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Plant growth and development are complex processes modulated by multi-level regulation mechanisms that depend on both internal and external signals. Such signals with the capacity to alter the morphogenesis of plant cells are perceived at the cellular level. Based on mitogen-activated protein kinase (MAPK) cascades, signalling of diverse stress-related factors is responsible for the generation of cell- and tissue-specific responses, and can be expected to participate also in the signal-mediated growth of plant cells, like tip growth of root hairs. Investigating stress-induced MAPKs in growing root hairs revealed a new field of complexity in plant signalling networks, and thus, MAPKs supplemented the collection of tip growth-specific factors with putative regulatory functions. The emerging scenario appears to be more complex; MAPK signalling pathways can support the reorganization of root epidermal cells during formation of tip growing hairs, but can also participate in the harmony of root hair tip growth with the changing environment. [ABSTRACT FROM AUTHOR]

Dynamic actin cytoskeleton and myosins, especially of the plant-specific class VIII, emerge as the most relevant candidates for plant gravi-and mechanosening as well as transduction of received physical signals into multitude of biological processes. Root apices are extremely sensitive to mechanical stimuli and show adaptive responses optimalizing root behaviour for their searching/avoidance and exploratory growth modes. Recently, the actin cytoskeleton and myosins have been proposed to participate in plant gravi- and mechanosening also via their roles in endocytosis and vesicle recycling, processes which are essential for the polar transport of auxin. Mechanosensitive endocytic vesicle recycling might act as an effective gravisensing system with a power to unite the two conflicting theories, Statolith-Starch Theory and Cholodny-Went Theory, under one umbrella – the unifying concept of gravisensing plant synapses. [ABSTRACT FROM AUTHOR]

Nonselective cation channels (NSCC) in the plasma membrane of higher plants form a large and diverse group of plant cation channels which are permeable for K+, Na+ and Ca2+. They include four classes: (1) constitutive NSCC; (2) NSCC activated by reactive oxygen species (ROS); (3) ligand-activated NSCC; and (4) mechanosensitive NSCC. Our understanding of physiological functions of NSCC has significantly progressed in the last few years. NSCC were demonstrated to be involved in nutritional uptake of K+, NH4+, Ca2+, Mg2+, micronutrients and trace elements, toxic Na+ influx, in ROS-, amino acid, purine- and cyclic nucleotide induced signalling, growth and development. Here, physiological roles of plant NSCC are reviewed and discussed. [ABSTRACT FROM AUTHOR]

Plant-plant signalling is discussed from a tritrophic perspective, with special reference to results from experiments with a model system consisting of barley, aphids and ladybirds. Experimental support for the following statements is discussed: (1) barley plants communicate via volatile substances and, in certain combinations of genotypes, this communication leads to changes in biomass allocation, (2) communication between barley plants of certain genotypes changes the pattern of host plant acceptance by the bird cherry-oat aphid, Rhopalosiphum padi, (3) odour stimuli from barley and common weeds affect the searching behaviour of the seven-spotted ladybird, Coccinella septempunctata. The results indicate that an active response of the barley plant to exposure to weed Cirsium arvense volatiles may be involved. The tritrophic effects of plant-plant communication in barley add a new dimension to the term allelopathy. Thus, we use the term allelobiosis to denote interactions in which exchange of plant chemicals has an informative value for the receiving plant, and the response of the receiving plant affects its growth strategy, and relations with herbivores and their natural enemies. [ABSTRACT FROM AUTHOR]

Plant root exudates have important effects on plant and soil communities, mediating positive and antagonistic plant-plant and plant-microbe interactions, and altering soil processes. Here, we discuss the potential for a single plant secondary metabolite to serve multiple functions in the rhizosphere. Secondary metabolites that serve multiple functions may provide plants with multiple benefits at relatively low metabolic cost, with consequences for competitive ability, disease resistance, and resource availability. Specifically, we describe recent research on (±)-catechin, a secondary metabolite exuded from the roots of the invasive weed Centaurea maculosa. Depending on concentration and the species that are present, (±)-catechin has the potential to act as an allelochemical, autoinhibitor, and antimicrobial agent, and to increase soil nutrient availability. [ABSTRACT FROM AUTHOR]

The focus is on the most widely studied wound response: the systemic synthesis in tomato of the defence proteins known as proteinase inhibitors (PIs). In wounded tomato seedlings, the severity of the wound is a major determinant of the type of wound signalling. Minor wounds produce outcomes that are consistent with the transport in the phloem of a chemical elicitor of PI synthesis. Severe wounds produce outcomes that are consistent with the distribution of elicitors of PI synthesis and of electrical activity by hydraulic dispersal in the xylem. The main electrical events associated with severe wounds are action-potential-like depolarisations in the sieve-tube-element/companion cell complexes; these events are considered in relation to the use of the terms “action potential” and “variation potential”. [ABSTRACT FROM AUTHOR]

