Aa. Bulychev et Wj. Vredenberg, Photoactivation of electrogenic activity in chloroplasts and its relation to photoinduced swelling of thylakoids, RUSS J PL P, 47(6), 2000, pp. 754-760
In patch-clamp experiments on isolated chloroplasts of Peperomia metallica
Lind, ct Rodig, (Piperaceae), the replacement of 50 mM KCL in a medium with
50 mM NH4Cl strongly influenced the parameters of photocurrent known to re
flect the generation of electric potential in thylakoids. The addition of N
H4+ to the medium modified the induction curves of the photocurrent as well
as the currents induced by single-turnover flashes in preilluminated chlor
oplasts. Under the action of a prolonged light pulse (similar to1 s), the s
teady-state current was much higher in the ammonium-containing medium than
in the presence of Kf. Preillumination of a dark-adapted chloroplast with a
1-s light pulse suppressed the current induced by a single-turnover flash
(6 Cls) in the presence of K+ but caused an elevation (by 50-150%) of the f
lash-induced current and shortening of its relaxation time in the presence
of NH4+ The origin of different induction kinetics for the photocurrent in
K+ and NH4+ media is partly clear, because ammonium prevents generation of
the pH gradient and, subsequently, eliminates the Delta pH-dependent suppre
ssion of the electron transport rate. However, this does not explain the or
igin of NH4+-dependent photostimulation of the current generated by single-
turnover flashes, This phenomenon arises from the thylakoid swelling caused
by the accumulation of NH4+ in the lumen and from the respective changes i
n the network resistances. The network element most sensitive to thylakoid
swelling is the lateral resistance of the lumen: it decreases upon enlargem
ent of the cross-section area. Stimulation of the flash-induced current by
preillumination in the presence of NH4+ was accompanied by accelerated rela
xation of the current, indicating that the phenomena observed are caused by
the reduction of network resistance involved in the discharge of the membr
ane capacity. Thus, the light-induced structural changes in the thylakoid s
ystem have a marked effect on the currents measured with the patch-clamp te
chnique.