Characterization of sodium transport at the plasma membrane of Spergularia marina
Wickens, Linda Karolyn
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Permalink
https://hdl.handle.net/2142/19644
Description
Title
Characterization of sodium transport at the plasma membrane of Spergularia marina
Author(s)
Wickens, Linda Karolyn
Issue Date
1991
Doctoral Committee Chair(s)
Cheeseman, John M.
Department of Study
Plant Biology
Discipline
Biology, Botany
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Botany
Biology, Cell
Biology, Plant Physiology
Language
eng
Abstract
Plants vary in their whole plant strategies for managing the sodium (Na$\sp{+}$) present in their environment, but current models predict that they have a common strategy at the cellular level: exclusion from the cytoplasm. This entails either passive exclusion mediated by low plasma membrane permeability or active exclusion of passively acquired Na$\sp{+}$ which has moved down its electrochemical gradient into the cell. Whole plant studies using the halophyte Spergularia marina have demonstrated high permeability for Na$\sp{+}$ into and out of root cells and the research reported here investigates these mechanisms by directly measuring Na$\sp{+}$ transport using radiolabelled Na$\sp{+}$ and sealed plasma membrane vesicles from the roots of this species. This is the first report to characterize $\sp{22}$Na$\sp{+}$ transport using plant plasma membrane vesicles.
Fractions from discontinuous and linear gradients of sucrose that were enriched in plasma membrane vesicles were identified for use in the $\sp{22}$Na$\sp{+}$ transport studies by the orthovanadate-sensitive P-type H$\sp{+}$-ATPase activity associated with them. Both the scalar and vectoral components of the activity were found to have negligible Na$\sp{+}$-dependent modifications. Indirect Na$\sp{+}$-dependent effects were observed, suggesting that Na$\sp{+}$ could disrupt K$\sp{+}$-dependent modifications of both components of the ATPase activity. An alternative explanation of the data was that an ATP-dependent, orthovanadate-insensitive, Na$\sp{+}$-dependent transport mechanism(s) mediated vesicle acidification. Radiolabelled Na$\sp{+}$ transport studies using similar conditions failed to show the presence of ATP-dependent transport of Na$\sp{+}$; and in addition, vesicle acidification studies, as well as $\sp{22}$Na$\sp{+}$ transport studies, suggested that a Na$\sp{+}$/H$\sp{+}$ antiport mechanism was not present in the plasma membrane. Two $\Delta\mu\sb{\rm Na+}$-driven transport mechanisms were identified using efflux studies. Lineweaver-Burk analysis predicted the presence of a Na$\sp{+}$ transport mechanism with a K$\sb{\rm m}$ of 93 mM Na$\sp{+}$ and a V$\sb{\rm max}$ of 52.6 nmoles Na$\sp{+}$ $\cdot$ (mg prot $\cdot$ min)$\sp{-1}$. Studies with monensin suggested that there was a Na$\sp{+}$-dependent alkalinization of the vesicles when the $\Delta\mu\sb{\rm Na+}$ was high.
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