(Questions)(n) on phloem biology. 1. Electropotential waves, Ca2+ fluxes and cellular cascades along the propagation pathway

Transcriptome analysis of green and purple fruited pepper provides insight into novel regulatory genes in anthocyanin biosynthesis

Background

Pepper (Capsicum annuum L.) is a valuable horticultural crop with economic significance, and its purple fruit color is attributed to anthocyanin, a phytonutrient known for its health-promoting benefits. However, the mechanisms regulating anthocyanin biosynthesis in pepper have yet to be fully elucidated.

Methods

RNA sequencing (RNA-seq) was utilized to analyze the transcriptome of fruits from three purple-fruited varieties (HN191, HN192, and HN005) and one green-fruited variety (EJT) at various developmental stages. To determine the relationships between samples, Pearson correlation coefficients (PCC) and principal component analysis (PCA) were calculated. Differential expression analysis was performed using the DESeq2 package to identify genes that were expressed differently between two samples. Transcription factors (TF) were predicted using the iTAK program. Heatmaps of selected genes were generated using Tbtools software.

Results

The unripe fruits of HN191, HN192, and HN005, at the stages of 10, 20, and 30 days after anthesis (DAA), display a purple color, whereas the unripe fruits of variety EJT remain green. To understand the molecular basis of this color difference, five transcriptome comparisons between green and purple fruits were conducted: HN191-10 vs EJT-10, HN191-20 vs EJT-20, HN191-30 vs EJT-30, HN192-30 vs EJT-30, and HN005-30 vs EJT-30. Through this analysis, 503 common differentially expressed genes (DEGs) were identified. Among these DEGs, eight structural genes related to the anthocyanin biosynthesis pathway and 24 transcription factors (TFs) were detected. Notably, one structural gene (MSTRG.12525) and three TFs (T459_25295, T459_06113, T459_26036) exhibited expression patterns that suggest they may be novel candidate genes involved in anthocyanin biosynthesis. These results provide new insights into the regulation of anthocyanin biosynthesis in purple pepper fruit and suggest potential candidate genes for future genetic improvement of pepper germplasm with enhanced anthocyanin accumulation.

Phloem Sap Composition: What Have We Learnt from Metabolomics?

Phloem sap transport is essential for plant nutrition and development since it mediates redistribution of nutrients, metabolites and signaling molecules. However, its biochemical composition is not so well-known because phloem sap sampling is difficult and does not always allow extensive chemical analysis. In the past years, efforts have been devoted to metabolomics analyses of phloem sap using either liquid chromatography or gas chromatography coupled with mass spectrometry. Phloem sap metabolomics is of importance to understand how metabolites can be exchanged between plant organs and how metabolite allocation may impact plant growth and development. Here, we provide an overview of our current knowledge of phloem sap metabolome and physiological information obtained therefrom. Although metabolomics analyses of phloem sap are still not numerous, they show that metabolites present in sap are not just sugars and amino acids but that many more metabolic pathways are represented. They further suggest that metabolite exchange between source and sink organs is a general phenomenon, offering opportunities for metabolic cycles at the whole-plant scale. Such cycles reflect metabolic interdependence of plant organs and shoot-root coordination of plant growth and development.

Electrical Signaling and Its Functions Under Conditions of Abiotic Stress: A Review of Methodological Approaches and Physiological Implications

In contrast to chemical messengers, electrical signals such as action potentials and variation potentials can transmit information much faster over long distances. Electrical signals can be triggered by various abiotic stress factors and are propagated via plasmodesmata over short distances and within the phloem over long distances. Thus, in addition to assimilate transport from sources to sinks, the phloem serves as a communication highway for various types of information. Key factors for systemic signaling in the phloem are peptides, RNAs, hormones, and electrical signals. In recent years, there has been increasing evidence that rapid communication by means of electrical signals is essential for various plant physiological processes. Thus, this chapter focuses on electrical signaling and various associated physiological effects, such as regulation of leaf movements, assimilate transport, photosynthesis, and gas exchange, as well as plant water status.

