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Cyclical Ketogenic Diets Review

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There is a wide range of inorganic and organic solutes in plants. Chapter 2 is an introduction to methods for their extraction and analysis. Inorganic elements can be measured by optical properties (by flame emission and atomic absorption spec-troscopy), mass spectroscopy, X-ray fluorescence, with ion-specific electrodes, and by ion chromatography. Analysis of organic solutes is usually achieved by chro-matographic separation, often in conjunction with mass spectroscopy and nuclear magnetic resonance. Intracellular localisation can be achieved either via transmission or scanning electron microscopy preceded by precipitation, freezing or freeze-substitution. Ion-specific intracellular electrodes can also be used, as can direct sampling using a modified pressure probe. Individual ions can be monitored in cells loaded with fluorescent probes, and tracer fluxes can be interpreted using analysis of compartmental models. Chapter 2 also introduces the roles of solutes in the vacuole, cytoplasm, organelles and cell walls.

Chapter 3 begins by describing the properties of water that are important to its behaviour in biological systems: the hydrogen-bonding that confers structure and order, latent heat, thermal capacity, tensile strength, surface free energy (tension) and incompressibility. The large dielectric constant gives water its solvent properties, its ability to perform charge shielding and provide hydration shells, which link to its roles in maintaining the higher order structure of macromolecules.

It is difficult to understand how plants acquire and transport solutes without understanding the physical bases of ion and water movement. What are the driving forces? Which way do ions and water 'want' to go? How do plants move and accumulate solutes against physical and chemical gradients? Chapter 3 continues with a consideration of Gibbs free energy and chemical potential, water potential and water potential gradients, osmosis and other colligative properties. It includes the derivation of equations for water movement in cells and in the soil-plant-atmosphere system (resistances and the Ohm's law analogy), and of how surface tension develops negative hydrostatic pressures in drying soils and cell walls. The chapter then moves on to solute movement; diffusion and Fick's law, and to permeabilities and fluxes. The contribution of electrical charge is explored in the derivation of the Nernst equation, Donnan systems and the Goldman equation. Finally, irreversible thermodynamics is introduced as it applies to the analysis of coupled flows of solutes and solvents. With this background, the subsequent section of chapters (4 to 10) looks at how solutes are moved at individual membranes and, on an increasingly integrative scale, within and between cells and around the plant, both up in the xylem and down in the phloem.

Chapter 4 considers the structure and composition of plant membranes - of which there are about 20 types, all comprised of lipids, proteins and carbohydrates in the approximate ratio of 40:40:20. The amphiphatic nature (both hydrophobic and hydrophilic) of lipids underlies the formation of bilayer membranes. These have little intrinsic solute permeability. This is conferred in biological membranes by embedded transporter proteins mediating either active or passive movement and providing varying degrees of regulation. The overall structure of membranes is currently considered to consist of lipid-ordered microdomains, with rather less freedom of movement in the plane of the membrane that was inherent in the first fluid mosaic model. The transport proteins are often multimeric and distributed in membranes in clusters.

Techniques for studying solute transport in membranes are discussed next beginning with those applicable to intact (or semi-intact) tissues and moving on to adaptation of these techniques for use with isolated membranes. There is an emphasis on design and composition of experimental solutions, particularly their os-molarity, and on consideration of unstirred layers, and the difference between the study of net transport and unidirectional fluxes. Methods available include inhibitor studies, radioactive tracers, fluorescent probes and electrophysiology - the last including multi-barrelled electrodes. Individual membrane types can be isolated via protoplasts, and sometimes by direct mechanical means, separated by differential centrifugation and identified by marker analysis. Aqueous polymer two-phase isolation provides information regarding sidedness. Analysis can be performed on vesicles or tiny pieces of a membrane attached to a micro-electrode. Techniques such as fluorescence microscopy and patch-clamping can yield considerable spatial and temporal resolution, enabling the detection of the activities of single ion channels.

Molecular techniques now allow the in silico characterisation of the possible function of membrane proteins where there is sufficient information on available databases. Forward and reverse genetic screens can be used to endeavour to relate gene to function, as can the use of over-expression and expression in heterologous systems (generally in yeasts or in Xenopus laevis oocytes). The location of proteins within the plant and cell can sometimes be determined by expression using reporter gene constructs. For all techniques of investigations there is a compromise between resolution and invasiveness (or distance from physiological reality). The importance of confirming a result obtained with one technique using a different approach, cannot be overstated.

