Mechanisms of phloem unloading: shaped by cellular pathways, their conductances and sink function

Milne, Ricky J.; Grof, Christopher P. L.; Patrick, John W.

doi:10.1016/j.pbi.2017.11.003

Cited by 87 publications

(96 citation statements)

References 56 publications

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“…Recently, Ross‐Elliott et al () proposed a new model of convective phloem unloading through plasmodesmata, combining bulk flow (requiring a pressure differential) as the dominant mechanism, with also diffusion induced in parallel to bulk flow. In contrast, no symplastic unloading occurs from metaphloem, therefore supporting the existence of an apoplastic unloading pathway beyond the root tip (Milne et al ).…”

Section: Sugar Unloading Pathway In Rootsmentioning

confidence: 76%

“…Interestingly, many of the sucrose transporter genes that are expressed in the leaf phloem are also expressed in roots where their exact role (reloading of sucrose into the phloem or unloading of sucrose from the phloem) is not clearly defined and may be linked to the localization of the corresponding proteins. Due to the high‐sucrose concentration in sieve tubes, sucrose exit does not require energy, therefore, either facilitators (such as SWEET proteins; Durand et al ) or H + /sucrose cotransporters functioning as effluxers (such as SUC/SUT transporters; Carpaneto et al , Milne et al ) may be involved. Once unloaded in the apoplasm, the fate of sucrose depends on the presence of cell‐wall invertases (CWI): if present, sucrose is cleaved to hexoses and taken up by specific transporters (monosaccharide transporters, STPs or SWEETs); if absent, sucrose is directly taken up, as in sugar beet (see below and Lemoine et al ).…”

Section: Sugar Unloading Pathway In Rootsmentioning

confidence: 99%

“…(B) Details of the phloem specialization at the root tip of Arabidopsis according to Ross-Elliott et al (2017). In the other parts of the roots, sucrose is unloaded from metaphloem cells apoplastically (Milne et al 2018). These unloading pathways are found also in lateral roots.…”

Section: Sugar Unloading Pathway In Rootsmentioning

confidence: 99%

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Sugars en route to the roots. Transport, metabolism and storage within plant roots and towards microorganisms of the rhizosphere

Hennion

Durand

Vriet

et al. 2018

Physiologia Plantarum

128

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In plants, the root is a typical sink organ that relies exclusively on the import of sugar from the aerial parts. Sucrose is delivered by the phloem to the most distant root tips and, en route to the tip, is used by the different root tissues for metabolism and storage. Besides, a certain portion of this carbon is exuded in the rhizosphere, supplied to beneficial microorganisms and diverted by parasitic microbes. The transport of sugars toward these numerous sinks either occurs symplastically through cell connections (plasmodesmata) or is apoplastically mediated through membrane transporters (MST, mononsaccharide tranporters, SUT/SUC, H+/sucrose transporters and SWEET, Sugar will eventually be exported transporters) that control monosaccharide and sucrose fluxes. Here, we review recent progresses on carbon partitioning within and outside roots, discussing membrane transporters involved in plant responses to biotic and abiotic factors.

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Section: Sugar Unloading Pathway In Rootsmentioning

confidence: 76%

Section: Sugar Unloading Pathway In Rootsmentioning

confidence: 99%

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Sugars en route to the roots. Transport, metabolism and storage within plant roots and towards microorganisms of the rhizosphere

Hennion

Durand

Vriet

et al. 2018

Physiologia Plantarum

128

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“…At the unloading end of phloem transport, two types of mechanisms have been identified in sink organs: (1) symplastic or (2) apoplastic unloading. The mechanisms seem to be sink specific, with, for example, unloading in meristems following a symplastic way, while seeds receive phloem sugars apoplastically (Milne et al 2018). However, most of the knowledge on phloem unloading is based on crops and information on trees is scarce.…”