The integration of activity of Chenopodium plants on a hydraulic-electrochemical level is expressed by a diurnal rhythm in the resting membrane potential measured with contact electrodes. The membrane state could be gated by the energy state of cells. From earlier studies we compiled evidence in favour of a circadian rhythm in overall energy transduction producing a circadian rhythm in energy charge and redox state (NADPH2/NADP). The ratio of metabolic coupling nucleotides would be relatively temperature independent and thus could fulfil the requirements for precise temperature-compensated time-keeping. The phytochrome photoreceptors, involved in photoperiodic control of development, could via changes in pyridine nucleotide pool sizes and changes in nucleotide ratios regulate transcription-translational loops by redox and phosphorylation controlled transcription factors. Spontaneous action potentials (APs) have been shown to correlate with turgor-controlled growth movements. The accumulation of spontaneous APs at specific times during daily light-dark spans were recorded, giving specific electrophysiograms, representative for flower-inducing and vegetative conditions. It is anticipated that hydraulic changes at the apex leading to flower initiation are mediated by a specific hydro-electrochemical communication between leaves, the shoot apex and the root system. These results have been used to substitute a flower-inducing photoperiod by specific timing of electric stimulation via surface electrodes. [ABSTRACT FROM AUTHOR]

Plants continually gather information about their environment. The conduction of bioelectrochemical excitation is a fundamental property of living organisms. Cells, tissues, and organs transmit electrochemical signals over short and long distances. The sensitive membranes in phloem cells facilitate the passage of electrical excitations in the form of action potentials. We have created a unique electrophysiological workstation that can effectively register this electrical activity in real time. It allows basic properties of electrical communication in green plants to be established. Our workstation has very high input impedance and a resolution of 0.01 ms. Excitation waves in higher plants are possible mechanisms for intercellular and intracellular communication in the presence of environmental changes. Ionic channels, as natural nanodevices, control the plasma membrane potential and the movement of ions across membranes regulating various biological functions. Some voltage-gated ion channels work as plasma membrane nanopotentiostats. Blockers of ionic channels, such as tetraethylammonium chloride and ZnCl2, stop the propagation of action potentials in soybean induced by blue light and inhibit phototropism in soybean plants. Voltage-gated ionic channels control the plasma membrane potential and the movement of ions across membranes regulating various biological functions. These biological nanodevices play vital roles in signal transduction in higher plants. Tetraethylammonium chloride and ZnCl2 block K+ and Ca2+ ionic channels. These blockers inhibit the propagation of action potentials induced by blue light, and inhibit phototropism in soybean plants. The irradiation of soybean plants at 450 ± 50 nm induces action potentials with duration times of about 1 ms and amplitudes around 60 mV. The role of the electrified interface of the plasma membrane in signal transduction is discussed. [ABSTRACT FROM AUTHOR]

Long-distance transmission of signals is a critical event in the life of trees. Many physiological studies have deduced that hormone-like substances together with hydraulic/ electrical signals mediate this important function of the plant life. However the nature of the complex network of signalling in trees has remained essentially unexplored. Recent molecular and genetic studies offer new approaches to understanding the mechanisms underlying the transmission of signals. [ABSTRACT FROM AUTHOR]

Electrical signalling along the phloem has been studied in a number of species like maize, willow and Mimosa. It appears that sieve tubes with their large sieve pores are used to transmit information over long distances, while plasmodesmata serve as a means for the propagation of electrical signals over short distances between cells. By using the aphid technique the phloem pathway has been shown to transmit action potentials with a velocity up to 10 cm s−1. With regard to the ion fluxes which create the conditions necessary for the generation of an action potential, we found that calcium influx as well as potassium and chloride efflux are involved. Some of their corresponding ion channels were identified. AKT2/3-like channels, expressed in the phloem and capable of mediating both uptake and release of K+ in response to changes in membrane potential, were identified in several species such as Arabidopsis, maize and broad bean. Concerning physiological functions of electrical signalling, evidence was found for a link between the signals and photosynthetic response in Mimosa, apart from the regulation of rapid leaf movements. In addition, electrical signals in maize play a role in the regulation of phloem transport as well as in root-to-shoot communication of entire plants. [ABSTRACT FROM AUTHOR]

When plant tissue is abiotically injured by crushing, cutting, heat-wounding, electrical stimulation, or by several other means, the injured (perceiving) tissue generates electrical signals (action potentials and variation potentials) and transmits them to distant (responding) tissue. Here they evoke apparently disparate responses, such as callose formation, closing of plasmodesmata, stoppage of cytoplasmic streaming, inhibition of ribosome movement along messenger RNA (mRNA), and ultrarapid but transient accumulation of over 100 transcripts, which are degraded without being translated. These apparently disparate responses can be reconciled by one fundamental hypothesis that assumes that “the plant does not know what hit it” and thus “expecting the worst” mounts a holistic defense response against its most potent nemesis, a putative viral invasion. We postulate that the basis for this response is calcium influx into the cytoplasm via voltage-gated channels (action potential) associated with the microtubules, or via mechano-sensitive channels (variation potential) associated with microfilaments. The calcium interacts with calcium and/or calmodulin-dependent cytoskeleton-associated protein kinases. This causes the phosphorylation of myosin, which stops cytoplasmic streaming, and of elongation factor 2F, which slows elongation and termination and causes ribosomes to pile up on polyribosomes. This decreases protein synthesis, but protects preexisting “host” transcripts from degradation. The phosphorylation signal then passes into the nucleus, where it phosphorylates RNA polymerase II, which goes into overdrive (i.e., does not stop at accuracy checkpoints), thus causing the synthesis of large amounts of mismade mRNA. The mRNA is transported into the cytoplasm, where it is scanned (checked for accuracy) by ribosomes, and found to be incorrect. This surveillance mechanism stimulates ribonuclease activity, which degrades the free (non-polysome-associated), mismade RNA, but leaves the original, “host” transcripts unscathed since they are protected by ribosomes. The ribonuclease also (and here is the crux of the matter) attacks other free mRNAs, including viral mRNAs, so these are disposed of before they can be translated. Within minutes this reaction is over, cytoplasmic steaming resumes, translation continues, ribosomes are released and so can be used to translate new (correctly made) transcripts. [ABSTRACT FROM AUTHOR]