The details of transport across membranes is considered for simple inorganic solutes, anions and cations (see Chapter 5). Any membrane protein involved in cross-membrane movement of substrates is defined as a transporter. Transporters can be classified as to whether the event they mediate is active or passive, and if it is active, whether it is a primary process or a secondary one utilising energy already stored in proton gradients. Transporters may also be classified as pumps, ion channels, or carriers - the last includes the provision of passive transport at higher selectivity but lower capacity than ion channels. A further form of classification is that of uniport, symport and antiport. All these terms will be met in different combinations in the literature. Broadly speaking, primary, ATP-driven, pumps set up proton gradients to drive secondary transport. Primary pumps are also directly involved in the transport of calcium and heavy and transition metals. Secondary transport in plants is generally coupled to proton gradients, and participates in the uptake and movement of hundreds, if not thousands, of different substrates. Finally, it is the ion channels that are almost exclusively responsible for passive transport. They mediate only passive transport and are either open or closed, known as gating, which may be regulated by voltage, ligands, or may be mechano-sensitive (e.g. stretch-activated). In addition, there is a selectivity filter that operates on the basis of physical size and charge properties. Channels may be inward- or outward-rectifying according to whether they are permitting passage into or out of the cell. The transport rate of channels may be millions per second. Water movement across membranes is always passive and directed by the gradient in water potential. Water may cross membranes via their intrinsic permeability to water and also through proteinaceous pores: aquaporins. The selectivity of aquaporins is related to size and they have very high capacity (over 109 per second).

Primary pumps use chemical, redox or light energy to move solutes against their electrochemical gradient. ATPases have low capacity (around 100 per second), and consequently large numbers of these proteins are required. There are also primary pumps for calcium and some other metals, such as for copper in chloro-plasts. Transport rates of primary pumps are hundreds per second. Secondary active transport pumps solutes against their free energy gradient, but the energy derives from coupling to the proton gradient set up by the primary pump(s) and can be either symport (in the same direction as protons) or antiport (in opposite directions). These are often termed carriers, as they are neither primary pumps nor channels. Such carriers have higher selectivity than channels but lower capacity (hundreds or thousands per second). Major nutrients, such as potassium, are taken up through channels at high external concentrations, and by active processes that are induced upon potassium starvation, at low external concentrations.

Plants have many transport processes that need to operate in different ways to address different environmental conditions and developmental stages, as well as differences between different cells and tissues. The processes require regulation (Chapter 6), which occurs at several levels, e.g. gene expression, mRNA degradation, protein turn-over, protein activity and membrane trafficking. Regulation involves both positive and negative feedback, and the transporters themselves are both components and targets of signalling pathways (e.g. calcium, auxin and ABA). Chapter 6 considers examples of the regulation of transporters in adaptive processes, the molecular mechanisms underlying transcriptional and post-transcriptional regulation, and the regulation of transporters by membrane trafficking.

Regulation of solute transport is required to effect changes in cell volume, both for sustained growth and for the cyclical changes in volume needed in stomatal guard cells for control of stomatal aperture. The pathways leading to co-ordinated regulation of potassium and chloride channels during stomatal closure are examined. High-affinity uptake of nutrients is often induced by deficiency situations, since there may be less costly pathways of uptake when the same nutrient is in abundant supply. Some transporters are induced by a change from high supply to low supply, and some transporters are induced by a change from nil to low supply. Fine-tuning may be via differential regulation of apparently functionally redundant isoforms. Nutrient transport is regulated not only by availability but by the nutrient status of the plant. Transport is also linked to carbon status, and thus is controlled indirectly by environmental factors that affect photosynthesis.

