Source physiology and assimilate transport: the interaction of sucrose metabolism, starch storage and phloem export in source leaves and the effects on sugar status in phloem

2000 ◽  
Vol 27 (6) ◽  
pp. 497 ◽  
Author(s):  
Ewald Komor
Keyword(s):  
Genetic Manipulation ◽  
Sieve Tube ◽  
Phloem Loading ◽  
Assimilate Transport ◽  
Vascular Bundles ◽  
Ambient Conditions ◽  
Sucrose Transport ◽  
Carbon Export ◽  
Export Rate ◽  
Long Day

Phloem loading of sucrose is decisive for the speed of mass flow, because sucrose is the dominant solutein the sieve tube sap of nearly all plant species. The export rate of carbon is linearly correlated to the concentration of sucrose in green leaves. Saturation of export was not observed, because surplus of assimilates is converted to starch, a process which is regulated by the sucrose level in the cytosol. Consequently, an increase of sucrose synthesis by overexpression of SPS did not enhance carbon export (at least under normal ambient conditions). Saturation of sucrose export could be observed only in experimental systems, where sucrose was fed directly to the phloem (e.g. in Ricinus seedling) or where constraints on transport activity were imposed by genetic manipulation either on the transporters (e.g. in sucrose transporter antisense plants) or on the path of sucrose (e.g. in plants trans ormed with TMV movement protein, or by incubation in salts). The balance between carbon storage and carbon export is subject to adaptation to meet growth requirements under special circumstances. For example, in a starch-deficient mutant, the day time export rate is nearly doubled compared to wild type plants. Furthermore, plants under short day illumination greatly accelerated starch storage compared to plants under long day illumination (a modulation which persists even a few days after a shift to long day conditions). Plants with a higher assimilation rate due to elevated ambient CO2 increase the nightly carbon export rate, whereas the export rate in day time rate appeared to work at its upper limit. The overall efficiency of sucrose export and incorporation into biomass is ca 0.65, which is close to the theoretical value of 0.75. Sucrose transport along the phloem strands is modulated according to the input at the source, but the individual phloem strands show also partial coordination with respect to sucrose concentrations (as revealed by NMR-imaging), especially obvious after physical interruption of some vascular bundles.

The paraveinal mesophyll: a specialized path for intermediary transfer of assimilates in legume leaves

2000 ◽  
Vol 27 (9) ◽  
pp. 757 ◽  
Author(s):  
Alexis J. Lansing ◽  
Vincent R. Franceschi
Keyword(s):  
Vascular System ◽  
Phloem Loading ◽  
Assimilate Transport ◽  
Vascular Bundles ◽  
Transport Pathway ◽  
Mesophyll Cells ◽  
Diffusion Limitations ◽  
Unit Leaf ◽  
Paraveinal Mesophyll ◽  
Leaf Surface Area

This paper originates from a presentation at the International Conference on Assimilate Transport and Partitioning, Newcastle, NSW, August 1999 The distance between sites of synthesis of assimilates and the site of phloem loading can be large, and specialized leaf cell layers such as the paraveinal mesophyll (PVM) might act to enhance the efficiency of transport. A number of techniques were used to analyse PVM of legume leaves with respect to a hypothesized function in transfer of assimilates between tissues. Of 39 legume species examined, PVM was found in 22. Leaves of all PVM-containing species had multiple palisade parenchyma layers, while non-PVM species generally had only one distinct palisade layer. Morphometric analysis identified a significant correlation between PVM presence and greater numbers of palisade cells per unit leaf surface area. Comparison of photosynthetic rates of four PVM and four non-PVM species showed the PVM species had higher rates on a leaf area basis than all but one of the non-PVM species. Microautoradiography of 14CO2 pulse–chase studies in soybean demonstrated PVM is an intermediary tissue in transfer of assimilates to vascular bundles. In addition, PVM cells but not mesophyll cells, were enriched in a sucrose binding protein previously found to be associated with sucrose-transporting tissues. The structural, positional and transport data support the hypothesis that the PVM acts as a transport pathway between the vascular system and photoassimilatory cells of the leaf, and has probably evolved to overcome diffusion limitations imposed by multiple palisade layers.

