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.

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.

scholarly journals Carbon export from leaves is controlled via ubiquitination and phosphorylation of sucrose transporter SUC2

2020 ◽  
Vol 117 (11) ◽  
pp. 6223-6230 ◽  
Author(s):  
Qiyu Xu ◽  
Shijiao Yin ◽  
Yue Ma ◽  
Min Song ◽  
Yingjie Song ◽  
...  
Keyword(s):  
Vascular Tissue ◽  
Carbon Sink ◽  
Phloem Loading ◽  
Sucrose Transporter ◽  
Dependent Manner ◽  
Carbon Export ◽  
Sucrose Transporters ◽  
Ubiquitin Conjugating Enzyme

All multicellular organisms keep a balance between sink and source activities by controlling nutrient transport at strategic positions. In most plants, photosynthetically produced sucrose is the predominant carbon and energy source, whose transport from leaves to carbon sink organs depends on sucrose transporters. In the model plantArabidopsis thaliana, transport of sucrose into the phloem vascular tissue by SUCROSE TRANSPORTER 2 (SUC2) sets the rate of carbon export from source leaves, just like the SUC2 homologs of most crop plants. Despite their importance, little is known about the proteins that regulate these sucrose transporters. Here, identification and characterization of SUC2-interaction partners revealed that SUC2 activity is regulated via its protein turnover rate and phosphorylation state. UBIQUITIN-CONJUGATING ENZYME 34 (UBC34) was found to trigger turnover of SUC2 in a light-dependent manner. The E2 enzyme UBC34 could ubiquitinate SUC2 in vitro, a function generally associated with E3 ubiquitin ligases.ubc34mutants showed increased phloem loading, as well as increased biomass and yield. In contrast, mutants of another SUC2-interaction partner, WALL-ASSOCIATED KINASE LIKE 8 (WAKL8), showed decreased phloem loading and growth. An in vivo assay based on a fluorescent sucrose analog confirmed that SUC2 phosphorylation by WAKL8 can increase transport activity. Both proteins are required for the up-regulation of phloem loading in response to increased light intensity. The molecular mechanism of SUC2 regulation elucidated here provides promising targets for the biotechnological enhancement of source strength.

scholarly journals Sucrose Transporter ZmSut1 Expression and Localization Uncover New Insights into Sucrose Phloem Loading

2016 ◽  
Vol 172 (3) ◽  
pp. 1876-1898 ◽  
Author(s):  
R. Frank Baker ◽  
Kristen A. Leach ◽  
Nathanial R. Boyer ◽  
Michael J. Swyers ◽  
Yoselin Benitez-Alfonso ◽  
...  
Keyword(s):  
Phloem Loading ◽  
Sucrose Transporter

scholarly journals Effects of vegetation structure on biomass accumulation in a Balanced Optimality Structure Vegetation Model (BOSVM v1.0)

2013 ◽  
Vol 6 (3) ◽  
pp. 4603-4663 ◽  
Author(s):  
Z. Yin ◽  
S. C. Dekker ◽  
B. J. J. M. van den Hurk ◽  
H. A. Dijkstra
Keyword(s):  
Water Stress ◽  
Leaf Area Index ◽  
Leaf Area ◽  
Vegetation Structure ◽  
Bare Soil ◽  
Feedback Mechanism ◽  
Carbon Gain ◽  
Area Index ◽  
Local Climate ◽  
Vegetation Structures

Abstract. A myriad of interactions exist between vegetation and local climate for arid and semi-arid regions. Vegetation function, structure and individual behavior have large impacts on carbon-water-energy balances, which consequently influence local climate variability that, in turn, feeds back to the vegetation. In this study, a conceptual vegetation structure scheme is formulated and tested in a new carbon-water-energy coupled model to explore the importance of vegetation structure and vegetation adaptation to water stress on equilibrium biomass states. Surface energy, water and carbon fluxes are simulated for a range of vegetation structures across a precipitation gradient in West Africa and optimal vegetation structures that maximizes biomass for each precipitation regime are determined. Two different strategies of vegetation adaptation to water stress are included. Under dry conditions vegetation tries to maximize the Water Use Efficiency and Leaf Area Index as it tries to maximize carbon gain. However, an important negative feedback mechanism is found as the vegetation also tries to minimize its cover to optimize the surrounding bare ground area from which water can be extracted, thereby forming patches of vertical vegetation. Under larger precipitation, a positive feedback mechanism is found in which vegetation tries to maximize its cover as it then can reduce water loss from bare soil while having maximum carbon gain due to a large Leaf Area Index. The competition between vegetation and bare soil determines a transition between a "survival" state to a "growing" state.

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.

Potato Sucrose Transporter Expression in Minor Veins Indicates a Role in Phloem Loading

1993 ◽  
Vol 5 (11) ◽  
pp. 1591 ◽  
Author(s):  
Jorg W. Riesmeier ◽  
Brigitte Hirner ◽  
Wolf B. Frommer
Keyword(s):  
Phloem Loading ◽  
Sucrose Transporter ◽  
Transporter Expression
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