Plants have at least three kinds of propagating electrical signals. In addition to a sustained wound potential (WP) that stops a few millimeters from dying cells, these signals are action potentials (APs) and slow wave potentials (SWPs). All three signals consist of a transient change in the membrane potential of plant cells (depolarization and subsequent repolarization), but only SWPs and APs make use of the vascular bundles to achieve a potentially systemic spread through the entire plant. The principal difference used to differentiate SWPs from APs is that SWPs show longer, delayed repolarizations. Unfortunately, SWP repolarizations also show a large range of variation that makes a distinction difficult. SWPs and APs do differ more clearly, however, in the causal factors stimulating their appearance, the ionic mechanisms of their depolarization and repolarization phases as well as the mechanisms and pathways of propagation. The depolarizations of a SWP arise with an increase in turgor pressure cells experience in the wake of a hydraulic pressure wave that spreads through the xylem conduits after rain, embolism, bending, local wounds, organ excision and local burning. The generation of APs occurs under different environmental and internal influences (e.g. touch, light changes, cold treatment, cell expansion) that — mediated through varying generator potentials — trigger a voltage-dependent depolarization spike in an all-or-nothing manner. While APs and WPs can be triggered in excised organs, SWPs depend on the pressure difference between the atmosphere and an intact plant interior. High humidity and prolonged darkness will also suppress SWP signaling. The ionic mechanism of the SWP is thought to involve a transient shutdown of a P-type H+-ATPase in the plasma membrane and differs from the mechanism underlying APs. Another defining characteristic of SWPs is the hydraulic mode of propagation that enables them — but not APs — to pass through killed or poisoned areas. Unlike APs they can easily communicate between leaf and stem. SWPs can move in both directions of the plant axis, while their amplitudes show a decrement of about 2.5% cm−1 and move with speeds that can be slower than APs in darkness and faster in bright light. The SWPs move with a rapid pressure increase that establishes an axial pressure gradient in the xylem. This gradient translates distance (perhaps via changing kinetics in the rise of turgor pressure) into increasing lag phases for the pressure-induced depolarizations in the epidermis cells. Haberlandt (1890), after studying propagating responses in Mimosa pudica, suggested the existence of hydraulically propagated electric potentials at a time when only APs were conceivable. It took a century to realize that such signals do exist and that they coincide with the characteristics of SWPs rather than those of APs. Moreover, we begin to understand that SWPs are not only ubiquitous among higher plants but represent a unique, defining characteristic without parallels in lower plants or animals. [ABSTRACT FROM AUTHOR]

Action potentials (APs) belong to long-distance signals in plants. They fulfill the all-or-none law, propagate without decrement and their generation is limited by refractory periods. The ion mechanism of APs was elaborated in giant Characean algae and extended by another model plant — the liverwort Conocephalum conicum. It consists of an increase in cytoplasmic Ca2+ concentration ([Ca2+]cyt) which activates anion channels responsible for Cl− efflux and for membrane depolarization. Repolarization occurs after the opening of potassium channels and K+ efflux. The resting potential is restored by the electrogenic proton pump. A number of ion channels which may play a role in AP were identified by the patch-clamp technique. APs propagate on the principle of local electrical circuits. They cover whole plants, plant organs or definite tissues, mainly phloem, phloem parenchyma and protoxylem. APs mediate between local stimulation and movements in carnivorous Dionaea muscipula, Aldrovanda vesiculosa, and tigmonastic Mimosa pudica. The role of APs in regulation of respiration, photosynthesis, growth, pollination, fertilization and gene expression is well documented. An AP-coupled increase in [Ca2+]cyt seems to play a central role in signal transduction. [ABSTRACT FROM AUTHOR]

Despite the fact that the rhythmical behaviour is a quintessential pattern of life itself, most researchers still treat oscillations in plants as some unwanted “physiological noise”. In this review, an attempt is made to summarise recent progress in this area and highlight the paramount role of oscillatory processes in plant life. First, diversity and hierarchy of oscillations in plants are examined, then a general overview of oscillatory phenomena is given, with the main emphasis on the physiological role of oscillatory processes in plants. The areas covered include leaf and stomata movement, nutations, nutrient acquisition, growth and differentiation, photosynthesis and osmotic adjustment. A possible role for ultradian rhythms in timekeeping is also briefly discussed. The importance of ultradian oscillations is further illustrated by discussing their involvement in the encoding mechanism, mediating plant-environment interaction. Finally, advantages and principles of oscillatory control are considered in the context of plant physiology, with a major emphasis on feedback control and self-sustained oscillations, as well as on deterministic chaos and “strange” behaviour in plants. [ABSTRACT FROM AUTHOR]

