Effect of Potassium on the Assimilate Conduction to Storage Tissue

The promoting effect of K+ on phloem transport has been shown by various authors for different plant species. This effect was also obtained in cases in which C0₂-assimilation was not improved by K+. Experiments with Ricinus communis provided evidence that K+ favoured the flux rate of phloem sap without diluting its content in organic and inorganic solutes. This all over effect results in higher flux rates of all phloem solutes. In this respect the long distance transport of sucrose, amino acids and Mg²+ is of particular interest. The long distance transport of these compounds was shown to be favoured by K+. Potassium is known to activate starch and protein synthezising enzymes involved in the metabolism of storage tissue. There is evidence, that also in cases of a suboptimum K+ supply, the K+ level in the storage tissue probably is still high enough to provide an optimum activation of the K+ requiring enzymes. For this reason it is unlikely that the favourable effect of K+ on phloem transport is related to processes in the physiological sink. It is more likely that K+ favours the phloem loading process. Indirectly K+ could promote the phloem loading process by improving the provision of energy (ATP). It is, however, also feasible that K+ acts more directly by being involved in the ATPase reaction which is supposed to drive the sucrose uptake of sieve tubes.     

The phloem, a miracle of ingenuity

This review deals with aspects of the cellular and molecular biology of the sieve element/companion cell complex, the functional unit of sieve tubes in angiosperms. It includes the following issues: (a) evolution of the sieve elements; (b) the specific structural outfit of sieve elements and its functional significance; (c) modes of cellular and molecular interaction between sieve element and companion cell; (d) plasmodesmal trafficking between sieve element and companion cell as the basis for macromolecular long‐distance signalling in the phloem; (e) diversity of sieve element/companion cell complexes in the respective phloem zones (collection phloem, transport phloem, release phloem); (f) deployment of carriers, pumps and channels on the plasma membrane of sieve element/companion cell complexes in various phloem zones; and (g) implications of the molecular‐cellular equipment of sieve element/companion cells complexes for mass flow of water and solutes in a whole‐plant frame.     

Allocation,stress tolerance and carbon transport in plants: how does phloem physiology affect plant ecology?

Despite the crucial role of carbon transport in whole plant physiology and its impact on plant–environment interactions and ecosystem function, relatively little research has tried to examine how phloem physiology impacts plant ecology. In this review, we highlight several areas of active research where inquiry into phloem physiology has increased our understanding of whole plant function and ecological processes. We consider how xylem–phloem interactions impact plant drought tolerance and reproduction, how phloem transport influences carbon allocation in trees and carbon cycling in ecosystems and how phloem function mediates plant relations with insects, pests, microbes and symbiotes. We argue that in spite of challenges that exist in studying phloem physiology, it is critical that we consider the role of this dynamic vascular system when examining the relationship between plants and their biotic and abiotic environment.     

Transport of photoassimilates in plants under unfavourable environmental conditions

Transport of photoassimilates linking functionally plant, as a whole system, is discussed as a target for different environmental stresses. Anatomical, physiological and biochemical aspects of phloem transport, phloem loading and unloading are taken into consideration. In the light of modern theoríes of assimilate transport some historical hypotheses are also shown, due to their input into the progress of transport science. The role of phloem unloading in plant acclimation to environment stress is not clear, however changes in source/sink ratio was often observed as the effect of stress. The blockage of sieve tubes found as the result of given stress may be of secondary importance. On the other hand, phloem loading process seems to be an important target for different environmental stresses.     

The nicotianamine molecule is made-to-measure for complexation of metal micronutrients in plants