scholarly journals Loading regulation prevents phloem failure during drought and widens the range of phloem and stomatal traits

2021 ◽  
Author(s):  
Ryan Stanfield ◽  
Megan Bartlett
Keyword(s):  
Water Stress ◽  
Phloem Loading ◽  
Feedback Mechanism ◽  
Total Carbon ◽  
Sucrose Transporter ◽  
Sucrose Transport ◽  
Carbon Export ◽  
Stomatal Traits ◽  
Soil Dryness ◽  
Operational Range

Plant carbon transport is controlled by a multitude of parameters both internal and external to the sugar transporting phloem tissue. Sucrose transporter kinetics, conduit hydraulic resistance, and xylem water stress are all hypothesized to impact the amount of carbon delivered to sink tissues. However, the most important traits determining carbon export under drought are not well understood, especially for species with active molecular regulation of sucrose transport. This in turn limits our ability to assess species’ resistances to phloem dysfunction under drought. Here, we use an integrated xylem-phloem-stomatal model to calculate leaf water potential from soil dryness, which is then used to determine gas exchange and phloem pressure gradients. We quantitatively compare the impacts of phloem loading kinetics, including feedbacks between loading and phloem pressure, phloem conduit resistances, and stomatal responses to water stress, on the total carbon export to sinks during drought. Regulating sucrose transporter kinetics which downregulates loading at high phloem pressures prevented runaway viscosity in the phloem sap and was the most important determinant of export rates under drought. In contrast to previous models, we found this feedback mechanism decoupled stomatal traits from phloem export efficiency during drought and increased the operational range of phloem hydraulic resistances.

scholarly journals Anatomy of Brazilian Cereeae (subfamily Cactoideae, Cactaceae): Arrojadoa Britton & Rose, Stephanocereus A. Berger and Brasilicereus Backeberg

2007 ◽  
Vol 21 (4) ◽  
pp. 813-822 ◽  
Author(s):  
Patricia Soffiatti ◽  
Veronica Angyalossy
Keyword(s):  
Secondary Xylem ◽  
Sieve Tube ◽  
Vascular Bundles ◽  
Campos Rupestres ◽  
Espinhaço Range ◽  
Diagnostic Characters ◽  
Bordered Pits ◽  
Companion Cells ◽  
Southern Bahia ◽  
Perforation Plates

(Anatomy of Brazilian Cereeae (subfamily Cactoideae, Cactaceae): Arrojadoa Britton & Rose, Stephanocereus A. Berger wâBrasilicereus Backeberg). Arrojadoa, Stephanocereus and Brasilicereus are endemic Brazilian Cereeae, occurring along the Espinhaço Range, in the campos rupestres, cerrados and caatingas, from northern Minas Gerais to southern Bahia. The genera are columnar, erect to semi-erect cacti, except for one species, A bahiensis, which is globose. This study describes the anatomy of dermal, fundamental and vascular systems, aiming to find diagnostic characters for the genera and species. Basal portions of stems were sectioned transversely and longitudinally, and stained with Astrablue and Safranin. The species share a uniseriate epidermis, with thick cuticle; well developed collenchymatic hypodermis, containing prismatic crystals; cortex with numerous mucilage cells, druses and vascular bundles; outside cortex as a palisade parenchyma; periderm composed of lignified cork cells alternating with suberized cells; pheloderm consisting of a few layers of thin-walled cells; phloem composed of solitary or multiple of two to three sieve tube elements, companion cells, axial and radial parenchyma; secondary xylem with solitary to multiple vessels, with simple perforation plates and alternate bordered to semi-bordered pits; axial parenchyma scanty vasicentric to incomplete; libriform septate fibres; large rays. Unlignified parenchyma is seen in the secondary xylem, varying from a few cells to bands among axial and radial elements. The following are considered diagnostic characters: the shape of lignified phellem cells, cubic to radially elongate, which individualizes S. leucostele; an underdeveloped hypodermis and the occurrence of sclereids in the cortex are exclusive to Brasilicereus markgrqfii.

