range of auxin signaling components than ethylene.

CONCLUSIONS

Upward growth in seedlings depends strongly on the internal hormonal balance. Endogenous ethylene stimulates, whereas BRs reduce negative gravitropism in a manner that depends on the function of different, yet overlapping sets of auxin signaling components.

A role for lipid-mediated signaling in plant gravitropism

Gravitropism is a universal plant response. It is initiated by the sensing of the primary signal (mass or pressure), which is then converted into chemical signals that are transduced and propagated in a precise spatial and temporal fashion, resulting in a differential growth response. Our thesis is that membrane lipids and lipid-mediated signaling pathways play critical roles in the initial signaling and in the establishment of polarity. In this review, we highlight results from recent literature and discuss the major questions that remain unanswered.

Measurement of inositol (1,4,5) trisphosphate in plant tissues by a competitive receptor binding assay

The phosphoinositide signaling pathway is important for plant responses to many different stresses. As part of the responses to a stimulus, InsP3 levels may increase rapidly and transiently. The receptor binding assay for InsP3 described here is easy to use and an ideal method to monitor and compare InsP3 levels in multiple samples from large scale experiments. The method is based on competitive binding of InsP3 to the mammalian brain InsP3 specific receptor protein. This chapter describes a protocol for extracting and neutralizing plant samples and performing the receptor binding assay (using a commercially available kit). The protocol described has been used effectively to monitor InsP3 levels in plant tissues of different origin and in response to different stresses.

Gravity sensing and signal transduction in vascular plant primary roots

During gravitropism, the potential energy of gravity is converted into a biochemical signal. How this transfer occurs remains one of the most exciting mysteries in plant cell biology. New experiments are filling in pieces of the puzzle. In this review, we introduce gravitropism and give an overview of what we know about gravity sensing in roots of vascular plants, with special highlight on recent papers. When plant roots are reoriented sideways, amyloplast resedimentation in the columella cells is a key initial step in gravity sensing. This process somehow leads to cytoplasmic alkalinization of these cells followed by relocalization of auxin efflux carriers (PINs). This changes auxin flow throughout the root, generating a lateral gradient of auxin across the cap that upon transmission to the elongation zone leads to differential cell elongation and gravibending. We will present the evidence for and against the following players having a role in transferring the signal from the amyloplast sedimentation into the auxin signaling cascade: mechanosensitive ion channels, actin, calcium ions, inositol trisphosphate, receptors/ligands, ARG1/ARL2, spermine, and the TOC complex. We also outline auxin transport and signaling during gravitropism.

Multiple roles for membrane-associated protein trafficking and signaling in gravitropism

Gravitropism is a process that allows plant organs to guide their growth relative to the gravity vector. It requires them to sense changes in their orientation and generate a biochemical signal that they transmit to the tissues that drive organ curvature. Trafficking between the plasma membrane and endosomal compartments is important for all of these phases of the gravitropic response. The sedimentation of starch-filled organelles called amyloplasts plays a key role in sensing reorientation, and vacuolar integrity is required for amyloplast sedimentation in shoots. Other proteins associated with the vesicle trafficking pathway contribute to early gravity signal transduction independently of amyloplast sedimentation in both roots and hypocotyls. Phosphatidylinositol signaling, which starts at the plasma membrane and later affects the localization of auxin efflux facilitators, is a likely second messenger in the signal transduction phase of gravitropism. Finally, membrane-localized auxin influx and efflux facilitators contribute to a differential auxin gradient across the gravistimulated organs, which directs root curvature.

Plant phospholipases: an overview

Plant phospholipases can be grouped into four major types, phospholipase D, phospholipase C, phospholipase A1 (PLA(1)), and phospholipase A2 (PLA(2)), that hydrolyze glycerophospholipids at different ester bonds. Within each type, there are different families or subfamilies of enzymes that can differ in substrate specificity, cofactor requirement, and/or reaction conditions. These differences provide insights into determining the cellular function of specific phospholipases in plants, and they can be explored for different industrial applications.

Phosphoinositide signaling

"All things flow and change…even in the stillest matter there is unseen flux and movement." Attributed to Heraclitus (530-470 BC), from The Story of Philosophy by Will Durant. Heraclitus, a Greek philosopher, was thinking on a much larger scale than molecular signaling; however, his visionary comments are an important reminder for those studying signaling today. Even in unstimulated cells, signaling pathways are in constant metabolic flux and provide basal signals that travel throughout the organism. In addition, negatively charged phospholipids, such as the polyphosphorylated inositol phospholipids, provide a circuit board of on/off switches for attracting or repelling proteins that define the membranes of the cell. This template of charged phospholipids is sensitive to discrete changes and metabolic fluxes-e.g., in pH and cations-which contribute to the oscillating signals in the cell. The inherent complexities of a constantly fluctuating system make understanding how plants integrate and process signals challenging. In this review we discuss one aspect of lipid signaling: the inositol family of negatively charged phospholipids and their functions as molecular sensors and regulators of metabolic flux in plants.