Electrical Signals, Plant Tolerance to Actions of Stressors, and Programmed Cell Death: Is Interaction Possible?

In environmental conditions, plants are affected by abiotic and biotic stressors which can be heterogenous. This means that the systemic plant adaptive responses on their actions require long-distance stress signals including electrical signals (ESs). ESs are based on transient changes in the activities of ion channels and H⁺-ATP-ase in the plasma membrane. They influence numerous physiological processes, including gene expression, phytohormone synthesis, photosynthesis, respiration, phloem mass flow, ATP content, and many others. It is considered that these changes increase plant tolerance to the action of stressors; the effect can be related to stimulation of damages of specific molecular structures. In this review, we hypothesize that programmed cell death (PCD) in plant cells can be interconnected with ESs. There are the following points supporting this hypothesis. (i) Propagation of ESs can be related to ROS waves; these waves are a probable mechanism of PCD initiation. (ii) ESs induce the inactivation of photosynthetic dark reactions and activation of respiration. Both responses can also produce ROS and, probably, induce PCD. (iii) ESs stimulate the synthesis of stress phytohormones (e.g., jasmonic acid, salicylic acid, and ethylene) which are known to contribute to the induction of PCD. (iv) Generation of ESs accompanies K⁺ efflux from the cytoplasm that is also a mechanism of induction of PCD. Our review argues for the possibility of PCD induction by electrical signals and shows some directions of future investigations in the field.

Stochastic Spatial Heterogeneity in Activities of H+-ATP-Ases in Electrically Connected Plant Cells Decreases Threshold for Cooling-Induced Electrical Responses

H⁺-ATP-ases, which support proton efflux through the plasma membrane, are key molecular transporters for electrogenesis in cells of higher plants. Initial activities of the transporters can influence the thresholds of generation of electrical responses induced by stressors and modify other parameters of these responses. Previously, it was theoretically shown that the stochastic heterogeneity of individual cell thresholds for electrical responses in a system of electrically connected neuronal cells can decrease the total threshold of the system (“diversity-induced resonance”, DIR). In the current work, we tested a hypothesis about decreasing the thresholds of generation of cooling-induced electrical responses in a system of electrically connected plant cells with increasing stochastic spatial heterogeny in the initial activities of H⁺-ATP-ases in these cells. A two-dimensional model of the system of electrically connected excitable cells (simple imitation of plant leaf), which was based on a model previously developed in our works, was used for the present investigation. Simulation showed that increasing dispersion in the distribution of initial activities of H⁺-ATP-ases between cells decreased the thresholds of generation of cooling-induced electrical responses. In addition, the increasing weakly influenced the amplitudes of electrical responses. Additional analysis showed two different mechanisms of the revealed effect. The increasing spatial heterogeneity in activities of H⁺-ATP-ases induced a weak positive shift of the membrane potential at rest. The shift decreased the threshold of electrical response generation. However, the decreased threshold induced by increasing the H⁺-ATP-ase activity heterogeneity was also observed after the elimination of the positive shift. The result showed that the “DIR-like” mechanism also participated in the revealed effect. Finally, we showed that the standard deviation of the membrane potentials before the induction of action potentials could be used for the estimation of thresholds of cooling-induced plant electrical responses. Thus, spatial heterogeneity in the initial activities of H⁺-ATP-ases can be a new regulatory mechanism influencing the generation of electrical responses in plants under actions of stressors.