Über den Zuckergehalt der Siebröhren- bzw. Siebzellensäfte von Heracleum Mantegazzianum und Picea abies (L.) KARST.

1959 ◽  
Vol 14 (4) ◽  
pp. 278-281 ◽  
Author(s):  
Hubert Ziegler ◽  
Tom E. Mittler
Keyword(s):  
Picea Abies ◽  
Sieve Tube ◽  
Vascular Bundles ◽  
Heracleum Mantegazzianum ◽  
Sieve Tubes

Sieve-tube sap from the petioles of Heracleum Mantegazzianum and Picea abies stems was obtained by severing the proboscides of aphids tapping the sieve tubes of these plants. Sucrose was the only sugar detected in the sieve-tube sap, and occurred at concentrations of 24% (Heracleum) and 10% (w/v) (Picea). Volumes of sieve-tube sap equal to 5500 sieve-tube cells of Heracleum and 50 sievetube cells of Picea exuded from severed aphid probiscides.The mouth-parts of the aphids living within hollow Heracleum petioles normally penetrate the xylem of the vascular bundles in order to reach the phloem sieve-tubes. The aphids also tap the sievetubes of isolated phloem strands.

A theory of translocation in phloem of Heracleum by contractile protein microfibrillar material

1972 ◽  
Vol 50 (3) ◽  
pp. 479-497 ◽  
Author(s):  
D. S. Fensom
Keyword(s):  
Electron Microscopy ◽  
Surface Layer ◽  
Light Microscopy ◽  
Mass Flow ◽  
Sieve Tube ◽  
Flow Mode ◽  
Sucrose Transport ◽  
Small Surface ◽  
Callose Formation ◽  
Mode A

Recent findings based on light microscopy, electron microscopy, electrical gradients, microinjection, and tracer feeding, all on isolated but living phloem strands of Heracleum are here summarized and collated. A new theory of translocation is presented in which sucrose transport occurs in two chief modes. The first is based on microperistaltic movement of contractile lipoprotein which is thought to extend axially through the sieve tube. The second is a mass flow of solution around the contractile microfibrillar material, but chiefly activated by it. The microfibrillar material transports sucrose in pulses at about 400 cm h−1 and this mode of translocation is stopped by cold blocks and desiccation, but not by callose. The mass flow mode is slower and is stopped by callose formation and also by the cessation of activity of the microfibrillar or pulsing mode. A third small surface-layer component of translocation is also indicated, operating at speeds above 1000 cm h−1.

Numerical Modeling of Fluidic Pumping in Micronetworks of Plants

2013 ◽  
Author(s):  
Tsun-kay Jackie Sze ◽  
Jin Liu ◽  
Prashanta Dutta
Keyword(s):  
Transport Model ◽  
Sucrose Concentration ◽  
Electrical Potential ◽  
Sugar Transport ◽  
Phloem Loading ◽  
Electrical Potential Difference ◽  
Transport Mechanisms ◽  
Sucrose Transport ◽  
Membrane Electrical Potential ◽  
Plant Transport

Plant transport mechanisms are of interest in developing micropump for engineering devices. We present a two-dimensional phloem loading and transport model incorporating protein level mechanics with cellular level fluid mechanics. Governing Navier-Stokes, continuity, and Nernst-Planck equations are numerically solved to determine fluid flow and sugar transport. Phloem loading mechanics for active loading is incorporated through a six-state proton sucrose pump kinetic model. The influence of binding rates constants, concentrations, and membrane electrical potential differences on resulting sucrose transport is studied. Numerical results show that increasing rates of the sucrose transporter will noticeably increase outflow. Simulation result also show that a lower leaf sieve sucrose concentration improves outflow. In addition, a more negative membrane electrical potential difference will increase outflow. This numerical model offers insight on parameters that may be significant for implementing plant transport mechanisms in microfluidic devices.