Response to many environmental stresses is dependent upon regulation. For example, 'unwanted' entry of sodium into the root cells in saline conditions will lead to membrane depolarisation, which will open depolarisation-activated calcium channels leading to a rise in cytoplasmic calcium activity, which is in turn a signal to enhance the activity of the sodium-proton antiport carrier at the plasma membrane, which pumps sodium out again. The limited information regarding the molecular components of the transcriptional regulation of nutrient transporters are summarised. Post-transcriptional regulation involves auto-inhibitory domains, protein-protein interactions (e.g. with protein kinases, calmodulins and 14-3-3 proteins), and ligand binding (e.g. ion channel gating by cyclic nucleotides). The 14-3-3 proteins are highly conserved and regulate a wide range of targets including a number of ion channels. Calmodulins are small calcium-binding proteins that are able to translate intracellular calcium signals into a variety of cellular responses. Cyclic nucleotides are widely used in signal transduction, and evidence is building that higher plants use cGMP as a secondary messenger. Finally, the role of membrane trafficking is reviewed. SNARE (soluble NSF attachment receptor) proteins have been identified in higher plants; they are a group of membrane proteins that are highly conserved in eukaryotes and are at the centre of the molecular machinery involved in vesicle trafficking and membrane fusion.

Plant processes involve a complex traffic between organelles, and between organelles and the cytoplasm. Organelles have their own transport systems and these are integrated with cellular metabolism (Chapter 7).

Chloroplasts are part of the plastid family that includes storage plastids and amyloplasts. They contain the light-harvesting centre and the photosynthetic electron transport chain. Chloroplasts have distinct outer and inner membranes, plus the thylakoid system. The outer envelope (OE) has a range of proteins (OEPs) which are selective channels for solutes essential to plastid function. The inner membrane contains the phosphate translocator family and members of the major facilitator superfamily. There are transporters for di- and tri-carboxylates and carbohydrates, for ATP/ADP exchange, and for a range of specific ions (including nitrate and sulphate which are reduced in the plastid) and there are also symporters for transition metals.

Mitochondria are semi-autonomous organelles with a smooth outer membrane and a much-folded inner membrane, which is the energy-transducing membrane. The compartments are the intermembrane space and the protein-rich core, or matrix. One key role of mitochondria is the synthesis of ATP formed by oxidative phos-phorylation - the PMF generated by the respiratory chain drives the ATP synthase complex. The outer membrane contains the VDAC porin which is freely permeable to solutes of up to 4-5 kDa: specific permeability barriers reside with the inner membrane. Carriers on the inner membrane include the phosphate carrier, the ATP/ADP carrier, and carriers for intermediates of the tricarboxylic acid cycle, amino acids, and a carrier for succinate/fumarate (which links j-oxidation in the peroxisomes with the TCA cycle). There are also ion channels for potassium and calcium.

Peroxisomes are bounded by a single membrane. They are involved in j-oxidation and are part of the photorespiratory cycle; they also generate reactive oxygen species and contain appropriate protective mechanisms. Glyoxisomes convert lipid reserves to sucrose. The peroxisome family has a 'specific porin' as well as transporter proteins including the peroxisomal ATP/ADP carrier. The pho-torespiratory pathway is split between the chloroplast, mitochondrion and per-oxisome.

Vacuoles are multifunctional and are involved in the storage of different metabolites, quantitatively extreme examples being malate (in CAM) and sucrose (in storage tissue). The vacuole is the largest organelle and usually comprises the major volume fraction: it is bounded by a single membrane, the tonoplast. The tonoplast contains proton ATPases and pyrophosphatases which together generate a PMF. A major facilitator imports malate. Tonoplast intrinsic proteins (TIPs, aquaporins) mediate water flow. There are ABC transporters for the accumulation of secondary metabolites and xenobiotics. There are a range of ion channels and carriers mediating the movement of solutes needed for cell expansion, guard cell movement, and compartmentalisation (such as of sodium).

Chapter 8 addresses the main factors affecting and controlling the uptake of charged solutes by plants, from the soil solution to the transpiration stream. It describes root anatomical and physiological responses to the availability of nutrients in the soil and the general processes involved in the transport of solutes into and out of root cells. The Casparian strip blocks apoplastic radial movement of water and solutes when it develops, and in many species this barrier is backed up with an hypodermis. Some leakage may occur, particularly when lateral roots are initiated. There is also a symplastic continuity from cell to cell via plasmodesmata. Root hairs are modified epidermal cells that increase surface area and root radius, and appear to be most important in the acquisition of immobile nutrients. In some instances, epidermal cells are modified as transfer cells. The cells of the cortex may be involved in nutrient uptake depending upon whether the epidermal cells can satisfy the needs of the plant and upon whether they have already depleted the concentration to which the cortex is exposed. The tissue and cell expression pattern of high-affinity transporters varies between different nutrients. Cortical cells may also be involved in re-uptake of nutrients that have been effluxed by cells in outer layers of the root.

