Sucrose transporter localization and function in phloem unloading in developing Stems

Ricky J. Milne, Jai M. Perroux, Anne L. Rae, Anke Reinders, John M. Ward, Christina E. Offler, John W. Patrick, Christopher P L Grof

Research output: Contribution to journalArticle

18 Citations (Scopus)

Abstract

How sucrose transporters (SUTs) regulate phloem unloading in monocot stems is poorly understood and particularly so for species storing high Suc concentrations. To this end, Sorghum bicolor SUTs SbSUT1 and SbSUT5 were characterized by determining their transport properties heterologously expressed in yeast or Xenopus laevis oocytes, and their in planta cellular and subcellular localization. The plasma membrane-localized SbSUT1 and SbSUT5 exhibited a strong selectivity for Suc and high Suc affinities in X. laevis oocytes at pH 5—SbSUT1, 6.3 ± 0.7 mM, and SbSUT5, 2.4 ± 0.5 mM Suc. The Suc affinity of SbSUT1 was dependent on membrane potential and pH. In contrast, SbSUT5 Suc affinity was independent of membrane potential and pH but supported high transport rates at neutral pH. Suc transport by the tonoplast localized SbSUT4 could not be detected using yeast or X. laevis oocytes. Across internode development, SUTs, other than SbSUT4, were immunolocalized to sieve elements, while for elongating and recently elongated internodes, SUTs also were detected in storage parenchyma cells. We conclude that apoplasmic Suc unloading from de-energized protophloem sieve elements in meristematic zones may be mediated by reversal of SbSUT1 and/or by uniporting SWEETs. Storage parenchyma localized SbSUT1 and SbSUT5 may accumulate Suc from the stem apoplasms of elongating and recently elongated internodes, whereas SbSUT4 may function to release Suc from vacuoles. Transiting from an apoplasmic to symplasmic unloading pathway as the stem matures, SbSUT1 and SbSUT5 increasingly function in Suc retrieval into metaphloem sieve elements to maintain a high turgor to drive symplasmic unloading by bulk flow.

Original languageEnglish (US)
Pages (from-to)1330-1341
Number of pages12
JournalPlant physiology
Volume173
Issue number2
DOIs
StatePublished - Feb 2017

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Phloem
phloem
sieve elements
Sucrose
transporters
Xenopus laevis
internodes
sucrose
Oocytes
stems
oocytes
membrane potential
Membrane Potentials
parenchyma (plant tissue)
Yeasts
yeasts
Sorghum
tonoplast
apoplast
turgor

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Sucrose transporter localization and function in phloem unloading in developing Stems. / Milne, Ricky J.; Perroux, Jai M.; Rae, Anne L.; Reinders, Anke; Ward, John M.; Offler, Christina E.; Patrick, John W.; Grof, Christopher P L.

In: Plant physiology, Vol. 173, No. 2, 02.2017, p. 1330-1341.

Research output: Contribution to journalArticle

Milne, RJ, Perroux, JM, Rae, AL, Reinders, A, Ward, JM, Offler, CE, Patrick, JW & Grof, CPL 2017, 'Sucrose transporter localization and function in phloem unloading in developing Stems', Plant physiology, vol. 173, no. 2, pp. 1330-1341. https://doi.org/10.1104/pp.16.01594
Milne, Ricky J. ; Perroux, Jai M. ; Rae, Anne L. ; Reinders, Anke ; Ward, John M. ; Offler, Christina E. ; Patrick, John W. ; Grof, Christopher P L. / Sucrose transporter localization and function in phloem unloading in developing Stems. In: Plant physiology. 2017 ; Vol. 173, No. 2. pp. 1330-1341.
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