The non-proteinogenic amino acid nicotianamine (NA) is ubiquitous among plants. In meristematic tissues it reaches concentrations of about 400mol (g fresh weight)^–1. NA forms complexes, among others, with the metal micronutrients (MN) copper, zinc, iron and manganese (logK _MeNA 18.6-8.8). Calculations of the dissociation curves of the metal-NA complexes based on the complex formation constants and on the acid dissociation constants of NA revealed their stability at the neutral or weak alkaline pH of cytoplasm and sieve tube sap. For the Mn-NA complex, dissociation begins at about pH 6.5, for all others dissociation occurs at more acid pHs. Thus, metal-NA complexes could theoretically persist also in the apoplasm and in xylem sap. The octanol water partition coefficient of NA is about 1 and those of its metal complexes are in the range of 0.3–0.4. The reason for this shift is perhaps the negative charge of the complexes. The higher lipophilicity of the free NA indicates that the NA supply to sites of requirement is faster than the removal of the complexes as long as membranes are an integral part of the transport paths. Changing phloem transport rates of MN-NA complexes by manipulation of the cotyledon apoplasm of Ricinus commuais L. suggest a competition of MN for NA at the site(s) of phloem loading. Thus, NA could control MN transport via phloem including recirculation.     

Nicotianamine: mediator of transport of iron and heavy metals in the phloem?

Recent work has demonstrated that minerals in plants are circulated between root and shoot. This occurs during the whole life time and renders possible response to changing environmental conditions. This mineral circulation occurs through intensive solute exchange between xylem and phloem in roots, stems, and leaves. The transport form of heavy metals such as iron, manganes, zinc and copper in the phloem, whether ionic or chelated, is unclear in most cases.
The unusual amino acid nicotianamine (NA) is ubiquitous throughout the plant kingdom. It is a chelator of several divalent transition metals. Its physiological role was investigated with the tomato mutant chloronerva, the only known NA-free multicellular plant. The mutant also exhibits disturbances of its iron metabolism and that of other heavy metals. This leads, among others, to a typical intercostal chlorosis and progressive iron accumulation in the leaves. From the heavy metal chelating properties of NA and from the phenotype of the mutant chloronerva it is concluded that NA is needed for normal distribution of heavy metals in young growing tissues fed via the phloem. This function could be fulfilled by mediating phloem loading or unloading of heavy metals as well as by preventing their precipitation in the alkaline phloem sap. An attempt is made to explain the chloronerva phenotype in the light of the phloem transport hypothesis of chelated iron.     

Evolution, polymorphology and multifunctionality of the phloem system

Current research discloses that the phloem system is not only responsible for the allocation of photoassimilates, but has several other functions. Despite the knowledge acquired recently, the phloem remains the most puzzling plant tissue due to its inaccessability to experimental approach. Since well-preserved fossile remnants of phloem tissue are rare, evolution of sieve elements and the whole phloem was inferred from the phloem structure in present plant taxa. Special attention is paid to the evolution of the sieve elements being the conducting modules of the phloem. Development of sieve elements probably was a polyphyletic event. It may have occurred independently in various groups of algae and in the land plants. The emergence of highly specialized accessory cells sustaining the sieve element is restricted to the Spermatophyta. An attempt is made to explain the presumptive evolutionary development of the phloem system in terms of physiological fitness. In particular, the diversity of the leaf phloem in dicotyledons is discussed. It is an example of progressive phloem evolution in a plant organ that is permanently challenged by daily variations and more persistent environmental changes.     

Oxygen isotope enrichment of organic matter in Ricinus communis during the diel course and as affected by assimilate transport

The oxygen isotope composition in leaf water and organic compounds in different plant tissues is useful for assessing the physiological performance of plants in their environment, but more information is needed on Delta(18)O variation during a diel course. Here, we assessed Delta(18)O of the organic matter in leaves, phloem and xylem in stem segments, and fine roots of Ricinus communis during a full diel cycle. Enrichment of newly assimilated organic matter in equilibrium with leaf water was calculated by applying a nonsteady-state evaporative enrichment model. During the light period, Delta(18)O of the water soluble organic matter pool in leaves and phloem could be explained by a 27 per thousand enrichment compared with leaf water enrichment. Leaf water enrichment influenced Delta(18)O of phloem organic matter during the night via daytime starch synthesis and night-time starch remobilization. Phloem transport did not affect Delta(18)O of phloem organic matter. Diel variation in Delta(18)O in organic matter pools can be modeled, and oxygen isotopic information is not biased during transport through the plant. These findings will aid field studies that characterize environmental influences on plant water balance using Delta(18)O in phloem organic matter or tree rings.     