Debunking a myth: plant consciousness

Claims that plants have conscious experiences have increased in recent years and have received wide coverage, from the popular media to scientific journals. Such claims are misleading and have the potential to misdirect funding and governmental policy decisions. After defining basic, primary consciousness, we provide new arguments against 12 core claims made by the proponents of plant consciousness. Three important new conclusions of our study are (1) plants have not been shown to perform the proactive, anticipatory behaviors associated with consciousness, but only to sense and follow stimulus trails reactively; (2) electrophysiological signaling in plants serves immediate physiological functions rather than integrative-information processing as in nervous systems of animals, giving no indication of plant consciousness; (3) the controversial claim of classical Pavlovian learning in plants, even if correct, is irrelevant because this type of learning does not require consciousness. Finally, we present our own hypothesis, based on two logical assumptions, concerning which organisms possess consciousness. Our first assumption is that affective (emotional) consciousness is marked by an advanced capacity for operant learning about rewards and punishments. Our second assumption is that image-based conscious experience is marked by demonstrably mapped representations of the external environment within the body. Certain animals fit both of these criteria, but plants fit neither. We conclude that claims for plant consciousness are highly speculative and lack sound scientific support.

Debunking a myth: plant consciousness

Claims that plants have conscious experiences have increased in recent years and have received wide coverage, from the popular media to scientific journals. Such claims are misleading and have the potential to misdirect funding and governmental policy decisions. After defining basic, primary consciousness, we provide new arguments against 12 core claims made by the proponents of plant consciousness. Three important new conclusions of our study are (1) plants have not been shown to perform the proactive, anticipatory behaviors associated with consciousness, but only to sense and follow stimulus trails reactively; (2) electrophysiological signaling in plants serves immediate physiological functions rather than integrative-information processing as in nervous systems of animals, giving no indication of plant consciousness; (3) the controversial claim of classical Pavlovian learning in plants, even if correct, is irrelevant because this type of learning does not require consciousness. Finally, we present our own hypothesis, based on two logical assumptions, concerning which organisms possess consciousness. Our first assumption is that affective (emotional) consciousness is marked by an advanced capacity for operant learning about rewards and punishments. Our second assumption is that image-based conscious experience is marked by demonstrably mapped representations of the external environment within the body. Certain animals fit both of these criteria, but plants fit neither. We conclude that claims for plant consciousness are highly speculative and lack sound scientific support.

Species-Specific and Distance-Dependent Dispersive Behaviour of Forisomes in Different Legume Species

Forisomes are giant fusiform protein complexes composed of sieve element occlusion (SEO) protein monomers, exclusively found in sieve elements (SEs) of legumes. Forisomes block the phloem mass flow by a Ca2+-induced conformational change (swelling and rounding). We studied the forisome reactivity in four different legume species-Medicago sativa, Pisum sativum, Trifolium pratense and Vicia faba. Depending on the species, we found direct relationships between SE diameter, forisome surface area and distance from the leaf tip, all indicative of a developmentally tuned regulation of SE diameter and forisome size. Heat-induced forisome dispersion occurred later with increasing distance from the stimulus site. T. pratense and V. faba dispersion occurred faster for forisomes with a smaller surface area. Near the stimulus site, electro potential waves (EPWs)-overlapping action (APs), and variation potentials (VPs)-were linked with high full-dispersion rates of forisomes. Distance-associated reduction of forisome reactivity was assigned to the disintegration of EPWs into APs, VPs and system potentials (SPs). Overall, APs and SPs alone were unable to induce forisome dispersion and only VPs above a critical threshold were capable of inducing forisome reactions.

Long-distance electrical signals as a link between the local action of stressors and the systemic physiological responses in higher plants

Our review is devoted to the analysis of the role of long-distance electrical signals in the development of the fast systemic physiological responses in higher plants. The characteristics and mechanisms of basic electrical signals (variation potential, action potential and system potential) are analyzed, and a potential schema of the generation and propagation of the system potential is proposed. The review summarizes the physiological changes induced by the variation potential, action potential and system potential in higher plants, including changes in gene expressions, the production of phytohormones, photosynthesis, phloem mass-flow, respiration, ATP content, transpiration and plant growth. Potential mechanisms of the changes are analyzed. Finally, a hypothetical schema, which describes a hierarchy of the variation potential, action potential and system potential, in the development of the fast systemic non-specific adaptation of plants to stressors, is proposed.