Uptake varies along the length of the root, being minimal at the apex (which is phloem-supplied). Root hairs are usually concentrated behind the apex. Uptake of mobile nutrients may occur along the root but uptake of immobile nutrients is mainly near the tip. Uptake of calcium occurs in young roots only where an apoplastic radial pathway remains available. Xylem loading varies longitudinally, clearly affected by the stage of xylem development. Xylem loading is independent of initial uptake, at least for some solutes. There is evidence that shoot requirements can dictate root uptake and translocation rates. Net uptake is the sum of influx and efflux, and the latter can be a very high percentage of the former. Analysis of tracer uptake is usually related to a three-compartment model: (1) the cytosol of cells of the outer root, (2) vacuoles and (3) transport to the shoot. After the initial uptake, filling of vacuoles and transport to the shoot are in parallel. The kinetics of tracer uptake have been interpreted as dual isotherms since the 1960s. This is now considered to represent the co-existence of low-capacity-high-affinity systems at low external concentration, and high-capacity-low-affinity systems at higher external concentration. These may be either channels or carriers.

The xylem has evolved for long-distance upward transport of water and solutes (Chapter 9). The xylem has a large capacity to carry the replacement of transpira-tional losses and is a leak-proofed conduit with the mechanical strength to avoid collapse under negative hydrostatic pressure. Xylem comprises vessels and tracheids (collectively, tracheary elements, the conducting pathway), fibres and parenchyma (the only living cells in the xylem). The xylem parenchyma cells are densely cytoplasmic with ER, ribososmes and mitochondria. Vessel elements are 5-500 ^m (typically 40-80 ^m) in diameter and joined end-to-end via perforation plates into vessels that may be several metres long. Tracheids are 10-25 ^m and are interconnected by pit fields at their overlapping, tapered ends. The classic interpretation of water movement in the xylem is the cohesion-tension theory. There have also been additional mechanisms suggested which include: mucopolysaccharides to help maintain water flow, osmotic water lifting (root pressure), ionic control of xylem conductance and an electrical driving force.

The concentration of major solutes in the xylem is mostly in the mM range though these concentrations are variable between species and may depend upon shoot demand. The osmotic pressure of the xylem is usually not considerable. Sampling of the xylem is difficult because most methods are very invasive, though there has recently been use of xylem-feeding insects. Loading of potassium into the xylem is probably via depolarisation-activated outward-rectifying potassium channels. There are three types of anion channels involved in xylem loading, and this is mostly a passive process. Sodium loading could be via a non-selective channel but probably via sodium-proton antiport. Unloading of solutes from the xylem into the leaf is plausibly under hormonal control, and a complex network of veins exists to reduce damage due to excessive concentration of xylem contents when water is withdrawn. Proton ATPases are probably the driving force behind both active and passive unloading, with co-transport processes important, for example, for sugars.

The other long-distance transport system in plants is the phloem (Chapter 10). The transport pathway consists of sieve tubes which are an end-to-end arrangement of sieve elements (each 40-500 pm by 5-50 pm) joined at a sieve plate. The plate is perforated by pores and is the major resistance to flow. The sieve tube contains mitochondria but is anucleate. Sieve tubes may live many years and have protection against oxidative damage. The other main component of the phloem is the companion cells which are connected to the sieve tubes via plasmodesmata, through which all proteins destined for the anucleate sieve element must pass from the companion cell.

Analysis of sieve tube contents has been made mostly using phloem-feeding insects or by bark incision. The major carbohydrate in most species is sucrose, at hundreds of mM, though in some species the major transported carbohydrate differs; for example, sorbitol, raffinose or mannitol. Potassium and sucrose are the major osmotica and there is a reciprocity between them, maintaining turgor pressure with varying carbon supply. Phloem transports many other nutrients and, recently, the implications of the transport of mRNA and proteins is complementing and revising the understanding of the phloem. Over 200 proteins have been identified, although sieve elements are unable to synthesise proteins themselves.

