C. Dordas et al., Permeability and channel-mediated transport of boric acid across membrane vesicles isolated from squash roots, PLANT PHYSL, 124(3), 2000, pp. 1349-1361
Boron is an essential micronutrient for plant growth and the boron content
of plants differs greatly, but the mechanism(s) of its uptake into cells is
not known. Boron is present in the soil solution as boric acid and it is i
n this form that it enters the roots. We determined the boron permeability
coefficient of purified plasma membrane vesicles obtained from squash (Cucu
rbita pepo) roots and found it to be 3 x 10(-7) +/-1.4 x 10(-8) cm s(-1), s
ix times higher than the permeability of microsomal vesicles. Boric acid I,
permeation of the plasma membrane vesicles was partially inhibited (30%-39
%) by mercuric chloride and phloretin, a non-specific channel blocker. The
inhibition by mercuric chloride was readily reversible by 2-mercaptoethanol
. The energy of activation for boron transport into the plasma membrane ves
icles was 10.2 kcal mol(-1). Together these data indicate that boron enters
plant cells in part by passive diffusion through the lipid bilayer of the
plasma membrane and in part through proteinaceous channels. Expression of t
he major intrinsic protein (MIP) PIP1 in Xenopus laevis oocytes resulted in
a 30% increase in the boron permeability of the oocytes. Other MIPs tested
(PIP3, MLM1, and GlpF) did not have this effect. WE postulate that certain
MIPs, like those that have recently been shown to transport small neutral
solutes, may also be the channels through which boron enters plant cells.