The abilities of plants to perceive stimuli in their environment are often overlooked. Yet hints of exquisite sensitivities abound. Indeed, although it is not generally appreciated, probably all plants can perceive and respond to simple mechanostimulation like touch. We will briefly review some of the more spectacular touch responses of specialized plants and then discuss developmental and molecular responses to touch that occur in nonspecialized plants. Many of these data and concepts have recently been reviewed by [Braam] (New Phytol. 165:373–389, 2005). [ABSTRACT FROM AUTHOR]

The traditionally emphasized roles of ATP and other nucleotides are in energy metabolism, and they all happen within the borders of the plasma membrane of cells. However, recent findings, first in animals and then in plants, indicate that ATP and ADP can have significant roles outside the plasma membrane as agonists that do not have to be hydrolyzed to induce diverse signaling responses in cells. This chapter reviews results obtained mainly in Arabidopsis that point to a likely role of extracellular nucleotides as regulators of plant growth and development. Concentrations of ATP in the 40–60-µM range are released from cells into intercellular spaces of plant tissues by wounds, and lower levels of applied ATP, but not AMP or phosphate, can induce increases in the concentration of cytoplasmic calcium ions and such calcium-dependent downstream responses as increased superoxide production and increased levels of messenger RNAs associated with wound responses such as lipoxygenase and 1-aminocyclopropane-1-carboxylic acid synthase. Inhibitors of calcium signaling (calcium chelators, La3+, Gd3+, calmodulin antagonists) and inhibitors of animal purinoceptors suppress the inductive effects of ATP in plants. Because growth zones of plants are regions of high secretory activity and because secretory vesicles contain millimolar levels of ATP that are delivered to the plant extracellular matrix when they fuse with the plasma membrane, extracellular ATP could also play a signaling role during growth of unwounded tissues, and published evidence for this is reviewed and discussed here. Although extracellular nucleotides are well-established agonists in animal cells, the evidence for their role in plants is thus far only indirect. The discovery and characterization of purinoceptors was the breakthrough that confirmed extracellular nucleotides were signaling agents in animals, and a similar identification of ATP-/ADP-responsive receptors in plants will be required to confirm that these agents are extracellular regulators of plant metabolism. [ABSTRACT FROM AUTHOR]

N-Acylethanolamines (NAEs) are minor lipid constituents of plant and animal cells, and their roles in mammalian physiology and neurobiology have been studied intensively for many years. However, corresponding studies on the function of NAEs in plants have appeared only recently. Within the last decade significant progress has been made in quantifying NAEs in plant tissues, characterizing their potential targets in plant cells and identifying the relevant enzyme involved in their degradation, but much remains to be determined regarding the role of these fatty acid amides in plant physiology. Although our understanding of the specific functions of NAE in plants is far from complete, recent advances in plant NAE biochemistry are pointing to intriguing similarities between animals and plants in the metabolism and perception of NAE. In this chapter we discuss NAEs as prospective signaling and regulatory molecules in plant cells. Advances in mammalian NAE research are presented when appropriate in order to draw parallels as well as to highlight differences between plant NAE metabolism and the endocannabinoid signaling system, the major pathway by which NAE exerts its physiological effects in animal cells. [ABSTRACT FROM AUTHOR]

The 20 genes that encode the Arabidopsis thaliana glutamate-like receptor family (AtGLR) share significant similarity in amino acid coding sequence and predicted secondary structure with animal ionotropic glutamate receptor (iGluR) subunits. In animals, iGluR subunits form glutamate-gated non-selective cation channels (NSCCs) catalysing Na+ and/or Ca2+ influx into cells; in one iGluR subfamily glycine also is required as a coagonist. In Arabidopsis, both glutamate and glycine have been demonstrated to depolarise the plasma membrane and increase [Ca2+]cyt, and iGluR antagonists blocked these effects. AtGLRs are therefore predicted to function in an analogous manner to iGluR. Attempts to functionally characterise AtGLRs in heterologous expression systems have proved inconclusive with no ligand-gated activity detected. Research into the glutamate receptor-like family has been hindered by the lack of phenotypes associated with the AtGLR genes but several phenotypes associated with AtGLR overexpression and knockout have recently given hints as to their function. AtGLR have been implicated in light and C:N signalling, hypocotyl detiolation, root growth, abscisic acid (ABA) metabolism, stress responses, and general ion transport. This review will concentrate on recent developments in the AtGLR field, including the roles and effects of glutamate and glycine and related metabolites in plant physiology relative to potential roles for AtGLRs. It will examine progress made toward defining the functions of particular AtGLRs and will conclude by recommending potentially fruitful avenues of future research. [ABSTRACT FROM AUTHOR]

γ-Aminobutyric acid (GABA) is a four-carbon non-protein amino acid conserved from bacteria to plants and vertebrates. In the latter it is mainly known as a neurotransmitter. The enzymes that synthesize and catabolize GABA constitute a metabolic pathway known as the GABA shunt which bypasses two steps of the tricarboxylic acid cycle. Functional genomics tools using Arabidopsis as a model system revealed that the GABA shunt is imperative for normal plant development and for response to stress, and suggest roles for GABA as an important metabolite as well as a potential signaling molecule. Moreover, γ-hydroxybutyrate, a by-product of the GABA shunt and a neurotransmitter in animals, was recently discovered in plants. Here we discuss the possible roles of these two neurotransmitters in plants with focus on components that underlie their roles as signaling molecules. [ABSTRACT FROM AUTHOR]