Molecular mechanisms of root gravity sensing and signal transduction

Plants use gravity as a guide to direct their roots down into the soil to anchor themselves and to find resources needed for growth and development. In higher plants, the columella cells of the root tip form the primary site of gravity sensing, and in these cells the sedimentation of dense, starch-filled plastids (amyloplasts) triggers gravity signal transduction. This generates an auxin gradient across the root cap that is transmitted to the elongation zone where it promotes differential cell elongation, allowing the root to direct itself downward. It is still not well understood how amyloplast sedimentation leads to auxin redistribution. Models have been proposed to explain how mechanosensitive ion channels or ligand-receptor interactions could connect these events. Although their roles are still unclear, possible second messengers in this process include protons, Ca(2+), and inositol 1,4,5-triphosphate. Upon gravistimulation, the auxin efflux facilitators PIN3 and PIN7 relocalize to the lower side of the columella cells and mediate auxin redistribution. However, evidence for an auxin-independent secondary mechanism of gravity sensing and signal transduction suggests that this physiological process is quite complex. Furthermore, plants must integrate a variety of environmental cues, resulting in multifaceted relationships between gravitropism and other directional growth responses such as hydro-, photo-, and thigmotropism.

The cellular language of myo-inositol signaling

The simple polyol, myo-inositol, is used as a building block of a cellular language that plays various roles in signal transduction. This review describes the terminology used to denote myo-inositol-containing molecules, with an emphasis on how phosphate and fatty acids are added to create second messengers used in signaling. Work in model systems has delineated the genes and enzymes required for synthesis and metabolism of many myo-inositol-containing molecules, with genetic mutants and measurement of second messengers playing key roles in developing our understanding. There is increasing evidence that molecules such as myo- inositol(1,4,5)trisphosphate and phosphatidylinositol(4,5)bisphosphate are synthesized in response to various signals plants encounter. In particular, the controversial role of myo-inositol(1,4,5)trisphosphate is addressed, accompanied by a discussion of the multiple enzymes that act to regulate this molecule. We are also beginning to understand new connections of myo-inositol signaling in plants. These recent discoveries include the novel roles of inositol phosphates in binding to plant hormone receptors and that of phosphatidylinositol(3)phosphate binding to pathogen effectors.

Arabidopsis phosphatidylinositol monophosphate 5-kinase 2 is involved in root gravitropism through regulation of polar auxin transport by affecting the cycling of PIN proteins

Phosphatidylinositol monophosphate 5-kinase (PIP5K) catalyzes the synthesis of PI-4,5-bisphosphate (PtdIns(4,5)P(2)) by phosphorylation of PI-4-phosphate at the 5 position of the inositol ring, and is involved in regulating multiple developmental processes and stress responses. We here report on the functional characterization of Arabidopsis PIP5K2, which is expressed during lateral root initiation and elongation, and whose expression is enhanced by exogenous auxin. The knockout mutant pip5k2 shows reduced lateral root formation, which could be recovered with exogenous auxin, and interestingly, delayed root gravity response that could not be recovered with exogenous auxin. Crossing with the DR5-GUS marker line and measurement of free IAA content confirmed the reduced auxin accumulation in pip5k2. In addition, analysis using the membrane-selective dye FM4-64 revealed the decelerated vesicle trafficking caused by PtdIns(4,5)P(2) reduction, which hence results in suppressed cycling of PIN proteins (PIN2 and 3), and delayed redistribution of PIN2 and auxin under gravistimulation in pip5k2 roots. On the contrary, PtdIns(4,5)P(2) significantly enhanced the vesicle trafficking and cycling of PIN proteins. These results demonstrate that PIP5K2 is involved in regulating lateral root formation and root gravity response, and reveal a critical role of PIP5K2/PtdIns(4,5)P(2) in root development through regulation of PIN proteins, providing direct evidence of crosstalk between the phosphatidylinositol signaling pathway and auxin response, and new insights into the control of polar auxin transport.

