Structural and functional heterogeneity in phloem loading and transport

Slewinski, Thomas L.; Zhang, Cankui; Turgeon, Robert

doi:10.3389/fpls.2013.00244

Cited by 73 publications

(70 citation statements)

References 97 publications

(159 reference statements)

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“…3B). This supports the concept that the elevated density of plasmodesmata observed in active symplastic loaders relative to passive ones evolved to accommodate the limitation on flux imposed by the narrow pores required for gradient inversion (Haritatos and Turgeon, 1995;Slewinski et al, 2013). Moreover, the fact that the export rate isoline (green line, Fig.…”

Section: Export Rates Are Compatible With Those Observed Experimentallysupporting

confidence: 77%

“…Interestingly, most, if not all, symplastic loaders polymerize some Suc into RFOs whether or not they display the gradient inversion associated with polymer trapping (Slewinski et al, 2013). Our model provides a possible rational for this observation: With or without segregation and gradient inversion, localized reduction of Suc in the phloem by polymerization increases export rates relative to the completely passive case (as for points 2 and 3 in Fig.…”

Section: Discussionmentioning

confidence: 70%

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Phloem Loading through Plasmodesmata: A Biophysical Analysis

2017

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In many species, Suc en route out of the leaf migrates from photosynthetically active mesophyll cells into the phloem down its concentration gradient via plasmodesmata, i.e. symplastically. In some of these plants, the process is entirely passive, but in others phloem Suc is actively converted into larger sugars, raffinose and stachyose, and segregated (trapped), thus raising total phloem sugar concentration to a level higher than in the mesophyll. Questions remain regarding the mechanisms and selective advantages conferred by both of these symplastic-loading processes. Here, we present an integrated model-including local and global transport and kinetics of polymerization-for passive and active symplastic loading. We also propose a physical model of transport through the plasmodesmata. With these models, we predict that (1) relative to passive loading, polymerization of Suc in the phloem, even in the absence of segregation, lowers the sugar content in the leaf required to achieve a given export rate and accelerates export for a given concentration of Suc in the mesophyll and (2) segregation of oligomers and the inverted gradient of total sugar content can be achieved for physiologically reasonable parameter values, but even higher export rates can be accessed in scenarios in which polymers are allowed to diffuse back into the mesophyll. We discuss these predictions in relation to further studies aimed at the clarification of loading mechanisms, fitness of active and passive symplastic loading, and potential targets for engineering improved rates of export.

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Section: Export Rates Are Compatible With Those Observed Experimentallysupporting

confidence: 77%

Section: Discussionmentioning

confidence: 70%

Phloem Loading through Plasmodesmata: A Biophysical Analysis

2017

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“…2A). Arabidopsis SHR-SCR complex has been suggested to control the development of vasculature in all tissues, including the Kranz anatomy in C4 leaves (15)(16)(17). Consistent with this hypothesis, the maize SHR and SCR genes not only were highly expressed during the development of Kranz anatomy in embryonic and foliar leaves (7) but also were preferentially expressed in the developing BS strands that contain both BS and vasculature cells (5).…”

Section: Resultsmentioning

confidence: 79%

Transcriptome dynamics of developing maize leaves and genomewide prediction ofciselements and their cognate transcription factors