Collection and Analysis of Arabidopsis Phloem Exudates Using the EDTA-facilitated Method

The plant phloem is essential for the long-distance transport of (photo-) assimilates as well as of signals conveying biotic or abiotic stress. It contains sugars, amino acids, proteins, RNA, lipids and other metabolites. While there is a large interest in understanding the composition and function of the phloem, the role of many of these molecules and thus, their importance in plant development and stress response has yet to be determined. One barrier to phloem analysis lies in the fact that the phloem seals itself upon wounding. As a result, the number of plants from which phloem sap can be obtained is limited. One method that allows collection of phloem exudates from several plant species without added equipment is the EDTA-facilitated phloem exudate collection described here. While it is easy to use, it does lead to the wounding of cells and care has to be taken to remove contents of damaged cells. In addition, several controls to prove purity of the exudate are necessary. Because it is an exudation rather than a direct collection of the phloem sap (not possible in many species) only relative quantification of its contents can occur. The advantage of this method over others is that it can be used in many herbaceous or woody plant species (Perilla, Arabidopsis, poplar, etc.) and requires minimal equipment and training. It leads to reasonably large amounts of exudates that can be used for subsequent analysis of proteins, sugars, lipids, RNA, viruses and metabolites. It is simple enough that it can be used in both a research as well as in a teaching laboratory.     

Phloem unloading and ‘sink strength’: The parallel between the site of attachment of Cuscuta and developing legume seeds

Sink regions play a central role in determining assimilate distribution patterns. Two factors are discussed which have a strong effect on the sink strength of a sink, viz. phloem unloading and turgor-sensitive transport. Sink strength may be defined as the capacity of phloem in the sink region to import assimilates from other parts of the plants and to release the imported substances into the sink apoplast.A stem parasitized by Cuscuta represents a very strong sink. A review is presented of data on enhanced phloem unloading, at the site of attachment of Custuta. Recent data on metabolically controlled sucrose and amino acid unloading into the seed coat apoplast of developing legume seeds show a remarkable parallel with phloem unloading in a parasitized Vicia faba stem. Data on turgor-sensitive sucrose and amino acid transport into developing seeds are presented, which throw new light on the pressure flow theory of phloem transport.     

Physiological implications of the Münch-Horwitz theory of phloem transport: effects of loading rates*

Abstract Predicted effects of phloem loading rates on the five profiles of unloading rate, osmotic water flux, pressure, transport speed and concentration, in hypothetical sieve tubes with different sink properties, were calculated using the steady-state mathematical expression of the Münch hypothesis of phloem transport. The prediction that increased loading rates always increases the concentration, and generally increase the speed of translocates through the sieve tube, is emphasized since these parameters are accessible for experimental testing. This particular prediction contrasts with a previous prediction (Tyree, Christy, & Ferrier, 1974), that where concentration was held constant at the loading end, concentration along the rest of the sieve tube would decrease, while speed would increase greatly. Where the unloading mechanism was assigned saturable (enzyme-like) kinetics, increased loading rates (in the range well below the V_max of the sink) caused both transport speed and concentration to increase. However, as loading rates approached the V_max of the sinks, speed reached a maximum and then declined, and concentration increased substantially. This was particularly true at very high values of Km, e.g. > 0.1 mol cm^?3.     

Analysis of protein transport in the Brassica oleracea vasculature reveals protein-specific destinations

We investigated the vascular transport properties of exogenously applied proteins to Brassica oleracea plants and compared their delivery to various aerial parts of the plant with carboxy fluorescein (CF) dye. We identified unique properties for each protein. Alexafluor-tagged bovine serum albumin (Alexa-BSA) and Alexafluor-tagged Histone H1 (Alexa-Histone) moved slower than CF dye throughout the plant. Interestingly, Alexa-Histone was retained in the phloem and phloem parenchyma while Alexa-BSA moved into the apoplast. One possibility is that Alexa-Histone sufficiently resembles plant endogenous proteins and is retained in the vascular stream, while Alexa-BSA is exported from the cell as a foreign protein. Both proteins diffuse from the leaf veins into the leaf lamina. Alexa-BSA accumulated in the leaf epidermis while Alexa-Histone accumulated mainly in the mesophyll layers. Fluorescein-tagged hepatitis C virus core protein (fluorescein-HCV) was also delivered to B. oleracea plants and is larger than Alexa-BSA. This protein moves more rapidly than BSA through the plant and was restricted to the leaf veins. Fluorescein-HCV failed to unload to the leaf lamina. These combined data suggest that there is not a single default pathway for the vascular transfer of exogenous proteins in B. oleracea plants. Specific protein properties appear to determine their destination and transport properties within the phloem.     