Cell wall modifications in maize pulvini in response to gravitational stress

Changes in cell wall polysaccharides, transcript abundance, metabolite profiles, and hormone concentrations were monitored in the upper and lower regions of maize (Zea mays) pulvini in response to gravistimulation, during which maize plants placed in a horizontal position returned to the vertical orientation. Heteroxylan levels increased in the lower regions of the pulvini, together with lignin, but xyloglucans and heteromannan contents decreased. The degree of substitution of heteroxylan with arabinofuranosyl residues decreased in the lower pulvini, which exhibited increased mechanical strength as the plants returned to the vertical position. Few or no changes in noncellulosic wall polysaccharides could be detected on the upper side of the pulvinus, and crystalline cellulose content remained essentially constant in both the upper and lower pulvinus. Microarray analyses showed that spatial and temporal changes in transcript profiles were consistent with the changes in wall composition that were observed in the lower regions of the pulvinus. In addition, the microarray analyses indicated that metabolic pathways leading to the biosynthesis of phytohormones were differentially activated in the upper and lower regions of the pulvinus in response to gravistimulation. Metabolite profiles and measured hormone concentrations were consistent with the microarray data, insofar as auxin, physiologically active gibberellic acid, and metabolites potentially involved in lignin biosynthesis increased in the elongating cells of the lower pulvinus.

Role of inositol 1,4,5-triphosphate signalling in gravitropic and phototropic gene expression

Plants sense light and gravity to orient their direction of growth. One common component in the early events of both phototropic and gravitropic signal transduction is activation of phospholipase C (PLC), which leads to an increase in inositol 1,4,5-triphosphate (InsP(3)) levels. The InsP(3) signal is terminated by hydrolysis of InsP(3) through inositolpolyphosphate-5-phosphatases (InsP 5-ptases). Arabidopsis plants expressing a heterologous InsP 5-ptase have low basal InsP(3) levels and exhibit reduced gravitropic and phototropic bending. Downstream effects of InsP(3)-mediated signalling are not understood. We used comparative transcript profiling to characterize gene expression changes in gravity- or light-stimulated Arabidopsis root apices that were manipulated in their InsP(3) metabolism either through inhibition of PLC activity or expression of InsP 5-ptase. We identified InsP(3)-dependent and InsP(3)-independent co-regulated gene sets in response to gravity or light stimulation. Inhibition of PLC activity in wild-type plants caused similar changes in transcript abundance in response to gravitropic and phototropic stimulation as in the transgenic lines. Therefore, we conclude that changes in gene expression in response to gravitropic and phototropic stimulation are mediated by two signal transduction pathways that vary in their dependence on changes in InsP(3).

Quantitative trait locus mapping of gravitaxis behaviour in Drosophila melanogaster

Drosophila melanogaster, like other organisms, move and orient themselves in response to the earth's gravitational force. The ability to sense and respond to gravity is essential for an organism to navigate and thrive in its environment. The genes underlying this behaviour in Drosophila remain elusive. Using 88 recombinant inbred lines, we have identified four quantitative trait loci (QTLs) that contribute to adult gravitaxis (geotaxis) behaviour in Drosophila. Candidate genes of interest were selected from the QTLs of highest significance based on their function in chordotonal organ formation. Quantitative complementation tests with these candidate genes revealed a role for skittles in adult gravitaxis behaviour in D. melanogaster.

Inhibition of phospholipase C disrupts cytoskeletal organization and gravitropic growth in Arabidopsis roots

The phospholipase protein superfamily plays an important role in hormonal signalling and cellular responses to environmental stimuli. There is also growing evidence for interactions between phospholipases and the cytoskeleton. In this report we used a pharmacological approach to investigate whether inhibiting a member of the phospholipase superfamily, phospholipase C (PLC), affects microtubules and actin microfilaments as well as root growth and morphology of Arabidopsis thaliana seedlings. Inhibiting PLC activity using the aminosteroid U73122 significantly inhibited root elongation and disrupted root morphology in a concentration-dependent manner, with the response being saturated at 5 μM, whereas the inactive analogue U73343 was ineffective. The primary root appeared to lose growth directionality accompanied by root waving and formation of curls. Immunolabelling of roots exposed to increasingly higher U73122 concentrations revealed that the normal transverse arrays of cortical microtubules in the elongation zone became progressively more disorganized or depolymerized, with the disorganization appearing within 1 h of incubation. Likewise, actin microfilament arrays also were disrupted. Inhibiting PLC using an alternative inhibitor, neomycin, caused similar disruptions to both cytoskeletal organization and root morphology. In seedlings gravistimulated by rotating the culture plates by 90°, both U73122 and neomycin disrupted the normal gravitropic growth of roots and etiolated hypocotyls. The effects of PLC inhibitors are therefore consistent with the notion that, as with phospholipases A and D, PLC likewise interacts with the cytoskeleton, alters growth morphology, and is involved in gravitropism.