Amino acids (AA) are essential elements in animals and plants. In a genome-wide analysis in yeasts, plants and animals five AA transporter superfamilies could be identified. Transporters of one superfamily (ATF1, SLC38) which includes animal and plant members correspond to proteins involved in neurotransmitter transport in animals. Their close relation with plant genes suggests that a closely related transport mechanism could be involved in AA transport in plants. This chapter summarizes current knowledge on AA transport in animals and plants and the question of a possible common mechanism is discussed. [ABSTRACT FROM AUTHOR]

The transmission of signals between living cells is essential for the life of an organism as it provides the mechanism by which cells respond to external stimuli. Plants produce a wide range of phytochemicals that mediate cell function and translate environmental cues for survival and many of these are human neuroregulatory molecules. For example, the human neurotransmitter melatonin (N-acetyl-5-methoxytryptamine) is a ubiquitous, highly conserved molecule associated with timing of circadian rhythms in many organisms, including higher plants. Other compounds such as hyperforin, now isolated from several plant species, may function as serotonin transport or nonspecific cation channel activators in human brains and potentially in higher plants. A different group of neuroregulatory molecules produced by plants overstimulate human neurons, resulting in neuronal cell damage and death. Excitotoxins such as β-methylamino-L-alanine not only affect human health but are also regulatory molecules redirecting plant growth. Many fascinating questions in future research will be defining the role of neurotransmitters, neuroregulators and neurotoxins in the growth and development of plants. As newer technologies emerge, it will become possible to understand more about the role of neurological compounds in the inner workings of plant metabolism, plant environment interactions and the impact of plant neurosystems on human neurology. [ABSTRACT FROM AUTHOR]

Roots are plant organs that mainly function to acquire water and nutrients from soil. Root development is under the control of a regulated cell proliferation and morphogenesis, and auxin is the central plant hormone that governs those processes. In this review we discuss new aspects of the mechanisms that operate during root organogenesis. We particularly emphasize the analyses of downstream signals involved in the auxin control of root development. Nitric oxide (NO), an emerging chemical messenger that plays a significant role in a broad spectrum of plant developmental processes, is a key component in the signal transduction pathways that determine root architecture. Lateral root development as well as adventitious root formation are strictly NO-dependent processes in the auxin-promoted root organogenesis. [ABSTRACT FROM AUTHOR]

Most plants resist potential parasite attack using a variety of biochemical responses that often lead to a localized cell death termed the hypersensitive response, and include production of antimicrobial compounds, lignin formation, oxidative and nitrosative burst, and increased expression of genes related to pathogenesis. In this framework, nitric oxide (NO) functions together with reactive oxygen species in triggering hypersensitive cell death, and works independently of such intermediates in the induction of defense-related genes. In this chapter, we will examine the synthesis of NO and its signaling functions in the hypersensitive response and in the establishment of systemic acquired resistance. [ABSTRACT FROM AUTHOR]

Plants often respond to stimuli with dramatic rearrangements of their actin cytoskeleton. Identification of the signals and transduction machinery that lead to changes in actin dynamics and organization is therefore of considerable interest. The self-incompatibility (SI) response in the field poppy, Papaver rhoeas L., involves signal-mediated inhibition of pollen tube growth in response to interaction with incompatible S proteins. This triggers larges increases in [Ca2+]i and downstream a number of signalling components and targets are modified in incompatible pollen. We have observed dramatic alterations to the actin cytoskeleton in response to SI and established that this involves massive and sustained actin depolymerization. We have begun to identify and characterize several actin-binding proteins, including profilin and gelsolin, that may cooperate to transduce the signal from a Ca2+ wave into destruction of the cytoskeletal network that is essential for tip growth. Recently, we identified a role for programmed cell death (PCD) signalling cascades being triggered in SI and that a caspase-like activity is involved in mediating irreversible pollen tube inhibition. Our data suggest that there is evidence for crosstalk between the SI-induced signalling cascades. We are currently investigating whether the signalling cascades for actin alterations and PCD are linked and whether the actin cytoskeleton functions as a sensor of cellular stress and can initiate PCD. Our current knowledge of the signalling cascade in P. rhoeas pollen therefore involves both early and late responses that work in concert to ensure that pollen does not effect fertilization. Early cessation of tip growth is mediated by destruction of the actin cytoskeleton, and this appears to cross-talk with a subsequent PCD cascade that commits the pollen to die. [ABSTRACT FROM AUTHOR]

Innate immunity is an ancient form of defense against microbial infection that is shared by plants, insects and vertebrates. Like animals, plants have evolved perception systems for multiple, highly invariant pathogen-associated molecular patterns (PAMP) that trigger basal or non-cultivar-specific defense responses in plants. Non-self-recognition is brought about by specific plant receptors that are structurally similar to Toll-like receptors mediating PAMP perception and activation of innate immune responses in animals. In addition to PAMP-mediated plant innate immunity, disease-resistance programs are often also initiated through plant cultivar-specific recognition of microbial race-specific virulence factors, a recognition specificity that is not known from animals. Plant species and plant cultivar-specific resistance represent evolutionarily linked types of immunity that are collectively referred to as the plant innate immune system. Signal transduction cascades that mediate activation of innate immune responses comprise elements that are common to both forms of plant immunity, such as alterations in cytoplasmic calcium levels, mitogenactivated protein kinase activities or reactive oxygen species production. Forward genetic approaches have, however, identified loci that contribute specifically to either plant species or plant cultivar-specific immunity. [ABSTRACT FROM AUTHOR]

