Tonoplast K+ channels of Chara corallina are well characterized but only a
few reports mention anion channels, which are likely to play an important r
ole in the tonoplast action potential and osmoregulation of this plant. For
experiments internodal cells were isolated. Cytoplasmic droplets were form
ed in an iso-osmotic bath solution according to a modified procedure. Ion c
hannels with conductances of 48 pS and 170 pS were detected by the patch-cl
amp technique. In the absence of K+ in the bath solution the 170 pS channel
was not observed at negative pipette potential values. When Cl- on either
the vacuolar side or the cytoplasmic side was partly replaced with F-, the
reversal potential of the 48 pS channel shifted conform to the Cl- equilibr
ium potential with similar behavior in droplet-attached and excised patch m
ode. These results showed that the 48 pS channel was a Cl- channel. In drop
let-attached mode the channel rectified outward current flow, and the slope
conductance was smaller. When Chara droplets were formed in a bath solutio
n containing low (10(-8) M) Ca2+, then no Cl- channels could be detected ei
ther in droplet-attached or in inside-out patch mode. Channel activity was
restored if Ca2+ was applied to the cytoplasmic side of inside-out patches.
Rectification properties in the inside-out patch configuration could be co
ntrolled by the holding pipette potential. Holding potential values negativ
e or positive to the calculated reversal potential for Cl- ions induced opp
osite rectification properties. Our results show Ca2+-activated Cl- channel
s in the tonoplast of Chara with holding potential dependent rectification.