Osmotic pressure in sieve tubes at ground level is generally 1-2 MPa. Phloem is loaded at sources (sites where there is synthesis, as in photosynthesis, or else breakdown of storage compounds) and solutes are removed at sinks (where contents are diluted, metabolised, or stored elsewhere). Turgor pressure differences between sources and sinks underlie the pressure flow hypothesis of bulk movement in the phloem. Solutes move into the sieve element from the companion cell. Entry of solutes into the companion cell can take one of the two routes, apoplastic or symplastic. Much evidence favours the former. Sucrose is loaded by a proton co-transport carrier, powered by the PMF set up by the proton ATPase. Loading of potassium into the sieve element-companion cell complex is important both in the transport of potassium in the phloem and in the regulation of the loading process itself. Aquaporins are also present, as are transporters for the loading of many other substances. Unloading may be by either symplastic or apoplastic routes; this differs with species, organ, and stage of development.

In the third section of the book we set out to put this information in an ecological and agricultural context (Chapters 11-15). We describe the factors, other than the transport processes themselves, which limit the supply of nutrients to plants in field conditions and even when growing in carefully tended artificial environments. Next, we look at deficiency and toxicity; some of the ways in which plants have evolved to cope with the 'not enough' and 'too much' of elements and minerals in their growth environment. We then go on to look at how the use of solutes, both in quantity and quality, has been adapted to more extreme environments: the demands of hot, dry deserts, freezing mountains and saline marshes. All of these entail dealing (by avoidance or tolerance) with some form of externally imposed dehydration. There is also a crucial stage in the life cycle of most plants, the internally controlled dehydration concomitant with seed formation. This is true desiccation tolerance and, while this is common place during reproduction, it is very rare in the vegetative tissues of vascular plants.

Many factors, in addition to the properties of the transport processes themselves, affect the rate of uptake of nutrients by plants (Chapter 11). Plants are able to take up nutrients from concentrations that are very low in comparison with those in the soil solution, certainly in fertile soils; except in the case of phosphorus which is commonly at limiting concentrations. Although present, nutrients are not always available: many processes affect the supply of nutrients from the bulk (soil) solution to uptake sites on the roots of plants. These include bioavailability and mobility (the rate of diffusion is impeded by absorption on, and chemical interaction with, the soil). Mass flow of soil solution provides a large-capacity route of nutrient supply, but the contribution of bulk flow to nutrient supply decreases with the decrease in soil water content. There will always be boundary (unstirred) layers around the root in which movement is principally by diffusion. Whenever the flux density of uptake exceeds the flux density of supply, there will be depletion zones around the root, greater than the unstirred layer, across which nutrients must also diffuse. Since diffusion becomes less effective as the distance increases, such supply is commonly limiting, and in many situations the rate of transfer across boundary and depletion zones limits the rate of uptake by the plant.

Distribution of nutrients in the soil is also heterogeneous in both space and time, and interception of nutrients also involves roots exploring and exploiting new volumes of soil. Uptake of nutrients depends on both affinity and capacity (flux density) of transport processes. High affinity transporters may provide enough capacity to avoid deficiency of major nutrients and sufficiency of trace nutrients, but are not able to supply the quantitative needs of the plant to support rapid growth. A spectrum of transport processes exists with lower affinity, higher capacity alternatives providing the uptake at higher external concentrations. Concentrations of major nutrients in the xylem are generally in the mM range, and external concentrations in the same range are generally needed to support maximal growth, even in well-mixed solutions, even though Km values for high-affinity transporters are often in the /M range. Maintaining optimal growth in horticulture increasingly relies upon the controlled supply of nutrient solution to the plant in hydroponics, which has many advantages as well as some disadvantages. Phosphorus stands out as the major nutrient that is commonly limiting to plant growth in field situations, mainly due to low bioavailability rather than chemical deficiency.