Analysis of a promoter for the NADH - glutamate synthase gene in rice (Oryza sativa): cell type-specific expression in developing organs of transgenic rice plants

2000 ◽  
Vol 27 (9) ◽  
pp. 787 ◽  
Author(s):  
Soichi Kojima ◽  
Michiko Kimura ◽  
Yukine Nozaki ◽  
Tomoyuki Yamaya
Keyword(s):  
Oryza Sativa ◽  
Oryza Sativa L ◽  
Glutamate Synthase ◽  
Gus Activity ◽  
Start Codon ◽  
Assimilate Transport ◽  
Upstream Region ◽  
Vascular Bundles ◽  
Specific Expression ◽  
Gus Reporter

This paper originates from a presentation at the International Conference on Assimilate Transport and Partitioning, Newcastle, NSW, August 1999 The entire 3.7 kbp 5´-upstream region (–2840 to +886) from the translational start codon of NADH–glutamate synthase (NADH–GOGAT, EC 1.4.1.14) gene from rice (Oryza sativa L.) or the region sequentially deleted from the 5´-end was fused with the β−glucuronidase (GUS) reporter gene. The chimeric gene was introduced into calli derived from rice scutellum via Agrobacterium tumefaciens-mediated transformation and tissue-specific GUS activity determined in T0 generations. When the entire region was fused, GUS activity was detected in vascular bundles of the developing leaf blade and in dorsal and lateral vascular bundles of developing grains. This corresponds with our previous immunodetection of NADH–GOGAT protein (Hayakawa et al., Planta 193, 455–460, 1994). A series of deletion experiments showed that a 149-nucleotide region between –142 and +7 was essential for promoter activity in the NADH–GOGAT gene.

Cellular pathways of source leaf phloem loading and phloem unloading in developing stems of Sorghum bicolor in relation to stem sucrose storage

2015 ◽  
Vol 42 (10) ◽  
pp. 957 ◽  
Author(s):  
Ricky J. Milne ◽  
Christina E. Offler ◽  
John W. Patrick ◽  
Christopher P. L. Grof
Keyword(s):  
Sorghum Bicolor ◽  
Cell Walls ◽  
Phloem Loading ◽  
Vascular Bundles ◽  
Phloem Unloading ◽  
Parenchyma Cells ◽  
Storage Parenchyma ◽  
Sucrose Storage ◽  
Cellular Pathways ◽  
And Storage

Cellular pathways of phloem loading in source leaves and phloem unloading in stems of sweet Sorghum bicolor (L.) Moench were deduced from histochemical determinations of cell wall composition and from the relative radial mobilities of fluorescent tracer dyes exiting vascular pipelines. The cell walls of small vascular bundles in source leaves, the predicted site of phloem loading, contained minimal quantities of lignin and suberin. A phloem-loaded symplasmic tracer, carboxyfluorescein, was retained within the collection phloem, indicating symplasmic isolation. Together, these findings suggested that phloem loading in source leaves occurs apoplasmically. Lignin was restricted to the walls of protoxylem elements located in meristematic, elongating and recently elongated regions of the stem. The apoplasmic tracer, 8-hydroxypyrene-1,3,6-trisulfonic acid, moved radially from the transpiration stream, consistent with phloem and storage parenchyma cells being interconnected by an apoplasmic pathway. The major phase of sucrose accumulation in mature stems coincided with heavy lignification and suberisation of sclerenchyma sheath cell walls restricting apoplasmic tracer movement from the phloem to storage parenchyma apoplasms. Phloem unloading at this stage of stem development followed a symplasmic route linking sieve elements and storage parenchyma cells, as confirmed by the phloem-delivered symplasmic tracer, 8-hydroxypyrene-1,3,6-trisulfonic acid, moving radially from the stem phloem.

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