Forisome performance in artificial sieve tubes

In the legume phloem, sieve element occlusion (SEO) proteins assemble into Ca(2+)-dependent contractile bodies. These forisomes presumably control phloem transport by forming reversible sieve tube plugs. This function, however, has never been directly demonstrated, and appears questionable as forisomes were reported to be too small to plug sieve tubes, and failed to block flow efficiently in artificial microchannels. Moreover, plugs of SEO-related proteins in Arabidopsis sieve tubes do not affect phloem translocation. We improved existing procedures for forisome isolation and storage, and found that the degree of Ca(2+)-driven deformation that is possible in forisomes of Vicia faba, the standard object of earlier research, has been underestimated substantially. Forisomes deform particularly strongly under reducing conditions and high sugar concentrations, as typically found in sieve tubes. In contrast to our previous inference, Ca(2+)-inducible forisome swelling certainly seems sufficient to plug sieve tubes. This conclusion was supported by 3D-reconstructions of forisome plugs in Canavalia gladiata. For a direct test, we built microfluidics chips with artificial sieve tubes. Using fluorescent dyes to visualize flow, we demonstrated the complete blockage of these biomimetic microtubes by Ca(2+)-induced forisome plugs, and concluded by analogy that forisomes are capable of regulating phloem flow in vivo.     

Action potentials in maize sieve tubes change phloem translocation

The inhibition of phloem translocation caused by electric- andcold-shock was studied by macro- and microautoradiography inmature leaves of maize (Zea mays L.). In addition, both typesof stimulation triggered action potentials with amplitudes ofmore than 50 mV which were transmitted without diminution insieve tubes with velocities of 3-5 cm s^-1. By utilizing X-raymicroanalysis ion concentrations of sieve tubes were measuredin non-stimulated and stimulated leaves. It was shown that potassiumand chloride diminished about 3-fold after stimulation whilethe amount of cytoplasmic calcium may have increased. Thesedisplacements lead to the conclusion that calcium influx aswell as potassium and chloride efflux are involved in the propagationof action potentials. A possible relationship between electricsignalling and the reduction of phloem translocation is discussed. Key words: Action potential, electric and cooling stimuli, energy-dispersive X-ray microanalysis, ion shifts, phloem translocation     

The cell wall-plasmalemma interface in sieve tubes of barley

Both thick- and thin-walled sieve tubes in leaf-blade veins of Hordeum vulgare L. exhibit a distinct, electron-opaque inner wall layer after fixation in glutaraldehyde-osmium tetroxide and staining with uranyl acetate and lead citrate. This inner wall layer is thickest at the sieve plates and lateral sieve areas where it is permeated by a labyrinth of tubules formed by the plasmalemma. Along the lateral walls between sieve areas the inner wall layer apparently is penetrated by numerous microvilli-like evaginations of the plasmalemma, giving the cell wall-plasmalemma interface the appearance of a brush border. It is suggested that a similar brush-border-like structure may occur at the cell wall-plasmalemma interface of sieve elements in a wide variety of vascular plants.Abbreviation ER endoplasmic reticulum     

Fine structure of the proteinaceous substance in sieve tubes

Summary The dense pore-filling substance in the sieve plates of dicotyledons seems to consist of fibrils, rather than systems of membranes.

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Zusammenfassung Die dichte Substanz, welche bei Dicotyledonen die Poren der Siebplatten ausfüllt, scheint eher aus Fibrillen als aus Membransystemen zu bestehen.

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With 6 Figures in the Fext     

Analysis of particle motion in sieve tubes of Heracleum

Abstract. We have cine-filmed the random motion of microscopic particles, mostly starch grains from ruptured plastids, in sieve tubes of Heracleum mantegazzianum L. and H. sphohdylium Somm. and Lev. Our frame-by-frame analysis of the positions of the particles shows that they move much less than calculated when generally accepted estimates for the viscosity of sieve-lube sap are inserted in the Stokes-Einstein and other equations for Brownian motion. Our analysis of a film, of similar particles, made by previous workers leads us to disagree with their conclusion that particle movement in sieve tubes was greater than should be expected for ordinary Brownian motion. Particles in their film and in ours moved less than expected even when we allow for the possibility that the particles are restricted by cell walls and by each other. We suggest that the particles move less than expected because the viscosity of sieve-lube sap may be higher than has been assumed by physiologists.     

Bidirectional translocation of sugars in sieve tubes of squash plants

Two streams of sugars moving in opposite directions in the petiole of a half-grown leaf were demonstrated by feeding tritiated glucose to a fully grown leaf of a squash plant (Cucurbita melopepo Bailey) and ¹⁴CO₂ to the half-grown one. Autoradiographic evidence indicates that the movement of both streams occurred within the same sieve tubes. The data do not fit the mass flow theory of translocation which requires unidirectional flow of sugar solution in the lumen of the sieve tube.     

Strand characteristics in sieve tubes of some common tree species

Summary A means of staining the inner phloem of tree barks of four common tree species without first sectioning the bark is described. Sieve tubes stained in this manner with a protein stain, brom phenol blue-HgCl₂, were shown to contain clearly-defined strands which connected at individual pores and extended the full length of many of the elements observed. Some of the strands were seen passing over rows of pores in line. Strand masses or groups of strands could be micromanipulated and shown to consist of separate strands. Strands were quite elastic to a bending motion as well as to a longitudinal movement. Some strands, barely visible under oil immersion, appeared to be similar in size to the tubules observed in some elements or cells with the electron microscope. The presence of strands in mature sieve tubes could provide a pathway for protoplasmic streaming. At least, without this pathway, the streaming theory would be much weakened.     