A total of sixteen elements are essential to the growth of all plants, and a further four have been demonstrated to be essential in some species (Chapter 12). Plants have evolved mechanisms to maximise uptake of minerals that are in limiting supply. The two strategies for the acquisition of iron in neutral and alkaline soils, where iron is present in quantity but unavailable, are discussed. Plant responses centre either on reduction of ferric to ferrous iron in the soil or on chelation of ferric iron, uptake and subsequent reduction. Another example concerns the acquisition of phosphate, which also includes alteration of conditions in the soil as well as the development of cluster roots and symbiotic associations.

Even 'essential' elements may be present at external concentrations that can elicit uptake to toxic internal concentrations. This is most widespread for aluminium and manganese in acidic soil. Aluminium is used as an example of how plants can detoxify metals in the soil and tolerate them in the plant. The process commonly involved is chelation, either pre-emptively in the soil by secreting exudates, or within the plant by using chelates combined with compartmentalisation. Toxicity also arises from non-essential elements, particularly transition and heavy metals. Although locally significant, these are rare events both geologically and anthropogenically. Because of this, there will have been little selection pressure to develop specific metal detoxification systems and such tolerance as exists is thought to have arisen from serendipitous recruitment of existing processes (the phytochelatins). A group of plants known as hyperaccumulators achieve enormous concentrations of metals in their tissues. Although phytochelatins, which may have evolved to provide homeostasis for essential metals, can cope with low-level chronic exposure, the hyperac-cumulators function by compartmentalisation of metals in the vacuole. There is evidence that high concentrations of metals in the leaves can deter herbivory and this has been advanced as an evolutionary explanation for their extraordinary metal contents.

Water availability (Chapter 13) is a major factor in the zonation and distribution of plants, with nearly half of all land being classified as dry land. Terminology around avoidance and tolerance is confusing and difficult, but a three-stage concept of drought has a clear mechanistic and physiological basis. Essentially, these stages are (I) water status can be maintained even with stomata open, (II) stomatal control of water status as water availability decreases, and (III) inability to control water status even with stomata closed. The model also helps clarify the blurred distinction between the agricultural and ecological agendas when it comes to coping with drought: these agendas are often in opposition. Ecological success is linked with survival to complete the life cycle, even if this means slowing down or shutting down as water availability decreases, including pre-emptive adaptations to reduce water usage. Agricultural success is concerned with using as much water as available, and maintaining photosynthesis under drought, in order to produce maximum yield.

Plants respond to water deficit in many ways ranging from rapid regulation of stomatal conductance to constitutive anatomical modifications seen in desert species.

Reduction in stomatal conductance and leaf area will not only reduce water loss, and conserve soil water, but also will reduce growth and yield. Transpiration is also central to heat dissipation in hot climates, and there are thermal considerations linked intimately with water conservation. Solute accumulation is most often considered in relation to osmotic adjustment. Although osmotic adjustment is clearly important in ecological survival, its role in improving yield in agricultural contexts has been severely challenged. Solutes may also be important as compatible solutes, in drought as in any situation that leads towards reduced hydration, though demonstration of a physiological role requires that compartmentalisation be sufficient.

Solute transport underpins the photosynthetic adaptations of CAM and C4 photosynthesis through the storage and transport of fixed carbon as malate. Both provide substantial increase in water use efficiency, and C4 photosynthesis is associated with high productivity.

Water deficit has been shown to affect the expression of numerous genes. Significantly, the way in which deficit is applied accounts for most of the differences in expression. This underlines the essentiality of applying treatments that are physiologically relevant. Commonly-affected genes were generally involved in down-regulation of growth, again emphasising the difference between agricultural objectives and what plants 'naturally tend to do'.

Salinity (Chapter 14) is unusual amongst stresses in that the adapted native flora is not particularly stressed - salinity stress is mainly an agricultural event. Excess salt uptake damages plants in the long term when salt accumulates in the cytoplasm or cell walls, particularly in leaves that are at the end of the line in the transpiration stream. Even halophytes 'exclude' most (perhaps 90%) of the salt in the medium, but are well able to manage the remainder. Species that are more salt-sensitive rely on limiting salt uptake and require near-perfect exclusion, close to 98%; this is both very expensive in terms of active extrusion and leaves many questions about achieving osmotic adjustment unanswered. Exclusion is a viable option only at very low external concentrations, and halophytes optimise the regulation of salt transport to the shoot rather than depending on exclusion. However, variation in salt tolerance in crops is usually associated with reduced salt uptake and so is diametrically opposed to the mechanisms that confer salt tolerance in halophytes.