Branch death is an important component of the processes that generate tree form. Many branches are formed and the plant appears to “choose” the most promising alternatives for survival and further development. The purpose of this chapter is to consider how this choice could occur in the absence of a central decision center. Experiments were carried out on a model system of pea seedlings with two shoots. In most plants only one of these shoots continued to grow. We studied the conditions that influence the choice of the “winning” branch. These included removing entire branches or leaves at different ages and modifying the environment of the entire plant or of individual shoots. The evidence from experiments and from comparative observations of many trees supports the following hypothesis. All components of a shoot are sources of auxin and possibly of other signals. The level of auxin synthesis depends on the immediate environment and the developmental stage of individual leaves. The responses to auxin include the orientation of vascular differentiation towards organs that are its strongest source. This oriented auxin response results in competition between alternative organs. This is a prominent example of “developmental selection,” an alternative to developmental programs and prepatterns, which also has various other roles in the generation of biological form. [ABSTRACT FROM AUTHOR]

This chapter reviews the potential and actual role of plant neurobiological activity for integrating function at the whole plant level. Some support is provided for the drawing of analogies between the signaling roles of plant long-distance transport systems and animal nervous systems. However, the specific findings reviewed here do not indicate that root apices function as essential neurobiological command centers involved in regulating shoot growth responses to adverse changes in the root environment. [ABSTRACT FROM AUTHOR]

All principal metabolic biochemical pathways are conserved in animal and plant cells. Besides this, plants have been shown to be identical to animals from several other rather unexpected perspectives. For their reproduction, plants use identical sexual processes based on fusing sperm cells and oocytes. Next, plants attacked by pathogens develop immunity using processes and mechanisms corresponding to those operating in animals. Last, but not least, both animals and plants use the same molecules and pathways to drive their circadian rhythms. Currently, owing to the critical mass of new data which has accumulated, plant science has reached a crossroads culminating in the emergence of plant neurobiology as the most recent area of plant sciences. Plants perform complex information processing and use not only action potentials but also synaptic modes of cell-cell communication. Thus, the term ‘plant neurobiology’ appears to be justified. In fact, the word neuron was taken by animal neurobiologists from Greek, where the original meaning of this word is ‘vegetal fibre’. Several surprises emerge when applying a ‘neurobiological’ perspective to illustrate how the plant tissues and the plant body are organized. Firstly, root apices are specialized not only for the uptake of nutrients but they also seem to support neuronal-like activities based on plant synapses. These synapses transport auxin via synaptic processes, suggesting that auxin is a plant-specific neurotransmitter. Altogether, root apices emerge as command centres and represent the anterior pole of the plant body. In accordance with this perspective, shoot apices act as the posterior pole. They are specialized for sexual reproduction and the excretion of metabolic products via hydathodes, trichomes, and stomata. Next, vascular elements allow the rapid spread of hydraulic signals and classical action potentials resembling nerves. As plants are capable of learning and they take decisions about their future activities according to the actual environmental conditions, it is obvious that they possess a complex apparatus for the storage and processing of information. [ABSTRACT FROM AUTHOR]

Charles Darwin was always pleased to exalt plants in the scale of organised beings, drawing particular attention to the sensory properties of their roots. He even went so far as to say that the root tip acts like a plant brain, located within the anterior end of the plant body. What impressed Darwin was the ability of the root to perceive, often simultaneously, multiple vectorial stimuli, and then make a ‘decision’ about which bending response to follow. According to J.G. Miller’s ‘living systems theory’ (LST), developed mainly for human organisms and human societies, there are similar sets of 20 subsystems supporting each level of organisation, from cellular to organismic. If LST is a universal theory, it should apply to plant organisms also. About half of all the LST subsystems concern the processing of information. In the present plant-neurobiological context, the information-processing subsystem of particular interest is ‘channel and net’. In the light of recent discoveries from plant cell biology, earlier designations of structures to this subsystem are confirmed. They reinforce the idea that plants possess a form of nervous system — even though Darwin denied this particular proposition-which, moreover, makes use of molecules and organelles similar to those found in the neurotransmission systems of animals. The LST approach to plant life converges upon that already recognised for animals and, hence, provides a coherent conceptualisation for the structuring of the two major kingdoms of plants and animals. [ABSTRACT FROM AUTHOR]

Intelligence is an aspect of complex adaptive behaviour and a term not normally applied to plants. This chapter indicates a change in concept is long overdue and if poets can recognize it (above) so should scientists. Networks that control information flow are described as intelligent and such networks exist in all single living cells and in more complex multicellular organisms. Phosphoneural bacterial networks are briefly considered and these exist in a slightly different molecular but more complex form in higher plant and a nimal cells. Intelligent behaviour involves the whole organism and such integration involves complex communication. Evidence that plants forage and act intelligently in acquiring resources is indicated. The phenotype is actively (not passively) constructed in response to a complex changing environment by decisions that best secure the well-being of the individual plant within the life cycle goal of optimal fitness. [ABSTRACT FROM AUTHOR]

