An update on phloem transport: a simple bulk flow under complex regulation

Liesche, Johannes; Patrick, John W.

doi:10.12688/f1000research.12577.1

Cited by 37 publications

(26 citation statements)

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“…Moreover, the ubc34 mutant phenotype hints at additional target proteins, as it displayed coregulation of phloem loading and photosynthetic sugar production. Previously analyzed mutants deficient in phloem loading, in contrast, were shown to accumulate sugars in the leaf (2). Investigation of potential interaction partners will be essential to understand how UBC34 fulfils its apparent role as master regulator of growth.…”

Section: Discussionmentioning

confidence: 99%

“…Sucrose is produced in photosynthetically active tissues of the leaf and stem and transported to carbon sink organs in the highly specialized cells of the phloem vascular system. Since transport inside the phloem happens by osmotically driven mass flow, it is the loading and unloading reactions that determine the transport rate (2). In most crops and the model plant Arabidopsis thaliana, phloem loading is mediated by secondary active SUCROSE TRANSPORTERs (SUCs or SUTs) after sucrose secretion into the cell-wall space by passive SUGARS WILL EVENTUALLY BE EXPORTED TRANSPORTERs (SWEETs) from cells surrounding the phloem (3).…”

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

Yin

et al. 2020

Proc. Natl. Acad. Sci. U.S.A.

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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 plant Arabidopsis 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. ubc34 mutants 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.carbon allocation | phloem loading | sucrose transporter | posttranslational regulation | Arabidopsis

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Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

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

Yin

et al. 2020

Proc. Natl. Acad. Sci. U.S.A.

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“…Despite these essential functional roles, the sieve element is one of the least understood cell types in plants. Many questions remain unresolved as to how phloem structure relates to function (Liesche & Patrick, ; Turgeon, ). This has been due to the relative obscurity of the tissue for direct measurement (Van Bel, ).…”

Section: Introductionmentioning

confidence: 99%

Computational models evaluating the impact of sieve plates and radial water exchange on phloem pressure gradients

Stanfield

Schulte

Randolph

et al. 2018

Plant Cell & Environment

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The sugar conducting phloem in angiosperms is a high resistance pathway made up of sieve elements bounded by sieve plates. The high resistance generated by sieve plates may be a trade-off for promoting quick sealing in the event of injury. However, previous modeling efforts have demonstrated a wide variation in the contribution of sieve plates towards total sieve tube resistance. In the current study, we generated high resolution scanning electron microscope images of sieve plates from balsam poplar and integrated them into a mathematical model using Comsol Multiphysics software. We found that sieve plates contribute upwards of 85% towards total sieve tube resistance. Utilizing the Navier-Stokes equations, we found that oblong pores may create over 50% more resistance in comparison with round pores of the same area. Although radial water flows in phloem sieve tubes have been previously considered, their impact on alleviating pressure gradients has not been fully studied. Our novel simulations find that radial water flow can reduce pressure requirements by half in comparison with modeled sieve tubes with no radial permeability. We discuss the implication that sieve tubes may alleviate pressure requirements to overcome high resistances by regulating their membrane permeability along the entire transport pathway.

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“…Also other biological factors could lead to an underestimation as well as an overestimation of . For example, in so-called active symplasmic phloem loaders, such as the cucurbits, sucrose moves symplasmically from bundle sheet cells (BSC) to intermediary cells (IC), where it is polymerized into the larger oligomers raffinose and stachyose, that do not diffuse back in detectable amounts ( Haritatos et al, 1996 ; Liesche and Patrick, 2017 ). Two explanations have been suggested: 1) a discriminating PD SEL at this interface, which prevents the back transport of raffinose and stachyose Liesche and Schulz ( 2013 ), or 2) open PDs combined with a directional flow which could be sustained by the xylem flow Comtet et al ( 2017 ).…”

Section: Discussionmentioning

confidence: 99%

From plasmodesma geometry to effective symplasmic permeability through biophysical modelling

Deinum¹,

Benitez‐Alfonso

Mulder

2019

Preprint

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11Regulation of molecular transport via intercellular channels called plasmodesmata (PDs) is 12 important for both coordinating developmental and environmental responses among 13 neighbouring cells, and isolating (groups of) cells to execute distinct programs. Cell-to-cell mobility 14 of fluorescent molecules and PD dimensions (measured from electron micrographs) are both used 15 as methods to predict PD transport capacity (i.e., effective symplasmic permeability), but often yield 16 very different values. Here, we build a theoretical bridge between both experimental approaches 17 by calculating the effective symplasmic permeability from a geometrical description of individual 18 PDs and considering the flow towards them. We find that a dilated central region has the strongest 19 impact in thick cell walls and that clustering of PDs into pit fields strongly reduces predicted 20 permeabilities. Moreover, our open source multi-level model allows to predict PD dimensions 21 matching measured permeabilities and add a functional interpretation to structural differences 22 observed between PDs in different cell walls. 23 24 51 are embryo or seedling lethal, highlighting the importance of these structures for normal plant 52 development (Kim et al., 2002; Benitez-Alfonso et al., 2009; Xu et al., 2012). 53 Small molecules can move via PD by diffusion (non-targeted transport). This is considered to 54 be predominantly symmetrical (Schönknecht et al., 2008; Maule, 2008), while in certain tissues, 55 such as secreting trichomes (Waigmann and Zambryski, 1995; Gunning and Hughes, 1976) and the 56 phloem (Ross-Elliott et al., 2017;Comtet et al., 2017), hydrodynamic flow may create directionality. 57 The maximum size of molecules that can move by this generic "passive" pathway is often referred to 58 2 of 38 Manuscript submitted to eLife as the "size exclusion limit" (SEL), which obviously depends on PD properties and structural features 59 (Dashevskaya et al., 2008). Large molecules can move through PD via an ''active" or "targeted" 60 pathway overriding the defined SEL. This may involve additional factors that temporarily modify 61 these substrates, target them to the PDs, or induce transient modifications of the PDs to allow for 62 the passage of larger molecules in a highly substrate dependent fashion (Zambryski and Crawford, 63 2000; Maule et al., 2011). 64 Computational modelling approaches have been applied to model PD transport but, so far, 65 these have mainly focused on hydrodynamic flow and the specific tissues where that matters (Blake, 66 1978; Bret-Harte and Silk, 1994; Jensen et al., 2012; Ross-Elliott et al., 2017; Comtet et al., 2017; 67 Foster and Miklavcic, 2017; Couvreur et al., 2018). The few existing studies on diffusive transport 68 do not consider neck constrictions or the approach to PDs from the cytoplasmic bulk. Most models 69 consider PDs as straight channels, with advective/diffusive transport through an unobstructed 70 cytosolic sleeve and typically, but not always, accou...

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An update on phloem transport: a simple bulk flow under complex regulation

Cited by 37 publications

References 123 publications

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

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

Computational models evaluating the impact of sieve plates and radial water exchange on phloem pressure gradients

From plasmodesma geometry to effective symplasmic permeability through biophysical modelling

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