Aphid (Sitobion yakini) investigation suggests thin-walled sieve tubes in barley (Hordeum vulgare) to be more functional than thick-walled sieve tubes

Barley, like most other grasses that have been studied, contains two kinds of sieve tube. The first formed are called thin-walled sieve tubes because of their thin wall compared to the late-formed, and are associated with companion cells. The late-formed are thick-walled sieve tubes, which differentiate next to the metaxylem vessels and lack companion cells. Aphid ( Sitobion yakini (Eastop) feeding was studied using light microscopy to determine if they preferentially feed from thin- or thick-walled sieve tubes in the barley leaf. Penetration of the stylets through the leaf epidermis and mesophyll was largely intercellular, becoming partly intercellular and, partly, intracellular inside the vascular bundle. Sixteen of 19 pairs of stylets (84%), and 293 of 317 (92%) stylet tracks terminated at the thin-walled sieve tubes, suggesting that Sitobion yakini feeds preferentially on the thin-walled sieve tubes which seem to be more attractive to the aphid. These thin-walled sieve tubes are thus probably the most functional in terms of phloem loading and transport.     

Modified clearing method to show sieve tubes in minor veins of leaves

Leaf samples of Glycine max and numerous other dicotyledonous species were cleared by a common, well established procedure modified by using more concentrated (10% w/v) aqueous NaOH, and by leaving samples in NaOH for 2-4 weeks and in chloral hydrate for 3 days, all at room temperature. A single dye, chlorazol black E (1 g/100 ml absolute ethanol), is used to stain for 3-6 min. Samples are mounted with the lower epidermis upward. Sieve tubes in favorable material can be seen in minor veins and vein endings.     

Thermodynamic battle for photosynthate acquisition between sieve tubes and adjoining parenchyma in transport phloem

In transport phloem, photoassimilates escaping from the sieve tubes are released into the apoplasmic space between sieve element (SE)/companion cell (CC) complexes (SE/CCs) and phloem parenchyma cells (PPCs). For uptake respective retrieval, PPCs and SE/CCs make use of plasma membrane translocators energized by the proton motive force (PMF). Their mutual competitiveness, which essentially determines the amount of photoassimilates translocated through the sieve tubes, therefore depends on the respective PMFs. We measured the components of the PMF, membrane potential and DeltapH, of SE/CCs and PPCs in transport phloem. Membrane potentials of SE/CCs and PPCs in tissue slices as well as in intact plants fell into two categories. In the first group including apoplasmically phloem-loading species (e.g. Vicia, Solanum), the membrane potentials of the SEs are more negative than those of the PPCs. In the second group including symplasmically phloem-loading species (e.g. Cucurbita, Ocimum), membrane potentials of SEs are equal to or slightly more positive than those of PPCs. Pure sieve tube sap collected from cut aphid stylets was measured with H(+)-selective microelectrodes. Under our experimental conditions, pH of the sieve tube saps was around 7.5, which is comparable to the pH of cytoplasmic compartments in parenchymatous cells. In conclusion, only the membrane potential appears to be relevant for the PMF-determined competition between SE/CCs and PPCs. The findings may imply that the axial sinks along the pathway withdraw more photoassimilates from the sieve tubes in symplasmically loading species than in apoplasmically loading species.     

Diffusion effects in calcium-regulated strain fields

The Goodwin-Trainor equations for cellular morphogenesis, based on calcium ion regulation of the visco-elastic properties of the cellular cortex, are generalized to the situation where the concentration of calcium ions (free plus bound) is allowed to change locally. A stability analysis is presented which shows that the generalized equations are also stable against perturbations at low and high wave lengths and may, for certain parameter values, develop instabilities at intermediate wave lengths.     

A method for estimating the proportion of sieve tubes in the phloem of higher plants

Summary A statistical method has been developed for the estimation of the proportion of phloem area occupied by sieve tube lumen which is applicable to most higher plants. By simple probability, the number of sieve tubes in a given area of phloem is equal to the number of sieve plates present in a series of transverse sections whose total thickness equals the mean sieve element length. The case of oblique sieve plates, where the plate is divided and occurs in more than one section, has also been dealt with and a solution obtained. Estimates of the proportion of phloem area occupied by sieve tubes have been made by this method in willow (Salix viminalis L.), sugar beet (Beta vulgaris L.) and a cucurbit (Ecballium elaterium L.) and the values obtained discussed in relation to estimates made previously by other methods.     

Translocation velocity and specific mass transfer in the sieve tubes of Fraxinus americana L.

Summary The ratios of sugar concentrations (sucrose/stachyose, raffinose/stachyose and sucrose/raffinose) in the sieve-tube exudate of Fraxinus americana L. undergo slight diurnal fluctuations. These ratio waves have been found to move down along the stem of trees at a velocity of 30–70 cm/h. They are, in contrast to the concentration waves of Huber et al. (1937), independent of the absolute exudate concentration and thus unaffected by hydrodynamic pressures changes in the xylem. The velocity is of the order of magnitude required by the mass transfer equation and thus indicates that sieve tube exudate is a moving solution. Furthermore, the phenomenon enabled us for the first time to follow phloem transport over distances of 12 meters.