Section: Box 1: the Basis Of Phloem Function And Transportmentioning

confidence: 99%

Drought impacts on tree phloem: from cell-level responses to ecological significance

et al. 2019

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On-going climate change is increasing the risk of drought stress across large areas worldwide. Such drought events decrease ecosystem productivity and have been increasingly linked to tree mortality. Understanding how trees respond to water shortage is key to predicting the future of ecosystem functions. Phloem is at the core of the tree functions, moving resources such as non-structural carbohydrates, nutrients, and defence and information molecules across the whole plant. Phloem function and ability to transport resources is tightly controlled by the balance of carbon and water fluxes within the tree. As such, drought is expected to impact phloem function by decreasing the amount of available water and new photoassimilates. Yet, the effect of drought on the phloem has received surprisingly little attention in the last decades. Here we review existing knowledge on drought impacts on phloem transport from loading and unloading processes at cellular level to possible effects on long-distance transport and consequences to ecosystems via ecophysiological feedbacks. We also point to new research frontiers that need to be explored to improve our understanding of phloem function under drought. In particular, we show how phloem transport is affected differently by increasing drought intensity, from no response to a slowdown, and explore how severe drought might actually disrupt the phloem transport enough to threaten tree survival. Because transport of resources affects other organisms interacting with the tree, we also review the ecological consequences of phloem response to drought and especially predatory, mutualistic and competitive relations. Finally, as phloem is the main path for carbon from sources to sink, we show how drought can affect biogeochemical cycles through changes in phloem transport. Overall, existing knowledge is consistent with the hypotheses that phloem response to drought matters for understanding tree and ecosystem function. However, future research on a large range of species and ecosystems is urgently needed to gain a comprehensive understanding of the question.

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“…MlSUC4 in M. lupulina roots is very low. The mechanisms for sucrose unloading in roots (including SUC4 expression) have been studied very fragmentarily and partially based on indirect modeling [61].…”

Section: Discussionmentioning

confidence: 99%

AM-Induced Alteration in the Expression of Genes, Encoding Phosphorus Transporters and Enzymes of Carbohydrate Metabolism in Medicago lupulina

Yurkov

Kryukov

Gorbunova

et al. 2020

Plants

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Plant–microbe interactions, including those of arbuscular mycorrhiza (AM), have been investigated for a wide spectrum of model plants. The present study focuses on an analysis of gene expression that encodes phosphate and sugar transporters and carbohydrate metabolic enzymes in a new model plant, the highly mycotrophic Medicago lupulina MLS-1 line under conditions of phosphorus deficiency and inoculation with Rhizophagus irregularis. Expression profiles were detected by RT-PCR at six plant stages of development (second leaf, third leaf, shooting, axillary shoot branching initiation, axillary shoot branching, flowering initiation). In comparison to control (without AM), the variant with AM inoculation exhibited a significant elevation of transcription levels of carbohydrate metabolic enzymes (MlSUS, MlHXK1) and sucrose transporters (MlSUC4) in M. lupulina leaves at the shooting stage. We suggest that this leads to a significant increase in the frequency of AM infection, an abundance of mycelium in roots and an increase in AM efficiency (which is calculated by the fresh weight of aerial parts and roots at the axillary shoot branching initiation stage). In roots, the specificity of MlPT4 and MlATP1 gene expressions were revealed for effective AM symbiosis. The level of MlPT4 transcripts in AM roots increased more than tenfold in comparison to that of non-specific MlPT1 and MlPT2. For the first time, MlPT1 expression was shown to increase sharply against MlPT2 in M. lupulina roots without AM at the shooting initiation stage. A significant increase in MlRUB expression was revealed at late stages in the host plant’s development, during axillary shoot branching and flowering initiation. The opposite changes characterized MlHXK1 expression. Alteration in MlHXK1 gene transcription was the same, but was more pronounced in roots. The obtained results indicate the importance of genes that encode phosphate transporters and the enzymes of carbohydrate metabolism for effective AM development at the shooting stage in the host plant.

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Mechanisms of phloem unloading: shaped by cellular pathways, their conductances and sink function

Cited by 87 publications

References 56 publications

Sugars en route to the roots. Transport, metabolism and storage within plant roots and towards microorganisms of the rhizosphere

Sugars en route to the roots. Transport, metabolism and storage within plant roots and towards microorganisms of the rhizosphere

Drought impacts on tree phloem: from cell-level responses to ecological significance

AM-Induced Alteration in the Expression of Genes, Encoding Phosphorus Transporters and Enzymes of Carbohydrate Metabolism in Medicago lupulina

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