The Ins and Outs of Nondestructive Cell-to-Cell and Systemic Movement of Plant Viruses

Propagation of viral infection in host plants comprises two distinct and sequential stages: viral transport from the initially infected cell into adjacent neighboring cells, a process termed local or cell-to-cell movement, and a chain of events collectively referred to as systemic movement that consists of entry into the vascular tissue, systemic distribution with the phloem stream, and unloading of the virus into noninfected tissues. To achieve intercellular transport, viruses exploit plasmodesmata, complex cytoplasmic bridges interconnecting plant cells. Viral transport through plasmodesmata is aided by virus-encoded proteins, the movement proteins (MPs), which function by two distinct mechanisms: MPs either bind viral nucleic acids and mediate passage of the resulting movement complexes (M-complexes) between cells, or MPs become a part of pathogenic tubules that penetrate through host cell walls and serve as conduits for transport of viral particles. In the first mechanism, M-complexes pass into neighboring cells without destroying or irreversibly altering plasmodesmata, whereas in the second mechanism plasmodesmata are replaced or significantly modified by the tubules. Here we summarize the current knowledge on both local and systemic movement of viruses that progress from cell to cell as M-complexes in a nondestructive fashion. For local movement, we focus mainly on movement functions of the 30 K superfamily viruses, which encode MPs with structural homology to the 30 kDa MP of Tobacco mosaic virus, one of the most extensively studied plant viruses, whereas systemic movement is primarily described for two well-characterized model systems, Tobacco mosaic virus and Tobacco etch potyvirus. Because local and systemic movement are intimately linked to the molecular infrastructure of the host cell, special emphasis is placed on host factors and cellular structures involved in viral transport.     

The Effects of Fusicoccin,Abscisic Acid,and Benzyladenine on the Transport and Partitioning of Substances as Related to Cytokinin Sink Activity

We tested the possible cytokinin effect on the functioning of the active transport system involved in the assimilate loading into the phloem as a cause for the cytokinin sink and retention effect. This effect is manifested in the deceleration of substance export from and the stimulation of substance import to the sites of local phytohormone application to the mature detached leaf from untreated leaf areas. To affect the membrane mechanisms of the substance transport, we used leaf treatment with the phytotoxin fusicoccin, an enhancer of plasmalemmal H⁺-ATPase and a potential stimulator of assimilates export, and with the phytohormone ABA affecting transport, metabolism, and plant growth. However, fusicoccin did not enhance ¹⁴C-sucrose export from the leaf blade and did not interfere with the cytokinin-induced export deceleration. ABA reduced substantially ¹⁴C export from the leaf but eliminated the cytokinin effect on this process. Similar results were obtained for broad bean (Vicia faba L.) leaves with apoplastic phloem loading, involving H⁺-ATPase activity, and pumpkin (Cucurbita pepo L.) leaves with symplastic phloem loading, that is, occurring without sucrose transmembrane translocation and without H⁺-ATPase involvement. The conclusion is that the cytokinin-induced development of sink zones in source leaves is not related to the membrane mechanisms of the substance transport in the mesophyll–phloem system. The data obtained support the idea that the cause for the cytokinin sink and retention effect is the enhancement of elongation growth and total activation of metabolism in the mesophyll cells of the detached leaf.     

How nutritionally imbalanced is phloem sap for aphids?