Chen²,

Chang³

et al. 2015

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

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Maize is a major crop and a model plant for studying C4 photosynthesis and leaf development. However, a genomewide regulatory network of leaf development is not yet available. This knowledge is useful for developing C3 crops to perform C4 photosynthesis for enhanced yields. Here, using 22 transcriptomes of developing maize leaves from dry seeds to 192 h post imbibition, we studied gene up-and down-regulation and functional transition during leaf development and inferred sets of strongly coexpressed genes. More significantly, we developed a method to predict transcription factor binding sites (TFBSs) and their cognate transcription factors (TFs) using genomic sequence and transcriptomic data. The method requires not only evolutionary conservation of candidate TFBSs and sets of strongly coexpressed genes but also that the genes in a gene set share the same Gene Ontology term so that they are involved in the same biological function. In addition, we developed another method to predict maize TF-TFBS pairs using known TF-TFBS pairs in Arabidopsis or rice. From these efforts, we predicted 1,340 novel TFBSs and 253 new TF-TFBS pairs in the maize genome, far exceeding the 30 TF-TFBS pairs currently known in maize. In most cases studied by both methods, the two methods gave similar predictions. In vitro tests of 12 predicted TF-TFBS interactions showed that our methods perform well. Our study has significantly expanded our knowledge on the regulatory network involved in maize leaf development. maize transcriptomes | coexpressed genes | cis binding site M aize (Zea mays) is a major crop and a model plant for studying C4 photosynthesis and leaf development. However, the regulatory network that controls maize leaf development is still not well understood. In fact, the number of known maize transcription factor (TF)-binding sites (TFBSs) is far smaller than that for Arabidopsis thaliana (1-3).To understand better the regulation of maize leaf development, several recent studies used next-generation sequencing (NGS) technologies to survey transcriptomic differences among maize leaf cell types and developmental stages. The first large-scale study was by Li et al. (7) investigated the transcriptomes of Kranz (i.e., the foliar leaf blade) and non-Kranz (the husk leaf sheath) maize leaves to identify cohorts of genes associated with procambium initiation and vascular patterning. Recently, Wang et al. (8) conducted comparative transcriptomic and metabolomic analyses of developing leaves in maize and rice and identified putative structural and regulatory components important for C4 and C3 photosynthesis. These studies provided insights into the regulatory mechanisms underlying the development of Kranz leaf anatomy in maize.In the present study, we obtained nine transcriptomes of the second leaf from 84-192 h post imbibition at 12-h (6:00 AM and 6:00 PM) or 24-h (6:00 PM only) intervals. Together with the 13 SignificanceMaize is a major crop and a model plant for studying C4 leaf development. However, its regulatory network of leaf ...

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“…The CmGAS promoter activates expression specifically in minor veins in the species that have been tested, and this is also the pattern in poplar (Slewinski et al, 2013). Two lines (Pta-GAS5 and Pta-GAS8) were determined to have significantly higher activities of apoplastic invertase than wild-type poplar.…”

Section: Effects Of Cell Wall Invertase On Poplarmentioning

confidence: 82%

Symplastic Phloem Loading in Poplar

et al. 2014

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Sap is driven through phloem sieve tubes by an osmotically generated pressure gradient between source and sink tissues. In many plants, source pressure results from thermodynamically active loading in which energy is used to transfer sucrose (Suc) from mesophyll cells to the phloem of leaf minor veins against a concentration gradient. However, in some species, almost all trees, correlative evidence suggests that sugar migrates passively through plasmodesmata from mesophyll cells into the sieve elements. The possibility of alternate loading mechanisms has important ramifications for the regulation of phloem transport and source-sink interactions. Here, we provide experimental evidence that, in gray poplar (Populus tremula 3 Populus alba), Suc enters the phloem through plasmodesmata. Transgenic plants were generated with yeast invertase in the cell walls to prevent Suc loading by this route. The constructs were driven either by the constitutive 35S promoter or the minor vein-specific galactinol synthase promoter. Transgenic plants grew at the same rate as the wild type without symptoms of loading inhibition, such as accumulation of carbohydrates or leaf chlorosis. Rates of photosynthesis were normal. In contrast, alfalfa (Medicago sativa) plants, which have limited numbers of plasmodesmata between mesophyll and phloem, displayed typical symptoms of loading inhibition when transformed with the same DNA constructs. The results are consistent with passive loading of Suc through plasmodesmata in poplar. We also noted defense-related symptoms in leaves of transgenic poplar when the plants were abruptly exposed to excessively high temperatures, adding to evidence that hexose is involved in triggering the hypersensitive response.

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Structural and functional heterogeneity in phloem loading and transport

Cited by 73 publications

References 97 publications

Phloem Loading through Plasmodesmata: A Biophysical Analysis

Phloem Loading through Plasmodesmata: A Biophysical Analysis

Transcriptome dynamics of developing maize leaves and genomewide prediction ofciselements and their cognate transcription factors

Symplastic Phloem Loading in Poplar

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