Effect of simultaneously induced environmental stimuli on electrical signalling and gas exchange in maize plants

Electrical signalling in response to environmental stimuli is a well-known phenomenon in higher plants. For example, in maize, different stimuli, such as wounding or re-irrigation after drought, incite characteristic electrical signals which have quite particular effects on gas exchange. What is less well understood is how plants (specifically maize) respond when two different environmental stimuli are applied simultaneously. To explore this, a three-stage experiment was designed. In the first stage, drought conditions were simulated by decreasing the soil water content to 30-40 % of field capacity. In these conditions, and in contrast to well-watered plants, the maize exhibited only 60-70% of the original level of stomatal conductance and 50-60 % of the original photosynthesis rate. In the second stage of the experiment the plants were re-irrigated and heat stimulated separately. Re-irrigation led to specific electrical signals followed by a gradual increase of gas exchange. In contrast, after heat stimulation of a leaf an electrical signal was evoked that reduced the net CO₂-uptake rate as well as stomatal conductance. In the third stage, to elucidate how plants process simultaneous re-irrigation and heat stimulation, the drought-stressed maize plants were re-watered and heat-stimulated at the same time. Results showed a two phase response. In the first phase there was a rapid decrease in both the CO₂ uptake rate and the stomatal conductance, while in the second phase each of these parameters increased gradually. Thus, the results strongly support the view that the responses from both stimuli were combined, indicating that maize plants can process simultaneously applied stimuli.

Plant ion channels and transporters in herbivory-induced signalling

In contrast to many biotic stresses that plants face, feeding by herbivores produces unique mechanical and chemical signatures. Plants have evolved effective systems to recognise these mechanical stimuli and chemical elicitors at the plasma membrane (PM), where this recognition generates ion fluxes, including an influx of Ca2+ that elicits cellular Ca2+ signalling, production of reactive oxygen species (ROS), and variation in transmembrane potential. These signalling events also function in propagation of long-distance signals (Ca2+ waves, ROS waves, and electrical signals), which contribute to rapid, systemic induction of defence responses. Recent studies have identified several candidate channels or transporters that likely produce these ion fluxes at the PM. Here, we describe the important roles of these channels/transporters in transduction or transmission of herbivory-induced early signalling events, long-distance signals, and jasmonic acid and green leaf volatile signalling in plants.

Ca Distribution Pattern in Litchi Fruit and Pedicel and Impact of Ca Channel Inhibitor, La3

Calcium (Ca) deficiency in fruit causes various physiological disorders leading to quality loss. However, disorders related to Ca deficiency are not simply caused by a shortage of calcium supply. Ca distribution is also an important relation. This study examined Ca distribution pattern in fruit and pedicel in litchi (Litchi chinensis Sonn.) and the influence of Ca channel inhibitor La³⁺ on fruit Ca uptake and distribution. In situ distribution of Ca in the phloem and xylem tissues of the pedicel was visualized by Ca mapping with X-ray microanalyzer. Ca²⁺ analogy Sr²⁺ was used to trace Ca²⁺ transport pathway to fruit as well as distribution pattern. The results showed Ca was more distributed in the pericarp, especially the distal part. Ca level in the bark/phloem was always significantly higher than in the xylem and increased with stem age, suggesting constant influx of Ca into the phloem from the xylem. La³⁺ increased the ratio of Ca in the xylem to that in the bark in the pedicel and significantly reduced Ca accumulation by 55.6% in fruit, suggesting influx of Ca into the symplast was involved in fruit Ca uptake. Sr²⁺ introduced from fruit stalk was found to be transported to fruit through the phloem as Sr was largely distributed in the phloem, and fruit stalk girdling significantly reduced Sr accumulation in the pericarp. Ca mapping across the pedicel revealed Ca-rich sites in the parenchyma cells in the phloem and along the cambium, where abundant Ca oxalate crystals were found. The results suggested extensive influx of Ca from xylem/apoplast pathway into the phloem/symplast pathway in the pedicel, which enables phloem/symplast pathway to contribute a considerable part to Ca uptake in litchi fruit.