Aphids harbour intracellular symbionts (Buchnera) that provide their host with amino acids present in low amounts in their diet, phloem sap. To find out the extent to which aphids depend on their symbionts for synthesis of individual essential amino acids, we have evaluated how well phloem sap amino acid composition matches the aphids' needs. Amino acid compositions of the ingested phloem sap were compared to amino acids in aphid body proteins and also to available information about optimal diet composition for other plant feeding insects. Phloem sap data from severed stylets of two aphid species, Rhopalosiphum padi (L.) (Homoptera: Aphididae) feeding on wheat, and Uroleucon sonchi (L.) (Homoptera: Aphididae) feeding on Sonchus oleraceus (L.), together with published information on phloem sap compositions from other plant species were used.Phloem sap has in general only around 20% essential amino acids, with a range from 15–48%. Aphid body proteins and optimal diets for two other plant feeding insects have around 50%. The phloem sap of early flowering S. oleraceus ingested by U. sonchi contained 48%, which seems to be exceptional. Aphids feeding on different plants appear to be very differently dependent on their symbionts for their overall essential amino acid synthesis, due to the large variation in proportion of essential amino acids in phloem sap from different plants.The profile of the essential amino acids in phloem sap from different plant species corresponds rather well to profiles of both aphid body proteins and optimal diets determined for plant feeding insects. However, methionine and leucine in phloem sap are in general low in these comparisons, suggesting a higher dependence on the symbiont for synthesis of these amino acids. Concentrations of several essential amino acids in phloem from different plant species seem to vary together, suggesting that levels of symbiont provisioning of different amino acids are adjusted in parallel.     

Münch,morphology, microfluidics – our structural problem with the phloem

The sieve tubes of the phloem are enigmatic structures. Their role as channels for the distribution of assimilates was established in the 19th century, but their sensitivity to disturbations has hampered the elucidation of their transport mechanisms and its regulation ever since. Ernst Münch's classical monograph of 1930 is generally regarded as the first coherent theory of phloem transport, but the ‘Münchian’ pressure flow mechanism had been discussed already before the turn of the century. Münch's impact rather rested on his simple physical models of the phloem that visualized pressure flow in an intuitive way, and we argue that the downscaling of such models to realistic, low‐Reynolds‐number sizes will boost our understanding of phloem transport in this century just as Münch's models did in the previous one. However, biologically meaningful physical models that could be used to test predictions of the many existing mathematical models would have to be designed in analogy with natural phloem structures. Unfortunately, the study of phloem anatomy seems in decline, and we still lack basic quantitative data required for evaluating the plausibility of our theoretical deductions. In this review, we provide a subjective overview of unresolved problems in angiosperm phloem structure research within a functional context.     

Whole Plant Regulation of Sulfur Nutrition of Deciduous Trees-Influences of the Environment

Abstract: The current view of sulfur nutrition is based on the source‐to‐sink relationship of carbohydrates. SO₄^2‐ reduction is thought to occur mainly in leaves. Surplus reduced sulfur must be transported out of the leaves, loaded into the phloem and transported to other tissues, in particular tissues assumed to be sink organs. However, it has not been proved that tissues which are sinks for carbohydrates are also sink organs for reduced sulfur. It is evident that sinks must communicate with sources, and vice versa, to signal demand and to transport the surplus of reduced sulfur that is produced. The demand‐driven control model of sulfur nutrition proposes that the tripeptide glutathione is the signal which regulates S nutrition of the whole plant at the level of SO₄^2‐ uptake. Acclimatization to environmental changes has been shown to result in several changes in S nutrition of deciduous trees: (i) Drought stress diminished SO₄^2‐ transport into the xylem, although the GSH content in lateral roots remained unaffected, possibly due to an overall reduction in water status. (ii) Flooding decreased APS reductase activity in the anoxic roots. This may be due to enhanced GSH transport to the roots, but it is more likely to be the result of a change in metabolism leading to diminished energy gain in the roots. (iii) Mycorrhization enhanced the GSH content in the phloem, while SO₄^2‐ uptake was not affected. This clearly goes against the demand‐driven control model. (iv) Under both short‐ and long‐term exposure to elevated pCO₂, the APS reductase activity in leaves and lateral roots did not correlate with the GSH contents therein. Therefore, it must be assumed that, under these conditions, regulation of S nutrition goes beyond the demand‐driven control model, and occurs within the network of other nutrient metabolism. (v) Atmospheric S in the form of H₂S enhanced the reduced sulfur content of the phloem and lateral roots. Under these conditions, the SO₄^2‐ loaded into the xylem decreased. It would appear that the demand‐driven control model of sulfur nutrition is not always valid in the case of deciduous trees.     

