PLANT PHYSIOLOGY , Vol 103, Issue 3 763-769, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Effects of Boron on Proton Transport and Membrane Properties of Sunflower (Helianthus annuus L.) Cell Microsomes
N. Ferrol, A. Belver, M. Roldan, M. P. Rodriguez-Rosales and J. P. Donaire
Departamento de Bioquimica Vegetal, Estacion Experimental del Zaidin, Consejo Superior de Investigaciones Cientificas, 18008 Granada, Spain
Boron deficiency and toxicity inhibit ATP-dependent H+ pumping and
vanadate-sensitive ATPase activity in sunflower roots and cell suspensions.
The effects of boron on H+ pumping and on passive H+ conductance, as well
as on fluorescence anisotropy in KI-washed microsomes isolated from
sunflower (Helianthus annuus L. cv Enano) cell suspensions, have been
investigated. Boron deficiency reduced the total and vanadate-sensitive
ATPase activities as well as the vanadate-sensitive ATP-dependent H+
pumping without affecting the amount of antigenic ATPase protein as
measured by immunoblotting with an Arabidopsis thaliana plasma membrane
anti-H+-ATPase polyclonal antibody. Kinetic studies revealed that boron
deficiency reduced Vmax of vanadate-sensitive ATPase activity with little
change in the apparent Km for Mg2+-ATP. Proton leakage was greater in
microsomal vesicles isolated from cells grown without boron and incubated
in reaction medium without added boron, and this effect was reversed by
addition of boron to the reaction medium. Fluorescence anisotropy indicated
that diphenyl hexatriene and
1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene probes were
immobilized to a greater extent in microsomes from cells grown without
boron than in those from cells grown with 100 [mu]M H3BO3. The apparent
decrease of membrane fluidity in microsomes from cells grown without boron
was reversed by the addition of boron to the reaction medium. Taken
together these data suggest that inhibition of H+ gradient formation in
microsomes from sunflower cells grown in the absence of boron could be due
to the combined effects of reduced H+-ATPase activity and increased passive
conductance across the membrane, possibly resulting from increased membrane
rigidity.