High-resolution non-contact measurement of the electrical activity of plants in situ using optical recording

The limitations of conventional extracellular recording and intracellular recording make high-resolution multisite recording of plant bioelectrical activity in situ challenging. By combining a cooled charge-coupled device camera with a voltage-sensitive dye, we recorded the action potentials in the stem of Helianthus annuus and variation potentials at multiple sites simultaneously with high spatial resolution. The method of signal processing using coherence analysis was used to determine the synchronization of the selected signals. Our results provide direct visualization of the phloem, which is the distribution region of the electrical activities in the stem and leaf of H. annuus, and verify that the phloem is the main action potential transmission route in the stems of higher plants. Finally, the method of optical recording offers a unique opportunity to map the dynamic bioelectrical activity and provides an insight into the mechanisms of long-distance electrical signal transmission in higher plants.

A New Application of the Electrical Penetration Graph (EPG) for Acquiring and Measuring Electrical Signals in Phloem Sieve Elements

Electrophysiological properties of cells are often studied in vitro, after dissociating them from their native environments. However, the study of electrical transmission between distant cells in an organism requires in vivo, artifact-free recordings of cells embedded within their native environment. The transmission of electrical signals from wounded to unwounded areas in a plant has since long piqued the interest of botanists. The phloem, the living part of the plant vasculature that is spread throughout the plant, has been postulated as a major tissue in electrical transmission in plants. The lack of suitable electrophysiological methods poses many challenges for the study of the electrical properties of the phloem cells in vivo. Here we present a novel approach for intracellular electrophysiology of sieve elements (SEs) that uses living aphids, or other phloem-feeding hemipteran insects, integrated in the electrical penetration graph (EPG) circuit. The versatility, robustness, and accuracy of this method made it possible to record and study in detail the wound-induced electrical signals in SEs of central veins of the model plant Arabidopsis thaliana(1). Here we show that EPG-electrodes can be easily implemented for intracellular electrophysiological recordings of SEs in marginal veins, as well as to study the capacity of SEs to respond with electrical signals to several external stimuli. The EPG approach applied to intracellular electrophysiology of SEs can be implemented to a wide variety of plant species, in a large number of plant/insect combinations, and for many research aims.

Plants respond to leaf vibrations caused by insect herbivore chewing

Plant germination and growth can be influenced by sound, but the ecological significance of these responses is unclear. We asked whether acoustic energy generated by the feeding of insect herbivores was detected by plants. We report that the vibrations caused by insect feeding can elicit chemical defenses. Arabidopsis thaliana (L.) rosettes pre-treated with the vibrations caused by caterpillar feeding had higher levels of glucosinolate and anthocyanin defenses when subsequently fed upon by Pieris rapae (L.) caterpillars than did untreated plants. The plants also discriminated between the vibrations caused by chewing and those caused by wind or insect song. Plants thus respond to herbivore-generated vibrations in a selective and ecologically meaningful way. A vibration signaling pathway would complement the known signaling pathways that rely on volatile, electrical, or phloem-borne signals. We suggest that vibration may represent a new long distance signaling mechanism in plant-insect interactions that contributes to systemic induction of chemical defenses.

Spatio-temporal mapping of variation potentials in leaves of Helianthus annuus L. seedlings in situ using multi-electrode array

Damaging thermal stimuli trigger long-lasting variation potentials (VPs) in higher plants. Owing to limitations in conventional plant electrophysiological recording techniques, recorded signals are composed of signals originating from all of the cells that are connected to an electrode. This limitation does not enable detailed spatio-temporal distributions of transmission and electrical activities in plants to be visualised. Multi-electrode array (MEA) enables the recording and imaging of dynamic spatio-temporal electrical activities in higher plants. Here, we used an 8 × 8 MEA with a polar distance of 450 μm to measure electrical activities from numerous cells simultaneously. The mapping of the data that were recorded from the MEA revealed the transfer mode of the thermally induced VPs in the leaves of Helianthus annuus L. seedlings in situ. These results suggest that MEA can enable recordings with high spatio-temporal resolution that facilitate the determination of the bioelectrical response mode of higher plants under stress.