Structural sterols are integral components of biological membranes. They regulate membrane permeability and fluidity, and they influence the activity of membrane proteins. In Arabidopsis, their composition is critical for normal plant development. The endocytosis and recycling of plasma membrane sterols display similar pathways as some polarly distributed proteins, and thus sterol-dependent trafficking can be an integral part of the polarity establishment in plants. Here, we summarise recent data about sterol endocytosis and sterol trafficking within endocytic pathways in different aspects of cell development in plants. [ABSTRACT FROM AUTHOR]

Recent advances in molecular cell biology have provided new insights into different cellular processes that all turn out to contribute to polarized cell growth in a variety of model systems used to analyse growth, differentiation and development. Polarized cell growth, although a general feature of the living cell, can be found in a pronounced fashion during pollen tube outgrowth and root hair development in plants, during neurite outgrowth, and during filamentous hyphal growth. Filamentous fungi represent excellent model systems to analyse polarized cell growth owing to their genetic tractability and the ease of generating and keeping mutant strains. Contributing to this is the fact that already a number of fungal genomes have been sequenced, which allows the rapid analysis and comparison of gene function. This has led to the finding that polarized cell growth can be influenced by perturbations in different cellular pathways. Control of polarity establishment and the maintenance of polarized cell growth are exerted by a number of conserved GTP-binding proteins of the Ras/Rho subfamily and their specific regulators that organize the actin cytoskeleton. Hyphal tip growth requires coordination of vesicle transport using actin and microtubule cytoskeletons. Recent evidence has shown that hyphal growth not only depends on polarized secretion but also requires endocytosis, suggesting that the recycling of the membrane and sorting of vesicles is required for fast elongation of hyphal tubes. Key players on the molecular level that direct tip growth and endocytosis in the fungal hyphae based on differential regulation of the actin cytoskeleton are discussed. [ABSTRACT FROM AUTHOR]

Stomatal movement requires large and repetitive changes to cell volume and consequently surface area. These alterations in surface area are accomplished by addition and removal of plasma membrane material. Recent studies of membrane turnover in guard cell protoplasts using electrophysiology and fluorescence imaging techniques implicate that exocytosis and endocytosis are sensitive to changes in membrane tension. This may provide a regulatory mechanism for the adaptation of surface area to osmotically driven changes in cell volume in guard cell protoplasts as well as turgid guard cells. In addition guard cells also exhibit constitutive membrane turnover. Constitutive and tension-driven membrane turnover were found to be associated with addition and removal of K+ channels. This implies that some of the exocytosis and endocytic vesicles carry K+ channels. Together the results demonstrate that exocytosis and endocytosis are essential for stomatal movement and thus gas exchange in plants. [ABSTRACT FROM AUTHOR]

In plants, tip-growing cells are an ideal system to investigate signal transduction mechanisms and, among these, pollen tubes are one of the favourite models. Many signalling pathways have been identified during germination and tip growth and, not surprisingly, the apical secretory machinery, essential for tip growth, seems to be an intersection point for all these pathways. Here we review previous data on the pollen tube endocytic machinery and its coupling to the exocytic delivery of new cell wall material. Additionally, we discuss new methodologies and how these are shaping our current working hypothesis to explain endocytosis in pollen tubes. [ABSTRACT FROM AUTHOR]

Symbioses are widespread in nature and occur between organisms that belong to a large variety of taxonomic divisions (Hentschel et al. 2000). Most often, only two partners are involved and the outcome may be either beneficial to both, i.e. mutualism, or detrimental to one of them, i.e. parasitism. Mutualism varies from simple protection against a hostile environment to an intimate cohabitation with exchange of essential nutrients. Important and well-studied examples are the symbiosis between nitrogen-fixing bacteria and plants of the Leguminosae family (approximately 750 genera and 20 000 species) and the arbuscular mycorrhizal interactions that involve more than 80% of land plants with fungi of the Glomeromycota. In the first case, plants profit through the supply of a nitrogen source, and in the second, through an uptake of phosphate. The microsymbionts benefit through the acquisition of carbon sources in a specific and exclusive ecological niche. In both types of interactions, the microsymbionts invade the plant host and the nutrient exchange takes place inside specialised plant cells. The establishment of the symbiosis is a complex process that requires the coordinated action of both symbionts and most probably the involvement of endocytosis in a number of critical events. In this chapter, we will describe both types of endosymbiosis in view of endocytosis and endocytosis-like processes. [ABSTRACT FROM AUTHOR]

Over the past decade, it has become evident that multiple endocytic pathways operate in eukaryotic cells, and several of these are dependent on dynamins and dynamin-related proteins (DRPs). Many members of the DRP superfamily possess the ability to self-assemble into long spiral polymers that wrap around lipid bilayers and thus facilitate tubulation and vesicle pinching from the plasma membrane and other membrane compartments, a process that is fundamental for endocytosis. Here, we discuss the roles of dynamins and DRPs in plants. DRPs have been shown to be present at different subcellular locations in plant cells including the cell plate, plasma membrane, Golgi apparatus, vesicles, mitochondria, chloroplasts, and peroxisomes. Arabidopsis contains 16 DRP members that are grouped into six functional subfamilies (DRP1--6) on the basis of their phylogeny and the presence of functional motifs. Members of the DRP1 subfamily are closest to soybean phragmoplastin and mediate membrane tubulation at the cell plate. The DRP2 subfamily members represent the bona fide plant dynamins characterized by the presence of a pleckstrin homology (PH) domain in the middle of the molecule and a proline-rich (PR) motif near the C-terminus ; they are involved in membrane recycling at the cell plate and the trans-Golgi region. The DRP3 subfamily does not contain PH or PR motifs; their function has been implicated in the division of mitochondria and peroxisomes, whereas the DRP5 subfamily in Arabidopsis is likely to play a role in plastid division. A DRP5 ortholog from the red alga Cyanidioschyzon has recently been shown to be a component of the chloroplast outer division-ring on the cytoplasmic face of the plastid double membrane. Finally, the DRP4 subfamily contains orthologs of the animal antiviral Mx proteins, but their function has not yet been established, and the role of DRP6 is entirely unknown so far. It is obvious that plant cells employ unique DRP subfamilies to carry out the mechanochemical work required for membrane deformation and segregation in various membranous compartments. However, to understand the function of DRPs in further detail, much is yet to be learned about the proteins that apparently interact with them to regulate their activity and specify their functions. [ABSTRACT FROM AUTHOR]