Effects of Acetylcholine, Cytochalasin B and Amiprophosmethyl on Phloem Transport in Radish （Raphanus sativas）

We investigated the role of the ＂sieve tube-companion cell complex＂ lining the tube periphery, particularly the microfilament and microtubule, in assisting the pushing of phloem sap flow. We made a simple phloem transport system with a living radish plant, in which the conducting channel was exposed for local treatment with chemicals that are effective in modulating protoplasmic movement （acetylcholine, （ACh） a neurotransmitter in animals and insects; cytochalasin B, （CB） a specific inhibitor of many cellular responses that are mediated by microfilament systems and amiprophos-methyl, （APM） a specific inhibitor of many cellular responses that are mediated by microtubule systems）. Their effects on phloem transport were estimated by two experimental devices： （i） a comparison of changes in the amount of assimilates in terms of carbohydrates and ^14C-labeled photosynthetic production that is left in the leaf blade of treated plants; and （ii） distribution patterns of ^14C-labeled leaf assimilates in the phloem transport system. The results indicate that CB and APM markedly inhibited the transfer of photosynthetic product from leaf to root via the leaf vein, while ACh enhanced the transfer of photosynthetic product in low concentrations （5.0×10^-4 mol/L） but inhibited it in higher concentrations （2.0×10^-3 mol/L） from leaf to root via the leaf vein. Autoradiograph imaging clearly reveals that ACh treatment is more effective than the control, and both CB and APM treatments effectively inhibit the passage of radioactive assimilates. All of the results support the postulation that the peripheral protoplasm in the sieve tube serves not only as a passive semi-permeable membrane, but is also directly involved in phloem transport.     

Sucrose carrier RcSCR1 is involved in sucrose retrieval, but not in sucrose unloading in growing hypocotyls of Ricinus communis L

The transport of assimilates from source to sink tissues is mediated by the phloem. Along the vascular system the phloem changes its physiological function from loading phloem to transport and unloading phloem. Sucrose carrier proteins have been identified in the transport phloem, but it is unclear whether the physiological role of these transporters is phloem unloading of sucrose or retrieval of apoplasmic sucrose back into the sieve element/companion cell complex. Here, we describe the dynamic expression of the Ricinus communis sucrose carrier RcSCR1 in the hypocotyl at different sink strengths. Our results indicate that phloem unloading in castor bean is not catalysed by the phloem loader RcSCR1. However, this sucrose carrier represents the molecular basis of the sucrose retrieval mechanism along the transport phloem, which is dynamically adjusted to the sink strength. As a consequence, we assume that other release carrier(s) exist in sink tissues, such as the hypocotyl, in R. communis.     

Improvement of pea biomass and seed productivity by simultaneous increase of phloem and embryo loading with amino acids

The development of sink organs such as fruits and seeds strongly depends on the amount of nitrogen that is moved within the phloem from photosynthetic‐active source leaves to the reproductive sinks. In many plant species nitrogen is transported as amino acids. In pea (Pisum sativum L.), source to sink partitioning of amino acids requires at least two active transport events mediated by plasma membrane‐localized proteins, and these are: (i) amino acid phloem loading; and (ii) import of amino acids into the seed cotyledons via epidermal transfer cells. As each of these transport steps might potentially be limiting to efficient nitrogen delivery to the pea embryo, we manipulated both simultaneously. Additional copies of the pea amino acid permease PsAAP1 were introduced into the pea genome and expression of the transporter was targeted to the sieve element‐companion cell complexes of the leaf phloem and to the epidermis of the seed cotyledons. The transgenic pea plants showed increased phloem loading and embryo loading of amino acids resulting in improved long distance transport of nitrogen, sink development and seed protein accumulation. Analyses of root and leaf tissues further revealed that genetic manipulation positively affected root nitrogen uptake, as well as primary source and sink metabolism. Overall, the results suggest that amino acid phloem loading exerts regulatory control over pea biomass production and seed yield, and that import of amino acids into the cotyledons limits seed protein levels.