Symplastic intercellular transport from a developmental perspective

Plant cells have channel-like structures named plasmodesmata that allow for the symplastic molecular transport between neighbouring cells. The importance of plasmodesmata in whole plant development is well acknowledged. They mediate the cell-to-cell and vascular loading and unloading of metabolites, proteins, and other signalling molecules. However, it is still not clear how, mechanistically, these channels are regulated in response to developmental and environmental cues. This review aims to bring together knowledge acquired in recent years on plasmodesmata composition, regulation, and function. Progress in the discovery of factors that regulate symplastic transport and plant development in particular are discussed. This will hopefully highlight the challenges faced by the scientific community to unveil the mechanisms controlling symplastic communication during the formation and maintenance of plant meristems.

Spread the news: systemic dissemination and local impact of Ca²⁺ signals along the phloem pathway

We explored the idea of whether electropotential waves (EPWs) primarily act as vehicles for systemic spread of Ca(2+) signals. EPW-associated Ca(2+) influx may trigger generation and amplification of countless long-distance signals along the phloem pathway given the fact that gating of Ca(2+)-permeable channels is a universal response to biotic and abiotic challenges. Despite fundamental differences, both action and variation potentials are associated with a sudden Ca(2+) influx. Both EPWs probably disperse in the lateral direction, which could be of essential functional significance. A vast set of Ca(2+)-permeable channels, some of which have been localized, is required for Ca(2+)-modulated events in sieve elements. There, Ca(2+)-permeable channels are clustered and create so-called Ca(2+) hotspots, which play a pivotal role in sieve element occlusion. Occlusion mechanisms play a central part in the interaction between plants and phytopathogens (e.g. aphids or phytoplasmas) and in transient re-organization of the vascular symplasm. It is argued that Ca(2+)-triggered systemic signalling occurs in partly overlapping waves. The forefront of EPWs may be accompanied by a burst of free Ca(2+) ions and Ca(2+)-binding proteins in the sieve tube sap, with a far-reaching impact on target cells. Lateral dispersion of EPWs may induce diverse Ca(2+) influx and handling patterns (Ca(2+) signatures) in various cell types lining the sieve tubes. As a result, a variety of cascades may trigger the fabrication of signals such as phytohormones, proteins, or RNA species released into the sap stream after product-related lag times. Moreover, transient reorganization of the vascular symplasm could modify cascades in disjunct vascular cells.

Plant Vascular Biology 2013: vascular trafficking

About 200 researchers from around the world attended the Third International Conference on Plant Vascular Biology (PVB 2013) held in July 2013 at the Rantapuisto Conference Center, in Helsinki, Finland (http://www.pvb2013.org). The plant vascular system, which connects every organ in the mature plant, continues to attract the interest of researchers representing a wide range of disciplines, including development, physiology, systems biology, and computational biology. At the meeting, participants discussed the latest research advances in vascular development, long- and short-distance vascular transport and long-distance signalling in plant defence, in addition to providing a context for how these studies intersect with each other. The meeting provided an opportunity for researchers working across a broad range of fields to share ideas and to discuss future directions in the expanding field of vascular biology. In this report, the latest advances in understanding the mechanism of vascular trafficking presented at the meeting have been summarized.

Calcium powered phloem protein of SEO gene family "Forisome" functions in wound sealing and act as biomimetic smart materials