Non-selective cation channels, high affinity potassium transporters, and LCT1 have emerged as the potential pathways for sodium entry. In non-halophytes, this (largely unwanted) sodium entry is opposed almost entirely by active extrusion. Despite the damaging consequences, sodium is, in most scenarios, moved actively into the xylem by proton antiport. This is perhaps demanded by the needs of root ion homeostasis. A range of 'scavenging' processes (reabsorption and retransloca-tion) exist to recover excess salt uptake, but are of limited capacity; large capacity would depend on the ability to actively efflux the recovered sodium in situations where efflux is already unable to limit net uptake. Compartmentalisation of salt in the vacuole, together with synthesis and localisation of a compatible solute in the cytoplasm, is central to the tolerance seen in halophytes. Compartmentalisa-tion depends upon minimising leakage across the tonoplast, rather than continually pumping sodium back in.

Overexpression of sodium-proton antiporters has been reported to increase the salt tolerance of some species, but the evidence is confusing and equivocal. The ion relations cannot be separated from the response to osmotic shock (an artefact of some experimental designs) and further limitations in analyses and experiments compromise interpretation. Nevertheless, there is, theoretically, potential to enhance the tolerance of the more tolerant species by manipulating their ion transport. In this scenario, halophytes will be a source of expertise on how to coordinate ion transport, rather than a source of cherry-picked genes. There is also some potential for minimising sodium influx pathways at the lower end of the salinity spectrum, where osmotic stress is not an issue. The majority of crop species lie, however, in the middle ground, where exclusion-based tolerance takes them further away from the successful halophytes, and this poses a dilemma in plant breeding.

The tolerance of desiccation (Chapter 15) is common in the development of seeds and of pollen, but is rare in the vegetative tissues of vascular plants. Desiccation differs from water deficit in a qualitative manner; it means the absence of cytoplasmic water. Water is no longer present to shield charges and the surfaces of macromolecules, and the hydrophobic effect no longer exists; the physical chemistry of the cell is entirely different. Tolerance of desiccation also implies tolerance of the metabolic disruption entailed during de- and re-hydration.

Tolerance in orthodox seeds requires tolerance of mechanical damage (shrinkage), metabolic damage, of the desiccated state itself and of rehydration. It is a slow and progressive process requiring the programmed and pre-emptive shut down of the cellular machinery. This depends in a co-ordinated way upon intracellular physical characteristics, de-differentiation of the cell, switching off metabolism, effective antioxidant systems, development of protective molecules (low molecular weight carbohydrates and LEA proteins that can preserve the cytoplasm in the desiccated state), oleosins to surround lipid bodies, and mechanisms for repairing damage. Protective molecules function in water-replacement or in glass-formation (the vitrified state). The failure of recalcitrant seeds to develop desiccation tolerance can be due to a deficit in any one of these mechanisms, in overall co-ordination, or simply be prevented by anatomy.

Vegetative tolerance in vascular plants is exemplified in the resurrection plants. Tolerance to desiccation is developed slowly, as in seeds, and so differs from the desiccation tolerance of bryophytes, which is constitutive, and can be moved in and out of quite rapidly. Vegetative desiccation tolerance depends, as in seed development, on physical characteristics that permit shrinkage, on metabolic shut-down and an-tioxidants, and on solutes, including LEA proteins, that can act in water replacement and vitrification. Desiccation tolerance in both seeds and vascular plants is slow, coordinated and pre-emptive. Competitive advantage rests with the predictability that shutting down is 'worthwhile'. Whilst this is clear for orthodox seeds, it may limit the advantage of vegetative desiccation tolerance to rare niches. This may help explain the rarity of desiccation tolerance in vascular plants. It may also be the case that few species have the mechanical ability to shrink. Another limitation is how roots dehydrate without damage when in contact with the soil matrix, and this has been little investigated.

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