Mutual interactions between actin and endocytic assembly machineries are essential for successful clathrin-mediated endocytosis in yeast and mammals. The actin cytoskeleton is indispensable for endocytic internalization and for short-range transport of endocytic vesicles. In plants as well, actin seems to be essential for endocytic recycling of plasma membrane proteins and sterols, but surprisingly also for the turnover of cell wall pectins, which have been identified as a major cargo of endocytic vesicles. Endosomes in animal cells perform long-range movements along microtubules, whereas plant endosomes use preferentially an actin polymerization mechanism but also actin tracks for their short- and long-range movements, respectively. Thus, the actin cytoskeleton not only assists endocytic internalization and is in fact inherently associated with endosomal vesicles and endosomes, but also is responsible for their movements at the cell cortex and for their targeted delivery into the cell interior. [ABSTRACT FROM AUTHOR]

In eukaryotic cells, the transport vesicles carry various cargo proteins from a donor compartment to a target compartment, and discharge the cargo into the target compartment by fusing with the membrane of the target compartment. SNARE molecules have a central role for initiating membrane fusion between transport vesicles and target membranes by forming a specific trans-SNARE complex in each transport step. In higher plants, the numbers of SNARE molecules are greater than those of yeast and mammals, suggesting a higher complexity of membrane traffic in higher plant cells. In this chapter, we will focus on the functions and subcellular localizations of plant SNARE molecules and discuss the complexity and evolution of endocytosis and the post-Golgi traffic in the higher plant cells. [ABSTRACT FROM AUTHOR]

Auxin is an essential regulator of plant growth and development. Polarized transport of auxin is responsible for apical dominance, tropic growth, and organ development. Previous studies have demonstrated that the polarized movement of auxin is dependent upon the action of polarly localized, endocytotically cycled PIN auxin efflux facilitator proteins. More recently, plant orthologs of mammalian multidrug-resistance (MDR)/P-glycoprotein (PGP) type ABC transporters have been shown to function in auxin transport. In this review, the PGP nomenclature/numbering system established by Martinoia et al., (Planta 214:345--355,2002) is used, as there is increasing evidence that in plants MDR/PGPs function as PGPs and not as multiple specificity MDR proteins. Defects in PGP1 and PGP19 (MDR1)genes result in decreased auxin transport and reduced growth phenotypes in Arabidopsis (pgp1, pgp19), maize (br2), and sorghum (dw3). Further, dwarf phenotypes are more severe in Arabidopsis double mutants, indicating that PGPs have overlapping functions. More recently, MDR/PGPs have been shown to function as ATP-activated hydrophobic anion transporters capable of auxin transport. Further, MDR/PGPs have been shown to stabilize PIN1 in detergent-resistant membrane microdomains, and synergistic MDR/PGP-PIN interactions have been shown to increase the rate and specificity of MDR/PGP-mediated auxin transport. Several lines of evidence indicate that, like their mammalian counterparts, Arabidopsis MDR/PGPs are regulated via endocytic cycling. Here we review the evidence for endocytic cycling of MDR/PGPs in planta and provide a model by which this cycling could occur. [ABSTRACT FROM AUTHOR]

The Rab family is part of the Ras superfamily of small GTPases. In eukaryotes Rab GTPases are present as members of gene families, and the different Rab GTPase isoforms are localized specific intracellular membranes, where they function as regulators of distinct steps in membrane traffic pathways. They perform these regulatory functions through the specific recruitment of cytosolic effector proteins onto membranes. This recruitment occurs when the Rab GTPase is in the GTP-bound, or active, form. Through these recruited effector proteins, Rab GTPases regulate many aspects of membrane trafficking including vesicle formation, actin- and tubulin-dependent vesicle movement, and membrane fusion. The recent sequencing of complete genomic sequences from animal, yeast, and plant organisms has revealed that a number of Rab GTPase families are conserved from yeast to animals and plants. The plant model system, Arabidopsis thaliana, contains 57 Rab GTPases, of which 40 distinct Rab GTPase members of four subfamilies RabA (26 members), RabC (three members), RabF (three members), and RabG (eight members) share significant similarity with Rab GTPases implicated in endocytic events in animals and yeast. In this review we will highlight recent observations of the function of some of these plant Rab GTPases during endocytosis in plants, and discuss possible roles of plant endocytic Rab GTPases in relation to what is currently known in animal and yeast systems. [ABSTRACT FROM AUTHOR]