Forisomes protein belongs to SEO gene family and is unique to Fabaceae family. These proteins are located in sieve tubes of phloem and function to prevent loss of nutrient-rich photoassimilates, upon mechanical injury/wounding. Forisome protein is also known as ATP independent, mechanically active proteins. Despite the wealth of information role of forisome in plants are not yet fully understood. Recent reports suggest that forisomes protein can act as ideal model to study self assembly mechanism for development of nanotechnological devices like microfluidic system application in space exploration mission. Improvement in micro instrument is highly demanding and has been a key technology by NASA in future space exploration missions. Based on its physical parameters, forisome are found to be ideal biomimetic materials for micro fluidic system because the conformational shifts can be replicated in vitro and are fully reversible over large number of cycles. By the use of protein engineering forisome recombinant protein can be tailored. Due to its unique ability to convert chemical energy into mechanical energy forisome has received much attention. For nanotechnological application and handling biomolecules such as DNA, RNA, protein and cell as a whole microfluidic system will be the most powerful technology. The discovery of new biomimetic smart materials has been a key factor in development of space science and its requirements in such a challenging environment. The field of microfludic, particularly in terms of development of its components along with identification of new biomimetic smart materials, deserves more attention. More biophysical investigation is required to characterize it to make it more suitable under parameters of performance.

Calcium-energized motor protein forisome controls damage in phloem: potential applications as biomimetic "smart" material

Forisomes are ATP independent, mechanically active proteins from the Fabaceae family (also called Leguminosae). These proteins are located in sieve tubes of phloem and function to prevent loss of nutrient-rich photoassimilates, upon mechanical injury/wounding. Forisomes are SEO (sieve element occlusion) gene family proteins that have recently been shown to be involved in wound sealing mechanism. Recent findings suggest that forisomes could act as an ideal model to study self assembly mechanism for the development of nanotechnological devices like microinstruments, the microfluidic system frequently used in space exploration missions. Technology enabling improvement in micro instruments has been identified as a key technology by NASA in future space exploration missions. Forisomes are designated as biomimetic smart materials which are calcium-energized motor proteins. Since forisomes are biomolecules from plant systems it can be doctored through genetic engineering. In contrast, "smart" materials which are not derived from plants are difficult to modify in their properties. Current levels of understanding about forisomes conformational shifts with respect to calcium ions and pH changes requires supplement of future advances with relation to its 3D structure to understand self assembly processes. In plant systems it forms blood clots in the form of occlusions to prevent nutrient fluid leakage and thus proves to be a unique damage control system of phloem tissue.

Involvement of the sieve element cytoskeleton in electrical responses to cold shocks

This study dealt with the visualization of the sieve element (SE) cytoskeleton and its involvement in electrical responses to local cold shocks, exemplifying the role of the cytoskeleton in Ca(2+)-triggered signal cascades in SEs. High-affinity fluorescent phalloidin as well as immunocytochemistry using anti-actin antibodies demonstrated a fully developed parietal actin meshwork in SEs. The involvement of the cytoskeleton in electrical responses and forisome conformation changes as indicators of Ca(2+) influx was investigated by the application of cold shocks in the presence of diverse actin disruptors (latrunculin A and cytochalasin D). Under control conditions, cold shocks elicited a graded initial voltage transient, ΔV1, reduced by external La(3+) in keeping with the involvement of Ca(2+) channels, and a second voltage transient, ΔV2. Cytochalasin D had no effect on ΔV1, while ΔV1 was significantly reduced with 500 nm latrunculin A. Forisome dispersion was triggered by cold shocks of 4°C or greater, which was indicative of an all-or-none behavior. Forisome dispersion was suppressed by incubation with latrunculin A. In conclusion, the cytoskeleton controls cold shock-induced Ca(2+) influx into SEs, leading to forisome dispersion and sieve plate occlusion in fava bean (Vicia faba).

MRI of plants and foods

The importance and prospects for MRI as applied to intact plants and to foods are presented in view of one of humanity's most pressing concerns, the sustainable and healthy feeding of a worldwide increasing population. Intact plants and foods have in common that their functionality is determined by complex multiple length scale architectures. Intact plants have an additional level of complexity since they are living systems which critically depend on transport and signalling processes between and within tissues and organs. The combination of recent cutting-edge technical advances and integration of MRI accessible parameters has the perspective to contribute to breakthroughs in understanding complex regulatory plant performance mechanisms. In food science and technology MRI allows for quantitative multi-length scale structural assessment of food systems, non-invasive monitoring of heat and mass transport during shelf-life and processing, and for a unique view on food properties under shear. These MRI applications are powerful enablers of rationally (re)designed food formulations and processes. Limitations and bottlenecks of the present plant and food MRI methods are mainly related to short T2 values and susceptibility artefacts originating from small air spaces in tissues/materials. We envisage cross-fertilisation of solutions to overcome these hurdles in MRI applications in plants and foods. For both application areas we witness a development where MRI is moving from highly specialised equipment to mobile and downscaled versions to be used by a broad user base in the field, greenhouse, food laboratory or factory.

Sampling and analysis of phloem sap

The transport tubes of the phloem are essential for higher plants. They not only provide the route for the distribution of assimilates produced during photosynthesis from source to sink organs but also (re-) distribute mineral nutrients. Additionally, the phloem is essential for sending information between distant plant organs and steering developmental and defense processes. For example, flowering and tuberization time are controlled by phloem-mobile signals and important defense reactions on the whole plant level, like systemic acquired resistance or systemic gene silencing, are spread through the phloem. In addition, recent results demonstrate that also the allocation of mineral nutrients is coordinated by phloem mobile signaling molecules. However, in many studies the important analysis of phloem sap is neglected, probably because the content of sieve tubes is not easy to access. This chapter will describe the current methods for sampling and analysis of phloem sap in order to encourage researchers to include the analysis of this crucial compartment in their relevant studies.

Interaction of xylem and phloem during exudation and wound occlusion in Cucurbita maxima

Collection of cucurbit exudates from cut petioles has been a powerful tool for gaining knowledge on phloem sap composition without full notion of the complex exudation mechanism. Only few publications explicitly mentioned that exudates were collected from the basal side of the cut, which exudes more copiously than the apical side. This is surprising since only exudation from the apical side is supposedly driven by phloem pressure gradients. Composition of carbohydrates and pH values at both wounding sides are equal, whereas protein concentration is higher at the basal side. Apparently, exudation is far more complex than just the delivery of phloem sap. Xylem involvement is indicated by lower protein concentrations after elimination of root pressure. Moreover, dye was sucked into xylem vessels owing to relaxation of negative pressure after cutting. The lateral water efflux from the vessels increases turgor of surrounding cells including sieve elements. Simultaneously, detached parietal proteins (PP1/PP2) induce occlusion of sieve plates and cover wound surface. If root pressure is strong enough, pure xylem sap can be collected after removal of the occlusion plug at the wound surface. The present findings provide a mechanism of sap exudation in Cucurbita maxima, in which the contribution of xylem water is integrated.

Soluble and filamentous proteins in Arabidopsis sieve elements

Phloem sieve elements are highly differentiated cells involved in the long-distance transport of photoassimilates. These cells contain both aggregated phloem-proteins (P-proteins) and soluble proteins, which are also translocated by mass flow. We used liquid chromatography-tandem mass spectrometry (LC-MS/MS) to carry out a proteomic survey of the phloem exudate of Arabidopsis thaliana, collected by the ethylenediaminetetraacetic acid (EDTA)-facilitated method. We identified 287 proteins, a large proportion of which were enzymes involved in the metabolic precursor generation and amino acid synthesis, suggesting that sieve tubes display high levels of metabolic activity. RNA-binding proteins, defence proteins and lectins were also found. No putative P-proteins were detected in the EDTA-exudate fraction, indicating a lack of long-distance translocation of such proteins in Arabidopsis. In parallel, we investigated the organization of P-proteins, by high-resolution transmission electron microscopy, and the localization of the phloem lectin PP2, a putative P-protein component, by immunolocalization with antibodies against PP2-A1. Transmission electron microscopy observations of P-proteins revealed bundles of filaments resembling strings of beads. PP2-A1 was found weakly associated with these structures in the sieve elements and bound to plastids. These observations suggest that PP2-A1 is anchored to P-proteins and organelles rather than being a structural component of P-proteins.